Metals oxo compounds

Many metal oxo-compounds (nitrates, oxides and particularly sulfates) and sulfides are reduced violently or explosively (i.e. undergo thermite reaction) on heating an intimate mixture with aluminium powder to a suitably high temperature to initiate the reaction. Contact of massive aluminium with molten salts may give explosions [1], Application of sodium carbonate to molten (red hot) aluminium caused an explosion [2]. 

Late Transition Metal-Oxo Compounds and Open-Framework Materials that Catalyze Aerobic Oxidations Rui Cao, Jong Woo Han, Travis M. Anderson, Daniel A. Hillesheim, Kenneth I. Hardcastle, Elena Slonkina, Britt Hedman, Keith O. Hodgson, Martin L. Kirk, Djamaladdin G. Musaev, Keiji Morokuma, Yurii V. Geletii and Craig L. Hill... 

Computational studies of alkene oxidation reactions by metal-oxo compounds, 38, 131 Computational studies on the mechanism of orotidine monophosphate decarboxylase,... 

Osmium tetraoxide and permanganate are the textbook example reactants for the direct addition of the hydroxyl function to double bonds as shown in Figure 1. Several reagents such as hydrogen peroxide, periodate, hexacyanoferrate(III) or recently also molecular oxygen [2-6] have been used to reoxidize the different metal-oxo compounds. 

The reaction mechanisms of these transition metal mediated oxidations have been the subject of several computational studies, especially in the case of osmium tetraoxide [7-10], where the controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally [11-20]. Based on the early proposal of Sharpless [12], that metallaoxetanes should be involved in alkene oxidation reactions of metal-oxo compounds like Cr02Cl2, 0s04 and Mn04" the question arose whether the reaction proceeds via a concerted [3+2] route as originally proposed by Criegee [11] or via a stepwise [2+2] process with a metallaoxetane intermediate [12] (Figure 2). 

LATE TRANSITION METAL-OXO COMPOUNDS AND OPEN-FRAMEWORK MATERIALS THAT CATALYZE AEROBIC OXIDATIONS... 

In general, the catalytic properties of transition metals for the oxidation of hydrocarbons are strongly governed by the existence and nature of the metal-oxygen intermediates. Among the various possible reactive intermediates, two families of metal-oxygen species emerge as the most important ones, i.e. metal peroxides and metal-oxo compounds. 

Computational studies of alkene oxidation reactions by metal-oxo compounds,... 

Scheme 1 Proposed transition states for the interaction of metal-oxo compounds with alkenes.

The resolution of the hot debate on the mechanism of metal-oxo mediated oxidations is one of the success stories of DFT calculations. An early publication by Sharpless on chromylchloride oxidations of alkenes10 started a long ongoing discussion11-25 on the mechanism of metal-oxo mediated oxidations. Sharpless proposed an interaction between the chromium metal and the alkene and generalized his proposal to include all metal-oxo compounds, especially osmium tetroxide and permanganate. Especially the mechanism of the reaction of osmium tetroxide with alkenes was the subject of an intense debate within the community of experimental organic chemists (Scheme 2). 

Quite recently it was reported that in addition to hydrogen peroxide, periodate or hexacyanoferrat(III), molecular oxygen21,31-34 can be used to reoxidize these metal-oxo compounds. New chiral centers in the products can be created with high enantioselectivity in the dihydroxylation reactions of prochiral alkenes. The development of the catalytic asymmetric version of the alkene dihydroxylation was recognized by Sharpless receipt of the 2001 Nobel prize in Chemistry. 

---