FeCl₃-Promoted Cyclization of o-Alkylaryl Isocyanates to Oxindoles

Significance

Reported is a new route to 3-(chloromethylene)oxindoles 2 relying on the FeCl₃-promoted cationic cyclization of isocyanates 1, which are subsequently stereoselectively converted into the corresponding amino oxindoles 4. The formation of the exocyclic double bond in compound 2 is envisaged to occur by addition of chloride to a developing vinylic carbocation resulting from the π-nucleophilic attack of the alkyne onto an imidoyl cation. Products 2 were isolated in good to excellent yields as a mixture of E/Z-isomers, but with high regioselectivity (5-exo-dig cyclization). The low stereoselectivity was in fact of little importance for the subsequent displacement of chloro substituents with amines 3 because in all cases only the single Z-isomer 4 (thermodynamic control) was formed in good to excellent yields and in large scope (cyclic and acyclic amine). Interestingly, the n-alkyl isocyanate 5 undergoes FeCl₃-promoted cationic cyclization to give the dichloro product 6.