Chemistry, kinetics, and spectroscopy of highly hindered diarylcarbenes. The case of decachlorodiphenylcarbene
作者:Hideo Tomioka、Katsuyuki Hirai、Takehito Nakayama
DOI:10.1021/ja00057a010
日期:1993.2
Bis(pentachlorophenyl)diazomethane (1a) was prepared, and reactivities of perchlorodiphenylcarbene (2a) generated by photolysis of 1a were investigated not only in terms of product analysis but also by using flash photolysis techniques. Although the major reaction found for 2a was dimerization forming perchlorotetraphenylethylene and was essentially similar to that observed for 2,2',4,4',6,6'-hexachlorodiphenylcarbene (2b), product distribution clearly indicated that the carbenic center in 2a is more rigidly protected than that in 2b by the four ortho chloro substituents which are buttressed by the four meta chloro groups. Irradiation of 1a in a 2-methyltetrahydrofuran glass at 77 K resulted in the appearance of the absorption due to triplet 2a at 356, 493, and 525 nm. Flash photolysis of 1a in benzene solution produced a transient absorption due to 2a at 357 nm, which decayed in second order in accordance with the product analysis data. The rate constant for dimerization of 2a was determined to be 2.5 +/- 0.1 x 10(6) M-1 s-1, 3 orders of magnitude smaller than that of unsubstituted diphenylcarbene. The triplet carbene (2a) was trapped by oxygen to generate the perchlorobenzophenone oxide showing its maximum at 390 nm (t1/2 = 510 +/- 9 ms) and also by 1,4-cyclohexadiene to produce bis(perchlorophenyl)methyl radical (lambda(max) 376 nm with the rate constant of 6.2 x 10(3) M-1 s-1.