Product studies triplet carbenes

Some of the earliest studies of triplet carbenes in frozen media by epr spectroscopy revealed that these intermediates react rapidly with molecular oxygen (Trozzolo and Gibbons, 1967). This should not come as a surprise since the combination of a triplet carbene with triplet oxygen is a spin-allowed process. Indeed, recent measurements show that this reaction proceeds with a rate that is approximately at the diffusion limit. The product of this reaction (17) is the expected carbonyl oxide (Werstiuk et al., 1984 ... 

Since the half-life estimated by dynamic ESR studies is in the order of 10 min, we assume that the first-decaying component is assigned to the triplet carbene, while the slower decaying one is assigned to a secondary product formed during the decay pathway of the triplet carbene. Thus, a triplet carbene having a lifetime over several minutes was realized for the hrst time. ... 

There are a series of photochemical investigations in which product ratios indicate that, in some cases, it is neither the singlet nor the triplet carbene in their ground states that forms certain products with a given substrate. The results indicate that photoexcited diazo compounds or carbenes in excited states may react. Evidence for such mechanisms came hitherto only from product studies, some of these cases will be mentioned only in the sections on the corresponding products. Potential reactive intermediates in carbene formation are summarized in Scheme 8-11. The latter includes the excited singlet and triplet carbenes, as well as the formation of triplets by triplet sensitizers such as benzophenone or thioxanthone ( Sens) and the direct formation of products from the excited diazo compound with the substrate S . Product... 

Direct generation of triplet carbene from the diazo compound is usually by irradiation with a triplet photosensitizer. Although the distinctions between reactions of singlet carbenes and triplet carbenes can be clearly made , product studies are always subject to the limitation that the final product depends on the relative speeds of singlet-triplet interconversion and the rates of the reactions of these species to yield the observed products. 

Until the last decade, product studies formed the main evidence for carbene formation singlet carbenes formed cyclopropanes from alkenes stereospecifically, while triplet carbenes formed cyclopropanes non-stereospecifically. Formation of a cyclopropane (though not by addition to an alkene) via a carbocation route was demonstrated and, more recently, it has been shown that p values for insertion-addition selectivity and for cyclopropanation stereoselectivity vary as to photochemical or thermal generation of the carbene. The authors of this latter study suggest that a ground state diazo compound could be masquerading as a carbene in its thermal reaction with olefins, possibly by electrocyclic... 

Laser flash photolysis has, as usual, illuminated the problem. Jones and Rettig photodecomposed 9-diazofluorene (188) in hexafluorobenzene and cw-4-methyl-2-pentene mixtures, and showed that the degree of stereoselectivity in the cyclopropane products depended on the concentration of the alkene. Laser flash photolysis showed that the first detectable intermediate in the photolysis reaction is the triplet carbene, and suggests that the product studies are consistent with initial formation of a singlet fluorenylidene which has an extremely short life (less than 5 ns) before forming the triplet. The singlet can be trapped only by high alkene concentrations while the more stable triplet is easily trapped. 

Actually, in spite of those highly favorable structural factors, 54 is very ephemeral [68] its lifetime in degassed benzene is 0.5 is [69] which is shorter even than that of parent triplet DPC. Product analysis studies have shown that 54 forms a trimer of dianthrylcarbene (55) as the main product (50-60 %) (Scheme 19). [70] The structure of the trimer is characterized as the one formed as a result of a three-fold coupling at position 10 of the carbene in a cyclic way. This observation suggest that delocalization of the unpaired electrons in 54 leads to their leaking out from the carbene center to position 10, where sufficient spin density builds up for the trimerization to take place. At the same time, the lack of formation of olefin-type dimers through coupling two units of 54 at their carbene centers indicates that the carbene center itself is indeed well shielded and stabilized. 

EPR studies of diphenylmethylene and a number of other arylmethylenes have indicated that these carbenes have triplet ground states.<30) Photolysis of diphenyldiazomethane in olefin matrices results in the formation of triplet diphenylmethylene, which undergoes primarily abstraction reactions with the olefins. Cyclopropanes are produced as minor products. 

The chemical behavior of PM is characteristic of a carbene for which equilibration of spin-states is faster than irreversible reaction. In particular, direct and triplet-sensitized irradiations of 2-n-butylphenyldiazomethane give exactly the same products (Baer and Gutsche, 1971). A similar conclusion was reached in the study of ANM (Hadel et al., 1983). 

Although the equilibration of singlets and triplets has been many times inferred from classical studies of reaction products, and taken as a given by the community, this tour de force work for the first time allows the direct observation of the many interrelated reactive intermediates involved when a carbene precursor is irradiated. 

Fluorescence quenching studies can establish the rate constant at which a certain substrate interacts with the excited carbene, but they cannot provide any independent mechanistic information. Absorption studies are somewhat more informative in that the primary product of reaction can sometimes be detected directly. In the reaction of di-p-tolylcarbene with CCI4, the radical, (MeC6H4)2CCl, obviously formed as a result of abstraction of Cl atom from the substrate, is detected. Its formation can be monitored to give a rate constant of 1.1 x 10 M s for the excited state, which should be compared with a rate of 2 x 10 M s for ground-state triplet DPC. ... 

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