An alkene with these constraints receives the OH and NHX groups from above, i.e. from the β-face, in the case of DHQD derived ligand and from the bottom, i.e. from the α-face, in the case of DHQ derivative. For example, the asymmetric aminohydroxylation of methyl cinnamate gives the following face selectivity based on the chiral ligand (Scheme 16).
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Scheme 16 |
With respect to the yield, regio- and enantioselectivity, reaction depend on number of parameters, e.g. the nature of starting material, the ligand, the solvent, the type of nitrogen source (sulfonamides), carbamates and carboxamides as well as the size of its substituent. For some examples (Scheme 17):
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Scheme 17 |
Mechanism
OsO4 may undergo reaction with chloroamine to give an active imido-osmium intermediate a that could readily co-ordinate with chiral ligand ‘L’ to afford chiral imido-osmium intermediate b (Scheme 18).The latter may react with alkene to yield c via (2+2)-cycloaddition that may rearrange to give d that could undergo hydrolysis with water to give the target hydroxylamine derivative.
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Scheme 18 |