Reductive elimination reactions, involving

C-X Oxidative Addition and Reductive Elimination Reactions Involving d /d Metal Complexes ... 

Reductive elimination reactions are responsible for the final C-C/C-X bond formation step in catalytic processes. In principle, C-bound or any other anionic pincer ligands could engage in reductive couphng reactions, bringing about the decomposition of the pincer complex. In order to enable catalysis by such complexes, it is mandatory to avoid reductive elimination reactions involving the pincer ligand itself Fortunately, this seems to be usually the case. For example, reductive elimination of ethane from the Pd(IV) pincer complex 18 (Scheme 2.3) is preferred... 

Reductive elimination reactions involving the intermediacy of a o-alkane or ti -arene complex... 

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 proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Cleavage reactions, classified according to the nature of the attacking group, are collected together in Table X. Besides these, adduct formation (mode 3) and reductive elimination reactions also involve cleavage of Si-M bonds, but it is convenient to treat these separately in... 

Recently An et al. disclosed a palladium(II)-catalyzed bis(peroxidation)/cycli-zation method for the synthesis of 3-(peroxymethyl)-3-peroxyoxindoles 209 from N - ar y 1 aery I a m i d e s 208 (Fig. 52) [235]. Using 5 mol% of Pd(OAc)2 in the presence of terf-butyl hydroperoxide, 46-96% of products 209 were obtained. The reactions were proposed to involve a Pd-catalyzed radical bis(peroxidation) of the acrylic unit [236] followed by a two-electron directed cyclometalation/reductive elimination reaction of intermediate bis(peroxide) 208A. 

These reactions involve a decrease in the oxidation number of the metal (in this case Ir to Ir ), and a decrease in coordination number for the metal. Reductive elimination reactions are very important in catalysis as the product-removing step. Frequently, the reductive elimination reactions are rapid, making detailed study difficult. 

The generic equations representing the different types of organic reactions you have learned—substitution, elimination, addition, oxidation-reduction, and condensation—can be used to predict the products of other organic reactions of the same types. For example, suppose you were asked to predict the product of an elimination reaction in which 1-butanol is a reactant. You know that a common elimination reaction involving an alcohol is a dehydration reaction. 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

Reductive elimination of 5 and 6 has been studied in detail in connection with the nickel-catalyzed cross-coupling reaction of aryl halides (ArX) and alkylmagnesium halides (RMgX) (eq (18)). The reaction of tram-5 is induced by addition of aryl halides (ArX) to the system, where the reductive elimination process involving para-... 

The conversion of the Ir(III) cyclohexyl hydride complex to an Ir/cyclohexane system involves a change in the formal oxidation state of Ir from + 3 to +1 (i.e., a formal two-electron reduction). As a result, this elementary reaction step is generally called a reductive coupling (Chart 11.4). From a metal hydrocarbyl hydride complex (i.e., M(R)(H)), the overall process of C H bond formation and dissociation of free hydrocarbon (or related functionalized molecule) is called reductive elimination (Chart 11.4). The reverse process, metal coordination of a C—H bond and insertion into the C—H bond, is called oxidative addition. Note Oxidative addition and reductive elimination reactions are not limited to reactions involving C and H.)... 

The isotopic studies of the mechanism of this reductive elimination process, involving breaking of M—C bonds followed by formation of C—C bonds, have been initiated by synthesis of (dimethyl-d6)(2,4-pentanedionato)gold, 73-(CD3)2, and identification of coupled reaction products produced in the pyrolytic decomposition carried out in sealed quartz NMR tubes in CD3OD and CV,Di2 solvents. 

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