REACTION OF PHENYL-SUBSTITUTED O-QUINODIMETHANES WITH NITRIC-OXIDE - ARE BENZOCYCLOBUTENES SUITABLE PRECURSORS FOR NITRIC-OXIDE CHELETROPICTRAPS

In order to elucidate the potential of substituted o-quinodimethanes a s reagents far the trapping of nitric oxide (NO) in biological systems , the reaction of alkoxyl- and alkyl-substituted 7,8-diphenyl- and 7,7 ,8-triphenyl-o-quinodimethanes with nitric oxide in solution was inves tigated by ESR spectroscopic and UV/vis stopped-flow techniques. Photo lytic decarbonylation of 1,3-diphenyl- and 1,1,3-triphenylindan-2-ones gave the corresponding phenyl-substituted benzocyclobutenes as the ma jor products and low photostationary concentrations of o-quinodimethan es. During 266-nm laser flash photolysis (LFP) of 1,3-dimethoxy-1,3-di phenylindan-2-one and 1-methoxy-1,3,3-triphenylindan-2-one in acetonit rile, species absorbing in the 400-600 nm range were produced, which w ere attributed to configurational isomers of the corresponding 7,7,8,8 -substituted o-quinodimethanes. The isomeric o-quinodimethanes decayed at significantly different rates, indicating a strong influence of th e relative orientation of the terminal substituents on their stability . Reaction of the raw photolysates of the 2-indanones with NO produced strong ESR spectra of the corresponding cyclic nitroxide radicals, is oindolin-2-oxyls. The nitroxide radicals were generated in a two-phase process, the first, rapid phase being attributed to the reaction of N O with the photolytically formed o-quinodimethanes and the second, slo w phase reflecting the reaction with small amounts of o-quinodimethane s, generated by thermal ring opening of the phenyl-substituted benzocy clobutenes and probably a direct reaction of NO with the benzocyclobut enes. The kinetics of both steps, as evaluated by stopped-flow UV/vis and ESR spectroscopy, revealed a strong dependence of the rate constan ts of the o-quinodimethane + NO reaction on the substitution pattern o f the o-quinodimethanes, with rate constants spanning a range of 10-40 00 M(-1) s(-1). The rate constants ((0.4-7.5) x 10(-4) s(-1)) for the reaction of NO with the 7,7,8,8-tetrasubstituted benzocyclobutenes are much less influenced by the substitution pattern. The utility of phen yl-substituted benzocyclobutenes as ''reservoirs'' for o-quinodimethan e-type nitric oxide traps is discussed.