Addition of Allenyl and Propargyl Stannanes to Aldehydes

Propargylic halides are converted to allenyl and propargylic tin halides upon exposure to SnCl2 in a mixture of A(,A(-dimethylformamide (DMF) and l,3-dimethyl-2-imidazo-lidinone (DMI) [87]. Subsequent addition of aldehydes leads to homopropargylic and/or allenic carbinols (Table 47). The ratio of the two regioisomeric adducts depends on the nature of and R. Alkyl substitution on the alkyne (R = Me) strongly favors the allenic adduct. On the other hand, the ratio of adducts from the TMS substituted alkyne (R = TMS) is dependent on the aldehyde substituent. 

A diallenyltin dibromide reagent can be prepared from propargyl bromide, metallic tin, and aluminum (Eq. 73) [88]. Addition to aldehydes leads to homopropargylic alcohols as the exclusive products. 

Similar treatment of TMS propargyl iodide affords a mixture of allenic and pro-pargylic adducts (Table 48). The regioselectivity is strongly solvent-dependent. Allenic adducts predominate in MeCN-DMSO whereas in 1,2-dimethoxyethane (DME) the propargylic adducts are the major products. In these additions the structure of the aldehyde plays a minor role. 

Additions of allenic tributylstannanes to aldehydes, like those of their allylic counterparts, require Lewis-acid promoters [83]. The favored promoter is BF3-OEt2 (Table 50). Promotion can also be effected by MgBr2, although less effectively. The use of other common Lewis acids, for example TiCU or AICI3, is complicated by competing exchange reactions. This type will be covered in a later section. 

The MgBr2-promoted additions to the (5)-a-benzyloxy aldehyde proceed via a chelation-controlled transition state (Fig. 18). Approach to the carbonyl face of the nearly planar five-membered magnesium chelate is directed by the methyl substituent resulting in re attack by the allenylstannane through the antiperiplanar geometry for the (P) stannane and the synclinal geometry for the (M) stannane. An antiperiplanar orientation of the (A/) stannane would place the vinylic methyl substituent in close proximity to the chelate ring. 

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