C-H bond-forming reductive elimination

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. 

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 Effect of Ancillary Ugands on C-H Bond-Forming Reductive Elimination... 

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. 

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 method of transition metal induced hydroacylation of olefins with aldehydes has its origins in the observation that aldehydes are decarbonylated by Wilkinson s catalyst31. Mechanistically, decarbonylation is believed to start with an oxidative addition of a low valent transition metal unit to the aldehyde C —H bond. This individual step has been observed in several cases32 " 37. The resulting metal acyl hydride system can then be decarbonylated to form a new C - H bond upon reductive elimination, steps which are also well known. 

In 2002, Yamamoto published the first account of C-O bond-forming reductive elimination from an isolated Pd(IV) complex (11) [37]. As shown in Eq. 14,11 was synthesized by the reaction of Pd2(dba)3 (dba = dibenzylideneacetone), tetra-chloro-l,2-benzoquinone, and benzonorbomadiene, followed by the addition of pyridine. This Pd adduct is stable in the solid state to >100°C and was characterized by H and C NMR spectroscopy as well as X-ray crystallography [38]. 

This same report also described the synthesis of diorgano Pd complex (p-Tol-BIAN)Pd (CH3)2(I)2. It was characterized by H and NMR spectroscopy and elemental analysis. The authors report that this species is unstable at room temperature in solution. While the decomposition products were not studied in detail, they are likely to include compounds derived from C-I bond-forming reductive elimination. 

The first literature example of C-Cl bond-forming reductive elimination fi om Pd involved the mono-organo Pd complex 36 (Eq. 35) [67]. This pincer adduct was accessed by van Koten and coworkers via oxidation of the Pd precursor (35) with excess CI2 at room temperature in CHCI3. The Pd trichloride intermediate was characterized by H NMR spectroscopy, but underwent fast reductive eliminatiMi to form 37. However, the decomposition was not very clean, which the authors attributed to the presence of excess dissolved CI2. 

The distribution of reductive elimination products from 44 and 45 varied depending on the solvent. For example, when 44 was heated at 80°C for 12 h in pyridine, C-C coupling predominated in contrast, C-Cl bond-forming reductive elimination was favored in AcOH and CH3CN (Eq. 39). The best selectivity for 46 over 47 was observed in AcOH, where these products were formed in a 5 1 ratio. 

In 2011, Yoshikai and Liu developed a Pd(ii)-catalyzed oxidative cyclization reaction of a 2-arylphenol to dibenzofuran derivatives respectively (Scheme 2.27). ° The catalytic system features a simple combination of a palladium(ii) salt and a pyridine ligand and the use of pero g benzoate as an inexpensive and convenient oxidant. In their mechanistic experiments, they showed that the reaction involves Pd(ii)-mediated C-H bond cleavage as the rate-limiting step, which is followed by oxidation to a high-oxidation-state palladium species and subsequent C-O bond-forming reductive elimination. 

Presumably, the oxidative cyclization of 1 commences with direct palladation at the orfAo-position, forming o-arylpalladium(II) complex 3 in a fashion analogous to a typical electrophilic aromatic substitution (this notion is useful in predicting the regiochemistry of oxidative cyclizations). The mechanism of the second formal C—H bond functionalization step is not fully elucidated, but may occur either via (a) an intramolecular carbopalladation reaction (migratory insertion) followed by czHft-P-hydride elimination from 4 (Path A) (b) by o-bond metathesis (through a four-centered transition state) followed by reductive elimination (Path B) (c) by electrophilic aromatic substitution followed by C—C bond-forming reductive elimination (PathC) [9]. 

The first case involves direct arylation of arenes with aryl halides via a Pd(0)/Pd (II) cycle (Scheme 8). After initial oxidative addition of Pd(0) into the aryl halide, the C-H bond activation event takes place at the Ar-Pd(II)-X species 4 and is followed by C-C bond forming reductive elimination and regeneration of Pd(0). 

Both methods can also be used in intramolecular ring closure reactions to form cyclic ketones. Similar, but not identical reaction mechanisms are assumed. The first reaction resembles hydroformylation and requires carbon monoxide insertion and an additional metal acyl alkene insertion step, while in the second reaction the carbon monoxide unit is already present in the substrate. This reaction starts with an oxidative addition to the aldehyde C-H bond, forming an acyl metal hydride, which then undergoes alkene insertion and reductive elimination. 

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. ... 

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. 

Carbon-heteroatom bond-forming reductive elimination from transient intermediates has been proposed as the product release step of a variety of important Pd-catalyzed transformations, including arene and alkane C-H bond functionahza-tion [1,2], ally lie acetoxylation [3], alkene borylation [4], and olefin difunctionalization [5]. Over the past 25 years, a variety of Pd " model complexes have been synthesized to study reductive elimination reactions at Pd centers. For instance, in 1986, Canty reported the first example of a crystallographically characterized organometallic Pd complex, /ac-[(bpy)Pd (CH3)3(l)] (bpy = 2,2 -bipyridine) (1). In addition, his group has demonstrated that this species undergoes facile C-C bond-forming reductive elimination to release ethane (Eq. 1) [6],... 

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