Asymmetric catalytic cycle

Scheme 8. Catalytic cycle for the BINAP-Rh(i)-catalyzed asymmetric isomerization of allylic amines.

The first attempt at a catalytic asymmetric sulfur ylide epoxidation was by Fur-ukawa s group [5]. The catalytic cycle was formed by initial alkylation of a sulfide (14), followed by deprotonation of the sulfonium salt 15 to form an ylide 16 and... 

Scheme 5-14 Stoichiometric reactions of Pt(Me-Duphos) complexes relevant to the proposed catalytic cycle for asymmetric hydrophosphination...

Figure 9 The proposed catalytic cycle of asymmetric aminohydroxylation.

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

As for the mechanism of asymmetric aminohydroxylation, it has been proposed that there are at least two catalytic cycles in the reaction system (Scheme 4-38).77b It is also suggested that both electronic and steric factors play important roles in the reaction. In the first cycle, in which the turnover occurs, effects of the ligand on selectivity are possible. For the ligand-independent... 

In asymmetric hydrogenation, the pressure of hydrogen may have a substantial impact on both the rates and the stereoselectivities of the reaction. These effects may be attributed either to the formation of different catalytically competing species in solution or to the operation of kinetically distinct catalytic cycles at different pressures. 

The solvent employed in asymmetric catalytic reactions may also have a dramatic influence on the reaction rate as well as the enantioselectivity, possibly because the solvent molecule is also involved in the catalytic cycle. Furthermore, the reaction temperature also has a profound influence on stereoselectivity. The goal of asymmetric hydrogenation or transfer hydrogenation studies is to find an optimal condition with a combination of chiral ligand, counterion, metal, solvent, hydrogen pressure, and reaction temperature under which the reactivity and the stereoselectivity of the reaction will be jointly maximized. 

Arai et al.51 reported that by using a catalytic amount of chiral quaternary ammonium salt as a phase transfer catalyst, a catalytic cycle was established in asymmetric HWE reactions in the presence of an inorganic base. Although catalytic turnover and enantiomeric excess for this reaction are not high, this is one of the first cases of an asymmetric HWE reaction proceeding in a catalytic manner (Scheme 8-20). 

Figure 3.16. Catalytic cycle of Rh/(5)-binap-catalyzed asymmetric 1,4-addition of orga-noboronic acids to a,P-enones.

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