C-H bonds and reductive elimination

Carbometallation of alkynes by Cp2TiMe2 affords vinyl complexes which serve as intermediates for the formation of titanacyclobutenes (Scheme 546). Alkyne insertion to form the vinyl species followed by oxidative addition into the 7-C-H bond and reductive elimination of methane is proposed.1437... 

The mechanism of this transformation was not investigated, however a possible mechanism was proposed (Scheme 32). Transmetalation of the organoboron reagent to a rhodium(I) center could be followed by coordination of the imine, oxidative addition of the ortho C-H bond and reductive elimination to afford the ortho arylated product and a rhodium(I) hydride. Reoxidation would then follow through insertion of the imine in to the Rh-H bond followed by protonation with NH4C1. 

Rapid conversion between C-Ir/H-C 19 and C-H/lr-C 21 is observed, a reaction that involves oxidative addition of the agostic C-H bond and reductive elimination of the aryl hydride. This is a rare example of reversible metalation of an agostic group under mild conditions. The tautomeric equilibration was suggested to go via the doubly metalated species 20 having a non-classical hydride but an Ir(V) dihydride could not be excluded. 

The first one, (A), includes (b) insertion of CO into the Pd-S bond (c) insertion of the C C triple bond of the enyne into the Pd-C(0)SR bond whereby Pd binds to the terminal carbon and the RSC(O) group to the internal carbon, and (d) C-H bond-forming reductive elimination or protolysis by the thiol to form 29 (Scheme 7-7). 

These reactions could proceed either via (1) insertion of the alkyne or the allene into the M-Se bonds, and (2) C-H bond-forming reductive elimination (or protolysis by selenol) or via (1) insertion of the alkyne or the aUene into the M-H bond, and (2) C-Se bond-forming reductive elimination. 

Cyclization of 2-(l-alkynyl)XV-alkylidene anilines is catalyzed by palladium to give indoles (Equation (114)).471 Two mechanisms are proposed the regioselective insersion of an H-Pd-OAc species to the alkyne moiety (formation of a vinylpalladium species) followed by (i) carbopalladation of the imine moiety and /3-hydride elimination or (ii) oxidative addition to the imino C-H bond and reductive coupling. 

Although other transition metals have been found to catalyze hydroborations with HBcat, early studies have shown that rhodium complexes are the most effective for reactions of simple alkenes. The catalytic cycle proposed resembles one suggested previously for the rhodium-catalyzed addition of carborane B-H bonds to the C=C unit in acrylate esters. The reaction is believed to proceed via initial loss of phosphine and oxidative addition (see Oxidative Addition) of the B-H bond of HBcat to the coordinatively unsaturated (see Coordinative Saturation Unsaturation) rhodium center. Coordination ofthe alkene (seeAlkene Complexes) and subsequent insertion (see Insertion) into the Rh-H bond and reductive elimination (see Reductive Elimination) of the B-C bond then generates the organoboronate ester product(s) (Scheme 1). 

Consider the reaction of 2,3-dimethyl-2-butene with H2 catalyzed by Pd/C. Two new C-H bonds are made, and a C=C 7r bond breaks. The fact that the addition is stereospecifically syn suggests that an insertion reaction is occurring. The Pd metal is in the (0) oxidation state (i.e., d10), so it can react with H2 by an oxidative addition to give two Pd-H bonds. At this point Pd is in the (II) oxidation state. Coordination and insertion of the alkene into one of the Pd-H bonds gives Pd-C and C-H bonds. Finally, reductive elimination gives the product and regenerates Pd°, which begins the catalytic cycle anew. 

A possible mechanism was proposed as shown in Figure 14.10. The catalytic process involves oxidative addition of the C-H bond to the nickel centre, insertion of styrene into the Ni-H bond, and reductive elimination. In the absence AlMes, the oxidative addition species of the C-H bond to the nickel electronically and thermodynamically favours hydride insertion at the Cp position of styrene, resulting in a branched product. However, in the presence of AlMes an adduct of AlMes and the oxidative addition species is formed. The steric hindrance of adduct compels hydride insertion at the C position of styrene to give a linear product. ... 

Much information has been gained on the mechanism of C-H bond-forming reductive elimination (see Equation 8.9). In addition to creating an understanding of C-H bond formation, this information has been used to understand the mechanism of the opposite reaction, the oxidative addition of C-H bonds. Because reductive eliminations of alkanes are faster from three- and five-coordinate species than from four- and six-coordinate species, square planar and octahedral complexes often dissociate or associate a dative ligand prior to reductive elimination. However, elimination to form a C-H bond from a four- or six-coordinate complex can also be fast enough that it occurs directly from the alkylmetal-hydride complexes prior to ligand dissociation. 

The reactions of benzyne complexes of zirconium " also occur by electrophilic attack at an M-C bond. The isolated phosphine adduct of a zironocraie-benzyne complex reacts with ketones to imdergo insertion into one of the M-C bonds and with alcohol to make an aryl alkoxo complex, as shown in Equation 12.67. An electron-rich ruthenium-benzyne complex also reacts with electrophiles, such as borzaldehyde or carbon dioxide, to form products from insertion, as shown at the top of Equation 12.68. It also reacts with weak acids, such as aniline, to form products from formal protonation at the Ru-C bond, as shown at the bottom of Equation 12.68. - This reaction with aniline could occur by initial protonation at the metal, followed by C-H bond-forming reductive elimination, or by direct protonation of the M-C bond. Initial protonation of the metal center was proposed. 

The mechanism of hydrocyanation of alkenes catalyzed by soluble complexes is closely related to the mechanism of hydrogenation and hydrosilation. Hydrocyanation occurs by a sequence consisting of oxidative addition of HCN, olefin insertion into the M-H bond, and reductive elimination to form the new C-C bond. The mechanism of the original hydrocyanation catalyzed by cobalt carbonyl has not been studied in depth, but the mechanism of the reactions catalyzed by nickel complexes has been studied in depth and is better defined. 

The C-H bond addition reactions across alkynes using a Rh catalyst such as RhC PPhjjj described earlier appear to proceed through oxidative addition of C-H toward a Rh species, alkyne insertion into the resulting Rh-H bond, and reductive elimination (path A in Scheme 18.82). An alternative pathway may be a sequence involving C-H metallation by an electrophilic Rh species, alkyne insertion into the resulting Rh-C bond, and protodemetallation (path B). The latter mechanism was proposed by Schipper ei al. [82] for their reaction of N-carbamoylindoles with alkynes in the presence of a cationic Rh catalyst (Scheme 18.83). 

The use of CsDs-Py showed a partial deuterium retention in the resulting alkyl group. Thus the authors suggested that the reaction occurs via the formal insertion of the ruthenium(II) into the ortho C-H bond, alkene insertion into the Ru-H bond and reductive elimination with C-C bond formation similarly to a Ru(0) catalysed Murai type reaction [36, 39]. However this possible mechanism needs to be demonstrated as it may also result from a classical Ru(II) C-H bond activation, cyclometallation and insertion of the C=C bond into the Ru-C bond. The later process corresponds to the addition of the Ru-C bond to the less hindered face of the alkene (opposite to the Ph substituent). The last step involves a protonation of the Ru-C bond after alkene insertion [122, 123]. 

The proposed mechanism (Scheme 8) involves initial coordination of the pyridine to direct sp C-H bond oxidative addition to the Ru(0) centre followed by alkene insertion into a Ru-H bond and reductive elimination as for the Murai reaction involving sp C-H bond alkylation [3]. 

Several useful reviews have appeared. Mondal and Blake have collected thermochemical data on oxidative addition, Halpern has investigated the formation of C-H bonds by reductive elimination, while in a thought-provoking article on activation of C[5/ ]-X bonds, Chanon stresses the importance of electron transfer in oxidative addition (among other topics). In a discussion of oxidation addition and reductive elimination involving two metal centers, Halpem classifies and gives examples of three mechanisms whereby binuclear reductive elimination can occur concerted two center (77), concerted one center (78), and free-radical [(79)-(81)] reactions given in Scheme 6. 

Reviews on transition-metal-alkyl bond dissociation energies and the formation of C-H bonds by reductive elimination are also relevant to the mechanisms of catalytic processes. It has been proposed that [Ti(Me2PCH2CH2PMe2)(Et)Cl3] contains a direct-bonding interaction between the titanium atom and the j8-C-H system the evidence for such agostic C-H-M bonds in other systems has been reviewed. Obviously, such bonds have important implications in alkane activation and it is particularly exciting that kinetic studies of the exchange reaction shown in equation (1) indicate that a bimolecular pathway predominates. ... 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

So we propose a general mechanism (Scheme 3) for the reaction of vinylsilanes with lithium metal, which should also allow a general access to vicinal and geminal dilithiovinylsilanes by repetitive addition of lithium metal to the C=C-double bond and subsequent elimination of lithium hydride. In order to explore this synthetic approach the reduction of a series of either a- or P-substituted vinylsilanes with lithium was examined, here the substituent R H) in 15 and 19 is introduced to prevent the last lithium hydride elimination. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

In a related reaction, photolysis of the 18-electron complexes fy -CpMo(dmpe)2H3 and (t -j-PrCp)MoL2H3 (L2 = two P-donor ligands or one chelating diphosphine) induces reductive elimination of H2 to produce the monohydrido complexes . These 16-electron complexes can oxidatively add aryl C-H bonds and catalyze H-D exchange in the aryl ring. 

Carbon-hydrogen bonds are commonly formed by reductive elimination when an alkyl or aryl group and a hydride occupy mutually cis positions. Although intramolecular oxidative additions of C—H bonds and reactions of activated C—H bonds are well known for Ni, Pd, and Pt, additions of C—H bonds in simple alkanes and arenes are less common. 

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