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DOI: 10.1055/sos-SD-036-00589

Nativi, C.; Roelens, S.Science of Synthesis, (200836758.

To minimize problems associated with the toxicity and the high cost of osmium(VIII) oxide, which makes the stoichiometric osmylation dangerous and uneconomical, catalytic variants that employ inexpensive reagents for the reoxidation of the osmium(VI) product have been developed.[‌10‌] One of the most effective methods was developed by VanRheenen, Kelly, and Cha at Upjohn, who used tertiary amine N-oxides, such as 4-methylmorpholine N-oxide, as both an oxidant for the osmium(VI) product, and the decomposing agent of the osmium ester.[‌11‌] Osmylation reactions using 4-methylmorpholine N-oxide can be performed at room temperature with as little as 1mol% of osmium(VIII) oxide. This method is superior to other syn-hydroxylation procedures as only small amounts of osmium(VIII) oxide are required and, in addition, overoxidation byproducts are minimized.[‌10‌] The solvents generally employed are aqueous acetone, tetrahydrofuran, or tert-butyl alcohol (as either one- or two-phase mixtures). Although these solvents can be used in different combinations the optimized system uses acetone/tert-butyl alcohol/water in a ratio of 18:1:1. With hindered alkenes, osmylation with 4-methylmorpholine N-oxide is often sluggish; in such cases the use of trimethylamine N-oxide gives better results. The catalytic osmylation of alkenes has been extended to potassium aryl- and alkyltrifluoroborates such as 2 (Scheme 2). The resulting diols 3 are effective coupling partners in SuzukiMiyaura coupling reactions.[‌12‌]

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