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Generation and reactivities of triplet diphenylcarbenes protected by bulky groups as para substituents
Author(s) -
Hirai Katsuyuki,
Hatanaka Keiji,
Yamaguchi Tsuyoshi,
Miyajima Akiko,
Kitagawa Toshikazu,
Tomioka Hideo
Publication year - 2011
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1898
Subject(s) - chemistry , flash photolysis , carbene , reaction rate constant , photochemistry , diazomethane , ketone , electron paramagnetic resonance , photodissociation , benzene , medicinal chemistry , kinetics , organic chemistry , nuclear magnetic resonance , catalysis , physics , quantum mechanics
Bis(2,6‐dimethylphenyl)diazomethane (1b‐N 2 ) having 10‐(4‐ tert ‐butyl‐2,6‐dimethylphenyl)‐2,7‐di(phenylethynyl)‐9‐anthrylethynyl groups at the 4,4′‐positions was synthesized. Triplet diphenylcarbene 3 1b was generated by photolysis of the precursor and was characterized by electron spin resonance and UV–Vis spectroscopies at low temperature, and laser flash photolysis techniques at room temperature. Irradiation of 1b‐N 2 in a 2‐methyltetrahydrofuran matrix at 77 K gave electron spin resonance signals ascribable to triplet carbene 3 1b. UV–Vis spectra of 3 1b were obtained by irradiating 1b‐N 2 under identical conditions. The laser flash photolysis of 1b‐N 2 in a degassed benzene solution gave transient absorption bands ascribable to 3 1b, which decayed in a first‐order fashion with a rate constant of 0.46 s –1 . The carbene was shown to be less reactive than the triplet bis(2,6‐dimethylphenyl)carbene ( 3 1a) having 9‐anthrylethynyl groups at the 4,4′‐positions, which decayed in a second‐order fashion with a rate constant (2 k/εl ) of 0.69 s –1 . Triplet carbene 3 1b was trapped by oxygen to generate ketone oxide 1b‐O with a rate constant of 1.9 × 10 7 M –1 s –1 and also by 1,4‐cyclohexadiene with a rate constant of 1.2 M –1 s –1 . Similar studies with diphenyldiazomethanes (1c‐N 2 and 1 d‐N 2 ) having bulky substituents, 10‐(4‐ tert ‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis[3,5‐di(phenylethynyl)phenylethynyl]‐9‐anthrylethynyl and 10‐(4‐ tert ‐butyl‐2,6‐dimethylphenyl)‐2,7‐bis(9‐triptycylethynyl)‐9‐anthrylethynyl groups at the 4,4′‐positions respectively indicated that the corresponding triplet carbenes 3 1c and 3 1 d were more persistent than 3 1b. Steady‐state irradiation of 1a‐N 2 , 1c‐N 2 , and 1 d‐N 2 in degassed benzene afforded tetraarylethenes 2a, 2c, and 2d in 83, 50, and 51% yield, respectively. Copyright © 2011 John Wiley & Sons, Ltd.