Homogeneous catalysis tertiary phosphine complexes

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

Such diene complexes can be used to prepare homogeneous hydrogenation catalysts in situ, especially where a variable tertiary phosphine/rhodium ratio is required3 or where an asymmetric tertiary phosphine is employed for asymmetric synthesis.4 The cyclooctadiene complex is also the starting point for the preparation a number of complexes of the type [Rh(l, 5-C8H12)L2]+ (L represents a variety of P— and N— donor ligands) of interest in homogeneous catalysis.s... 

At about the time of Wilkinson s discovery, new schemes were developed by others for the preparation and configurational correlation of chiral phosphines (lOa-e). The combination of advances in homogeneous catalysis and chiral phosphine technology prompted research on chiral phosphine complexes. Horner et al. (11) were the first to hypothesize in print that rhodium complexes containing optically active tertiary phosphine ligands should effect the asymmetric hydrogenation of unsymmetrically substituted olefins. 

Recently it has become clear that tertiary phosphine-metal complexes are reactive and liable to undergo carbon-phosphorus bond scission. The reaction between the C— P " bond and the transition metal to which the tertiary phosphine is bound has profound implications on homogeneous catalysis, particularly on the mode of homogeneous catalyst deactivation in hydroformylation (Rh- and Co-catalyzed) and various other hydrogenation/dehydrogenation reactions, including asymmetric hydrogenation. 

The phosphine metal complexes now finding application in homogeneous catalysis are those of the tertiary organic phosphines. They were discovered around the middle of the last century and their ability to combine with heavy metal salts noted almost immediately, but the application of their metal complexes to homogeneous catalysis came only after the lapse of about 100 years. 

Although the oxidation of tertiary phosphines by these catalytic processes has minimal useful application, it needs to be considered as a problematic side reaction in homogeneous catalysis. Much effort is being currently expended to immobilize platinum metal phosphine complexes on heterogenized tertiary phosphine supports, and irreversible oxidation at phosphorus on these supports effectively destroys the supported catalyst. Recent observations that the compound Rh6(CO)i6 catalyzes the oxidation of tertiary phosphines correlate with the report that phosphine oxidation occurs with molecular oxygen on Rh6(CO)i6 bound to diphenylphosphino-functionalized poly(styrenedivinylbenzene). Thus, in order to use these phosphinated polymer-supported rhodium catalysts, one needs either to rigorously exclude oxygen, or to find a way to inhibit the simultaneous catalyzed phosphine oxidation. 

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