Reaction of Phenyl-Substituted o-Quinodimethanes with Nitric Oxide. Are Benzocyclobutenes Suitable Precursors for Nitric Oxide Cheletropic Traps?

In order to elucidate the potential of substituted o-quinodimethanes as reagents for 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 investigated by ESR spectroscopic and UV/vis stopped-flow techniques. Photolytic decarbonylation of 1,3-diphenyl- and 1,1,3-triphenylindan-2-ones gave the corresponding phenyl-substituted benzocyclobutenes as the major products and low photostationary concentrations of o-quinodimethanes. During 266-nm laser flash photolysis (LFP) of 1,3-dimethoxy-1,3-diphenylindan-2-one and 1-methoxy-1,3,3-triphenylindan-2-one in acetonitrile, species absorbing in the 400-600 nm range were produced, which were 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 the 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, isoindolin-2-oxyls. The nitroxide radicals were generated in a two-phase process, the first, rapid phase being attributed to the reaction of NO with the photolytically formed o-quinodimethanes and the second, slow phase reflecting the reaction with small amounts of o-quinodimethanes, generated by thermal ring opening of the phenyl-substituted benzocyclobutenes and probably a direct reaction of NO with the benzocyclobutenes. The kinetics of both steps, as evaluated by stopped-flow UV/vis and ESR spectroscopy, revealed a strong dependence of the rate constants of the o-quinodimethane + NO reaction on the substitution pattern of the o-quinodimethanes, with rate constants spanning a range of 10-4000 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 phenyl-substituted benzocyclobutenes as "reservoirs" for o-quinodimethane-type nitric oxide traps is discussed.

Absolute Rate Constants for Radical Additions to Alkenes in Solution. The Synergistic Effect of Perfluorination on the Reactivities of n-Alkyl Radicals1.

Laser flash photolysis has been used to determine the absolute rate constants for addition of several partially fluorinated n-alkyl radicals to three styrenes at 25 °C in Freon 113. Fluorination at the γ-position (RCF2CH2CH2•) gives radicals with essentially the same reactivity as non-fluorinated n-alkyls. The RCH2CF2CH2• and RCH2CH2CHF• radicals are both about three times as reactive as RCF2CH2CH2•, but the RCH2CH2CF2• radical is ca. five to six times rather than ca. three times as reactive as RCH2CH2CHF•. Similarly, the perfluorinated radical CF3CF2CF2• is much more reactive than would be expected on the basis of the reactivities of the RCH2CF2CH2• and RCH2CH2CF2• radicals. Thus, perfluorinated n-alkyl radicals are very considerably more reactive than would be predicted from the individual effects of α-, ß-, and γ-fluorination.