a short time a yellow precipitate separates; this is collected on a cooled frit with exclusion of oxygen. The yield of (2) is cu. 40%.

Received. June 22, 1973 [Z X74 IE] German version: Angew. Chem. NS, 910 (1973)

Tetrameric q-Cyclopentadienylcobalt Hydride - A New Type of Four-center Cluster with p,-Hydrido Bridges[**]

By Jiirit Miiller and Horst D o r n c ~ ~ ' ]

Relatively little is known about reactions of coordinated nitric oxide although theoretically a variety of such reac- tions is possible. Our earlier successful experiments with addition of carbanions to the N atom of a nitrosyl ligand['I induced us to study the possibility of a similar hydride addition. The reduction of an N O to an N H 2 ligand has already been described for the case of reaction of cyclopen- tadienyldinitrosylchromium chloride with NaBH,r2l. We report here the quite different course of reduction of an q-cyclopentadienylnitrosylmetal complex by a hydride rea- gent. If the dimeric cyclopentadienylnitr~sylcobalt[~~, [ C S H 5 C ~ N 0 ] 2 ( I ), is treated in tetrahydrofuran at 20 C under argon with LiAIHl in presence of AIC13, evolution of gas ( H 2 and N2) and change of color from dark olive to dark brown occur. After hydrolysis, a complex of com- position C,,H,,Co,, which forms dark violet shiny crys- tals, readily soluble in pentane, benzene or ether, can be isolated in 43%) yield; when heated under N z at 300 C this neither melts nor undergoes any other change. In the solid state the compound is stable in air, but in solution it decomposes on access of air. Because of its properties and spectroscopic data we assign structure (2) to the corn plex.

a 7 I

co

The dipole moment of only 0.69k0.1 D, measured in ben- zene-the true value is probably lower rather than higher--indicates a very symmetrical structure for the compound. The base peak in the mass spectrum is formed by the molecular ion C 2 0 H 2 4 C ~ $ (m/e 500); except for the ions produced by loss of H therefrom, the intensities of other fragments are relatively low. The, IR spectrum (KBr) shows the occurrence of equivalent, x-bonded, sym- metrical cyclopentadienyl ligands by the following absorp- tions (the corresponding bands for cobaltocene are shown

[* ] Doz Dr. J . Muller and DiplLChem. H . Dorner Anorgaiiisch-chemisches Laborator ium dei Tcchntschcii Universitit 8 Munchen 2. Arcisstrasse 71 (Germany)

[**I Rcactions of Nitrosyl Complexes, Part 2. This work was supported by the Deutsche Forschungsgeineinschalt and the Fonds der Chemischen industric. ~ Part I - see Ref. [ 11.

in parentheses for comparison): 3090 (3060), 1416 (1412), 1103 (1103), 998 (993), 817 (859) and 785 (777) cm- I. Corre- spondingly, in the 'H-NMR spectrum ([D,]-tetrahydro- furan) there is one singlet for the five-membered ring pro- tons at T = 5.08. The existence of bridging hydride ligands follows on the one hand from the metal-hydride vibration at only 950cm- (expected region 1 1 0 0 ~ 3 0 0 c m - ' I%

which characteristically appears as a broad band in the IR spectrum (a shoulder at 890cm- ' and a further weaker band at 1052 cm- could also be assigned to Co-H vibra- tions, but proof of this through the deuteriated compound is not yet available); secondly, a signal a t the unusually high T value of 33.06 is observed in the 'H-NMR spectrum. This and the high symmetry ofthe complex are best brought into accord with occurrence of p3-hydrido bridges. The diamagnetism is also in agreement with the proposed struc- ture in which the Co atoms have noble-gas configurations and are each involved in three metal-metal bonds. The complex (2) is thc first organometallic four-center cluster without carbonyl ligands. The primary step in its formation is probably nucleophilic attack by hydride on the nitrosyl-nitrogen of ( I ). The catalytic role of the alu- minum chloride in the redox reaction probably consists of activation of the N O ligand by coordinative interaction with its 0 atom. Analogous interactions have been observed with carbonyl-metal complexes[51.

E.xpcv+inental: All operations must becarried out under argon or nitrogen. A solution of ( I ) (6.16g, 20mmol) and AICI.3 (5.3g) in THF (200 ml) is added dropwise to a stirred solution of LiAIH? (1.6 g, 42 mmol) and A1CI3 (5.3 g) in THF (200 nil), whereupon gas is evolved (cu. 1.41). After 24 h the solvent is removed, thc residue is stirred vigorously with benzene and water, and the benzene phase is washed with NaHCO3 solution and dried over Na2S0,. Removal of the benzene affords almost pure (2) (2.15 g, 43 %). For further purifica- tion the product is dissolved in pentane/benzene ( 1 : 1) and filtered from a layer (lOcm long, 3cm wide) of A l l 0 3 (Woelm, 7%) of water): it may then be recrystallized from pentane/THF (4: 1 ) with severe cooling

Received: June 20, 1973 [Z 876 IF] German vcrsion: Angew. Chcm. X i . 867 i 1973)

[ I ] J . Miil ler and H . Doriier, Chcm Ber. 106, I 122 (1973)

[2] N . F/ircrofi, J . Organomctal . Chem. 1 5 , 254 (1968). [3] H . Eri i i i i iw , J. Organomctal . Chcin 1-7. 517 (196X) 141 H D. K w x and R . €5. S u i i i u ~ . Chcm. Rcv. 72, 231 (1972). [S] D. F . Shrilcr and A . .4/ld1. Coord. Chem. Rcv. 8. I S (1972)

Concerted Fragmentation of a 1,6-Diradical on Thermolysis of 1,2-Dioxanes[l][***]

By Wuldernur Admi and Jui?ies Scincihiu[*]

I,2-Dioxanes such as the tetramethyl derivative ( I r r ) i ' l should be potential precursors of the 1,6-diradical ( 2 " ) or the 1,4-diradical(3u) provided that the concerted three-

[*] Prof. Dr. W. Adam [**] and J. Sanabia Department of Chemistry. Univcrsity of Puerto Rico Rio Piedras. Puerto Rico 00931 (USA)

["'I A. P. Sloan Fellow, 1968--1972: J. S. Guggenheim Fellow. 1972 to I973

["*I This work was supported by thc National Science Foiindation and the Petrolcum Rcscarch Fund of t he American Chcmical Society.

843

fold fragmentation of ( I u ) into acetone and ethylene by way oftheactivated complex (4u)does not predominate[’]. In order to learn about the chemical fate of such diradicals we have studied the thermolysis and photolysis of (l)iar,

When ( l a ) is heated in benzene solution at 200“C, it is fragmented quantitatively into acetone and ethylene. 2,2-Dimethyloxetane, the cyclization product of the 1,4-di- radical (3u ) , could not be detected even in traces, although control experiments showed that it is stable under the conditions of thermolysis of ( I a ) . The acetone and ethylene could have been formed, at least in principle, from the 1,6-diradical ( Z u ) , from the 1,4-diradical ( 3 u ) or by the concerted fragmentation pic- tured in ( 4 ~ ) . To clarify the mechanism we have measured 1. the activation parameters, 2. the secondary isotope effect and 3. the stereochemistry of the olefin formation on ther- molysis of ( I ) : 1. AH’=32&1 kcaljmol, AS* = -12.6k1 cal deg-’ mol-I, AG* (500“K)=38.3*1 kcal/mol for ( l u ) , 2. k l l lkD = I .03 kO.03, determined for ( I a ) and ( I b ) . 3. 70 & 2 %) inversion of the ethylenes (5) on thermolysis of ( I c) and ( 1 d ) , determined by IR spectroscopy from the ratio of c is - (S) to t runs - (S ) . Control experiments showed that c i s - ( 5 ) and rruns-(5) d o not isomerize under the reaction conditions. [Photolysis of ( I c ) and ( I d ) in benzene at 300nm affords 50% each of cis-(5) and rrtrris-(S) in both cases; the two ethylenes are photostable at this wavelength.] The thermolytic results cannot be explained by a concerted three-fold fragmentation of ( I ) by way of (4) because in that event the configurations of the olefins should have been quantitatively retainedl3I and the secondary isotope effect should have been considerably largeris1. The results are also not in accord with production of the 1,4-diradical ( 3 ) because this precursor should have led to complete isomcrization of the olefin ( 5 ) and a considerably larger secondary i~otopeeffect1~1. The I .6-diradical(2) best meets the requirements (see Scheme I) . The stereospecificity of

c i s - ( 5 1 trans-: 5 )

!s

the fragmentation excludes stepwise fission of the 1,6-dira- dical (2) by way of the lA-diradical (3). The preponderance of inverted ethylene indicates that the activation barrier for concerted double fission to ketone is greater for the cisoid diradicals ( 2 c ) and ( 2 d ) than for the transoid diradicals (2c’) and (Zd’). EHT calcula- tionsC6] showed that for the cisoid 1,6-diradical (2) the antisymmetric (a) and the symmetric (s) “diradical” MO’s are degenerate, but that for the transoid (2) they are split by through-bond coupling['^ [see Scheme 2; there (2) is shown with H in place of CH’]. Now the antisymme- tric combination is ca. 0.8 eV more stable than the symme- tricone;more important, however, is that theantisymmetric “diradical” MO in the transoid conformation is favorably situated for the fragmentation since rc-bonded ethylene is formed, whereas the cisoid conformation leads to dis- torted ethylene. Thus this simple analysis would suggest that concerted fragmentation is more easily undergone by the transoid than by the cisoid conformer.

H

H H

(2). cisoid (2) , trunsoid

Scheme 2

If fragmentation of the 1,6-diradical (2) and not homolysis of the peroxide bond determines the rate of reaction, a small isotope effect would be expected since hybridization changes during this fragmentation. In other words, our value of k , + / k D = 1.03 indicates that the energy barrier for concerted fragmentation of the cisoid and transoid I ,6-dira- dicals (2j is higher than that for reclosure to the 1,2-diox- ane ( I ) ; a similar situation exists for competition between rearrangement and cyclization of the trimethylene diradi- calc’l. The ready ring opening and recyclization would explain not only the small although not negligible isotope effect but also the negative activation entropy for thermo- lysis of 1,2-dioxanes ( I ). For example, 1,2-dioxanes ( I ), like 1,2-dioxolane~‘~J, are 10’ times more stable than rrrr- butyl peroxide. This stabilization is controlled by entropy rather than by enthalphy since for ( I ) AH* is cu. 6 kcal/mol smaller than for rw-butyl peroxide[”]. If reclosure of the 1,6-diradical (2) is easier than its fragmentation, then only special conformations of the diradical could decom- pose to acetone and ethylene and a negative AS* value is to be expected. Our kinetic and stereochemical results

thus show that 1,6-diradicals ( 2 ) , formed by simple homol- ysis of the peroxide bond of 1,2-dioxane ( I ) , undergo the novel concerted fragmentation described here in prefer- ence to the stepwise loss of acetone by way of the IA-diradi- cal (3).

Received: June 27, 1973 [Z 883 iE] German version: Angew Chem. N5.914 11973)

111 Cyclic Peroxides, Part 28 -Part 27: W Adorn and N . Dururi, J . Org. Chem. 3X. 1434 (1973). [2] R . Crirgrr and G . Paulig, Chem. Ber 8N, 712 (1955) 131 R . 8. Woodwnrd and R Hoflmorinr The Conservation of Orbital Symmetry. Verlag Chemie, Weinheim. 1970.

141 L. Solem and C. Rowlund, Angew. Chem. 84, 86 (1972): Angew. Chem. internat Edit. f I . 92 (1972).

[5] L. M . Szmdrey, Dissertation, University of Puerto Rico 1972.

[6] We thank Prof. R . G(eiter, Darmstadt, for the EHT calculations. 171 R . Hi,ffmum, Accounts Chem. Res. 4, 1 (1971).

[8] R . G . Brrgmon and W L. Corrrr. J . Amer. Chem. Soc. Y f , 741 I (1969) and references cited there. [9] W 4. Pryor. Free Radicals. McGraw-Hill, New York 1966, p. 51.

Phase-transfer-catalyzed Production of Sulfur Ylides in an Aqueous System

By Andreas Merz and Gottfried Markl[*]

There is an increasing number of reports of phase-transfer- catalyzed two-phase reactions"] in which anionic reactants are transferred from an aqueous to an organic phase by quaternary ammonium salts (benzyltriethylammonium[21 and n-hexadecyltrimethylammonium chloride['] and tetra- n-butylammonium iodidef4'). Excellent examples are the cycloadditions['. 3. 51 and insertion reactions[6' of halocar- benes produced with aqueous sodium hydroxide. We recently"] reported on Wittig olefinations by non-stabi- lized alkylidenetriphenylphosphoranes in the two-phase system dichloromethane/aquesous sodium hydroxide. Our assumption that phase-transfer catalysis is involved in these reactions, brought about by the phosphonium salts, is supported by similar reactions of sulfonium- and oxosulfo- nium salts described below, which proceed through inter- mediate sulfonium and oxosulfonium ylides['I. Like tetra- alkylammonium salts containing small alkyl substituents, the trimethylsulfonium ion should not be an effective phase- transfer catalyst ; accordingly, trimethylsulfonium iodide (1) does not react with benzaldehyde in the two-phase system CH,CI,/NaOH, whereas on addition of 1-5 mol-% of tetrabutylammonium iodide (TBAI) 2-phenyloxirane ( 2 u ) (b. p. 77 T / 1 1 torr ; nio = 1 .5359[']) can be isolated in >90% yield.

+,CH3 H CHB-S, I + R-CL

CH3 0

In spite of the long reaction time of 48 h at 50 'C neither hydrolysis of the sulfonium salt nor a Cannizzaro reaction of the benzaldehyde can be observed.

[*] Prof. Dr. G . MPrkl and Dr. A. Merz Fachhereich Chemie der Universitit X4 Regensburg, UniversitPtsstrasse 31 (Germany)

Use of cinnamaldehyde affords equally smoothly an 85 'In

yield of 2-styryloxirane (Zb) , b. p. 64 C/O. 1 torr. 'H-NMR spectrum of ( 2 h ) in CDCl3 (ppm against internal TMS): aromatic H: 7 .2G7.55 (5H); olefinic H : AB portion of an ABX spectrum, centered at 6.37 (2 H) (JA - x = 2, J B - x = 8, J A - , ~ = l 6 Hz); cyclopropyl-H: 3.3G3.60m ( 1 H), 2.85- 3.15m (1 H), 2.6&2.80m ( 1 H). Ketones give only small yields even after prolonged reac- tion. Acetophenone undergoes 36% reaction in 72 h, and benzophenone 18 %, the products again being oxiranes. The reactions of the trimethyloxosulfonium salt (3) are interesting. With benzaldehyde it gives only 2&30% of the oxirane ( 2 ~ ) ; but this is accompanied by a 12% yield of a colorless, crystalline compound, m.p. 180 C, which was identified as 2,6-diphenyl- 1,4-oxathiane 4-oxide ( 4 ) by elemental analysis and spectroscopic data.

(cH,),~=o I - + c~H,-c, P

,r+ (Za) 2 0 - 3 0 %

TI1AI; 20 h , SOo<'

IR spectrum of ( 4 ) (KBr disc): vS=O 1025[81, vC-0 1076 cm- '. 'H-NMR spectrum: 7.38s (IOH), AA:BB'X spectrum with SX=5.60, DXD (2H) (JA-.=2, J B - x = 10 Hz), AB potion 2.34-3.20 m (4H) ppm. Mass spec- trum: m/e=272 (M'), 104 (C,H,-cH-CH;). Molecular weight (osmometric)=272 (calc. 272). (3) reacts remarkably smoothly with x,P-unsaturated aro- matic ketones:

M.P [ C l Yield ['YO]

R = H [9] 37-43 R = C H , 85--87 R = O C H , colorless oil

86 78 74

Here the same selectivity is evident as was observed by Corey et ~ l . ' ' ~ : no oxirane is formed, but instead c,is/rrtins- mixtures of cyclopropane derivatives. According to the molecular ratio of the reactants, 1,5- diphenyl- 1,4-pentadien-3-one affords mono- or bis-cycio- propane ketones.

2-Phenyloxirane ( 2 a ) : Benzaldehyde (10.6 g, 0.1 mol) and TBAI (0.5 g, 1.35 mol) are dissolved in dichloromethane (l00ml) and a layer of 50 '4 aqueous sodium hydroxide is introduced underneath this solution. Trimethylsulfonium iodide (20.4 g, 0.1 mol) is then added and the whole is warmed at 50 C with

Arigrw. Chem. i n t w i i o f . Edit. Vo/ I2 (1973) N o . I0 845