Carbon Bond Cleavage Reactions

Takahashi T, Kanno K (2005) Carbon-Carbon Bond Cleavage Reaction Using Metallocenes. 8 217-236... 

Shyadehi AZ, DC Lamb, SL Kelly, DE Kelly, W-H Schunck, JN Wright, D Corina, M Akhtar (1996) The mechanism of the acyl-carbon bond cleavage reaction catalyzed by recombinant sterol 14a-demethyl-ase of Candida albicans (other names are lanosterol 14a-demethylase, P-450]4p, and CYP51). J Biol Chem 271 12445-12450. 

Elimination of an alkoxy group or of a halogen in the case of zirconacyclopentenes has been investigated in combination with the (3, (3-carbon—carbon bond-cleavage reaction. As shown in Eq. 2.73, an OR group or a halogen at a (3-position is eliminated and trapped by the zirconium metal center [53],... 

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

Out of a number of preparative routes described in Section II for preparing metal alkoxides, those involving (a) the metal-carbon bond cleavage reaction (Section II.F), (b) the chloride-alkoxide exchange reaction (Section II.C. 1), (c) the amido-alkoxo exchange reaction (Section II.E), and (d) the alcoholysis reaction (Section II.D) appear to be more convenient and versatile. Schemes 1-6 summarize some of the typical reactions studied for the preparation of metal complexes of alcohol ligands with large steric requirements. 

Two additional intermediates, the ferric peroxy anion and ferric hydroperoxo complex, have been proposed to substitute for the ferryl as the actual oxidizing species in at least some P450 reactions. The role of the ferric peroxy anion in some reactions is supported by good evidence and is discussed in the section on carbon-carbon bond cleavage reactions (see Section 8), but the proposed role of the ferric hydroperoxide in electrophilic double bond and heteroatom oxidations is discussed here. 

A carbon-carbon bond cleavage reaction of pyrimidinediones of type 74 has been described [93CPB(41)2073). 

The pioneering work on the calibration of intramolecular cy-clization of the 5-hexenyl radical by Ingold and co-workers provided the basis for the development of a large number of radical clocks." These are now used both for the calibration of rate constants for intermolecular radical reactions and as mechanistic probes to test for the intermediacy of radical intermediates in a variety of processes. Furthermore, the ready availability of bimolecular rate constants from competitive product studies using free radical clocks without the use of time-resolved experiments has greatly enhanced the synthetic utility of free radical chemistry. The same concept has recently been extended to radical ion chemistry. For example, rate constants for carbon—carbon bond cleavage reactions of a variety of radical cations and anions derived from substituted diarylethanes have been measured by direct time-resolved techniques. " ... 

Cleavage, as depicted, would give a PPhj cobalt complex. Such PPh3 would only build up in solution to a small degree since it would compete with the resin for bonding to the Co and would itself be susceptible to the phosphorus-carbon bond cleavage reaction. 

This rearrangement was first reported by von Liebig in 1838. It is the conversion of benzil (a-diketone) into the salt of a-hydroxy acid by means of base treatment and is generally known as the benzilic acid rearrangement or benzil-benzilic acid rearrangement. It is one of the oldest and most widely studied carbon-carbon bond cleavage reactions" and... 

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