Kinetics of the Protodestannylation of Substituted Vinylstannanes

The set of vinylstannanes of general formula (E)/(Z)-R3SnC(R')=CHR" (R3Sn/R'/R" = Ph3Sn/H/OEt 1a/1b, Bu3Sn/H/OEt 2a/2b, Bu2ClSn/H/OEt 3a/3b, Bu3Sn/OEt/OEt 4a, Bu3Sn/H/Bu 5a/5b, Bu3Sn/H/t-Bu 6a/6b, Bu3Sn/H/Ph 7a/7b, Bu3Sn/H/H 8) was prepared and the kinetics of reactions with acetic, chloroacetic and trifluoroacetic acid in CDCl3 at 25degreesC were studied by means of H-1 NMR spectroscopy in order to find an explanation of the difference in reactivity of stereoisomers. The reaction starts with the rate determining step whereat the proton of carboxylic acid attacks the alpha-carbon of the double bond and consequently the transition state rises, where a positive charge is developed partly at the tin atom and partly at the beta-carbon of the double bond. It was shown that the stabilisation of this positive charge is at least as significant for the overall rate of reaction as an electron density on the alpha-carbon of vinyl group. Furthermore, it was suggested that for weak acids the nucleophilic assistance of the anionic moiety to the departing tin atom takes place and that this is probably responsible for the difference in reactivity of E- and Z-isomers. It is supposed that, due to conjugation of the tin atom with the pi-electron system of the vinyl group, mutual trans-position of an electron-releasing substituent (OEt or alkyl) and the tin centre makes this nuclecleophilic assistance easier than in the case of mutual cis-position, while an electron-withdrawing substituent (Ph) has the reverse effect.