Hydride, iridium complex platinum complexes

In many other cases, oxidative additions of alkanes occur readily to transition-metal-alkyl complexes to generate hydride dialkyl intermediates that subsequently eliminate alkane and form a new metal-alkyl complex. For example, cations related to the alkyl hydrides of iridium formed by oxidative addition undergo reaction with alkanes at or below room temperature to generate new alkyl complexes (Equation 6.34). Cationic platinum complexes undergo similar reactions with substrates containing aromatic and aliphatic C-H bonds (Equation 6.35). " The C-H activation of the platinum complexes has been studied, in part, to understand and to develop systems related to the ones reported by Shilov that lead to H/D exchange, and oxidation and halogenation of alkanes. 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

The intramolecular migration of a hydride to an olefin is fast enough that few complexes contain an olefin and a hydride ligand cis to each other. Some olefin hydride complexes, such as the iridium and platinum examples below, are stable because they contain hydride and olefin Hgands that are mutually trans. These complexes must isomerize before insertion can take place. 

Prior to 1982, Crabtree s report of the reaction of cyclopentane with a solvated IrH2(PPh3)2+ species to give a cyclopentadienyl-iridium product stood as the only well characterized example of a reaction of an alkane with a homogeneous transition metal, in contrast to the widespread reactivity of arenes [2]. Based upon the instability of the platinum methyl hydride complex Pt(PPh3)2(CH3)H, it was believed that alkane oxidative addition might not be a thermodynamically feasible process, and consequently few attempts were made to attempt such a reaction [3]. It was not until the discovery of the formation of stable alkane oxidative addition products in 1982 that it was realized that reactions of hydrocarbons were in fact feasible. 

Proton magnetic resonance studies have also shown the presence of metal-hydrogen species in cyanide solutions of rhodium, platinum, and iridium (Table IX). In particular, the addition of CN- to a boiled aqueous solution of rhodium trichloride, followed by reduction with sodium boro-hydride, yields a solution that contains an Rh—H complex in moderately high concentrations and is stable in the absence of oxygen for several years (108). The observed coupling of the proton with the Rh10 nucleus (spin ) confirms the presence of an Rh—H bond (108). 

Early forays into this reaction were described by Cavell and Yates. Experimental studies demonstrated that oxidative addition of both C2 H and C2-I imidazolium ions to platinum was feasible. The C2-I substituted imidazolium ion 5 also underwent oxidative addition to [Pd(PPh3)4] however, an attempted reaction with the C2-H imidazolium met with failure (Scheme 3.4). Palladium has been shown to oxidatively add into C2 H imidazolium ions in bidentate systems resulting in palladium complexes 6 and 7 (Scheme 3.5). The oxidative addition of imidazolium ions to iridium, generating (NHC)Ir -hydrides was also reported. ... 

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

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