Hetero‐Wolff Rearrangement of an α‐Sulfinyl Carbene: Thermally Activated Intersystem Crossing of the Lowest Excited Triplet State of a Ground‐State Singlet Carbene

Abstract

AbstractLaser flash photolysis (λexc = 266 nm) of α‐sulfinyl diazo compound 1b results in the formation of two transient phenomena. Transient absorption A (intense, λmax = 275 nm) shows jump‐and‐growth behaviour with a growth lifetime of τ = 2 μs. Transient absorption B (weak, λmax = 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 kcal mol–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 kcal mol–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.