Bonding orthometallation

Subtle differences in the behavior of azoarenes toward cobalt carbonyl derivatives are observed in regard to metal-complex formation. Azobenzene is transformed by dicobalt octacarbonyl in processes of orthometallation and carbonyl insertion into 2-phenylindazolin-3-one (see Section IV,D,2). In contrast, cyclopentadienylcobalt dicarbonyl effects N—N bond cleavage, and carbonylation of the isolable complex 88a provides 1 -phenylbenzimid-azolin-2-one (Scheme 106).171... 

Good yields of l-arylaminoquinolin-2-ones are obtained when the azo-arene complexes 116 are treated with hexafluorobut-2-yne (Scheme 141).214 This procedure is particularly novel in the chemistry of orthometallated complexes in that the primary organometallic products (117) arise from the formal insertion of a three carbon unit [CO + C2(CF3)2] into the aryl-cobalt bond of 116. A wider study of the scope of this type of process using a variety of orthometallated complexes would be desirable. 

The addition to a double bond is observed in aromatic substrates where the reaction is assisted by chelation. The initial success of such reactions was achieved with the double alkylation of phenol with ethene (Equation (2)).1 This reaction occurs at the or/ -positions selectively by using an orthometallated ruthenium phosphate complex 1. 

A similar range of reactions has also been reported for the ruthenium carbonyl-triphenylphosphine systems (148). In these systems, a high percentage of the products were dinuclear, reflecting the weaker bonding in the ruthenium system, and as for some of the osmium complexes discussed above, some contain orthometallated phenylphos-phine groups (see Fig. 29, structures I, IV, X). 

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. 

Several chapters deal with the synthesis of and the synthetic applications of organolithium compounds such as orthometallation, arene catalysed lithiation, addition to carbon-carbon double bonds, their reaction with oxiranes, and asymmetric deprotonation with lithium (-)-sparteine. We gratefully acknowledge the contributions of ah the authors of these chapters. 

Aromatic trans-diazenes easily form orthometallated complexes, sometimes accompanied by cleavage of the N=N bond to give the o-semidine containing products. These reactions have been recently reviewed by Bruce and Goodall2a ... 

The agostic interaction is considered to be important in such reactions as a elimination, /1-hydrogen elimination, and orthometalation. For example, in a/i-hydrogen elimination, a H atom on Lhc/i-C atom of an alkyl group is transferred to the metal atom and an alkene is eliminated. This reaction proceeds through an agostic-bond intermediate, as shown below ... 

In a similar way, triscarbene-chelate Ir(III) complexes can be obtained by the reaction of an aryl-enetetramine with [IrCl(cod)]2. In the formation of 7 (Scheme 5) the reaction proceeds by the insertion of the metal into the C = C bond of the olefin followed by spontaneous orthometallation of the N-aryl substituent [6]. 

Complexes 98 [L = PPh3, P(Ph-p-F)3, P(Ph-p-Me)3] react with methyl-lithium to give, after methanolysis, the orthometallated complexes 99 (Scheme 5). Complex 98 (L = PPh3) also leads to 99 by reaction with phenyllithium or Red-Al 54). The formation of 99 suggests that the initial reduction of 98 leads to a 16-electron ruthenium (0) intermediate followed by C—H bond activation as for the transformations of 90 and 91. Treatment of complex 98 (L = P-i-Pr3) with methyllithium produces the cyclo-metallated diastereoisomers 100. Complexes 101 and 102 are obtained by treatment of 98 (L = PPh2-f-Bu) with methyllithium at -78°C and at +70°C, respectively. Complex 101 isomerizes to 102 by a first-order process (k 0.2 hour-1 in C6D6 at 50°C) when L is PPh2-i-Pr 98 leads to 103 which isomerizes to the orthometallated complex 104 54). 

Reactions of C—H bonds attached to donor ligands. These may lead to the formation of rings of various sizes. The orthometallation of aryl substituents is particularly common those involving PPh3 may contribute to catalyst deactivation reactions. The metallation of ligands may be reversible, particularly where 3- and 4-membered rings are formed.117... 

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