Substituent effects in the cobalt-catalyzed oxidative carbonylation of aromatic amines

The N,N-bis(salicylidene)ethylenediaminocobalt(II)-catalyzed oxidative carbonylation of ortho-, meta- and para-substituted aromatic primary amines in methanol gives ureas, isocyanates, carbamates, azoderivatives. The conversion is correlated for six substrates with Hammett sigma giving a rho = -0.5. This suggests the electrophilic attack of carbon monoxide at a nitrogen anion complexed to cobalt in the transition state. Force field calculations show that octahedral trans-di-aminocobalt(III)salen intermediates have stability very similar to that of the corresponding cis derivatives. These latter are suggested to be intermediates in the formation of ureas. Steric effects in an unknown transition state may explain the fact that high yields in ureas are obtained only using para-substituted anilines as starting materials