Oxidative addition sequence

Carbon-oxygen bond cleavage also proceeds by oxidative addition sequences. For example, lactones oxidatively add to Ir(I) to give an iridacycloester ... 

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. ... 

Dinuclear Cp Rh(III) and Ir(III) complexes show high activity at room temperature and atmospheric pressure for the reduction of olefins. Arenes are also reduced at higher pressures. The process is cocatalyzed by bases and the mechanism seems to involve both heterolytic and oxidative addition sequences [4] ... 

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. 

Without question, the most significant advance in the use of sulfur-centered nucleophiles was made by Shibasaki, who discovered that 10 mol% of a novel gallium-lithium-bis(binaphthoxide) complex 5 could catalyze the addition of tert-butylthiol to various cyclic and acyclic meso-epoxides with excellent enantioselectiv-ities and in good yields (Scheme 7.11) [21], This work builds on Shibasaki s broader studies of heterobimetallic complexes, in which dual activation of both the electrophile and the nucleophile is invoked [22]. This method has been applied to an efficient asymmetric synthesis of the prostaglandin core through an oxidation/ elimination sequence (Scheme 7.12). 

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]. 

Anionic/oxidative reaction sequences have been developed in addition to the domino anionic/reductive processes. For example, with regard to the synthesis of novel diaryl heterocycles as COX-2 inhibitors [500], including rofecoxib (Vioxx) 2-972 [501] (which has recently been withdrawn from the market) or the pyrrolin-2-one derivative 2-973 [494], Pal and coworkers reported on a so-far unique domino aldol condensation/oxidation sequence (Scheme 2.218) [503]. 

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). 

For the C-H activation sequence, the different possibilities to be considered are shown in Scheme 5 (a) direct oxidative addition to square-planar Pt(II) to form a six-coordinate Pt(IV) intermediate and (b, c) mechanisms involving a Pt(II) alkane complex intermediate. In (b) the alkane complex is deprotonated (which is referred to as the electrophilic mechanism) while in (c) oxidative addition occurs to form a five-coordinate Pt(IV) species which is subsequently deprotonated to form the Pt(II) alkyl product. 

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]. 

The ion 28 loses H2 by CID with argon to form [(PHOX)Ir(styrene)]+ (29). Compound 29 then undergoes H-D exchange with D2 gas to form the mixture of iso-topomers 29, 29-dh and 29-d2 (Scheme 13.3). When combined, these observations show that the oxidative addition of H2 to 29 is followed by alkene hydride insertion, and that both these steps occur rapidly and reversibly in the gas phase. These results thereby provide gas-phase analogues for catalytic elementary steps that are proposed to occur in solution. Support for this proposed sequence of steps was obtained from a solution-phase catalytic deuteration of styrene. Analysis showed no deuterium incorporation in the unreacted styrene at various conversions, and clean formation of dideuterio ethylbenzene as sole product. 

A new pyrrole ring synthesis developed by Arcadi involves the addition of ammonia or benzylamine to 4-pentynones, the latter of which are conveniently prepared via a palladium oxidative coupling sequence as shown below for the synthesis of 40 [39,40]. 

This indole C-7 Heck cyclization strategy was employed by Shao and Cai in a synthesis of anhydrolycorine-7-one from the requisite N-aroylindoline [275], by Miki in syntheses of pratosine and hippadine from substrates like 262 [276], and by Rigby to synthesize anhydrodehydrolycorine from an N-benzylhydroindolone [277, 278]. Thai and co-workers constructed examples of the new ring systems, pyrido[2 ,3-d ]pyridazino[2,3-a]indole (264) and pyrido[2 ,3 -Heck cyclizations on the appropriate 2-bromopyridine precursors (e.g., 263) at C-2 or C-7, respectively [279, 280]. Compound 264 undergoes oxidative-addition with methyl acrylate at the C-3 position. This resulting product (not shown) can also be obtained from 263 in a tandem Heck sequence with methyl acrylate (62% yield). 

The efficiency of the [2 + 2]-cycloadditions of 417 was utilized in a strategy for the synthesis of cephalosporin derivatives that carry an acetone or acetic acid ester group in the 3-position (Scheme 6.88) [175]. Liberated in the presence of 2-(trimethylsilyl-oxy)propene, 417 underwent cycloaddition leading to 435, treatment of which with tetrabutylammonium or hydrogen fluoride furnished the A3-cephalosporin 436 admixed with the A2-isomer. This mixture was converted to pure 436 by an oxidation-reduction sequence. In addition to the trimethylsilylenol ether of acetone, the... 

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. 

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