Hypervalent allyltin

The ability of allyltin halides to extend their coordination sphere allowed the preparation of chiral hypervalent complexes with diamine ligands, which have been efficient in the asymmetric synthesis of homoallylic alcohols with up to 82% ee100. Similarly, a chiral hypervalent allyltin was prepared from a low valent tin (II) catecholate, chiral dialkyl tartrate and ally lie halide101. The allylation of aldehydes and activated ketones proceeded with high enantiomeric excess. Allyltins prepared from Lappert s stannylene and allylic halides were shown to be efficient as well, although the Lewis acid character of the tin atom is much less marked in that case102,103. 

An intermediate allyltin trichloride, possibly stabilized by an oxygen-tin hypervalent interaction, which then reacts with the aldehyde via a chair-like six-membered ring transition state with the substituent a to tin in an axial position, may be involved94. On heating with aldehydes, the 4-benzyloxypentenylstannane shows only modest diastereofacial selectivity22. 

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