Chelates external reagents

The structure of titanium complexes is helpfid in understanding the interaction mechanism between Lewis acidic titanium ion and substrate [4]. Some crystal structures of chelate complexes have been reported [5]. It has been shown that most of this kind of titanium complexes take in-plane coordination marmer rather than out-of-plane [6] (Scheme 14.1). The attacking nucleophile is either a ligand bonded totitanimn complex or an external reagent The stereoselectivity results solely from the bias of the chiral titanium center. 

Achiral allylic indium reagents and chiral aldehydes. Allylindium reagents generated in water react smoothly with aldehydes and ketones (Scheme 10-99) [196], The reaction of achiral aldehydes and a-oxygenated aldehydes 290 with the allyl indium reagents proceeds smoothly to homoallylic alcohols without the need for external promoters. It is interesting to note that the a-hydroxyl aldehyde was se-leetive for the syn (chelation-eontrolled) product even in water. 

These results, along with quantum mechanical calculations, supported the conclusion that a six-membered chelated flat-chairlike transition state operated under the kinetic conditions. The diastereoselectivity of reactions between Li-salts of the (/ )-methyl p-tolyl sulfoxide formed by a-deprotonation with -BuLi, could be further enhanced by the addition of external C2-synunetrical bidentate ligand (7 ,7 )-l,2-M,M-bis(trifluoromethanesulfonyla-mino)cyclohexane in the form of the lithium M,M-dianion providing the amine in 80% yield with a diastereoselectivity of 99 1, apparently achieving a match between the chirality of the additive and the chirality of the sulfoxide reagent. 

A more general approach to get access to 1,3-syn-diols is the yn-selective Narasaka-Prasad reduction [77]. It utilizes the chelating capabilities of boron reagents and delivers the hydride externally, usually from NaBH (Scheme 2.144). Both a chair-like transition state and a boat transition state explain the observed yn-selective reduction. The mcommg... 

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