Preparation of Allylic Trialkylstannanes

A variety of allylic tributylstannanes has been synthesized through in situ displacement of allylic mesylates with BusSnLi (Eq. 7) [9]. The method is quite general for primary allylic stannanes. 

A free-radical approach has also been successfully applied to the synthesis of primary allylic tributylstannanes (Eq. 8) [10]. The sequence involves a thermal [3,3] rearrangement of an allylic methyl xanthate then addition of a BusSn radical to the double bond of the derived dithiocarbonate intermediate and subsequent loss of COS in a chain-propagating step. 

As noted in the introduction, the first addition of an allylic stannane to carbonyl compounds was reported over thirty years ago and involved allyl triethyltin and mainly aromatic aldehydes at elevated temperatures without solvent (Table 1) [1], In these experiments the triethylstannyl ether of the alcohol adduct was isolated by distillation before conversion to the final product. 

A more extensive study of the thermal addition revealed a strong preference for anti (Z) adducts when substituted allylic stannanes are employed (Table 2) [11], These findings are consistent with a cyclic transition state in which the allylic substituent (Me) of the stannane adopts an axial orientation in a chair-like transition state. The preference for this axial orientation is the result of unfavorable steric interactions between an equatorial allylic substituent and the adjacent R groups of the trigonal bipyramidal stannane (Eq. 9). 

Additions of allylic stannanes to aldehydes can also be effected imder high pressure without an added catalyst or promoter [12]. Interestingly, and in apparent contradiction to the thermal additions, mixtures of syn and anti adducts are formed from both (E)- and (Z)-allylic stannanes (Table 3). Moreover, the proportion of (E) syn and (Z) — anti products actually increases as the pressure is increased in contrast with what might have been expected from the thermal reactions. These findings have been interpreted as reflecting reduced activation volumes in the boat vs chair transition states, as illustrated in Eq. (10). The effect is also observed with propanal, 2-ethylbu-tanal, and 2-phenylpropanal. 

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