Single bond cleaving reaction

In all the alkene addition reactions we ve seen thus far, the carbon-carbon double bond has been converted into a single bond but the carbon skeleton has been left intact. There are, however, powerful oxidizing reagents that will cleave C=C bonds and produce two carbonyl-containing fragments. 

With R = R = Ph and using complexes 1 or 2a, the central N -N single bond of the azine is cleaved by both metals. In this case, the bis(imido) complexes 81 were formed, treatment of which with complexes such as CpCo(C2H2)2 can give heterobimetallic bis(alkylideneamido)-bridged complexes such as 82. Mach has used this concept for the reaction of methyl-substituted titanocenes with acetoneazine. With 3, monomeric Ti(III) complexes 83 and, after activation of the methyl groups, coupled products such as 84 could be obtained [44],... 

Hydrolysis reactions occur by nucleophilic attack at a carbon single bond, involving either the water molecule directly or the hydronium or hydroxyl ion. The most favorable conditions for hydrolysis, e.g. acidic or alkaline solutions, depend on the nature of the bond which is to be cleaved. Mineral surfaces that have Bronsted acidity have been shown to catalyze hydrolysis reactions. Examples of hydrolysis reactions which may be catalyzed by the surfaces of minerals in soils include peptide bond formation by amino acids which are adsorbed on clay mineral surfaces and the degradation of pesticides (see Chapter 22). 

In the propagation steps shown above, the radical propagates a further radical by causing fission of a single bond in the substrate. Many important radical reactions actually involve compounds with double bonds as substrates, and the n bond is cleaved during the radical addition reaction. 

Numerous studies have dealt with the mechanism of metathesis.6,7 14 20-31 The first important question to answer in an understanding of the mechanism is whether the carbon-carbon single bonds or the carbon-carbon double bonds are cleaved during the reaction. The corresponding transformations taking place are transalkylation [Eq. (12.9)] or transalkylidenation [Eq. (12.10)], respectively ... 

Unsymmetrical alkyl phenyl tellurium derivatives were prepared in good yields from phenyl trimethylsilyl tellurium and epoxides, carboxylic acid esters, and linear or cyclic ethers under very mild conditions. In these reactions, which proceed in dichloromethane in the presence of a catalytic amount of zinc iodide, a carbon-oxygen single bond is cleaved. The highly nucleophilic benzenetellurolate binds to the carbon fragment, whereas the trimethylsilyl group becomes linked to the oxygen. 

Further investigations on the field of oxidative bond cleavage even made single bonds accessible. Thus, biaryls 10 and 11 were similarly obtained by electrooxidation of 9,10-dihydrophenanthrene. Moreover, the cleaving reaction of benzylic carbons was also exploited in the synthesis p-tert-butylbenzaldehyde dimethyl acetale (3) starting from l,2-di-(p-ferf-butylphenyl)ethane (4, 1,2-DPTE) (Fig. 5.8) (Zollinger et al. 2004b). 

Fragmentation reactions cleave C-C single bonds by a combination of electron push and electron pull so that both electrons in the bond move in the same direction as the bond breaks. In the next chapter we shall see reactions that break C-C bonds in a quite different way. No electron push or pull is required because one electron goes one way and one the other. These are radical reactions. 

The reaction conditions have been quite vigorous, reflecting the difficulty of cleaving the strained carbon-carbon single bond. The bond which is cleaved is the one best situated for simultaneous overlap with both carbonyl activating groups. 

Usually the C-0 single bond in esters is cleaved at the acyl-0 bond, whereas examples of cleavage at the other point in esters have been reported. An electron-rich iron(O) complex produced on reductive elimination of naphthalene from a hydrido(naphthyl)iron complex undergoes oxidative addition reaction with methyl benzoate to give a methyliron benzoate complex (Eq. 22) [63]. 

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