Reaction mechanism monodentate phosphine ligands

The mechanism of hydrogenation of enamides by rhodium catalysts with monoden-tate phosphorous ligands was investigated computationally by means of the functional M05-2X taking into account the role of trans intermediates. Since the study suggested that cis intermediates played the major role in the mechanism of the reaction, the results with monodentate phosphorous ligands needed interpretation without involvement of structures with rrun -phosphine arrangements. ... 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

Recent research has shown that this mechanism does not hold for all catalyst and substrate combinations. For instance, the rhodium-catalysed hydrogenation with electron-rich bidentate phosphine (2.11) is thought to proceed via initial oxidative addition of hydrogen to a catalyst-substrate complex. Furthermore, enantioselectivity arises from reaction of the major catalyst-ligand complex, in a lock-and-key process, when using monodentate phosphites as ligands. 

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