B. Hermans et L. Hevesi, LEWIS-ACID ACTIVATED REACTIONS OF MIXED (O,SE) ACETALS WITH ALLYLTRIMETHYLSILANE AND ALLYLTRIBUTYLSTANNANE, Journal of organic chemistry, 60(19), 1995, pp. 6141-6147
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.