Reference

Regioselective photoprotonation of 1,3-dimethoxybenzenes in 1,1,1,3,3,3-hexafluoroisopropyl alcohol

Regioselective photoprotonation of 1,3-dimethoxybenzenes in 1,1,1,3,3,3-hexafluoroisopropyl alcohol. H-D isotope-exchange and laser flash photolysis studies. Mathivanan, N.; Cozens, Frances; McClelland, Robert A.; Steenken, Steen (Dep. Chem., Univ. Toronto, Toronto, ON M5S 1A1, Can.). J. Am. Chem. Soc., 114(6), 2198-203 (English) 1992. CODEN: JACSAT. ISSN: 0002-7863. DOCUMENT TYPE: Journal CA Section: 22 (Physical Organic Chemistry) Laser flash photolysis of 1,3-dimethoxybenzene, and five substituted derivs. (2-Me, 4-Me, 5-Me, 2-MeO, 4-MeO) in 1,1,1,3,3,3-hexafluoroisopropyl alc. (HFIP) results in a single transient with lmax in the range 395-460 dependent on the addnl. substituent. This transient forms in a monophotonic process, is unaffected by oxygen, but quenched by methanol and bromide, and decays by returning to the ground-state compd. These are characteristics of an intermediate cyclohexadienyl cation, produced by protonation of the arom. excited state by the solvent. The lmax values of the cyclohexadienyl cations produced by protonation of these dimethoxybenzenes in concd. H2SO4, however, lie in the range 315-340 nm. The 400-MHz 1H NMR of the latter solns. reveals that the cations obtained here are 2,4-dimethoxybenzenonium ions, obtained by protonation at the 4-position. Photolysis at 254 nm in HFIP-d reveals that, for the four compds. that are unsubstituted at C-2, there is a relatively efficient exchange of the hydrogen at this carbon; with the two compds bearing 2-substituents there is no net photochem. reaction. Thus, the cyclohexadienyl cations obsd. as transient intermediates in the photochem. expts. are 2,6-dimethoxybenzenonium ions, obtained by protonation at C-2 of the excited state. The change in selectivity between ground and excited states is explained by a shift of p-electron d. upon excitation. The different spectra of 2,4- and 2,6-dimethoxybenzenonium ions have their origins in very different effects of o- and p-methoxy substituents, the former causing a ~40-nm bathochromic shift in a benzenonium ion and the latter a ~55-nm hypsochromic shift. Photolysis of 1-methoxynaphthalene in HFIP-d results in exchange at C-5, and to a lesser extent at C-8, as has been previously obsd. in acidic aq. solns. A transient is obsd. upon laser flash photolysis of this compd.Except for chemicals metioned above, 2216-69-5 and 634-36-6 are also used. in HFIP with two lmax at 360 and 550 nm. This transient is assigned to the cyclohexadienyl cation obtained by photoprotonation at C-5. .

Selective saturation and inversion of multiple resonances in high resolution solid state carbon-13 CP/MAS experiments

Selective saturation and inversion of multiple resonances in high resolution solid state carbon-13 CP/MAS experiments. Hu, Jian Zhi; Pugmire, Ronald J.; Orendt, Anita M.; Grant, David M.; Ye, C. (Dep. Chem., Univ. Utah, Salt Lake City, UT 84112, USA). Prepr. Pap. - Am.There are some reagents with their cas registry numbers 634-36-6 and 87-85-4 are used in this study. Chem. Soc., Div. Fuel Chem., 37(2), 646-59 (English) 1992. CODEN: ACFPAI. ISSN: 0569-3772. DOCUMENT TYPE: Journal CA Section: 51 (Fossil Fuels, Derivatives, and Related Products) Section cross-reference(s): 77 Taking advantage of the long 13C T1 relaxation values generally encountered in solids, selective satn. and inversion of >1 resonance in 13C cross-polarization magic angle spinning expts. can be achieved by sequentially applying several DANTE pulse sequences centered at different frequency offsets. A selective satn. pulse sequence was introduced composed of a series of 90° DANTE sequences sepd. by interrupted decoupling periods, during which the selected resonance was destroyed. Applications of this method, including the simplification of the measurement of the 13C chem. shift tensor under slow magic angle spinning conditions were described. The method can be used for the detn. of coal aromaticity. .