Imines titanium catalyzed hydrogenation

Table 6.13 Asymmetric, titanium catalyzed hydrogenation of imines...

Particularly noteworthy is the discovery of a new type of the active catalyst 99,103,104 a crystalline, air-stable yellow-orange solid, which can serve as a highly enantioselective tool in the titanium-catalyzed hydrosilylation of imines. The reaction can be highly stereoselective for both acyclic and cyclic imines under a wide range of hydrogen pressures (Scheme 6-46). 

Another variation in the reaction of imines involves the titanium catalyzed condensation of a Schiff base with a ketene silyl acetal to give a p-lactam. The TiCU catalyzed reaction of 2.114 (derived from propanal and phenethylamine) and 2.775 led to 2.116. The reaction was mediated by a nitrogen-TiCla intermediate. Hydrolysis of the P-lactam and reduction of the N-benzylic group with hydrogen and palladium gave 2,2-dimethyl-3-aminopentanoic acid, 2.117. In this study, chiral... 

Alkyl and aryl substituted imines have received the most attention as substrates for asymmetric hydrogenation, and the development of the field can therefore be outlined by examining their reductions. These are usually catalyzed by chiral complexes of titanium, ruthenium, rhodium, or iridium, though gold catalysts have also recently proven useful for this purpose [31]. New catalysts are generally tested for the reductions of substrates A-D (Scheme 6.1). 

Titanium complexes that are similar to Duthaler s ( 2.5.2) can be generated from TiCl4, Ti(Or-Pr)4 and diacetoneglucose 1.48. These complexes catalyze asymmetric hetero-Diels-Alder reactions, and give high enantiomeric excesses [827], Corey and coworkers [828] also prepared a chiral titanium catalyst derived from cis-/V-sulfonyl-2-amino-1 -indanol and used this to catalyze asymmetric Diels-Alder reactions. Buchwald and coworkers [829, 830] have proposed the use of titanocene-binaphthol catalysts for asymmetric hydrogenation of imines or trisubsti-tuted olefins. 

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