Acid- and Base-Mediated Hydrolysis of Dichloroacetamide Herbicide Safeners

Safeners are used extensively in commercial herbicide formulations. Although safeners are regulated as inert ingredients, some of their transformation products have enhanced biological activity. Here, to fill gaps in our understanding of safener environmental fate, we determined rate constants and transformation products associated with the acid- and base-mediated hydrolysis of dichloroacetamide safeners AD-67, benoxacor, dichlormid, and furilazole. Second-order rate constants for acid- (HCl) and base-mediated (NaOH) dichloroacetamide hydrolysis (2.8 × 10^-3 to 0.46 and 0.3-500 M^-1 h^-1, respectively) were, in many cases (5 of 8), greater than those reported for their chloroacetamide herbicide co-formulants. In particular, the rate constant for base-mediated hydrolysis of benoxacor was 2 orders of magnitude greater than that of its active ingredient co-formulant, S-metolachlor. At circumneutral pH, only benoxacor underwent appreciable hydrolysis (5.3 × 10^-4 h^-1), and under high-pH conditions representative of lime-soda softening, benoxacor's half-life was 13 h─a timescale consistent with partial transformation during water treatment. Based on Orbitrap LC-MS/MS analysis of dichloroacetamide hydrolysis product mixtures, we propose structures for major products and three distinct mechanistic pathways that depend on the system pH and compound structure. These include base-mediated amide cleavage, acid-mediated amide cleavage, and acid-mediated oxazolidine ring opening. Collectively, this work will help to identify systems in which hydrolysis contributes to the transformation of dichloroacetamides, while also highlighting important differences in the reactivity of dichloroacetamides and their active chloroacetamide co-formulants.

Computational Approaches for the Prediction of Environmental Transformation Products: Chlorination of Steroidal Enones

There is growing interest in the fate and effects of transformation products generated from emerging pollutant classes, and new tools that help predict the products most likely to form will aid in risk assessment. Here, using a family of structurally related steroids (enones, dienones, and trienones), we evaluate the use of density functional theory to help predict products from reaction with chlorine, a common chemical disinfectant. For steroidal dienones (e.g., dienogest) and trienones (e.g., 17β-trenbolone), computational data support that reactions proceed through spontaneous C4 chlorination to yield 4-chloro derivatives for trienones and, after further reaction, 9,10-epoxide structures for dienones. For testosterone, a simple steroidal enone, in silico predictions suggest that C4 chlorination is still most likely, but slow at environmentally relevant conditions. Predictions were then assessed through laboratory chlorination reactions (0.5-5 mg Cl₂/L) with product characterization via HRMS and NMR, which confirmed near exclusive 4-chloro and 9,10-epoxide products for most trienones and all dienones, respectively. Also consistent with computational expectations, testosterone was effectively unreactive at these same chlorine levels, although products consistent with in silico predictions were observed at higher concentrations (in excess of 500 mg Cl₂/L). Although slight deviations from in silico predictions were observed for steroids with electron-rich substituents (e.g., C17 allyl-substituted altrenogest), this work highlights the potential for computational approaches to improve our understanding of transformation products generated from emerging pollutant classes.

Building Shape-Persistent Arylene Ethynylene Macrocycles as Scaffolds for 1,4-Diiodobutadiyne

Two shape-persistent arylene ethynylene macrocycles have been designed and synthesized as scaffolds to bind the nonpolar molecule 1,4-diiodobutadiyne. Binding via halogen bonding interactions between the pyridine moieties of the macrocycle and 1,4-diiodobutadiyne is predicted by density functional theory calculations and has been demonstrated in solution by ¹³C NMR titrations. The binding constant for the macrocycle-monomer complex (K = 10.5 L mol^-1) is much larger than for other comparable halogen bonds, strongly supporting cooperative binding of both ends of the diyne. These results demonstrate a fully inserted geometry of 1,4-diiodobutadiyne in the complex.

Photocatalytic Radical Aroylation of Unactivated Alkenes: Pathway to β-Functionalized 1,4-, 1,6-, and 1,7-Diketones

We report the development of a photocatalytic strategy for the synthesis of β-functionalized unsymmetrical 1,4-, 1-6 and 1,7-diketones from aroyl chlorides and unactivated alkenes at room temperature. The mild reaction conditions not only tolerate a wide range of functional groups and structural moieties, but also enable migration of a variety of distal groups including (hetero)arenes, nitrile, aldehyde, oxime-derivative, and alkene. The efficiency of chirality transfer, factors that control the distal-group migration, and synthesis of carbo- and heterocycles from the diketones are also described. Mechanistic studies suggest a reaction pathway involving a photocatalytic radical aroylation of unactivated alkenes followed by a distal-group migration, oxidation, and deprotonation to afford the desired diketones.

Intramolecular [2 + 2] Photocycloaddition of Altrenogest: Confirmation of Product Structure, Theoretical Mechanistic Insight, and Bioactivity Assessment

While studying the environmental fate of potent endocrine-active steroid hormones, we observed the formation of an intramolecular [2 + 2] photocycloaddition product (2) with a novel hexacyclic ring system following the photolysis of altrenogest (1). The structure and absolute configuration were established by X-ray diffraction analysis. Theoretical computations identified a barrierless two-step cyclization mechanism for the formation of 2 upon photoexcitation. 2 exhibited progesterone, estrogen, androgen, and pregnane X receptor activity, albeit generally with reduced potency relative to 1.

Atropselective Oxidation of 2,2',3,3',4,6'-Hexachlorobiphenyl (PCB 132) to Hydroxylated Metabolites by Human Liver Microsomes and Its Implications for PCB 132 Neurotoxicity

Polychlorinated biphenyls (PCBs) have been associated with neurodevelopmental disorders. Several neurotoxic congeners display axial chirality and atropselectively affect cellular targets implicated in PCB neurotoxicity. Only limited information is available regarding the atropselective metabolism of these congeners in humans and their atropselective effects on neurotoxic outcomes. Here we investigate the hypothesis that the oxidation of 2,2',3,3',4,6'-hexachlorobiphenyl (PCB 132) by human liver microsomes (HLMs) and their effects on dopaminergic cells in culture are atropselective. Racemic PCB 132 was incubated with pooled or single donor HLMs, and levels and enantiomeric fractions of PCB 132 and its metabolites were determined gas chromatographically. The major metabolite was either 2,2',3,4,4',6'-hexachlorobiphenyl-3'-ol (3'-140), a 1,2-shift product, or 2,2',3,3',4,6'-hexachlorobiphenyl-5'-ol (5'-132). The PCB 132 metabolite profiles displayed inter-individual differences and depended on the PCB 132 atropisomer. Computational studies suggested that 3'-140 is formed via a 3,4-arene oxide intermediate. The second eluting atropisomer of PCB 132, first eluting atropisomer of 3'-140, and second eluting atropisomer of 5'-132 were enriched in all HLM incubations. Enantiomeric fractions of the PCB 132 metabolites differed only slightly between the single donor HLM preparations investigated. Reactive oxygen species and levels of dopamine and its metabolites were not significantly altered after a 24 h exposure of dopaminergic cells to pure PCB 132 atropisomers. These findings suggest that there are inter-individual differences in the atropselective biotransformation of PCB 132 to its metabolites in humans; however, the resulting atropisomeric enrichment of PCB 132 is unlikely to affect neurotoxic outcomes associated with the endpoints investigated in the study.

Reversible Photohydration of Trenbolone Acetate Metabolites: Mechanistic Understanding of Product-to-Parent Reversion through Complementary Experimental and Theoretical Approaches

Photolysis experiments (in H2O and D2O) and quantum chemical calculations were performed to explore the pH-dependent, reversible photohydration of trenbolone acetate (TBA) metabolites. Photohydration of 17α-trenbolone (17α-TBOH) and 17β-trenbolone (17β-TBOH) occurred readily in simulated sunlight to yield hydrated products with incorporated H(+) at C4 and OH(-) at either C5 (5-OH-TBOH) or C12 (12-OH-TBOH) in the tetracyclic steroid backbone. Although unable to be elucidated analytically, theory suggests preferred orientations of cis-12-OH-TBOH (relative to C13 methyl) and trans-5-OH-TBOH, with the former most thermodynamically stable overall. Both experiment and theory indicate limited stability of trans-5-OH-TBOH at acidic pH where it undergoes concurrent, carbocation-mediated thermal rearrangement to cis-12-OH-TBOH and dehydration to regenerate its parent structure. Experiments revealed cis-12-OH-TBOH to be more stable at acidic pH, which is the only condition where its reversion to parent TBA metabolite occurred. At basic pH cis-12-OH-TBOH decayed quickly via hydroxide/water addition, behavior that theory attributes to the formation of a stable enolate resistant to dehydration but prone to thermal hydration. In a noteworthy deviation from predicted theoretical stability, 17α-TBOH photohydration yields major trans-5-OH-TBOH and minor cis-12-OH-TBOH, a distribution also opposite that observed for 17β-TBOH. Because H(+) and OH(-) loss from adjacent carbon centers allows trans-5-OH-TBOH to dehydrate at all pH values, the presumed kinetically controlled yield of 17α-TBOH photohydrates results in a greater propensity for 17α-TBOH reversion than 17β-TBOH. Additional calculations explored minor, but potentially bioactive, trenbolone analogs that could be generated via alternative rearrangement of the acidic carbocation intermediate.

H2S-mediated thermal and photochemical methane activation

Sustainable, low-temperature methods for natural gas activation are critical in addressing current and foreseeable energy and hydrocarbon feedstock needs. Large portions of natural gas resources are still too expensive to process due to their high content of hydrogen sulfide gas (H2S) mixed with methane, deemed altogether as sub-quality or "sour" gas. We propose a unique method of activation to form a mixture of sulfur-containing hydrocarbon intermediates, CH3SH and CH3SCH3 , and an energy carrier such as H2. For this purpose, we investigated the H2S-mediated methane activation to form a reactive CH3SH species by means of direct photolysis of sub-quality natural gas. Photoexcitation of hydrogen sulfide in the CH4 + H2S complex resulted in a barrierless relaxation by a conical intersection to form a ground-state CH3SH + H2 complex. The resulting CH3SH could further be coupled over acidic catalysts to form higher hydrocarbons, and the resulting H2 used as a fuel. This process is very different from conventional thermal or radical-based processes and can be driven photolytically at low temperatures, with enhanced control over the conditions currently used in industrial oxidative natural gas activation. Finally, the proposed process is CO2 neutral, as opposed to the current industrial steam methane reforming (SMR).

Product-to-parent reversion of trenbolone: unrecognized risks for endocrine disruption

Trenbolone acetate (TBA) is a high-value steroidal growth promoter often administered to beef cattle, whose metabolites are potent endocrine-disrupting compounds. We performed laboratory and field phototransformation experiments to assess the fate of TBA metabolites and their photoproducts. Unexpectedly, we observed that the rapid photohydration of TBA metabolites is reversible under conditions representative of those in surface waters (pH 7, 25°C). This product-to-parent reversion mechanism results in diurnal cycling and substantial regeneration of TBA metabolites at rates that are strongly temperature- and pH-dependent. Photoproducts can also react to produce structural analogs of TBA metabolites. These reactions also occur in structurally similar steroids, including human pharmaceuticals, which suggests that predictive fate models and regulatory risk assessment paradigms must account for transformation products of high-risk environmental contaminants such as endocrine-disrupting steroids.

Backbone-base interactions critical to quantum stabilization of transfer RNA anticodon structure

Transfer RNA (tRNA) anticodons adopt a highly ordered 3'-stack without significant base overlap. Density functional theory at the M06-2X/6-31+G(d,p) level in combination with natural bond orbital analysis was utilized to calculate the intramolecular interactions within the tRNA anticodon that are responsible for stabilizing the stair-stepped conformation. Ten tRNA X-ray crystal structures were obtained from the PDB databank and were trimmed to include only the anticodon bases. Hydrogenic positions were added and optimized for the structures in the stair-stepped conformation. The sugar-phosphate backbone has been retained for these calculations, revealing the role it plays in RNA structural stability. It was found that electrostatic interactions between the sugar-phosphate backbone and the base provide the most stability, rather than the traditionally studied interbase stacking. Base-stacking interactions, though present, were weak and inconsistent. Aqueous solvation was found to have little effect on the intramolecular interactions.

Computational studies of CO2 activation via photochemical reactions with reduced sulfur compounds

Reactions between CO(2) and reduced sulfur compounds (RSC), H(2)S and CH(3)SH, were investigated using ground and excited state density functional theory (DFT) and coupled cluster (CC) methods to explore possible RSC oxidation mechanisms and CO(2) activation mechanisms in the atmospheric environment. Ground electronic state calculations at the CR-CC(2,3)/6-311+G(2df,2p)//CAM-B3LYP/6-311+G(2df,2p) level show proton transfer as a limiting step in the reduction of CO(2) with activation energies of 49.64 and 47.70 kcal/mol, respectively, for H(2)S and CH(3)SH. On the first excited state surface, CR-EOMCC(2,3)/6-311+G(2df,2p)//CAM-B3LYP/6-311+G(2df,2p) calculations reveal that energies of <250 nm are needed to form H(2)S-CO(2) and CH(3)SH-CO(2) complexes allowing facile hydrogen atom transfer. Once excited, all reaction intermediates and transition states are downhill energetically showing either C-H or C-S bond formation in the excited state whereas only C-S bond formation was found in the ground state. Environmental implications of these data are discussed with a focus on tropospheric reactions between CO(2) and RSC, as well as potential for carbon sequestration using photocatalysis.

2-Amino-5-(3,4-dimethoxybenzylidene)-1-methylimidazol-4(5H)-one N,N-dimethylformamide monosolvate

The crystal structure of the title compound, C(13)H(15)N(3)O(3) x C(3)H(7)NO, was determined as part of a larger project focusing on creatinine derivatives as potential pharmaceuticals. The molecule is essentially planar, in part because of intramolecular hydrogen bonding. Inversion-related pairs of molecules result from intermolecular hydrogen bonding. The pi systems of 2-amino-5-(3,4-dimethoxybenzylidene)-1-methylimidazol-4(5H)-one and an inversion-related molecule overlap slightly, indicating a small amount of pi-pi stacking. Bond lengths, angles and torsion angles are consistent with similar structures, except in the imidazolone ring near the doubly bonded C atom, where significant differences occur.

Impact of solvent polarity on N-heterocyclic carbene-catalyzed beta-protonations of homoenolate equivalents

N-Heterocyclic carbenes have been demonstrated to react through divergent pathways under the same conditions. Experimental and computational evidence demonstrates that the ability to favor generation of homoenolate equivalents from alpha,beta-unsaturated aldehydes versus the oxidation of aldehydes to esters is highly dependent upon the choice of solvent. The solvation environment plays an important role due to the mechanistic differences in these processes, with polar protic solvent favoring the oxidation process due to solvation of intermediates with greater charge separation.

On Gas Phase α-Effects. 1. The Gas-Phase Manifestation and Potential SET Character

The possibility of a gas-phase alpha-effect has been explored for the methyl transfer from methyl formate to hydroxide, hydroperoxide, and ethoxide by computing barrier heights at the HF/6-311++G(2df,2p) level of theory. The alpha-nucleophile (hydroperoxide) is found to have a lower barrier than the gas-phase-acidity-matched normal nucleophile (ethoxide) by 3.6 kcal/mol, offering evidence for a gas phase alpha-effect. A Shi-Boyd analysis for these reactions indicates that there is more single-electron-transfer character in the hydroperoxide transition state than for either hydroxide or ethoxide, further bolstering the existence of a gas-phase alpha-effect and the appropriateness of the Hoz model for the alpha-effect.

Ab initio molecular orbital and density functional studies on the solvolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX-like compound

[reaction: see text] Potential energy surfaces for the alkaline hydrolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX model compound, and the perhydrolysis of the latter have been computed at the MP2/6-31+G(d)//mPW1K/MIDI! level of theory. The effect of aqueous solvation was accounted for via the integral equation formalism polarizable continuum model (IEF-PCM) at the HF/6-31+G(d) level. Excellent agreement with the experimental enthalpy of activation for alkaline hydrolysis of sarin was found. For the alkaline hydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that the P-O and P-S bond cleavage processes are kinetically competitive but that the products of P-S bond cleavage are thermodynamically favored. For the perhydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that P-O bond cleavage is not kinetically competitive with P-S bond cleavage. In both cases, the data support initial formation of trigonal bipyramidal intermediates and demonstrate kinetic selectivity for nucleophilic attack on the face opposite the more apicophilic methoxide ligand.

Ionic liquids based on FeCl(3) and FeCl(2). Raman scattering and ab initio calculations

We have prepared ionic liquids by mixing either iron(II) chloride or iron(III) chloride with 1-butyl-3-methylimidazolium chloride (BMIC). Iron(II) chloride forms ionic liquids from a mole ratio of 1 FeCl(2)/3 BMIC to almost 1 FeCl(2)/1 BMIC. Both Raman scattering and ab initio calculations indicate that FeCl(4)(2-) is the predominant iron-containing species in these liquids. Iron(III) chloride forms ionic liquids from a mole ratio of 1 FeCl(3)/1.9 BMIC to 1.7 FeCl(3)/1 BMIC. When BMIC is in excess, Raman scattering indicates the presence of FeCl(4-). When FeCl(3) is in excess, Fe(2)Cl(7-) begins to appear and the amount of Fe(2)Cl(7-) increases with increasing amounts of FeCl(3). Ionic liquids were also prepared from a mixture of FeCl(2) and FeCl(3) and are discussed. Finally, we have used both Hartree-Fock and density functional theory methods to compute the optimized structures and vibrational spectra for these species. An analysis of the results using an all-electron basis set, 6-31G, as well as two different effective core potential basis sets, LANL2DZ and CEP-31G is presented.

Reductive dechlorination of hexachloroethane in the environment: mechanistic studies via computational electrochemistry

Ab initio and density functional levels of electronic structure theory are applied to characterize alternative mechanisms for the reductive dechlorination of hexachloroethane (HCA) to perchloroethylene (PCE). Aqueous solvation effects are included using the SM5.42R continuum solvation model. After correction for a small systematic error in the electron affinity of the chlorine atom, theoretical predictions are accurate to within 23 mV for four aqueous reduction potentials relevant to HCA. A single pathway that proceeds via two successive single-electron transfer/barrierless chloride elimination steps, is predicted to be the dominant mechanism for reductive dechlorination. An alternative pathway predicted to be accessible involves trichloromethylchlorocarbene as a reactive intermediate. Bimolecular reactions of the carbene with other species at millimolar or higher concentrations are predicted to potentially be competitive with its unimolecular rearrangement to form PCE.

Rearrangements of C(7)H(6) Isomers: Computational Studies of the Interconversions of Bicyclo[3.2.0]hepta-1,3,6-triene, Bicyclo[3.2.0]hepta-3,6-diene-2-ylidene, Bicyclo[3.2.0]hepta-2,3,6-triene, and Cyclohepta-1,2,4,6-tetraene

The structures of several bicyclo[3.2.0] isomers on the C(7)H(6) potential energy surface, and the transition states which interconnect them, have been fully optimized using density functional theory (BLYP/6-31G). Relative energies were determined using single-point calculations at higher levels of theory (CCSD(T), CASPT2N). These calculations show that bicyclo[3.2.0]hepta-3,6-diene-2-ylidene (2) readily undergoes ring opening to cycloheptatetraene (4) with a barrier of 5 kcal/mol. The rearrangement of 2 to 4 occurs without intervention of bicyclo[3.2.0]hepta-1,3,6-triene (1). Triene 1 is much more stable than carbene 2, but faces a much higher barrier (35 kcal/mol) to rearrangement to cycloheptatetraene (4). The singlet and triplet states of carbene 2, along with the triplet state of triene 1 and the singlet state of bicyclo[3.2.0]hepta-2,3,6-triene (3), lie very close in energy, ca. 55 kcal/mol higher than the singlet state of 4. The computed transition states for the electrocyclic ring closure of cycloheptatetraene (4) to bicyclo[3.2.0]hepta-1,3,6-triene (1) and for the degenerate 1,5-sigmatropic H-shift in triene 1 occur at very similar energy. This finding provides a rationalization for the experimental observation of (13)C-label scrambling upon high-temperature pyrolysis of various C(7)H(6) isomers. The calculated IR frequencies and intensities for 1, (1)()2, (3)()2, and 3 may aid future identification of these species.