Cleaving paraoxon with hydroxylamine: Ammonium oxide isomer favors a Frontside attack mechanism

Abstract

A detailed molecular understanding of the mechanisms of the dephosphorylation of phosphate triesters due to nucleophilic attack can be very useful for the design of nucleophiles that are more efficient. In this work, we report a combined experimental and theoretical study of the reaction of hydroxylamine with paraoxon. The profile of the reaction rate according to pH was determined, and it was found that protonated hydroxylamine was unreactive, while the neutral form reacted via the zwitterion tautomer, with an overall free energy barrier of 23.0 kcal mol‐1. The anionic form was the most reactive, with a free energy barrier of 18.1 kcal mol‐1. Computational calculations revealed 2 mechanisms for the nucleophilic attack of the zwitterion form: the usual backside attack and a new frontside attack mechanism. The former proceeded according to a 2‐step associative mechanism, while the latter was a concerted single‐step mechanism involving attack of the hydroxylamine oxygen on the phosphorus center and interaction of the NH3+ group with the oxygen of the P═O group. The calculations indicated that the free energy barrier for the frontside attack was more favorable than the backside attack by 3 kcal mol‐1, supporting the notion that the observed reaction occurs by frontside attack. The reaction with the anionic hydroxylamine form occurred according to a single‐step concerted ANDN mechanism.

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

“…H 2 O [38] À 1.54 100 % P No 7 years À OH [38] 15.54 100 % P No 4 min IMZ [38] 7.00 100 % P Yes 8 min NH 2 OH [70c] 5.96 100 % P No 3 min BHA [50] 9. H 2 O [97] À 1.54 100 % P No > 20 years e À OH [55] 15.54 100 % P No 46 s IMZ [55] 7.00 100 % P Yes 77 h NH 2 OH [75] 5.96 100 % P No 5 min Piperidine [51] 10.82 100 % P No 2 h 1-formyl piperazine [51] 7. 63 100 % P No 32 h 4-hydroxy-pyridine [51] 11.5 100 % P No 27 min 3,4-dimethyl pyridine [51] 5.…”

“…Considering only the attack at phosphorous, the reaction of hydroxylamine with EDNPP is over 2 × 10 5 times faster than the spontaneous hydrolysis. [67] On the other hand, the dephosphorylation reactions of DEDNPP and Paraoxon by hydroxylamine under mild conditions are selective (at P) and effective (fast): comparing with respective spontaneous hydrolysis, rate enhancements of 2 × 10 6 and 9 × 10 5 -fold are observed in the presence of NH 2 OH for DEDNPP [73] and Paraoxon, [75] respectively. Although, hydroxylamine does not act as a catalyst, and leads to hydrazine.…”

Degradation of Organophosphates Promoted by 1,2,4-Triazole Anion: Exploring Scaffolds for Efficient Catalytic Systems

“…(A) Kinetic pH profile at 25 °C for the reaction of TAZ(−) (0.5 mol L –1 ) with DEDNPP (■, left axis) and the hydrolysis DEDNPP (□, right axis). (B) Kinetic pH profile at 25 °C for the reaction of TAZ(−) (0.5 mol L –1 ) with Paraoxon (●, left axis) and the hydrolysis of Paraoxon (○, right axis). Hydrolyses were included for comparison purposes.…”

Hydroperoxo on the Niobium Oxyhydroxide Surface as the Active Species in the Catalyzed Oxidation of Organic Sulfide by Hydrogen Peroxide

“…Here, an oxygen atom from H 2 O 2 on the reaction path attacks a niobium atom nucleophilically, and a simultaneous migration of a proton from H 2 O 2 to the hydroxyl on the surface occurs, leading to the formation of the hydroperoxo group (−OOH) on the surface and the release of a water molecule to the medium. This mechanism is also a S N 2-like process that occurs on the niobium atom with the groups entering and leaving on the same side …”

“…The other This mechanism is also a S N 2-like process that occurs on the niobium atom with the groups entering and leaving on the same side. 33 The determination of the free energy barrier (ΔG ‡ ) for both processes was done using acetonitrile as a solvent, thus corresponding to the conditions present in catalysis. 12 The calculations for the ΔG ‡ point out that the second mechanism (TS1b) presents the lowest barrier of just 25.5 kcal mol −1 , as opposed to a barrier of 38.6 kcal mol −1 for TS1a.…”