Mechanism of the Aminolysis of Fischer Alkoxy and Thiocarbene Complexes: A DFT Study

DIEGO M. ANDRADA; J. OSCAR C. JIMENEZ-HALLA; MIQUEL SOLÁ

JOURNAL OF ORGANIC CHEMISTRY

AMER CHEMICAL SOC

Año: 2010 vol. 75 p. 5821 - 5836

0022-3263

B3LYP calculations have been carried out to study the reaction mechanism of the aminolysis of Fischercarbene complexes of the type (CO)5CrdC(XMe)R (X=O and S; R=Me and Ph).We have exploreddifferent possible reaction mechanisms either through neutral or zwitterionic intermediates as well as ageneral base catalysis assisted by an ammonia molecule. Our results show that the most favorablepathway for the aminolysis of Fischer carbene complexes is througha stepwise reaction via a zwitterionicintermediate generated by the initial nucleophilic attack. We have found that the ammonia-catalyzedmechanism entails a significantly lower barrier for the rate-determining step than the uncatalyzed one.Atlower pressure gas-phase conditions, the rate-determining step corresponds to the concerted protontransfer and MeXH elimination. Thiocarbene complexes show a higher energy barrier for this ratedeterminingstep due to the lower basicity of theMeS- substituent. At higher pressure or in solution, therate-determining step corresponds to the initial nucleophilic attack.Our results indicate that the transitionstate of the nucleophilic attack is more advanced and has a higher barrier for alkoxycarbene than thiocarbenecomplexes due to the stronger π-donor character of the alkoxy group that reduces the electrophilicityof the attacked carbene atom making the nucleophilic attack more difficult.