Cyclometallation reactions, formation

When the phosphonium ylide 81 is reacted with zinc amide, the corresponding a-zincated phosphorus yUde is formed. Thermally unstable, it evolves almost quantitatively to zincatacyclobutane 82 which in presence of pyridine leads to the formation of the zincataphosphoniaindane 83. In order to explain this unprecedented cyclometallation reaction, a mechanism is proposed involving a low coordinated zinc center. The new product, reacted with benzaldehyde leads to the diphenylallene 84 (Scheme 27) [106-108]. 

Reactions that involve significant bond formation in the ratedetermining step are in general expected to exhibit large and negative volumes of activation. This was for instance found for a series of cyclometallation reactions of benzylidenebenzylamines, -anilines, and -propylamine with palladium acetate in toluene and acetic acid solution (171, 172). The cyclometallated compounds are formed via C-H electrophilic bond activation to produce different types of metalla-... 

Although cyclometallation reactions have seldom been observed with d° metal compounds, intramolecular metallation involving early transition metal dialkylamides with formation of azametallacyclopropane, which occurs by /3 H abstraction, has been reported.281... 

The tetrahedral [160-163] and cis-planar [100,134,164-167] structures are characteristic for chelates of type 874 with coordination units NiN4 and MN2S2, respectively, as well as chelates 868 discussed above. Original polyhedral forms were discovered by x-ray diffraction for nickel and palladium ICC of the discussed type 874. It is accepted that, in case of a nickel complex, the compound with a carbon-carbon bond 875 is formed [165,166] formation of palladium chelates is accompanied by the cyclometallation reaction leading to tetranuclear clusters 876, where the tridentate ligand behaves as C,N,S-donor [168]. 

Discovered over a century ago, electrophilic mercuration is probably the oldest known C-H bond-activation reaction with a metal compound. The earliest examples of aromatic mercuration were reported by Volhard (mercuration of thiophene) [1], Pesci (mercuration of aromatic amines) [2], and Dimroth [3], who was the first to mercurate benzene and toluene, generalize the reaction, and assign the correct structures to the products originally observed by Pesci. Since the work of Dimroth electrophilic aromatic metalation reactions with compounds of other metals, for example Tl(III), Pb(IV), Sn(IV), Pt(IV), Au(III), Rh(III), and Pd(II), have been discovered [4], In this chapter, we will focus on intermolecular SEAr reactions involving main-group metal electrophiles and resulting in the formation of isolable metal aryls which find numerous important applications in synthesis [5], Well-known electrophilic cyclometalation reactions, for example cyclopalla-dation can be found in other chapters of this book and will not be reviewed here. 

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... 

Methods of Synthesis. Cyclometallation reactions in the original sense [19] comprise the formation of one cycle per metal through spontaneous... 

The mechanism for cyclometallation was proposed to involve a concerted heterolytic process with hydrogen atom abstraction and metallacycle formation occurring in a four-center transition state. Kinetic and labeUng studies in the cyclometallation reactions indicate that intramolecular y-C-H activation is the rate-limiting step [290]. 

The fifth item concerns agostic interactions, C-H activation, C-X activation, C-H functionalization, chelation-assisted reactions, cross-coupling reactions, etc., which are indicated as titles. The reactions indicated by these titles are mostly related to cyclometalation reactions. The reaction mechanisms of these reactions include metal activation by the coordination of a hetero atom to the central metal atom and the chelate effect of the formation of a five-membered ring. 

These results show very high selectivity in the formation of five-membered rings in cyclometalation reactions, and this high selectivity is understandable based on data retrieved from the Cambridge Structural Database, which are shown in Table 5.3 (selectivity 95 %). 

Verification of the Formation of Intramolecular-Coordination Bonds in Cyclometalation Reactions with Transition Metal Compounds... 

In 1997, Gomez, Granell, and Martinez [6] also reported on similarly easy five-membered ring formation. The cyclometalation reactions of imines with palladium acetate have been studied in toluene solutions, as shown in Fig. 6.2. 

Fig. 6.2 Formation of ring compounds in cyclometalation reactions of imines with palladium acetate [6]...

In cyclometalation reactions, as shown in Eq. (6.7), a central atom is activated by the coordination of lone electron pair of the coordinating atom, such as nitrogen, phosphoras, oxygen, or sulfur. Metal activation 63 and agostic interactions 6.4 consequently occur, and bond formation between the y-carbon atom and the metal atom follows. Final cyclization proceeds via agostic interactions and CH activation through the chelate effect to cause bidentate coordination of the central metal atom with the coordinating atom and the y-carbon atom [23]. 

As for the computer chemistry of cyclometalation reactions, the reaction with the most representative substrate, A,A-dimethylbenzylamine in a palladium compound, was studied. This reaction proceeds very easily, and its intermediate state, or agostic interaction, is therefore not actually isolated. As shown in Scheme 6.3, however, the activation energy for the agostic interaction is only 13 kcal/mol. It may be pointed out that the acyl group assists the formation of the agostic interaction in the reaction, as exhibited by the agostic intermediate 6.6 shown in Scheme 6.3 [24]. 

Generally, 2,2 -bipyridines are representative -chelating ligands that are similar to ethylenediamine and 1,10-phenanthroline as inorganic chelate ligands [30]. Only recently has a new coordinating behavior called a rollover cyclometalation reaction appeared in the literature. A rollover reaction involves y-C-H bond activation and formation of a y-C-M bond, as shown in Eq. (7.7) [31]. 

Two factors are thought to explain why these organometaUic intramolecular-coordination flve-membered ring compounds are very easily synthesized through cyclometalation reactions The flrst is metal activation by the coordination of the lone electron pair to the metal atom, and the second is a chelate effect caused by the formation of a flve-membered ting due to bonding between the metal atom and a carbon atom at the y-position to the hetero atom. 

In 2000, Milstein and coworkers reported the synthesis of the first stable metallo-quinone [48], a compound in which one of the oxygen atoms of quinone has been replaced by a metal. The procedure for the synthesis of such species consisted first in the formation of (PCP)Ru(II) pincer complexes (120a) and (120b) (Scheme 2.54) from the cyclometallation reaction of the PCP ligand 3,5-bis(di-fert-butylphosphinomethy-... 

Reactions of metal complexes whose ligands (typically 0-, N-, P-, S-, and As- donors) contain hydrocarbon groups that are subject to metallation with the formation of metal-carbon o-bonds are called cyclometallation reactions,53,54 Scheme 12.11 ... 

The majority of N- and 0-donor ligands form five-membered rings in cyclometallation reactions. In contrast, S- and P-containing ligands show a tendency to form three- and four-membered rings, especially when formation of a five-membered ring is precluded. 

N7H = 11.57 ppm). While these reactions are rather unusual with regard to metal-nucleobase chemistry, they are in fact examples of classical cyclometalation (97). Such reactions often involve a pre-coordination step, as observed in the formation of 25. 

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