Matrix isolation study of reactive o-quinoid compounds: Generation, detection and reactions

A series of o-quinoid compounds are generated by photolysis of phenylwbene and phenylnitrene precursors bearing appropriate functional groups at the ortho position in an argon matrix at 10 K and are analyzed by IR and UV/vis spectra. Attempts to convert these quinoid compounds to their valence isomers show that photointerconvertibility is possible only with the quinone methide system. Experimental IR spectra of the observed products are shown to match quite well with the theoretical frequencies calculated by ab initw method. Optimized geometries and energies of the quinoid compounds and their valence isomers are also calculated and are employed to interprete the observed dependence of the reversibility between the isomers on the structure. INTRODUCTION o-Quinoid compounds (A) have been invoked as highly reactive molecules in a variety of chemical reactions and have been shown to have potential synthetic utility (ref. 1). Another interesting aspect of these molecules lies in their isomerization between benzo-condensed four-membered heterocyclic ring systems (B). Recent attempts to characterize these species by direct spectroscopic observation coupled with theoretical calculation have began to unfold their nature. While a relatively variety of o-quinoid isomers have been generated and characterized, little is known concerning the strained benzenoid form. The stability of the closed isomer very much depends on the nature of element, and, so far, only derivatives with at least one substituent being CH2 have been isolated (ref. 2). In this paper, we wish to report our research along this line where we generated a series of new o-quinoid compounds by photdysis of phenyl azides and diazomethanes carrying appropriate substituents at the ortho position and characterized not only by matrix isolation technique but also by theoretical studies in terms of the valence isomerization. 1. PHOTOLYSIS OF PHENYL AZIDES HAVING 0-HYDROXY AND AMINO GROUPS /A) o-Benzoauinone imine Irradiation (h > 370 nm) 2-hydroxyphenyl azide (1) matrix-isolated in an argon at 10 K resulted in the formation of three products. The two products showing characteristic absorption bands due to carbonyl group were assigned to Eand Z-mixture of 6-imino-2,4-cyclohexadien-l-ones (3), which must be formed as a result of 1 , 4 H migration in photolytically generated 2-hydroxyphenylnitrene (2), while the remaining product exhibiting the bands due to cumulenic bond was assigned to 6-imino1, 3, 5-hexatrien1-one (4)