The BF3 and B(C6F5)3-catalyzed 1,3-isomerization of allylic stannanes

A spectroscopic and chemical study of the stereospecific B(C6F5)3-catalyzed 1,3-isomerization of (E,S)-1-methoxymethoxy-2-butenyl tributylstannane to (Z,S)-1-methyl-3-methoxymethoxy-2-propenyl tributylstannane is described. A pathway involving an intermediate B(C6F5)3 adduct is proposed based upon 1H- and 119Sn-NMR data. Analogous spectral evidence was not observed in the seemingly related BF3·OEt2-catalyzed isomerization. However, the similar product ratios obtained in B(C6F5)3 and BF3·OEt2-promoted additions of the latter stannane to o- and p-methoxybenzaldehyde reveals a close similarity between the two and prompts speculation that the BF3·OEt2-catalyzed 1,3-isomerization proceeds by an analogous pathway. A pathway for the B(C6F5)3-promoted addition of allylic stannanes to o-methoxybenzaldehyde is also presented.