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 [WCl6] 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)] (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 WO bond and the CN 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·PhCN + CO2 is exothermic (ΔH = −20.7 kJ mol−1) and is favoured from free energy considerations (ΔG = −69.3 kJ mol−1).

Benzamide and benzoyl isocyanate act as oxygen transfer agents to metal halide complexes and form benzonitrile stoichiometrically in the process. Aroyl isocyanates react stoichiometrically with [WOCl4] to give nitrile complexes [WOCl4(NCAr)], and catalytically to give complete conversion to nitriles. DFT studies show structures and intermediates for a possible catalytic mechanism.Figure optionsDownload as PowerPoint slideResearch highlights
► Benzonitrile coordinated to metal complexes can be formed in a stoichiometric manner by the reaction of PhCONH2 with [WCl6], [WOCl4] or [NbCl5].
► Similarly, aroyl isocyanates produce coordinated aryl nitriles with [WOCl4].
► This latter reaction can be made catalytic to give quantitative yields of the appropriate aryl nitrile.
► DFT studies show structures and intermediates in support of a possible catalytic mechanism.