1.1.1 Introduction
Osmium is the densest (density 22.59 gcm^-3) transition metal naturally available. It has seven naturally occurring isotopes, six of which are stable: ¹⁸⁴Os, ¹⁸⁷Os, ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and ¹⁹²Os. It forms compounds with oxidation states ranging from -2 to +8, among them, the most common oxidation states are +2, +3, +4 and +8. Some important osmium catalyzed organic oxidation reactions follow:

1.1.2 Dihydroxylation of Alkenes

Cis-1,2-dihydroxylation of alkenes is a versatile process, because cis-1,2-diols are present in many important natural products and biologically active molecules. There are several methods available for cis-1,2-dihydroxylation of alkenes, among them, the OsO₄-catalzyed reactions are more valuable (Scheme 1).

OsO4 vapours are poisonous and result in damage to the respiratory tract and temporary damage to the eyes. Use OsO4powder only in a well-ventilated hood with extreme caution. Y. Gao, Encylcopedia of Reagents for Organic Synthesis, John Wiley and Sons, Inc., L. A. Paquette, New York, 1995, 6, 380.

Scheme 1

The use of tertiary amine such as triethyl amine or pyridine enhances the rate of reaction (Scheme 2).

Scheme 2

Catalytic amount of OsO₄ can be used along with an oxidizing agent, which oxidizes the reduced osmium(VI) into osmium(VIII) to regenerate the catalyst.  A variety of oxidizing agents, such as hydrogen peroxide, metal chlorates, tert-butyl hydroperoxide, N-methylmorpholine-N-oxide, molecular oxygen, sodium periodate and sodium hypochlorite, have been found to be effective (Scheme 3-7).

Scheme 3