- Spectroscopic and product studies of the effect of para substituents on the reactivity of triplet bis(2,6-dimethylphenyl)carbenes
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A series of diazobis(2,6-dimethylphenyl)methanes (1) bearing eight symmetrical para di-substituents have been prepared and photolyzed to generate the corresponding carbenes (2). Product analysis studies showed that carbenes (2) decay mainly either by dimerization to form tetra(aryl)ethylene (3) or by attack at an o-methyl group to afford 1,2-dihydrobenzocyclobutenes (4) by way of o-quinodimethanes (6) in solution. The zero-field splitting parameters, D and E, were measured in matrices of different viscosities and are analyzed in terms of a sigma-dot (σ.) scale of spin-delocalization substituent constants. Fairly good correlation with σ. was found for the D values of 32 in its minimum energy geometry. Stabilities of 32 were estimated either by measuring the temperature at which the triplet carbene signals disappeared upon thawing the matrix or by analyzing the decay kinetics of 32 in a degassed solution at room temperature. They are examined in terms of the D values in matrix at low temperature and in terms of product distributions in solution at room temperature.
- Hu, Yingmo,Ishikawa, Yui,Hirai, Katsuyuki,Tomioka, Hideo
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p. 2207 - 2218
(2007/10/03)
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- Aqeous and Nonaqueous Polarographic Studies of Substituted 2,6-Dimethylbenzonitrile N-Oxides1)
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Aqueous and nonaqueous polarographic properties of 4-substituted 2,6-dimethylbenzonitrile N-oxides (stable nitrile N-oxides) have been studied and compared with those of the substituted pyridine N-oxides and benzylidenemethylamine N-oxides (nitrones) investigated previously by us.The first reduction wave in both the aqueous and N,N-dimethylformamide (DMF) solvent systems is due to the deoxygenation reaction of the nitrile N-oxide group except when certain substituents are present (see text).This conclusion has also been verified by controlled potential electrolysis in aqueous solution and by cyclic voltammetry in DMF solvent.A plot of the Hammett ? constants of the substituents against E1/2 values was linear with a positive slope for both the aqueous and DMF solvent systems.The slope is smaller than in the case of pyridine N-oxides and nitrones, this being reasonably attributable to the triple bond nature of the CNO group.Half-wave reduction potentials of the nitrile N-oxides are positively shifted compared with those of pyridine N-oxides, particularly in an aqueous solvent.Molecular orbital calculations were applied to interpret the substituent effect on the reduction potentials of the N-oxides. Keywords --- aqueous polarography; nonaqueous polarography; substituent effect on half-wave potential; cyclic voltammetry; controlled potential electrolysis; PPP-SCFMO; CNDO/2; LUMO energy; 4-substituted 2,6-dimethylbenzonitrile N-oxide
- Kubota, Tanekazu,Hiramatsu, Sadaaki,Kano, Kenji,Uno, Bunji,Miyazaki, Hiroshi
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p. 3830 - 3839
(2007/10/02)
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