C-H reductive elimination

The P-H oxidative addition, acrylonitrile insertion, and C-H reductive elimination steps were observed directly with the dcpe catalyst, and the potential intermediates Pt(diphos)(PHMes )(CH2CH2CN) (7, diphos = dppe, dcpe) were shown not to undergo P-C reductive elimination. The generality of this proposed mechanism for less bulky phosphine substrates, or for Pt catalysts supported by monodentate ligands, remains to be investigated [9]. 

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). 

At this point it is worthy of mention that solutions of these alkenyl-hydrido isomers react with hydrogen, at room temperature, to yield styrene and the starting [lrH2(NCMe)3(P Pr3)]BF4 complex. Deuterium treatment of the alkenyl-hydrido isomers shows an easy H/D hydride exchange, which suggests that the reaction with hydrogen is more favorable than C—H reductive elimination. Therefore, the hydrogenahon is dominated by an iridium(lll) species, and most probably iridium(l) species are not involved under catalytic conditions. 

Complex 24 was used in thermal and Lewis acid-catalyzed decomposition experiments. Intramolecular C—H reductive elimination from 24 to form exo-2-phenyl-aminonorbornane was demonstrated with labeling experiments [7]. 

In closely related experiments it was shown that sp C—H activation takes place reversibly within the coordinahon sphere of the electron-rich Ir(I)-diphosphine complex 58 (Scheme 6.9) to form an alkyl-amino-hydrido derivative 57 reminiscent of the CCM intermediate 24 the solid-state structure of 57 is shown in Figure 6.13 [40]. It appears that C—H activation only takes place after coordination of the amine function to the Ir(I) center (complex 58, NMR characterized). Amine coordination allows to break the chloro bridge of 59 and to augment the electron density of the metal center, thus favoring oxidative addihon of the C—H bond. Most importantly, the microscopic reverse of this C—H activation process (i.e. C—H reductive elimination) models the final step of the CCM cycle (see Scheme 6.1) indeed, the reaction of Scheme 6.10 is cleanly reversible at 373 K. 

Jun s proposed mechanism was probed by a deuterium labeling experiment using N-methyl co-catalyst 66 (Equation 9.8). Hydridoimidoyl complexes (e.g., 64), which are implicated by Jun in C—C bond formation, cannot form when secondary amine 66 is used as a cocatalyst. Instead, C—H reductive elimination from a complex... 

Reaction of the oct-4-yne complex with HBF4 liberates cis -oct-4-ene, and this is postulated as occurring via initial protonation of the metal (to yield [Cpf Ta(H)2(alkyne)]+), followed by insertion of acetylene into the Ta-H bond and C-H reductive elimination, cis-Oct-4-ene also results when H2 is reacted with the hydrido complex at 100°C 178). 

Reductive elimination of benzene from rhodium complexes shows some differences to other C-H reductive elimination systems. Formation of an complex, which is... 

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... 

SCHEME 11.18 Net C—H reductive elimination and the microscopic reverse, oxidative addition equilibria are typically governed by four rate constants. 

SCHEME 11.19 Two scenarios that lead to inverse kinetic isotope effect for overall C—H reductive elimination (HE = inverse isotope effect NIE = normal isotope effect). 

SCHEME 11.23 Proposed mechanism for C—H reductive elimination from observable Pt(IV) intermediate. 

When the reductive elimination proceeds by a concerted mechanism involving a three-center transition state, the two ligands to be eliminated must be situated cis to each other. This geometrical requirement has been clearly demonstrated in the thermolysis reactions of cis- and tran5-PdEt2L2 complexes, which afford entirely different products from one another (Eqs. 9.3 and 9.4) [2]. The cis complexes exclusively provide butane as the reductive elimination product, whereas the trans isomers selectively afford a 1 1 mixture of ethylene and ethane, which are formed by -hydrogen elimination followed by C-H reductive elimination [3]. 

Owing to the extremely high reactivity of hydrido ligand, direct observations of H-H and C-H reductive elimination have been limited to platinum complexes. H-H reductive elimination from c 5-PtH2L2 proceeds without notable activation barrier [13]. C-H reductive elimination is also a facile process cA-PtH(Me)(PPh3)2 and cA-PtH(CH2CF3)(PPh3)2 readily decompose at -40°C and room temperature, respectively [14]. These reactions involve a modest kinetic isotope effect ( h/ d =... 

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... 

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