The reaction. h the presence of soluble lithium salts, of non-stabllfzed ylides at low temperature with altphatic derbatbes of acyManes g&s moderate to goad 137-82%) yields of Z-1.Pclisubstituted vin&~s trt exdlent I >96% 1 tmnerlc pwtttes for normal alkyl derbatbes. Impllcatfons of these results on the mechantstfc aspects of the Wittfg ol.t$natron are dfxussed Since Its dtscovery over thirty years ago, the Wltttg ole8natton2 has engendered conttnuous synthettc and mechanistic intereA3 Non-stabtltsed ylides react with aliphattc aldehydes to give matnly cts alkenes. w&h thfs selectivity being enhanced by "salt-free" or "ltthlum ton-sequestering conditions as well as wtth increasing bulk on the aldehyde or phosphorus portton of the combtntng partners.3e** Recently. it has been demonstrated that acylsflanes can function as stertcally-modified aldehydes tn certain reactions4 It occurred to us that the stereochemical consequence of substttuttng an acylsilane for an aldehyde in thts process should be to gtve Zallcenylstlanes selecttvely. In such products. the alkyl groups would have the opposite (trans) stereochemical relationship. as that which would be obtained from the corresponding aldehyde. Moreover, these compounds would also contatn the additional synthetic versatthty of the vlnylstlane functtonality as compared to normal alkene products.5
Early studies by Brooks clearly established that altphatic acylstlanes underwent the normal olefination with methylenetrtphenylphosphorane, but wtth thts ylide the stereochemistry of the process could not be determined. Thus, we chose to examtne representattve ylide/altphattc acylsllane combtnattons. first with respect to yield opttmisatlon (Table 1) and second, to determine the scope and stereoselecttvky of the process (Table 2). This data reveals that the best condttions for the preparation of 2 are obtatned when 1 is added to the ylide at low temperature in the presence of soluble ltthium salts. As expected, no Brook rearrangement products were observed for these altphattc acylstlanes. To ascertain the product con8gurattons. 2 was phototsomerlscd to a mixture employing Zweifel's procedure which enabled us to quantitatively analyze the product composition by capillary GC.'
Corroborative NMR data reveals the following generalizations: (1) lH NMR The stltcon methyls absorb at slightly higher field for the E us the 2 isomer (eg. 2a: 0.06 vs 0.10 1. (2) 2gSt NMRI The Z isomer absorbs at higher field than the corresponding E isomer (eg. !& -7.8 vs -4.9 ) wtth the exception of 2e (R' = OMe) where the opposite is true (k. 2~: -4.8 us -6.7 ). (3) 13C NMR: The allyhc carbon atoms in the the E isomer are consistently found at higher field than the corresponding carbon atoms in the Zisomer (eg. 2a: 30.6: 29.6 us 34.2: 38.3 for C-3,6).* Formation of the enolate of the acylsllane was found to be a major competing process. While the monodeuteration of the enolate was consistently Incomplete (cf: Table 21.' the silylation process was more conclusive in that no unreacted acylsilane was found and 2a and 9 were formed as the only signtflcant volatile products. Skmlarly, under "salt-free" conditions (c$. Table 1. Entry 9). silylation gives 3 exclusively. 1. PhsPCHPr/UX 2. TMSCl + 2a la(58% Drl 3