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

Abstract

Reaction of PhCONH 2 with [WCl 6] under reflux in benzene gives a near quantitative yield of [WOCl 4(NCPh)] ( 1) which can be prepared directly by the reaction of PhCN and [WOCl 4]. PhCONH 2 reacts with [WOCl 4] under reflux in benzene to give [WO 2Cl 2(NCPh)] ( 2) and with [NbCl 5] under similar conditions to give [NbOCl 3(NCPh)] ( 3). PhCONH 2 and TiCl 4 give [TiCl 4(NH 2COPh) 2] ( 4). Reaction of the aroyl isocyanate PhCONCO with [WOCl 4] gave [WOCl 4(NCPh)] ( 1) and 4-Me 3CPhCONCO and [WOCl 4] gave [WOCl 4(NCPhCMe 3-4)] ( 5). PhCONCO reacts catalytically with [WOCl 4] or [WOCl 4(NCPh)] to give quantitative yields of PhCN. Similarly, 4-Me 3CPhCONCO and 2-ClCPhCONCO react catalytically with [WOCl 4] to give complete conversion to 4-Me 3CPhCN 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 CO 2 leads to an amidate complex that undergoes C–O bond scission and O migration to the W atom to give PhCN bound to the WOCl 4 moiety in a cis arrangement. This complex is unstable with respect to dissociation which completes the catalytic cycle. The overall reaction WOCl 4 + PhC(O)NCO → WOCl 4·PhCN + CO 2 is exothermic (Δ H = −20.7 kJ mol −1) and is favoured from free energy considerations (Δ G = −69.3 kJ mol −1).