Mengnan Zhang

Diaryl sulfoxides from aryl benzyl sulfoxides: a single palladium-catalyzed triple relay process.

A novel approach to produce diaryl sulfoxides from aryl benzyl sulfoxides is reported. Optimization of the reaction conditions was performed using high-throughput experimentation techniques. The [Pd(dba)2 ]/NiXantPhos catalyst system successfully promotes a triple relay process involving sulfoxide α-arylation, CS bond cleavage, and CS bond formation. The byproduct benzophenone is formed by an additional palladium-catalyzed process. It is noteworthy that palladium-catalyzed benzylative CS bond cleavage of sulfoxides is unprecedented. A wide range of aryl benzyl sulfoxides, as well as alkyl benzyl sulfoxides with various (hetero)aryl bromides were employed in the triple relay process in good to excellent yields (85-99 %). Moreover, aryl methyl sulfoxides, dibenzyl sulfoxides, and dimethylsulfoxide could be utilized to generate diaryl sulfoxides involving multiple catalytic cycles by a single catalyst.

Palladium Catalyzed Diaryl Sulfoxide Generation from Aryl Benzyl Sulfoxides and Aryl Chlorides

Diaryl sulfoxides are synthesized from aryl benzyl sulfoxides and aryl chlorides via three sequential catalytic cycles all promoted by a NiXantPhos-based palladium catalyst. The key step is S-arylation of a sulfenate anion. An air- and moisture-stable precatalyst derived from NiXantPhos efficiently facilitates the transformation. Various functional groups, including those with acidic protons, were tolerated. This method can also be extended to methyl and dibenzyl sulfoxides substrates.

tert-Butyl phenyl sulfoxide: a traceless sulfenate anion precatalyst.

tert-Butyl phenyl sulfoxide is employed as a traceless precatalyst for the generation of sulfenate anions under basic conditions and has been used to catalyze the coupling of benzyl halides to trans-stilbenes. The advantage of this precatalyst over previous precatalysts is that the byproduct generated on catalyst formation is a gas, facilitating product isolation in high purity. Using this second generation catalyst, a variety of trans-stilbenes were generated in 39-98% isolated yield.

Palladium-Catalyzed Arylation of Alkyl Sulfenate Anions

A unique palladium-catalyzed arylation of alkyl sulfenate anions is introduced that affords aryl alkyl sulfoxides in high yields. Due to the base sensitivity of the starting sulfoxides, sulfenate anion intermediates, and alkyl aryl sulfoxide products, the use of a mild method to generate alkyl sulfenate anions was crucial to the success of this process. Thus, a fluoride triggered elimination strategy was employed with alkyl 2-(trimethylsilyl)ethyl sulfoxides to liberate the requisite alkyl sulfenate anion intermediates. In the presence of palladium catalysts with bulky monodentate phosphines (SPhos and Cy-CarPhos) and aryl bromides or chlorides, alkyl sulfenate anions were readily arylated. Moreover, the thermal fragmentation and the base promoted elimination of alkyl sulfoxides was overridden. The alkyl sulfenate anion arylation exhibited excellent chemoselectivity in the presence of functional groups, such as anilines and phenols, which are also known to undergo palladium catalyzed arylation reactions.

A New Class of Organocatalysts: Sulfenate Anions†

Sulfenate anions are known to act as highly reactive species in the organic arena. Now they premiere as organocatalysts. Proof of concept is offered by the sulfoxide/sulfenate-catalyzed (1-10 mol%) coupling of benzyl halides in the presence of base to generate trans-stilbenes in good to excellent yields (up to 99%). Mechanistic studies support the intermediacy of sulfenate anions, and the deprotonated sulfoxide was determined to be the resting state of the catalyst.

Organocatalytic Synthesis of Alkynes

Carbon-carbon triple bonds of alkynes are ubiquitous. They serve as valuable starting materials that can be transformed into a vast array of diverse materials, with applications ranging from medicinal chemistry to electronic materials. The methods used to prepare alkynes involve stoichiometric reactions and the most popular install only a single carbon rather than uniting larger fragments. These methods are useful, but they are limited by harsh conditions or the need to prepare reagents. Introduced herein is the first catalytic method to prepare carbon-carbon triple bonds from precursors that do not contain such linkages. By coupling benzaldehyde and benzyl chloride derivatives under basic conditions with an organocatalyst, good yields of alkynes are obtained. The catalyst, a highly reactive sulfenate anion, is readily generated under the reaction conditions from air-stable precursors. This method represents an attractive organocatalytic alternative to well-established stoichiometric approaches to alkynes and to transition-metal-based alkyne functionalization methods in various applications.

Palladium-Catalyzed Arylation of Aryl Sulfenate Anions with Aryl Bromides under Mild Conditions: Synthesis of Diaryl Sulfoxides

A palladium-catalyzed arylation of aryl sulfenate anions generated from aryl 2-(trimethylsilyl)ethyl sulfoxides and CsF has been developed. This protocol is effective for the synthesis of diaryl sulfoxides and heteroaryl aryl sulfoxides under mild conditions employing aryl bromides. Various functional groups, including those with acidic protons, are well tolerated.

Palladium-Catalyzed Enantioselective Arylation of Aryl Sulfenate Anions: A Combined Experimental and Computational Study

A novel approach to produce chiral diaryl sulfoxides from aryl benzyl sulfoxides and aryl bromides via an enantioselective arylation of aryl sulfenate anions is reported. A (JosiPhos)Pd-based catalyst successfully promotes the asymmetric arylation reaction with good functional group compatibility. A wide range of enantioenriched diaryl, aryl heteroaryl, and even diheteroaryl sulfoxides were generated. Many of the sulfoxides prepared herein would be difficult to prepare via classic enantioselective oxidation of sulfides, including Ph(Ph-d5)SO (90% ee, 95% yield). A DFT-based computational study suggested that chiral induction originates from two primary factors: (i) both a kinetic and a thermodynamic preference for oxidative addition that places the bromide trans to the JosiPhos-diarylphosphine moiety and (ii) Curtin-Hammett-type control over the interconversion between O- and S-bound isomers of palladium sulfenate species following rapid interconversion between re- and si-bound transmetalation products, re/si-Pd-OSPh (re/si-PdO-trans).

Palladium-Catalyzed Asymmetric Allylic Alkylations with Toluene Derivatives as Pronucleophiles

The first two highly enantioselective palladium-catalyzed allylic alkylations with benzylic nucleophiles, activated with Cr(CO)3 , have been developed. These methods enable the enantioselective synthesis of α-2-propenyl benzyl motifs, which are important scaffolds in natural products and pharmaceuticals. A variety of cyclic and acyclic allylic carbonates are competent electrophilic partners furnishing the products in excellent enantioselectivity (up to 99 % ee and 92 % yield). This approach was employed to prepare a nonsteroidal anti-inflammatory drug analogue.

Palladium-Catalyzed Direct C–H Arylation of 3-(Methylsulfinyl)thiophenes

A palladium-catalyzed direct arylation of (3-thiophene)S(O)Me derivatives has been developed. This protocol is effective for the selective synthesis of 2-arylated and 2,5-diarylated sulfinylthiophene derivatives with as low as 0.5 mol % catalyst loading. Various functional groups are well tolerated. A method to install two different aryl groups on 3-(methylsulfinyl)thiophenes is also introduced.