The addition of alkyl nucleophile to aldehydes can be accomplished with high optical purity. For an example, the chiral cyclpentadienyltitanium fluoride complex catalyzes the addition of methyltitanium triisopropoxide to benzaldehdye with excellent enantioselectivity (Scheme 7).
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Scheme 7 |
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Scheme 8 |
The synthesis of cyanohydrins can be achieved with high optical purity using chiral titanium-polymer complex (Scheme 8). In these reactions, the catalyst catalyzes the reactions under homogeneous conditions, however, after completion, the catalyst can be precipitated from the reaction mixture and collected for recyclability without loss of activity and selectivity using MeOH. This catalytic system has the advantages of both homogeneous as well as heterogeneous processes.
3.8.5.2 Cycloadditions
Chiral titanium(IV) complexes have been found to be excellent Lewis acid catalysts for cycloaddition reactions. For example, chiral titanium(IV) complex prepared from TADDOL and TiCl2 (OiPr)2 catalyzes Diels-Alder reactions of dienes with oxazolidinone derivatives of several α,β -unsaturated carboxylic acids with excellent optical purity (Scheme 9). These systems could also be used for [2+2]-cycloadditions between unsaturated N-acyl-oxazolidinones and electron-rich alkenes. For example, fumaryol derivative reacts with alkynyl sulfide to give cyclobutene derivative with excellent yield and enantioselectivity (Scheme 10).
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Scheme 9 |
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Scheme 10 |