Platinum-metal complexes reaction with tertiary phosphine

The reactions with tertiary amines or phosphines that have no active hydrogen atoms result in platinacyclobutene cations, a rare species for late transition metal (Scheme 39). Substituted carbanions are added to the jj -aUenyl/propargyl platinum complex to yield the neutral substituted- ) -TMM derivatives that undergo huther [3 + 2] cycloaddition with good tt-acids as TCNE or maleic anhydride to produce highly substituted cyclopentanoids (Schemes 40, 41). 

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. 

Tertiary phosphines are among the easiest molecules to catalytically oxidize with molecular oxygen. The reactions can usually be effected under ambient temperature and pressure conditions. Metal complexes of both the platinum metal group and from the first row transition series have been used as catalysts, although in only a few cases have detailed mechanistic... 

Primary and secondary phosphines can oxidatively add to low-valent transition metal complexes to form phosphido complexes. In contrast to phosphines, metal phosphido complexes are known to undergo fast pyramidal inversion, often on the NMR timescale. Inversion barriers for some platinum complexes, determined by NMR spectroscopy, range from 42 to 67kJmol. As a consequence, phosphido complexes containing other chiral ligands are mixtures of interconverting diastereomers. Reaction of these complexes with electrophiles yields tertiary phosphines and derivatives (Scheme 6.1). 

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. 

Several other recent reviews contain material relevant to this section. An article by Blandamer and Burgess on the thermodynamics, kinetics, and mechanisms of solvation, solvolysis, and substitution in nonaqueous solvents contains a contribution on the controversial dissociative mechanism for isomerization of square-planar molecules. This is outlined in Section 5.5. A review of ligand substitution reactions at low-valency transition-metal centers contains sections on five-coordinate metal carbonyl complexes and on ML4 complexes (mainly tetrahedral configurations with L being a tertiary phosphine), as well as on acid- and base-catalyzed reactions. A review by Constable " surveying the reactions of nucleophiles with complexes of chelating heterocyclic imines contains a sizable section on square-planar palladium and platinum derivatives. Most discussion centers on [Pt(bipy)2] and [Pt(phen)2] (bipy = 2,2 -bipyridine phen = 1,10-phenanthroline). The metal center, ligand, or both are susceptible to nucleophilic attack and the mechanisms involved are critically assessed. 

The reactions of binary and mixed-ligand bis(iS-diketonato)-palla-dium(II) and -platinum(II) chelates with various nitrogen bases and tertiary phosphines have been studied extensively. Many types of complexes have been obtained depending on the nature of the central metals, jS-dik ligands, and the attacking Lewis bases the possible reaction routes are shown in Scheme 5. 

---