Carbenes singlet-triplet equilibration

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

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. 

Triplet sensitization of two 1,2-diaryIdiazoethanes in methanol solution gave rise to stilbenes, by intramolecular 1,2-H shifts, and ethers, by reaction of the corresponding carbenes with methanol.This method of generating the intermediate carbenes bypasses the singlet excited diazo compounds, and thus eliminates the possibility that the products arise by direct reaction of these excited species. The results go some way towards establishing that 1,2-H and 1,2-C shifts can compete with alcohol trapping of a spin-equilibrated carbene. 

These results are quite distinct from the chemistry of phenyl carbene (PC) [100]. The chemistry of PC in hydrocarbons is independent of initial singlet or triplet carbene generation. Equilibration of singlet and triplet PC is faster than their intermolecular reactions. 

At that time, it was not possible to measure any of the rate constants of Scheme 1 directly but in some cases it was possible to measure ratios of rate constants or to determine if spin equilibration was much faster or slower than intermolecular reactions. Organic chemists could then only dream of determining the absolute rate constants of Scheme 1. This would become possible around 1980 with the invention of laser flash photolysis with nanosecond (ns) time resolution. But successful application of this tool would require knowledge of the electronic spectra of singlet and triplet carbenes. Low temperature spectroscopy was enormously helpful in this regard. 

Irradiation of the homochiral pyrazoline (51) leads to the elimination of nitrogen and the stereospecific formation of the cyclopropane (52) (Jimenez et al). The reaction proceeds by way of triplet diradical intermediates but evidently, closure to give the cyclopropane ring is faster than C-C bond rotation. The multiplicity of the diradical from the low-temperature photolysis of dihydropyr-rolo[3,4-d(]pyridazines such as (53) is reported to be wavelength dependent and even at 77K, the singlet and triplet states do not equilibrate to a measurable extent (Bush et ai). The results reported by Motschiedler et al. from the triplet sensitised irradiation of 1,2-diaryldiazoethanes in methanol solution go some way to establishing that 1,2-H and 1,2-C shifts in a spin-equilibrated carbene compete... 

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