Nickel-Catalysed Cross-Electrophile Coupling of Benzyl Bromides and Sulfonium Salts towards the Synthesis of Dihydrostilbenes

A nickel-catalysed reductive cross-coupling reaction between benzyl sulfonium salts and benzyl bromides is reported. Simple, stable and readily available sulfonium salts have shown their ability as leaving groups in cross-electrophile coupling, allowing the formation of challenging sp³–sp³ carbon-carbon bonds, towards the synthesis of interesting dihydrostilbene derivatives. In addition, benzyl tosyl derivatives have been demonstrated to be suitable substrates for reductive cross-coupling by in-situ formation of the corresponding sulfonium salt.     

MULTIPLE PATHWAYS IN THE BASE-CATALYZED REACTIONS OF S-ALLYL SULFONIUM COMPOUNDS

Abstract

Various S-allyl sulfonium salts were allowed to react with bases (EtONa/EtOH, NaH/THF). From product analyses, it was found that modes of reaction depend on both the structure of sulfonium salts and the bases employed. Among the observations, it is worthy to note the formation of cyclopropane derivatives from some allyl sulfonium salts.     

Effects of conjugated systems on UV-visible light-sensitive D-<Emphasis Type="Italic">π</Emphasis>-A type sulfonium salt photoacid generators

A series of D-π-A type sulfonium salt photoacid generators with different π-conjugated structures, such as triphenyl, phenylstilbene, styryl-biphenyl, and stilbene, were designed to determine the effect of molecular structures on the photochemical and photophysical properties. The mechanisms of photochemical generation of H⁺ were studied by UV-Vis spectroscopy, theoretical calculations, and fluorescence spectroscopy. It is found that the frontier orbits determine the absorption, the molar extinction coefficients, and the quantum yields of photoacid generation. Triphenyl systems connected with sulfonium are beneficial to increase the quantum yields of acid generation. The photoreactivity of four sulfonium salts was further evaluated through the polymerizations of various epoxide monomers at different irradiation wavelengths (365–425 nm) by using the real-time infrared spectroscopy with light-emitting diodes. The high quantum yields for acid generation (Φ _H ⁺ = ~0.32 to 0.58) and the high molar extinction coefficients (ε = ~23500 L?mol^?1?cm^?1 to 31000 L?mol^?1?cm^?1) of the sulfonium salts lead to high conversion rates (over 50%–80%). Hence, these photoinitiators exhibit potential for the photocuring applications.     

Novel benzyl sulfonium salt having an aromatic group on sulfur atom as a latent thermal initiator

Three benzyl p-hydroxyphenyl methylsulfonium salts with different counter anions were synthesized as novel latent thermal initiators. Syntheses of the sulfonium salts ( 2 ) were performed by the reaction of p-hydroxyphenyl methyl sulfide with benzyl chloride followed by exchange of the counter anion (Cl^?) with SbF^?₆ ( 2a ), PF^?₆ ( 2b ), or BF^?₄ ( 2c ). In the bulk polymerization of glycidyl phenyl ether (PGE) with 2 , initiator activity of the sulfonium salts was evaluated by comparison with that of benzyl tetramethylenesulfonium hexafluoroantimonate ( 1 , R = H). Among the initiators, 2a showed the highest activity, and was much more active than 1 (R = H). Since the polymerization of PGE with 2a proceeded efficiently at more than 80°C but not at all at less than 60°C, 2a was suggested to be a good latent thermal initiator.     

On the nature and geometry of the ylidic intermediates in the methylation of six-membered ring sulfonium salts

¹³C N.M.R. data indicate that different types of intermediates are generated by deprotonation of sulfonium salts with lithium bases $\text{viz}$ almost ‘free’ or ‘tight-ion-pair’ ylids, with different geometries at the carbanionic carbon     

Reactions of aromatic and heteroaromatic compounds bearing electron-acceptor substituents

Dimethyl(2-thienyl)sulfonium salts were synthesized for the first time. The corresponding triiodomercurates were obtained by the reaction of methyl 2-thienyl sulfide or its substituted derivatives with methyl iodide and mercuric iodide. Sulfonium perchlorates are formed as a result of the reaction of thiophene compounds with dimethyl sulfoxide in the presence of POCl₃ and perchloric acid. The HgI₃ ^– anion was replaced by NO₃ ^– by means of an anion-exchange resin. The methods for the dimethylation of sulfonium salts with conversion of them to the corresponding sulfides were studied; the conditions for obtaining quantitative yields from these reactions by means of an anion exchange resin in the acetate form were found.See [1] for communication VII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 165–168, February, 1972.     

An Interrupted Pummerer/Nickel‐Catalysed Cross‐Coupling Sequence

An interrupted Pummerer/nickel‐catalysed cross‐coupling strategy has been developed and used in the elaboration of styrenes. The operationally simple method can be carried out as a one‐pot process, involves the direct formation of stable alkenyl sulfonium salt intermediates, utilises a commercially available sulfoxide, catalyst, and ligand, operates at ambient temperature, accommodates sp‐, sp²‐, and sp³‐hybridised organozinc coupling partners, and delivers functionalised styrene products in high yields over two steps. An interrupted Pummerer/cyclisation approach has also been used to access carbo‐ and heterocyclic alkenyl sulfonium salts for cross‐coupling.     

Grafting of a bicycloorthoester onto polymers bearing sulfonium salt structures

Graft copolymerization of a bicycloorthoester (BOE) with polymer-supported sulfonium salts was studied. Several polymer-supported sulfonium salts were prepared by the homopolymerizations of p-vinylbenzyl tetramethylenesulfonium hexafluoroantimonate ( 2 ) and 4-(p-vinylphenyl)butyl tetramethylenesulfonium hexafluoroantimonate ( 3 ), and by the copolymerizations of 2 with some vinyl monomers (n-butyl vinyl ether, styrene, acrylonitrile, and p-styrenesulfonic acid potassium salt). These sulfonium salts could initiate the polymerization of BOE to give grafted polymers. Temperature dependences of the catalytic activity of them were not so dramatic as that of benzyl tetramethylenesulfonium hexafluoroantimonate ( 1 ), but the activities of them were higher than that of 1 at temperatures lower than 80°C. The conversion of BOE in the polymerizations with these polymer initiators was ca. 30–70% at 120°C for 7 h. An effect of the comonomer structure on the catalytic activity was observed and styrene was the best comonomer for 2 in terms of the reactivity of the copolymer. The spacer-modified sulfonium salt (homopolymer of 3 ) was slightly lower than polymer-supported benzyl type sulfonium salt (homopolymer of 2 ) in the catalytic activity.     

Electroorganic chemistry—XV : Electroreduction of sulfonium and ammonium salts. Formation of sulfonium ylids

In the electroreduction of sulfonium and ammonium salts, sulfonium ylid was generated from the sulfonium salt, whereas in the case of the ammonium salt, nitrogen ylid was not formed but the N+C bond was reductively cleaved. On the basis of the controlled potential electrolysis, it seems acceptable that the mechanism of formation of sulfonium ylid involves the direct electron transfer to the sulfonium salt as the initiation step.     

New Synthetic Capabilities of Sulfonium Salts

The review summarizes published data on the synthesis and reactivity of new functionally substituted sulfonium salts: dimethyl(trifluoromethylsulfonyloxy)sulfonium trifluoromethanesulfonate, dimethyl(trifluoromethylsulfonyl)sulfonium trifluoromethanesulfonate, various disulfonium dications, dimethyl sulfidesulfur trioxide complex, acyl(dimethyl)sulfonium salts, dimethyl(trifluoroacetyl)sulfonium salts, and boron trifluoride complexes with alkylthio-substituted acyl fluorides as intramolecular analogs of acylsulfonium salts. A theoretical approach is described, which explains electrophilic reactivity of sulfonium salts.     

Biomimetic methyl transfer to olefins

Conditions have been found under which trisubstituted olefins can be methylated with diaryl methyl sulfonium salts in 2,6-di-$\text{t}$-butylpyridine. The pattern of methylated compound's formed is similar to that of the enzymatic methylation of Δ²⁴ steroids side chains with $\text{S}$-adenosylmethionine as a methyl donor.     

Mechanisms of elimination reactions—XXVI: The α'-β mechanism in elimination reactions of sulforium salts

Tracer studies with β-deuterated sulfonium salts have shown that the α'-β, or ylid mechanism can be the major path of elimination in the reaction of sulfonium salts with t-butoxide in t-butyl alcohol, while the E2 reaction is dominant with hydroxide in water or n-butoxide in n-butyl alcohol. The structure of the sulfonium salt also affects the propensity toward α'-β elimination, with 3-pentyl > 3-propyl and eyclopentyl > 3-pentyl > cyclohexyl. The S-methyl protons of the sulfonium salt exchange at a rate much faster than that of the elimination reaction. A strongly basic medium and a syn-periplanar arrangement of the α-C-S and β-C-H bonds seem to be the two most important factors favoring the α'-β mechanism.     

Nucleophilic Substitution at Tricoordinate Sulfur—Alkaline Hydrolysis of Optically Active Dialkoxysulfonium Salts: Stereochemistry,Mechanism and Reaction Energetics

Optically active dialkoxyisopropylsulfonium salts were obtained by methylation (ethylation) of optically active alkyl isopropanesulfinates using methyl (ethyl) trifluoromethanesulfonate. Alkaline hydrolysis of a series of methoxy(alkoxy)sulfonium salts afforded the two sulfinate products methyl isopropanesulfinate and alkyl isopropanesulfinate, both formed with a slightly prevailing inversion of configuration at the sulfur atom. DFT calculations revealed that this substitution reaction proceeded stepwise according to an addition-elimination (A–E) mechanism involving the formation of high tetracoordinate S^IV sulfurane intermediates. In addition, the DFT calculations showed that recombination of the hydroxy anion with the methoxy(alkoxy)sulfonium cation—leading to the parallel formation of the two most stable primary sulfuranes, with the hydroxy and alkoxy groups in apical positions and their direct decomposition—is the most energetically favorable pathway.     

Novel Cyclic Sulfonium‐Based Ionic Liquids: Synthesis,Characterization, and Physicochemical Properties

A new series of ionic liquids composed of three cyclic sulfonium cations and four anions has been synthesized and characterized. Their physicochemical properties, including their spectroscopic characteristics, ion cluster behavior, surface properties, phase transitions, thermal stability, density, viscosity, refractive index, tribological properties, ion conductivity, and electrochemical window have been comprehensively studied. Eight of these salts are liquids at room temperature, at which some salts based on [NO₃]^? and [NTf₂]^? ions exhibit organic plastic crystal behaviors, and all the saccharin‐based salts display relatively high refractive indices (1.442–1.594). In addition, some ionic liquids with the [NTf₂]^? ion exhibit peculiar spectroscopic characteristics in FTIR and UV/Vis regions, whilst those salts based on the [DCA]^? ion show lower viscosities (34.2–62.6 mPa s at 20 °C) and much higher conductivities (7.6–17.6 mS cm^?1 at 20 °C) than most traditional 1,3‐dialkylimidazolium salts.     

Photocationic and radical polymerizations of epoxides and acrylates by novel sulfonium salts

Novel sulfonium salts [methyl‐, 2‐indany‐, or 1‐ethoxycarbonylethyl methyl‐2‐naphthylsulfonium hexafluorophosphate and 2‐indany‐, 1‐ethoxycarbonylethyl‐, 2‐methyl‐2‐phenylpropyl‐, 2‐phenylpropyl‐, 2‐phenylethyl‐, 2‐(4‐methoxyphenyl)‐ethyl‐, or 3‐(4‐methoxyphenyl)‐2‐propyl methylphenylsulfonium hexafluorophosphates] were synthesized by the reaction of dimethylsulfate and the corresponding sulfides followed by anion exchange with KPF₆. These sulfonium salts could polymerize epoxy monomers at lower temperatures than previously reported for benzylsulfonium salt initiators. In particular, sulfonium salts with naphthyl groups showed higher photoactivity than already reported for di(4‐tert‐butylphenyl)iodonium and triphenylsulfonium hexafluorophosphates. These sulfonium salts showed higher activity in photoradical polymerization and photocationic polymerization. The photopolymerization was accelerated by the addition of 4‐methoxy‐1‐naphthol, N‐ethylcarbazole, 2,4‐dimethylthioxanthone, phenothiazine, and 2‐ethyl‐9,10‐dimethoxyanthracene as photosensitizers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3816–3827, 2003     

Effect of alkyl substituents on initiator activity in cationic polymerization of styrene with p-methoxybenzyldialkylsulfonium salts as initiators

The effect of alkyl substituents on cationic polymerization of styrene with p-methoxybenzyldialkysulfonium salts was studied. p-Methoxybenzyl tetramethylene ( 1 ), dimethyl ( 2 ), diethyl ( 3 ), dibuty ( 4 ), and diisopropylsulfonium salts ( 5 ) were synthesized by the reaction of p-methoxybenzyl bromide with the corresponding sulfides, followed by exchange of the counter anion Br^? with SbF^?₆. These sulfonium salts served as potent cationic thermal initiators of which activity was estimated by the bulk and solution polymerizations of styrene. The bulk polymerizations with 1–4 (0.1 mol %) for 30 min gradually proceeded at 30–50°C, but the exothermic polymerization occurred vigorously at 40–60°C. The Polymerization with 5 took place exothermically even at room temperature. Temperature-conversion curves of the polymerizations for 30 and 5 min revealed that the activity of the sulfonium salts was in the following order: 5 > 4 > 3 > 2 ≈ 1 . This order was explained by the order of the bulkiness of the alkyl substituents on the sulfur atom. Number-average molecular weight (M?_n) of polystyrene obtained by the polymerization undergoing no exothermic process was in a range of 6600–16000, which depended on the structure of the alkyl substituents: the more bulky the substituent was, the higher M?_n was.     

Reactions of aromatic and heteroaromatic compounds bearing electron-acceptor substituents

The specificity of the nitration and bromination of dimethyl(2-thienyl)sulfonium salts was studied. It was found that, in contrast to methyl 2-thienyl sulfide, which reacts to form 3- and 5-substituted derivatives, the sulfonium salts give a mixture of 4- and 5-substituted products. Total suppression of the activity of the position under the influence of the sulfonium grouping is not observed.See [1] for communication VIII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 310–314, March, 1972.     

A DFT Study of the Structure and Relative Stability of 1,3-Thiasilacycloalkanes and Their S-Functional Derivatives

A series of 1,3-thiasilacycloalkanes and their S-oxides, S,S-dioxides, and sulfonium salts were studied by the DFT (B3LYP/6-31G*) method. In six-membered sulfoxides, the oxygen atom preferentially occupies the axial position, whereas in the corresponding sulfonium salts the S-alkyl group occupies the equatorial position. The calculation results explain the lower hydrolytic stability of the five-membered rings compared to the six-membered analogs and the lower stability of sulfoxides compared to sulfones.     

Efficient photoacids based upon triarylamine dialkylsulfonium salts

New triarylamine dialkylsulfonium salts that are photosensitive in the near-ultraviolet have been prepared. The quantum yields of photoacid generation were found to be approximately 0.5 and are independent of the counterion. On the other hand, the efficiencies of the sulfonium salts toward the photopolymerization of cyclohexene oxide depend on the counterion and the sulfonium substituents. Photopolymerization kinetic studies demonstrate that these triphenylamine sulfonium salts are highly efficient cationic photoinitiators.     

Fluorinated Vinyl Sulfonium Salts and Their Synthetic Utilization

The fluorinated building block strategy and the direct fluorination strategy are of great importance for the synthesis of new fluorinated molecules. These strategies complement each other and can be combined to develop a new methodology for the construction of a wide variety of fascinating organofluorine compounds. In our opinion, the versatile building blocks used in this method should satisfy the following conditions: 1) readily prepared from commercially available reagents; 2) easy to handle; 3) storage under ordinary conditions without noticeable decomposition; 4) wide applicability. Based on the aforementioned requirements, we focused on the use of vinyl sulfonium salts. This brief review describes the synthesis of five types of fluorinated vinyl sulfonium salts and their reactions. Especially, we featured typical fluorinated groups containing trifluoromethyl (CF₃), difluoromethyl (CF₂H), monofluoromethyl (CFH₂), monofluoro (F) or β-bromotetrafluoroethyl (BrCF₂CF₂) moieties in combination with vinyl sulfonium salts.