Α-carbonyl Group Research Articles

Neighbouring group participation by a carbonyl group in the hydrolysis of methyl benzil-2-carboxylates

Intramolecular catalysis by the neighbouring carbonyl group occurs in the alkaline hydrolysis of methyl benzil-2-carboxylates. The α-carbonyl group participates by giving rise to an intermediate having a five-membered ring.

Thermomechanical Pulp and Light -Photoactivity of α-Carbonyl Group in Solid Lignin

ABSTRACT Interaction of sunlight and UV light with thermomechanical (TMP) pulps, milled wood lignin (MWL) and various lignin model compounds in solid state was studied. It was found that TMP fibers were sensitive to these lights. During exposure, TMP lost its brightness as a function of irradiation time and increased its yellowness. The α-carbonyl group functioned as a light absorption center to promote photocleavage of Cα—Cβ and β–0–4 linkages. Electron spin resonance (ESR) studies demonstrated that free radicals were generated in the interim. Phenoxy radicals appeared to be important radical intermediates that ultimately transformed into o—quinonoid structures.

Gas-phase pyrolytic reactions. Part 5. Rate data for pyrolysis of N-t-butyl- and N-acetyl-benzamide, N-acetyl-N-methylacetamide, and N-ethyl- and N-prop-2-yl-thioacetamide

The rates of gas-phase elimination of N-t-butylbenzamide 1, N-acetylbenzmide 2 an N-acetyl-N-methylacetamide have been measured in the ranges 674–734, 580–620 and 696–765 K, respectively. The compounds undergo unimolecular first order elimination reactons for which log A= 11.1, 13.7 and 10.5 s–1 and Ea= 1 72.6, 171.7 and 167 kJ mol–1, respectively. At 600 K, the following reactivity ratios are observed:1:N-t-butylacetamde 3, 260; t-butyl benzoate 4: t-butyl aceate 5, 2.3; N-acetylcetamide 6N-acetyl-N-methylacetamide, 290; and 2:6, 3.6. These relative rate factors show that the phenyl group increases the rate of thermolysis due to its electron-withdrawing ability 250 times more for simple amides than for esters and diamides. These relative rate differences are highly affected by the nature of the Cα–X bond. The pronounced effect of the phenyl group on simple amides could be explained in terms of the low polarity of the Cα,–NH bond relative to the more polar Cα,–O bond in esters. On the other hand the pronounced deactivation effect of te methyl group in N-acetyl-N-methylacetamide is highly reflected in the reactivity ratio of 290 beween 6 ad N-acetyl-N-methylacetamide which could be explained in terms of the greater bond order of the Cα–X bond in the latter than in the former. Furthermore, the small reactivity ratio in the diamides 2 and 6 is consistent with the fact that resonance between the lone-pair of electrons on X and the α-carbonyl group increases the Cα–X bond order, thus rendering the Cα–X bond breakage more difficult. We have also measured the rates of thermolysis of N-ethythioacetamide and N-prop-2-ylthioacetamide. The relative primary secondary: tertiary rates at 600 K of 1 :1.3:1.5 for the thioamides suggests that the transition state for this class of compound is much less polar than that for the thioacetates.

A sensitivity analysis of the kinetic mechanism for the photodegradation of a model system of relevance to lignin yellowing

A sensitivity analysis of the kinetic mechanism for a model system designed to mimic the photosensitized oxygenation of lignin was performed. The model consisted of a phenolic lignin model compound ( tert-butyl-guaiacylcarbinol) as substrate and a lignin-type sensitizer ( p-methoxypropiophenone) containing an α-carbonyl group. The kinetic behaviour of this model system when irradiated in aerated dimethoxyethane solution with sunlamps has previously been studied experimentally and by computer modelling. It was found that a reaction set consisting of 91 elementary reactions and 58 different species could reproduce the experimentally measured kinetics. Sensitivity calculations for this model were performed at six different reaction times. It was found that 29 elementary reactions out of the 91 steps of the complete reaction system showed comparatively large sensitivities with respect to all species included in the model. Most of the remaining elementary reactions displayed larger sensitivity coefficients only with regard to some of the species, thus indicating their importance for the formation of certain reaction products. The singlet oxygen mode of reaction was found to be more important than the hydrogen abstraction pathway. Independent computer modelling using a reaction set including only the 29 elementary reactions showing the largest sensitivities clearly indicated that such a simplified mechanism was too restricted to account for all the experimentally observed features. Thus, in spite of being more lucid, this simplified mechanism was found to be less well suited for modelling purposes. Only three elementary reactions of the original reaction set were found to be unimportant and could be discarded, leaving a final mechanism with 88 elementary reactions. The sensitivity coefficients showed only small variations with reaction time.

Phenyl nucleus-exchange method for the degradation of lignin

The phenyl nucleus-exchange method (NE-method) is a procedure for the degradation of lignin which allows the C−C linkages between the side chains and the phenyl nuclei to be cleaved selectively, and the phenyl nuclei to be liberated finally as polyhydric phenols. The important characteristic of this method is to take advantage of the dealkylation in diphenylmethane type structures in the presence of boron trifluoride and excess phenol, for the degradation of lignin. The lignin building units which give phenol monomers (guaiacol and/or catechol in softwood lignin) almost quantitatively by this method are noncondensed types and diphenylmethane types, and each of these units has any of the benzyl alcohol, the benzyl ether, the conjugated double bond, the α-carbonyl group and the Cα-aryl bond, in the side chain. The yields of phenyl nuclei are about 25–30% in softwood protolignins and about 8–13% in technical lignins. In this paper, the reaction theory and the degradation mechanism of lignins in the NE-method are outlined.

Stability of the experimental anticancer agent [[(4-methoxyphenyl)sulfonyl]hydrazono]acetic acid (NSC-267213). II. Effect of structural modification on stability and mechanism of degradation

The stability and mechanism of degradation of selected derivatives of [[(4-methoxyphenyl)sulfonyl]hydrazono]acetic acid ( I) were studied in order to evaluate the structural features contributing to its aqueous instability and the possibility of stabilizing this agent by prodrug derivatization. II, [[(4-methylphenyl)sulfonyl]hydrazono]acetic acid ethyl ester, the ethyl ester of a compound closely related to I, was found to degrade more rapidly than the parent acid via unimolecular elimination of the corresponding sulfinic acid. Electron delocalization to the α-carbonyl group was proposed to contribute to the instability of the ester. The substitution of a methyl group for a methoxy group on the benzene ring had no significant effect on the kinetics and mechanism of degradation for the parent hydrazone. The N-methyl derivative, [[(4-methoxyphenyl)sulfonyl]- N-methylhydrazono]acetic acid ( III), demonstrated much greater stability than I and appeared to degrade primarily by hydrolysis. These results indicated that without the possible ionization of the NH group, the unimolecular elimination was not an important degradation route. Consideration of these structural features and the degree of instability of I led to a proposal to stabilize this agent by formation of an N-substituted bioreversible derivative.

Light-Induced Bleaching of High-Yield Pulps Part I. Sensitized Bleaching of Pulp Sheets

Abstract Bleached high-yield pulps undergo a much higher rate of light-induced brightness reversion than their chemical counterparts. This tendency to discolor is one of the main factors limiting the application of high-yield pulps to short-term products such as newsprint, tissue, toweling, and foodboard. The present study was conducted to determine whether the light absorption responsible for aging could be utilized to activate and subsequently destroy the chromophores in high-yield pulps. This would permit production of a bleached pulp. Aspen thermomechanical pulp (TMP) and white oak chemimechanical pulp (CMP) sheets were bleached by irradiation with near ultraviolet light (300–400 nm) while saturated with alcohols in the presence of oxygen. Addition of benzophenone and/or hydrogen peroxide as photosensi-tizers to the alcohols prior to irradiation accelerated the bleaching rates and increased the sheet reflectance (457 nm) 20 to 30 points. High light intensity enhanced bleaching; alkali and water retarded the brightening process. Benzophenone and hydrogen peroxide accelerate bleaching, presumably by utilizing light energy to synthesize significant amounts of organic peroxides (α-hydroxyhydroperoxides) for light-induced bleaching. It is suggested that in the absence of added benzophenone or hydrogen peroxide, chemical groups in lignin (i.e., α-carbonyl groups) are responsible for sensitizing the formation of adequate quantities of these peroxides for bleaching to occur. The light also accelerates bleaching, possibly by activating lignin chromophores, and this enhances their reactivities towards the alcohol peroxides.

Effects of Cα -Oxidation in the Fungal Metabolism of Lignin

Abstract Cα-Oxidation (benzyl alcohol oxidation) is a prominent reaction in the degradation of lignin by white-rot fungi. This study showed that such oxidation markedly retards metabolism of a nonphenolic β-O-4 model compound, 1-(3-methoxy-4-ethoxyphenyl)-2-(o-methoxyphenoxy)propane-1,3-diol, by cultures of Phanerochaete chrysosporium Burds. Surprisingly, however, selective chemical Cα -oxidation of spruce lignins enhanced their depolymerization by the cultures. Thus the decrease in intrinsic degradability of substructures is more than compensated by another effect of Cα-oxidation in lignin. One possibility is that the oxidation increases the accessibility of the lignin to enzymes by decreasing its steric complexity. This study also revealed that the β-O-4 model, like lignin in wood, is degraded in part via Cα-oxidation by P. chrysosporium. Reduction of the α-carbonyl groups thus formed does not occur. Addition of L-glutamate to ligninolytic cultures completely suppresses their competence to degrade the m...

Chemical mechanism of an important cleavage reaction in the fungal degradation of lignin

Previous research established that lignin substructure model compounds of the non-phenolic β-1 type (R-C αHOH-C βHR′-C γH 2OH, in which R,R′ = ▪ O-alkyl) are cleaved between C α and C β by Phanerochaete chrysosporium. The C β-derived product is a racemic phenylglycol (R′-CHOH-CH 2OH) in which the benzyl hydroxyl oxygen atom is derived from molecular oxygen. Investigations described here have established, through the use of an isotope-trapping technique and the same organism, that the C α-derived product is the aromatic aldehyde (R-C αHO); this aldehyde is rapidly reduced in cultures to the corresponding alcohol. A β-1 substrate without the C γH 2OH moiety is also cleaved, showing that the C γ-group is not required. A β-1 substrate with a C α-carbonyl group is only slowly degraded (to polar products), probably not via C α-C β cleavage. Mass spectrometric analysis of the products formed from deuterated β-1 substrates established that hydrogen (deuterium) atoms at C α and at C β are not lost during C α-C β cleavage. These results establish that neither C α-oxidation, C β-hydroxylation nor formation of a C α-C β double bond occurs prior to C α-C β cleavage, and that cleavage does not involve C γ. A concerted mechanism is indicated, in which C β-hydroxylation, with a hydroxyl derived from O 2, occurs simultaneously with, or after, C α-C β cleavage.

Studies on water solubilization of lignin. 1.

In order to develop a new process for the preparation of a water soluble kraft lignin, sulfonation mechanism of lignin model compounds by oxidizing reagents-sulfite system and its applicability to the sulfonation of kraft lignin were investigated. Among the lignin structural units, a catecholic structure was turned out to be most reactive to this sulfonation reaction. A guaiacyl structure without α-carbonyl group showed much higher reactivity than that of a guaiacyl structure withα-carbonyl group. Kraft lignin was totally water solubilized by this sulfonation reaction, regardless the kind of oxidizing reagent, such as MnO2, KMnO4, K2Cr2O7 and K3Fe (CN) 6.

Addition thiophile exclusive d'organomagnesiens satures et insatures sur un α-thioxoester

Resume Ethyl 3,3-dimethyl-2-thioxobutanoate 4 , a thioketone bearing an α-carbonyl group, has been synthesized and isolated. It undergoes selective thiophilic addition with alkyl, vinyl and benzyl-magnesium bromides at room temperature to give, after hydrolysis, α-alkylthio esters 7 . Reaction of the thioxoester 4 with 2-buten-1-yl-magnesium bromide was investigated. At -70°C, thiophilic addition with “inversion” of the allylic chain, giving product 8 , is demonstrated, in contrast with previous observations of carbophilic addition with thiocarbonyl compounds. When the reaction mixture is warmed to 20°, the intermediate sulphur carbanion undergoes a [2.3] sigmatropic shift to afford compound 10 , bearing a linear allylic chain. A mechanism, involving 1,4-conjugate addition, is discussed.

Zur theorie der α-ketosäuren: Sekundäre thermodynamische und kinetische deuterium isotopieeffekte bie nucleophilen additionsreaktionen an die α-carbonyl-gruppe von pyruvamid

Equilibrium constants for the addition of a series of O, N and S-nucleophilic reagents to the α-carbonyl group of pyruvamide are reported. The secondary deuterium isotope effects of the addition constants are nearly independent of the nature of the O- and S-nucleophilic reagent; the corresponding isotope effects of N-nucleophilic reagents are significantly larger. The values of the kinetic secondary deuterium isotope effects for the attack of H 2O to the α-carbonyl group are useful as a guide to the transition state of the uncatalysed and the OH −-catalysed reaction.

Studies on the Color of Lignin Part.X

Lignin is a phenolic compound and contains a small amount of catechol group. Especially, the catechol group exists more abundantly in thiolignin because of the demethylation during sulfate cooking. Therefore, the color of lignin and pulp is influenced by the formation of lignin-metal chelate. This investigation was undertaken to clarify the effects of iron and copper on the color of lignin and pulp.The results are as follows : (1) The residual lignin in mangrove sulfate pulp forms the lignin-iron chelate which affects the color of pulp. The amount of chelate site in pulp, however, is a little. The all chelate sites in sulfate pulp are completely blocked through the lignin-iron chelate with the iron in wood and the iron introduced during pulping process. Even if the iron would be added into sulfate pulp, the iron could not form a chelate and would be merely absorbed in pulp. Consequently, the color of sulfate pulp is slightly affected by the addition of iron (Fig. 1 and Table 1).(2) 7080 ppm of iron is introduced in lignin from the stainless steel autoclave during the soda or sulfate cooking. Moreover, a considerable amount of heavy metal is introduced into pulp from mill water and pulping apparatus (Table 2).(3) The molar extinction coefficients of thiolignin at each wavelength increase linearly with increasing the amount of iron added. This, however, does not vary with more than 7, 000 ppm of iron. Based on 7, 000 ppm of iron, 0.025 per phenyl propane unit is estimated as the chelate site in softwood thiolignin which affects the coloration (Fig. 3 and Table 3).(4) The darkening ratio at each wavelength was calculated as follows : D= (e-e0) /e×100where D=darkening ratioe=molar extinction coefficient of original lignine0=molar extinction coefficient of lignin which does not contain the ironThe darkening ratio in the range of 400700 mμ is less than 10% in softwood thiolignin and is 4050% in softwood lignosulfonate. The darkening ratio of MWL is almost 100% in the range of longer wavelength than 570 mμ. The color of sulfomethylated thiolignin increases with increasing the molecular weight. This, however, can not be explained on the basis of the amount of α-carbonyl group and heavy metal in lignin (Fig. 5, 6 and Table 5).(5) Even if lignin is methylated with diazomethane and dimethyl sulfate, the amount of heavy metal in lignins does not vary, but decreases considerably by sodium borohydride or sodium hydrosulfite reduction. By the reduction treatment, the lignin-iron chelate is destroyed, and iron is released from lignin. Consequently, it is conceivable that the brihgtening of lignin by the reduction with sodium borohydride or sodium hydrosulfite is due to the destruction of lignin-iron chelate together with the reduction of quinoid structure (Fig. 79 and Table 4).

Studies on the Color of Lignin Part. IX

Lignin is a phenolic compound and contains a small amount of catechol group. Especially, the catechol group exists more abundantly in thiolignin because of the demethylation during sulfate cooking. Therefore, the color of lignin and pulp is influenced by the formation of lignin-metal chelate. This investigation was undertaken to clarify the effects of iron and copper on the color of lignin and pulp.The results are as follows : (1) The residual lignin in mangrove sulfate pulp forms the lignin-iron chelate which affects the color of pulp. The amount of chelate site in pulp, however, is a little. The all chelate sites in sulfate pulp are completely blocked through the lignin-iron chelate with the iron in wood and the iron introduced during pulping process. Even if the iron would be added into sulfate pulp, the iron could not form a chelate and would be merely absorbed in pulp. Consequently, the color of sulfate pulp is slightly affected by the addition of iron (Fig. 1 and Table 1).(2) 7080 ppm of iron is introduced in lignin from the stainless steel autoclave during the soda or sulfate cooking. Moreover, a considerable amount of heavy metal is introduced into pulp from mill water and pulping apparatus (Table 2).(3) The molar extinction coefficients of thiolignin at each wavelength increase linearly with increasing the amount of iron added. This, however, does not vary with more than 7, 000 ppm of iron. Based on 7, 000 ppm of iron, 0.025 per phenyl propane unit is estimated as the chelate site in softwood thiolignin which affects the coloration (Fig. 3 and Table 3).(4) The darkening ratio at each wavelength was calculated as follows : D= (e-e0) /e×100where D=darkening ratioe=molar extinction coefficient of original lignine0=molar extinction coefficient of lignin which does not contain the ironThe darkening ratio in the range of 400700 mμ is less than 10% in softwood thiolignin and is 4050% in softwood lignosulfonate. The darkening ratio of MWL is almost 100% in the range of longer wavelength than 570 mμ. The color of sulfomethylated thiolignin increases with increasing the molecular weight. This, however, can not be explained on the basis of the amount of α-carbonyl group and heavy metal in lignin (Fig. 5, 6 and Table 5).(5) Even if lignin is methylated with diazomethane and dimethyl sulfate, the amount of heavy metal in lignins does not vary, but decreases considerably by sodium borohydride or sodium hydrosulfite reduction. By the reduction treatment, the lignin-iron chelate is destroyed, and iron is released from lignin. Consequently, it is conceivable that the brihgtening of lignin by the reduction with sodium borohydride or sodium hydrosulfite is due to the destruction of lignin-iron chelate together with the reduction of quinoid structure (Fig. 79 and Table 4).

Zur theorie der α-ketosäuren : Beziehungen zwischen struktur und UV-Spektren von α-ketosäuren und verwandten α-dicarbonylverbindungen

In homopolar solvents α-keto acids form a stable proton chelate between the carboxyl and α-carbonyl groups. This chelate causes a major bathochrome shift of the n-π*-bands of the α-carbonyl group as compared to the spectrum of the non-chelated molecule, and moreover it shows a remarkable vibrational band structure. The UV spectra of α-keto acid amides also suggest an intramolecular -NH⋯OC interaction. A study of the action of the alkyl group on the n-π*-bands in compounds of the type R 1COCOR 2 (R 1 = Alkyl; R 2 = OH, NH 2 and CH 3) shows that methyl diketo compounds (CH 3COCOR 2) occupy a special position, as hyperconjugation is possible.

Studies on Color of Lignin (VIII)

In order to clarify how cooking time affects the color of lignosulfonate, six fractions of fir lignosulfonate were isolated from acid sulfite waste liquor withdrawn at regular intervals. In this connection, it should be emphasized that each fraction of lignosulfonate is isolated very soon after dissolution into the cooking liquor, in other words it is not much influenced by heating after dissolution. The color, the amounts of functional groups and the molecular weight distribution curve of each lignosulfonate were compared each other, and the changes of molecular weight distribution curve by recooking with acid sulfite cooking liquor were also discussed. The results are summarized as follows : 1. The amounts of methoxyl and non-conjugated phenolic hydroxyl groups show no significant difference among lignosulfonate, obtained at various cooking stages. However, the amount of α-carbonyl group in lignosulfonate at the earlier cooking stages is higher than that at the later cooking stage. 2. The chromophoric structures in lignosulfonate are not practically formed inthe solid state of protolignin, but formed after the dissolution from wood. The rate of coloration of lignosulfonate obtained at the earlier stage is more remarkable than that at the later stage. 3. The molecular weight distribution curve of lignosulfonate was obtained by gel filtration. At the earlier cooking stage, lignosulfonate of lower molecular weight fraction dissolves from wood more easily than that of higher molecular weight fraction. The peaks of these gel filtration curves which correspond to the fractions of higher molecular weight, are located at the same amount of elution volume. This means that the pore size of cell wall determine the rate of delignification. 4. The molecular weight distribution curve of lignosulfonate changes to the lower side of the molecular weight during recooking with acid sulfite cooking liquor, and this tendency is especially remarkable in the case of lignosulfonates obtained at the earlier cooking stage. This shows that the different accessibilities of lignosulfonates to the acid hydrolysis are present even among specimens which are obtained at various cooking stages and then fractionated at the same elution volume.

Multiplicity and regulation of amino acid transport in Penicillium chrysogenum

Penicillium chrysogenum possesses a highly specific sulfur-regulated permease for l-methionine (Benko, Wood, and Segel, Arch. Biochem. Biophys. 122, 783–804 (1967)). In addition, the mycelium has several other relatively specific amino acid permeases. For example, l-phenylalanine- 14C transport by sulfur- and nitrogen-sufficient mycelium is not significantly inhibited by a 10-fold excess of unlabeled l-methionine, l-leucine, l-serine, or l-α-aminobutyric acid. Similarly, l-leucine- 14C transport is not significantly inhibited by a 10-fold excess of any of the other four amino acids. Nitrogen-starvation results in the development of a nonspecific amino acid permease ( V max for l-methionine, l-leucine, and l-phenylalanine ca. 10 μmoles/g-min). The development is prevented by actidione. Structural requirements for interaction with the nonspecific amino acid permease are: (a) an unsubstituted l-α-amino group, (b) a single α-hydrogen atom, (c) no secondary negatively charged group, and (d) an α-carbonyl group. The K m values of the nonspecific permease for l-phenylalanine and l-methionine are ca. 10 −5 m. The K i for l-leucine (with l-methionine as substrate) is 2.4 × 10 −5 m; the K i for l-methionine (with l-leucine as substrate) is 1.4 × 10 −5 m. Other characteristics of the permease are pH-dependence (optimum at 6), temperature-dependence (Q 10 of 2–5 between 15 and 32 °), and energy-dependence (DNP and azide sensitive). The general amino acid permease is independent of monovalent and divalent metal ions and of concurrent protein synthesis for activity. Ammonia (NH 4 +) was the only non-amino acid inhibitor of the permease. The permease activity can be markedly and rapidly reduced by preloading the nitrogen-deficient mycelium with NH 4 + or any one of several amino acids. The results suggest that the nonspecific amino acid permease is subject to feedback regulation by intracellular NH 4 + and amino acid substrates.

Studies on Color of Lignin (4)

In order to clarify how cooking time affects the color of lignosulfonate, six fractions of fir lignosulfonate were isolated from acid sulfite waste liquor withdrawn at regular intervals. In this connection, it should be emphasized that each fraction of lignosulfonate is isolated very soon after dissolution into the cooking liquor, in other words it is not much influenced by heating after dissolution. The color, the amounts of functional groups and the molecular weight distribution curve of each lignosulfonate were compared each other, and the changes of molecular weight distribution curve by recooking with acid sulfite cooking liquor were also discussed. The results are summarized as follows : 1. The amounts of methoxyl and non-conjugated phenolic hydroxyl groups show no significant difference among lignosulfonate, obtained at various cooking stages. However, the amount of α-carbonyl group in lignosulfonate at the earlier cooking stages is higher than that at the later cooking stage. 2. The chromophoric structures in lignosulfonate are not practically formed inthe solid state of protolignin, but formed after the dissolution from wood. The rate of coloration of lignosulfonate obtained at the earlier stage is more remarkable than that at the later stage. 3. The molecular weight distribution curve of lignosulfonate was obtained by gel filtration. At the earlier cooking stage, lignosulfonate of lower molecular weight fraction dissolves from wood more easily than that of higher molecular weight fraction. The peaks of these gel filtration curves which correspond to the fractions of higher molecular weight, are located at the same amount of elution volume. This means that the pore size of cell wall determine the rate of delignification. 4. The molecular weight distribution curve of lignosulfonate changes to the lower side of the molecular weight during recooking with acid sulfite cooking liquor, and this tendency is especially remarkable in the case of lignosulfonates obtained at the earlier cooking stage. This shows that the different accessibilities of lignosulfonates to the acid hydrolysis are present even among specimens which are obtained at various cooking stages and then fractionated at the same elution volume.

MECHANISM OF THE BIMOLECULAR NUCLEOPHILIC DISPLACEMENT REACTION ON α-HALOCARBONYL COMPOUNDS

Two mechanisms have previously been postulated for the high reactivity of a-halocarbonyl compounds in the nucleophilic displacement reaction. Kinetic data for α-halocarbonyl compounds and various nucleophiles (cyanate, acetate, azide, and thiocyanate) in anhydrous methanol are presented. The new data, in conjunction with previously reported results, are in better agreement with the concept of neighboring group overlap than with that of Pearson's electrostatic facilitation between the nucleophile and the α-carbonyl group.