Stereoselective catalysts

The chemistry of Rh(DIPAMP) and mechanism has been reviewed.914 1617 2 28 Marginally higher catalyst efficiencies are observed with higher alcohols compared to methanol, whereas the presence of water can result in the reduction of slurries. Filtration of the product can improve the % ee while the catalyst and D,L-product remain in the mother liquor. The catalyst stereoselectivities decrease as hydrogen pressure increases. [Rh(COD)(R,W-DIPAMP) +BF4 (13) affords the. S -con-figuration of amino acids on reduction of the enamide substrate. Reduction of enamides in the presence of base eliminates the pressure variances on the stereoselectivities, but the rate of reaction under these conditions is slow.17... 

The addition to 1,3-dienes afforded a new class of allylboron compounds. The platinum(0)-phosphine catalysts stereoselectively yielded or-1,4-addition products 131233 234 and 133216 235 for 2,3-disubstituted butadiene, 1,3-cyclohex-adiene, and 1,3-pentadiene by Txr-coordination of a diene to a platinum catalyst (Equations (30) and (31)). In contrast, phosphine-free Pt(dba)2 resulted in the selective formation of a 1,2-addition product 134216 for 1,3-pentadiene (Equation (31)). The corresponding chiral allyl boronates were synthesized by diboration of dienes with 123 or 124.234 235... 

Finally, the regio- and stereoselectivities of the intramolecular hydrosilylation of allyl alcohols, homoallyl alcohols, and allylamlnes are summarized in Scheme 1. While these results have been obtained with the dimethylsilyl (HMe2Si) group in the presence of a platinum catalyst, stereoselectivity, but not regioselectivity, can be controlled by other silyl groups such as the diphenyl (HPh2Si) or di(isopropyl)silyl [H(i-Pr)2Si] groups. 

Since every propene insertion, whatever its orientation, creates a new stereogenic center, the catalyst stereoselectivity and the stereoregularity or tacticity of the polymer) is determined by the stereochemical relationship(s) between the stereogenic carbon atoms in the polymer chain. 

Other chapters of this book discuss the manner in which well-defined stereorigid metal complexes with chirotopic sites can be converted into highly efficient catalysts for the stereoselective polymerization of propylene. For anifl-metallocenes, in particular, the relationship between precursor symmetry and catalyst stereoselectivity has been deeply studied and is now well-understood. ... 

The additions of monohydrosilanes to terminal alkynes catalysed by chloro-platinic acid involves stereoselective (100%) cw-addition. However, in the presence of [RhCl(PPh3)3l as catalyst stereoselective /ra j-addition is observed, results which resemble peroxide-catalysed reaction of trichlorosilanes with alkynes. The rhodium-catalysed reactions may involve radical species. ... 

Rhodium-catalyzed Additions. The title compound has been a test reagent in several rhodium-catalyzed additions to the triple bond. For example, a rhodium-catalyzed silylcarbamoylation in the presence of a secondary amine afforded the silylated product in reasonable yield (eq 40). In the presence of a CO atmosphere and with 5% rhodium on alumina as catalyst, stereoselective hydrosilylation has been reported (eq 41). ... 

The first commercial processes for making PP used alkanes as diluents in the reactor. Catalyst stereoselectivity was such as to make PP with a 90% isotactic index. Catalyst efficiency was low... 

Screening of a library of seven EREDs enabled the enantioselective synthesis of N-acyl protected amino acids 55 with excellent optical purities for both antipodes (Scheme 9.18a). Although no single enzyme could be established as perfect catalyst, stereoselectivity could be effectively tuned by size variation of the protective group (>99% ee for (S) using ethyl carbamate 54, 92% ee for (R) employing benzyl carbamate 54). The authors attributed the tendency of... 

Lindlar catalyst stereoselectively gives the Z-isomer contaminated with up to 5% of the -isomer. Reduction of acetylenes to yield >98% Z-olefins has been accomplished by hydroboration with hindered reagents such as disiamylborane (188) or 9-BBN (189) followed by hydrolysis with acetic acid. 

Table 18 Influence of Ru-catalysts stereoselectivity in cyclopropanation of styrene with ethyl diazoacetate...

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