Deoxygenation of Sulfoxides to Sulfides with Thionyl Chloride and Triphenylphosphine: Competition with the Pummerer Reaction

Triethyl amine as an effective reducing agent for sulfoxide deoxygenation

Enabled by triethyl amine (Et3N) and thionyl chloride (SOCl2), an efficient and practical protocol for deoxygenation of sulfoxide to sulfide was developed. This new method features a wide range of substrate scope, including diaryl, dialkyl and aryl alkyl substituted sulfoxides. Detailed mechanistic investigations reveal the crucial role played by Et3N as an electron-donating reductant rather than a hydrogen-atom donor.

Methionine-Containing Peptides: Avoiding Secondary Reactions in the Final Global Deprotection

The solid-phase synthesis of Met-containing peptides using a fluorenylmethoxycarbonyl (Fmoc)/tert-butyl (tBu) protection scheme is inevitably accompanied by two stubborn side reactions, namely, oxidation and S-alkylation (tert-butylation), which result in the formation of Met(O) and sulfonium salt impurities of the target peptide, respectively. These two reactions are acid-catalyzed, and they occur during the final trifluoroacetic (TFA)-based acidolytic cleavage step. Herein, we developed two new cleavage solutions that eradicate the oxidation and reduce S-alkylation. TFA-anisole-trimethylsilyl chloride (TMSCl)-Me₂S-triisopropylsilane (TIS) containing 1 mg of triphenyl phosphine per mL of solution was the optimal mixture for Cys-containing peptides, while for the remaining peptides, TIS was not required. Both cleavage solutions proved to be excellent when sensitive amino acids such as Cys and Trp were involved. TMSCl did not affect either of these sensitive amino acids. Reversing the sulfonium salt to free Met-containing peptide was achieved by heating the peptide at 40 °C for 24 h using 5% acetic acid.

Homocouplings of Sodium Arenesulfinates: Selective Access to Symmetric Diaryl Sulfides and Diaryl Disulfides

Symmetrical diaryl sulfides and diaryl disulfides have been efficiently and selectively constructed via the homocoupling of sodium arenesulfinates. The selectivity of products relied on the different reaction systems: symmetrical diaryl sulfides were predominately obtained under the Pd(OAc)2 catalysis, whereas symmetrical diaryl sulfides were exclusively yielded in the presence of the reductive Fe/HCl system.

Catalytic Chemo-, Regio-, Diastereo-, and Enantioselective Bromochlorination of Unsaturated Systems Enabled by Lewis Base-Controlled Chloride Release

A new strategy is described for the Lewis base-catalyzed bromochlorination of unsaturated systems that is mechanistically distinct from prior methodologies. The novelty of this method hinges on the utilization of thionyl chloride as a latent chloride source in combination with as little as 1 mol % of triphenylphosphine or triphenylphosphine oxide as Lewis basic activators. This metal-free, catalytic chemo-, regio-, and diastereoselective bromochlorination of alkenes and alkynes exhibits excellent site selectivity in polyunsaturated systems and provides access to a wide variety of vicinal bromochlorides with up to >20:1 regio- and diastereoselectivity. The precision installation of Br, Cl, and I in various combinations is also demonstrated by simply varying the commercial halogenating reagents employed. Notably, when a chiral Lewis base promoter is employed, an enantioselective bromochlorination of chalcones is possible with up to a 92:8 enantiomeric ratio when utilizing only 1-3 mol % of (DHQD)₂PHAL.

2-Methoxy-4-methylsulfinylbenzyl Alcohol as a Safety-Catch Linker for the Fmoc/tBu Solid-Phase Peptide Synthesis Strategy

Fmoc and Boc group are the two main groups used to protect the α-amino function in Solid-Phase Peptide Synthesis (SPPS). In this regard, the use of the Mmsb linker allows the combination of these two groups. Peptide-O-Mmsb-Resin is stable to the piperidine and trifluoroacetic acid (TFA) treatment used to remove Fmoc and Boc, respectively. The peptide is detached in a two-step protocol, namely reduction of the sulfoxide to the sulfide with Me₃SiCl and Ph₃P, and then treatment with TFA. The advantage of this strategy has been demonstrated by the following: preparation of peptide with no diketopiperazine formation in sequences prone to this side reaction; on-resin cyclization without the concourse of common organic reagents such as Pd(0) but of difficult use in a biological laboratory; and on-resin disulfide formation in a total side-chain unprotected peptide. The use of Mmsb linker together with Msib (4-(methylsulfinyl)benzyl) and Msbh (4,4'-bis(methylsulfinyl)benzhydryl) described in the accompanying manuscript add a fourth dimension to the SPPS protecting group scheme.

CoCe/N–C hybrids constructed via Ce–O–Co solid solution for the deoxygenation of sulfoxide

CeO2-promoted Co–N–C hybrids were prepared by the strategy of solid solution construction for the deoxygenation of sulfoxide.

Reductively disilylated N-heterocycles as versatile organosilicon reagents

The reductively disilylated N-heterocyclic systems 1,4-bis(trimethylsilyl)-1-aza-2,5-cyclohexadiene (1Si), 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2Si) and its methyl derivatives (3Si and 4Si), and 1,1'-bis(trimethylsilyl)-4,4'-bipyridinylidene (5Si) are proficient organosilicon reagents owing to their low first vertical ionization potentials and the heterophilicity of the polarized N-Si bonds. These have prompted their reactivity as two-electron reductants or reductive silylations. These reactions benefit from the concomitant rearomatization of the N-heterocycles and liberation of trimethylsilyl halides or (Me₃Si)₂O, which are mostly volatile or easily removable byproducts. In this perspective, we have discussed the utilization of these reductively disilylated N-heterocyclic systems as versatile reagents in the salt-free reduction of transition metals (A) and main-group halides (B), in organic transformations (C) and in materials syntheses (D).

Synergistic Effect of Squaric Acid in Bromine-Catalyzed Deoxygenation of Sulfonyl Derivatives: Mechanistic Investigations and Synthetic Applications in Electrophilic (Fluoroalkyl)sulfenylation

A method for electrophilic (fluoroalkyl)sulfenylation of nucleophiles by collaborative CTAB- and squaric acid-promoted deoxygenation of sulfonyl derivatives is reported. Mechanistic studies indicate that squaric acid dramatically decreased the energy barrier in the first step of deoxygenation. The mild deoxygenation process enables the reduction of a wide range of functionalized sulfonyl chlorides as well as sulfonic anhydrides. The novel method represents an operationally simple protocol using readily available reagents and exhibits broad functional group tolerance.

Aldehyde to Ketone Homologation Enabled by Improved Access to Thioalkyl Phosphonium Salts

Phosphines were previously unusable as Pummerer-type nucleophiles due to competing redox chemistry with sulfoxides. Here we circumvent this problem to achieve a formal phosphine Pummerer reaction that offers thioalkyl phosphonium salts that, in turn, give rise to diverse vinyl sulfides via Wittig olefinations. Thirty vinyl sulfides are thus prepared from (alkylthioalkyl)triphenyl phosphonium salts and aldehydes. The hydrolysis of these vinyl sulfides offers an efficient and versatile two-step one-carbon homologation of aldehydes to ketones.

Nickel phosphide nanoalloy catalyst for the selective deoxygenation of sulfoxides to sulfides under ambient H2 pressure

Exploring novel catalysis by less common, metal-non-metal nanoalloys is of great interest in organic synthesis. We herein report a titanium-dioxide-supported nickel phosphide nanoalloy (nano-Ni2P/TiO2) that exhibits high catalytic activity for the deoxygenation of sulfoxides. nano-Ni2P/TiO2 deoxygenated various sulfoxides to sulfides under 1 bar of H2, representing the first non-noble metal catalyst for sulfoxide deoxygenation under ambient H2 pressure. Spectroscopic analyses revealed that this high activity is due to cooperative catalysis by nano-Ni2P and TiO2.

Electrophilic Chlorine from Chlorosulfonium Salts: A Highly Chemoselective Reduction of Sulfoxides

Herein, we describe a selective late-stage deoxygenation of sulfoxides based on a novel application of chlorosulfonium salts and demonstrate a new process using these species generated in situ from sulfoxides as the source of electrophilic chlorine. The use of highly nucleophilic 1,3,5-trimethoxybenzene (TMB) as the reducing agent is described for the first time and applied in the deoxygenation of simple and functionalized sulfoxides. The method is easy to handle, economic, suitable for gram-scale operations, and readily applied for poly-functionalized molecules, as demonstrated with more than 45 examples, including commercial medicines and analogues. We also report the results of competition experiments that define the more reactive sulfoxide and we present a mechanistic proposal based on substrate and product observations.

Photocatalytic Deoxygenation of Sulfoxides Using Visible Light: Mechanistic Investigations and Synthetic Applications

The photocatalytic deoxygenation of sulfoxides to generate sulfides facilitated by either Ir[(dF(CF₃)ppy)₂(dtbbpy)]PF₆ or fac-Ir(ppy)₃ is reported. Mechanistic studies indicate that a radical chain mechanism operates, which proceeds via a phosphoranyl radical generated from a radical/polar crossover process. Initiation of the radical chain was found to proceed via two opposing photocatalytic quenching mechanisms, offering complementary reactivity. The mild nature of the radical deoxygenation process enables the reduction of a wide range of functionalized sulfoxides, including those containing acid-sensitive groups, in typically high isolated yields.

Synthesis and nano-Pd catalyzed chemoselective oxidation of symmetrical and unsymmetrical sulfides

A highly chemoselective, efficient and nano-Pd catalyzed protocol for the rapid construction of sulfoxides and sulfones via the oxidation of symmetrical and unsymmetrical sulfides using H2O2 as an oxidant has been developed, respectively. The ready availability of starting materials, easy recovery and reutilization of the catalyst, wide substrate scope, and high yields make this protocol an attractive alternative. The process also involves the metal-free and microwave-promoted synthesis of symmetrical diarylsulfides, and FeCl3-mediated preparation of symmetrical diaryldisulfides through the reaction of arenediazonium tetrafluoroborates with Na2S·9H2O as a sulfur source. In addition, unsymmetrical sulfides were generated via the K2CO3-mediated reaction of arenediazonium tetrafluoroborates with symmetrical disulfides.

Application of Silicon-Initiated Water Splitting for the Reduction of Organic Substrates

The use of water as a donor for hydrogen suitable for the reduction of several important classes of organic compounds is described. It is found that the reductive water splitting can be promoted by several metalloids among which silicon shows the best efficiency. The developed methodologies were applied for the reduction of nitro compounds, N-oxides, sulfoxides, alkenes, alkynes, hydrodehalogenation as well as for the gram-scale synthesis of several substrates of industrial importance.

Deoxygenation of sulphoxides to sulphides with trichlorophosphane

An efficient route to deoxygenation of sulphoxides to sulphides with PCl₃ under mild reaction condition was developed.

Disilanes as oxygen scavengers and surrogates of hydrosilanes suitable for selective reduction of nitroarenes, phosphine oxides and other valuable substrates

In this report, we demonstrate that the reaction of nitroarenes with hexamethyldisilane under various conditions affords a different range of compounds with excellent selectivity.

Homolytic Cleavage of a B-B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the B-B σ bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals. Our experimental verification provides supportive evidence for this new B-B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B2 (pin)2 at mild conditions.

A practical and chemoselective Mo-catalysed sulfoxide reduction protocol using a 3-mercaptopropyl-functionalized silica gel (MPS)

3-Mercaptopropyl-functionalized silica gel (MPS) has been described as a new reducing agent for the chemoselective deoxygenation of sulfoxides under dioxomolybdenum(vi)-catalysis.

Iodine-catalyzed thiolation of electron-rich aromatics using sulfonyl hydrazides as sulfenylation reagents

Iodine-catalyzed thiolation of electron-rich aromatics, including substituted anisole, thioanisole, phenol, toluene, and naphthalene, using sulfonyl hydrazides as sulfenylation reagents was carried out.

Improved Schmidt Conversion of Aldehydes to Nitriles Using Azidotrimethylsilane in 1,1,1,3,3,3-Hexafluoro-2-propanol

The Schmidt reaction of aromatic aldehydes using a substoichiometric amount (40 mol %) of triflic acid is described. Low catalyst loading was enabled by a strong hydrogen-bond-donating solvent hexafluoro-2-propanol (HFIP). This improved protocol tolerates a broad scope of aldehydes with diverse functional groups and the corresponding nitriles were obtained in good to high yields without the need for aqueous work up.

The Zwitterionic Structure of the Parent Amidinium Tetrazolide and a Rare Tetrazole Ring-Opening Reaction

A crystallization method for 5-amidinium tetrazolide (1) was developed. Crystals of the pure zwitterion 1 enabled the unambiguous assignment of the structure as an inner-salt rather than the amidine tetrazole postulated previously. The solid-state structure of 1 consists of two sets of hydrogen bonds that form a tightly networked two-dimensional sheet. The reaction of amidinium tetrazolide 1 with SOCl2 in the presence of 2.0 equiv. of Et3N produces 3-amino-4-azido-1,2,5-thiadiazole (2) as the sole product. The required tetrazole ring-opening reactivity suggests that the tetrazole moiety may function as a masked geminal azido-nitrene. The thionyl chloride reaction with the amidinium zwitterion 1 occurs not exclusively at the amidine, but rather in the Bay-region of 1, which comprises both the amidinium group and tetrazolide ring. Thiadiazole 2 has a planar structure with a 2D network of hydrogen bonds between the amino group and nitrogen atoms and an S⋯N interaction between the azido group and sulfur.