Reports: AC4
47527-AC4 Multiplicity Control in Carbenes Aided by the Trifluoromethyl Group
Aryl(trifluoromethyl)carbenes are among the most widely utilized reactive intermediates in photoaffinity labeling of biological systems. Their precursors, trifluoromethyldiazirines, are relatively straightforward to prepare, are chemically stable under physiological conditions, can be attached to a variety of biologically relevant moieties, and undergo efficient photocleavage at convenient wavelengths. Despite the broad application of aryl(trifluoromethyl)carbenes, however, only a handful of studies have probed their chemistry, reactivity, and electronic structures.
We have found that trifluoromethylcarbenes are generally more photochemically and thermally robust than corresponding H-substituted carbenes. For example, although phenylcarbene and substituted derivatives are particularly photolabile, and ring-expand readily at low temperatures, substituted phenyl(trifluoromethyl)carbenes can be easily characterized in low temperature matrices and are much less photochemically reactive. We have found via DFT calculations that trifluoromethyl groups actually stabilize the singlet states of carbenes compared to H. Aryl(trifluoromethyl)carbenes thus have smaller S-T energy gaps than phenylcarbenes, and we have found that singlet or triplet multiplicities can be finely tuned through appropriate substitution.
We have been able to generate a variety of substituted phenyl(trifluoromethyl)carbenes in inert matrices at cryogenic temperatures, and probe their spectroscopy and reactivity. Through a combination of these data, we have been able to determine the spin states of these carbenes convincingly. We have found that the ground state multiplicity of the various carbenes fits predictions from DFT calculations. We are currently preparing the results of our first studies in this area for publication.
