Vitaliy I. Timokhin

The influence of different irradiation sources on the treatment of nitrobenzene

Abstract This work describes the influence of light on the treatment of nitrobenzene (NB) in aqueous solutions. Three different experimental devices were used to perform a detailed study: a photoreactor with four low-pressure mercury lamps (λ=253.7xa0nm), a tubular photoreactor with a polychromatic Xe lamp (290

Rate equation for the degradation of nitrobenzene by ‘Fenton-like’ reagent

Abstract This paper describes the effect of temperature and initial concentration of H 2 O 2 , Fe(II), PhNO 2 and dissolved oxygen on the degradation rate of PhNO 2 in homogeneous aqueous solution by ‘Fenton-like’ reagent ([H 2 O 2 ] o ≫[Fe(II)] o ). The oxidation products o -, m - and p -nitrophenol were found as intermediates in the ratio 1:1.3–2.8:1.4–2.7 as compared with PhNO 2 when conversion of the latter was less than 25%. This fact suggests that hydroxylation of PhNO 2 was promoted by HO radicals. The reaction was investigated in a completely mixed-batch reactor under a wide range of experimental conditions: pH ∼3.0; 278–318 K; 1.5 2 O 2 ] o o 2 ] o 2 ] o 2 was determined to be 59.7 kJ mol −1 . The degradation rate of PhNO 2 follows pseudo-first-order kinetics. The results of this study demonstrate that the degradation rate of PhNO 2 in the ‘Fenton-like’ system could be predicted with sufficient precision by the equation R D =1.05×10 11 exp(−59.7/ RT )[H 2 O 2 ] o 0.68 [Fe(II)] o 1.67 [PhNO 2 ] o −0.32 . The second-order rate constant for the overall rate of H 2 O 2 decomposition by Fe(III) was found to be 0.83 M −1 s −1 at 298 K. The value of the steady-state HO radical concentration in the ‘Fenton-like’ reaction was found to be ∼10 −13 M, as estimated by two independent methods.

In Search of Efficient 5-Endo-dig Cyclization of a Carbon-Centered Radical: 40 Years from a Prediction to Another Success for the Baldwin Rules

Despite being predicted to be stereoelectronically favorable by the Baldwin rules, efficient formation of a C-C bond through a 5-endo-dig radical cyclization remained unknown for more than 40 years. This work reports a remarkable increase in the efficiency of this process upon beta-Ts substitution, which led to the development of an expedient approach to densely functionalized cyclic 1,3-dienes. Good qualitative agreement between the increased efficiency and stereoselectivity for the 5-endo-dig cyclization of Ts-substituted vinyl radicals and the results of density functional theory analysis further confirms the utility of computational methods in the design of new radical processes. Although reactions of Br atoms generated through photochemical Ts-Br bond homolysis lead to the formation of cyclic dibromide side products, the yields of target bromosulfones in the photochemically induced reactions can be increased by recycling the dibromide byproduct into the target bromosulfones through a sequence of addition/elimination reactions at the exocyclic double bond. Discovery of a relatively efficient radical 5-endo-dig closure, accompanied by a C-C bond formation, provides further support to stereoelectronic considerations at the heart of the Baldwin rules and fills one of the last remaining gaps in the arsenal of radical cyclizations.

Evidence from Serpula lacrymans that 2,5-Dimethoxyhydroquinone Is a Lignocellulolytic Agent of Divergent Brown Rot Basidiomycetes

ABSTRACT Basidiomycetes that cause brown rot of wood are essential biomass recyclers in coniferous forest ecosystems and a major cause of failure in wooden structures. Recent work indicates that distinct lineages of brown rot fungi have arisen independently from ligninolytic white rot ancestors via loss of lignocellulolytic enzymes. Brown rot thus proceeds without significant lignin removal, apparently beginning instead with oxidative attack on wood polymers by Fenton reagent produced when fungal hydroquinones or catechols reduce Fe3+ in colonized wood. Since there is little evidence that white rot fungi produce these metabolites, one question is the extent to which independent lineages of brown rot fungi may have evolved different Fe3+ reductants. Recently, the catechol variegatic acid was proposed to drive Fenton chemistry in Serpula lacrymans, a brown rot member of the Boletales (D. C. Eastwood et al., Science 333:762-765, 2011). We found no variegatic acid in wood undergoing decay by S. lacrymans. We found also that variegatic acid failed to reduce in vitro the Fe3+ oxalate chelates that predominate in brown-rotting wood and that it did not drive Fenton chemistry in vitro under physiological conditions. Instead, the decaying wood contained physiologically significant levels of 2,5-dimethoxyhydroquinone, a reductant with a demonstrated biodegradative role when wood is attacked by certain brown rot fungi in two other divergent lineages, the Gloeophyllales and Polyporales. Our results suggest that the pathway for 2,5-dimethoxyhydroquinone biosynthesis may have been present in ancestral white rot basidiomycetes but do not rule out the possibility that it appeared multiple times via convergent evolution.

Kinetic studies on the formation of sulfonyl radicals and their addition to carbon-carbon multiple bonds.

The reactions of α-hydroxyl and α-alkoxyl alkyl radicals with methanesulfonyl chloride (MeSO(2)Cl) have been studied by pulse radiolysis at room temperature. The alkyl radicals were produced by ionizing radiation of N(2)O-saturated aqueous solution containing methanol, ethanol, isopropanol, or tetrahydrofuran. The transient optical absorption spectrum consisted of a broad band in the region 280-380 nm with a maximum at 320 nm typical of the MeSO(2)(•) radical. The rate constants in the interval of 1.7 × 10(7)-2.2 × 10(8) M(-1) s(-1) were assigned to an electron-transfer process that leads to MeSO(2)Cl(•-), subsequently decaying into MeSO(2)(•) radical and Cl(-). The rate constants for the addition of CH(3)SO(2)(•) to acrolein and propiolic acid were found to be 4.9 × 10(9) M(-1) s(-1) and 5.9 × 10(7) M(-1) s(-1), respectively, in aqueous solutions and reversible. The reactivity of tosyl radical (p-CH(3)C(6)H(4)SO(2)(•)) toward a series of alkenes bearing various functional groups was also determined by competition kinetics in benzene. The rate constants for the addition of tosyl radical to alkenes vary in a much narrower range than the rate constants for the reverse reaction. The stabilization of the adduct radical substantially contributes to the increase of the rate constant for the addition of tosyl radical to alkenes and, conversely, retards the β-elimination of tosyl radical.

Lignin decomposition is sustained under fluctuating redox conditions in humid tropical forest soils

Lignin mineralization represents a critical flux in the terrestrial carbon (C) cycle, yet little is known about mechanisms and environmental factors controlling lignin breakdown in mineral soils. Hypoxia is thought to suppress lignin decomposition, yet potential effects of oxygen (O2 ) variability in surface soils have not been explored. Here, we tested the impact of redox fluctuations on lignin breakdown in humid tropical forest soils during ten-week laboratory incubations. We used synthetic lignins labeled with 13 C in either of two positions (aromatic methoxyl or propyl side chain Cβ ) to provide highly sensitive and specific measures of lignin mineralization seldom employed in soils. Four-day redox fluctuations increased the percent contribution of methoxyl C to soil respiration relative to static aerobic conditions, and cumulative methoxyl-C mineralization was statistically equivalent under static aerobic and fluctuating redox conditions despite lower soil respiration in the latter treatment. Contributions of the less labile lignin Cβ to soil respiration were equivalent in the static aerobic and fluctuating redox treatments during periods of O2 exposure, and tended to decline during periods of O2 limitation, resulting in lower cumulative Cβ mineralization in the fluctuating treatment relative to the static aerobic treatment. However, cumulative mineralization of both the Cβ - and methoxyl-labeled lignins nearly doubled in the fluctuating treatment relative to the static aerobic treatment when total lignin mineralization was normalized to total O2 exposure. Oxygen fluctuations are thought to be suboptimal for canonical lignin-degrading microorganisms. However, O2 fluctuations drove substantial Fe reduction and oxidation, and reactive oxygen species generated during abiotic Fe oxidation might explain the elevated contribution of lignin to C mineralization. Iron redox cycling provides a potential mechanism for lignin depletion in soil organic matter. Couplings between soil moisture, redox fluctuations, and lignin breakdown provide a potential link between climate variability and the biochemical composition of soil organic matter.

A Highly Diastereoselective Oxidant Contributes to Ligninolysis by the White Rot Basidiomycete Ceriporiopsis subvermispora

ABSTRACT The white rot basidiomycete Ceriporiopsis subvermispora delignifies wood selectively and has potential biotechnological applications. Its ability to remove lignin before the substrate porosity has increased enough to admit enzymes suggests that small diffusible oxidants contribute to delignification. A key question is whether these unidentified oxidants attack lignin via single-electron transfer (SET), in which case they are expected to cleave its propyl side chains between Cα and Cβ and to oxidize the threo-diastereomer of its predominating β-O-4-linked structures more extensively than the corresponding erythro-diastereomer. We used two-dimensional solution-state nuclear magnetic resonance (NMR) techniques to look for changes in partially biodegraded lignin extracted from spruce wood after white rot caused by C. subvermispora. The results showed that (i) benzoic acid residues indicative of Cα—Cβ cleavage were the major identifiable truncated structures in lignin after decay and (ii) depletion of β-O-4-linked units was markedly diastereoselective with a threo preference. The less selective delignifier Phanerochaete chrysosporium also exhibited this diastereoselectivity on spruce, and a P. chrysosporium lignin peroxidase operating in conjunction with the P. chrysosporium metabolite veratryl alcohol did likewise when cleaving synthetic lignin in vitro. However, C. subvermispora was significantly more diastereoselective than P. chrysosporium or lignin peroxidase-veratryl alcohol. Our results show that the ligninolytic oxidants of C. subvermispora are collectively more diastereoselective than currently known fungal ligninolytic oxidants and suggest that SET oxidation is one of the chemical mechanisms involved.

Rate constants for the reaction of cumylperoxyl radicals with group 14 hydrides

Rate constants for the reaction of a cumylperoxyl radical with a variety of group 14 hydrides were measured by using inhibited hydrocarbon oxidation methodologies. In the silane series the rate constants at 345.5 K are in the range 0.10–0.90 M−1 s−1 for t-BuSiHMe2, PhSiH3, PhSiHMe2, Ph2SiH2, Ph2SiHMe and Ph3SiH, whereas the rate constant for (TMS)3SiH is 2–3 orders of magnitude higher. The rate constants for Bu3GeH and Bu3SnH are 19.4 and 1.8xa0× 103 M−1 s−1, respectively, at 345.5 K. The trends in reactivity for these reactions are similar to those observed for other radicals with group 14 hydrides although the reactions are several orders of magnitude slower.

Rate constants for the reaction of cumylperoxyl radicals with Bu3SnH and (TMS)3SiH

The rate constants for the reaction of a cumylperoxyl radical with Bu3SnH and (TMS)3SiH were determined at 72.5 °C to be 1600 and 66 M–1 s–1, respectively, by using inhibited hydrocarbon oxidation methodologies.

Thirty Years of (TMS)3SiH: A Milestone in Radical-Based Synthetic Chemistry

This review is an update on tris(trimethylsilyl)silane, TTMSS, in organic chemistry, focusing on the advancements of the past decade. The overview includes a wide range of chemical processes and synthetic strategies under different experimental conditions, including functional group insertion and transformations, as well as preparation of complex molecules, natural products, polymers, surfaces, and new materials. These results reveal how TTMSS has matured over the past 30 years, and they further establish its value as a free radical reagent with widespread academic and industrial applications.