Both cationic and neutral rhodium catalytic systems for the hydroiminoacylation of alkenes were studied using NMR spectroscopy and DFT-based methods. With neutral systems, the oxidative addition step was shown to be thermodynamically favored. With the cationic system, the oxidative addition reaction was shown to be endothermic by DFT calculations, and the corresponding intermediate was not detected by NMR. The energy barriers in the cationic and in the neutral pathway are relatively similar. This indicates that the role of chloride when it is coordinated to the rhodium complex is to increase the stability of the oxidative addition product, enabling the reaction to continue. This is not the case in the cationic complexes since the low stability of the reaction products promotes the back reaction, and therefore, low conversions are obtained. The alkene insertion step was also investigated for both systems with styrene as the substrate. The novel neutral complex 21 was detected and fully characterized by multinuclear NMR spectroscopy. In the cationic system, the rhodium hydride styrene intermediate 25 was detected and characterized by NMR spectroscopy. However, no evidence for alkene insertion was found. This study shows that each step of the catalytic cycle is favored in the case of the neutral system when compared with the cationic system.
Rhodium-catalyzed intermolecular hydroiminoacylation of alkenes: Comparison of neutral and cationic catalytic systems
Organometallics 2009, 28, 2976-2985.