Triplet carbenes oxygen reactions

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 ... 

The susceptibility of a carbene to reaction with oxygen can be used as a probe of its spin multiplicity. If a carbene is insensitive to oxygen, this is taken to be an indication that it is a ground-state singlet and that the singlet does not readily form the triplet carbene. 

The other advantage of matrix IR spectroscopy is that one can follow the subsequent reaction (either thermally or photochemically) of triplet carbenes relatively precisely, again with the aid of computational methods. " This complementary approach will reinforce the assignment of triplet species. For example, triplet carbenes react with oxygen even at very low temperature to give oxidation products such as carbonyl oxides (see Section 6), which are easUy characterized by comparing the IR spectra with those calculated. 

Molecular oxygen is an important participant in reactions of triplet carbene because of its triplet ground state and its ubiquity as an impurity in reaction... 

The role of spin in the reaction is an especially interesting and important issue in carbene chemistry. Apparent violation of the spin selection rule, such as the reaction of ground state singlet carbene with triplet molecular oxygen in matrices as well as in solution, and the reaction of triplet ground-state carbenes with (singlet) CO, CO2, and N2 in matrices, are challenging issues for the near future. 

Attempts to trap carbenes 3b and 3d with molecular oxygen, a reaction frequently used to characterize triplet carbenes, were successful at high (>5%) concentrations of O2 in the argon matrix22. In both cases, the corresponding silyl formate was detected, and it was assumed that a carbonyl oxide and a dioxirane are intermediates in this carbene trapping... 

Oxygen has frequently been added to reaction mixtures as a trap for radicals (Hamilton and Giacin, 1966) and triplet carbenes (see, for example, Tang and Howland, 1966, McKnight et al., 1967 R. L. Russell and Rowland, 1968). Nitric oxide has also been used in this way (Halberstadt and McNesby, 1967). 

One approach, which has proved revealing in the case of 9-carbena-fluorene and which may be generally applicable, is to examine the influence of additives which can deactivate electronically excited carbenes without undergoing reaction themselves (e.g. hexafluoro-benzene) or which react preferentially with carbenes of a particular spin state (e.g. oxygen and butadiene, which are both thought to react preferentially with triplet carbenes). The technique is analogous to well-... 

Consideration of the reactions outlined in Scheme 2 permits an estimate of AGjt for XA to be obtained. With the knowledge that triplet carbenes react irreversibly with Oj with a rate constant close to the diffusion limit can be expressed as the quotient shown in (24) where is the rate constant for reaction of XA in oxygen saturated solution, and is the rate constant in the absence of Oj. Since no significant difference in these rate constants can be detected, the estimate of is a limit set in part by the experimental uncertainty in k (Table 8). This value, 1 x 10" ... 

Molecular oxygen is a triplet and, therefore, the reaction of 2 is an exceptional case in the sense that triplet carbenes react faster with than singlet carbenes, as... 

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