Benzyne, transition metal complex

The rich addition and insertion chemistry exhibited by 5 is unusual for late transition-metal complexes and, as mentioned, more closely resembles the reactivity of benzyne complexes of electrophilic d° metals. 

Transition-metal complex of benzyne, It is well known that some very unstable compounds (e.g., cyclobutadiene) can be isolated as complexes with transition... 

An important use of transition metal complexation has been in the stabilization of reactive molecules including cyclobutadiene, trimethylene-methane, o-xylylene, carbenes, carbynes, and others. In many cases, release of the ligand by chemical means has proven possible, making the complexed precursors valuable as shelf-stable sources of the reactive species for structural study or synthetic deployment. There also has been considerable interest in analogous complexes of benzyne, other strained cycloalkynes, and highly reactive acetylenes. 

Benzynes are highly strained molecules, which are recognized as useful intermediates in organic synthesis.44 They can be isolated by coordination to transition metals.45 Similar to the reaction of the cyclohexyne species 66, Ni-benzyne complex such as 85 reacted with C02 to give the corresponding five-membered oxanickelacyle complex 86 (Scheme 31).46... 

The tantalum-benzyne complex (130) is much less reactive than other early transition-metal aryne complexes. It shows no reaction with acetone, benzophenone, benzaldehyde, acetonitrile, 3-hexyne, or methanol. The lack of reactivity of 130 was attributed to nonlability of the PMe3 ligand. Indeed, no phosphine exchange was observed when 130 was mixed with an excess of PMe3-d9. Refluxing 129 in a mixture of methanol and toluene (3 10 v/v) leads to clean formation of 131. This presumably results from reaction of a 16-electron benzyne complex with the alcohol. 

Eisenberg et al have reported studies on the reaction of the benzyne-hydride complex (Cp 2Ta(CgH4)H) with p-Hj [24]. At 273 K, the complex reacted to form a new phenyl dihydride complex, Cp 2Ta(CgH5)H2, which yielded enhanced hydride resonances with p-Hj. This complex went on to eliminate benzene and react with H2 to form the trihydride complex Cp 2TaH3- The central component of the proton resonance due to the central hydride ligand was again absent under PHIP conditions. This report is especially noteworthy because it represented the first example of PHIP at an early transition metal centre. 

Exchange reactions of aromatic compounds catalyzed by base are also well-documented. The generation of benzyne intermediates by loss of HX (or DX) from haloaromatics in the presence of a strong base such as NH2 (or NDi) is responsible for H—D exchange. Complexation of aromatics to zero-valent transition metals to give complexes such as 7i-C6H6Cr(CO)3 enhances the acidity of the protons of the complexed aromatic and permits base-catalyzed exchange. 

The triple bond in ortHo-benzyne can be stabilized by complexation with transition metals. Aryne-metal complexes were originally proposed as intermediates in the decomposition of various aryl derivatives of early transition metals, and the first fuUy characterized mononuclear ortho-benzyne complex, TaMe2(q -C5Me5) (q -CjH4), was prepared. Although this method does not appear general for all transition metals, various complexes of zirconium, rhenium, and niobium have been characterized. More recently, complexes of nickel and platinum have also been... 

Like strained olefins, strained alkynes, some of which are unstable when free in solution, form stable complexes with transition metals. Cycloheptyne, cyclohexyne, cyclopentyne, and benzyne complexes have aU been prepared. The lengths of the coordinated and uncoordinated C-C bonds of most benz)me complexes are equivalent. Thus, the structures and reactions of the benzyne complexes are more like those of arene dianions than those of a fragment containing a strained C-C triple bond. In other words, these complexes are better described by resonance structure B in Figure 2.27 than by resonance structure A. 

On the other hand, carboryne, l,2-dehydro-t -carborane, is a three-dimensional relative of benzyne (Fig. 7.1) [6]. It can react with alkenes, dienes, and alkynes in [2-1-2], [4-1-2] cycloaddition and ene-reaction patterns [7], similar to those of benzyne [8], Although these reactions show the potential for the preparation of functionalized carboranes in a single operation, they are complex and do not proceed in a controlled manner. In view of the spectacular role of transition metals in synthetic chemistry, we envisage that the aforementioned reactions may work efficiently and in a controlled way with the help of transition metals. In this connection, we initiated a research program to develop transition-metal-mediated/catalyzed synthetic methodologies for the functionalization of carboranes. This chapter summarizes the recent progress in this research area. 

Like benzyne, carboryne can be trapped and stabilized by transition metals. Salt metathesis between Li2C2BioHio and metal halides is a useful method for the preparation of metal-carboryne complexes [10]. Density functional theory (DFT) calculations suggest that such bonding interactions between the metal atom and carboryne ligand are best described as a resonance hybrid of both the M-C a and M-C n bonding forms (Fig. 7.2) [11], similar to those described for metal-benzyne complexes [12]. 

Transition-metal-alkyl bonds can be formed by a variety of reactions that include metathetical replacement of a halide ion, oxidative addition, and insertion of an alkene into a metal-hydride bond. " A similar set of reactions is available for the synthesis of transition-metal-aryl bonds, although the analogous insertion of a benzyne intermediate into a metal-hydride bond is not particularly viable as a synthetic route. For alkyl complexes that have longer chains than methyl, thermal decomposition to give the metal-hydride complex by a j5-hydrogen transfer reaction is frequently observed at ambient temperature. 

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