Elimination reactions from coordinated alkanes

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. 

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... 

Puddephatt etal. [41] have studied the C-H or C-C bond activation in the alkane complexes [PtMe(CH4)L2] or [PtMe(CHjCH3)L2] (L = NH3 or PH3) as well as the reductive elimination of methane or ethane from the five-coordinate model complexes [PtHMe2L2] or [PtMesLi], respectively, by carrying out extended Hiickel molecular orbital calculations and density functional theory. The oxidative addition and reductive elimination reactions occur by a concerted mechanism, probably with a pinched trigonal-bipyramidal complex on the... 

Kinetic studies of C-H reductive elimination from the alkyl-hydrido complexes bearing a d metal center have been reported [80-82], Similarly to the reactions of d metal complexes, the reductive elimination proceeds via an alkane-coordinate intermediate, as supported by the observation of an inverse kinetic isotope effect. Representative data are as follows = 0.75 for Cp2W(H)(Me) in... 

The catalytic cycle presented in Figure 2 was proposed to explain these dehydrogenation reactions. The postulated active intermediates are the coordinatively unsaturated [M]H 2 16 and [M]H 4 17 which would be formed by stripping hydrogen atoms from the polyhydrides [M]H . It was assumed that the key step of the reaction would be the insertion of the 14-e species 17 into the CH bond of the alkane molecule RH to give the hydrido alkyl intermediate 18. This latter could undergo a elimination reaction leading to the olefin R(—H) via the olefin complex 19. 

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). 

This reaction, like dicarbene recombination, also has its analog in coordination chemistry, that is, reductive elimination of tetramethylene and pentamethylene ligands from platinum complexes yields cyclobutane and cyclopentane, respectively (777). According to this direct ring closure mechanism, the observed selectivity for dehydrocyclization of n-alkanes on metals (nonformation of quaternary-secondary and tertiary-secondary C-C bonds in reactions of type A and B) should be interpreted in terms of simple steric effects. However, although, in the case of platinum, the concepts of steric hindrance could account for the change of selectivity that occurs with decreasing metal particle size (i.e., cyclization of n-hexane takes place on... 

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