The suppression of the secondary cycle relies oneffective hydrolysis.1.2.3.

The proposed mechanism for the asymmetric aminohydroxylation is closely basedon mechanistic studies of its forerunner, the AD reaction. The mechanism is 3+2cycloaddition of ligand-bound complex 6 to the alkene, analogous to the Criegeemechanism for osmium-mediated dihydroxylation. In this, ligand co-ordination withimidotrioxo osmium (VIII) followed by 3+2 cycloaddition with olefin gives 7. Based onthese results, a mechanistic scheme has been proposed in which two catalytic cycles, givedifferent results for selectivity of the transformation (Figure 4). The primary cycle ismediated by the alkaloid derived ligand and in all but one of the AA methods reported todate,21 the ligand is observed to improve catalytic turnover relative to the non-ligandmediatedreaction. Ligand mediated addition of imidotrioxoosmium(VIII) species 6 to thealkene gives azaglycolate species 7. Reoxidation of 7 by the nitrogen source gives 8, whichcan undergo hydrolysis to regenerate the initial osmium species and liberate product. Theoxidized azaglycolate species 8 may also enter the secondary cycle and add to a secondalkene to give the bis(azaglycolate)osmium species 9. The addition step of this cycle isindependent of the Cinchona alkaloid derived ligand and as a result, gives additionproducts with low enantio and regio-selectivity. Hydrolysis of 9 leads back to 7, which canthen re-enter either the primary or secondary cycle. The turnover-limiting step in both catalytic cycles is the hydrolysis of azaglycolatecomplexes 8 or 9.22 Control of the oxidation pathway is achieved by conducting thereaction in aqueous solvent mixtures, thereby favouring hydrolysis of 822a and dominanceof the primary cycle. In comparison, all of the AA processes reported to date have beencarried out under homogeneous conditions and suppression of the secondary cycle relies oneffective hydrolysis.1.2.3. Nitrogen sourcesThere are three main classes of nitrogen source that have been used to date in the AAreaction. The iV-halogenated species derived from (i) sulfonamides (ii) carbamates and (iii)amides. All are converted into the respective alkali metal salt prior to addition to the alkene(Figure 5). (i) Sulfonamide variant 10: The sulfonamide method was first to be developed, stemmingdirectly from the use of chloramine-T TsN(Na)Cl in the catalytic but non-asymmetricforerunner to the AA.23 Chloramine-T remains the most frequently used reagent, due to itslow cost and commercial availability. Subsequent studies have revealed that the size of thesulfonamide group has a tremendous influence on the outcome of the reaction, the smallerthe residue the better the results.60 Thus the methane sulfonamide based chloramine-Mreagent generally gives superior results in terms of enantio and regioselectivity, catalyticturnover, and yield, compared to chloramine-T. Additionally, the chloramine-M systemshows ligand acceleration, while the toluene sulfonamide based system is liganddeaccelerated. The robust nature of the sulfonamide product requires harsh deprotectioncondition such as reductive cleavage of sulfonamides under Birch conditions24 or with RedAl.25 In addition, 33% HBr/CH3COOH has been used to cleave toluene sulfonamides.6bSulfonamide method is limited in its substrate scope, encompassing a, p-unsaturated esters,