Preparation of carbamates via cobalt-catalyzed oxidative carbonylation of aromatic or aliphatic primary amines and aliphatic secondary amines

The N,N-bis(salicylidene)ethylenediaminocobalt(II)-catalysed oxidative carbonylation of para-substituted aromatic primary amines at 100°C in methanol gives carbamates in high yields. In presence of excess dimethylamine also N-aryl-N',N'-dimethylureas are formed. In dichloromethane isocyanates and ureas are formed. The conversion is correlated with Hammett's sigma giving slightly negative Hammett's rho values. This suggests the electrophilic attack of carbon monoxide at a cobalt amido complex in the transition state. Also the aliphatic secondary amines piperidine, pyrrolidine, morpholine and the aliphatic primary amine di-i-propylamine undergo in methanol in the same conditions oxidative carbonylation to a carbamate. The urea is formed in dichloromethane. Turnover frequency for p-toluidine in methanol is ca. 40