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PUBLISHED ONLINE: 22 JULY 2011 | DOI: 10.1038/NCHEM.1089
Efcient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO
Ekambaram Balaraman1, Chidambaram Gunanathan1, Jing Zhang1, Linda J. W. Shimon2
and David Milstein1*
Catalytic hydrogenation of organic carbonates, carbamates and formates is of signicant interest both conceptually and practically, because these compounds can be produced from CO2 and CO, and their mild hydrogenation can provide alternative, mild approaches to the indirect hydrogenation of CO2 and CO to methanol, an important fuel and synthetic building block. Here, we report for the rst time catalytic hydrogenation of organic carbonates to alcohols, and carbamates to alcohols and amines. Unprecedented homogeneously catalysed hydrogenation of organic formates to methanol has also been accomplished. The reactions are efciently catalysed by dearomatized PNN Ru(II) pincer complexes derived from pyridine- and bipyridine-based tridentate ligands. These atom-economical reactions proceed under neutral, homogeneous conditions, at mild temperatures and under mild hydrogen pressures, and can operate in the absence of solvent with no generation of waste, representing the ultimate green reactions. A possible mechanism involves metalligand cooperation by aromatizationdearomatization of the heteroaromatic pincer core.
The hydrogenation of polar bonds, in particular organic carbonyl groups, has captured much attention during the past four decades13, mainly due to its synthetic signicance as an envir
onmentally benign approach to fundamental synthetic building blocks such as alcohols and amines. Much progress has been made in the hydrogenation of ketones and aldehydes and, more recently, rare examples of the signicantly more difcult hydrogenation of esters4,5 and amides6,7 have also been reported. However, the hydrogenation of organic carbonates and carbamates remains a major challenge. Indeed, as far as we know, catalytic hydrogenation of these important families of compounds has never been reported, be it under heterogeneous or homogeneous catalysis. In fact, organic carbonates have been used as solvents in catalytic hydrogenation reactions8. Moreover, the popular amine protecting groups, benzyl carbamates, undergo deprotection by heterogeneous hydrogenation, which involves cleavage of the benzylO bond, but with the carbonyl group not being reduced9. In addition, hydrogenation of cyclic N-acylcarbamates leads to cleavage of the CN bond without affecting the carbamate group10.
Hydrogenation of carbonates and carbamates is of considerable interest conceptually and practically, because these compounds can be...