LEWIS-ACID ACTIVATED REACTIONS OF MIXED (O,SE) ACETALS WITH ALLYLTRIMETHYLSILANE AND ALLYLTRIBUTYLSTANNANE

On the basis of preferential complexation of oxygen or selenium atoms by different Lewis acids, it was expected that, depending on the Lewis acid employed, the title reactions would lead to selective formation of homoallyl ethers or homoallyl selenides. This has not been confirme d by experiment: in almost all the reactions tried homoallyl ethers la rgely predominated; or were the exclusive allylation products, even wh en strongly oxygenophilic Lewis acids such as TiCl4 were used. In the cases of the latter type of Lewis acids, the results observed with (O, Se) and a couple of(O,S) mixed acetals are-interpreted in terms of two major factors operating in opposite directions. H-1 NMR data of a mix ture of TiCl4 and (O,Se) acetal indicate that preferential (but not ex clusive) complexation of the oxygen moiety takes place indeed. However , because of the much stronger C-O bond as compared to the C-Se bond, this latter.(also activated by the Lewis acid to some extent) undergoe s cleavage by allyl metals to give homoallyl ethers as predominating p roducts. In contrast with BF3 . OEt(2), boron trichloride and boron tr ibromide were found to react with (O,Se) acetals to give the correspon ding alpha-halo selenides, which in turn were cleanly transformed into homoallyl selenides on reaction with allyltrimethylsilane in the pres ence of tin tetrachloride.