Preparation of bis(diazo) compounds incorporated into butadiyne and thiophene units and generation and characterization of their photoproducts

Abstract

(2,6-Dimethyl-4-tert-butylphenyl)(2,4,6-tribromophenyl)diazomethane(1-N₂) was found to be stable enough to survive under Sonogashira coupling reaction conditions, and aryldiazomethyl substituents were introduced at the 1,4-positions of butadiyne (4-2N₂) and the 2,5-positions of thiophene(5-2N₂). Irradiation of those bis(diazo) compounds generated bis(carbenes), which were characterized by using ESR and UV/vis spectroscopic techniques in a matrix at low temperature as well as time-resolved UV/vis spectroscopy in solution at room temperature. These studies revealed that both of the bis(carbenes), 4 and 5, have singlet quinoidal diradical ground states with a very small singlet–triplet energy gap of less than 1 kcal mol⁻¹. A remarkable increase in the lifetime of bis(carbenes), as opposed to that of the monocarbene (2), was noted and was interpreted to indicate that bis(carbenes) are thermodynamically stabilized as a result of delocalization of unpaired electrons throughout the π net framework. In spite of the stability, both bis(carbenes) are readily trapped by molecular oxygen to afford bis(ketones). Presumably, the reaction of the upper-lying localized quintet states with oxygen is much faster than that for lower-lying states.