Alkyl hydrides

The catalytic cycle, which is supported by stoichiometric and labeling experiments, is shown in Scheme 38. Loss of 2 equiv. of N2 from 5 affords the active species a. Reaction of a with the 1,6-enyne gives the metallacycle complex b. Subsequently, b reacts with H2 to give the alkenyl hydride complex c or the alkyl hydride complex d. Finally, reductive elimination constructs the C-H bond in the cyclization product and regenerates intermediate a to complete the catalytic cycle. 

Consistent with the mechanism proposed earlier (Section 5.2.2, Scheme 5-8), in stoichiometric experiments, the intermediates phosphide hydride 12 and diastere-omeric mixture of alkyl hydrides 13 could be isolated and observed by NMR at -20°C, respectively (Scheme 5-14). 

ALKYL, HYDRIDE, AND HYDROXIDE DERIVATIVES OF THE s- AND p-BLOCK ELEMENTS SUPPORTED BY POLY(PYRAZOLYL)BORATO LIGATION MODELS FOR CARBONIC ANHYDRASE, RECEPTORS FOR ANIONS, AND THE STUDY OF CONTROLLED CRYSTALLOGRAPHIC DISORDER... 

Zinnalkyle sind mit Dialkylalanen nicht leicht zu hydrieren, da das Gleichgewicht der Reaktion (220) auf der rechten Seite liegt. Deshalb gelingt eine Hydrierung von Aluminiumtrialkylen mit Stannanen zu Dialkylalanen leicht (722). Man muB annehmen, daB der Alkyl-Hydrid-... 

In 1978, Schwartz and Gell found that CO would induce reductive elimination of alkane in various zirconocene alkyl hydride complexes with concurrent formation of Cp2Zr(CO)2 (2) (52,53). It was postulated that CO initially coordinates to the 6-e complex 23 forming the coordina-tively saturated species 24 which can then reductively eliminate alkane and/or rearrange to a zirconocene acyl hydride intermediate. When R = cyclohexylmethyl, methylcyclohexane reductively eliminated and Cp2Zr(CO)2 was isolated in 25% yield. 

Metal alkyl hydrides 15 (R2A1H)2 or 3 (R = Me, Et), R2AlH(NMe3) Sources for growth of Al-based III-V alloys and A1 CVD... 

Other organometallic compounds of aluminum include the alkyl hydrides, R2A1H. Molecular association of these compounds leads to cyclic tetramers. When the dimeric and trimeric compounds are dissolved in a basic aprotic solvent, the aggregates separate as a result of formation of bonds between A1 and the unshared pair of electrons on the solvent molecule. Toward Lewis bases such as trimeth-ylamine, aluminum alkyls are strong Lewis acids (as are aluminum halides). 

Tin alkyl hydrides can be prepared from the halides by the reaction with lithium aluminum hydride. 

Alkyl, Hydride, and Hydroxide Derivatives in the s- and p-Block Elements Supported by Poly(pyrazolyl)borato Ligation Models for Carbonic Anhydrase, Receptors for Anions, and the Study of Controlled Crystallographic Disorder Gerard Parkin... 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

An important study using cyelopentadienyl (Cp) molybdenum species (196) has shown that reductive elimination of saturated product from an alkyl-hydride complex occurs with retention of configuration at the... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

Direct oxidative addition of C-H bonds to square-planar Pt(II) to form octahedral Pt(IV) alkyl hydrides (Scheme 5, a) appears to be extremely uncommon. In fact, there are no unequivocal examples of this reaction pathway. However, a couple of examples have been reported wherein such a direct mechanism may provide the best explanation for the available data. 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

Thus to date, virtually all studies of C-C reductive elimination to form alkanes from Pt(IV) have found that these reactions proceed via five-coordinate intermediates. Only very recently have stable examples of Pt(IV) alkyl hydrides been synthesized (53-69). Detailed studies of C-H reductive elimination to form alkanes from these related complexes have identified similar five-coordinate intermediates on the reaction pathway (see following section). 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

The complex TpPtMeH2 was synthesized by reacting TpPtMe(CO) with water (66). While it is stable towards reductive elimination of methane at 55 °C, deuterium incorporation from methanol-c/4 solvent occurs rapidly into the hydride positions and subsequently, more slowly, into the methyl position (Scheme 15). The scrambling into the methyl position has been attributed to reversible formation of a methane complex which does not lose methane under the reaction conditions (75,76). Similar scrambling reactions have been observed for other metal alkyl hydrides at temperatures below those where alkane reductive elimination becomes dominant (77-84). This includes examples of scrambling without methane loss at elevated temperature (78). 

The observation of stable Pt(IV) alkyl hydrides upon protonation of Pt(II) alkyls has provided support for the idea that the methane which had been observed in earlier studies (89-92) of protonation of Pt(II) methyls could be produced via a reductive elimination reaction from Pt(IV). An extensive study of protonation of Pt(II) methyl complexes was carried out in 1996 (56) and an excellent summary of these results appeared in a recent review article (14). Strong evidence was presented to support the involvement of both Pt(IV) methyl hydrides and Pt(II) cr-methane complexes as intermediates in the rapid protonolysis reactions of Pt(II) methyls to generate methane. The principle of microscopic... 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

The classic platinum(O) approach to C-H activation, yielding platinum(II) alkyl hydrides as the oxidative addition products, contributed significantly to our understanding of C-H activation. However, the platinum(II)/(IV) approach has proven capable of achieving oxidative functionalization of hydrocarbons, and so this review focuses on the higher oxidation state. 

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