Dipole-Stabilized Carbanions from Thioesters, Evidence for Stabilization by the Carbonyl Group
Journal of the American Society
Reactions are reported which establish that metalation can occur a to the sulfur of methyl and ethyl thioesters to give synthetically useful species. The methyl thiobenzoates 4a, 4b, 4c, and methyl thiopivalate (4d) react with lithium 2,2,6,6- tetramethylpiperidide (LiTMP) in tetrahydrofuran at ambient temperature to give the corresponding dibenzoylmethanes, 5a, 5b, Sc, and dipivaloylmethane (5d), respectively, in yields of 60-80%. Ethyl thiobenzoate (6) on reaction with LiTMP gives 23% methyldibenzoylmethane (7) at ambient temperature and 22% 2-thiobenzoylpropiophenone (8) at -98 'C. A double labeling experiment with 6 and 6-dlo gives 8 and 16 consistent with the intermediacy of the dipole-stabilized species 12 and inconsistent with formation of the homoenolate 15. Accordingly, the conversions of 4a-d and 6 to the 0-diketones are considered to proceed as shown in Scheme I. The isomerization of the vinylogous thioester 23 to 3-thia-tert-butylindanone( 24) in 43% yield is suggested to involve the metalated intermediate 25. Stable a-lithioalkyl thioesters, represented by 31, are produced by metalation of methyl 2,4,6-triethylthiobenzoate( 27b) and ethyl 2,4,6-triethylthiobenzoate( 28) with butyllithiums at -78 and -98 'C. Reactions of the dipole-stabilized carbanions from 27b and 28 with alkyl halides and with carbonyl compounds provide the expected adducts 32. Conversions of some of these adducts to a primary thiol, a secondary thiol, a 2,3-disubstituted thiirane, and an olefin illustrate the synthetic potential of 31. Evidence that the carbonyl group of the thioester provides significant thermodynamic stabilization for the formal carbanion 36 is provided by the observation that methyl 2,4,6-triisopropylthiobenzoate is completely metalated by thiomethylmethyllithium.
Metalation, carbanions, thiosters, carbonyl group
Helmick, L. S. (1978). Dipole-Stabilized Carbanions from Thioesters: Evidence for Stabilization by the Carbonyl Group. Journal of the American Chemical Society, 100, 5428-5436.