Diphenylmethylene, triplet

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. 

Diphenylmethylene is certainly the most exhaustively studied of the aromatic carbenes. Low temperature epr spectroscopy (Trozzolo et al., 1962) clearly established the ground state of this carbene as the triplet. The optical spectrum of the triplet was recorded first in a 1,1-diphenylethylene host crystal (Closs et al., 1966) and later in frozen solvents (Trozzolo and Gibbons, 1967). 

Products which can be ascribed to the intermediate formation of radicals have long been observed in carbene reactions. In the gas phase these products could arise by homolytic decomposition of excited primary products before collisional deactivation rather than from radicals generated in the course of insertion. This is not so in solution. It is found that, in the thermal decomposition of diphenyldiazomethane (Bethell et al., 1965) or photolysis of diphenylketene (Nozaki et al., 1966) in toluene solution, the product of insertion of diphenylmethylene into the benzylic carbon-hydrogen bonds, 1,1,2-triphenylethane, is accompanied by substantial amounts of 1,1,2,2-tetraphenylethane and bibenzyl. This is a strong indication that discrete diphenylmethyl and benzyl radicals are formed, and, taken in conjunction with EPR (Section IIB) and other evidence (Etter et al., 1959) that diphenylmethylene is a ground-state triplet, would support the view that equation (20) is an adequate representation of triplet insertion. 

Tr. To date, all the photoexcited triplet CIDEP observed satisfy these conditions with k-p having a value of 10 -10 s-- -and D < coz. While almost all of the photoexcited triplet CIDEP in solution involved the carbonyl chromophore, the singular case of solid-state photochemical decomposition of diphenyldiazo-methane at 1.2°K giving a ground-state triplet diphenylmethylene with electron spin polarization was reported by Doetschman et al. in 1976 (44). There is a genuine potential that the photoexcited triplet mechanism can operate in systems containing chromophores other than the carbonyl and in solid-state photochemical systems. 

Diphenylcarbene (diphenylmethylene) can be generated from diphenyldiazomethane (373) by direct irradiation or by triplet sensitization.1156 The intermediate multiplicity then controls the subsequent reactions the singlet carbene inserts into the O—H bond of methanol, whereas the triplet carbene adds to an a I Irene (Scheme 6.173). It has been found that singlet and triplet diphenylcarbenes are in rapid equilibrium relative to the rates of reactions.1157,1158 Competitive quenching experiments (to obtain k1 and kTS) and laser flash spectroscopy (Section 3.7 to obtain k2 and kST) allowed the determination of the free energy difference between the singlet and triplet states of carbene ( 20kJ mol J). 

Table 9.3 Zero-field splitting parameters for the reactive species during the thermal solid state polymerization of TS [20], pme methylene [16], diphenylmethylene [21], and benzene [22] in their first excited triplet state.

The triplet carbene diphenylmethylene gives very complex products with... 

Eisenthal, K.B., Turro, N.J., Sitzmann, E.V., Gould, I.R., Hefferon, G., Langan, J., and Cha, Y., Singlet-Triplet Interconversion of Diphenylmethylene. Energetics, Dynamics and Reactivities of Different Spin States, Tetrahedron, 41, 1543,1985. 

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