Enantioselective Indole Insertion Reactions of α-Carbonyl Sulfoxonium Ylides

Catalyst-Free [4+1] Annulation of α-Imidoyl Sulfoxonium Ylides and Diazo Compounds Enabling the Modular Synthesis of 2-Indanones and 3(2H)-Furanones

A novel substrate-regulated [4+1] annulation of α-imidoyl sulfoxonium ylides with diazoketones under catalyst-free conditions is described. The reaction proceeds through a coupling of sulfoxonium ylides and in situ-generated ketenes to form the key reactive zwitterionic intermediates, followed by selective formation of C-C or C-O bonds to achieve five-membered ring systems. The cascade reaction permits the direct synthesis of synthetically useful 2-indanones and 3(2H)-furanones, which expands the reaction pattern of sulfoxonium ylides in annulation transformation.

Synthesis of gem-Difluorinated Keto-Sulfoxides from Sulfoxonium Ylides

Organic molecules containing fluorine and sulfur atoms represent a large percentage of approved pharmaceuticals. Those with combination of both S and F atoms in their structure such as Xtandi, approved in 2012 for prostate cancer, indicates the importance of synthetic methods that accommodates both atoms in an organic moiety. In this study, a novel aspect of sulfoxonium ylide reactivity was explored, unveiling a streamlined and mild synthesis method for gem-difluorinated keto-sulfoxides. Our protocol offers a direct and practical approach to prepare these compounds in 14-80 % chemical yields, that were represented by 21 examples. NMR studies and Hammett correlations gave strong evidence about the mechanism of this transformation.

Catalytic Enantioselective Nucleophilic α-Chlorination of Ketones with NaCl

Catalytic enantioselective α-chlorination of ketones is a highly desirable process. Different from the conventional approaches that employ corrosive electrophilic chlorination reagents, the process disclosed here employs nucleophilic chloride, aqueous NaCl solution, and even seawater, as green inexpensive chlorine sources. This mechanistically distinct and electronically opposite approach provides facile access to diverse highly enantioenriched acyclic α-chloro ketones that are less straightforward by conventional approaches. With a chiral thiourea catalyst, a range of racemic α-keto sulfonium salts underwent enantioconvergent carbon-chlorine bond formation with high efficiency and excellent enantioselectivity under mild conditions. The sulfonium motif plays a crucial triple role by permitting smooth dynamic kinetic resolution to take place via a chiral anion binding mechanism in a well-designed phase-transfer system. This protocol represents a new general platform for the asymmetric nucleophilic α-functionalization of carbonyl compounds.

Pathway to Construct α-Acyloxy Esters by B(C6F5)3-Catalyzed O-H Insertion of Carboxylic Acids with Sulfoxonium Ylides

We report the first example of B(C6F5)3-catalyzed O-H insertion reaction of sulfoxonium ylides and carboxylic acids, achieving efficient construction of diester moieties under metal-free condition. This protocol is characterized by broad substrate tolerance, particularly for various phenylacetic acids, and good compatibility with water/air condition, which is superior to most other methods.

Divergent Synthesis of Tetrasubstituted Phenols via [3 + 3] Cycloaddition Reaction of Vinyl Sulfoxonnium Ylides with Cyclopropenones

Two categories of tetrasubstituted phenols were prepared via the cycloaddition reaction of vinyl sulfoxonnium ylides with cyclopropenones in a switchable manner. Copper carbenoid was proposed as the active intermediate in the process of 2,3,4,5-tetrasubstituted phenols formation, while 2,3,5,6-tetrasubstituted phenols were generated via the direct [3 + 3] annulation of vinyl sulfoxonnium ylides with cyclopropenones under metal-free conditions. Further synthetic applications were also demonstrated.

Organocatalytic Transformations from Sulfur Ylides

Sulfur ylides are an important class of organic compounds due to their ability to perform many different transformations that can give diverse and interesting products with a high degree of complexity. Although metal-catalyzed transformations are frequent in this class of compounds, organocatalyzed transformations remain scarce. From initial works, this review aims to show organocatalyzed transformations from sulfur ylides, involving cyclopropanation and formal N–H, S–H, and C–H insertion reactions, including enantioselective versions. The proposed mechanisms and the modes of activation of these organocatalysts will be covered. Furthermore, advances in this area and potential challenges to be circumvented in the near future will also be discussed.

Catalytic asymmetric synthesis of α-tertiary aminoketones from sulfoxonium ylides bearing two aryl groups

Disclosed herein is an efficient organocatalytic formal N-H insertion reaction of arylamines with α-keto sulfoxonium ylides bearing two aryl groups, delivering a broad range of α-tertiary aminoketones with good to excellent yields and enantioselectivities (up to 90% yield and 94% ee). The utilities of this protocol were also demonstrated by facile preparation of enantioenriched 2-amino-1,2-diarylethanol bearing two different aryl groups, a type of important building block lacking efficient access.

Visible-light-induced organocatalytic enantioselective N-H insertion of α-diazoesters enabled by indirect free carbene capture

While asymmetric insertion of metal carbenes into H-X (X = C, N, O, etc.) bonds has been well-established, asymmetric control over free carbenes is challenging due to the presence of strong background reactions and lack of any anchor for a catalyst interaction. Here we have achieved the first photo-induced metal-free asymmetric H-X bond insertion of this type. With visible light used as a promoter and a chiral phosphoric acid used as a catalyst, α-diazoesters and aryl amines underwent smooth N-H bond insertion to form enantioenriched α-aminoesters with high efficiency and good enantioselectivity under mild conditions. Key to the success was the use of DMSO as an additive, which served to rapidly capture the highly reactive free carbene intermediate to form a domesticated sulfoxonium ylide.

Bidirectional Iterative Approach to Sequence-Defined Unsaturated Oligoesters

Herein, we describe the development of a new strategy for the synthesis of unsaturated oligoesters via sequential metal- and reagent-free insertion of vinyl sulfoxonium ylides into the O-H bond of carboxylic acid. Like two directional coupling of amino acids (N- to C-terminal and C- to N-terminal) in peptide synthesis, the present approach offers a strategy in both directions to synthesize oligoesters. The sequential addition of the vinyl sulfoxonium ylide to the carboxylic acids (acid iteration sequence) in one direction and the sequential addition of the carboxylic acids to the vinyl sulfoxonium ylide (ylide iteration sequence) in another direction yield (Z)-configured unsaturated oligoesters. To perform this iteration, we have developed a highly regioselective insertion of vinyl sulfoxonium ylide into the X-H (X = O, N, C, S, halogen) bond of acids, thiols, phenols, amines, indoles, and halogen acids under metal-free reaction conditions. The insertion reaction is applied to a broad range of substrates (>50 examples, up to 99% yield) and eight iterative sequences. Mechanistic studies suggest that the rate-limiting step depends on the type of X-H insertion.

Dearomatization of benzopyrylium triflates with sulfoxonium ylides

Benzopyrylium triflates react with sulfoxonium ylides to give rise to cyclopropanated products in up to 90% yield as a single diastereomer. The cyclopropanated products can easily undergo acid-mediated ring-expansion to afford benzo[b]oxepines. Control over the absolute stereochemistry of the process is possible when the reaction is executed under the influence of a suitable anion-binding catalyst.

Synthesis of I(III)/S(VI) reagents and their reactivity in photochemical cycloaddition reactions with unsaturated bonds

The development of novel methodologies for the introduction of the sulfoxonium group under mild conditions is appealing but remains underexplored. Herein we report the synthesis of a class of hypervalent iodine reagents with a transferrable sulfoxonium group. These compounds enable mixed iodonium-sulfoxonium ylide reactivity. These well-defined reagents are examined in visible-light-promoted cyclization reactions with a wide range of unsaturated bonds including alkenes, alkynes, nitriles, and allenes. Two distinct cyclization pathways are identified, which are controlled by the substituent of the unsaturated bond. The cycloaddition protocol features simple operation, mild reaction conditions, and excellent functional group tolerance, affording a broad range of sulfoxonium-containing cyclic structures in moderate to excellent yields. Furthermore, the sufoxonium group in the product can be transformed into diverse functional groups and structural motifs via single electron transfer and transition-metal catalysis.

Sulfoxonium Ylides in Aminocatalysis: An Enantioselective Entry to Cyclopropane-Fused Chromanol Structures

The 1,1a,2,7b-tetrahydrocyclopropa[c]chromene, arising from fusion of chromane and cyclopropane rings is the core of medicinally relevant compounds. Engaging sulfoxonium ylides in enantioselective aminocatalytic reactions for the first time, a convenient entry to this scaffold is presented. Several ring-fused derivatives were obtained in moderate-to-good yields and enantioselectivities and with perfect diastereoselectivity at the cyclopropane, using an α,α-diphenylprolinol aminocatalyst. The versatility of the hemiacetal moiety in the products was leveraged to effect various synthetic manipulations.

Condition-Dependent Selective Synthesis of Indolo[1,2-c]quinazolines and Indolo[3,2-c]quinolines from 2-(1H-Indol-2-yl)anilines and Sulfoxonium Ylides

In this paper, a selective synthesis of indolo[1,2-c]quinazolines and indolo[3,2-c]quinolines through the cascade reactions of 2-(1H-indol-2-yl)anilines with sulfoxonium ylides is presented. The formation of products involves the generation of a carbene species from sulfoxonium ylide and its N-H bond insertion reaction with 2-(1H-indol-2-yl)aniline followed by deoxygenative imine formation, intramolecular N- or C- nucleophilic addition and deoxygenative aromatization. This switchable synthesis was condition-dependent. In the presence of K₂CO₃ in CH₃CN, the reaction mainly furnished indolo[1,2-c]quinazolines. In the presence of HOAc in dioxane, it selectively afforded indolo[3,2-c]quinolines. In addition, direct C-H/N-H functionalization of the products obtained provides a convenient and direct access to polycyclic heteroaromatic compounds. These novel protocols have advantages such as readily accessible substrates, easily tunable selectivity, good compatibility with diverse functional groups, and the use of air as a cost-free and sustainable oxidant.

Asymmetric transformations from sulfoxonium ylides

Sulfoxonium ylides are important surrogates for diazo compounds, and their use in industry as safer alternatives has been evaluated during recent years. Beyond the known classical transformations, these ylides have also been used in a surprising plethora of novel and intrinsic chemical reactions, especially in recent years. Bench stability and handling are also an advantage of this class of organosulfur molecules. Despite this, efficient asymmetric transformations, specifically catalytic enantioselective versions, have only recently been reported, and there are specific reasons for this. This perspective article covers this topic from the first studies up to the latest advances, giving personal perspectives and showing the main challenges in this area in the coming years.

Organocatalytic asymmetric azidation of sulfoxonium ylides: mild synthesis of enantioenriched α-azido ketones bearing a labile tertiary stereocenter

Disclosed here is a catalytic asymmetric azidation reaction for the efficient synthesis of α-azido ketones bearing a labile tertiary stereocenter. With a superb chiral squaramide catalyst, a mild asymmetric formal H–N₃ insertion of α-carbonyl sulfoxonium ylides proceeded with excellent efficiency and enantioselectivity. This organocatalytic process not only complements the previous α-azidation approaches for the formation of quaternary stereocenters and mostly for 1,3-dicarbonyl compounds, but also has advantages over the well-known metal-catalyzed asymmetric carbene insertion chemistry using α-diazocarbonyl compounds. Detailed mechanistic studies via control reactions and NMR studies provided important insights into the reaction pathway, which features reversible protonation and dynamic kinetic resolution. The curiosity in mechanism also led to the development of a simplified alternative protocol with a cheaper HN₃ source.

An organocatalytic asymmetric H–N₃ insertion of α-carbonyl sulfoxonium ylides has been developed, providing efficient access to α-azido ketones bearing labile tertiary stereocenters and complementing the metal carbene insertion chemistry.

Catalytic asymmetric cyclopropanation of sulfoxonium ylides catalyzed by a chiral-at-metal rhodium complex

An efficient asymmetric cyclopropanation of sulfoxonium ylides with α,β-unsaturated 2-acyl imidazoles catalyzed by a chiral-at-metal rhodium complex has been developed.