Flash vacuum pyrolysis of methoxy-substituted lignin model compounds

The flash vacuum pyrolysis (FVP) of methoxy-substituted beta-O-4 lignin model compounds has been studied at 500 degrees C to provide mechanistic insight into the primary reaction pathways that occur under conditions of fast pyrolysis. FVP of PhCH(2)CH(2)OPh (PPE), a model of the dominant beta-O-4 linkage in lignin, proceeds by C-O and C-C cleavage, in a 37:1 ratio, to produce styrene plus phenol as the dominant products and minor amounts of toluene, bibenzyl, and benzaldehyde. From the deuterium isotope effect in the FVP of PhCD(2)CH(2)OPh, it was shown that C-O cleavage occurs by homolysis and by 1,2-elimination in a ratio of 1.4:1, respectively. Methoxy substituents enhance the homolysis of the beta-O-4 linkage, relative to PPE, in o-CH(3)O-C(6)H(4)OCH(2)CH(2)Ph (o-CH(3)O-PPE) and (o-CH(3)O)(2)-C(6)H(3)OCH(2)CH(2)Ph ((o-CH(3)O)(2)-PPE) by a factor of 7.4 and 21, respectively. The methoxy-substituted phenoxy radicals undergo a complex series of reactions, which are dominated by 1,5-, 1,6-, and 1,4-intramolecular hydrogen abstraction, rearrangement, and beta-scission reactions. In the FVP of o-CH(3)O-PPE, the dominant product, salicylaldehyde, forms from the methoxyphenoxy radical by a 1,5-hydrogen shift to form 2-hydroxyphenoxymethyl radical, 1,2-phenyl shift, and beta-scission of a hydrogen atom. The 2-hydroxyphenoxymethyl radical can also cleave to form formaldehyde and phenol in which the ratio of 1, 2-phenyl shift to beta-scission is ca. 4:1. In the FVP of o-CH(3)O-PPE and (o-CH(3)O)(2)-PPE, products (ca. 20 mol %) are also formed by C-O homolysis of the methoxy group. The resulting phenoxy radicals undergo 1,5- and 1,6-hydrogen shifts in a ratio of ca. 2:1 to the aliphatic or benzylic carbon, respectively, of the phenethyl chain. In the FVP of (o-CH(3)O)(2)-PPE, o-cresol was the dominant product. It was formed by decomposition of 2-hydroxy-3-hydroxymethylbenzaldehyde and 2-hydroxybenzyl alcohol, which are formed from a complex series of reactions from the 2, 6-dimethoxyphenoxy radical. The key step in this reaction sequence was the rapid 1,5-hydrogen shift from 2-hydroxy-3-methoxybenzyloxy radical to 2-hydroxymethyl-6-methoxyphenoxy radical before beta-scission of a hydrogen atom to give the substituted benzaldehyde. The 2-hydroxybenzyl alcohols rapidly decompose under the reaction conditions to o-benzoquinone methide and pick up hydrogen from the reactor walls to form o-cresol.     

An anomalous x-ray diffraction study of the hydration structures of Cs+ and I- in concentrated solutions

Anomalous x-ray diffraction experiments were carried out on concentrated aqueous solutions of sodium iodide (6 molal) and cesium iodide (3 molal). Data were gathered at two energies below the absorption edges of the Cs+ and I- ions in order to avoid contributions from fluorescence. The statistics and quality of the raw data were improved by the use of a focusing analyzer crystal. Differences were taken between the data sets and used to calculate the hydration structures of Cs+ and I-. The structures found are more complex than anticipated for such large ions with relatively low charge densities and show evidence of ion-pair formation in both solutions. A two-Gaussian fit to the Cs+ data gives information about the Cs+-O and Cs+-I- correlations. The central position of the Gaussian representing the Cs+-O was fixed at 3.00 A, that is, the maximum of this contribution. The other parameters were allowed to vary freely, giving a Cs+-I- distance of 3.84+/-0.05 A and coordination numbers of 7.9 and 2.7, respectively, for the Cs+-O and Cs+-I- correlations. The results on the structure of I- in the 6 molal NaI aqueous solution were also fitted to a model based on Gaussians; this gives correlations for I- -O and I- -Na+ at 3.17+/-0.06 and 3.76+/-0.06 A with respective coordination numbers of 8.8 and 1.6. The structure of I- in the 3 molal CsI solution shows overlapping contributions due to I- -H, I- -O, and I- -Cs+. The best Gaussian fit gives two peaks centered at 3.00+/-0.08 and 3.82+/-0.04 A and shows that the latter two correlations are unresolved. The hydration structures are compared with those of other alkali and halide ions. The results are also found to be in good agreement with those obtained from standard x-ray diffraction and computer simulation.     

Synthesis of 4′-Carboxy-benzo-30-crown-10 and a Thermodynamic Study of Its Complexes with Thallium and Alkali Cations in Acetonitrile Solution

A synthetic procedure has been developed for the preparation of 4-carboxy-benzo-30-crown-10. The formation of Na+, K+, Rb+, Cs+ and Tl+ complexes with the large crown ether was investigated conductometrically in acetonitrile solution at various temperatures. The formation constants of the resulting 1:1 complexes were determined from the molar conductance-mole ratio data. It was found that the stability of the complexes vary in the order Tl+ > K+ > Rb+ > Cs+ > Na+. The data obtained in this study support the existence of a wrap around structure for the above complexes in solution. The enthalpy and entropy of complexation reactions were determined from the temperature dependence of the formation constants. In all cases, the complexes were enthalpy stabilized but entropy destabilized. The resulting TS° vs. H°plot showed a fairly good linear correlation, indicating the existence of an entropy-enthalpy compensation in the large crown complexation reactions.     

Highly stereoselective oxy-Michael additions to alpha,beta-disubstituted nitro olefins: asymmetric synthesis of pseudo-norephedrine derivatives and THP* protected alpha-hydroxy ketones

The "naked" anion of (S)-6-methyl delta lactol undergoes efficient oxy-Michael addition to alpha,beta-disubstituted nitro olefins to give the THP* protected Henry products with excellent (95-->98% de) stereocontrol at the beta-position and moderate (up to 3 : 1) stereocontrol at the alpha-position in favour of the syn-diastereoisomer. Nitro group reduction with in situN-Boc protection and THP* removal provides alpha,beta-disubstituted ethanolamine derivatives, while treatment with tetrapropylammonium perruthenate gives THP* protected alpha-hydroxy ketone derivatives in high diasteromeric excess.     

Photolysis of cellulose and related compounds by 253.7 nm light. I. Rates of gas production from hydrocellulose

The initial stages of the photolysis of cellulose under vacuum can be followed by the gas production. A photolysis cell is described which allows a rigorous temperature control and gives a reproducible photolysis surface of the solid carbohydrate sample. A mass spectrometer was used to analyze the gases. To measure accurately the small amounts produced in a small volume requires careful calibration of this instrument, together with a fixed routine for the experimental procedure. Using this apparatus, we have photolyzed hydrocellulose under vacuum with 253.7 nm light. Only H₂, CO, and CO₂ were evolved, the rate of H₂ production increasing to a constant value, that of CO increasing more slowly without becoming constant, and that of CO₂ decreasing rapidly to a constant value. No induction periods were observed. The gas evolution was not caused by residual O₂ or CO₂ adsorbed on the polymer, but the rates were altered by leaving the photolyzed sample under vacuum overnight or heating the unphotolyzed sample under vacuum at 100°C. The rates of gas production increased markedly when the light filter was changed to allow a combined photolysis by 253.7 and 184.9 nm light. The results illustrate that stringent precautions must be taken to filter off the latter line in polymer photolyses with Hg arc light if the results are to be meaningfully interpreted in terms of 253.7 nm light photolysis. On photolyzing hydrocellulose previously boiled in NaOH solution, a larger H₂ production rate and a smaller CO₂ rate (compared with untreated hydrocellulose) were observed. It is suggested that this standard alkali treatment is the reason for the differences between our results and the results of the photolysis of cellulose reported in the literature.     

A photoelectrochemical study of the anodic oxidation of lead in alkaline solution

The oxidation of lead in sodium tetraborate and in sulphuric acid solution has been studied. The analysis of photocurrent spectra has shown that tetragonal PbO is formed in both electrolytes. Evidence for photoemission from the metal substrate into the anodic oxide film is presented and discussed. The photocurrent behaviour is related to changes in electrode impedance which accompany the interconvertion of PbO and PbO₂.