SYNTHESIS AND ELECTROPHILIC DESTANNYLATION REACTIONS OF TRIMETHYLSTANNYL-SUBSTITUTED METHYL CROTONATES

Four methyl(trimethylstannyl)crotonates have been prepared. Methyl (E)-2-(trimethylstannyl)crotonate and methyl (E)-3-(trimethylstannyl)crotonate were obtained from the Pd(0)-catalyzed hydrostannation of methyl-2-butynoate. Methyl (Z)-2-(trimethylstannyl)crotonate and methyl (Z)-3-(trimethylstannyl)-crotonate were obtained from the AIBN-catalyzed hydrostannation of the same alkynoic ester. Structures were confirmed by H-1 and C-13 NMR spectra. Reactivity in protodestannylation was determined from kinetic studies and the stereochemistry of the reaction determined by deuteriodestannylation. The 2-trimethylstannyl derivatives react by the allenol mechanism, while the 3-trimethylstannyl derivatives react by the normal S(E)2 mechanism. Bromodestannylation of the four isomers is accomplished with retention of configuration. Methyl (Z)-3-(trimethylstannyl)crotonate is the least reactive to electrophilic destannylation, and methyl cleavage from tin is competitive with vinyl cleavage for reaction with both acid and bromine.