Asymmetric aminohydroxylation mechanism

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... 

Scheme 4-38. Proposed mechanism for asymmetric aminohydroxylation. Sequence of steps in the first catalysis cycle (left) (1) addition (a1), (2) reoxidation (O), (3) hydrolysis (h1) in the second catalysis cycle (right) (1) addition (a2), (2) hydrolysis (h2), (3) reoxidation (O). The first cycle proceeds with high ee, the second with low ee. L = chiral ligand X = CH3SO2. ... 

Osmium-catalysed dihydroxylation has been reviewed with emphasis on the use of new reoxidants and recycling of the catalysts.44 Various aspects of asymmetric dihydroxylation of alkenes by osmium complexes, including the mechanism, acceleration by chiral ligands 45 and development of novel asymmetric dihydroxylation processes,46 has been reviewed. Two reviews on the recent developments in osmium-catalysed asymmetric aminohydroxylation of alkenes have appeared. Factors responsible for chemo-, enantio- and regio-selectivities have been discussed.47,48 Osmium tetraoxide oxidizes unactivated alkanes in aqueous base. Isobutane is oxidized to r-butyl alcohol, cyclohexane to a mixture of adipate and succinate, toluene to benzoate, and both ethane and propane to acetate in low yields. The data are consistent with a concerted 3 + 2 mechanism, analogous to that proposed for alkane oxidation by Ru04, and for alkene oxidations by 0s04.49... 

Chiral amines and K20sC>2(0H)4 are also used to catalyze the Sharpless asymmetric aminohydroxylation. The stoichiometric oxidant in aminohydroxylation is a deprotonated /V-lialoamide, whose mechanistic behavior is very similar to NMO. The reaction proceeds by a mechanism essentially identical to that of Sharpless dihydroxylation. 

HP as the oxygen source, have been reviewed, with major focus on the their synthetic potential. Recent experimental investigations of the nature of catalytically active species and mechanisms of their action are summarized.Asymmetric oxidation reactions, viz., sulfoxidation, epoxidation, dihydroxylation, and aminohydroxylation in water have been reviewed. The focus is on the development of catalytic oxidation in water, particularly the use of HP in the presence of metal catalysts. The enantioselective oxidation of sulfides to sulfoxides is also included. ... 

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