Suicide Nucleophilic Attack: Reactions of Benzohydroxamate Anion with Bis(2,4-dinitrophenyl) Phosphate

Ultraviolet dechlorination of tetrachloro-p-benzoquinone by hydrogen sulfide: Theoretical confirmation of the significance of hydrosulfide radical

In this work, a systematical investigation on the role of hydrogen sulfide (H₂S) on the transformation of tetrachloro-p-benzoquinone (TCBQ) under ultraviolet (UV) irradiation (at 253.7 nm) in aqueous solution has been conducted through quantum chemical calculations. Under the UV irradiation, with the forward energy barrier (E_a,f, 11.7 kcal mol^-1) much lower than the reverse one (E_a,r, 22.3 kcal mol^-1), the first triplet state of TCBQ was kinetically feasible to react with bisulfide anion (HS^-) via the Michael addition, and the addition of HS^- could promote the release of Cl^- and the formation of primary dechlorination product (HS-TriCBQ). During the UV photolysis of the primary dechlorination products (HO-TriCBQ and HS-TriCBQ) in the presence of H₂O and H₂S, the addition of nucleophile (OH^- or HS^-) to the ortho-position of the hydroxyl or thiol group might be the most efficient pathway for the dechlorination, and their respective E_a,f were 9.2 kcal mol^-1 (for HS^--hydroxyl), 1.1 kcal mol^-1 (for OH^--thiol) and 8.9 kcal mol^-1 (for HS^--thiol). Moreover, the electron transfer from HS^- to the first triplet states could generate hydrosulfide radical for the dechlorination of TCBQ. The findings in the present study may provide some important theoretical foundation for the dehalogenation of TCBQ as well as other halobenzoquinones.

Waste-derived biocatalysts for pesticide degradation

A green approach to produce a cellulose-derived biocatalyst containing hydroxamic acids targeted for the neutralization of toxic organophosphates is shown. The cellulose source, rice husk, is among the largest agricultural waste worldwide and can be strategically functionalized, broadening its sustainable application. Herein, rice husk was oxidized in different degrees, leading to carboxylic acid-based colloidal and solid samples. These were functionalized with hydroxamic acids via amide bonds and fully characterized. The hydroxamic acid derived biocatalysts were evaluated in the cleavage of toxic organophosphates, including the pesticide Paraoxon. Catalytic increments reached up to 10⁷-fold compared to non-catalyzed reactions. Most impressively, the materials showed P atom-selectivity and recyclability features. This guarantees only one reaction pathway that leads to less toxic products, hereby, detoxifies. Overall, highly sustainable catalysts are presented, that benefits from waste source, its green functionalization and is successfully employed for the promotion of chemical security of threatening organophosphates. To the best of our knowledge, this is the first report of a hydroxamate-derived rice husk (selectively modified at the C6 of cellulose) and its application in organophosphates reaction.

Ultraviolet photolysis of monochloro-p-benzoquinone (MCBQ) in aqueous solution: Theoretical investigation into the dechlorination

In this work, the UV-induced transformation of monochloro-p-benzoquinone (MCBQ) in aqueous solution has been systematically investigated through quantum chemical calculations. During the UV irradiation at 253.7 nm, the first triplet state of MCBQ (³MCBQ*) was from the intersystem crossing of its first excited singlet state (¹MCBQ*). In aqueous solution, the nucleophilic attack of OH^- on carbon atoms in ³MCBQ* was the central reaction. The addition of OH^- to olefinic carbon atoms was much more kinetically feasible than that to carbonyl carbon atoms, even though the carbonyl carbon atoms were more positively charged. Moreover, OH^- preferred to add to the ortho-position of C-Cl bond, where the unchlorinated atom was more negatively charged than the chlorinated one. The UV photolysis of the primary intermediate (HO-CBQ) was not the same as that of MCBQ. The attack of OH^- on the para-position of C-Cl bond was the most efficient pathway. The addition of OH^- to the chlorinated atom of ³HO-CBQ* was much more efficient than that in the case of ³MCBQ*, which reveals that more UV irradiation may promote the dechlorination. The findings in the present study may be helpful to enrich the understanding of the halobenzoquinones transformation in aqueous solution.

Nucleophilic Neutralization of Organophosphates: Lack of Selectivity or Plenty of Versatility?

Neutralization of organophosphates is an issue of public health and safety, involving agrochemicals and chemical warfare. A promising approach is the nucleophilic neutralization, scope of this review, which focuses on the molecular nucleophiles: hydroxide, imidazole derivatives, alpha nucleophiles, amines and other nucleophiles. A reactivity mapping is given correlating the pathways and reaction efficiency with structural dependence of the nucleophile (basicity) and the organophosphate (electrophilic centers, P=O/P=S shift, leaving and non-leaving group). Reactions extremely unfavorable (>20 years) can be reduced to seconds with various nucleophiles, some which are catalytic. Although there is no universal nucleophile, a lack of selectivity in some cases accounts for plenty of versatility in other reactions. The ideal neutralization requires a solid mechanistic understanding, together with balancing factors such as milder conditions, fast process, selectivity and less toxic products.

Towards catch-up therapy: evaluation of nucleophilic active pharmaceutical ingredients for the treatment of percutaneous VX poisoning, in-vial and in-vitro studies

Dermal exposure to low volatility organophosphorus chemical warfare agents (OP CWA) poses a great risk to the exposed person. Due to their lipophilic nature, these compounds rapidly absorb into the skin, leading to the formation of a "dermal reservoir" from which they slowly enter the bloodstream causing prolonged intoxication. Traditionally, strategies to counter the toxicity of such substances consist of chemical decontamination/physical removal of the residual agent from the skin surface (preferably as soon as possible following the exposure) and administration of antidotes in the case of intoxication signs. Hence, these strategies are unable to counter a substantial amount of the agent, which accumulates inthe dermal reservoir. More than a decade ago, the concept of a "catch-up therapy" intended to neutralize the dermal reservoir was suggested. Herein, we describe examples of potential "catch-up therapy" lotions - vehicles designed to deliver small nucleophilic molecules into the skin and potentially decompose the remaining CWA before it reaches the blood stream. Eleven nucleophilic compounds, based on approved drugs, were initially screened. They were then tested in various binary solutions, for their detoxification efficacy and degradation ability towards lipophilic OP CWA models such as dibutylphosphofluoridate and o-nitro-phenyl diphenyl phosphate, as well as the nerve agent VX, by means of kinetic ³¹P NMR and UV-Vis spectroscopy. Of these, the potassium and diethyl ammonium salts of acetohydroxamic acid (AHAK and AHA DEA) in (DMSO/H₂O 1:4) were found to be the most active nucleophiles, hydrolyzing VX in practical time scales (t_1/2 = 5.28 and 6.78 min, respectively). The vehicle solution DMSO/H₂O 1:4 promoted the penetration of substantial amounts of AHA K and AHA DEA through excised pig skin in in-vitro studies, suggesting that such formulations may serve as useful CWA nucleophilic scavengers for both on and within -skin detoxification. These findings may pave the way to a more efficacious treatment against low volatility OP CWA percutaneous poisoning.

Enhancing Phosphate Diester Cleavage by a Zinc Complex through Controlling Nucleophile Coordination

Metal-ion complexes are the most effective artificial catalysts capable of cleaving phosphate diesters under mild aqueous conditions. A central strategy for making these complexes highly reactive has been to use ligand-based alcohols that are coordinated to the ion, providing an ionised nucleophile under neutral conditions but at the expense of deactivating it. We have created a highly reactive Zn complex that is 350-fold more reactive than an alcohol analogue by preventing the nucleophile binding to the metal ion. This strategy successfully delivers the benefits of efficient nucleophile delivery without strongly deactivating the metal ion Lewis acidity nor the oxyanion nucleophilicity. Varying the leaving group reveals that the transition state of the reaction is much further advanced than the reaction with hydroxide.

From α-nucleophiles to functionalized aggregates: exploring the reactivity of hydroxamate ion towards esterolytic reactions in micelles

Hydroxamate ions as α-nucleophiles for esterolytic reactions in water and micelles.

Ionic liquids: anion effect on the reaction of O,O-diethyl O-(2,4-dinitrophenyl) phosphate triester with piperidine

The ionic liquids can be considered as designer solvents in the titled reaction because by an appropriate choice of the anion it is possible to steer the selectivity of this reaction.

A combined experimental and computational investigation on the unusual molecular mechanism of the Lossen rearrangement reaction activated by carcinogenic halogenated quinones

The classic Lossen rearrangement is a well-known reaction describing the transformation of an O-activated hydroxamic acid into the corresponding isocyanate. In this study, we found that chlorinated benzoquinones (CnBQ) serve as a new class of agents for the activation of benzohydroxamic acid (BHA), leading to Lossen rearrangement. Compared to the classic one, this new kind of CnBQ-activated Lossen rearrangement has the following unique characteristics: (1) The stability of CnBQ-activated BHA intermediates was found to depend not only on the degree but also on the position of Cl-substitution on CnBQs, which can be divided into two subgroups. (2) It is the relative energy of the anionic CnBQ-BHA intermediates that determine the rate of this CnBQ-activated rearrangement, which is the rate-limiting step, and the Cl or H ortho to the reaction site at CnBQ is crucial for the stability of the anionic intermediates. (3) A pKa-activation energy correlation was observed, which can explain why the correlation exists between the rate of the rearrangement and the acidity of the conjugate acid of the anionic leaving group, the hydroxlated quinones. These findings may have broad implications for future research on halogenated quinoid carcinogens and hydroxamate biomedical agents.

Functionalized graphene oxide as a nanocatalyst in dephosphorylation reactions: pursuing artificial enzymes

The first report on the use of thiol-functionalized graphene oxide as a recyclable nanocatalyst in dephosphorylation reactions with impressive activity (>10⁵-fold).

Polymers containing hydroxamate groups: nanoreactors for hydrolysis of phosphoryl esters

A polyhydroxamicalkanoate (PHA) polymer containing the functional groups hydroxamic acid and carboxylic acid with the ability to accelerate dephosphorylation reactions is proposed. The methodology used to prepare this polymer favored the position of the two functional groups next to each other, which allows for the cooperativity between these groups. This cooperative effect has an important role when one wants to mimic enzymes. The catalytic effect promoted by the polymer was evaluated on the cleavage of the bis(2,4-dinitrophenyl) phosphate (BDNPP) and diethyl 2,4-dinitrophenyl phosphate (DEDNPP) esters. Indeed, PHA was very efficient and promiscuous because it increased the rate of both reactions by a factor of up to 10(6)-fold. Isothermal titration calorimetry (ITC) experiments showed that the presence of PHA aids the formation of cetyltrimethylammonium bromide (CTABr) micelles. Thus, the effect of the cationic surfactant CTABr on the dephosphorylation of BDNPP by PHA was also investigated, and it was observed that, when CTABr is added to PHA, the reaction is ca. 15-fold faster compared to the reaction when only PHA is present.

Detoxifying carcinogenic polyhalogenated quinones by hydroxamic acids via an unusual double Lossen rearrangement mechanism

Hydroxamic acids, which are best-known for their metal-chelating properties in biomedical research, have been found to effectively detoxify the carcinogenic polyhalogenated quinoid metabolites of pentachlorophenol and other persistent organic pollutants. However, the chemical mechanism underlying such detoxication is unclear. Here we show that benzohydroxamic acid (BHA) could dramatically accelerate the conversion of the highly toxic tetrachloro-1, 4-benzoquinone (p-chloranil) to the much less toxic 2,5-dichloro-3, 6-dihydroxy-1, 4-benzoquonine (chloranilic acid), with rate accelerations of up to 150,000-fold. In contrast, no enhancing effect was observed with O-methyl BHA. The major reaction product of BHA was isolated and identified as O-phenylcarbamyl benzohydroxamate. On the basis of these data and oxygen-18 isotope-labeling studies, we proposed that suicidal nucleophilic attack coupled with an unexpected double Lossen rearrangement reaction was responsible for this remarkable acceleration of the detoxication reaction. This is the first report of an unusually mild and facile Lossen-type rearrangement, which could take place under normal physiological conditions in two consecutive steps. Our findings may have broad biological and environmental implications for future research on hydroxamic acids and polyhalogenated quinoid carcinogens, which are two important classes of compounds of major biomedical and environmental interest.