Arenes cyclometallation

Electron-rich carbyne complexes can react at the carbyne carbon atom with electrophiles to yield carbene complexes. Numerous examples of such reactions, mostly protonations, have been reported [519]. Depending on the nucleophilicity of the carbyne complex, such reactions will occur more or less readily. The protonation of weakly nucleophilic carbyne complexes requires the use of strong acids, such as triflic [533], tetrafluoroboric [534] or hydrochloric acid [535,536]. More electron-rich carbyne complexes can, however, even react with phenols [537,538], water [393,539], amines [418,540,541], alkyl halides, or intramolecularly with arenes (cyclometallation, [542]) to yield the corresponding carbene complexes. A selection of illustrative examples is shown in Figure 3.25. 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Binuclear [RuX2(arene)]2 (1) and mononuclear RuX2(L) (arene) (3) derivatives have been shown to be useful precursors for access to alkyl-or hydrido(arene)ruthenium complexes. The latter are key compounds for the formation of arene ruthenium(O) intermediates capable of C—H bond activation leading to new hydrido and cyclometallated ruthenium arene derivatives. Arene ruthenium carboxylates appear to be useful derivatives of alkyl-ruthenium as precursors of hydrido-ruthenium complexes their access is examined first. 

Under the conditions leading to RuH(Cl)(L)(arene) complexes, the derivative 105 gives the cyclometallated compound 106 (55) [Eq (10)]. The bidentate complex 108 is formed from the evolution of the methyl derivative 49 in solution (Scheme 6). The reaction is consistent with ethylene... 

Cyclometalation occurs with other kinds of C-H bonds besides arene C-H bonds. The 8-methylquinoline ligand and its derivatives provide many such examples of metalation at sp -carbon atoms. Treatment of 8-methylquinoline with Li2PdCLt, followed by addition of PEt3 to split the chlorine bridge, provides the cyclometalated complex (equation 69). 

Biaryls can also be obtained by oxidation of arenes, as long as these are electron-rich enough. Some very short dinuclear complexes have been obtained in this way. Thus, a bis-cyclometallated dinuclear complex has been obtained by a smooth oxidation of a nonfunctionalized precursor by silver(I) ions (Figure 7b) [17a]. 

Many types of aromatic substrate are known to undergo a cyclometallation reaction when exposed to alkylpentacar-bonylmanganese complexes under thermal conditions. It is well established that the treatment of ligand appended arenes with alkylmanganesepentacarbonyl complexes can lead to the formation of [C,Y] heterochelates of Mn(CO)4 (Y being a two-electron donor ligand) (Equation 6). For instance, aromatic compounds such as W,W-dimethylbenzyl-amine, alkyl benzyl thioethers, 2-phenylpyridine, acetophenone, benzaldehyde, and diazobenzene can be readily... 

A number of research groups have developed rhodium-catalyzed methods for the C-H bond arylation of arenes that likely proceed through cyclometalated intermediates. Aryl phosphites and phosphinites undergo facile orthometalation... 

An important catalytic process that relies on cyclometalation is the Murai reaction [136]. This involves heteroatom directed cyclometalation of an arene followed by insertion of an alkene and reductive elimination to give a net alkylation of the arene. The most common catalyst is RuH2(CO)(PPh3)3. An example of transformation brought about by this catalyst is shown in Eq. 2.51. 

These cyclometalation products also are used as catalysts. The borations of arenes proceed in high yields, for example, as shown in Eq. (4.8). On the other hand, dehydrogenation proceeds without the use of an acceptor such as t-butylethylene (see Eqs. 8.33, 8.38, and 8.39), as shown in Eq. (4.9). The TON 68 reaction proceeds without use of an acceptor [25]. 

Iwasawa et al. [21] also reported chelation-assisted reactions in an article entitled Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with CO2 via Chelation-Assisted C-H Bond Activation, in which the cyclometalation reactions proceed easily and form cyclometalation intermediates. The metal atoms are active centers in their intermediates. Hence, the active metal atom reacts easily with inert carbon dioxide to give carboxylic acid derivatives. Examples include the cyclometalation of 2-phenylpyridine as a substrate in the presence of a rhodium intermediate. Carbon dioxide can be inserted into the rhodium-phenyl carbon bond, and a methyl ester is formed with TMSCH2N2 from a rhodium carboxylate, as shown in Eq. (6.5). The reaction mechanism is proposed as shown in Scheme 6.2 [21]. 

It is well documented [95] that direct halogenation of arenes, bromination in particular, is one of the most selective electrophilic reactions yielding almost exclusively para-substituted products. Evidently, however, the use of cyclometalated compounds might drastically change the selectivity in favor of ort/io-halogenated compounds, as shown in Eq. (7.43) [76, 96]. This halogenation is also widely applied as a regioselective reaction method, as shown below in Eq. (7.44) [97]. 

The coordination chemistry of this anionic, cyclometallated precatalyst and its derivatives, along with its stoichiometric reactions with anthracene, have been reported by Halpem. On the basis of these studies, a plausible mechanism can be proposed for the hydrogenation of polycyclic arenes by this catalyst. Although kinetic studies of the complete catalytic cycle have not been reported, Halpem s investigation of the stoichiometric reactions of this system provides strong evidence that the reaction occurs by a series of steps involving soluble complexes. 

Reductive elimination of ethane from five-coordinate Pt(IV) alkyl complexes has also led to the generation of three-coordinate complexes that have shown catalytic activity in the hydroarylation of olefins. In contrast to the f-Bu or Ph substituted pypyr ligands which underwent facile cyclometalation and trapping with ethylene (Scheme 7), when the Me-substituted ( pypyr)PtMe3 (4c) was heated in benzene solvent under a pressure of ethylene, ethyl benzene product was produced with a TON of 26 [94]. Other combinations of arenes and olefins were also observed to yield hydroarylation products when ( pypyr)PtMe3 complex 4c was used as a catalyst precursor. Presumably C-C reductive elimination of ethane is followed by C-H activation of the arene, reductive elimination of methane, and then... 

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