Diphenylcarbene Protected by Four ortho-Iodine Groups: An Unusually Persistent Triplet Carbene

作者：Hirai Katsuyuki^*; Bessho Kana; Tsujita Kosaku; Kitagawa Toshikazu^*

来源：Molecules, 2016, 21(11): 1545.

DOI：10.3390/molecules21111545

• triplet carbene
• diazo compound
• photolysis
• steric hindrance
• substituent effect

Diphenyldiazomethane with four iodine groups at the ortho positions and two tert-butyl groups at the para positions, i.e., bis(4-tert-butyl-2,6-diiodophenyl) diazomethane (1a-N-2), was synthesized as a sterically hindered triplet carbene precursor. Irradiation of 1a-N-2 in solution effectively generated the corresponding triplet diphenylcarbene (3)1a, which was characterized by UV-vis spectroscopy at low temperature, along with laser flash photolysis techniques at room temperature. The UV-vis spectrum of (3)1a was obtained by irradiating 1a-N-2 in a 2-methyltetrahydrofuran matrix at 77 K. The ESR spectrum showed no triplet carbene signals, while a radical species was observed at the anticipated temperature of the decomposition of triplet carbene (3)1a. Transient absorption bands ascribable to (3)1a were observed by laser flash photolysis of 1a-N-2 in a degassed benzene solution and decayed very slowly with a second-order rate constant (2k/epsilon l) of 5.5 x 10(-3).s(-1). Steady-state irradiation of 1a-N-2 in degassed benzene afforded 9,10-diarylphenanthrene derivative 2a in a (3)1% yield. Triplet carbene (3)1a was also trapped by either oxygen (k(O2) = 6.5 x 10(5) M-1.s(-1)) or 1,4-cyclohexadiene (k(CHD) = 1.5 M-1.s(-1)) to afford the corresponding ketone 1a-O or the diarylmethane 1a-H-2. The carbene was shown to be much less reactive than the triplet diphenylcarbene that is protected by two ortho-iodo and two ortho-bromo groups, (3)1b.

• 出版日期2016-11