Edited by A. Stephen K. Hashmi and Dean F. Toste Cover.jpg ......2.3.3 Strong Metal–Support Interactions: Effect of Electronic Transfers and Decoration on the Gold Nanoparticles

Modern Gold Catalyzed Synthesis

Edited by A. Stephen K. Hashmiand Dean F. Toste

57268File AttachmentCover.jpg

Edited by

A. Stephen K. Hashmi and

F. Dean Toste

Modern Gold Catalyzed

Synthesis

Further Reading

Mohr, F. (ed.)

Gold Chemistry

Highlights and Future Directions

2009

ISBN: 978-3-527-32086-8

Laguna, A. (ed.)

Modern SupramolecularGold Chemistry

Gold-Metal Interactions and Applications

2008

ISBN: 978-3-527-32029-5

Dupont, J., Pfeffer, M. (eds.)

Palladacycles

Synthesis, Characterization

and Applications

2008

ISBN: 978-3-527-31781-3

Ertl, G., Knözinger, H., Schüth, F.,Weitkamp, J. (eds.)

Handbook of HeterogeneousCatalysis

8 Volumes

2008

ISBN: 978-3-527-31241-2

Ding, K., Uozumi, Y. (eds.)

Handbook of AsymmetricHeterogeneous Catalysis2008

ISBN: 978-3-527-31913-8

Edited by A. Stephen K. Hashmiand F. Dean Toste

Modern Gold Catalyzed Synthesis

The Editors

Prof. Dr. A. Stephen K. HashmiOrganisch-Chemisches InstitutUniversität HeidelbergIm Neuenheimer Feld 27069120 Heidelberg

Prof. F. Dean TosteDepartment of ChemistryUniversity of CaliforniaBerkeley, CA 94720-1460USA

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Contents

List of Contributors XIII

1 Hydrochlorination of Acetylene Catalyzed by Gold 1Marco Conte and Graham J. Hutchings

1.1 Introduction 11.2 Reactions of Alkynes Using Gold Chloride as Catalyst 21.3 The Correlation of E 88 with the Activity of Gold for the

Hydrochlorination of Acetylene 41.3.1 The Initial Correlation 41.3.2 Conceptual Developments of the E8 Correlation 51.3.3 Further Study of the Correlation of E8 with the Activity

of Platinum Group Metals 81.3.4 The E8 Correlation Applied to Homogeneous and

Nonhomogeneous Gold Nanoalloys 91.4 Central Role of Au3þ and Regeneration of Au/C Catalysts 121.5 Reaction Mechanism of Alkynes Over Au/C Catalysts 141.5.1 Effect of the Individual Components of the Reactants to Au/C 141.5.2 Reaction of Higher Alkynes Over Au/C 161.5.3 Hydrochlorination of 1-hexyne, phenylacetylene, and 2-hexyne

Over Au/C Catalyst 171.5.4 Computational Studies of the Reaction of Acetylene Over Au/C 211.6 Chemical Origin of the E8 Correlation and General Remarks 221.7 Commercial Processes and Economic Aspects of

Vinyl Chloride Monomer Manufacture 24References 25

2 Gold-Catalyzed Reduction Reactions 27Avelino Corma and Pedro Serna

2.1 Introduction 272.2 Hydrogenation of Multiple C¼C Bonds. Role of the Gold

Oxidation State 282.2.1 Introduction 28

V

2.2.2 First Solid Gold Catalysts: from Extended Au Surfaces to HighlyDispersed Nanoparticles 28

2.2.3 Recent Advances in Supported Gold Chemistry: from HighlyDispersed Nanoparticles to Individual Supported Au Atoms 31

2.2.4 Summary 342.3 Hydrogenation of a,b-Unsaturated Aldehydes 342.3.1 Introduction 342.3.2 Chemistry of Gold Nanoparticles: First Studies and Hypotheses 362.3.3 Strong Metal–Support Interactions: Effect of Electronic Transfers

and Decoration on the Gold Nanoparticles 372.3.4 Effect of Morphological Factors: Size and Shape 382.3.5 Summary 392.4 Hydrogenation of Substituted Nitroaromatic Compounds 412.4.1 Introduction 412.4.2 Gold Catalysts for the Production of Substituted Nitro Compounds 422.4.3 Hydrogenation of –NO2 Groups on Gold Catalysts:

Reaction Pathway 432.4.4 Chemoselectivity of Gold Catalysts for the Hydrogenation

of NO2 groups 472.4.5 Activity of Gold Catalysts for the Hydrogenation of NO2

groups 492.4.6 Summary 51

References 51

3 Gold-Catalyzed Benzannulations: Asao–Yamamoto BenzopyryliumPathway 55Naoki Asao and Yoshinori Yamamoto

3.1 Introduction 553.2 Acetylenic Compounds as 2p Systems 563.3 Enols as 2p Systems 603.4 Enol Ethers as 2p Systems 623.5 Benzynes as 2p Systems 633.6 Synthesis of Phthalazine Derivatives 643.7 Application to the Synthesis of Angucyclinone Antibiotics

and Other Applications in Total Synthesis 653.8 Copper-Catalyzed Benzannulations 673.9 Conclusion 68

References 69

4 Gold-Catalyzed Reactions of Propargyl Esters, Propargyl Alcohols,and Related Compounds 75Pablo Mauleon and F. Dean Toste

4.1 Introduction and Extent of This Chapter 754.2 Propargyl Esters 76

VI Contents

4.2.1 General Mechanistic Considerations 764.2.1.1 [2,3]- and [3,3]-Rearrangements 774.2.1.2 Reversibility 784.2.1.3 Ionization 804.2.1.4 Double [2,3]-Rearrangements 834.2.2 Reactions Initiated by [2,3]-Rearrangements 854.2.2.1 The Rautenstrauch Rearrangement 864.2.2.2 Alkene Cyclopropanations 874.2.2.3 Enantioselective Transformations After [2,3]-Rearrangements 924.2.2.4 Nucleophilic Attack on Gold Carbenoids Generated

After [2,3]-Rearrangements 944.2.3 Reactions Initiated by [3,3]-Rearrangements 964.2.3.1 Nucleophilic Double Bonds 974.2.3.2 Triple Bonds 994.2.3.3 Aromatic Groups 1004.2.3.4 Alkyl Groups 1004.2.3.5 Heteroatoms 1024.2.3.6 Electrophilic Trapping of Vinyl–Gold Intermediates 1034.2.3.7 Other Processes 1044.3 Propargyl Ethers 1074.3.1 Propargyl Vinyl Ethers 1074.3.2 Propargyl Alkyl Ethers 1104.3.3 Other Ether Substitution Patterns 1134.4 Propargyl Alcohols 1154.4.1 Alkyne Hydration 1154.4.2 Meyer–Schuster Rearrangements 1154.4.3 Nucleophilic Substitution at the Propargylic

Position 1174.4.4 Ring Expansions 1174.4.5 Other Reactions Involving Propargyl Alcohols 1194.5 Propargyl Amines 1214.6 Propargyl Carbonates, Amides, and Carbamates 1244.7 Other Propargyl Substitution Patterns 1274.8 Conclusion 129

References 130

5 Intramolecular Hydroarylation of Alkynes 135Paula de Mendoza and Antonio M. Echavarren

5.1 Introduction 1355.2 Intramolecular Reactions of Arenes with Alkynes 1375.3 Intramolecular Reactions of Electron-Rich Heteroarenes

with Alkynes 1425.4 Conclusion and Outlook 148

References 148

Contents VII

6 Gold–Alkyne Complexes 153Maria Agostina Cinellu

6.1 Introduction 1536.2 Description of the M–p-Bond Interaction in Alkene and Alkyne

Complexes 1546.3 Gold(0) Complexes 1556.4 Gold(I) Complexes 1556.4.1 Neutral Derivatives 1556.4.2 Cationic Derivatives 1586.4.3 p-Complexation of Gold(I) at Metal–Alkynyl Units M–C�C–R 1646.5 Gold(III) complexes 1676.6 Theoretical Studies 167

References 171

7 Gold–Alkene Complexes 175Maria Agostina Cinellu

7.1 Introduction 1757.2 Gold(0) Derivatives 1767.3 Gold(I) Complexes 1767.3.1 Neutral Derivatives 1767.3.1.1 14-Electron Species 1767.3.1.2 16-Electron Species 1787.3.2 Cationic Derivatives 1817.3.2.1 14-Electron Species 1817.3.2.2 16-Electron Species 1867.4 Gold(III) Complexes 1917.5 Theoretical Studies 192

References 196

8 Hydration and Hydroalkoxylation of CC Multiple Bonds 201J. Henrique Teles

8.1 Historical Perspective 2018.1.1 Addition of Water to Alkynes 2018.1.2 Addition of Alcohols to Alkynes 2028.2 Gold Catalysts 2028.2.1 First Reports of Gold Catalysts 2038.2.2 The Discovery of Au(I) Catalysts 2048.2.2.1 Catalyst Precursors 2068.2.2.2 The Importance of Chemical Equilibria 2088.3 Hydration and Hydroalkoxylation of CC Triple Bonds (Alkynes) 2098.3.1 Effect of Alcohol Structure 2098.3.2 Addition of Water to Alkynes 2118.3.3 Addition of Alcohols to Simple Alkynes 2158.3.4 Reactions Involving Propargylic Alcohols 2178.3.5 Additions to Homopropargylic Alcohols and Other Alkynols 220

VIII Contents

8.3.6 Reactions Involving Propargylic Ethers 2248.4 Hydration and Hydroalkoxylation of CC Double Bonds

(Allenes and Alkenes) 2268.4.1 Addition of Alcohols to Allenes 2268.4.2 Cyclization of Allenyl and Propargyl Ketones 2278.4.3 Addition of Alcohols to Alkenes 230

References 234

9 Gold-Catalyzed Aldol and Related Reactions 237Christoph Hubbert and A. Stephen K. Hashmi

9.1 The Gold-Catalyzed Aldol Reaction 2379.1.1 Synthetic Scope 2399.1.1.1 Reactions of Aldehydes with Methyl Isocyanoacetate 2399.1.1.2 Reactions of Aldehydes with a-Substituted Isonitriles 2419.1.1.3 Reactions of Aldehydes with Alkyl Isocyanoacetates 2429.1.1.4 Reactions of Aldehydes with Alkyl Isocyanoacetamides 2439.1.1.5 Reactions of Aldehydes with a-Isocyano Weinreb Amide 2449.1.1.6 Reactions of Aldehydes with Isocyano Phosphonates 2459.1.1.7 Reactions of Aldehydes with a-Keto Esters 2469.1.2 Structure of the Ligand 2479.1.2.1 Internal Cooperativity of Chirality 2489.1.2.2 Conformation of the Pendant Side Chain 2519.1.3 Mechanistic Aspects 2539.1.3.1 The First Transition-State Model 2539.1.3.2 Structure of the Ferrocenyl–Gold(I) Complex 2539.1.3.3 Mechanistic Aspects 2539.2 Related Reactions 2579.2.1 Synthesis of Dihydroimidazole 2579.2.2 Mannich Reactions 2589.2.3 Michael Reactions 259

References 260

10 Gold-Catalyzed Oxidation Reactions: Oxidation of Alkenes 263Yuanhong Liu

10.1 Introduction 26310.2 Epoxidation Reactions 26310.3 Aziridination Reactions 26810.4 Oxidative Cleavage of C¼C Double Bonds 26910.5 Oxygen Transfer to Carbenoids 270

References 271

11 Gold-Catalyzed Oxygen-Atom Transfer to Alkynes 273Maria Camila Blanco Jaimes and A. Stephen K. Hashmi

11.1 Introduction 27311.2 Oxygen-Atom Transfer from NO Groups 273

Contents IX

11.2.1 Nitrones 27411.2.2 Nitro Compounds 27611.2.3 N-Oxides 27711.3 Oxygen-Atom Transfer from Sulfoxides 28011.4 Oxygen-Atom Transfer from Epoxides 28211.5 Gold-Catalyzed Oxidative Coupling 28311.5.1 Introduction 28311.5.2 Functionalization of C(sp2)–H Bonds 28411.5.3 Gold-Catalyzed Nucleophilic Addition–Oxidative Coupling

Reactions 287References 295

12 Gold-Catalyzed Additions to Alkenes: N-Nucleophiles 297Zigang Li, David A. Capretto, and Chuan HeReferences 302

13 Gold-Catalyzed Additions to Alkenes: O-Nucleophiles 303Zigang Li, David A. Capretto, and Chuan HeReferences 307

14 Oxidation of Alcohols and Carbohydrates 309Cristina Della Pina, Ermelinda Falletta, and Michele Rossi

14.1 Introduction 30914.2 Selective Oxidation of Alcohols 31014.2.1 Catalyst Preparation 31114.2.2 Oxidation of Diols 31214.2.3 Oxidation of Other Polyols 31514.2.3.1 Glycerol 31514.2.3.2 Sorbitol 31714.2.3.3 Other Alcohols 31714.2.3.4 Amino Alcohols 31814.3 Selective Oxidation of Carbohydrates 32014.3.1 Oxidation of Glucose to Sodium Gluconate 32114.3.1.1 Kinetics and Modeling 32314.3.2 Synthesis of Free Gluconic Acid 32514.4 Future Applications 32614.5 Conclusion 327

References 328

15 Applications of Gold-Catalyzed Reactions to NaturalProduct Synthesis 331Matthias Rudolph

15.1 Introduction 33115.2 Addition of Heteroatom Nucleophiles to Alkynes 33215.2.1 Hydration of Alkynes: Pterosin B and C 33215.2.2 Tandem Reaction Including Hydration of Alkynes, Elimination, and

Conjugate Addition: (þ)-Andrachcinidine 332

X Contents

15.2.3 Hydroalkoxylation of Alkynes: Bryostatin 16 33315.2.4 Bis-spiroketalization of Alkynes: A–D Ring of Azaspiracid and

(�)-Ushikulide A 33415.2.5 Intramolecular Hydroamination of C–C Triple Bonds:

Solenopsin A, Comunesin B, Mersicarpine, and Nitidine 33615.3 Addition of Heteroatom Nucleophiles to Allenes 33915.3.1 Intermolecular Hydroalkoxylation of Allenes: Citreoviral,

(�)-Isocyclocapiteline, (�)-Isochrysotricine, and Bejarol 33915.3.2 Intermolecular Hydroamination of Allenes: Swainsonine 34115.3.3 Intermolecular Hydroarylation of Allenes: (�)-Rhazinilam 34215.4 Cycloadditions via Pyrylium Intermediates from

o-Alkynylacylarenes 34315.5 Rearrangements of Propargyl Esters 34615.5.1 1,2-Acyl Shift of Propargyl Esters: a-Diazoketone Equivalents 34615.5.2 1,3-Acyl Shift of Propargyl Esters and Subsequent Tandem

Cyclization of Ene Vinylallenes: D9(12)-Capnellene 34715.6 Skeletal Rearrangement of 3-Butynyl-N-Oxides: (�)-Cermizine

and (þ)-Lentiginosine 34915.7 Enyne Cyclizations 35015.7.1 Silylenol Ethers as Nucleophiles: Platencin, (þ)-Lycopaladin A,

and (þ)-Fawcettimine 35015.7.2 Iodoalkynes in Enyne Cyclizations: (þ)-Lycopladine A 35115.7.3 Furan–Yne Cyclization: (�)-Jungianol 35215.7.4 Tandem Process of Enyne Rearrangement and Prins Cyclization:

(þ)-Orientalol and (�)-Englerin A 35315.7.5 Tandem Enyne Cycloisomerization and Semipinacol Rearrangement:

Ventricos-7(13)-ene 35515.8 Propargyl Claisen Rearrangement: Azadirachtin 35615.9 Gold-Catalyzed C–H Activation: (�)-Pterocarpan and Crassifolone 35615.10 Gold-Catalyzed Allylic Amination: (�)-Angustureine 35815.11 Catalytic Asymmetric Aldol Reaction of Isocyanoacetates and

Aldehydes 359References 361

16 Gold-Catalyzed Addition Reactions to Allenes 363Christian Winter and Norbert Krause

16.1 Introduction 36316.2 Addition of Heteroatom Nucleophiles 36316.2.1 Addition of Oxygen Nucleophiles 36416.2.2 Addition of Nitrogen Nucleophiles 37616.2.3 Addition of Sulfur Nucleophiles 38116.3 Addition of Carbon Nucleophiles 38216.4 Conclusion 386

References 386

Index 391

Contents XI

List of Contributors

XIII

Naoki AsaoTohoku UniversityGraduate School of ScienceDepartment of ChemistrySendai 980-8578Japan

Maria Camila Blanco JaimesUniversität HeidelbergOrganisch-Chemisches InstitutIm Neuenheimer Feld 27069120 HeidelbergGermany

David A. CaprettoUniversity of ChicagoDepartment of Chemistry5735 South Ellis AvenueChicagoIL 60635USA

Maria Agostina CinelluUniversità di SassariDipartimento di ChimicaVia Vienna 207100 SassariItaly

Marco ConteCardiff UniversitySchool of ChemistryCardiff Catalysis InstituteCardiff CF10 3ATUK

Avelino CormaUniversidad Politécnica de ValenciaInstituto de Tecnología QuímicaAvenida de los Naranjos s/n46022 ValenciaSpain

Cristina Della PinaUniversità degli Studi di MilanoDipartimento di Chimica Inorganica,Metallorganica e AnaliticaVia Venezian 2120133 MilanItaly

Paula de MendozaInstitute of Chemical Research ofCatalonia (ICIQ)Avinguda Països Catalans 1643007 TarragonaSpain

Antonio M. EchavarrenInstitute of Chemical Research ofCatalonia (ICIQ)Avinguda Països Catalans 1643007 Tarragona

and

Universitat Rovira i VirgiliDepartament de Química Analítica iQuímica Orgànicac/ Marcel·li Domingo s/n43007 TarragonaSpain

Ermelinda FallettaUniversità degli Studi di MilanoDipartimento di Chimica Inorganica,Metallorganica e AnaliticaVia Venezian 2120133 MilanItaly

A. Stephen K. HashmiUniversität HeidelbergOrganisch-Chemisches InstitutIm Neuenheimer Feld 27069120 HeidelbergGermany

Chuan HeUniversity of ChicagoDepartment of Chemistry5735 South Ellis AvenueChicagoIL 60635USA

Christoph HubbertUniversität HeidelbergOrganisch-Chemisches InstitutIm Neuenheimer Feld 27069120 HeidelbergGermany

Graham J. HutchingsCardiff UniversitySchool of ChemistryCardiff Catalysis InstituteCardiff CF10 3ATUK

Norbert KrauseDortmund University of TechnologyOrganic Chemistry IIOtto-Hahn-Straße 644227 DortmundGermany

Zigang LiUniversity of ChicagoDepartment of Chemistry5735 South Ellis AvenueChicagoIL 60635USA

Yuanhong LiuChinese Academy of SciencesShanghai Institute of OrganicChemistryState Key Laboratory of OrganometallicChemistry354 Fenglin LuShanghai 200032China

Michele RossiUniversità degli Studi di MilanoDipartimento di Chimica Inorganica,Metallorganica e AnaliticaVia Venezian 2120133 MilanItaly

Matthias RudolphUniversität HeidelbergOrganisch-Chemisches InstitutIm Neuenheimer Feld 27069120 HeidelbergGermany

XIV List of Contributors

F. Dean TosteUniversity of California, BerkeleyDepartment of ChemistryLatimer HallBerkeleyCA 94720USA

Christian WinterDortmund University of TechnologyOrganic Chemistry IIOtto-Hahn-Straße 644227 DortmundGermany

Yoshinori YamamotoTohoku UniversityGraduate School of ScienceDepartment of ChemistrySendai 980-8578Japan

List of Contributors XV

1Hydrochlorination of Acetylene Catalyzed by GoldMarco Conte and Graham J. Hutchings

1.1Introduction

In recent years, there has been a revolution in the chemistry of gold. Until recently,gold had been considered to be less reactive than its neighbors in the periodic table(Figure 1.1) owing to it being the most noble of metals, having the highest standardelectrode potential. Indeed, until very recently, there had been the perception thatgoldwould be unreactive in catalytic applications. As gold does not chemisorbO2 as abulkmetal, it was considered that gold could not be effective in redox applications. Asthis was the perceived wisdom, then it is unsurprising that the exceptional catalyticactivity of gold lay undiscovered for so long. For it is only when gold is prepared innanoparticulate form or as soluble complexes that its high activity is observed.

The traditional view of gold as an unreactive immutable metal has inspired greatworks of art and literature. In the late nineteenth century Kipling wrote about therelative roles of the coinage metals (gold, silver and copper) and iron in society:

Gold is for the mistress – silver for the maid –

Copper for the craftsman, cunning at his trade.

Good! said the Baron, sitting in his hall,

But iron – cold iron – is ruler of them all.

from Cold Iron

Rudyard Kipling

At that time, gold was used mainly in coinage and in artwork, whereas iron hadunderpinned the industrial revolution and was at the heart of the manufacturingindustry. It is nowonder that ironwas the prized commercialmetal. This traditionalviewhas now been overturned and currently there is an amazingly rich chemistry ofcatalysis based on gold nanoparticles and complexes. It is over 25 years since theprediction was made that gold would be the best catalyst for the hydrochlorination

Modern Gold Catalyzed Synthesis, First Edition. Edited by A. Stephen K. Hashmi, and F. Dean Toste.� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

j1

Fischer Tropsch

reaction

Methanation

Steam reforming

Commercial catalysts

Methanol synthesis

Water gas shift reaction

Ethene

epoxidation

Car exhaust

catalysts

Acetylene

hydrochlorination

Fischer Tropsch

reaction

Methanation

Steam reforming

Commercial catalysts

Methanol synthesis

Water gas shift reaction

Ethylene

epoxidation

Car exhaust

catalysts

Acetylene

hydrochlorination

Figure 1.1 Examples of commercial catalysts using elements near gold in the periodic table.

of acetylene. Since then, there has been a phenomenal growth in gold-basedreactions of acetylenes and a very rich field of homogeneous catalysis based ongold has been established. To date, gold is still the best catalyst for acetylenehydrochlorination, in addition to being the catalyst of choice for the low-temper-ature oxidation of carbon monoxide. In this chapter, we introduce the reactions ofgold and acetylenes and set out in greater detail the discovery and chemistry of goldas a catalyst for acetylene hydrochlorination.

1.2Reactions of Alkynes Using Gold Chloride as Catalyst

The discovery that cationic gold supported on carbon is a highly effective catalyst forhydrochlorination of acetylene to vinyl chloride led over the years to a surge ofapplications of gold salts involving alkyne-based substrates [1, 2]. These includehydrofluorination [3], hydration [4, 5], hydroxylation [6], and hydrocarboxylation [7]reactions, and also hydroamination [8] and reactions over alkynes containingproximate oxygen nucleophiles [9]. Some of these reactions will be briefly describedin this section; however, it should be noted that one of the first uses of Au3þ salts forcatalytic purposes can be traced back to Thomas and co-workers in 1976 [10] in theoxidation of phenylacetylene (1) in aqueous methanol in presence of HAuCl4, whichgave the ketone 2 as the major product, followed by the ether 3 and the chlorinatedvinyl adduct 4 (Scheme 1.1).

However,despiteaturnovernumberof�6inthisreaction, theroleofgoldwasmerelyassignedasareagentandnotasacatalyst.Itwasalmost10yearslater,in1985[11],thattherole of cationic gold was recognized in full as it was the best catalyst for the hydro-chlorination reaction of acetylene (5) to vinyl chloridemonomer (VCM) (6) and can beconsideredasamilestoneforrevealingcationicgoldasaneffectivecatalyst(Scheme1.2).

2j 1 Hydrochlorination of Acetylene Catalyzed by Gold

O

OMe

Cl

7 mol% HAuCl4

MeOH/H2O

1 2

3

4

(38%)

(< 5%)

(< 5%)

Scheme 1.1 Reaction of phenylacetylene (1) with tetrachloroauric acid in aqueous methanol.The ketone 2 is the major product, which is obtained by Markovnikov addition. Minor amounts ofmethyl vinyl ethers (3) and vinyl chlorides (4) are obtained.

H H + HClAu/C H

Cl

H

H5 6

Scheme 1.2 Hydrochlorination of acetylene (5) to vinyl chloridemonomer (6). The reaction occursat 180 �C and is catalyzed by an Au/C catalyst on which the active sites are Au3þ centers.

5 mol% NaAuCl4

87

MeCNH2N

Pent

N

Pent

(100%)

Scheme 1.3 Intramolecular addition of amine 7 to cyclic adduct 8, using sodium tetrachloroaurateas the catalyst.

Soon afterwards, in 1987, the intramolecular addition of amines to triple bonds(7) to yield cyclic adducts (8) with NaAuCl4 as catalyst was reported [12](Scheme 1.3).

Addition of alcohols to alkynes has also been successfully achieved [13], as shownby the cyclization of (Z)-3-ethylallyl alcohols (9), which can cyclize to furans (10 and11) (Scheme 1.4).

Moreover, if two alcohol functional groups are present, such as in 12, it is evenpossible to obtain bicyclic ketals such as 13 by means of AuCl or AuCl3(Scheme 1.5) [14].

1.2 Reactions of Alkynes Using Gold Chloride as Catalyst j3

The interest in the latter reaction also relies on the circumstance that Au3þ isactive towards carbon–carbon triple bonds, but not towards carbon–carbon doublebonds. Analogies can be found also in the hydrochlorination reaction of acetylene,where Au/C-based catalysts are unreactive towards ethylene [15], making themsuitable for selective chemical synthesis.

1.3The Correlation of E�� with the Activity of Gold for the Hydrochlorination of Acetylene

1.3.1The Initial Correlation

In this section, we demonstrate how the activity of gold can be correlated with thestandard electrode potential (E�) of the metal used to carry out the hydrochlorinationreaction and how this correlation can predict gold to be the most effective metal forthe catalytic hydrochlorination of acetylene.

9 10

HO

R1

R5R4

R3

R2

O

R5 R4

R3

R2R1

O R3

R2R1

R4

11

0.1 mol% AuCl3

for R5 not H

for R5 = H

Scheme 1.4 Cyclization of (Z)-3-ethylallyl alcohols (9) to furans, with R5¼ hydrogen to yield 10or R5¼ alkyl to yield 11.

12 13

2 mol% AuCl

Ph OH

OH

Ph

O

O

MeOH, rt

Scheme1.5 Intramolecular reaction from the alcohol12 to the bicyclic ketal13. The reaction canbecatalyzed by AuCl or AuCl3.


1.3.2Conceptual Developments of the E� Correlation

Several metallic salts supported on carbon, such as HgCl2, are capable of carryingout the catalytic hydrochlorination of acetylene. The reaction is exothermic(DH� ¼�99 kJmol�1) but in the absence of a catalyst no reaction occurs. Detailedkinetic studies were carried out during the 1960s, identifying the addition of HClto adsorbed C2H2 to produce vinyl chloride as the rate-determining step of thereaction [16–18]. This led initially to a search for metallic salts that could lead toboth stable metal–acetylene and metal–hydrochloric acid complexes. A numberof metallic salts may lead to metastable acetylides, such as Cu, Ag, Au, Na, K, Rb,Zn, Cd, Hg, Pd, Os, Ce, Al, Mg, Ca, Sr, and Ba and also the better known examplesof Pt, Ru, Rh, and Ir. As the stability of the acetylide species can be considered asan essential parameter to control the activity, it was initially attempted to correlatethe activity of the catalyst with the electron affinity of the metal. However, whenthis correlation was extended to a wider set of metals, as carried out byShinoda [19] (Figure 1.2), the correlation with electron affinity was no longervalid even when the electron affinity of themetal was divided by its oxidation stateas a correction factor.

In the first instance, this relationship is important because it can correlate thecatalytic activity with the formation of metal–acetylene complexes, and consequentlyprovides information on which metal would be best to carry out the reaction. Only

11109876540

20

40

60

80

100

Acet

ylen

e co

nver

sion

%

Electron affinity cationn

K+Sr2+

Ba2+

Ca2+

La3+

Pb2+

Mn2+

Sb3+Bi3+Cd2+

Ag+

Cu+Hg2+

Pd2+

Cu2+

Ni2+

Al3+

Zn2+Fe3+

(eV)

Figure 1.2 Correlation of hydrochlorination activity of differentmetal chlorides supported on carbon(200 �C, GHSV 150 h�1), with the electron affinity of the cation divided by the metal oxidationstate [19].

1.3 The Correlation of E�� with the Activity of Gold for the Hydrochlorination j5

metals able to lead to the formation ofmetastable acetylides would be expected to forman active catalyst; in the plot shown in Figure 1.2, the metal species able to form themost stable acetylides are Pd2þ , Hg2þ , Cu2þ , and Agþ , whereas the lower setincludes metal cations that can form stable complexes with HCl and act as Friedel–Crafts catalysts.

However, this kind of approach has two strong limitations: (i) if the correlationexists then it consists of a set of two straight lines rather than one, hence, it cannot beused predictably, and (ii) the correlation parameter used is the electron affinity, whichby definition takes into account only a single electron process, whereas the addition ofHCl to acetylene involves a two-electron process (from acetylene to themetal center).Therefore, it was not considered chemically realistic to associate the formation of ametal acetylide unit to a single-electron process. It was considered that a moresuitable correlation would be obtained by using the standard electrode potentialrather than electron affinity. It is important to note that all the metal chloridesinvestigated exist in oxidation state 2þ or 3þ . Plotting the data using the E� value ofthe metal leads to the plot shown in Figure 1.3.

In this case, a single correlation curve is obtained, which,more importantly, can beused as a predictive model. In fact, metals with higher standard electrode potentialsthan Pd2þ and Hg2þ should lead to enhanced activity. This hypothesis, which is atthe basis of all the applications of gold as a catalyst, has been confirmed usingsupported Au3þ catalysts (Figure 1.4) [11, 20].

Although gold can be considered the best catalyst in terms of initial activity, it isunfortunately affected by deactivation phenomena, the most important of which has

10-1-2-3-40

20

40

60

80

100

Ace

tyle

ne c

onve

rsio

n %

E0/ v

KSr

Ba

Ca

La

Mg

Al

MnSb

Ni

Zn Fe

Bi

Pb

Cd

Cu

Pd

Hg

Ag

Figure 1.3 Correlation of hydrochlorination activity of different metal chlorides supported oncarbon (200 �C, GHSV 150 h�1), with the standard reduction potential of the metal,Mnþ þ e� ! M(n� 1)þ [11]. (conversion activity values after ref 19).


been identified as Au3þ reduction [20, 21]; this aspect will be described further in thischapter. A second deactivation pathway involves oligomer formation, and previousobservations showed that the deactivation rate is minimum at 100–120 �C(Figure 1.5), but at this temperature the catalyst is not sufficiently active compared

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5100

200

300

400

500In

itial

act

ivity

mol

/ m

ol/h

Standard electrode potential

Hg

Pd Pt

Ir

Au

Figure 1.4 Correlation of hydrochlorination activity of metal chlorides supported on carbon(180 �C,GHSV1140 h�1) with the standard electrode potential. Catalysts contain 0.0005molmetal/100 g catalyst. It should be noted that in this series, Pt is obtained from a Pt2þ precursor [20].

0 30 60 90 120 150 1800

1

2

3

4

5

6

Dea

ctiv

atio

n ra

te /

loss

HC

l %C

onve

rsio

n / h

Reaction temperature (oC)

Figure 1.5 Effect of reaction temperature on rate of deactivation (loss of HCl, conversion per hour,averaged over the initial 3 h). Reaction conditions: C2H2 :HCl¼ 1 : 1.1 [22].


with temperatures in the region of 180 �C, which is the standard temperature to carryout the reaction over gold on carbon catalysts [22].

Nevertheless, only catalysts containing gold have the interesting properties ofbeing reactivated on-stream by Cl2, NO, and NO2 [22] and off-line by aquaregia [23], making them still attractive for the catalytic hydrochlorination ofacetylene. In particular, the catalytic synthesis route used in the early studiesinvolved a relatively non-complex impregnation method and it is anticipated thatby using enhanced preparation methodologies, catalysts of higher activity anddurability will be produced.

1.3.3Further Study of the Correlation of E� with the Activity of Platinum Group Metals

The correlation of E� with activity has been recently further confirmed using otherplatinum group metals where the data for the chlorides were used (Figure 1.6) [24].

All the catalysts were obtained using thewetness impregnation technique and aquaregia as solvent. The precursor salts were HAuCl4, PdCl2, H2PtCl6, RhCl3�3H2O,IrCl3�3H2O, and RuCl3�3H2O. This trend confirms the previous findings for thisreaction, that is, the higher the standard electrode potential, the higher is theactivity [11, 25], with the trend: that the Au/C catalyst gave the highest conversionof acetylene, with the following order of initial activity: Au>Ru� Ir>Pd>Pt�Rh.However, there is a notable exception in the above series, as Pt, clearly does not fit the

0.4 0.5 0.6 0.7 0.8 0.9 1.00

20

40

60

80

100

120

140

160

mol

C2H

2 con

verte

d / m

ol m

etal

min

–1

E0 / V vs SHE

Rh

Ru

Pd

Pt

Ir

Au

Figure 1.6 Correlation between initialacetylene conversion and the standardelectrode potential of platinum group metals.Potentials were obtained from the reduction of

the following chloride salts: RhCl63�, RuCl5

2�,PdCl2, PtCl6

2�, IrCl63�, and AuCl4

�. Thereaction was carried out at 180 �C, GHSV870 h�1 [24].


data set. In contrast to the original set [11, 20], where Pt2þ was used, in this case Pt4þ

data were used, whereas all the othermetals had initial oxidation state 1þ or 2þ , andthe reductionofMnþ toM0was considered. In contrast, in this set [24], Pt has an initialoxidation state of 4þ , we used the reduction Pt4þ to Pt2þ because the reaction is atwo-electron process, and reduction to the metal is unlikely.

Moreover, it is also worth noting that the initial activity displayed by platinum ismuch lower than that expected from the correlation. This can be explained takinginto account that this catalyst deactivates very quickly, and can display dehydro-chlorination properties [25]. Moreover, Pt4þ usually does not form stable com-plexes with unsaturated species, and it is often considered inert towards alkynecomplex formation [26]. However, iodo–Pt4þ complexes, in acidic aqueous ormethanolic solutions, catalyze acetylene hydroiodination to vinyl iodide [27], andmechanically activated K2PtCl6 can catalyze the hydrochlorination reaction ofacetylene [28], but this latter case cannot take into account possible effects inducedby the carbon support.

1.3.4TheE� CorrelationApplied toHomogeneous andNonhomogeneousGoldNanoalloys

Au nanoalloys, such as Au–Pd alloys, can be very efficient catalysts for the directsynthesis of hydrogen peroxide [29] and the oxidation of alcohols to aldehydes [30].This is not the case for the hydrochlorination reaction. Despite a wide range of alloyscontaining Pd, Pt, Ru, Rh, and Ir, in different ratios, none except Au–Rh systems(with a 90 : 10 atomic ratio) are capable of increasing the activity of gold for thehydrochlorination reaction during the time on-stream. Apparently, at first sight this adisappointing result with respect to catalyst design. However, it is rather a furtherdemonstration of the validity of theE� correlation and the formation of homogeneousalloy systems. In order to clarify this important aspect of the hydrochlorinationreaction, the example of the bimetallic Au–Pd system will be illustrated.

Au and Pd are able to form an alloy at all Au : Pd ratios. Thismeans that, if the totalmetal loading per catalyst is kept constant, the addition of a secondmetal to gold leadsto dilution of gold by the secondmetal, in this case Pd, provided that a homogeneousalloy is present. This implies that the addition of a secondmetal leads to a decrease inthe standard electrode potential which is a linear function of the atomic fraction x ofthe second metal added. This can be explained by calculating E� for a generalhomogeneous alloy of composition PdxAu(1� x).

It is necessary to consider first the following equilibria:

xPdþ 1�xð ÞAu>PdxAuð1�xÞ ð1:1Þ

Pd2þ þ 2e� >Pd ð1:2ÞAu3þ þ 3e� >Au ð1:3Þ

The value of E� for any alloy composition PdxAu(1� x) is required:

xPd2þ þ 1�xð ÞAu3þ þ 3�xð Þe� >PdxAuð1�xÞ ð1:4Þ


The last equation can be obtained as a combination of the equilibria (1.1), (1.2)and (1.3):

ð4Þ ¼ ð1Þþ xð2Þþ ð1�xÞð3Þwhere 1–4 represent equilibria (1.1–1.4), and therefore

DG�4 ¼ DG�1 þ xDG�2 þ 1�xð ÞDG�3 ð1:5Þ

� 3�xð ÞFE�4 ¼ DG�1�2xFE�2�3 1�xð ÞFE�3 ð1:6Þ

By using the E� potentials for equilibria (1.2) and (1.3) reported below:

Pd2þðaqÞ þ 2e� >PdðsÞ E�2 ¼ 0:951 V ð1:7Þ

Au3þðaqÞ þ 3e� >AuðsÞ E�3 ¼ 1:498 V ð1:8Þ

E�4 ¼�DG�13�xð ÞF þ

2xE�23�x þ

3 1�xð ÞE�33�x ð1:9Þ

E�4 ¼�DG�13�xð ÞF þ

1:902x3�x þ

4:494 1�xð Þ3�x ð1:10Þ

E�4 ¼�DG�13�xð ÞF�

2:592x3�x þ

4:4943�x ð1:11Þ

In order to calculate finally the value of E�4, estimates of DG� for the differentalloy compositions are required. In the case of PdxAu(1� x), literature values areavailable [31], and using the form of the equation reported above it is possible toconstruct the plot shown in Figure 1.7. It is clear that E� decreases almost linearlyfrom the value for Au to that for Pd with change in composition of the homoge-neous nanoalloy.

In view of this effect, we consider that gold will invariably always be the best metalfor the hydrochlorination reaction. However, alloys such as Au–Rh with an atomicfraction ratio of 90:10 can actually be better catalysts than those containing only gold(Figure 1.8) [24].

Whereas the Au–Pd catalyst leads to the formation of homogeneous alloys, Au andRh are basically immiscible in each other. Binary-phase diagrams suggest that Rh hasvery limited solubility in Au (

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6E

0 /

V v

s S

HE

x / PdxAu1-x

Figure 1.7 Standard electrode potential versus palladiummole fraction in a Pd–Au alloy, accordingto reactions (1.9–1.11) [24].

0 50 100 150 200 2500

5

10

15

20

25

30

35

Ace

tyle

ne c

onve

rsio

n %

Time (min)

Figure 1.8 Acetylene conversion for (~) Au–Rh catalyst (90:10 molar ratio), (&) Au, and (.) Rhonly using carbon as support. Reaction conditions: 180 �C, GHSV 870 h�1 [24].


1.4Central Role of Au3þ and Regeneration of Au/C Catalysts

As described previously, twomain routes are responsible for the deactivation of Au/Ccatalysts: (i) the possible formation of oligomers on the catalyst surface withconsequent site blocking and (ii) the reduction of Au3þ to Au0.Whereas the oligomerformation can be minimized by the choice of a suitable reaction temperature, theAu3þ reduction is unavoidable. Moreover, it is possible to identify a direct correlationbetween the activity ofAu/Ccatalysts and the amount ofAu3þ . This correlation canbeobtained by treating Au3þ catalysts off-line with aqua regia. In fact, unlike othersupports, carbon presents the interesting property that no significant amount of goldis leached from the carbon even if it is treated with boiling aqua regia for a shorttime (20min) [23]. Moreover, such treatment can restore the initial gold activity(Figure 1.9). X-ray photoelectron spectroscopic (XPS) characterization of an Au/Ccatalyst before and after use, and before and after aqua regia treatment, indicated(Figure 1.10) that the activity is related to the presence of Au3þ . This experimentalresult was further confirmed by the use ofM€ossbauer spectroscopy (Figure 1.11) [20].

In addition to off-line treatment, it is also possible to proceed to time on-streamtreatment, by introducing into the reaction mixtures reagents such as HCl, Cl2, NO,and NO2, with the most marked effect being displayed by NO [22], which allows fullrecovery of catalytic activity and slower decay of activity after the catalyst wasregenerated. Moreover, when HCl and Cl2 are compared, Cl2 leads to a lowerdeactivation rate for the regenerated catalysts, but also to a slower deactivation if itis used to pretreat the catalysts (Figure 1.12). This indicates that control of the goldoxidation state is of essential importance for the acetylene hydrochlorination reaction.

0 30 60 90 120 150 180 210 240 2700

5

10

15

20

25

30

35

40

45

50

Acet

ylen

e co

nver

sion

%

Time (min)

Figure 1.9 Activity trend for the hydrochlorination reaction over Au/C, (&) fresh catalyst and (.)regenerated catalyst using a short treatment with aqua regia. Reaction conditions: 180 �C, GHSV870 h�1 [23].


Documents

Edited by A. Stephen K. Hashmi and Dean F. Toste Cover.jpg ......2.3.3 Strong Metal–Support Interactions: Effect of Electronic Transfers and Decoration on the Gold Nanoparticles