A mechanistic study of the acid- and base-catalysed cleavages of β-trimethylstannylstyrenes

Abstract

The cleavages of cis- and trans-PhCHCHSnMe 3 in 1/10 AcOD-MeOD have been shown to give cis- and trans-PhCHCHD, respectively. The rates of cleavage of some XC 6H 4CHCHSnMe 3 compounds in 1/10 AcOH-MeOH at 50°C have been measured; there is no significant difference between the rates for cis- and trans-PhCHCHSnMe 3, and the relative rates of cleavage as X is varied are (X  H), 1.0; p-OMe, 7.0; p-Me, 2.3; m-Cl, 0.34; m-Br, 0.36. There is an excellent correlation with σ + constants, with a ρ value of 1.1. The results are interpreted in terms of rate-determining proton transfer to the β-carbon atoms, and it is suggested that acid cleavages of vinylHgX bonds involve analogous mechanisms. PhSnMe 3 is cleaved 20 times as slowly as PhCHCHSnMe 3 in the 1/10 AcOH/MeOH. The rates of cleavage of XC 6H 4CHCHSnMe 3 compounds by a mixture of MeOH (3 vol.) and 2 M aqueous NaOH (2 vol.) have been measured; trans-PhCHCHSnMe 3 is cleaved about 1.3 times as rapidly as its cis-isomer, and about 12 times as rapidly as a mixture of cis- and trans-PhCHCHSnEt 3. The relative rates for the various XC 6H 4CHCHSnMe 3 compounds (mainly trans-isomers) are (X ) H, 1.0; p-OMe, 0.99; p-Me, 0.92; m-Cl, 1.67; m-Br, 1.65. Cleavage of trans-PhCHCHSnMe 3 by NaOD/D 2O/MeOD gives exclusively trans-PhCHCHD. For cleavages in methanolic NaOMe the values of the rate isotope effects, (the ratio k meOH/ k MeOD) are 2.3–2.6, and those of the product isotope effects, PIE (the product ratio RH/RD on cleavage of RSnMe 3 by NaOMe in 1 : 1 MeOH/MeOD) are 4.5–5.0. The results are interpreted in terms of proton transfer from the solvent to the leaving carbon atom in the rate determining step as the SnC bond breaks as a result of the attack of the base anion at tin in a prior or synchronous process. PhCHCHSnMe 3 is cleaved by the aqueous alcoholic base about 5 times as rapidly as PhSnMe 3. Cleavage of trans-PhCHCHSnMe 3 by PHCOCl in presence of AlCl 3 in CH 2Cl 2 gives trans-PhCHCHCOPh, and cleavage of a 90/10 mixture of trans- and cis-PhCHCHSnMe 3 by bromine in CCl 4 gives a corresponding mixture of trans- and cis-PhCHCHBr.