Intramolecular allylmetallation

Scheme 1 Formation of heterocycles by intramolecular allylmetallation of aldehydes...

Among the possible intramolecular allylmetallation reactions the intermolecular allylstannation (2) has been most widely studied by the group of Y. Yamamoto. (/) These reactions can be carried out under Lewis acid catalysis to proceed via transition states of the type 3, thermally via transition states of the type 4, and under Broensted acid catalysis, a reaction for which the nature of the transition state is not yet clear. In these reactions high to very high stereoselectivity has been attained. But there is not yet a uniform solution to reach predictable stereoselectivity in terms of attaining each of a number of possible stereoisomers selectively. It is for this reason that we wanted to evaluate the scope of the intramolecular allylboration reaction as a route to substituted tetrahydropyrans and piperidines. 

These examples again have some mechanistic implications in that they appear to rule out cyclization via 5n2 displacement of the halide by a samarium ketyl. However, one cannot distinguish between a mechanism based on allylsamarium addition to the carbonyl versus an electron transfer mechanism as outlined for the alkyl hdide substrates above. Both mechanisms allow for isomerization of the double bond (via 1,3-allylic transposition in the case of an allylmetallic, or configurational instability in an allylic radical in a diradical coupling mechanism) and also provide reasonable routes for generation of butadiene. Further mechanistic work is clearly required in order to provide a more detailed understanding of all of these intramolecular Barbier-type reactions. 

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