Homogeneous catalysis oxidative addition

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

The breaking of carbon-to-phosphorus bonds is by itself not a useful reaction in homogeneous catalysis. It is an undesirable side-reaction that occurs in systems containing transition metals and phosphine ligands and that leads to deactivation of the catalysts. Two reaction pathways can be distinguished, oxidative addition and nucleophilic attack at the co-ordinated phosphorus atom (Figure 2.35). 

Elimination. Two kinds of elimination reaction are important in homogeneous catalysis. A catalytic cycle which has involved an oxidative addition generally ends with the reverse process of reductive elimination (equation 6). Rhodium-catalyzed hydrogenations end with this step. 

Several reports of catalysis of the decomposition of formic acid involving homogeneous transition metal complexes and proceeding by means of metalloformate intermediates have recently appeared in the literature. For example, Rh(C6H4PPh2)(PPh3)2 (8) catalyzes the decomposition of formic acid to C02 and H2 via the intermediacy of the product of oxidative-addition of HCOOH, Rh(HC02)(PPh3)3 (56). -Elimination of the hydride from the... 

The high formal oxidation states of metals in some of these adducts is noteworthy, e.g., Fe(IV) (entries 17 and 18), Ru(IV) (entries 21 and 22), and Pt(IV) (entries 55 and 56). Such adducts are important because they provide definite examples of species often postulated as intermediates in oxidative addition-reductive elimination processes (compare Section II,G,1) and in homogeneous catalysis (134,220a, 410a). In the case of germanium, a tris(germyl) adduct of Pt(IV) has been described (57), but no more than two silyl groups per metal atom are known to result from oxidative addition. 

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