CARBENE-CARBENE INTERCONVERSION BETWEEN 1-PHENYL-2-PROPYNYLIDENE AND 3-PHENYL-2-PROPYNYLIDENE

1-Phenyl-3-diazopropyne (Id) and 3-phenyl-3-diazopropyne (2d) were prepared and photolyzed under various conditions. In ethanol at ambient temperatures, both 1d and 2d gave a 1:10 +/- 1 mixture of 1-phenyl-3-ethoxypropyne (3) and 3-phenyl-3-ethoxypropyne (4). The photolyses of matrix-isolated 1d and 2d at cryogenic temperatures were followed by EPR, IR, and UV-visible spectroscopy. EPR experiments in 2-methyltetrahydrofuran (MTHF), isopentane, and ethanol-d(6) matrices at 9 K showed spectra due to a mixture of the corresponding triplet carbenes 1c (\D/hc\ = 0.543 and \E/hc\ = 0.003 cm(-1)) and 2c (\D/hc\ = 0.526 and \E/hc\ = 0.010 cm(-1)). The ratio of the generated carbenes carried the memory of the starting diazo compounds; 1c and 2c were produced mainly from 1d and 2d, respectively. Carbene 2c isomerized to 1c at 70-90 K in MTHF and ethanol-d(6) and at 44-68 K in isopentane, indicating that 1c was thermodynamically more stable than 2c on the triplet ground-state potential energy surface. IR and UV-visible absorption experiments employing various media (argon, isopentane, N-2, CO/Ar, O-2/N-2, and O-2/Ar) revealed that the photolysis of 2d afforded mostly absorptions due to 2c. Photolysis of 1d produced similar spectra, due mainly to 2c, together with weak absorptions due to 1c. The different results observed in EPR and IR experiments were explained by the difference in the matrices in which the diazo groups were photolyzed. Calculations at the ab initio MP2/DZV(d) level of theory showed that the triplet ground state of 1c was more thermodynamically stable than 2c by 1.41 kcal/mol, in agreement with the experimental results. The situation is reversed in the singlet manifold, where 2c was computed to be much lower in energy than 1c. The energy differences (Delta E(ST)) between the singlet and triplet states were computed to be 15.7 and 11.0 kcal/mol for 1c and 2c, respectively, with the DZV(d) basis set.