Lewis Acid Catalysis of Allyltin Additions

When these addition reactions are promoted by the more conventional Lewis acids, e.g. BF3 OEt2, TiCU, SnCU, and MgBr2, they require at least a full molar equivalent of the promoter. Presumably the Lewis acid forms a stable alkoxide complex with the product of the addition which removes it from a possible catalytic cycle (Eq. 14). If the Lewis acid, MX , could somehow be regenerated in situ it should be possible to employ catalytic rather than stoichiometric quantities. 

A similar ploy has been used for the addition of cinnamyl tributyltin to isobutyral-dehyde [23], In that reaction InCls serves as the catalyst and TMSCl is used as the catalyst liberating reagent (Eq. 15). The diastereoselectivity of the addition is solvent-dependent, ranging from 88 12 anttsyn in acetonitrile at 25 °C, to 12 88 antisyn in dichloromethane at -30 °C. The former addition proceeds by transmetalation to the cinnamyl dichloroindium species, which adds to the aldehyde by way of a cyclic transition state. In the latter addition InCb serves as a Lewis acid and the reaction proceeds by the usual acyclic transition state to give the syn adduct in predominance. 

Both Sc(OTf)3 and Yb(OTf)3 have been employed as Lewis acid catalysts for additions of allylic stannanes to aldehydes. Reactions with the former catalyst can be conducted in a variety of solvents and are not sensitive to water [24]. All four allyl groups of tetraallyltin are consumed in the addition (Table 13). The latter reaction is performed with allyl tributyltin in CH2CI2 (Eq. 16) [25]. 

Evidently the lanthanide triflate is only weakly bound to the alkoxide oxygen of the product enabling protonolysis to occur in the former case with protic solvents or cosolvents. In the latter case the Yb(OTf)2 alkoxide product must react with Bu3SnOTf to regenerate the catalyst (Eq. 17). 

Bis-pi-allyl Pd and Pt complexes have been found to catalyze the addition of allyl tributyltin to aldehydes [26]. These catalysts are formed in situ from Pd- and PtCl2-phosphine complexes and the allylstannanes (Fig. 7, step 1). The allylation step is depicted as a metallocene reaction of the aldehyde and an his-allylmetal complex (Fig. 7, step 3). The catalyst is regenerated by attack of the allylic stannane on the alcoholate-palladium complex formed in step 3. Representative additions of allyl and methallyl tributyltin to aldehydes with the pi-allyl platinum catalyst are summarized in Table 14. 

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