Asymmetric hydrosilylation titanocene

Scheme 6-46. Titanocene-catalyzed asymmetric hydrosilylation of imines.

Keywords Asymmetric hydrosilylation, optically active alcohols, amines, Chiral Titanocene Catalysts, Acyclic Imines, Cyclic Imines, Chiral Rhodium Catalysts, aromatic ketones... 

Asymmetric hydrosilylation of several AT-alkyl ketimines with PMHS was effectively promoted by the chiral titanocene catalyst (S)-ll (Scheme 8) [24,25], The... 

Racemic N-methylimines derived from 4-substituted 1-tetralones were ki-netically resolved by asymmetric hydrosilylation with phenylsilane (1 equivalent) as a reducing agent using the titanocene catalyst (R)-ll (substrate Ti= 100 1) at 13 °C, followed by a workup procedure to afford the corresponding chiral ketones and chiral cis amines with very high enantio- and diastere-oselectivity (Scheme 12) [28], The extent of the enantiomeric differentiation, kfast/kslow was calculated to be up to 114. The ris-selectivity of this reaction was... 

The chiral titanocene complex 61 is an excellent catalyst for the enantioselective hydrosilyation of the ketone 131 with PMHS (133) to afford the alcohol 132 with 91 % ee [80]. Efficient asymmetric hydrosilylation of symmerical diketones is catalysed by the Rh complex coordinated by EtTRAP. Biacetyl (134) was converted to (2S,3S)-2,3-butanediol (135) with 95% ee in 69% yield [81]. 

Asymmetric hydrosilylation of ketimines in the presence of a chiral titanocene difluoride is improved by a nucleophilic additive (e.g., isobutylamine) which serves to release the chiral amine products and thereby generates less hindered amido complexes which are susceptible to reaction with the hydrosilane. ... 

Thus, the chiral titanocene complexes have been developed to exhibit a potentially high activity for the asymmetric hydrosilylation of ketones. 

A review on the industrial applications of homogeneous catalysts is particularly welcome. The proceedings of recent symposia and a general text have been published in addition to reviews on metal cluster catalysts, activation of saturated hydrocarbons by metal complexes in solution, catalysis by arene Group-VIB tricarbonyls, titanocene-catalysed reactions ofalkenes, transition-metal hydrides in catalysis, the mechanisms of the catalytic cyclization of aliphatic, hydrocarbons, asymmetric hydrosilylation and asymmetric synthesis. A n.m.r. study of the conformations of chelated Diop and a MO study of organo-metallic migration reactions are also of interest. Polymer supported catalysts have been reviewed and the relationship between cross-linking of the polymer and catalytic activity has been discussed. ... 

As well as rhodium-catalysed hydrosilylation, asymmetric ruthenium and titanium-catalysed hydrosilylation have also been reported. Amongst these, Buchwald s report of the hydrosilylation of ketones using titanocene catalysts and inexpensive polymethyUiydrosiloxane (PMHS) appear to be the most general. 

The basic compound of Brintzinger s ansa-titanocene complexes is ethylenebis-(tetrahydroindenyl)titanium dichloride, (EBTHI)TiCl2. Further analogues ((EBTHI)TiH, (EBTHI)Ti(Me)2, and (EBTHI)Ti(CO)2) have been wddely used for asymmetric hydrogenation, hydrosilylation, and Pauson-Khand reaction (121). Novel optically active titanium complexes containing a linked amido-cyclopentadienyl ligand have been developed and used for asymmetric hydrogenation (122). 

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