Oxidative addition sequence elimination

Oxidative addition and reductive elimination reactions play key roles in C—H activation reactions, where a strong C—bond is cleaved by a transition-metal complex. These are important reactions because they permit unfunctionalized hydrocarbons to be transformed into complex molecules. Bergman reported the following classic C—H reductive elimination/oxidative addition sequence. ... 

As a typical example, the catalytic reaction of iodobenzene with methyl acrylate to afford methyl cinnamate (18) is explained by the sequences illustrated for the oxidative addition, insertion, and /3-elimination reactions. 

Romakhin et al. [49] showed that anodically generated phosphoniumyl radicals can add onto alkenes to yield phosphonylated alkenes through an anodic oxi-dation/addition/anodic oxidation/elimination/nucleophilic attack sequence (Scheme 17). 

The Mizoroki-Heck reaction is a metal catalysed transformation that involves the reaction of a non-functionalised olefin with an aryl or alkenyl group to yield a more substituted aUcene [11,12]. The reaction mechanism is described as a sequence of oxidative addition of the catalytic active species to an aryl halide, coordination of the alkene and migratory insertion, P-hydride elimination, and final reductive elimination of the hydride, facilitated by a base, to regenerate the active species and complete the catalytic cycle (Scheme 6.5). 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

In an alternative sequence suggested by Eisenberg et al. (80), X in Eq. (19) is hydride. C02 production then proceeds via /3-elimination from the oxygen OH in 18, giving a rhodium(III) dihydrido species which can then reductively eliminate H2. Some support for this latter suggestion is provided by the observation that iridium(lll) species of type 19, formed by oxidative addition of formic acid to /ra/ s-[IrCI(CO)L2] (L = tertiary phosphine), rapidly lose C02 to give the dihydrido species 20 (81). 

The catalytic asymmetric hydrogenation with cationic Rh(I)-complexes is one of the best-understood selection processes, the reaction sequence having been elucidated by Halpern, Landis and colleagues [21a, b], as well as by Brown et al. [55]. Diastereomeric substrate complexes are formed in pre-equilibria from the solvent complex, as the active species, and the prochiral olefin. They react in a series of elementary steps - oxidative addition of hydrogen, insertion, and reductive elimination - to yield the enantiomeric products (cf. Scheme 10.2) [56]. 

Miyaura and co-workers reported the platinum-catalyzed diboration of allenes with bis(pinacolato)diboron (Scheme 16.52) [57]. The catalytic cycle involves a sequence of oxidative addition of bis(pinacolato)diboron to Pt(0), insertion of an allene into the B-Pt bond and reductive elimination of an allylic boronate, re-producing the Pt(0) species. (Z)-Allylic boronates are formed stereoselectively in the reaction with monosubstituted allenes, which strongly suggests a pathway via a vinylplatinum species rather than a Jt-allylplatinum species. 

Essentially the C1=C2 bond is inserted into the C5-H bond. This suggests that the Rh oxidatively adds across the C5-H bond. Rh can do this with aldehydes. After oxidative addition to the C5-H bond to give a Rh(III) complex, insertion and reductive elimination give the product and regenerate Rh(I). Solvent molecules may be associating or dissociating at any point in the sequence. 

The overall transformation involves insertion of the C5=C6 and the C2=C1 k bonds into the Si7-H bond. An oxidative addition of Si-H to Y, insertion, insertion, reductive elimination sequence might occur. The problem with this is that the d° Y complex can t do oxidative addition. The alternative by which the Si-H... 

The difference between this catalytic system and Wilkinson s catalyst lies in the sequence of the oxidative addition and the alkene complexation. As mentioned above, for the cationic catalysts the intermediate alkene (enamide) complex has been spectroscopically observed. Subsequently oxidative addition of H2 and insertion of the alkene occurs, followed by reductive elimination of the hydrogenation product. 

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. 

In spite of its formal similarity to the above mentioned annulation processes, the reaction shown in 4.37. includes a unique migration step. Oxidative insertion of the palladium into the phenyl-iodine bond is followed by the migration of the palladium onto the more electron rich indole ring. The 2-indolylpalladium complex than carbopalladates the pendant alkync moiety and the process ends by the formal activation of a C-H bond of the phenyl substituent and subsequent reductive elimination, furnishing the pentacyclic product.48 The same strategy has been utilised in the preparation of the indoloindolone framework from /V-bcnzoyl-3-(o-iodophcnyl)-indolc in an oxidative addition - palladium migration - C-H activation sequence.49... 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

Mercury(II) halides electrophilically attack Pt(CF3C CCF3)(PMePh2)2 at the alkynic carbon (equation 278).8SS Site migrations between PtCl(G=CR)(CO)L and Hg(C=CR )2 involve oxidative addition and reductive elimination sequences.856... 

An oxidative addition-reductive elimination sequence is expected to result in gold-methyl bond cleavage, so the latter was dismissed as a possible mechanism. 

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