Allenyl-tributylstannane

Chiral allenylmetal compounds provide convenient access to enantioenriched homopropargylic alcohols through Se2 additions to aldehydes. The syn adducts can be obtained through addition of allenyl tributylstannanes in the presence of stoichiometric boron trifluoride etherate (BF3-OEt2). The use of allenylmetal halides derivatives of Sn, Zn, and In lead to the anti diastereomers. The former additions proceed through an acyclic transition slate whereas the latter are thought to involve a cyclic transition state, thus accounting for the difference in diastereoselectivity. 

Table SO. Additions of an allenyl tributylstannane to aldehydes promoted by BF3 OEt2 or MgBf2.

The first report on the conceptually new asymmetric catalysis described that both a stoichiometric amount of SiCU and a catalytic amount of chiral phosphoramide (107) promote highly enantioselective allylation and propargylation of aromatic aldehydes with allyl- and allenyl-tributylstannane, respectively [41], The allylation does not proceed without (107). In the proposed mechanism, SiCU, a weak achiral Lewis acid, accepts the Lewis base (107) to form a strong chiral Lewis acid by polarization of the Si-Cl bonds. The active Lewis acid promotes the asymmetric reaction to give trichlorosilylated adducts with regeneration of (107) (Scheme 9.74). 

An analogous propargylation can be attained with allenyl tributylstannane however, this reaction is catalyzed by SiCk that is activated by a bis-phosphoramide catalyst. Note that, in this case, the role of the Lewis basic phosphoramide is to increase the Lewis acidity of SiCLj rather than to increase the nucleophiUcity of the stannane [66bj. For a discussion of these effects, see Section 15.4. ... 

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