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Different chemical reactivity and kinetic behaviour are observed for 2-azido-(1 ) and 3-azidothiophene (2) in either 1.3-cycloaddition reactions with (trimethylsilyl)acetylene and trimethyl-(viny1)silane or thermal decomposition. Compound 1 gives cyclo-A,A, adducts (silylated triazoie and triazoline) three times faster than compound 2. Triazolines are sensitive to silica and undergo ring-contraction on chromatography, with extrusion of nitrogen, to give the corresponding 1 -(t h ienyl ) -2-(trimet hylsily I) aziridine. Kinetic measurements of the u n i molecu lar thermal decompositions afford distinct activation parameters: EB = 22.6 and 30.6 kcal mol-', A S f = -8.2 and -0.7 cal mot-' K-' for 1 and 2, respectively. The modelling of the chemical reactivity for the a-or P-azido groups in the thiophene and the consequent geometric and electronic perturbation of the azidothiophenes are qualitatively supported by measurements of either 'H or 13C NMR chemical shifts.The chemistry and kinetics of organic azides, as precursors (or otherwise) of nitrenes, have been increasingly investigated this century.' Thermolysis and photolysis of phenyl azide, useful for many synthetic, biological and industrial applications, has been investigated primarily to understand its diverse behaviour. These reactions show a dependence on the nature of the substituents and experimental conditions, and the intermediacy of the highly reactive intermediate phenylnitrene (PhN), in either the singlet and/or triplet state, is always assumed to be responsible for the different product distributions (i. e. dimerization to azobenzene, ring-expansion to a dehydroazepine, ring contraction to a cyanocyclopentadienyl radical, hydrogen abstraction, or insertion into a secondary amine).2With five-membered heteroaromatic azides the product distribution is dependent on the position in the ring of the azido group. l b q 3 In fact, thermolysis of a-substituted heteroaryl azides generally results in ring-opening of the heteroaromatic substrate.t On the other hand, P-substituted azides appear to behave like aryl azides, forming nitrene intermediates which cyclize onto suitable ortho-substituents.Recently we published papers on the thermally induced ringopening of related 2-azidobenzo[b]thiophene (2-BTA),* 2azido-1 -methylindole and 2-azidobenzo[b]furan which undergo ring-opening. On the basis of chemical and kinetic studies with azidobenzothiophenes (2-BTA and 3-BTA) we suggested that the unimolecular decomposition and ring-opening of 2-BTA could occur in a concerted manner (path b, Scheme l)." Furthermore, we found that 2-BTA and 3-BTA undergo 1,3dipolar cycloadditions with silylated alkenes at comparable rates, as expected for phenyl azides substituted with electronwithdrawing groups, and faster than azides carrying electrondonating substituents.' ' Paper 3/00 179B

We have explored the changes in the phase stability, orientational order, and dynamics of the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) doped with either the trans or the cis form of different p-azobenzene derivatives using the ESR spin-probe technique. In particular, we have studied the effects induced by each of the seven nonmesogenic 4-R-phenylazobenzenes (R = H, F, Br, CH3, CF3, On-Bu, Ot-Bu) at 1% and 7% mole fraction on the order parameter and on the shift of the nematic-isotropic transition temperature (TNI), as reported by a nitroxide spin probe, and we have tried to relate them to the solute shape and charge distribution. In all the cases the presence of the azo-derivative causes a depression of T(NI), more pronounced for the cis isomers. The dependence of on the reduced temperature T* = T/T(NI) remains the same as that of pure 5CB in all trans-doped samples at 1% and 7% and decreases only slightly in the cis at 1%. However, we observe different and in some cases large variations (up to 25%) in for the cis at 7%, showing solute effects that go beyond the shift in T(NI). Surprisingly enough, even at the highest concentration, the probe dynamics appears to be essentially independent of the nature, the configuration, and the concentration of the different solutes and very similar to that observed in the pure 5CB.

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5-(Phenylsulfenyl)-2-pentanone. Aluminum chloride (2.5 mL, 1.4 M solution of AlClg in acetonitrile, 3.5 mmol) was added via syringe to a solution containing 0.195 g (2.31 mmol) of 1 and 10 mL of acetonitrile under nitrogen. Thiophenol (0.48 mL, 4.63 mmol) was added via syringe, and the solution was heated at 55 °C for 15 h, cooled to room temperature, poured into 10% NaHCOg, and extracted with ether. The organic phase was washed with water and brine and dried over magnesium sulfate. Removal of solvent in vacuo gave 0.688 g of a yellow oil. Purification by TLC (silica gel, 1000 /tm; CH2C12) gave pure 5-(phenylsulfenyl)-2-pentanone: 0.1169 g (28% yield);;