Alkyl Bond Fission

During the period under review, two very useful textbooks have appeared, by Kochi and by Collman and Hegedus. The reviewer has tried the latter in graduate teaching with good effect. 

A review of metal-alkyl bond cleavage (and other topics) has been made by Kochi.Halpem has discussed attempts to obtain both kinetic and thermodynamic data for M-C a-bond fission. A particular point of interest is the estimation of activation energies for the reverse reaction, equation (1), 

Grate and Schrauzer have found evidence for steric destabilization of the Co-C bond in alkylcobalamins. The rate of decomposition of compounds containing secondary alkyl groups to alkenes and hydridocobalamin (which occurs 

Base-on cobalamins dealkylate considerably faster than base-off, probably owing to the bulk of the base bending the corrin ring toward the alkyl group thereby destabilizing the Co-C bond. 

A 7T-bonded intermediate is postulated by Espenson and Wang to interpret kinetic studies on the decomposition of (p-hydroxyalkyl)cobaloximes to give alkenes. If the formation of the tt compex rather than its destruction is rate determining, which the authors consider is probable, ki has values at 25 C, of 3.09 X 10 and 0.10 s for R = H and CH3, respectively, while 2 (R 

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Sulfones are thermally very stable compounds, diaryl derivatives being more stable than alkyl aryl sulfones which, in turn, are more stable than dialkyl sulfones allyl and benzyl substituents facilitate the homolysis by lowering the C—S bond dissociation energy17. Arylazo aryl sulfones, on heating in neutral or weakly basic media at 100°C, yield an aryl and arenesulfonyl radical pair via a reversible one-bond fission followed by dediazoni-ation of the aryldiazenyl radical (see Scheme 2 below)20. However, photolysis provides a relatively easy method for generating sulfonyl radicals from compounds containing the S02 moiety. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

The effectiveness of micellar control on the rate and stereochemical course of hydrolysis at a saturated carbon atom was found to be fairly striking. Chiral 1-methylheptyl trifluoromethanesulfonate [45] undergoes hydrolysis via alkyl-oxygen bond fission, and the hydrolysis rate was only 1/300 (for CTAB) or 1/350 (for SDS) as fast as the rate in pure water (Okamoto et al., 1975). Interestingly, the 2-octanol formed shows net inversion (70%) in a nonmicellar... 

In reactions between methylmagnesium iodide and O-alkyl S-methyl phenylphos-phonothiolate, the mechanistic pathway, i.e. displacement of OR or SMe, depends on the bulk of the group R (Me, Pr1, or menthyl). The bulkier is R, the more extensive is the P—S bond fission.131... 

Since there are but two possibilities, acceptable evidence may be pos tive or negative. It is sufficient, for example, in order to establish acyl-oxygen fission, to show either that the acyl-oxygen bond is cleaved, or that the alkyl-oxygen bond is not, and both types of evidence have been used. Some of the evidence, for reactions involving alkyl-oxygen fission, has already been discussed in the sections dealing with the AA) 1 reaction (p. 87). 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

Peroxy nitric adds and organic peroxy nitrates are another precursors of free radicals, which may be introduced into polymers from polluted atmosphere. They produce both peroxy radicals and reactive nitrogen oxides (NO and N02) on decomposition. With alkyl peroxynitrates, decomposition proceeds via OO—N bond fission having activation energy 87 kJ/mol, their half-life being several seconds at 0 °C [12]. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

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