Cycloadditions and Cyclizations of Acetylenic, Allenic, and Conjugated Dienyl Sulfones

Photochemical Dual-Catalytic Synthesis of Alkynyl Sulfides

A photochemical dual-catalytic cross-coupling to form alkynyl sulfides via C(sp)-S bond formation is described. The cross-coupling of thiols and bromoalkynes is promoted by a soluble organic carbazole-based photocatalyst using continuous flow techniques. Synthesis of alkynyl sulfides bearing a wide range of electronically and sterically diverse aromatic alkynes and thiols can be achieved in good to excellent yields (50-96 %). The simple continuous flow setup also allows for short reaction times (30 min) and high reproducibility on gram scale. In addition, we report the first application of photoredox/nickel dual catalysis towards macrocyclization, as well as the first example of the incorporation of an alkynyl sulfide functional group into a macrocyclic scaffold.

Synthesis of Functionalized Pyrazoles via 1,3-Dipolar Cycloaddition of α-Diazo-β-ketophosphonates, Sufones and Esters with Electron-Deficient Alkenes

1,3-Dipolar cycloaddition of diazo compounds with olefinic substrates is a promising atom-economic strategy for the construction of functionalized pyrazoles. Over the last few years, our group has been engaged in the synthesis of phosphonyl/sulfonylpyrazoles and pyrazole esters by employing Bestmann-Ohira Reagent (BOR) and its sulfur and ester analogs as 1,3-dipole precursors with various dipolarophiles. This account describes the novel synthetic methods developed in our laboratory, in the perspective of closely related work by others, for the synthesis of phosphonyl/sulfonylpyrazoles, pyrazole esters and the total synthesis of Withasomnine, a natural product, by using 1,3-dipolar cycloaddition as the key step.

Visible light mediated metal-free thiol-yne click reaction

The carbon-sulfur bond formation reaction is of paramount importance for functionalized materials design, as well as for biochemical applications. The use of expensive metal-based catalysts and the consequent contamination with trace metal impurities are challenging drawbacks of the existing methodologies. Here, we describe the first environmentally friendly metal-free photoredox pathway to the thiol-yne click reaction. Using Eosin Y as a cheap and readily available catalyst, C-S coupling products were obtained in high yields (up to 91%) and excellent selectivity (up to 60 : 1). A 3D-printed photoreactor was developed to create arrays of parallel reactions with temperature stabilization to improve the performance of the catalytic system.

Synthesis of Extended Oxazoles III: Reactions of 2-(Phenylsulfonyl)methyl-4,5-diaryloxazoles

2-((Phenylsulfonyl)methyl)-4,5-diphenyloxazole is a useful scaffold for synthetic elaboration at the 2-methylene position thereby affording extended oxazoles. The corresponding α-sulfonyl anion reacts smoothly with diverse alkyl halides giving monoalkylated (47-90%), dialkylated (50-97%), and cyclic (59-93%) products. The reductive desulfonylation of the monoalkylated and selected dialkylated products was optimized with a magnesium/mercuric chloride reagent system and afforded desulfonylated products in the range of 66-97%. The anti-inflammatory Oxaprozin was prepared using the α-sulfonyl carbanion strategy along with optimized desulfonylation.

Visible-Light-Enabled Decarboxylative Sulfonylation of Cinnamic Acids with Sulfonylhydrazides under Transition-Metal-Free Conditions

Decarboxylative cross-coupling reactions of cinnamic acids with sulfonylhydrazides were explored using oxygen as the sole terminal oxidant, realizing a conceptually novel technology for vinyl sulfone synthesis under the synergistic interactions of visible light irradiation, organic dye-type photocatalyst eosin Y, KI, and Cs2CO3 at room temperature.

Electrochemical synthesis of 1-N-phenyl-4-(sulfonyl)benzene-1,2-diamine derivatives: a mild and regioselective protocol

Regioselective synthesis of 1-N-phenyl-4-(arylsulfonyl)benzene-1,2-diamine derivatives was carried out by the electrochemical oxidation of 2-aminodiphenylamine in aqueous solution in the presence of sulfinic acids as nucleophiles.

Synthesis of 1,4-dihydrophosphinoline 1-oxides by acid-promoted cyclization of 1-(diphenylphosphoryl)allenes

Phosphonoallenes in Brønsted acids (TfOH, FSO₃H, H₂SO₄) gave the corresponding 1,2-oxaphosphol-3-enium ions, that were studied by means of NMR and DFT calculations.

Regioselective carboannulation of electron-deficient allenes with dialkyl (2-formylphenyl)malonates leading to multisubstituted naphthalenes

A regioselective carboannulation of allenoates (or allenylphosphonates) with dialkyl 2-(2-formylphenyl)malonates leads to multi-substituted naphthalenes via Michael addition, cyclisation, dealkoxycarboxylation and tautomerisation.

A Direct Metal-Free Decarboxylative Sulfono Functionalization (DSF) of Cinnamic Acids to α,β-Unsaturated Phenyl Sulfones

A metal-free room temperature decarboxylative cross-coupling between cinnamic acids and arylsulfonyl hydrazides has been realized for the first time for the synthesis of (E)-vinyl sulfones. The scope and versatility of the reaction has been demonstrated by the regio- and stereoselective synthesis of 22 derivatives with diverse structural features.

One-pot, three-component arylalkynyl sulfone synthesis

A one-pot three-component protocol for the preparation of arylsulfonyl alkynes through the reaction of ethynyl-benziodoxolone (EBX) reagents, DABSO (DABCO·SO₂), and either organomagnesium reagents or aryl iodides with a palladium catalyst is reported. A broad range of aryl and heteroarylalkynyl sulfones were obtained in 46–85% overall yield.

Unexpected reactions of Grignard reagents with selected β-carboalkoxy substituted sulfinate esters

A series of six-membered rings bearing cis oriented vicinal carboxylate and sulfinate esters were treated with Grignard reagents with the intention of transforming them to β-carboalkoxy substituted sulfoxides. The expected outcome did not transpire and instead the substrates demonstrated the capacity to accept 3 equiv. of organometallic agent in an uncontrollable manner. As such, the substrates possessing eclipsing carboxylate and sulfinate esters accepted two organic ligands at the carboxylate functionality and one at the sulfinyl group. When isomer mixtures of sterically encumbered sulfinate esters were reacted, a single sulfoxide stereochemistry resulted. A mechanism involving the intermediacy of a sulfurane is proposed to account for the experimental observations.

Acid-promoted transformations of 1-(diphenylphosphoryl)allenes: synthesis of novel 1,4-dihydrophosphinoline 1-oxides

1-(Diphenylphosphoryl)alka-1,2-dienes in acids (TfOH, FSO₃H, H₂SO₄) give (3-hydroxyalk-1-en-1-yl)diphenylphosphine oxides, that are further converted into 1-phenyl-1,4-dihydrophosphinoline 1-oxides. The latter compounds are directly formed from these 1,2-dienes under the action of AlCl₃.

Bifunctionalized allenes. Part XV. Synthesis of 2,5-dihydro-1,2-oxaphospholes by electrophilic cyclization reaction of phosphorylated α-hydroxyallenes

This paper discusses a reaction of phosphorylated α-hydroxyallenes with protected or unprotected hydroxy groups involving 5-endo-trig cyclizations. Various electrophilic reagents such as sulfuryl chloride, bromine, benzenesulfenyl and benzeneselenenyl chlorides have been applied. The paper describes the reaction of 1-hydroxyalkyl-1,2-dienephosphonates with electrophiles that produces 2-methoxy-2-oxo-2,5-dihydro-1,2-oxaphospholes due to the participation of the phosphonate neighbouring group in the cyclization. On the other hand, (1E)-alk-1-en-1-yl phosphine oxides were prepared as mixtures with 2,5-dihydro-1,2-oxaphosphol-2-ium halides in a ratio of about 1:2 by chemo-, regio, and stereoselective electrophilic addition to the C(2)-C(3)-double bond in the allene moiety and subsequent concurrent attack of the external (halide anion) and internal (phosphine oxide group) nucleophiles. The paper proposes a possible mechanism that involves cyclization and additional reactions of the phosphorylated α-hydroxyallenes.

Bifunctionalized allenes. Part XIII. A convenient and efficient method for regioselective synthesis of phosphorylated α-hydroxyallenes with protected and unprotected hydroxy group

The paper describes a convenient and efficient method for regioselective synthesis of phosphorylated α-hydroxyallenes using an atom economical [2,3]-sigmatropic rearrangement of intermediate propargyl phosphites or phosphinites. These can be readily prepared via reaction of protected alkynols with dimethyl chlorophosphite or chlorodiphenyl phosphine respectively in the presence of a base.

Gold-catalyzed intermolecular C-S bond formation: efficient synthesis of α-substituted vinyl sulfones

A general method for the synthesis of α-substituted vinyl sulfones makes use of a combination of a triazole gold complex and gallium triflate. This efficient CS bond formation between simple terminal alkynes and sulfinic acids provides access to various α-substituted vinyl sulfones.