Sulfinyl Radical Research Articles

A-Band Absorption Spectrum of the ClSO Radical: Electronic Structure of the Sulfinyl Group.

Sulfur oxide species (RSOx) play a critical role in many fields, ranging from biology to atmospheric chemistry. Chlorine-containing sulfur oxides may play a key role in sulfate aerosol formation in Venus' cloud layer by catalyzing the oxidation of SO to SO2 via sulfinyl radicals (RSO). We present results from the gas-phase UV-vis transient absorption spectroscopy study of the simplest sulfinyl radical, ClSO, generated from the pulsed-laser photolysis of thionyl chloride at 248 nm (at 40 Torr of N2 and 292 K). A weak absorption spectrum from 350 to 480 nm with a peak at 385 nm was observed, with partially resolved vibronic bands (spacing = 226 cm-1), and a peak cross section σ(385 nm) = (7.6 ± 1.9) × 10-20 cm2. From ab initio calculations at the EOMEE-CCSD/ano-pVQZ level, we assigned this band to 12A' ← X2A″ and 22A' ← X2A″ transitions. The spectrum was modeled as a sum of a bound-to-free transition to the 12A' state and a bound-to-bound transition to the 22A' state with similar oscillator strengths; the prediction agreed well with the observed spectrum. We attributed the vibronic structure to a progression in the bending vibration of the 22A' state. Further calculations at the XDW-CASPT2 level predicted a conical intersection between the excited 12A' and 22A' potential energy surfaces near the Franck-Condon region. The geometry of the minimum-energy conical intersection was similar to that of the ground-state geometry. The lack of structure at shorter wavelengths could be evidence of a short excited-state lifetime arising from strong vibronic coupling. From simplified molecular orbital analysis, we attributed the ClSO spectrum to transitions involving the out-of-plane π/π* orbitals along the S-O bond and the in-plane orbital possessing a σ/σ* character along the S-Cl bond. We hypothesize that these orbitals are common to other sulfinyl radicals, RSO, which would share a combination of a strong and a weak transition in the UV (near 300 nm) and visible (400-600 nm) regions.

Iodine-mediated photoinduced tuneable disulfonylation and sulfinylsulfonylation of alkynes with sodium arylsulfinates.

An efficient transition-metal free iodine-mediated tuneable disulfonylation and sulfinylsulfonylation of alkynes with sodium arylsulfinates under visible light irradiation has been developed. This photochemical protocol offers broad substrate scope of 1,2-bissulfonylethenes with high functional group tolerance and good yields under mild reaction conditions. In addition, it was found that β-sulfinyl alkenylsulfones can be obtained in the absence of base. It is proposed that the reactions proceed via sulfonyl and sulfinyl radicals.

Visible-light-induced bifunctionalisation of (homo)propargylic amines with CO2 and arylsulfinates.

An unprecedented carboxylative sulfonylation of (homo)propargyl amines with CO2 and sodium arylsulfinates under visible light irradiation has been developed with high efficiency. This ruthenium-catalysed photochemical protocol offers broad substrate scope giving 2-oxazolidinones and 2-oxazinones bearing alkyl sulfones in good yields under ambient reaction conditions. An in situ double bond isomerisation occurs in tandem. A mechanistic rationale for these radical-initiated carboxylative cyclisations involving sulfinyl radicals is presented, supported by control and quenching experiments.

Aryl Ketone Mediated Light-Driven Naphthylation of C(sp3)–H Bonds Attached to either Oxygen or Nitrogen Substituents

AbstractA light-driven naphthylation was achieved at C(sp3)–H bonds attached to either oxygen or nitrogen substituents using sulfonylnaphthalenes as a naphthalene precursor in the presence of 4-benzoylpyridine at ambient temperature. The present transformation is proposed to proceed through the generation of a carbon radical species via chemoselective cleavage of the heteroatom-substituted C(sp3)–H bond by photoexcited 4-benzoylpyridine, the addition of the derived carbon radical to the electron-deficient sulfonylnaphthalene, and then rearomatization by releasing sulfinyl radical.

Fluoromethylsulfinyl radicals: spectroscopic characterization and photoisomerization via intramolecular hydrogen shift.

Two new sulfinyl radicals, CHF2SO˙ and CH2FSO˙, have been generated in the gas phase through homolytic cleavage of the weak S-S bonds in disulfane oxides CHF2S(O)SCF3 and CH2FS(O)SCF3 by high-vacuum flash pyrolysis (HVFP) at ca. 500 °C. The IR spectroscopy characterization of the two fluoromethylsulfinyl radicals in solid N2 (10 K), Ar (10 K), and Ne (3 K) matrices reveals the presence of two conformers for CHF2SO˙ (gauche and cis) and one conformer for CH2FSO˙ (gauche). Upon 266 nm laser irradiation, these radicals undergo both isomerization and decomposition in the matrices. In addition to the dominant formation of the elusive oxathiyl radicals CHF2OS˙ (gauche and cis) and CH2FOS˙ (gauche) via 1,2-alkyl migration, two higher-energy carbon-centered radicals ˙CF2SOH and ˙CHFSOH bearing similar molecular structures to hydroperoxyalkyl radicals (˙QOOH) form via intramolecular 1,3-hydrogen shift in the two sulfinyl radicals. Additionally, the involvement of 1,3-hydrogen shift in CHF2OS˙ and CH2FOS˙ is also indicated by the observation of the fragmentation species. The identification of these radicals by matrix-isolation IR and UV-vis spectroscopy is aided by the quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level of theory. The stability of the isomers of the two sulfinyl radicals CHF2SO˙ and CH2FSO˙ has been discussed according to the experimental observations and also based on the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) calculated energy profiles.

Exploring the synthetic application of sulfinyl radicals

This review summarized the applications of sulfinyl radicals in organic chemistry and thoroughly examined the challenges and future development trends of sulfinyl radicals in modern organic chemistry, as well as their structures and properties.

Synthetic exploration of sulfinyl radicals using sulfinyl sulfones

Sulfinyl radicals – one of the fundamental classes of S-centered radicals – have eluded synthetic application in organic chemistry for over 60 years, despite their potential to assemble valuable sulfoxide compounds. Here we report the successful generation and use of sulfinyl radicals in a dual radical addition/radical coupling with unsaturated hydrocarbons, where readily-accessed sulfinyl sulfones serve as the sulfinyl radical precursor. The strategy provides an entry to a variety of previously inaccessible linear and cyclic disulfurized adducts in a single step, and demonstrates tolerance to an extensive range of hydrocarbons and functional groups. Experimental and theoretical mechanistic investigations suggest that these reactions proceed through sequential sulfonyl and sulfinyl radical addition.

Synthesis of symmetrical / unsymmetrical thiosulfonates through the disproportionate coupling reaction of sulfonyl hydrazide mediated by phosphomolybdic acid

Phosphomolybdic acid (H3PMo12O40) is reported as a green low-cost catalyst for the synthesis of symmetrical / unsymmetrical thiosulfonates via the disproportionate coupling reaction of sulfonyl hydrazide. The attributes of this reported catalytic system include low catalyst loadings (1 mol%), efficient turnover, and high yields (up to 94%). Additionally, this reaction mechanism involves the formation of a thiyl radical and sulfonyl radical via sulfinyl radical disproportionation.

Spectroscopic characterization and photochemistry of the vinylsulfinyl radical.

The simplest α,β-unsaturated sulfinyl radical CH2[double bond, length as m-dash]C(H)SO˙ has been generated in the gas phase by high-vacuum flash pyrolysis (HVFP) of sulfoxide CH2[double bond, length as m-dash]C(H)S(O)CF3 at ca. 800 °C. Two planar cis and trans conformers of CH2[double bond, length as m-dash]C(H)SO˙ were isolated in cryogenic matrixes (N2, Ne, and Ar) and characterized with IR and UV/Vis spectroscopy. In addition to the photo-induced cis ⇋ trans conformational interconversion, CH2[double bond, length as m-dash]C(H)SO˙ displays complex photochemistry. Upon irradiation with a purple light LED (400 nm), CH2[double bond, length as m-dash]C(H)SO˙ isomerizes to novel radicals CH3SCO˙, ˙CH2SC(O)H, and ˙CH2C(O)SH with concomitant dissociation to a caged molecular complex CH3S˙CO. Subsequent UV-laser (266 nm) irradiation causes fragmentation to ˙CH3/OCS and additional formation of an elusive carbonyl radical CH3C(O)S˙, which rearranges to ˙CH2C(O)SH upon further UV-light irradiation (365 nm). The vibrational data and bonding analysis of the two conformers of CH2[double bond, length as m-dash]C(H)SO˙ suggest that both are floppy radicals in which the unpaired electron conjugates with the vicinal π(C[double bond, length as m-dash]C) bond, leading to significant contribution of the canonical resonance form of ˙CH2-C(H)SO. The mechanism for the isomerization of CH2[double bond, length as m-dash]C(H)SO˙ is discussed based on the observed intermediates along with a computed potential energy profile at the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level of theory.

Spectroscopic identification of the •SSNO isomers.

Elusive [S, S, N, O] isomers including the perthiyl radical •SSNO are S/N hybrid species in the complex bioinorganic chemistry of signaling molecules H2S and •NO. By mixing thermally generated disulfur (S2) with •NO in the gas phase, •SSNO was generated and subsequently isolated in cryogenic Ar- and N2-matrices at 10.0 K and 15.0 K, respectively. Upon irradiation with a 266 nm laser, •SSNO isomerizes to novel sulfinyl radicals cis-NSSO• and trans-NSSO• as well as thiyl radicals cis-OSNS• and trans-OSNS•, which have been characterized by combining matrix-isolation IR (15N-labeling) and UV/Vis spectroscopy and quantum chemical calculations at the CCSD(T)-F12/cc-pVTZ-F12 level of theory. The photo-induced reversible interconversion between NSSO• and OSNS• has also been observed.

Two-step reaction mechanism reveals new antioxidant capability of cysteine disulfides against hydroxyl radical attack

Cysteine disulfides, which constitute an important component in biological redox buffer systems, are highly reactive toward the hydroxyl radical (•OH). The mechanistic details of this reaction, however, remain unclear, largely due to the difficulty in characterizing unstable reaction products. Herein, we have developed a combined approach involving mass spectrometry (MS) and theoretical calculations to investigate reactions of •OH with cysteine disulfides (Cys-S-S-R) in the gas phase. Four types of first-generation products were identified: protonated ions of the cysteine thiyl radical (+Cys-S•), cysteine (+Cys-SH), cysteine sulfinyl radical (+Cys-SO•), and cysteine sulfenic acid (+Cys-SOH). The relative reaction rates and product branching ratios responded sensitively to the electronic property of the R group, providing key evidence to deriving a two-step reaction mechanism. The first step involved •OH conducting a back-side attack on one of the sulfur atoms, forming sulfenic acid (-SOH) and thiyl radical (-S•) product pairs. A subsequent H transfer step within the product complex was favored for protonated systems, generating sulfinyl radical (-SO•) and thiol (-SH) products. Because sulfenic acid is a potent scavenger of peroxyl radicals, our results implied that cysteine disulfide can form two lines of defense against reactive oxygen species, one using the cysteine disulfide itself and the other using the sulfenic acid product of the conversion of cysteine disulfide. This aspect suggested that, in a nonpolar environment, cysteine disulfides might play a more active role in the antioxidant network than previously appreciated.

Capture of the Sulfur Monoxide-Hydroxyl Radical Complex.

The elusive hydrogen-bonded sulfur monoxide-hydroxyl radical complex (•OH···OS), a missing intermediate in the atmospheric chemistry of SO2, was generated in the 266 nm laser photolysis of the sulfinyl radical HOSO• in cryogenic Ar-matrixes. In addition to the IR spectroscopic characterization with deuteration, its thermal conversion to HOSO• with an activation barrier of 0.33 ± 0.11 kcal mol-1 (calcd 0.32 kcal mol-1, CCSD(T)-F12a/AVTZ) in the temperature range of 15.0-21.0 K and a H/D kinetic isotope effect of 2.4 at 16.0 K have been observed.

Electrochemical synthesis of sulfonamides in a graphite powder macroelectrode

The electrochemical generation of sulfinyl radicals from commercially available and non-expensive sodium salts of sulfinic acids is described.

Disproportionate Coupling Reaction of Sodium Sulfinates Mediated by BF3·OEt2: An Approach to Symmetrical/Unsymmetrical Thiosulfonates.

The BF3·OEt2-mediated disproportionate coupling reaction of sodium sulfinates was found for the first time. In this reaction, various S-S(O)2 bonds can be formed, efficiently giving thiosulfonates in moderate to excellent yields. As a convenient protocol for the synthesis of symmetrical and unsymmetrical thiosulfonates, its reaction mechanism involves the formation of a thiyl radical and sulfonyl radical via a sulfinyl radical disproportionation. What is more, this transformation can also be applied practically as a gram-scale reaction and to the two-step synthesis of sulfone and sulfonamide in one pot in situ using thiosulfonate as an intermediate.

Nanostructured Silicas, a Platform for the Observation of Transient Radicals: Application to Sulfinyl Radicals

Diazenes, precursors of sulfinyl radicals, were used to functionalize nanostructured SBA-15 silicas either in the framework or on the pore by formation of covalent links, or by simple adsorption on the surface. Depending on their design and on the experimental conditions, these materials proved to be effective tools to study the behavior and the reactivity of arylsulfinyl radicals made persistent because of confinement effects. When the covalently linked precursors were irradiated at room temperature, a spectacular increase of the lifetime of the expected sulfinyl radicals was registered (up to 17 h). At higher temperature, upon thermal initiation at 473 K, the decomposition of the adsorbed precursor enabled visualization of the rearrangement of the corresponding arylsulfinyl radicals into sulfonyl radicals via O–S coupling.

Cover Feature: The Trifluoromethyl Sulfinyl and Oxathiyl Radicals (Chem. Eur. J. 7/2018)

Cover Feature: The Trifluoromethyl Sulfinyl and Oxathiyl Radicals (Chem. Eur. J. 7/2018)

The Trifluoromethyl Sulfinyl and Oxathiyl Radicals.

Two hitherto unreported sulfur-centered radicals CF3 SO. and CF3 OS. were generated in the gas phase through high-vacuum flash pyrolyses of sulfoxide CF3 S(O)X (X=CF3 , Cl, PhO) precursors. The CF3 OS. molecule is the first experimental example that constitutes an oxathiyl radical. It was isolated and characterized by combining matrix-isolation IR and UV/Vis spectroscopy with quantum chemical computations up to the UCCSD(T)-F12/cc-pVTZ-F12 level of theory. Upon UV light irradiation (254 or 266 nm), sulfinyl radical (CF3 SO. ) isomerizes to oxathiyl radical (CF3 OS. ) in cryogenic noble gas matrices (Ar and Ne). Natural population analyses at the BP86/def2-TZVPP//UCCSD(T)-F12/cc-pVTZ-F12 level suggest that the spin density in CF3 OS. is mainly localized on the sulfur atom (0.86), whereas, in CF3 SO. the spin density is almost equally distributed on the sulfur (0.55) and oxygen (0.43) atoms.

Parent Thioketene S-Oxide H2 CCSO: Gas-Phase Generation, Structure, and Bonding Analysis.

The parent thioketene S-oxide H2 CCSO has been generated in the gas phase through flash vacuum pyrolysis (ca. 1000 K) of vinyl sulfoxide H2 CC(Br)-S(O)CF3 via the intermediacy of a novel vinyl sulfinyl radical H2 C=C(Br)-SO (syn and anti conformers). Upon irradiation at 266 nm, H2 CCSO decomposes into HCCH/SO and H2 CS/CO in cryogenic Ar matrix. Whereas, visible-light irradiations result in syn↔anti conformational interconversion in H2 C=C(Br)-SO. The molecular structures of H2 CCSO and isomers are computationally studied at the CCSD(T)-F12/VTZ-F12 level of theory, and the bonding properties of H2 CCSO are analyzed with the EDA-NOCV method at the M06-2X/TZ2P level.

ChemInform Abstract: Copper‐Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals for the Synthesis of Sulfinates and Thiosulfonates.

ChemInform Abstract: Copper‐Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals for the Synthesis of Sulfinates and Thiosulfonates.

Copper‐Catalyzed Aerobic Formation of Unstable Sulfinyl Radicals for the Synthesis of Sulfinates and Thiosulfonates

AbstractCopper‐catalyzed aerobic coupling of thiols and alcohols affords sulfinates and thiosulfonates. These products are assumed to form via sulfinyl radicals which are not commonly found in oxidative coupling reactions of thiols. A reaction mechanism involving sulfinyl radicals is proposed, and mass and electron paramagnetic resonance (EPR) experimental results are provided.magnified image