6487
electronegativities on phosphorus would never occur without the constraint of the small ring.
coupled in high yields during the decomposition of diethyl(methylXtriphenylphosphine)gold,1, e.g. (CH3CHz)zCH3Au1"PPh3
--f
CH3CHzCHzCH3
Ph 1
The preparation of an epoxide has been reported by the use of singlet oxygen on an olefin in pinacolone as a solvent,l0 in analogy t o the isolation of dioxetanes from the action of ozone on olefins in the same solvent. l1 In each of these cases it has been postulated that the monodeoxygenation involves the removal of coordinate oxygen from a perepoxidelO or perdioxetane (Staudinger molozonide) intermediate 2a through BaeyerVilliger attack on pinacolone. The present report represents the first instance of reduction of a dioxetane to an epoxide through initial insertion of the oxygen acceptor into the ring. We shall report shortly on some metal insertions into the dioxetane ring followed by reductive fragmentation, which provide models for the extremely rapid catalytic dioxetane decompositions recently observed. l 3 Acknowledgments. This work was supported by grants from the National Science Foundation and the National Institutes of Health. We thank Charles L. Lerman for arousing our interest in the reactions of dioxetanes with trivalent phosphorus compounds. (10) A. P. Schaap and G. R. Faler, J . Amer. Chem. Soc., 95, 3381 (1973). (11) N. C. Yang and R. V. Carr, TetrahedronLett., 5143 (1972). (12) (a) P. R. Story, E. A. Whited, and J. A . Alford, J . Amer. Chem. Soc., 94, 2143 (1972); (b) but see K. R. Kopecky, P. A. Lockwood, J. E. Filby, and R. W. Reid, Can. J . Chem., 51,468 (1973); P. S.Bailey, T. P. Carter, Jr., C. M. Fischer, and J. A. Thompson, ibid., 51, 1278 (1973). (13) T. Wilson, M. E. Landis, A. L. Baumstark, and P. D. Bartlett, J . Amer. Chem. Soc., 95,4765 (1973).
Paul D. Bartlett,* Alfons L. Baumstark, Michael E. Landis Concerse Memorial Laboratory Department of Chemistry, Harvard Unicersity Cambridge, Massachusetts 02138 Receiaed July 5 , I973
+ CH3Au'PPha
(1)
Various isomeric trialkyltriphenylphosphinegold complexes can now be obtained, which have squareplanar configurations characteristic of four-coordinate gold(III). The known stereochemical structures of these complexes4f5allowed us to examine the stereochemistry and the manner in which alkyl groups are reductively eliminated from alkylgold(II1) species, and they have provided mechanistic insight into the coupling process. trans-Ethyldimethyl(tripheny1phosphine)gold was synthesized by oxidative addition of ethyl iodide to lithium dimethyl(triphenylphosphine)aurate(I).6 The corresponding cis isomer was prepared by stereospecific alkyl exchange between cis-dimethyliodotriphenylphosphinegold and ethyl Grignard. 40 Other analogs in Table I were synthesized by similar procedures. The Table I.
Decomposition of Stereoisomeric
Alkyldimethyl(tripheny1phosphine)gold' R(CH&AuPPh3 PPh3 (lo3 (lo3 mmol) mmol)
T,
Time,*
"C
min
0 0
70 90
75 45
57
90
180
83 93 96
70
100
58
51 56 trans-+Propyl 50
0 0
90
55
90
86
0
70
80 150 60
50
51
90 70 90 70 90 70 90
trans-Ethyl cis-Ethyl
cis-+Propyl trans-Isopropyl cis-Isopropyl
70 51 49 44
50 51 50 50 51 50
0 51
0 50 0 51
170 200 300 13 25 75
60
Alkane (mol RCH, CH3CH3 R R
54 65e 80e
60e 60e 95' 758 70h 85'
3 3 2 28 29 3
2 2 2 4
8 2
6 3 45 4 10 1 45 1
* Time required for major decomposition 5 Decalin solution. (arbitrary). Yields relative to R(CH3)2AuPPh3charged. Yields in excess of 100% due t o additional ethane formed by decomposition of CH3AuPPh3in the absence of added PPha (see text). e Including isobutane 5%. f n-Butane, 40%. 0 n-Butane, 20Z. n-Butane, 25%. n-Butane, 20%, yields formed relative to R(CH3)zAuPPh 3 .
Sir : Trialkylgold(II1) species are the key intermediates in the catalytic coupling of alkyl groups from organogold(1) complexes and alkyl halides.' They are also rather unique among transition metal alkyls in undergoing reductive elimination by alkyl coupling rather than by disproportionation. Thus, ethyl groups are
stereoisomeric pairs are readily distinguished by their proton nmr spectra. 4 , 6 Molecular weight measurements by vapor pressure osmometry indicated that these alkyldimethyltriphenylphosphine gold complexes are monomeric in solutions of benzene as well as nheptane. The decomposition of ~ ~ U ~ S - C H ~ C H ~ ( C H ~ ) ~ A U P afforded high yields of only propane. On the other hand, the decomposition of C ~ S - C H ~ C H ~ ( C H ~ ) ~ A U PPh3, which proceeded at significantly slower rates under the same conditions, produced a mixture of pro-
(1) A. Tamaki and J. ICH3AuPPh3 I (24 cessive additions of PPh, d o not alter the spectrum of I CH,CH, I (2b) GHj(CH&AuPPh3. The 31P nmr spectra of transCH CH3 C,H:(CH3)zAuPPh3 and free PPh3 both consist of singlets, the chemical shifts of which are unchanged R = CzHj, n-C3H7, i-C3H7 with various amounts of added PPh,. Furthermore, Table 1. Decomposition under these conditions is the and 'H nmr spectra show no indication of slightly complicated by the slow further decomposition any new phosphine adducts in sufficient concentrations of CH3AuPPh3to deposit a gold mirror and liberate to cause rate decreases of the magnitudes obtained. additional ethane' We conclude, therefore, that decomposition (eq 4) 2CHsAuPPh3 +CHaCH3 + 2Aua + 2PPh3 (3) of trialkyl(tripheny1phosphine)gold proceeds uia a dissociative process involving a three-coordinate trialkylNo metallic gold is deposited, however, when the gold(II1) intermediate such as that described in eq 8 reductive elimination of alkyldimethyl(tripheny1phosand 9. Furthermore, the trigonal intermediate must phine)gold is carried out in the presence of added PPh,. maintain sufficient stereochemical integrity to allow The rate of decomposition of alkyldimethyl(tripheny1for cis elimination of alkyl groups in the absence of phosphine)gold is increasingly retarded by successive added PPh, (cf. eq 2).9210 It is noteworthy that no sigamounts of added PPh3. The reaction under these nificant amounts of alkenes are formed during the conditions stops cleanly after the first stage decomposition of any of these trialkylgold(II1) comR(CH3)%Au1I'PPh3 +RCHs + CHaAu'PPh3 (4) pounds. The exchange of alkyl groups between gold(II1) and since the slower decomposition of CH3AuPPh3is segold(1) species' accounts for some of the minor anomverely retarded by PPh3,' and CH3AuPPh3 can be alies observed during the decomposition. Thus, the readily observed in the reaction mixture by nmr. formation of small amounts of n-butane from CH3CH,Strikingly, propane is the only major product ob~ ) ~ A U - suggests that diethylgold(II1) species are tained from both cis- and ~ ~ U ~ ~ - C H ~ C H ~ ( C H(CH&AuPPh3 present. Even more important, however, is the isomPPh3 in the presence of added PPh3 as shown in Table erization of alkyl groups, as shown in Table I by the I. Separate nmr studies established that at slightly formation of isobutane from the cis and trans isomers lower temperatures (where the decomposition is slower), of CH8CH2CH2(CH3)zAuPPh3 and n-butane from the cis- and rrans-CH3CH2(CH&AuPPh3are readily interisopropyl analogs. Both cases can be traced to the converted to an equilibrium mixture favoring the trans interconversion of isopropylgold(II1) and n-propylisomer. The rate of ,cis-trans isomerization is relagold(II1) species, e.g. tively insensitive to the concentration of added PPh3, although we were unable to study the isomerization at CH3 CHI CH, very low concentrations of PPh, due to the onset of the \ I 1 I
CHAuPPh3
CH3
I I
CH3CHsAuPPh3 CH3 (trans)
CH3
I +CHSAuPPh3 i
CH/ (5)
CHzCH3 (cis)
competing decomposition described above. Thus, the rate of cis-trans isomerization in eq 5 is clearly faster than the rate of decomposition in eq 4, in the presence of added PPh3. Under these conditions, the preponderant formation of only propane from either cis- or trans-ethyldimethyl(tripheny1phosphine)gold (see Table I) is readily attributable to rapid cis-trans isomerization followed by the preferential decomposition of the more labile trans isomers8 The retardation of the decomposition of alkyldimethyl(tripheny1phosphine)gold by added PPh3 can be accounted for by either of two mechanisms: (i) a preequilibrium formation of an inactive five-coordinate bisphosphine adduct R(CH&AuPPh3
+ PPh3 eR ( C H & A U ( P P ~ ~ ) ~( 6 )
I
(10)
CHI
the mechanism of which is analogous to the spontaneous rearrangement of trans-tert-Bu(CH&AuPPh3 to the isobutyl isomer." The latter has also bekn shown to occur uia a trigonal intermediate formed by the same dissociative route presented in eq 8 and 9. Acknowledgment. We wish to thank the National Science Foundation for financial support of this work. (9) A geometry for the cis isomer is required in which the CH3 groups are inequivalent. The presence of PPh3 in the outer coordination sphere is not excluded. (1 0) I t is possible that cis-trans isomerization of trialkyl(tripheny1phosphine)gold also proceeds by eq 8 and 9, if isomerization of the trigonal intermediate occurs faster than PPhs return. Nmr studies show that in the absence of PPh3, cis-ethyl(CHa)nAuPPha at 60" undergoes rearrangement and decomposition at competitive rates. The selective decompositions observed at higher temperatures (Table I) is consistent with this observation if the activation energy for reductive elimination from the trigonal intermediate is higher than that for rearrangement. Further studies are in progress. (1 1) A. Tamaki and J. K. Kochi, J . Chem. SOC.,Chem. Commun., 423 (1973).
(7) A. Tamaki and J. K. Kochi, J . Organometal. Chem., in press. ( 8 ) Cf.the relative trans effects of PPh3 and CH3 in the related Pt(I1) complexes: F. R. Hartley, "The Chemistry of Platinum and Palladium," Wiley, New York, N. Y.,1973, p 299ff.
Journal of the A m e r i c a n Chemical S o c i e t y 1 95:19
LH3
eCH3CHzCHzAuPPh3
1 S e p t e m b e r 19, 1973
A. Tamaki, S. A. Magennis, J. K. Kochi* Department of Chemistry, Indiana Uniaersity Bloomington, Indiana 47401 Received M a y 26, 1973
