Abstraction-recombination insertion

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. 

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). 

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. 

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). 

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 ... 

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). ... 

Singlet methylene is much more reactive than triplet methylene. In fact, methylene is more stable in its triplet ground state by some 9 kcal mol than its singlet state." " Apart from the well-known reaction of olefins, methylene indiscriminately inserts into C-H bonds, " From the time of the original discovery of this latter reaction by Meerwein et al." and Doering and coworkers," a question has remained whether the mechanism of the transformation is a direct concerted reaction [Eq. (6,126)] or a two-step abstraction-recombination pathway [Eq. (6.127)] ... 

The olefinic products which formally correspond to C—H insertion reactions are thought to arise by stepwise abstraction of hydrogen by triplet carbene and subsequent recombination ... 

Whether the reaction proceed by the concerted insertion into the C—H bond as in [1 ] or by abstraction and recombination as in [2] depends on whether the carbon atom of the attacking methylene impinges on the C—H bond or on the hydrogen atom to be transferred ... 

The four hitherto known routes of the C-H insertion are shown in Scheme 1. In general, the insertion by singlet carbenes proceeds via route a in one step, whereas the reaction by triplet carbenes proceeds sequentially via route b, i.e., hydrogen abstraction followed by recombination of the radical pairs.4 Other stepwise mechanisms are hydride abstraction (route c) and proton abstraction (route d), both being followed by the recombination of ion pairs. However, extended study on routes c and d for synthetic purposes had not been done before we started, except for a few earlier studies on carbanion-promoted P C-H insertion reactions.5,6 Recent advances in transition metal-catalyzed... 

E.Z-Selectivity in the insertion by unsymmetrical carbenoid 24, is specifically indicative of the transition state of the stepwise mechanism. Based on the evidence that carbenoid 24, which is generated from 42 or 43 (E Z = 84 16), exists nearly exclusively in the -configuration under the equilibrium even at —95°C,29 the observed stereoselectivity for E-isomers in the insertion products verifies that hydride abstraction takes place via an Sn2-like transition state 52 with inversion of configuration at the carbenoid carbon, followed by the recombination of menthone 40 and carbanion 53 (Scheme 19). 

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

Mechanisms [i] and [II] may occur through the recombination of unpaired electrons which are formed by the hydrogen abstraction of nitrenes, and the insertion reaction of nitrenes to C-H bonds, respectively. However, it has not been revealed in this study which C-H bond is attacked by the nitrene. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

---