Hetero-wolff rearrangement of an α-sulfinyl carbene: Thermally activated intersystem crossing of the lowest excited triplet state of a ground-state singlet carbene

Laser flash photolysis (λ_exc = 266 nm) of α-sulfinyl diazo compound 1b results in the formation of two transient phenomena. Transient absorption A (intense, λ_max = 275 nm) shows jump-and-growth behaviour with a growth lifetime of τ = 2 μs. Transient absorption B (weak, λ_max = 415 and 540 nm) decays with a lifetime of τ = 1.8 μs, which suggests that B decays into A. On the basis of a comparison with calculated UV/Vis spectra, transient absorption B is assigned to triplet carbene ³7. DFT and CCSD(T) calculations indicate that carbene 7 should have a singlet (S) ground state with a singlet-triplet energy gap of ca. 11 kcalmol^-1 in acetonitrile solution. The seemingly paradoxical observation of a triplet excited state of a ground-state singlet carbene is rationalised in terms of the very different geometries of the singlet and triplet carbene; the most prominent difference lies in the pyramidalisation at the sulfoxide sulfur atom. For the singlet carbene ¹7, the O-S-C:-C(=O) dihedral angle is calculated to be 172.8', whereas it is -80.0' for the triplet carbene ³7. Therefore, for efficient intersystem crossing (ISC) to occur, the triplet carbene must significantly change its geometry. A triplet-singlet minimum energy crossing point (MECP) was located with an O-S-C:-C(=O) dihedral angle of -141.3'. On the basis of the energy of this MECP (15.2 kcalmol^-1 above the S carbene in CH₃CN) and on the ISC rate constant of diphenyl carbene taken as preexponential factor, a rate for the ISC of ³7 is estimated that matches the experimental value within one order of magnitude.