Identification and Decay Kinetics of Photogenerated o-Xylylenes

Abstract

<p>The thesis consists of two parts, which have been combined in the Introduction and Discussion sections.</p> <p>Part A. Isoindenes</p> <p>Transient isoindenes absorbing in the 400-550 nm range have been observed in the flash photolysis of a series of 1,1-diarylindenes. They have been studied by ultraviolet spectroscopy, low temperature nmr, chemical trapping, and flash photolysis techniques. Irradiation of 1,1,3-triphenyl-indene at 254 nm in cyclopentane at -70ºC gives an orange solution (λ max 478 nm) which contains 1,2,3-triphenylisoindene whose methine and vinylic proton resonances in the nmr occur at 4.45 and 6.80 ppm respectively. Reaction of 1,2,3-triphenylisoindene with 4-phenyl-1,2,4-triazoline-3,5-dione at -70°C, gave a Diels-Alder adduct. The isoindene formed 1,2,3-triphenylindene quantitatively on warming to 20°C. The kinetics of the 1,5-hydrogen shift by which a series of isoindenes rearrange to stable indenes were studied by flash photolysis. The transient decay was first order, and rate constants (deoxygenated hexane, 20°C) are: 1,2-diphenylisoindene, 36 s⁻¹; 1-phenyl-2-p-cyanophenylisoindene, 14 s⁻¹; 1-phenyl-2-p-bromophenylisoindene, 27 s⁻¹; 1-phenyl-2-p-methoxyphenylisoindene, 44 s⁻¹ and 1,2,3-triphenylisoindene, 1.21 s⁻¹. Kinetic isotope effects of kH/kD = 3.7 and 6.5 for the decay of 1,2,3-triphenylisoindene and 1,2-diphenylisoindene (2-H and 2-D) respectively show that the H-shift is rate determining. The 1,5-hydrogen shift in 1,2-diphenylisoindene has Ea = 13.1 kcal/mol and ΔSº† = -9.0 eu, and in 1,2,3-triphenylisoindene has Ea = 11.8 kcal/mol and ΔSº‡= -19.6 eu. The ground state energy of isoindene relative to indene is estimated to be 20 kcal/mol, which is compared with theoretically calculated values.</p> <p>Part B. o-Xylylenes</p> <p>o-Xylylene derivatives have been generated in the flash photolysis of four indan-2-ones. Thus, 1,1,3,3-tetramethylindan-2-one gives 7,7,8,8- tetramethyl-o-xylylene while 1,1-dimethylindan-2-one gives 7,7-dimethyl-o-xylylene. Both these transients decay (thermally and photochemically) by a 1,5-hydrogen shift to yield alkyl isopropenylbenzenes. The rates of thermal decay are 0.002 s⁻¹ and 0.038 s⁻¹, for the tetra- and dimethyl compounds respectively, at ca. 20°C in deoxygenated hexane. The decay of tetramethyl-o-xylylene showed a kinetic isotope effect (kH/kD) of 5.4. Activation parameters were: Ea= 19 kcal/mol, ΔSº‡= -7.7 eu (tetramethyl o-xylylene); Ea= 15.5 kcal/mol, ΔSº‡= -13.9 eu (dimethyl-o-xylylene). Low temperature photolysis (-70°C, 254 nm) show absorption maxima at 350nm, for the tetramethyl compound, and 360 nm for the dimethyl compound. Photolysis of 1,1,3,3-tetramethyl-4,5-benzindan-2-one gives the thermally stable 9,9,10,10-tetramethyl-1,2-naphthoquinodimethane, which decays photochemically to 1-isopropenyl-2-isopropylnaphthalene. A transient with a lifetime of about 1 μs could be observed in the flash photolysis of 1,1,3,3-tetramethyl-5-6-benzindan-2-one, which gives two photoproducts, 2-isopropyl-3-isopropenylnaphthalene and 3,3,4,4-tetramethylnaphtho[b]cyclo-butene. The rates of thermal decay of the transients are consistent with thermochemical calculations. Spectroscopic evidence, together with theoretical calculations, indicates that the transients are twisted about the 'essential' single bonds of the non-aromatic ring, and cannot adopt the planar geometry necessary for the allowed suprafacial 1,5-H shift. This together with the photolability of the transients, suggests that the thermal decay maay involve the previously unknown antarafacial 1,5-hydrogen shift.</p>