New 1,1-amino hydroperoxides from regioselective oxygenation of 4-(n-arylmethyleneamino)-2,6-di-t-butylphenols.

The oxygenation of 4-(N-arylmethyleneamino)-2,6-di-$\text{t}$-butylphenols with Co(Salpr), a five coordinate Co(II0 Schiff base complex, in CH₂Cl₂ results in the regioselective hydroperoxylation at the imino carbon to give N-(l-aryl-l-hydroperoxymethyl)-3,5-di-$\text{t}$-butyl-$\text{p}$-benzoquinone monoimines, which give exclusively the corresponding amides and 2,6-di-$\text{t}$-butyl-$\text{p}$-benzoquinone in an aerobic solution of KOH in 90% EtOH.     

Mechanism of selective p-quinol formation in the cobalt schiff base complex-catalyzed oxygenation of 4-alkyl-2,6-di-t-butylphenols

The selective formation of $\text{p}$-quinols in the Co(Salpr)-catalyzed oxygenation of 4-alkyl-2,6-di-$\text{t}$-butylphenols in MeOH has been found to involve the rate determining reduction of peroxy-$\text{p}$-quinolato Co(III) complex formed in the initial fast step. An ionic mechanism of the reduction of the OO bond in the peroxy complex by MeOH has been discussed based on kinetic studies. The reactive species in the catalytic cycle is found to be [Co(III)(Salpr)(OH)].     

A new route to 3-alkyl-2,5-di-t-butyl-2,4-cyclopentadienones from 4-alkyl-2,6-di-t-butylphenols

Base-catalyzed reaction of 5-acylmethyl-2,5-di-$\text{t}$-butyl-4-oxa-2-cyclopentenones selectively derived from 4-alkyl-2,6-di-$\text{t}$-butylphenols $\text{via}$ three steps involving oxygenation, acetylation, and acid-treatment gave 3-alkyl-2,5-di-$\text{t}$-butyl-2,4-cyclopentadienones in excellent yield.     

Synthesis,structure, and dynamic behavior of cyclopentadienyl-lithium, -sodium,and -potassium annelated with bicyclo[2.2.2]octene units: a systematic study on site exchange of alkali metals on a cyclopentadienyl ring in tetrahydrofuran

Novel cyclopentadienyl (Cp)-alkali metal complexes 1-M and 2-M (M = Li, Na, K), in which the Cp ring is annelated with two bicyclo[2.2.2]octene units and substituted with a phenyl group for 1 and a tert-butyl group for 2, were synthesized, and their structures and dynamic behaviors were investigated by means of X-ray crystallography, dynamic (13)C NMR, and DFT calculations. The X-ray crystallography results indicated that 1-Li, 1-Na, and 2-Na form monomeric contact ion pairs (CIP) with three THF molecules coordinated to the metal atom. Also, in THF-d(8), all of the 1-M and 2-M form monomeric CIP in the ground state. However, variable-temperature (13)C NMR measurements of 1-M and 2-M in THF-d(8) demonstrated dynamic behavior in which the metal ion exchanges positions between the upper and lower faces of the Cp ring. From a study of the concentration dependence of the dynamic behavior, the exchange was found to proceed principally as an intramolecular process at concentration ranges lower than 0.2 M. The experimentally observed deltaG values for the intramolecular exchange process for all the 1-M and 2-M (except for 2-Li, whose intramolecular process was too slow to observe) were found to be quite similar in THF-d(8) solution and to fall within the range of 12-14 kcal mol(-)(1). Within this range, a tendency was observed for the deltaG values to increase as the size of the metal decreased. Theoretical calculations (B3LYP/6-31G(d)) afforded considerably large values as the gas-phase dissociation energy for 1-M (162.7 kcal mol(-)(1) for M = Li; 131.6 kcal mol(-)(1) for M = Na; 110.9 kcal mol(-)(1) for M = K) and for 2-M (170.0 kcal mol(-)(1) for M = Li; 137.5 kcal mol(-)(1) for M = Na; 115.4 kcal mol(-)(1) for M = K). These values should be compensated for by a decrease in the solvation energies for the metal ions with increasing size, as exemplified by the calculated solvation energy for M(+)(Me(2)O)(4), which serves as a model for metal ions solvated with four molecules of THF (-122.9 kcal mol(-)(1) for M = Li; -94.7 kcal mol(-)(1) for M = Na; -67.7 kcal mol(-)(1) for M = K). This compensation results in a small difference in the overall energy for dissociation of 1-M or 2-M in ethereal solutions, thus supporting the similar deltaG values observed for the intramolecular metal exchange.     

Synthesis and properties of p-benzoquinone-fused hexadehydro[18]annulenes

A series of hexadehydro[18]annulenes fused with different numbers of p-benzoquinone, 4-6, were synthesized by stepwise transformation of the p-dimethoxybenzene moiety of the precursor dehydroannulene 3 fused with three 3,6-dimethoxy-4,5-dimethylbenzene units at 1,2-positions into p-benzoquinone using ceric ammonium nitrate. The UV-vis spectra of compounds 4 and 5, which have both electron-donating p-dimethoxybenzene unit(s) and electron-accepting p-benzoquinone unit(s) in the π-systems, showed the maximum absorption bands bathochromically shifted in comparison with 3 having only p-dimethoxybenzene units and 6 having only p-benzoquinone units. However, the solvatochromism expected for 4 and 5 was found to be quite weak possibly because the HOMO and LUMO (B3LYP/6-31G(d)) are not localized but rather delocalized over the whole π-systems.     

THE ROLE OF DIOXYGEN SPECIES IN THE BASE- AND COBALT-CATALYZED OXYGENATION OF HINDERED PHENOLS

Abstract— The mechanisms by which 4-substituted 2,6-di- t -butylphenols are oxygenated by base- and Co(II) Schiff base complex-catalysis into o - or p -peroxyquinols and their Co(III) complexes, respectively, have been investigated. For the base-catalyzed oxygenation, a one-step ionic mechanism involving no radical species is suggested to be the most probable one. For the formation of the peroxycobalt(III) complexes, the following stoichiometry is concluded: ArOH + Co(II) + 5/4 O₂→ peroxycobalt(III) complex + 1/2 H₂O. A mechanism involving an electron transfer between the phenols and the Co(II)-O₂ complex followed by further electron transfer between the formed phenoxy radicals and the Co(II) complex to give the corresponding phenolate anions is proposed.     

Synthesis and structural, electronic, and optical properties of oligo(thienylfuran)s in comparison with oligothiophenes and oligofurans

[structure: see text] Alternate thiophene/furan oligomers having four and six heterocycles, i.e., oligo(thienylfuran) dimer and trimer 2 (n = 4 and 6), were newly synthesized by repetitive Stille coupling reactions. The structural, electronic, and optical properties of these oligomers were investigated by X-ray crystallography (for n = 4), cyclic voltammetry (CV), UV-vis and fluorescence spectroscopy, and DFT calculations, and the results were compared with those of corresponding oligothiophenes (1) and oligofurans (3). The inter-ring torsional energy profiles calculated for bithiophene 1 (n = 2), thienylfuran 2 (n = 2), and bifuran 3 (n = 2) at the B3LYP/6-31G(d) level indicated that the most stable conformers of 2 (n = 2) and 3 (n = 2) are fully coplanar with transoid structure while that of 1 (n = 2) is twisted with a dihedral angle of 158 degrees . In accord with this, X-ray crystallographic analysis of 2 (n = 4) revealed that the pi-conjugated system is nearly planar with the inter-ring C=C-C=C dihedral angles between the thiophene and furan rings of 173.6(7) degrees , -177.0(7) degrees , and 172.6(6) degrees . In the packing structure, these nearly planar molecules are arranged in a herringbone pattern. The CV on a series of oligo(thienylfuran)s 2 showed irreversible oxidation peaks at +0.90, +0.42, and +0.29 V vs Fc/Fc(+) for n = 2, 4, and 6, which were 0.15-0.18 V lower than those for corresponding oligothiophenes 1 and were closer to those for oligofurans 3. On the other hand, the UV-vis spectra of 2 showed the longest wavelength absorption to be almost identical with those of the corresponding 1, and more bathochromically shifted than those of the corresponding 3. The results of CV and UV-vis measurements were supported by DFT calculations (B3LYP/ 6-311+G(2d,p)//B3LYP/6-31G(d)). Thus, oligo(thienylfuran)s 2 have HOMOs which are higher than those of oligothiophenes 1 and close to those of 3, and HOMO-LUMO gaps which are close to those of 1 and smaller than those of 3. In fluorescence spectra, the quantum yield of 2 increased with elongation of the pi-system (n = 2 (3.5%), 4 (19%), 6 (24%)).     

Perfection and homogeneity of space-grown GaSb:Te crystals

A Te-doped GaSb crystal grown by the method of directed crystallization under microgravitation has been studied by X-ray topography. It is shown that the structural perfection of the crystal part grown without contact with the ampule walls is substantially higher than the structural perfection of the crystals grown under terrestrial conditions. The distribution of a Te dopant in the crystal is studied by quantitative X-ray topography, and it is shown that, if the gap between the growing crystal and the ampule is small, the Marangoni convection in the melt can be less intense than the convection provided by residual microgravitation. The relation between the formation of a flat face on the crystallization front (faceting) and the formation of twins is also established.