Samarium -based catalyst system

When certain cyclodipeptides are used as catalysts for the enantioselective formation of cyanohydrins, an autocatalytic improvement of selectivity is observed in the presence of chiral hydrocyanation products [80]. A commercial process for the manufacture of a pyrethroid insecticide involving asymmetric addition of HCN to an aromatic aldehyde in the presence of a cyclic dipeptide has been described [80]. Besides HCN itself, acetone cyanohydrin is also used (usually in the literature referred to as the Nazarov method), which can be activated cata-lytically by certain lanthanide complexes [81]. Acetylcyanation of aldehydes is described with samarium-based catalysts in the presence of isopropenyl acetate cyclohexanone oxime acetate is hydrocyanated with acetone cyanohydrin as the HCN source in the presence of these catalytic systems [82]. 

Another notable samarium(II)-based catalyst system is Sm(OAr)2(THF)3 (53). While it is inactive as a catalyst for styrene polymerization at 0.1 MPa,... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Overview A large number of catalysts based on vanadium [35-37], titanium [38 0], zirconium [41], hafnium [42], lanthanides (in particular neodymium, samarium, and ytterbium) [43], cobalt [44, 45], niobium [46], chromium [47], nickel [48], and palladium [49] provide well-defined polyolefins. Many of these systems are able to meet the requirements for living polymerizations by suppressing P-hydride and P-methyl eliminations, as well as chain transfer to cocatalysts, such as alkyl aluminums or methylaluminoxane (MAO). Since MAO is usually obtained as a liquid solution with residual trimethylalumi-num, drying MAO to a white powder and removing residual trimethylaluminum can help minimize chain transfer to cocatlysts. 

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