Inner- and Outer-Sphere Hydrogenation Mechanisms: A Computational Perspective

Homogeneous hydrogenation of organic compounds is ubiquitous in many chemical synthetic procedures; being one the most extensively studied reactions in homogeneous catalysis. Nevertheless, one important point difficult to be assessed from only experimental measurements is the placement of the substrate in the coordination sphere of the transition metal catalyst along the catalytic cycle. From the substrate's perspective the general reaction mechanisms for the hydrogenation process can be classified as inner-sphere (involving substrate coordination) and outer-sphere mechanisms (without substrate coordination). In this chapter first an overall scope from this viewpoint of the homogeneous hydrogenation mechanisms, including hydrogen-transfer reactions, is given. Later, we show that Density Functional Theory(DFT) calculations coupled with suitable solvation methods can provide a very efficient framework in order to unravel the operating reaction mechanisms of catalytic processes, allowing from a detailed microscopic description of the process to distinguish between inner- and outer-sphere mechanisms. In the second part several examples of how first principles calculations can provide very useful mechanistic insights into hydrogenation reactions are then discussed, giving hints on the different effects that may influence the reactivity and showing which reaction steps should be improved in order to increase the overall performance of transition-metal catalysts. © 2010 Elsevier Inc.