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Bifunctional catalysts salen based

An unsymmetrical salen ligand bearing a Lewis base catalyses Ti(OPr-i)4-promoted addition of TMSCN to benzaldehyde with as little as 0.05 mol% loading, quantitative conversion is achieved in 10 min at ambient temperature. Another salen catalyst - a bifunctional salen-phosphine oxide-Ti(IV) combination - promotes enantioselective cyanosilylation of aldehydes. Fine tweaking of the structure of another series of bifunctional chiral salen-Ti(IV) complexes allows the enantioselectivity to be reversed. Biaryl-bridged salen-titanium complexes are also highly efficient catalysts, one example giving 87% ee at room temperature. ... [Pg.42]

The catalytic enantioselective addition of diethylzinc to a-ketoesters was developed by Kozlowski [56]. Modular bifunctional Ti-salen [N,N -bis(salicylidene) ethylene-diamine] type catalysts that contain Lewis acid and Lewis base activating groups accelerate the carbonyl addition to a much greater extent than the competing carbonyl reduction. a-Hydroxyesters containing new quaternary stere-ogenic centers are obtained in high yields and moderate enantiomeric excesses (Scheme 14.10). [Pg.201]

Lu and coworkers have synthesized a related bifunctional cobalt(lll) salen catalyst similar to that seen in Fig. 11 that contains an attached quaternary ammonium salt (Fig. 13) [36]. This catalyst was found to be very effective at copolymerizing propylene oxide and CO2. For example, in a reaction carried out at 90°C and 2.5 MPa pressure, a high molecular weight poly(propylene carbonate) = 59,000 and PDI = 1.22) was obtained with only 6% propylene carbonate byproduct. For a polymerization process performed under these reaction conditions for 0.5 h, a TOF (turnover frequency) of 5,160 h was reported. For comparative purposes, the best TOF observed for a binary catalyst system of (salen)CoX (where X is 2,4-dinitrophenolate) onium salt or base for the copolymerization of propylene oxide and CO2 at 25°C was 400-500 h for a process performed at 1.5 MPa pressure [21, 37]. On the other hand, employing catalysts of the type shown in Fig. 12, TOFs as high as 13,000 h with >99% selectivity for copolymers withMn 170,000 were obtained at 75°C and 2.0 MPa pressure [35]. The cobalt catalyst in Fig. 13 has also been shown to be effective for selective copolymer formation from styrene oxide and carbon dioxide [38]. [Pg.14]

A chiral alumi n i u m - s a I e n - PI13 PO combination catalyses addition to ketones in up to 92% ee the catalyst system essentially acts as a Lewis acid-Lewis base bifunctional system.248 A similar chiral manganese(III)-salen-Ph3PO method is comparable.249... [Pg.31]

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. [Pg.487]


See other pages where Bifunctional catalysts salen based is mentioned: [Pg.205]    [Pg.205]    [Pg.66]   
See also in sourсe #XX -- [ Pg.39 , Pg.42 ]




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