Carbamates, from nitroarene carbonylation

In 1984, one of us first reported on the use of a Ru3(CO)i2-based catalytic system for the carbonylation of nitroarenes to carbamates [170]. This report was followed by several others, mostly from our and Grate s groups [171-176]. Since the catalytic systems are very similar, they are discussed together. Note that, although Ru3(CO)i2 is always used as a catalyst precursor, it is known that, under the reaction conditions, it is in equilibrium with the mononuclear Ru(CO)5 and it has become now clear that it is this last compound which is mainly responsible for catalytic activity [177]. However, Ru(CO)s is an unstable volatile liquid, which tends to aggregate to Ru3(CO)i2 within a few hours at room temperature and is thus an unsuitable starting catalyst for most applications. Anyway, in the only experiments we performed by using Ru(CO)s as catalyst, we obtained results essentially identical to those obtained under the same conditions... 

For M = Ru, formamides and amines were the principal products. Substitution of Fe3(CO)i2 for Ru3(CO)i2 results in the formation of carbamate esters (ArNHCOOMe) as the major products, with ureas as the main by-products. Without the metal carbonyl, nitroarenes are recovered imchanged from the reaction mixture. An imido-alcoxycarbonyl complex was suggested as an intermediate in the reaction and the difference between iron and ruthenium was proposed to be due to the different facility of this intermediate to undergo protonation and reductive elimination to give carbamate, or insertion of CO into the M-N bond, followed by hydrogenation, affording formamide (Scheme 13) ... 

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