Oxidative addition of the ortho C-H bond

This ring thereby constitutes an unusual yu-ij. ij -vinyl group whereby the phosphino carbon is also metalated. Intramolecular oxidative addition of the ortho C-H bond of the phenyl substituents is a common feature in crowded iridium phosphine complexes [65, 309, 310]. 

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. 

Formation of 2(biphenyl-2-yl)phenol (51) from 50 is explained by electrophilic attack of 57 to form 59 and its reductive elimination. As another explanation, oxidative addition of aromatic ortho C—H bond to 57 generates the palladacycle 58 and its reductive elimination affords 59. Domino arylations by a similar sequence of the reactions via 60 finally give rise to the pentaphenylated product 49 in 58 % yield. Certainly the reaction occurs by strong participation of OH group. It is surprising that efficient polyarylation of phenol with bromobenzene proceeds smoothly in the presence of CS2CO3 which is sparingly soluble in xylene and since it is difficult to abstract protons from phenol. 

Murai, Kakiuchi, and coworkers [83] reported the Ru-catalyzed C-H bond addition of aromatic ketones across alkynes (Scheme 18.84). This reaction is considered to be initiated by oxidative addition of the ortho-C-H of ketones, as in path A for the Rh-catalyzed reactions (Scheme 18.82). 

Our exploration of the reactivity of (PCP)Ir toward C-O bonds began with anisole. At room temperature, we observed immediate formation of product resulting from oxidative addition of the aryl C-H bond ortho to the methoxy substituent, which was spectroscopically analogous to (but significantly more thermodynamically stable than) the previously reported (PCP)Ir(Ph)(H) complex [8]. In an effort to effect subsequent C-O bond activation, (PCP)Ir(H)(o-C6H40CH3) was heated for 3 h at 90 °C, but this yielded exclusively the cyclometalated product arising from activation of the methoxy C(sp )-H bond (Scheme 4.7). 

Intramolecular C-H oxidative additions have been common for many years. These reactions are known as cyclometallations (or orthometallations when the metal undergoes addition of an ortho C-H bond of the aromatic group of a ligand). Examples are provided in Equations 6.22-6.25. " Cyclometallations of azobenzene are common and constitute some of the earliest examples of C-H activation. Two examples with Co2(CO)j and Cp Ni are shown in Equation 6.22 the product from the reac-... 

It is observed that the presence of a CF3 group at the arene mefa-position leads to a fast reaction likely by increasing the acidity of the ortho C-H bond thus favouring the ruthenacycle intermediate formation. The proposed mechanism, as for direct arylation of phenyl pyridine C-H bonds, involves initial cyclometalation, via carbonate-assisted C-H bond deprotonation, successive oxidative addition of CICO2R or CICONR2 and reductive elimination forming a C-CONu bond. 

Reaction of (TMEDA)2Nb 2Cl5Li(TMEDA) with the amide Ph2NK leads to the formation of several compounds including the Nb anion (85a) and neutral, diamagnetic Nb° (85b). (85a) is the product of oxidative addition (see Oxidative Addition) of two Nb centers to the ortho C-H bond of a Ph group. (85a) then undergoes reductive elimination (see Reductive Elimination) to restore the C-H bond to form (85b). ... 

In parallel with the directed hydroarylation of olefins, a series of papers described the formation of ketones from heteroarenes, carbon monoxide, and an alkene. Moore first reported the reaction of CO and ethylene with pyridine at the position a to nitrogen to form a ketone (Equation 18.28). Related reactions at the less-hindered C-H bond in the 4-position of an A/-benzyl imidazole were also reported (Equation 18.29). - Reaction of CO and ethylene to form a ketone at the ortho C-H bond of a 2-arylpyridine or an N-Bu aromatic aldimine has also been reported (Equations 18.30 and 18.31). Reaction at an sp C-H bond of an N-2-pyridylpiperazine results in both alkylative carbonylation and dehydrogenation of the piperazine to form an a,p-unsaturated ketone (Equation 18.32). The proposed mechanism of the alkylative carbonylation reaction is shown in Scheme 18.6. This process is believed to occur by oxidative addition of the C-H bond, insertion of CO into the metal-heteroaryl linkage, insertion of olefin into the metal-acyl bond, and reductive elimination to form the new C-H bond in the product. 

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

The following compounds with H—C and II—M bonds undergo oxidative addition to form metal hydrides. This is examplified by the reaction of 6, which is often called ortho-metallation, and occurs on the aromatic C—H bond at the ortho position of such donar atoms as N, S, 0 and P. Reactions of terminal alkynes and aldehydes are known to start by the oxidative addition of their C—H bonds. Some reactions of carboxylic acids and active methylene compounds are explained as starting with oxidative addition of their O—H and C—H bonds. 

The C-H activation step could, in principle, occur either by oxidative addition of the C-H bond - pathway (a) - or by electrophilic displacement - pathway (b). The oxidative addition pathway would proceed via the formation of a palla-dium(IV) species. Although such intermediates have been postulated in some coupling reactions catalyzed by palladacycles, as yet no conclusive experimental evidence has been presented [14], It is perhaps more likely that C-H activation results from electrophilic displacement of the ortho proton [15]. 

Iridium has been found to be a very robust late transition metal which can mediate or catalyze C—H bond activation reactions very efficiently. However, the highly enantioselective Ir-catalyzed C—H bond functionalization via a transient C—Ir species for the construction of C—C or C—X bonds only emerged recently. Mechanistically, the catalytic cycle starts with oxidative addition of the Ii catalyst to the inert C—H bonds (such as aromatic, olefinic, or aliphatic C—H bonds), which are usually assisted with an ortho directing group. Subsequently, the formed C—Ir species inserts into an unsaturated functionality such as alkene, alkyne, or imine, delivering a new C—Ir speeies. Finally, the reductive elimination releases the products and regenerates the Ir catalyst. 

A series of arylations of olefins by C-H bond cleavage without direction by an ortho functional group has also been reported, and these reactions can be divided into two sets. In one case, the C-H bond of an arene adds across an olefin to form an alkylarene product. This reaction has been called hydroarylation. In a second case, oxidative coupling of an arene with an olefin has been reported. This reaction forms an aryl-substituted olefin as product, and has been called an oxidative arylation of olefins. The first reaction forms the same t)q)es of products that are formed from Friedel-Crafts reactions, but with selectivity controlled by the irietal catalyst. For example, the metal-catalyzed process can form products enriched in the isomer resulting from anti-Markovnikov addition, or it could form the products from Markovnikov addition with control of absolute stereochemistry. Examples of hydroarylation and oxidative arylation of olefins are shown in Equations 18.63 - and 18.64. ... 

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