Modern Acetylene Chemistry - Startseite€¦ · Modern acetylene chemistry / ed. by P. J. Stang and F. Diederich. - Weinheim : New York ; ... chemistry. The reactions and transformations

Modern Acetylene Chemistry

Edited by P. J. Stang and F. Diederich

VCH 4b Weinheim . New York . Base1 . Cambridge . Tokyo

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ISBN 3-527-29084-2




Prof. Dr. Peter J . Stang Department of Cheinistry University of Utah Salt Lake City, UT 84 I 12 USA

Prof. Dr. FranCois Diederich Laboratorium fur Organische Cheinie EidgenBssische Technische Hochschule ETH-Zcntrum Universitiitstrasse I6 CH-8092 Zurich Switzerland

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Modern acetylene chemistry / ed. by P. J. Stang and F. Diederich. - Weinheim : New York ; Basel ; Cambridge; Tokyo: VCH, 1995

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Foreword

The carbon-carbon triple bond is one of the oldest and simplest functional groups in chemistry. The reactions and transformations of this humble functionality are intertwined with the history and development of organic chemistry. In the past dozen years, acetylene chemistry has experienced a major renaissance engendered by the incurrence of molecules with C = C bonds in the frontiers of modern organic chemistry - namely biochemistry and materials science. An entire family of powerful antitumor antibiotics with cis-enediynes as reactive fragments was discovered in the mid 1980s, and new members of this family continue to be found. The range and potential of these antitumor antibiotics has been greatly expanded by a family of synthetic enediynes capable, analogously to their natural counterparts, of undergoing the Bergmann cycloaromatization and efficiently nicking and cleaving DNA. On the other hand, acetylenic molecular scaffolding has been employed to prepare multinano- meter-sized molecular objects with unprecedented structures, functions, and properties. Some of these materials are being developed into components for molecular electronics; others form crystals with molecular pores for separation, inclusion, and catalysis, and thereby become the organic counterparts of zeolites. Acetylenic two- and three-dimensional carbon allotropes, with structures and functions different from the natural modifications of diamond and graphite, as well as fullerenes, are under construction. New, fully conjugated, acetylenic polymer backbones complement the functional property range of polyacetylenes and polydiacetylenes. The construction of organic ferromagnets based on acetylenic backbones and scaffolds is being explored intensively.

These developments, which offer plenty of fascinating perspectives at the two interfaces to materials science and biology, are efficiently fueled by the invention of powerful new synthetic methodology, based to a large extent on transition metal chemistry. The invention of new synthetic methods has particularly facilitated the cross-coupling between acetylenic sp-C atoms and alkene and arene sp2-C-atoms; reactions crucial to molecular scaffolding. Other impor- tant advances have been made in the formation of five-, six-, and higher-membered rings using alkyne transition metal chemistry. Small reactive acetylenes such as iodonium derivatives are increasingly used as reagents in organic synthesis since ways have now been found to control their reactivity and tame their previous tendency for spontaneous decomposition. The chemistry of heteroalkynes such as phosphaalkynes has emerged over the past decade.

Theoretical chemistry has been challenged by the broad new developments in modern acetylene chemistry. Structures and electronic configurations of acyclic and cyclic acetylenic rr-systems have attracted the interest of both experimentalists and theoreticians, and much of the current knowledge on homoconjugation, and on through-space orbital interactions between precisely aligned chromophores, has been gained in studies of acetylenic systems. The structures and electronic properties of acetylenic all-carbon rods and rings, which are formed in the laser vaporization of graphite and occur as intermediates in fullerene production processes, have attracted much interest from theoreticians, providing attractive and challenging targets to calibrate and improve computational methods.

VI Foreword

This multi-author monograph documents and critically analyzes these recent developments in contemporary acetylene chemistry in 13 chapters written by leading scientists in the various areas. With emphasis on the above-mentioned modern developments, the monograph does not duplicate previous treatises on alkyne chemistry such as Houben- Weyl-Miiller Vol. V/2a (Alkine, Di und Polyine, Allene, Kumulene), the volumes in the Patai series on The Chemistry of the Carbon-Carbon Triple Bond, the pioneering monograph on The Chemistry of Acetylenes by H. G. Viehe, or the book by Brandsma on Preparative Acetylenic Chemistry with a great variety of useful synthetic procedures. Rather, it builds upon these predecessors and complements them by updating the reader on the broad new developments in today’s acetylene chemistry. To enhance the practical value of the monograph, most experimental chapters include synthetic protocols which have been chosen for broad utility and application.

We anticipate and hope that this monograph will further stimulate the development and application of acetylene chemistry as one of the key synthetic, structural, and functional tools of future chemistry.

Salt Lake City and Zurich December 1994

Peter J. Stang Fransois Diederich

Contents

Foreword

List of Contributors

1 Modern Computational and Theoretical Aspects of Acetylene Chemistry

Dietmar A . Plattner, Yi Li. K . N . Houk

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 1.2.1 1.2.2 1.2.3

1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5

1.4 1.4.1 1.4.2 1.4.2.1 1.4.2.2 1.4.2.3 1.4.2.4 1.4.2.5 1.4.2.6 1.4.2.1

1.5

Electronic structures of acetylene and monoacetylenes . . . . . . . . . 1 Ground-state potential energy surfaces . . . . . . . . . . . . . . . . 3 Excited-state potential energy surfaces . . . . . . . . . . . . . . . . 5 Radicalions . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Reactivities and molecular interactions of acetylenes . . . . . . . . . . 7 Pericyclic reactions . . . . . . . . . . . . . . . . . . . . . . . . 7 Electrophilic reactions . . . . . . . . . . . . . . . . . . . . . . . 10 Nucleophilic additions . . . . . . . . . . . . . . . . . . . . . . 11 Radical additions . . . . . . . . . . . . . . . . . . . . . . . . . 11 Molecular complexes . . . . . . . . . . . . . . . . . . . . . . . 12

Polyacetylenes . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Diacetylene . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 C, and cyclic C, . . . . . . . . . . . . . . . . . . . . . . . . .

c4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 C5. C,. andC, . . . . . . . . . . . . . . . . . . . . . . . . . . 19

CI1 to c,, . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

15 Cz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 C3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

C,j. C8. and Clo . . . . . . . . . . . . . . . . . . . . . . . . . 20

c18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

VIII Contents

2 Functionalized Acetylenes in Organic Synthesis . The Case of the 1-Cyano- and the 1-Halogenoacetylenes

Henning HopJ Bernhard Witulski

2.1

2.2 2.2.1 2.2.2 2.2.2.1 2.2.2.2

2.2.2.3

2.3 2.3.1 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.3

2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

Synthesis and preparative use of cyanoacetylenes . . . . . . . . . . . Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparative use of cyanoacetylenes . . . . . . . . . . . . . . . . . A short summary of the older literature . . . . . . . . . . . . . . Novel cycloadditions with cyanoacetylenes . simple and efficient methods for the construction of complex carbon frameworks . . . . . . . . . . Cyanoacetylenes as precursors for reactive and interstellar intermediates . .

Synthesis and preparative use of I-halogenoacetylenes . . . . . . . . . Older review of the literature on halogenoacetylenes . . . . . . . . . . Synthesis of 1-halogenoacetylenes . . . . . . . . . . . . . . . . . . The preparation of the 1-halogeno- and 1, 2.dihalogenoethynes . . . . . More highly unsaturated halogenoacetylenes . . . . . . . . . . . . . Derivatives of 1-halogenoacetylenes . . . . . . . . . . . . . . . . . Novel preparative uses of 1-halogeno- and 1. 2.dihalogenoacetylenes . . . Experimental procedures . . . . . . . . . . . . . . . . . . . . . Cyanoacetylene (1) . . . . . . . . . . . . . . . . . . . . . . . . Dicyanoacetylene (2) . . . . . . . . . . . . . . . . . . . . . . . Dicyanodiacetylene (3) . . . . . . . . . . . . . . . . . . . . . . Chloroacetylene (93) . . . . . . . . . . . . . . . . . . . . . . . Dichloroacetylene (100) . . . . . . . . . . . . . . . . . . . . . . Diiodoacetylene (105) . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Alkynyliodonium Salts: Electrophilic Acetylene Equivalents

Peter .L Stang

3.1

3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.2.7

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .

Preparation and properties . . . . . . . . . . . . . . . . . . . . . Alkynyliodonium sulfonates . . . . . . . . . . . . . . . . . . . . Alkynyliodonium tetrafluoroborates . . . . . . . . . . . . . . . . . Heterocyclic alkynyliodonium species . . . . . . . . . . . . . . . . Mechanism of formation . . . . . . . . . . . . . . . . . . . . . Diynyliodonium and dialkynyliodonium triflates . . . . . . . . . . . Bis-iodonium species . . . . . . . . . . . . . . . . . . . . . . . Properties of alkynyliodonium salts . . . . . . . . . . . . . . . . .

33

34 34 38 38

39 46

48 48 48 48 50 52 53

60 60 60 61 61 62 62 63

67

68 68 69 70 71 72 72 73

Contents IX

3.3 3.3.1 3.3.2

3.4 3.4.1 3.4.1.1 3.4.1.2 3.4.1.3 3.4.1.4 3.4.1.5 3.4.1.6 3.4.2 3.4.3 3.4.3.1 3.4.3.2

3.5

3.6 3.6.1 3.6.2

3.6.3

3.6.4

3.6.5 3.6.6

3.6.7

4

4.1 4.2 4.2.1 4.2.2

4.3

4.4 4.4.1

Characterization and structure . . . . . . . . . . . . . . . . . . . Spectroscopic properties . . . . . . . . . . . . . . . . . . . . . . X-ray and molecular structure . . . . . . . . . . . . . . . . . . .

Reactions and uses of alkynyliodonium salts . . . . . . . . . . . . . Reaction with nucleophiles . . . . . . . . . . . . . . . . . . . . . Carbon nucleophiles . . . . . . . . . . . . . . . . . . . . . . . Nitrogen nucleophiles . . . . . . . . . . . . . . . . . . . . . . . Oxygen nucleophiles . . . . . . . . . . . . . . . . . . . . . . . Sulfur nucleophiles . . . . . . . . . . . . . . . . . . . . . . . . Phosphorus nucleophiles . . . . . . . . . . . . . . . . . . . . . Halogen nucleophiles . . . . . . . . . . . . . . . . . . . . . . . Reaction with organometallic species . . . . . . . . . . . . . . . . Cycloaddition reactions . . . . . . . . . . . . . . . . . . . . . . [2 + 41-Diels-Alder cycloadditions . . . . . . . . . . . . . . . . . 1. 3.Dipolar cycloadditions . . . . . . . . . . . . . . . . . . . . .

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .

Experimental procedures . . . . . . . . . . . . . . . . . . . . .

General procedure for the preparation of P-alkyl- and P-phenylethynyl(phe- ny1)iodonium triflates. 10 . . . . . . . . . . . . . . . . . . . . . General preparation of B-functionalized ethynyl(pheny1)iodonium triflates. 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General procedure for the preparation of bis-iodonium diyne bktriflates. 34 and 35 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of bis(phenyl[ [(trifluoromethyl)sulfonyl]oxy]iodo)ethyne, 30 . General procedure for the Diels- Alder reaction of alkynyl(pheny1)iodonium

General procedure for the preparation of cyclopentenones and y-lactams . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(Cyano[[(trifluoromethyl)sulfonyl]oxy)iodo)benzene, 7 . . . . . . . . .

salts. 11. with 1. 3.dienes. formation of cycloadducts 118-120 . . . . . .

The Chemistry of Metal-Alkyne Complexes

Gagik G . Melikyan. Kenneth M . Nicholas

74 74 75

76 77 78 80 81 83 86 87 88 90 90 91

92

92 92

93

93

93 94

94 94 95

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Bonding and structure . . . . . . . . . . . . . . . . . . . . . . . 99 Alkyne complexes . . . . . . . . . . . . . . . . . . . . . . . . 99 Propargylium-metal complexes . . . . . . . . . . . . . . . . . . . 101

Complexes of novel alkynes . . . . . . . . . . . . . . . . . . . . 104

Reactions of metal-alkyne complexes . . . . . . . . . . . . . . . . 107 Reactions at the C - C triple bond . . . . . . . . . . . . . . . . . 107

X Contents

4.4.1.1 4.4.1.2 4.4.1.3 4.4.1.4 4.4.1.5 4.4.1.6 4.4.1.7 4.4.1.8 4.4.1.9 4.4.2 4.4.2.1 4.4.2.2 4.4.3 4.4.3.1 4.4.3.2 4.4.3.2.1 4.4.3.2.2 4.4.3.2.3 4.4.3.2.4 4.4.4 4.4.5

4.5

4.6 4.6.1

4.6.2 4.6.3

Nucleophilic addition . . . . . . . . . . . . . . . . . . . . . . . Electrophilic addition . . . . . . . . . . . . . . . . . . . . . . . M . H addition/hydrogenation . . . . . . . . . . . . . . . . . . . M . C addition . . . . . . . . . . . . . . . . . . . . . . . . . . Coupling reactions with unsaturated substrates . . . . . . . . . . . .

Cluster substitution/expansion . . . . . . . . . . . . . . . . . . . Demetalation . . . . . . . . . . . . . . . . . . . . . . . . . . Nucleophilic addition to mononuclear q 3-propargylium-M complexes . . Reactions at the complexed acetylenic C . X bond . . . . . . . . . . . Alkyne-vinylidene isomerization . . . . . . . . . . . . . . . . . . Reactions of complexed terminal alkynes with base . . . . . . . . . . Reactions at the propargylic (a) carbon

Alkyne scission/metathesis/polymerization . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . Alkyne/allene isomerization . . . . . . . . . . . . . . . . . . . . Reactions of dinuclear propargylium complexes with nucleophiles . . . . General reaction features . . . . . . . . . . . . . . . . . . . . . Proton loss/elimination . . . . . . . . . . . . . . . . . . . . . . Coupling with noncarbon nucleophiles . . . . . . . . . . . . . . . . Coupling with carbon nucleophiles . . . . . . . . . . . . . . . . . Reactions remote from the complexed triple bond . . . . . . . . . . . Reaction summary . . . . . . . . . . . . . . . . . . . . . . . .

Special applications of metal-alkyne complexes . . . . . . . . . . . .

Selected experimental procedures . . . . . . . . . . . . . . . . . . p[(q ’, q 2-l-Methyl-2-propynylium)dicobalthexacarbonyl] tetrafluoroborate (126) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-(l-Methyl-2-propynyl)cyclohexanone (127) . . . . . . . . . . . . . p-[q ’, q 2.dl.3,4-Diphenyl.1,5.cyclooctadiyne].bi s.hexacarbony1dicob.t (128) . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 Organometallic Cycloaddition Reactions of Acetylenes

Joseph A . Casalnuovq Neil E . Schore

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.2 Cycloadditions of acetylenes with Fischer carbenes . . . . . . . . . . . 5.2.1 Naphthols - the Dtitz reaction . . . . . . . . . . . . . . . . . . . 5.2.2 Indenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Cyclobutenones . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Cyclopentenones . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Cycloheptadienones . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6 Cyclopropanes . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.7 Heterocyclic ring systems . . . . . . . . . . . . . . . . . . . . .

107 108 109 110 110 114 115 115 116 116 116 117 118 118 118 118 120 121 122 128 128

128

130

130 131 131 132

139

139 140 147 149 150 151 151 153

Contents XI

5.3 The Pauson-Khand reaction: cycloadditions of olefins. acetylenes. and CO 5.3.1 Background and mechanism . . . . . . . . . . . . . . . . . . . . 5.3.2 Intermolecular Pauson-Khand reaction . . . . . . . . . . . . . . . 5.3.3 Intramolecular Pauson-Khand reaction . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

6.1

6.2

6.3

6.4

6.5 6.5.1 6.5.1.1 6.5.1.2 6.5.1.3 6.5.1.4

6.5.1.5 6.5.1.6 6.5.2 6.5.2.1 6.5.2.2 6.5.2.3 6.5.3 6.5.3.1 6.5.3.2 6.5.3.3 6.5.3.4

6.6

6.7 6.7.1 6.7.2 6.7.3

6.7.4

Phosphaalkynes . Starting Point for the Synthesis of Phosphorus-Carbon Cage Compounds

Manfred Regitz. A . Hoffmann. L! BergstraJer

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

Syntheses of phosphaalkynes . . . . . . . . . . . . . . . . . . .

Reactivity of phosphaalkynes . . . . . . . . . . . . . . . . . . .

The history of phosphorus-carbon cage compounds from phosphaalkynes

Synthesis of phosphorus-carbon cage compounds . . . . . . . . . . . Construction by cycloaddition reactions . . . . . . . . . . . . . . . Diphosphatetracyclodecenes . . . . . . . . . . . . . . . . . . . . Phosphaprismanes and phosphabenzvalenes . . . . . . . . . . . . . Diphosphatricyclooctenes . . . . . . . . . . . . . . . . . . . . . Diphosphatetracycloundecadienones and oxadiphosphapentacyclononade- capentaenones (the tropone reaction of phosphaalkynes) . . . . . . . . Diphosphirenes as intermediates for phosphorus-carbon cage compounds Thermal cyclotetramerization . . . . . . . . . . . . . . . . . . . Construction by extrusion of Cp2Zr from phosphaalkyne dimer complexes Cp2Zr-phosphaalkyne dimer complexes . . . . . . . . . . . . . . . Tetraphosphacubanes and isomeric cage compounds P-functionalization of the tetraphosphacubane system . . . . . . . . . Cyclooligomerization with the aid of Lewis acids . . . . . . . . . . .

Phosphaalkyne tetramers from the spirocyclotrimer 71a Hexaphosphapentaprismane from the spirocyclotrimer 71a . . . . . . . Phosphorus-carbon-aluminum cage compounds . . . . . . . . . . .

. . . . . . . . .

Spirocyclotrimerization . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Experimental procedures . . . . . . . . . . . . . . . . . . . . . 2,2.Dimethyl. l.(trimethylsiloxy)propy~dene(trimethylsilyl)phosphane (10a) (2. 2.Dimethylpropylidyne)phosphane (9 a) . . . . . . . . . . . . . . Bis(q 5.cyclopentadienyl)(2,4.di.ter t.

2,4.6,8.Tetra.tert.butyl.l.3,5, 7.tetraphosphapentacyclo [4.2.0.02!' .O 3.* .04. '1 octane (53a) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

butyl-1,3-diphosphabicyclo[l.l.0]butan-2,4-diyl)zirconium (59a) . . . . .

154 155 157 161 167

173

174

175

176

177 177 177 178 180

182 183 184 185 186 186 188 189 189 190 192 194

195

196 196 196

197

197

XI1 Contents

6.7.5 2,4,6-Tri-tert-butyl-1,5-diphospha-3-phosphoniaspiro[3.4]hexa-1,4-diene-6-t- richloroaluminate (71 a) . . . . . . . . . . . . . . . . . . . . . . 197

6.7.6 2,5,6,8-Tetra-tert-butyl-l,3,4,7-tetraphosphatetracyclo[3.3.0.02~4.03~6]o~t-7_ene (76) 198 6.7.7 1,4,6.Tri.tert.butyl.2,5,7,7,8,8.hexaethyl.5,8.dialuminato.3.phosph a.

2,7-diphosphoniatetracyclo[3.3.0.~~4.03~6]~ctane (80) . . . . . . . . . . 198 6.7.8 2,5,7,9.Tetra.tert.butyl.3,3,4.triethyl.4.aluminato.3,6,8.triphospha.l.pho s.

phoniatetracyclo[4.2.1.0'~5.0479]octane (81) . . . . . . . . . . . . . . 198 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

7

7.1

7.2 7.2.1 7.2.2

7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.3.5 7.3.5.1 7.3.5.2 7.3.5.3

7.4 7.4.1 7.4.1.1 7.4.1.2 7.4.2 7.4.2.1

7.4.2.2 7.4.2.3

7.4.2.4 7.4.3 7.4.4

7.5

The Enediyne Antibiotics

K . C. Nicolaou. Adrian L . Smith

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

The aromaticity era . . . . . . . . . . . . . . . . . . . . . . . . The cycloaromatization of conjugated polyenyne systems . . . . . . . . Application to the synthesis of aromatic systems . . . . . . . . . . . .

The discovery of the enediyne antibiotics Neocarzinostatin . . . . . . . . . . . The calicheamicins . . . . . . . . . . The esperamicins . . . . . . . . . . . The dynemicins . . . . . . . . . . . The chromoprotein enediyne antibiotics . Kedarcidin . . . . . . . . . . . . .

Maduropeptin . . . . . . . . . . . . C-1027 . . . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

Theoretical and synthetic studies on the enediyne antibiotics . . . . . . Neocarzinostatin chromophore model systems . . . . . . . . . . . . Theoretical considerations . . . . . . . . . . . . . . . . . . . . . Synthetic studies . . . . . . . . . . . . . . . . . . . . . . . . . Calicheamicin/esperamicin theoretical and synthetic studies . . . . . . Synthetic and theoretical studies on the Bergman cycloaromatization of cyclic enediynes . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic approaches to the calicheamicin aglycone . . . . . . . . . . Synthetic approaches to the calicheamicin/esperamicin carbohydrate fragments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Total synthesis of calicheamicin y! . . . . . . . . . . . . . . . . . Dynemicin synthetic studies . . . . . . . . . . . . . . . . . . . . The chromoprotein enediyne antibiotics . . . . . . . . . . . . . . . Medical applications of the enediyne antibiotics . . . . . . . . . . . .

203

205 205 206

207 208 212 216 217 221 221 223 224

224 224 224 226 238

238 241

249 258 261 273

273

7.6 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 274 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

Contents XI11

8

8.1

8.2 8.2.1 8.2.1.1

8.2.1.2 8.2.2 8.2.2.1 8.2.2.2 8.2.2.3 8.2.3

8.3 8.3.1 8.3.2

8.4 8.4.1 8.4.2 8.4.3 8.4.3.1 8.4.3.2 8.4.3.3

8.5

8.6

8.7 8.7.1 8.7.1.1 8.7.1.2 8.7.1.3 8.7.2 8.7.2.1

8.7.2.2

8.7.3

Cyclic Alkynes: Preparation and Properties

Rolf Gleiter, Roland Merger

Introduction . . . . . . . . . . . . . . . . . . .

Synthesis of cyclic acetylenes . . . . . . . . . . . . . . . . . . . Cyclic alkynes from ring-closure reactions . . . . . . . . . . . . . . Using acetylenic reactivity: nucleophilic substitution with metal acetylides and related reactions . . . . . . . . . . . . . . . . . . . . . . . Employing propargylic cations. anions. and radicals . . . . . . . . . Cyclic alkynes from elimination reactions . . . . . . . . . . . . . . 1. 2.Elimination . . . . . . . . . . . . . . . . . . . . . . . . . Cycloelimination reactions . . . . . . . . . . . . . . . . . . . . . Ring contraction . . . . . . . . . . . . . . . . . . . . . . . . . Ring-enlargement reactions . . . . . . . . . . . . . . . . . . . .

Structural and spectroscopic properties . . . . . . . . . . . . . . . . Structures of cyclic mono- and dialkynes . . . . . . . . . . . . . . Photoelectron spectra of cyclic diacetylenes . . . . . . . . . . . . . .

Organic reactions of cyclic alkynes . . . . . . . . . . . . . . . . . Rearrangement of cyclic alkynes . . . . . . . . . . . . . . . . . . Transannular reactions . . . . . . . . . . . . . . . . . . . . . .

Homonuclear addition reactions . . . . . . . . . . . . . . . . . . Heteronuclear addition reactions . . . . . . . . . . . . . . . . . .

Addition reactions of cyclic alkynes . . . . . . . . . . . . . . . . .

Cycloaddition reactions . . . . . . . . . . . . . . . . . . . . . .

Reactions of cyclic alkynes with metal compounds . . . . . . . . . . .

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .

Experimental procedures . . . . . . . . . . . . . . . . . . . . . Preparation of cyclic dialkynes of ring size Cl2.Cl. . . . . . . . . . General procedure . . . . . . . . . . . . . . . . . . . . . . . . 1. 7.Cyclododecadiyne (3) . . . . . . . . . . . . . . . . . . . . . l&Cyclotetradecadiyne (120) . . . . . . . . . . . . . . . . . . . General procedure for Dewar benzenes 181 and 182 . . . . . . . . . Dimethyl tetracyclo[l2.2.0.0'~7.08~14]hexadeca-7,15-diene-15,16-dicarboxylate (182; n = 5 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . Dimethyl tetracyclo[7.5.2.0.02~8]hexadeca-2,15-diene-l5,16-dicarboxylate (181;

Cyclonon-2-ynone (91) and bicyclo[6.l.0]non-l(8)-en-9-one (92) . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n = 5 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

285

286 286

286 288 292 292 293 294 295

296 296 301

303 303 305 308 308 309 309

311

314

314 314 314 314 314 315

315

315 315 316

XIV Contents

9

9.1

9.2

9.3

9.4

9.5

9.6 9.6.1 9.6.2

9.6.3

9.6.4 9.6.5

9.6.6

Macrocyclic Homoconjugated Polyacetylenes

Lawrence T Scott. Mark J. Cooney

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pericyclynes . . . . . . . . . . . . . . . . . . . . . . . . . . .

“Exploded” pericyclynes . . . . . . . . . . . . . . . . . . . . .

Homoconjugated mixed polyalkyne/diyne macrocycles . . . . . . . . .

Heterocyclic cognates of pericyclynes . . . . . . . . . . . . . . . .

Experimental procedures . . . . . . . . . . . . . . . . . . . . .

Conversion of a terminal acetylene to a bromoalkyne using tosyl bromide (30 -, 50) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of a 1. 3.diyne by cross-coupling of a preformed copper ace- tylide with a bromoalkyne - 2 : 1 example (49 + 50 + 51) Oxidative cyclization of a long-chain a. o.diyne (53 + 44)

Conversion of a methyl ketone to a terminal acetylene (28-30) . . . .

. . . . . . .

. . . . . . . Coupling of a terminal acetylene with a tertiary propargylic chloride - 2: 1 example (47 + 69) . . . . . . . . . . . . . . . . . . . . . . Conversion of a 2, 2.dibromovinyl compound to a bromoalkyne - two-fold example (75 + 74) . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 Polyacetylene

Eric J. Ginsburg. Christopher B . Gorman. Robert H . Grubbs

10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

10.2 10.2.1 10.2.1.1 10.2.1.2 10.2.2 10.2.2.1 10.2.2.2 10.2.3

Syntheses and properties . . . . . . . . . . . . . . . . . . . . . . Routes from alkynes . . . . . . . . . . . . . . . . . . . . . . . Acetylene polymerization . . . . . . . . . . . . . . . . . . . . . Polymerization of substituted alkynes . . . . . . . . . . . . . . . . Routes from alkene precursors . . . . . . . . . . . . . . . . . . . Nonmetathetic routes . . . . . . . . . . . . . . . . . . . . . . . Routes using olefin metathesis . . . . . . . . . . . . . . . . . . . Ring-opening of cyclooctatetraene . . . . . . . . . . . . . . . . . .

10.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Experimental procedures . . . . . . . . . . . . . . . . . . . . . 10.4.1 10.4.2 10.4.3

Synthesis of substituted polycyclooctatetraenes . . . . . . . . . . . . Cis/?runs isomerization of soluble polycyclooctatetraenes . . . . . . . . A precursor route to polyacetylene . . . . . . . . . . . . . . . . . .

321

322

330

337

340

345 345

347

347 348

348

349 349

353

358 358 358 359 363 363 366 368

376

376 376 377 377

Contents XV

10.4.3.1 Synthesis of poly(diethy1 7-oxabicyclo[2.2.l]hepta-2,5-diene-2,3-dicarboxylate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

10.4.3.2 Solid-state production of polyacetylene from poly(diethy1 7-oxabicyclo [2.2.l]hepta-2,5-diene-2,3-dicarboxylate) . . . . . . . . . . . . . . . 378

10.4.3.3 Solution production of polyacetylene from poly(diethy1 7-oxabicyclo[2.2.1] hepta.2.5.diene.2,3.dicarboxylate) . . . . . . . . . . . . . . . . . . 378 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

11

11.1

11.2 11.2.1 11.2.2 11.2.3

11.3 11.3.1 11.3.2

11.3.3

11.4 11.4.1

11.4.2

11.4.3

11.4.3.1 11.4.3.2

11.5

11.6

11.7 11.7.1 11.7.2 11.7.2.1

Acetylenic Compounds as Building Blocks for High-Spin Molecules and Molecular Assemblies

Hiizu Iwamura. Kenji Matsuda

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

Alkynyl compounds carrying unpaired electrons . . . . . . . . . . . Alkynyl compounds carrying unpaired electrons in remote substituents . . Alkynes bonded to paramagnetic transition metals . . . . . . . . . . . 2-Propynylidenes . . . . . . . . . . . . . . . . . . . . . . . . .

Molecular crystals of organic free radicals that carry alkynyl substituents . What makes acetylenic compounds unique in assembling their molecules? Guiding principles on aligning electron spins in parallel between two neigh- boring molecules . . . . . . . . . . . . . . . . . . . . . . . . . Crystals of antiferromagnetic 1. 3.butadiyne and ferromagnetic 1.3.5.hex a. triyne both carrying 4-chloro-3-(N-tert-butyl-N-oxyamino)phenyl as a stable free-radical substituent . . . . . . . . . . . . . . . . . . . . . . Spin alignments in poly(phenylacety1enes) and poly(1. 3.butadiynes) . . . . Natural spins detected during the solid-state polymerization of 1.3.but a. diynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Topological control of the high-spin vs . low-spin ground states of A-con-

Attempts at introducing stoichiometric amounts of spins in poly(pheny1ace- tylenes) and poly(phenyldiacety1enes) . . . . . . . . . . . . . . . . Poly(phenylacety1enes) . . . . . . . . . . . . . . . . . . . . . . Poly(phenyldiacety1enes) . . . . . . . . . . . . . . . . . . . . . .

dary m i n e . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . .

Experimental procedures . . . . . . . . . . . . . . . . . . . . Characterization of magnetic properties . . . . . . . . . . . . . . Synthesis of dendritic “Starburst” dodecaketone 49 . . . . . . . . 1.(3,5.Dibenzoylbenzoyl).3.(3.trimethylsilyl. 2-propynoy1)benzene (52) .

jugated diradicals and dicarbenes . . . . . . . . . . . . . . . . . .

Cyclotrimerization reaction of benzoylacetylenes in the presence of a secon-

385

385 385 387 389

391 391

392

393

395

395

398

400 400 402

403

409

409 409 410 410

XVI Contents

11.7.2.2 1.(3,5.Dibenzoylbenzoy1).3.( 2.propynoyl)benzene (56) . . . . . . . . . 11.7.2.3 1,3,5.Tris[3.(3, 5.dibenzoylbenzoyl)benzoyl]benzene (49) . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12 Acetylenes in Nanostructures

James K . Young. Jeffrey S . Moore

12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.1 Structural parameters of phenylacetylenes . . . . . . . . . . . . . .

Synthetic considerations for phenylacetylene dendrimer construction . . . 12.2 Phenylacetylene dendrimers . . . . . . . . . . . . . . . . . . . . 12.2.1 12.2.1.1 The divergent and convergent synthetic approaches . . . . . . . . . . 12.2.1.2 Convergent synthesis of phenylacetylene dendrimers . . . . . . . . . 12.2.1.3 Effect of varying focal point functionality on the convergent synthesis of

phenylacetylene dendrimers . . . . . . . . . . . . . . . . . . . . 12.2.1.4 Synthesis of dendrimers by repetition of monomer enlargement (SYN-

DROME method) . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1.5 “Double exponential” dendrimer growth . . . . . . . . . . . . . .

12.3 Phenylacetylene macrocycles . . . . . . . . . . . . . . . . . . . . 12.3.1 Phenylacetylene macrocyclic framework . . . . . . . . . . . . . . . 12.3.2 Synthetic considerations for phenylacetylene macrocycle construction . . . 12.3.2.1 The double cyclization of branched phenylacetylene oligomers . . . . . 12.3.2.2

12.4

Tandem bimolecular coupling followed by intramolecular cyclization to form a foldable phenylacetylene macrotetracycle Synthesis of sequence-specific phenylacetylene oligomers and dendrimers on an insoluble solid support . . . . . . . . . . . . . . . . . . .

12.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . .

12.6 12.6.1

12.6.2 12.6.3 12.6.4 12.6.4.1

12.6.4.2 12.6.4.3 12.6.4.4

12.6.5 12.6.6

Experimental procedures . . . . . . . . . . . . . . . . . . . . . 48-Cascade: benzene[3.1.3. 51 : (5.ethynyl.l. 3.phenylene)G. 5-ethynyl- 1,3.di(ter t.butyl)benzene (8) . . . . . . . . . . . . . . . . . . . . General procedure for double cyclization . . . . . . . . . . . . . . Sample preparation for mass spectrometry . . . . . . . . . . . . . .

General procedure A: Pd(0)-catalyzed coupling reactions (except for trime- thylsilylacet ylene) . . . . . . . . . . . . . . . . . . . . . . . . General procedure B: Pd(0)-catalyzed coupling with trimethylsilylacetylene General procedure C : trimethylsilyl deprotection . . . . . . . . . . . General procedure D: liberation of the oligomeric sequence from the support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peptide linkage to aminomethylated polystyrene (26) . . . . . . . . . Ether linkage to chloromethylated polystyrene (28) . . . . . . . . . . .

Procedures for solid-supported phenylacetylene chemistry . . . . . . .

410 410 411

415 416

418 419 420 421

423

423 424

426 428 430 430

431

433

436

437

437 437 437 438

438 43 8 438

439 439 439

Contents XVII

12.6.7 Propylaminomethylated polystyrene (29) . . . . . . . . . . . . . . 12.6.8 Direct triazene linkage to propylaminomethylated polystyrene (31) . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 Oligoacetylenes

Franqois Diederich

13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .

13.2 13.2.1 The retro-Diels-Alder route to cyclo.C, . . . . . . . . . . . . . . . 13.2.2 The 3.cyclobutene.l. 2.dione route to the cyclocarbons . . . . . . . . . 13.2.3 The transition metal complex route to cycIo-Cl8 . . . . . . . . . . . 13.3 Tetraethynylethenes, fully cross-conjugated n-electron chromophores. and

other perethynylated molecules . . . . . . . . . . . . . . . . . . . 13.3.1 Synthesis of tetraethynylethene (20) and geminally bisdeprotected deriva-

tives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.2 Synthesis of monodeprotected tetraethynylethenes . . . . . . . . . . . 13.3.3 Synthesis of trans-bis(triisopropylsily1)-protected and trans-bisdeprotected

13.3.4 Synthesis of cis-bisdeprotected tetraethynylethenes . . . . . . . . . . . 13.3.5 Other perethynylated compounds as potential monomers for carbon net-

works . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Synthetic approaches to the cyclocarbons . . . . . . . . . . . . . . .

tetraethynylethenes . . . . . . . . . . . . . . . . . . . . . . . .

13.4

13.4.1 13.4.2

13.5 13.5.1 13.5.2

13.6

13.7 13.7.1 13.7.2

13.7.3 13.7.4 13.7.5 13.7.6

Perethynylated dehydroannulenes and expanded radialenes: large carbon cores on the way to all-carbon sheets . . . . . . . . . . . . . . . .

Perethynylated expanded radialenes . . . . . . . . . . . . . . . . . Molecular wires : from polytriacetylenes to carbyne . . . . . . . . . . Linear polyynes: short oligomers of elusive carbyne . . . . . . . . . Stable soluble conjugated carbon rods with a polytriacetylene backbone . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental procedures . . . . . . . . . . . . . . . . . . . . . 3,4.Bis[triisopropylsilyl)ethynyl].3.cyclobutene.l. 2.dione (12f) . . . . . Oxidative Hay coupling of 14 to the cyclobutene-fused dehydroannulenes

3.Dibromomethylene.l,5.bis(trimethylsily~).l, 4.pentadiyne (23) . . . . . (E).1,2.Diethynyl.l, 2.bis[(triisopropylsilyl)ethynyl]ethene (30a) . . . . . Eglinton-Glaser coupling of 54 to the expanded radialenes 51 and 53 . . . General procedure for solution-spray flash vacuum pyrolysis (SS-FVP) . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Perethynylated dehydroannulenes . . . . . . . . . . . . . . . . . .

15-17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

439 440 441

443

443 445 446 448

449

449 451

451 452

453

456 456 459

461 461 463

464

464 464

465 465 466 466 466 469

This Page Intentionally Left Blank

List of Contributors

U. BergstraDe Fachbereich Chemie der Universitat Erwin-Schrodinger-Stral3e D-67663 Kaiserslautern Germany

Joseph A. Casalnuovo Department of Chemistry California Polytechnic University Pomona CA 91768, USA

Mark J. Cooney Department of Chemistry Merkert Chemistry Center Boston College Chestnut Hill MA 02167, USA

Francois Diederich Laboratorium fur Organische Chemie ETH Zentrum UniversitatsstraRe 16 CH-8092 Zurich Switzerland

Eric J. Ginsburg Research Laboratories Eastman Kodak Company Rochester NY 14650, USA

Rolf Gleiter Organisch-Chemisches Institut der Universitat Heidelberg Im Neuenheimer Feld 270 D-69120 Heidelberg Germany

Christopher B. Gorman Department of Chemistry North Carolina State University Raleigh NC 27695, USA

Robert H. Grubbs Arnold and Mabel Beckman Laboratories of Chemical Synthesis California Institute of Technology Pasadena CA 91125, USA

A. Hoffmann Fachbereich Chemie der UniversitSlt Erwin-Schr6dinger-Stral3e D-67663 Kaiserslautern Germany

Henning Hopf Institute of Organic Chemistry Technical University of Braunschweig Hagennng 30 D-38106 Braunschweig Germany

K. N. Houk Department of Chemistry and Biochemistry University of California Los Angeles CA 90024, USA

Hiizu Iwamura Department of Chemistry Graduate School of Science The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113 Japan

XX List of Contributors

Yi Li Bristol-Myers Squibb Pharmaceutical Research Institute 5 Research Parkway P.O. Box 5100 Wallingford, CT 06492-7660 USA

Kenji Matsuda Department of Chemistry Graduate School of Science The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113 Japan

Gagik G. Melikyan Department of Chemistry and Biochemistry University of Oklahoma Norman OK 73019, USA

Roland Merger Organisch-Chemisches Institut der Universitat Heidelberg Im Neuenheimer Feld 270 D-69120 Heidelberg and Farbenlabor der BASE AG D-67056 Ludwigshafen Germany

Jeffrey S. Moore Departments of Chemistry and Materials Science & Engineering Roger Adams Laboratory Box 55 600 S. Mathews Urbana IL 61801, USA

K. C. Nicolaou Department of Chemistry The Scripps Research Institute La Jolla, CA 92037 and Department of Chemistry University of California San Diego, CA 92093 USA

Dietmar A. Plattner Department of Chemistry and Biochemistry University of California Los Angeles CA 90024, USA

Manfred Regitz Fachbereich Chemie der Universitat Erwin-Schrddinger-Strane D-67663 Kaiserslautern Germany

Neil E. Schore Department of Chemistry University of California Davis CA 95616, USA

Lawrence T. Scott Department of Chemistry Merkert Chemistry Center Boston College Chestnut Hill MA 02167, USA

Adrian L. Smith Merck Sharpe & Dohme Research Laboratories Terlings Park Harlow Essex CM20 2QR, UK

Kenneth M. Nicholas Department of Chemistry and Biochemistry University of Oklahoma Norman Salt Lake City OK 73019, USA

Peter J. Stang Department of Chemistry University of Utah

UT 84112, USA

List of Contributors XXI

Bernhard Witulski' James K. Young Institute of Organic Chemistry Technical University of Braunschweig Hagenring 30 Box 55 D-38106 Braunschweig 600 S. Mathews Germany Urbana

Department of Chemistry Roger Adams Laboratory

IL 61801, USA

' Present address: Department of Chemistry, Stanford University, Stanford, CA 94305, USA

1 Modern Computational and Theoretical Aspects of Acetylene Chemistry

Dietrnar A. Plattner, Yi Li, K. N. Houk

1.1 Introduction

Few organic molecules have been the object of more intensive physicochemical and theoretical scrutiny in recent years than acetylene. The focus of modern experimental, theoretical, and computational studies has been the characterization and elucidation of transition states and reactive intermediates, reaction potential energy surfaces and reaction dynamics of acetylene. The reason for this attention is clear. Acetylene is a simple polyatomic molecule only slightly more complex than a diatomic molecule, yet it has a variety of uses and undergoes a host of reactions like those of polyfunctional molecules. The rich and diverse chemical properties are amenable to high-level computational treatment, state-of-the-art spectroscopic measurements, and detailed theoretical interpretations of experimental data. Although acetylene is one of the most common molecules, its most fundamental properties such as bond strength are still subject to refinement, both experimentally and computationally. The rapid growing number of studies on the structures and stabilities of carbon clusters and rods have renewed interest in the bonding character of the acetylenic bond. Concepts of bonding in acetylene continue to evolve, and the understanding of complex varities of acetylenes will aid in the design of new molecules and materials.

In this chapter, we review some recent developments in the theoretical and computational aspects of acetylenes. There are several detailed reviews covering various aspects of the early work [l, 21. It will become self-evident in this review that modern experimental and computational studies of acetylene constitute a paradigm for the rivalry and interplay between theory and experiment. As the theoretical treatments become increasingly sophisticated, and as the experimental design becomes more and more ingenious and precise, the better is our understanding.

1.2 Electronic Structures of Acetylene and Monoacetylenes

Historically, the application of molecular orbital theory to the electronic structures of isoelec- tronic 14-electron molecules such as acetylene, HCN, N,, and 0, was an excellent pioneering demonstration of the value of quantum chemistry. Within the framework of molecular orbital theory, the C - C bond in acetylene is a triple bond involving one a-bond, and two orthogonal n-bonds. The a-bond is formed by two sp-hybrid orbitals from each carbon, and the two n- bonds are formed from the perpendicular p-orbitals. Alternatively, the so-called “bent” or “banana” bonds have been invoked to describe the multiple C-C bonds in acetylene (Fig. 1-1) [3-51. This creates a conceptual dilemma, though one bonding model can be transformed to the other by appropriate linear combinations. It is now realized that both

2 I Modern Computational and Theoretical Aspects of Acetylene Chemistry

models are useful for describing various aspects of bonding and reactivity [6] , but neither approach is perfect in describing the electronic structure of acetylene. Due to the electron correlation effects, a multiconfiguration wavefunction is necessary to describe fully the electronic structure of a molecule. The separation of 8- and n-orbitals in the molecular orbital treatment is an approximation, and thus has limitations. A generalized valence-bond theory was developed by the introduction of Pauling’s resonance theory, which took both models into the consideration [7]. For acetylene, the descriptions of 6-71 bonds or “banana” bonds comprise merely one configuration that contributes to the multiconfiguration wavefunction.

n n

H - C G C - H n

H - C q - H u

a-n bond model

Figure 1-1 The o-n and bent bond models for bonding in acetylene.

bent bond model

To address the question of which single-configuration bond description is a better starting point for the treatment of correlation effects, Karadakov et al. [8] examined the spin-coupled wavefunctions generated, respectively, from the Hartree-Fock molecular orbitals and the generalized valence-bond wavefunction with perfect-pairing and strong-orthogonality con- straints. The results using these wavefunctions were than compared with the calculations using a multiconfiguration wavefunction consisting of a complete-active space self-consistent field. From an energetic point of view, they found that both approaches were equally good for the treatment of correlation effects beyond the one-configuration approximation. The spin- coupled wavefunction from o-n orbitals recovers 63% of the CASSCF correlation energy, as compared with a 66% recovery of correlation energy using the equivalent bent orbitals. Other computational studies demonstrated the superiority of banana bonds for a variety of systems containing multiple bonds [9- 121. The superiority of one bond description over the other may depend upon the extent of conjugation of the multiple bonds [13].

In spite of the shortcomings of the single-configuration approach, the 6-71 concept has played an indispensable role in bridging theoretical understanding and chemical relevance. The HOMO-LUMO interactions in the frontier molecular orbital theory, the orbital energies either calculated by theory or measured from ionization potentials, the electron distribution and density of n-orbitals and bond orders, all have been used to understand and predict the molecular structural features, chemical stabilities, reactivities, regioselectivities, and stereoselectivities of acetylenes. Classical chemical concepts such as bond orders, the HOMO-LUMO energies, and electron densities have also been defined quantitatively and have been calculated for acetylene numerically from ab-initio calculations [14-201.

1.2 Electronic Structures of Acetylene and Monoacetylenes 3

-46 kcali'mol

1.2.1 Ground-state Potential Energy Surfaces

-43 kcal,'mol

On the singlet potential energy surface, acetylene (HC E CH) may undergo isomerization to vinylidene (H,C=C:). Whether singlet vinylidene exists as a bound intermediate has been the subject of extensive studies, both experimentally and theoretically [l, 21 -351. The simplest unsaturated carbene has been proposed to be involved in many chemical reactions, and is of great value in preparative organic chemistry [36-391. Because vinylidene is highly reactive, there has been limited direct experimental characterization of this species, and much debate about whether vinylidene is a minimum on the potential energy surface or a transition state for the degenerate hydrogen shift in acetylene. On the other hand, numerous computational studies have only recently provided a clear consensus on the classical barrier height for the isomerization process [26].

The lowest singlet state of vinylidene is an extremely shallow minimum on the potential energy surface. The best estimate of the classical barrier of isomerization to acetylene made by Gallo et al. is - 3 kcal/mol (1 kcal = 4.184 kJ) using large basis sets and the coupled cluster method including single and double excitations [26]. The energy of isomerization of acetylene is predicted to be - 43 kcal/mol at the same level of theory. Although an artifact at the MP2 level was noted, calculations at the high Msller-Plesset perturbation levels also predicted a diminishingly small barrier for the vinylidene isomerization [29, 311. Such a small barrier of 2-4 kcal/mol also led to a prediction of a lifetime of about 1 ps for the ground-state vinylidene [30, 321. The first direct observation of singlet vinylidene came from a photodetachment experiment involving the vinylidene radical anion [23]. Ervin et al. studied in detail the photoelectron spectra of the vinylidene anion and observed the vibrational structure of vinylidene [22]. The observed 2 + 0 CH, rock transition (450 cm-') indicated that the singlet vinylidene is a minimum with a barrier to rearrangement of > 1.3 kcal/mol. Its lifetime was estimated from these experiments to be 0.04-0.2 ps. Chen et al. also observed vinylidene in the high-resolution stimulated emission pumping spectrum of acetylene [24]. Although they were unable to determine the barrier height for vinylidene isomerization, a value of

H / /

H \

:c=c

Scheme 1-1 Energetics of the vinylidene-acetylene rearrangement [25, 401.

4 I Modern Computational and Theoretical Aspects of Acetylene Chemistry

44 kcal/mol was determined for the vinylidene-acetylene isomerization energy. This is in good agreement with the results of other measurements [21], and is consistent with the prediction by calculations.

One surprising feature in the transition structure obtained at various levels of theory for the acetylene-vinylidene isomerization is the extent of hydrogen migration (Scheme 1-1). For a low-barrier, highly exothermic reaction like the vinylidene rearrangement, an early transition state is expected according to the Hammond postulate. In other words, the transition structure should resemble vinylidene rather than displaying the reaction progressed halfway in terms of hydrogen transfer. This contradiction was first observed by Dykstra and Schaefer [40], and was apparently not due to the level of theory used. Petersson et al. offered a plausible explana- tion [25]. They considered two distinct processes that are involved in the isomerization: one corresponding to the hydrogen transfer and the other corresponding to the conversion of the carbene lone-pair electron to the R bonding electrons. The hydrogen transfer process, which breaks one C - H bond but creates another, is nearly thermoneutral, forming a species which is essentially a twisted zwitterion. The transition state for such a thermoneutral reaction should be midway according to the Hammond postulate. The second part of vinylidene isomerization is the electron reorganization from the twisted zwitterion to form acetylene, a very exothermic process. Therefore, in terms of the C - C bond length in the transition structure, the transition state closely resembles vinylidene, obeying the Hammond postulate. Petersson et al. suggested that the Hammond postulate should be applied to the energetics of individual processes, not to the total energy directly.

Besides the acetylene-vinylidene isomerization, other topological regions of the lowest singlet potential energy surface have been explored in a limited number of studies to date. The stimulated-emission pumping technique has been used to probe the potential energy surface up to 28000 cm-' [24, 41, 421. These studies indicated that acetylene at energy around 26500 cm-I undergoes the transition from the regular to the chaotic regime. Sibert and Mayrhofer carried out a variational calculation on highly excited vibrational states up to 8770 cm-I 1431. Binkley reported geometries and frequencies for two additional stationary points, bridged acetylene and planar bridged acetylene [33]. Halvick et al. investigated thoroughly the singlet acetylene energy surface up to 43000 cm-' using high level ab initio calculations [34]. They located eight stationary points and characterized the minimum energy paths connecting them. This information was then used to build a topologically consistent and complete configuration space, which included all three isomerization coordinates among acetylene, vinylidene, bridged acetylene, and planar bridged acetylene (Fig. 1-2).

Finally, calculations of potential energy surfaces involving bond dissociation reactions of acetylene are highly demanding on the level of theory, and have often been used as the testing ground for the development of the latest theoretical methods. Recent examples include the G2 theory by Pople and co-workers [44, 451, the coupled cluster methods [46], and the density functional theory [47-511. Several authors investigated in great detail the C -H bond dissociation [52-561, and the C-C bond dissociation as well [53, 57, 581. For the C-H bond dissociation of acetylene, high-level calculations, which range from 126 to 132 kcal/mol after zero-point energy correction, are in agreement with the upper end of the experimentally measured values. The C - C bond energy of acetylene is predicted to be 206 kcal/mol by the GVB method [%I, or 226 kcal/mol by G2 theory [45], as compared with 229 kcal/mol derived indirectly from experiments [21]. Table 1-1 summarizes the C - H and C - C bond dissociation energies obtained at various levels of theory and by experimental measurements.

1.2 Electronic Structures of Acetylene and Monoacetylenes 5

C2h 7 .. T /

H I \

, ,

H

/

- c=c 4

H TS(0); 2" saddle point(@]

Figure 1-2 Stationary points on the potential energy surface of C2H2. TS = transition structure.

Table 1-1 Calculated bond dissociation energies for acetylene (Do, kcal/mol)(a)

Method HCC - H HC = CH Reference

G-1 133.4 226.9 WI G-2 133.4 226.3 [451 GVB-CCCVDZP 129.7 206.3 [581 DFT-LDA/DN 131.1 [511 DlT-LDA/DNP 129.9 [511 Exptl. 126-132 (228.8 f 0.7) See text

(a) 1 kcal = 4.184 kJ.

1.2.2 Excited-state Potential Energy Surfaces

The lowest triplet potential energy surface of acetylene has also been studied by experiments and theory [59-621. The lowest excited state of acetylene is a cis-bent triplet state which was predicted theoretically and confirmed experimentally [62, 631. Although subsequent experimental studies by Lisy and Klemperer cast some doubts on this conclusion [64], more recent studies have resolved the apparent contradiction between the two experimental findings [59]. Theoretical work by several groups also extended to the tmns-bent triplet acetylene and its isomerization to the cis-bent triplet state [62, 651. In the case of triplet vinylidene, the energy gap between the lowest and first excited triplet state was determined to be 15 kcal/mol 1221, in good agreement with the theoretical predications [32,40]. In contrast to the singlet vinylidene, there is a significant barrier of - 54 kcal/mol predicted for the isomerization of the triplet vinylidene to the triplet acetylene [60], involving a nonplanar transition structure (Scheme 1-2). This is in agreement with the experimental evidence that the lifetime of the triplet vinylidene (> 0.4 ps) is much longer than that of the singlet state [66, 671.

6 1 Modern Computational and Theoretical Aspects of Acetylene Chemistry

H l.081A H / c--c - \

1.312A H L

LHCCH = 115' ii3B2 vinylidene b3B, acetylene

r H 128" trans-bent

H 127" 184" C--- - 1 1.327A

b3B, acetylene 1302 acetylene

Scheme 1-2 Computed structures of triplet C,H2 and transition structures for interconversions.

In contrast to the triplet excited state, singlet excited states of acetylene are less well characterized. Recent spectroscopic studies showed there is a strong singlet- triplet coupling in the singlet excited state [68-701. This led to the speculation that the lowest singlet excited state lies close in energy to one of the transition states on the triplet potential energy surface of acetylene [60]. Several computational studies have been reported on the singlet excited states of acetylene [33, 71-75].

1.2.3 Radical Ions

Ionization of acetylene gives a radical cation, for which many studies have been reported in the literature. The radical cation is a Renner-Teller molecule, and is predicted to have a degenerate X'll, electronic ground state [76]. It was observed experimentally by mass spectrometry [67, 771. The structures and energies of the C,H, radical cation have been studied in detail by several groups [78-811. The isomerization barrier from vinylidene cation to the more stable acetylene cation is predicted to be - 10 kcal/mol both at the UMP2/6-311G** level reported by Baker [SO] and at the CISD(+Q)/DZP level reported by Hamilton and Schaefer [79]. In addition, theoretical considerations led Ramasesha and Sinha to suggest that stacked acetylenic radical ions are prime candidates for the observation of organic fer- romagnetism, because of their stable high-spin ground state [82].

The acetylene anion radical undergoes autodetachment of the electron, but the vinylidene anion can be generated easily [83]. Since the calculated isomerization barrier is - 45 kcal/mol, the 'B, ground-state vinylidene anion radical is predicted to be stable with respect to the 1,2-hydrogen shift [30, 84, 851. As mentioned before, the vinylidene anion radical was used as the precursor for the generation of the singlet vinylidene in Lineberger's experimental studies.

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