Additions that Proceed by Transmetalation

When Lewis acids such as SnCU and TiCU are used to promote additions of allylic trialkyltin reagents to aldehydes several reaction outcomes are possible, depending on stoichiometry and the mode of addition. If the Lewis acid is added to the aldehyde followed by the allylic stannane, the typical product (syn for crotylstannanes) derived from an acyclic transition state is formed. If, however, the stannane and Lewis acid are premixed and left to equilibrate, metathesis can occur forming the allylic halome-tal compound which reacts with the subsequently added aldehyde to give products (anti for crotyl) consistent with a cyclic transition state (Eq. 22). The initially formed allylic halostannane gives rise to the linear adduct, but if aldehyde addition is delayed, this initial secondary allylic metal halide can equilibrate to the primary isomer which then reacts with the aldehyde to afford the branched product. 

Butyltin halides have also been used to mediate this process. One of the first examples involved addition of a 3 1 mixture of tram- and ds-crotyl tributyltin and a variety of conjugated aldehydes to Bu2SnCl2 without solvent to form (Z) homoallylic linear adducts (Table 22) [39], In this reaction, addition of the initially formed secondary allylic dibutylchlorostannane to the aldehydes must be faster than that of the tributyl crotylstannanes, and faster than 1,3-isomerization of the chlorostannane. Formation of the (Z) isomer is consistent with a chair transition state in which the allylic methyl group of the stannane adopts an axial orientation to avoid steric interactions with the adjacent stannane substituents (Eq. 23). 

The linear (Z) addition product was also formed from cis-crotyl tributyltin and BuSnCls at -78 °C (Eq. 24) [40]. Significant amounts of branched adducts ( 4 1-2 1 syn anti) were seen at 0 °C and when a 60 40 mixture of cis- and frans-crotyltributyltin was employed at -78 °C. Premixing the crotylstannane and the BuSnCla followed, after 8 h or more, by the aldehyde gave the branched adducts nearly exclusively. 

These findings are consistent with a process in which metathesis of the cw-crotyl stannane is faster than addition of that stannane to the aldehyde at -78 °C (Fig. 8). The resulting secondary BuSnCl2 intermediate adds rapidly to the aldehyde to afford the linear product. At elevated temperature, 1,3-isomerization of the secondary stannane competes with aldehyde addition resulting in more of the branched product. The trans crotyl stannane, on the other hand, is more reactive than the cis isomer and BuSnCls-promoted addition competes with metathesis when the aldehyde is added after short pre-equilibration (1 min). Longer pre-equilibration times enable metathesis and ensuing 1,3-isomerization to occur with the formation of cis- and tranj-crotyl BuSnCla intermediates which react with aldehydes by a cyclic transition state to form the syn and anti branched adducts. 

Transmetalation of cinnamyl and crotyl tributyltin reagents by SnCl2 in acetonitrile has been proposed to explain the predominant formation of anti homoallylic alcohols from aldehydes (Table 23) [41]. In contrast, the syn adducts predominate when these reactions are conducted in CH2CI2. The exchange reaction in acetonitrile was confirmed by the quantitative formation of BusSnCl. Presumably the putative allylic SnCl species is stabilized by complexation with acetonitrile (Eq. 25). 

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