Mechanisms of Reactions of Sulfur Hydride Hydroxide: Tautomerism, Condensations, and C-Sulfenylation and O-Sulfenylation of 2,4-Pentanedione

Abstract

The conformations, equilibrium structures, hydrogen bonds, and non-covalent interactions involved in the mechanisms of tautomerization, condensations, and C-sulfenylation and O-sulfenylation of 2,4-pentanedione by sulfur hydride hydroxide (hydrogen thioperoxide, oxadisulfane, H-SOH) have been studied using BD(T), CCSD(T), and QCISD(T) with the cc-pVTZ basis set and using B3LYP, B3PW91, CAM-B3LYP, PBE1PBE, PBEh1PBE, LC-ωPBE, M06-2X, and ωB97XD with the 6-311+G(d,p) basis set. All levels of theory predict the sulfenyl (H-SOH) tautomer of hydrogen thioperoxide to be lower in energy than the sulfinyl (H2S═O) tautomer. Four reasonable mechanisms were considered for the tautomerization of the sulfenyl tautomer of hydrogen thioperoxide to the sulfinyl tautomer: a cyclic three-membered water-free transition state (TS, CCSD(T) activation energy barrier E(⧧) = 65.1 kcal/mol), a cyclic five-membered transition state with one water molecule (TSH2O, E(⧧) = 31.1 kcal/mol), a cyclic seven-membered transition state with two water molecules (TS2H2O, E(⧧) = 14.5 kcal/mol), and a cyclic nine-membered transition state with three water molecules (TS3H2O, E(⧧) = 5.6 kcal/mol). The mechanisms involve hydrogen-bonded reactant complexes and hydrogen-bonded product complexes. The CCSD(T)-predicted energy barriers for the condensation of hydrogen thioperoxide to form thiosulfinic acid through transition states with zero, one, and two waters are E(⧧) = 42.0, 18.3, and 0 kcal/mol, respectively. Mixed condensation reactions are predicted to afford organosulfur products and compounds containing sulfur-selenium bonds. Hydrogen thioperoxide is predicted to add to 2,4-pentanedione to form C-sulfenylated (sulfide, thioether) and O-sulfenylated (sulfenate ester) products. Similar mechanistic trends and reaction pathways are observed in the tautomerism, condensations, and C-sulfenylation and O-sulfenylation reactions of hydrogen thioperoxide. The water molecules set up proton relay networks (bridges) that reduce ring strain, generate favorable conformations for reactivity, lower energy barriers, and increase the numbers of stabilizing hydrogen bonds and nonbonding interactions.

Characterization of Polysulfides, Polysulfanes, and Other Unique Species in the Reaction between GSNO and H2S

“…As in Scheme 3, these species may result from thiol addition to sulfinyl tautomers. Although rarely characterized, a recent density functional theory study [40] has shown that inclusion of water solvation dramatically lowers the energy of the S(IV)sulfinyl tautomers, suggesting that mixed-valence sulfide species may be more prevalent in aqueous solution. Alternatively, these bridging S(IV) species may be generated from equilibrating insertion reactions of a trisulfane species with a disulfide bond, as seen in Equation (6), and also predicted by gas-phase calculation to have a higher reaction enthalpy than S-S bond dissociation [41].…”

Kinetic evidence in favor of glyoxalase III and against deglycase activity of DJ‐1

“…Since the C106 residue is readily oxidized, the dissolved O 2 may induce the labile intermediate state of C106‐sulfenic acid, which could be a reason for the decreased K m of DJ‐1. It has been reported that sulfenic acid can react as both nucleophile and electrophile according to two different tautomers (sulfenyl and sulfinyl; Freeman, 2015 ; Kumar & Farmer, 2017 ). Since the sulfenyl form of the C106‐sulfenic acid might have a higher affinity for MG, we measured the K m and k cat values of DJ‐1 for PG under O 2 ‐depleted conditions using N 2 ‐purge.…”

Chemical trapping and characterization of small oxoacids of sulfur (SOS) generated in aqueous oxidations of H2S

“…The initial oxoacid produced by H 2 S oxidation is sulfenic acid, HSOH. It may exist in two tautomeric forms [38] , [39] , [40] , sulfenyl and sulfinyl, 1 and 2 , Scheme 2 , which should generate different derivatized products, as were observed in glutathione peroxidations [33 ]. Upon reaction of H 2 S with peroxymaleic acid, the sulfenyl tautomer is trapped by sequential reaction with dimedone and iodoacetamide yielding the thioether derivative, 4 in Scheme 3 and Fig.…”