Carbon-oxygen bond cleavage, attack

Our first thought is that two different mechanisms are involved here, SnI and S 2. But the evidence indicates pretty clearly that both are of the Sn2 type cleavage of the carbon-oxygen bond and attack by the nucleophile occur in a single step. (There is not only stereochemical evidence—complete inversion—but also evidence of several kinds that we cannot go into here.) How, then, are we to account for the difference in orientation—in particular, for 8 2 attack at the more hindered position in acid-catalyzed cleavage ... 

This observation clearly indicated that the substitution reaction followed the bimolecular (Sn2) mechanism. Nucleophilic attack on sulfonate esters may occur via two possible pathways either with carbon-oxygen bond cleavage (Equation 17) or by cleavage of the sulfur-oxygen bond (Equation 18). 

Several studies have been performed on the photodecomposition of diaryl sulfones and polysulfones Khodair, et. al., (21) demonstrated that the photodecomposition of diaryl sulfones proceeds by a free-radical mechanism with initial carbon-sulfur bond cleavage. This gives an aryl radical and an aromatic sulfonyl radical. The latter radical can react with oxygen and a hydrogen donor to eventually form the hydroxyl radical. The hydroxy radical may attack the aromatic nucleus in PET and forms the hydroxyterephthaloyl radical. 

Electron transfer from Me2C=C(OMe)OSiMe3 to Q is made possible by the strong interaction between Q" and Mg + (or 2Mg +) to produce the radical ion pair. Since the spin of the ketene silyl acetal radical cation is mainly localized on the terminal carbon atom [229], the carbon -oxygen bond is formed before the cleavage of the Si-0 bond to yield the adduct (Scheme 15). This contrasts with the 1,2-addition of nonsubstituted ketene silyl acetal [H2C=C(OEt)OSiEt3] via nucleophilic attack to the positively charged carbonyl carbon of the quinone rather than via an alternative electron transfer pathway [228]. 

On the other hand, with aryl sulfonates (58) (R = aryl), nucleophilic substitution occurs with preferential sulfur-oxygen bond cleavage since aryl substituents show little tendency to undergo nucleophilic attack (Scheme 44). The relative order of nucleophilicity towards the sulfur atom is similar to that obtaining at a carbonyl carbon atom4b and is reported to be as shown in Figure 3. 

If the other major product, 13, arises by nucleophilic attack of 14 on 12, it represents, as pointed out by Laughlin (207), an unusual mode of cleavage of a carboxylate ester. A neighboring group effect may be operative. Although elevated temperature (250°) is required for reaction of dodecyl acetate with dimethyl methylphosphonate, cleavage of the carbon-oxygen bond also occurs (207). 

The only leaving group in the substrate is bromide. Neither of the carbon-oxygen bonds is susceptible to cleavage by nucleophihc attack. 

Scheme 6.25. A representation of the oxidative hydroboration of 1-methylcyclopentene (as a typical alkene). The addition of boron and hydride (H ) is suprafacial with the double bond attacking the boron so as to build up charge on that carbon of the double bond that would be the most stable carbocation it is to this carbon of the double bond that the hydride (H") adds. In the oxidation step, the anion of hydrogen peroxide (H02 ) attacks the boron. Subsequent rearrangement of carbon to oxygen with oxygen-oxygen bond cleavage produces the alcohol. Only one ligand to boron is shown to emphasize the oxidation process.

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