Triplet ground state diphenylcarbenes

Interest in carbenes in rigid media can be dated to the 1960s when Murray et al. reported diphenylcarbene as the first organic species with a triplet ground state. ... 

A two-electron oxidation of allenes (1) (A = S, NMe) has been found to yield carbenes (2) (Scheme 1) dimerization and reaction with (2,2,6,6-tetraamethyl-piperidin-l-yl)oxyl (TEMPO) support a triplet ground state as predicted by computations. Triplet diphenylcarbenes bearing bulky substituents at the para positions have been generated and studied in rigid matrixes at low temperatures by electron spin resonance (ESR) and UV-Vis spectroscopy as well as by laser flash photolysis (LFP) in solution at room temperature. Their reactivity upon LFP was shown to be dominated by dimerization unless triplet carbene quenchers (such as oxygen and cyclohexadiene) were present. 

Diazo compounds decompose with extrusion of nitrogen under the effect of Hght or heat to produce carbenes. Phenylcarbene > as well as diphenylcarbene possess triplet ground states that are in equi-Hbrium with the singlet conhgurations. 

This experimental fact indicates that the singlet and triplet diphenylcarbene are in thermal equilibrium and that Aigc is fast because the Ph2C-state is the ground state. The relatively low nonstereospecifity can be explained by assuming a) a faster addition of the singlet or b) most of the triplet undergoing the abstraction reaction and only little of it the addition reaction. 

Comparison of the chemical reactions of various carbenes with those of methylene itself is also very interesting. Both diphenylcarbene52 and propargylene,52 which possess triplet configurations in their ground states, add nonstereospecifically to olefins. Moreover, diphenylcarbene reacts with oxygen54 to form an excited state of benzophenone.50... 

The hydrogen abstraction reaction and the reaction with oxygen presumably proceed through a triplet state, known to be the ground state of diphenylcarbene, but the alcohol and azine more probably derive from an initially formed singlet. 

For arylcarbenes, low-temperature EPR spectroscopy has shown that the ground state is a bent triplet with two orthogonal singly occupied orbitals The first excited state is an electrophilic singlet and, in the case of diphenylcarbene, the free energy difference between these two states has been estimated to be 5.1 1 kcal moTh... 

The thermal decomposition of diphenyldiazomethane is first order in aceto-nitrile , xylene and 1-methyl naphthalene and probably leads to the formation of diphenylcarbene . The rate-determining step is the formation of diphenyl-carbene. The nature of the solvent does not affect the rate, but does alter the subsequent reactions of the carbene. At 85°, k = 1.6 x 10 sec" and the Arrhenius parameters give Af/ == 27.2 kcal.mole" and AS = 0 . Diphenylcarbene displays a marked electrophilic character implying a singlet electronic state. In the absence of nucleophiles rapid spin inversion to the ground triplet occurs, and it was suggested that this spin inversion is reversible. 

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