The Upjohn Dihydroxylation allows the syn-selective preparation of 1,2-diols from alkenes by the use of osmium tetroxide as a catalyst and a stoichiometric amount of an oxidant such as NMO (N-methyl morpholine-N-Oxide).
Mechanism of the Upjohn Dihydroxylation
The toxic and volatile OsO4 can also be prepared in situ by the oxidation of K2OsO2(OH)4 with NMO. NMO is also the cooxidant that enables the use of a catalytic amount of OsO4, because this reagent is able to reoxidize an Os(VI) species to an Os(VIII) species:
The mechanism is simplified, for example in alkaline solutions, the catalyst is indeed hydrated.
The key step is the cycloaddition of OsO4 to the olefin. There has been some speculation regarding the actual addition step, for which experimental data suggest the possible involvement of two separate steps. Thus, the question arises during these discussions of whether the key step takes place via an initial (3+2)-addition (1,3-dipolar cycloaddition), or by a (2+2)-addition followed by expansion of the metallacycle.
Quantum chemical calculations have shown an initial (3+2)-addition of OsO4 to be energetically more favorable. However, this energy difference is substantially smaller in the related Re(VII) oxide additions, for example. (D. V. Deubel, G. Frenking, Acc. Chem. Res., 2003, 36, 645. DOI).
Tertiary amines such as DMAP and pyridine accelerate the addition reaction.
The use of chiral amines enables enantioselective conversions, such as (DHQ)2-PHAL and (DHQD)2-PHAL in the Sharpless Dihydroxylation.
An Osmium(III)/Osmium(V) Redox Couple Generating OsV(O)(OH) Center for cis-1,2-Dihydroxylation of Alkenes with H2O2: Os Complex with a Nitrogen-Based Tetradentate Ligand
H. Sugimoto, K. Kitayama, S. Mori, S. Itho, J. Am. Chem. Soc., 2012, 134, 19270-19280.
Catalytic Asymmetric Dihydroxylation of Olefins with Reusable OsO42- on Ion-Exchangers: The Scope and Reactivity Using Various Cooxidants
B. M. Choudary, N. S. Chodari, K. Jyothi, M. L. Kantam, J. Am. Chem. Soc., 2002, 124, 5341-5349.