Applications for Cyclometalation Reactions

The author has already reported in detail on cyclometalation reactions in many reviews and three monographs, as described in Chap. 1 and in the previous section. Applications for cyclometalation reactions in this section, therefore, describe only the following three new cyclometalation reactions  

cyclometalation reactions with new types of substrates that are formed during the reactions 

Second, cyclometalation reactions with substrates that are formed by mild oxidation Third, rollover cyclometalation reactions 

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The sixth item concerns applications for both cyclometalation reactions and their products, which are organometallic intramolecular-coordination five-membered ring compounds. The applications for cyclometalation reactions are syntheses of organometallic intramolecular-coordination five-membered ring compounds, and these products and their reaction intermediates are used to obtain their derivatives. Pincer products are also utilized for synthesis of their derivatives. 

Abstract There are two applications for cyclometalation reactions and five-manbered ring products for synthetic purposes. The first is synthesis of five-manbered ring products by cyclometalation reactions. The second is synthesis of the derivatives of five-membered ring products or their intermediates during cyclometalation reactions. Pincer products are also used for synthesis of their derivatives. 

Recently, organosynthetic applications for cyclometalation reactions have been expanding remarkably. Many recent articles include agostic interactions, C-H bond activations, C-X (C-F, C-Cl, C-Br, C-I, C-C, C-O, C-Si, etc.) bond activations, chelate-assisted reactions, and C-H bond functionalizations in their titles. These articles have reported on syntheses of derivatives of cyclometalation products or cyclometalation intermediates. 

As other applications for cyclometalation reaction five-membered ring products, they can be utilized for the compounds for CO2 fixation. For example, the pincer A,A-dimethylbenzylamine-type tin compound 9.44 readily absorbs CO2 at room temperature to yield organotin carbonate 9.45 [95]. Easy desorption and reversible CO2 fixation is achieved. 

The other applications for cyclometalation reaction five-membered ring products include dendrimers [108-112], liquid crystals (metallomesogens) [113-116], resolving agents (chiral auxiliaries) [117-119], and photosensitizers for hydrogen production [120,121]. 

Applications of Cyclometalation Reactions and Five-Membered Ring Products for Synthetic Purposes... 

As applications of cyclometalation reactions for producing organometalUc intramolecular-coordination five-membered ring compounds, two categories of uses are considered. These are applications of cyclometalation reactions and syntheses of the derivatives of their five-membered ring products. 

Applications of cyclometalation reactions for producing organometallic intramolecular-coordination five-membered ring products... 

Many articles on arylations have reported on synthetic applications of cyclometalation reactions, especially. For example. Song and Ackermann reported that the cobalt acac complex reacts with 2-phenylpyridine first to give a cobalt intermediate bonded with the y-carbon atom 7.87, and the intermediate then reacts with an aryl snlfamate to give a final substitntion product in a high yield, as shown in Eq. (7.63) [133, 134, 138, 139]. 

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. 

Ligand exchange has found a wide application in the syntheses of metal chelates. Metal-cycles containing C,N-donor atoms are formed as a result of cyclometallation reactions using ligand exchange. For example, the complex [Pt(DMSO)2Cl2], which is a universal cycloplatination reactant, takes part in such a process (3.104) [276] ... 

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. 

Cyclometalation reactions produce three-, four-, six-, and seven-membered ring compounds in addition to flve-membered ring compounds. Cyclometalation reactions are mainly used, however, for the preparation of flve-membered ring compounds. Since the preparation of flve-membered ring compounds is easily performed, many of these reactions were actually tried using many kinds of metal compounds, and many reaction products are used for convenient applications in various fields. Cyclometalation reactions are therefore widely utilized as reaction methods in recent synthetic organic chemistry. 

As concerns the second category, this monograph presents applications for both hve-membered ring prodncts and their intermediates produced by cyclometalation reactions from the author s two reviews as well as from recent articles. 

Applications of Five-Membered Ring Products in Cyclometalation Reactions for Other Purposes... 

Abstract Applications of five-membered ring products in cyclometalation reactions for other purposes include organic electronic devices, pharmaceuticals, dye-sensitized solar cells, carbon dioxide utilizations, sensors, dendrimers, liquid crystals, resolving agents, and photosensitizers for hydrogen production. 

Fig. 9.10 Some representative cyclometalation reaction five-membered ring products are used for the other applications such as dendrimers, liquid crystals and resolving agnets...

Most cyclometallated compounds of Pt and Pd contain the metals in the + 2 oxidation state (d8 configuration) with its strong tendency for planar coordination. Other oxidation states, notably +4, are also possible. A series of Pt(IV)-cyclometallated complexes have been obtained [55] from Pt(II) compounds through oxidative addition reactions. Details of the photochemical and photophysical properties of these systems are discussed later in this review. Here we restrict ourselves to the discussion of the structural aspects of the Pt(IV) and, as far as applicable, to Pd(IV) compounds. 

The first type of application is already described in Chap. 6. The metal elements in the active metal center react easily with CO2 to give carboxylic acid derivatives. For example, the cyclometalation of 2-phenylpyridine as a substrate in the presence of a rhodium compound proceeds easily to give a five-membered ring rhodium intermediate. CO2 can be inserted into the rhodium-phenyl carbon bond, and a methyl ester is then formed from rhodium and a carboxylate through reaction with TMSCH2N2, as shown in Eq. (6.5) [89]. The reaction mechanism is proposed as shown in Scheme 6.2 [89]. 

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