Transition states chiral allylic stannanes

Allylation reactions can be designed to effect high stereoselectivity in the case of chiral /3-alkoxy aldehydes, in which the ether oxygen provides for effective coordination with a Lewis acid. Multi-valent, oxophilic Lewis acids serve to pre-organize the aldehyde substrate in a six-membered chelation complex. As in the examples of a-chelation control, an open transition state is deployed with synchnal or antiperiplanar orientations based upon the consideration of steric interactions with placement of the small (hydrogen) vinyl substituent of the allylic stannane over the preformed metallocycle. Several examples are illustrated in Scheme 5.2.20. i... 

Chiral titanium complexes 4 and 5, which were developed as chiral catalysts for asymmetric carbonyl-ene reactions with prochiral glyoxylate esters [50], were first apphed to the catalytic asymmetric allylation of carbonyl compounds by Mikami and Nakai (Scheme 5) [9]. The titanium catalysts are prepared from (S)-binaphthol and diisopropoxytitanium dihahde (X=C1 and Br) in the presence of 4 A molecular sieves. Using these catalysts, glyoxylates are enantio- and diastereoselectively allylated with allylic trimethylsilanes or allylic tributylstan-nanes. High levels of enantioselectivity and syn selectivity are observed for (E)-crotylsilane and -stannane. The syn selective allylation reaction is believed to proceed mainly through an antiperiplanar transition state. 

Marshall has developed a collection of chiral a-alkoxy allyl stannanes 152 that provide facile access to various diastereomeric 1,2-diols (Scheme 5.26) [104-106]. For these reagents, Marshall has proposed that stereospecific rearrangement of 152 in the presence of Lewis acids gives the (Z)-y-alkoxy stannanes 153 prior to aldehyde addition. The transition state for the subsequent addition to the aldehyde is formulated as an open acyclic structure, leading to 1,2-syn-configured diols 154 (cf transition state 96) [107, 108]. 

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