Stoichiometric aryl nitrile formation from amides and aroyl isocyanates using high-valent early transition metal complexes and a catalytic process from the aroyl isocyanates

Reaction of PhCONH2 with [WCI6] under reflux in benzene gives a near quantitative yield of [WOCl4(NCPh)] (1) which can be prepared directly by the reaction of PhCN and [WOCl4]. PhCONH2 reacts with [WOCL4] under reflux in benzene to give [WO2Cl2(NCPh)] (2) and with [NbCl5] under similar conditions to give [NbOCl3(NCPh)](3). PhCONH2 and TiCl4 give [TiCl4(NH2COPh)(2)] (4). Reaction of the aroyl isocyanate PhCONCO with [WOCl4] gave [WOCl4(NCPh)l (1) and 4-Me3CPhCONCO and [WOCl4] gave [WOCl4(NCPhCMe3-4)] (5). PhCONCO reacts catalytically with [WOCl4] or [WOCL4(NCPh)] to give quantitative yields of PhCN. Similarly, 4-Me3CPhCONCO and 2-ClCPhCONCO react catalytically with [WOCl4] to give complete conversion to 4-Me3CPhCN and 2-ClCPhCN. DFT studies show structures and intermediates in support of a possible catalytic mechanism. This involves initial complexation followed by formation of a metallocycle from the W=O bond and the C=N bond of the isocyanate. Rearrangement and elimination of CO2 leads to an amidate complex that undergoes C-O bond scission and O migration to the W atom to give PhCN bound to the WOCl4 moiety in a cis arrangement. This complex is unstable with respect to dissociation which completes the catalytic cycle. The overall reaction WOCl4 + PhC(O)NCO -> WOCl4 center dot PhCN + CO2 is exothermic (Delta H = -20.7 kJ mol(-1)) and is favoured from free energy considerations (Delta G = -69.3 kJ mol(-1)). (C) 2010 Elsevier Ltd. All rights reserved.