[3 + 2] Versus [2 + 2] addition of metal oxides across CC bonds: A theoretical study of the mechanisms of oxidation of ethylene by osmium oxide complexes

The oxidation of ethylene by osmium tetroxide (OsO4), osmyl hydroxide (OsO2(OH)2) and osmyl chloride (OsO2Cl2) have been studied by hybrid density functional theory at the B3LYP/LACVP∗ level of theory. It was found that in the reaction of OsO4 with ethylene, the [3 + 2] addition pathway leading to a five-membered metallacycle intermediate is more favorable than the [2 + 2] addition to form a four-membered metallaoxetane and that the reaction leads ultimately to the formation of diols without epoxide formation, in agreement with earlier works on the subject. The reaction of ethylene with osmyl hydroxide (OsO2(OH)2), which is released when the ‘cyclic’ esters formed from [3 + 2] reaction of OsO4 with ethylene undergo hydrolysis, was found to be less favorable, kinetically and thermodynamically, than the [3 + 2] reaction of OsO4 with ethylene. In the reaction of OsO2Cl2 with ethylene it was found that the [2 + 2] addition pathway to form a four-membered metallacycle is more favorable than the [3 + 2] addition to form the five-membered metallacycle intermediate. It was also found that the formation of an epoxide precursor from the reaction of OsO2Cl2 with ethylene, though possible, is a very unfeasible reaction kinetically and therefore epoxide is not a likely product in the reaction. The results of this study indicates that the preference of the [3 + 2] pathway in the addition of metal oxides across CC bonds, which has been established for osmium tetroxide, is not pervasive in all osmium oxo complexes as publications on the subject seem to suggest.