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 OsO4 powder 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.
The use of tertiary amine such as triethyl amine or pyridine enhances the rate of reaction (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).
In the latter case, the resultant diols undergo oxidative cleavage to give aldehydes or ketones. This reaction is known as Lemieux-Johnson Oxidation. NaIO4 oxidizes the reduced osmium(VI) to osmium(VIII) along with the oxidative cleavage of the diols.
The reaction involves the formation of cyclic osmate ester, which undergoes oxidative cleavage with NaIO4 to give the dicarbonyl compounds (Scheme 9).
1.1.3 Sharpless Asymmetric Dihydroxylation
Although osmylation of alkenes is an attractive process for the conversion of alkenes to 1,2-diols, the reaction produces racemic products. Sharpless group attempted to solve this problem by adding chiral substrate to the osmylation reagents, with the goal of producing a chiral osmate intermediate (Scheme 10). The most effective chiral additives were found to be the cinchona alkaloids, especially esters of dihydorquinidines such as DHQ and DHQD. The % ee of the diol product is good to excellent with a wide range of alkenes.
If the alkene is oriented as shown in Scheme 11, the natural dihydroquinidine (DHQD) ester forces delivery of the hydroxyls from the top face (-attack). Conversely, dihydorquinine (DHQ) esters deliver hydroxyls from the bottom face (-attack).
The reactions are generally carried out in a mixture of tert-butyl alcohol and water at ambient temperature (Scheme 12).
The reaction is stereospecific leading to 1,2-cis-addition of two OH groups to the alkenes
It typically proceeds with high chemoselectivity and enantioselectivity
The reaction conditions are simple and the reaction can be easily scaled up
The product is always a diol derived from cis-addition.
It generally exhibits a high catalytic turnover number
Similar to cis-1,2-dihydroxylation, cis-1,2-aminohydroxylation of alkenes has been developed by reaction with chloroamine in the presence of catalytic amount of OsO4. In this process, alkene reacts with chloroamine in the presence of OsO4 to give sulfonamides that is readily converted into the cis-1,2-hydroxyamines by cleavage with sodium in liquid ammonia (Scheme 13). This process provides a direct cis-aminohydroxylation of alkenes, but the major problem is the poor regioselectivity for unsymmetrical alkenes.
The catalytically active species in the reaction most likely is an imidotrioxo osmium(VIII) complexes, which is formed in situ from the osmium reagent and the stoichiometric nitrogen source, i.e. chloroamine (Scheme 14). Experiments under stoichiometric conditions have been shown that imidotrioxo osmium(VIII) complexes transfer the nitrogen atom and one of the oxygen atoms into the substrate. The major regioisomer normally has the nitrogen placed distal to the most electron withdrawing group of the substrate.
1.1.5 Asymmetric Aminohydroxylation
The asymmetric cis-1,2-aminohydroxylation of alkenes with chloroamine has been explored using the chiral osmium catalyst derived from OsO4 and cinchona alkaloids, dihydroquinidine ligands (DHQD)2-PHAL and dihydroquinine ligands (DHQ)2-PHAL.
The face selectivity for the aminohydroxylation can too be reliably predicted (Scheme 15).
An alkene with these constraints receives the OH and NHX groups from above, i.e. from the -face, in the case of DHQD derived ligand and from the bottom, i.e. from the -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).
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):
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 cvia (2+2)-cycloaddition that may rearrange to give d that could undergo hydrolysis with water to give the target hydroxylamine derivative.
1.1.5 Reaction with Alkynes
Alkynes react with OsO4 in the presence of tertiary amines such as pyridine to give osmium(VI) ester complexes, which on hydrolysis with sodium sulfite yield the corresponding carbonyl compounds (Scheme 19-20). In the case of terminal alkynes, carboxylic acids are obtained (Scheme 21)
E. J. Corey, A. Guzman-Perez, M. C. Noe, J. Am. Chem. Soc.1995, 117, 10805.
Guzman-Perez, E. J. Corey, Tetrahedron Lett.1997, 38. 5941.
Give the major products for the following reactions:
Clayden, N. Creeves, S. Warren, P. Wothers, Organic Chemistry, Oxford University Press, New York, 2001.