Abstraction-recombination insertion triplet carbenes

It is not always easy to establish that an authentic insertion reaction has occurred. An alternative route to the same product is available it involves hydrogen-atom abstraction by the triplet carbene, followed by recombination of the fragments ... 

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... 

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 ... 

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 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... 

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... 

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... 

Insertion into C-H bonds is more probable than insertion into C-C bonds. Insertion into C-C bonds does not appear to occur at all. For example, photolysis of diazomethane in cyclopentane at —75 °C produced only methylcyclopentane, with cyclohexane not being observed. Singlet carbenes are thought to add to C- H bonds by a concerted process, while triplet carbenes can produce net addition through hydrogen abstraction and then recombination of the alkyl radicals. It was found that o-(2-endo-norbornyl)phenylcarbene inserts into the 3-position in such a way as to give a trans-junction (Scheme 5.47). 

D. Triplet carbene insertions go via radical abstraction followed by recombination. H- D., 5 hVr... 

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