Pier Carlo Montevecchi

A Kinetic Study of S‐Nitrosothiol Decomposition

Under anaerobic conditions S-nitrosothiols 1 a–e undergo thermal decomposition by homolytic cleavage of the S−N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k1, have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1 c (k1(69 °C)=13×10−3 min−1) and 1 d (k1(69 °C)=91×10−3 min−1) decomposed faster than the primary nitrosothiols 1 a (k1(69 °C)=3.0×10−3 min−1) and 1 b (k1(69 °C)=6.5×10−3 min−1). The activation energies (E#=20.5–22.8 Kcal mol−1) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S-nitrosothiols 1 a–e takes place by an autocatalytic chain-decomposition process catalyzed by N2O3. The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p-cresol, β-styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.Under anaerobic conditions S-nitrosothiols 1a-e undergo thermal decomposition by homolytic cleavage of the S-N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k(t), have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1c (k1(69 degrees C) = 13 x 10(-3) min(-1)) and 1d (k1(69 degrees C) = 91 x 10(-3) min(-1)) decomposed faster than the primary nitrosothiols 1a (k1(69 degrees C) = 3.0 x 10(-3) min(-1)) and 1b (k1(69 degrees C) = 6.5 x 10(-3) min(-1)). The activation energies (E# = 20.5-22.8 Kcal mol(-1)) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S-nitrosothiols 1a-e takes place by an autocatalytic chain-decomposition process catalyzed by N2O3. The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p-cresol, beta-styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.

A Study of Vinyl Radical Cyclization onto the Azido Group by Addition of Sulfanyl, Stannyl, and Silyl Radicals to Alkynyl Azides☆

Thermal radical reactions of azidoalkynes 2, 8, 14, and 21a–c with thiols 1a–c afford 2-sulfanylvinyl radicals by selective addition of sulfanyl radicals to the triple bond. 1-Phenylvinyl radicals 23 and 30a, as well as vinyl radical 30b, undergo fast 5-cyclization onto the aromatic azide function to give cyclized indoles. In contrast, both 1-phenyl (15, 17) and 1-alkyl (3a,b, 9) vinyl radicals fail to add to their aliphatic azido substituents and exclusively undergo cyclization onto the aromatic sulfanyl ring and H transfer from the thiol precursor. Azidoalkynes 14 and 21a react with Bu3SnH and TMSS under radical conditions to give instead the corresponding amines as a result of preferential attack of Bu3Sn · and (TMS)3Si · radicals on the azido group rather than on the triple bond. Evidence is provided that alkyl radical cyclizations onto azides are not feasible in the presence of thiol, in contrast with the reported utility of these cyclization reactions in the presence of Bu3SnH and TMSS.

Reduction of azides to amines by samarium diiodide

Abstract Amines can be prepared by reduction with samarium diiodide under mild conditions and in good yields.

Thiol radical addition to alkynes. Sulfanyl radical addition and hydrogen atom abstraction relative reaction rates

Abstract 2-(Toluenesulfanyl)- 1 and 2-(benzenesulfanyl)-phenylacetylene 10 reacted with benzenethiol and toluenethiol, respectively, in the presence of AIBN at 84 and 154 °C to give products deriving from vinyl radicals 2 which undergo hydrogen abstraction reaction and 5- ortho and 5- exo cyclization onto both the adjacent phenyl rings in competition with the β-fragmentation. Definitive evidence has been obtained that alkanesulfanyl radical addition to the alkyne triple bond is a non-reversible process, whereas arenesulfanyl radicals add in a reversible mode. Competing experiments involving several alkynes towards benzenethiol and benzeneethanethiol radical addition have been performed in order to determine the relative rate constants of the sulfanyl radical addition to the alkyne triple bond (k 1 ) and the hydrogen abstraction reaction by the resulting vinyl radicals (k H ). The k 1 values are mainly determined by the vinyl radical stabilization provided by the C-(α) vinyl radical substituent, whereas the k H values seem to be mainly determined by polarity factors. An unexpected different behavior between α-propyl and α-long-chain substituents is discussed in terms of different hybridization of the vinyl radical.

Synthesis of 5-Acetoxy-2(5H)-Furanones through Manganese(III)-Promoted Functionalization of Arylacetylenes

Abstract Reaction of phenylacetylenes 1a – e with manganese(III) triacetate in acetic acid/acetic anhydride at reflux gave the corresponding 5-acetoxy-5-phenyl-2(5 H )-furanones 2a – e in good yield (40–86%). Furanones 2 were derived from further oxidation of the initially formed 5-phenyl-2(3 H )-furanones 4 which were in turn obtained through regioselective addition of carboxymethyl radicals to the alkyne 1 triple bond and subsequent oxidative cyclization of the resulting α-phenylvinyl radical 3 . In contrast, the (trimethylsilyl)alkylacetylene 1f gave the corresponding furanone 2f in only 25% yield, whereas alkylacetylenes 1g – h totally failed to give the corresponding furanones 2f – h , probably due to the incapability of the α-alkyl vinyl radical intermediates 3g – h of undergoing oxidative cyclization.

Boron trifluoride promoted reaction of benzenesulphenanilides with alkenes-arylaminosulphenylation of alkenes.

Abstract The addition of benzenesulphenanilides to alkenes in the presence of boron trifluoride provides a practicable synthetic procedure for the arylaminosulphenylation of alkenes.

Synthetic utility of 4'-nitrobenzenesulfenanilide in the functionalization of carbon-carbon double and triple bonds : its use in the bromosulfenylation of alkenes and alkynes

Abstract The reaction of 4′-nitrobenzenesulfenanilide (NBSA) with hydrobromic acid, suitably carried out at room temperature in the presence of cyclohexene, trans -hex-3-ene, hex-1-ene and 3,3-dimethylbut-1-ene, results in quantitative isolation of corresponding 2-bromoalkyl phenyl sulfides which occur with trans -stereospecificity and anti-Markovnikov regiospecificity through electrophilic addition of initially-formed benzenesulfenyl bromide to the alkene double bond. Similar reaction in the presence of mono- and di- substituted alkyl-and phenyl-acetylenes generally affords (E)-2-bromovinyl phenyl sulfides in good yields, which become lower with decreasing nucleophilic power of the alkyne employed. However, in the presence of parent acetylene, no virtual formation of the corresponding sulfide adduct occurs, but almost exclusive formation of diphenyl disulfide essentially ascribable to preferred decomposition of the highly unstable benzenesulfenyl bromide intermediate. The present additions of benzenesulfenyl bromide to alkenes and alkynes are believed to involve the initial intermediacy of thiiranium- and thiirenium-like ions, respectively, by analogy with related AdE reactions of sulfenyl chlorides.

4′-Nitroarenesulphenanilides: Their use in the synthesis of unsymmetrical disulphides

Abstract The reaction of 4′-nitroarenesulphenanilides with thiols in the presence of boron trifluoride etherate can provide an effective route to unsymmetrical disulphides.

The Chemistry of Peroxynitrite: Involvement of an ET Process in the Radical Nitration of Unsaturated and Aromatic Systems

Reactions of peroxynitrous acid, HPN, with styrene under acidic conditions lead to the oxime 1, the nitrate 2, benzaldehyde (3), and α-nitroacetophenone (4) in overall yields that depend strongly on the pH value and with a product distribution that depends on the dioxygen concentration. The results are rationalized by assuming that HPN undergoes acid-catalyzed decomposition to give nitrous anhydride, or its synthetic equivalent, which is responsible for the regioselective nitration of the styrene double bond by an ET process. The resulting β-nitrobenzyl radical 6 can, depending on the reaction conditions, undergo reversible coupling with nitric oxide to afford the nitroso derivative 7 and then the tautomeric oxime 1, or trapping by dioxygen, eventually leading to products 2, 3, and 4 through the intermediacy of the peroxynitrite derivative 8. Oxime 1 and nitrate 2 are also obtained by treating styrene with nitrous anhydride under protic conditions, the latter being produced in situ from nitric oxide/dioxygen. Similarly to styrene, 1,4-diphenylbutadiene (14) gives radicals 22 and 21 by competitive trapping at the side chain and at the aromatic ring. In turn, radicals 22 and 21 undergo β-fragmentation reactions or trapping by dioxygen with eventual formation of nitrates 16 and 17, cinnamic aldehyde (18), and the diol 15. Finally, the HPN-promoted reaction of p-cresol (27) leads to the 2-nitro derivative 28 through an initial electron-transfer process followed by in cage recombination of the resulting radical ion pair.

A convenient synthesis of β-keto phenyl sulphides from alkynes.

Summary β-Keto sulphides can be conveniently prepared by BF 3 -promoted reaction of 4′-nitrobenzenesulphenanilide in acetonitrile or acetic acid and subsequent hydrolysis of the resulting β-acetamidino- or β-acetoxy-vinyl phenyl sulphides.