Elimination of alkane, reductive

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. 

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

Both the Cp and Cp hydridoalkyl compounds react with hydrogen, but different mechanisms have been proposed. In both cases there is no evidence for the reductive elimination of alkane followed by the oxidative addition of H2 to the Cp2Zr or Cpf Zr fragment. In the case of the Cp2Zr(H)R compounds, reaction with D2 leads to incorporation of D into the hydride position, and a mechanism involving deuteride abstraction... 

A continuation of the studies on Cp2Zr(H)R compounds has shown that in addition to hydrogen various tertiary phosphine ligands can also produce reductive elimination of alkane ... 

Insertion of alkenes into Os3H2(CO)10 gives compounds of the type Os3H(alkyl)(CO)10 which are usually reactive toward /(-elimination to regenerate alkene (see Section V) or reductive elimination of alkane to allow oxidative addition of the alkene (161,162). Sometimes, however, if the alkene is bi- or polyfunctional, stable insertion products are formed. For example, CH2=CHOMe inserts to give a mixture of diastereomers 71 and 72. Ether coordination reduces the rate of /(-elimination (243). Similar stabilization occurs on inserting a,/(-unsaturated esters (51), although a,/(-unsaturated ketones RCH—CHCOMe (R = H, Me, or Ph) insert, then eliminate the... 

Ans. (a) PR, and R. (b) Alkene insertion into Rh-H bond followed by reductive elimination of alkane, (c) No back-donation from Ti4+(d°) but back-donation from Pt2+(d8). (d) Facile /3-elimination from C2H5. 

Until recently, reductive elimination of alkanes from 18e complexes was not well studied. The reaction of Rh(PMc3 )3C1... 

More recently, the reductive elimination of alkanes from Pt(IV) complexes has become an intense area of investigation as it represents the reverse of the C-H oxidative addition reaction that has been shown to be catalytic in the presence of the Pt(II)/Pt(fV) couple. ... 

Activation parameters were determined for reductive elimination of H2 from Ir(CO)(dppe)(Et)(H)2, A/f = 16 + 1 kcal/mol and A5 = — 24 + 4 eu . The negative entropy of activation, unusual for a dissociative process, reflects the binding together of two hydrogen atoms. The reaction of Rh(PMe3)3Cl with propylene oxide led to one example of reductive elimination of alkanes . 

Alkyl(hydrido)platinum(II) complexes decompose via reductive elimination of alkane [184,185]. When the complex has abidentate phosphine ligand bearing bulky cyclohexyl groups, the platinum(O) species generated by reductive elimination causes C—H bond activation of a variety of hydrocarbons (eq (141)) [184]. 

In addition to the indirect evidence of O-complexes as intermediates in metal-mediated reductive elimination/oxidative addition transformations from isotopic scrambling, KIEs for C—H reductive elimination reactions lend credence to the intermediacy of coordinated alkane intermediates.20 Assuming coordinated alkane intermediates, the equilibria and rates of reductive elimination and oxidative addition can be described with four rate constants that correspond to reversible C—H reductive coupling/oxidative addition and dissociation/association of the hydrocarbon substrate (Scheme 11.18). Several groups have reported that the rates of C—H reductive elimination of alkanes of perprotio versus perdeuterio variants yield inverse KIEs (i.e., kyjki) < l).20... 

---