Abstraction recombination

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. 

Singlet Carbene C-H Insertions Although [1,2]-H shifts are formally carbene C-H insertions, these rearrangements have different orbital symmetry aspects than those of intramolecular insertions. As described above, overwhelming evidence exists that triplet carbenes undergo abstraction-recombination reactions to... 

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. 

Alternatively, if some of the reaction occurs by the hydrogen abstraction-recombination route proceeding through free radicals, then some mono-deuteriated (crossed) products will be formed (14). The former result is taken to be characteristic of the reaction of a singlet carbene, the latter that of a triplet. 

Xanthylidene does not react measurably with cyclohexane at room temperature (Table 5). Thermolysis of DAX at high temperature does, however, give some of the expected coupling product 9-cyclohexylxanthene. The crossover experiment (1 1 C6H12, C6D12) reveals that this product is not formed by the abstraction-recombination sequence. This observation is consistent with the direct insertion characteristic of a singlet carbene. 

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). 

Triplet carbenes react exclusively according to mechanism a), by abstraction-recombination involving single bonds such as C—H and C—Cl. 

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 triplet carbenes fluorenylidene 20 and anthronylidene 24, which can be generated from the diazoalkanes by photolysis, show a similar behaviour. Fluorenylidene in cyclohexane yields 9-cyclohexenyl-fluorene 27, and 9,9 -difluo-renyl 22 which are clearly formed by an abstraction-recombination process Another example is anthronylidene 24 in cyclohexane or toluene, which yields the products 25,26,27 resulting from an abstraction-recombination process Benzene, on the contrary, failed to give the radical pair product 20... 

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. 

As predicted, in reactions of 3-diazopyrroles, 3-diazoindoles, and 4-diazopyrazoles with benzene derivatives, azolylidene 35 also reacted in its triplet state to give the parent heterocycle by abstraction-recombination processes. 

Photochemical abstraction/ recombination can be used to effect direct five-membered ring formation. This reaction can be highly diastereoselective, as shown by the photocyclization of 2-(2-ethylphenyl)-1 -phenylethanone1 The diastereoselectivity of radical recombination is influenced by the reaction medium a 20 1 ratio in benzene becomes a 2 1 ratio in methanol. 

Abstraction/recombination reactions are not limited to radical- radical recombination. An intriguing alternative has been demonstrated19, zo, as illustrated by the intramolecular addition of the free radical 2, generated by remote abstraction, to an alkene. As previously observed for other radical cyclizations, there is a significant selectivity for the thermodynamically less preferred entfo-product21 from this cyclization. 

Abstraction/Recombination with Retention of Absolute Configuration... 

It would usually be assumed that abstraction/recombination reactions such as those illustrated in this section would proceed with racemization at the reacting centers. It has been reported22, however, that photocyclization of amide 1 proceeds with complete retention of absolute configuration. Racemization at the site of abstraction requires orbital rehybridization, passing through a planar intermediate. In this case rehybridization appears to be markedly slowed. This may be an electronic effect due to the heteroatom substituent on the intermediate radical, or simply a steric effect. The structure of product 2 was established by X-ray crystallographic analysis. 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

Thus, if a mixture of these hydrocarbons reacts with a singlet state, then the addition product will consist entirely of undeuterated (do) and deuterated(i i2) compounds. If some of the reaction occurs by the hydrogen abstraction-recombination route, then some crossed products (di and/or du compounds) will be formed. By using this technique, it has been shown that, while some of the DPC-cyclohexane adduct (34) is formed by combination of radical pairs, there is no evidence of crossover product present in the FL-cyclohexane adduct (37). The results are interpreted by assuming rapid spin state equilibration relative to the reaction of either spin state with solvent. The larger amount of singlet chemistry of FL relative to DPC can then be explained if the S-T gap is smaller in FL than in DPC. 

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. ... 

By analogy with the mechanism proposed for the reaction with alkenes, C—H insertion product formation can be explained in terms of a H abstraction-recombination process of triplet arylcarbenes. The observations that ground-state singlet carbenes, for example, chlorophenylcarbene (67), produce only O—H insertion... 

There has been a good deal of controversy about the so-called insertion reaction since it is difficult to distinguish between it and the following sequence of abstraction-recombination reactions ... 

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

The numerous minor products can be ascribed to abstraction, recombination, and addition reactions which involve principally CsHs-and CHr radicals. 

The reactions of diphenylmethylene and fluorenylidene with olefinic double bonds are not stereospecific. Photochemical or thermal decomposition of diphenyldiazomethane in the presence of alkenes is often accompanied by the formation of a substantial amount of non-cyclic products derived from abstraction-recombination reactions The extent of hydrogen abstraction relative to addition is highly dependent on the substitution pattern of the olefin In contrast, fluorenylidene generated from 9-diazofluorene usually gives cyclopropanes as the major product. Cyclopentadienylidene and its substituted analogues can be generated from the corresponding diazo precursors. They react with olefinic as well as with acetylenic substrates Cycloheptatrienylidene preferentially... 

Photochemical C—insertion, by abstraction-recombination, can also be used to construct heterocyclic rings. In the example illustrated, l,S-hydrogen atom abstraction leads to the six-membered lactam (117 equation 41). ... 

In some cases, abstraction-recombination can lead to still larger rings. In the example illustrated, 1,8-hydrogen atom extraction leads to the formation of a seven-membered ring product (119 equation 42). The 1,7-diradical would be thermodynamically favored because of the radical-stabilizing ability of the two sulfur atoms. The reaction may not, however, proceed by direct 1,8-abstraction. A 1, S-abstraction followed by a subsequent 1,4-abstraction would give the same product and would be sterically more reasonable. 

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