Abstraction-recombination mechanism

A/E) (Roth, 1972b). In contrast, no polarization is observed in the absence of a sensitizer, despite the fact that ethylbenzene is produced. This is consistent with a direct insertion of singlet methylene into the C—H bond, but it could also arise from an abstraction-recombination mechanism if the lifetime of the intermediate radical-pair were too short to permit a significant amount of Tq-S mixing. 

C/H-insertions have been reported to occur in copper-catalyzed reactions between diazomalonates and cyclohexene as well as some alkylated derivatives 9,57. Some acyclic alkenes behave similarly9, but not so 1,1-dicyclopropylethylene150), An abstraction/recombination mechanism via intermediates of type 103 has been proposed53 which would account not only for the three insertion products 104-106... 

The mechanism of the insertion is not clear, however, since both carbenes have triplet ground states, an abstraction-recombination mechanism with radical pairs as intermediates is most likely. The only other triplet carbene that has been reported to insert into CH4 in low temperature matrices is methylene.89,90 However, in this case it is not completely clear if the insertion is a thermal or photochemical reaction. 

The time-resolved, chemical behavior of FL depends on the solvent. Irradiation of DAF in cyclohexane gives FLH The lifetime of FL in cyclohexane is 1.4 ns, and the ratio of products obtained (26) indicates that both direct insertion and abstraction-recombination mechanisms are operating (Griller et al., 1984b). Replacement of the cyclohexane by its deuteriated counterpart reveals a kinetic isotope effect of ca 2 (Table 5). 

However, coUisional deactivation in solution is so effective that no vibration-ally excited species is present. The reaction of photochemicaUy generated methylene with 2-methylpropene-l-)- C yields, 2-methyl-butene, which is formed by allylic insertion. In the liquid phase 2 % of the rearranged product labeled in the 3-position are formed, whereas in the gas phase 8% of this olefin can be isolated. This can be interpreted as follows 4% of 2-methyl-butene in solution and 16% of 2-methyl-butene in the gas phase are formed by an abstraction-recombination mechanism involving triplet methylene 96). 

Evident cases of abstraction/recombination mechanism are observed with phenylsubstituted carbenes. Diphenyl-diazomethane, which is photolyzed to give the triplet diphenyl-carbene, very readily abstracts a hydrogen atom from the benzyl group of toluene. The primarily formed radicals can now recombine to give a formal "insertion product — 1,1,2-triphenylethane — or they can recombine to form 1,1,2,2-tetraphenylethane and 1,2-diphenylethane... 

The deuterium kinetic isotope effect for intramolecular CH insertion of the nitrene (87), generated by photolysis of the corresponding azide, is 14.7 0.3 at 20 °C and is consistent with the H-abstraction-recombination mechanism from the triplet state. The temperature dependence of the kinetic isotope effect suggests that quanmm mechanical tunnelling is important in this process. 

Other differences between singlet (concerted) insertion and triplet (abstraction-recombination) carbene insertion are seen in selectivity, stereochemistry, and the kinetic deuterium isotope effect. The triplet states are more selective in C—H insertion than the singlets. For example, the triplet shows higher tertiary to primary selectivity than the singlet in the insertion reaction with 2,3-dimethylbutane. Singlet carbene is shown to insert into C—H bond with retention of configuration, while racemization is expected for triplet insertion reaction from the abstraction-recombination mechanism. For example, the ratios of diastereomeric insertion product in the reaction of phenylcarbene with roc- and mcTO-2,3-dimethylbutanes are 98.5 1.5 and 3.5 96.5, respectively. ... 

For exumpte. an abstraction recombination mechanism has been demonstrated for the d iplieny leaf bene-propene insertion reaction (291 ... 

A large isotope effect supports the hydrogen abstraction-recombination mechanism involving the triplet state nitrene II-2. 

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