The emergence of sulfoxides as efficient ligands in transition metal catalysis

Germylene-sulfoxide as a potential hemilabile ligand: application in coordination chemistry

We describe here the synthesis of heteroleptic organogermylenes containing a sulfoxide donor function for their application in coordination chemistry. While complexation reaction with [W(cod)(CO)4] and [Mo(nbd)(CO)4] afforded bis(germanium)(ii) transition-metal complexes, a bidentate complex coordinated by germanium(ii) and the oxygen atom of the sulfinyl group was obtained from [Ru(PPh3)3Cl2].

Efficient modular phosphorus-containing ligands for stereoselective catalysis

Over several years, our research team has contributed to ligand design for metal-catalyzed stereoselective transformations. This has been achieved with the synthesis and application to organic transformations of interest of an array of structurally diverse P-containing ligands. These range from highly modular enantiopure phosphine–phosphite ligands to supramolecularly regulated enantioselective phosphorus-based catalysts. Our research in supramolecular interactions has also led to the discovery of an unprecedented halogen-bonded rhodium-catalyst.

Pummerer Cyclization Revisited: Unraveling of Acyl Oxonium Ion and Vinyl Sulfide Pathways

Two viable pathways (vinyl sulfide and acyl oxonium ion) for the Pummerer cyclization have been unraveled that expand the reaction scope and capabilities. Use of Brønsted- enhanced Lewis acidity was key to realization of the vinyl sulfide pathway, whereas selective complexation of the sulfur lone pair facilitated the unprecedented acyl oxonium ion pathway. Preliminary mechanistic investigations support these hypotheses. A range of substrates have been explored to understand the reaction parameters.

Electronic and Steric Tuning of an Atropisomeric Disulfoxide Ligand Motif and Its Use in the Rh(I)-Catalyzed Addition Reactions of Boronic Acids to a Wide Range of Acceptors

A novel chiral disulfoxide ligand pair bearing fluorine atoms at the 6 and 6' position of its atropisomeric backbone, ( M, S, S)- and ( P, S, S)- p-Tol-6F-BIPHESO, was synthesized. Complexation to a rhodium(I) precursor gave rise to μ-Cl- and μ-OH-bridged rhodium dimer complexes incorporating the new ( M, S, S)- p-Tol-6F-BIPHESO ligand, while its sibling ( P, S, S)- p-Tol-6F-BIPHESO was not complexed efficiently to the rhodium precursor. The performance of this disulfoxide ligand [( M, S, S)- p-Tol-6F-BIPHESO] in catalysis was tested in both 1,4- and 1,2-addition reactions of arylboronic acids. We show that addition to both cyclic and acyclic enones as well as N-tosylarylimines proceeds with high yields and high enantioselectivities to give the corresponding products. The synthesis of enantiomerically pure p-Tol-6F-BIPHESO is straightforward and inexpensive which, together with the high catalytic performance and wide substrate scope for these addition reactions, makes it a very attractive alternative to more classical chiral ligand entities.

Chemoenzymatic Deracemization of Chiral Sulfoxides

The highly enantioselective enzyme methionine sulfoxide reductase A was combined with an oxaziridine-type oxidant in a biphasic setup for the deracemization of chiral sulfoxides. Remarkably, high ee values were observed with a wide range of substrates, thus providing a practical route for the synthesis of enantiomerically pure sulfoxides.

Nondirected Copper-Catalyzed Sulfoxidations of Benzylic C-H Bonds

A copper-catalyzed sulfoxidation of benzylic C-H bonds by nondirected oxidative C(sp³)-H activation was developed. The process proceeds via sulfenate anions, which are generated by base-triggered elimination of β-sulfinyl esters and benzyl radicals. The functional group tolerance is high, and the product yields are good.

Crystal structure of hexa-kis-(dimethyl sulfoxide-κO)cobalt(II) bis-[tri-chlorido-(quinoline-κN)cobaltate(II)]

There are few reports that describe crystal structures of compounds containing cobalt complexed to either dimethyl sulfoxide (Me2SO) or quinoline (C9H7N). The title compound, [Co(C2H6OS)6][CoCl3(C9H7N)]2, is a cobalt salt in which the metal ion is complexed to both Me2SO and quinoline. In particular, we observed that anhydrous cobalt(II) chloride reacts with quinoline in Me2SO to form a salt that is to be formulated as [CoII(Me2SO)6]2+{[CoIICl3quinoline]2-}. The CoII atom in the cation portion of this mol-ecule lies on a inversion center and is bound to the O atoms of six Me2SO moieties in an octa-hedral configuration, while the CoII atom in the anion is attached to three chloride ligands and one quinoline moiety in a tetra-hedral arrangement.

Palladium/PC-Phos-Catalyzed Enantioselective Arylation of General Sulfenate Anions: Scope and Synthetic Applications

Herein we reported an efficient palladium-catalyzed enantioselective arylation of both alkyl and aryl sulfenate anions to deliver various chiral sulfoxides in good yields (up to 98%) with excellent enantioselectivities (up to 99% ee) by the use of our developed chiral O,P-ligands (PC-Phos). PC-Phos are easily prepared in short steps from inexpensive commercially available starting materials. The single-crystal structure of the PC4/PdCl₂ showed that a rarely observed 11-membered ring was formed via the O,P-coordination with the palladium(II) center. The salient features of this method include general substrate scope, ease of scale-up, applicable to the late-stage modification of bioactive compounds, and the synthesis of a marketed medicine Sulindac.

Chiral sulfoxides: advances in asymmetric synthesis and problems with the accurate determination of the stereochemical outcome

Chiral sulfoxides are in extremely high demand in nearly every sector of the chemical industry concerned with the design and development of new synthetic reagents, drugs, and functional materials. The primary objective of this review is to update readers on the latest developments from the past five years (2011-2016) in the preparation of optically active sulfoxides. Methodologies covered include catalytic asymmetric sulfoxidation using either chemical, enzymatic, or hybrid biocatalytic means; kinetic resolution involving oxidation to sulfones, reduction to sulfides, modification of side chains, and imidation to sulfoximines; as well as various other methods including nucleophilic displacement at the sulfur atom for the desymmetrization of achiral sulfoxides, enantioselective recognition and separation based on either metal-organic frameworks (MOF's) or host-guest chemistry, and the Horner-Wadsworth-Emmons reaction. A second goal of this work concerns a critical discussion of the problem of the accurate determination of the stereochemical outcome of a reaction due to the self-disproportionation of enantiomers (SDE) phenomenon, particularly as it relates to chiral sulfoxides. The SDE is a little-appreciated phenomenon that can readily and spontaneously occur for scalemic samples when subjected to practically any physicochemical process. It has now been unequivocally demonstrated that ignorance in the SDE phenomenon inevitably leads to erroneous interpretation of the stereochemical outcome of catalytic enantioselective reactions, in particular, for the synthesis of chiral sulfoxides. It is hoped that this two-pronged approach to covering the chemistry of chiral sulfoxides will be appealing, engaging, and motivating for current research-active authors to respond to in their future publications in this exciting area of current research.

Graphene Oxide Foam Supported Titanium(IV): Recoverable Heterogeneous Catalyst for Efficient, Selective Oxidation of Arylalkyl Sulfides to Sulfoxides Under Mild Conditions

An efficient and environmentally friendly method was designed for the oxidation of sulfides to sulfoxides with a recyclable, carbon-skeleton-based heterogeneous catalyst developed by titanium sulfate [Ti(SO₄)₂] mineralization on the surface of graphene oxide foam [Ti(SO₄)₂@GOF] by using 30 wt% H₂O₂ as oxidant. Several different substituted sulfides were examined to explore the scope of substrates of the selective oxidation. The excellent reusability and durability of Ti(SO₄)₂@GOF was demonstrated by recycling experiments and the catalyst was further applied in the preparation of pantoprazole sodium in a one-pot process.

Artificial Metalloenzymes: Reaction Scope and Optimization Strategies

The incorporation of a synthetic, catalytically competent metallocofactor into a protein scaffold to generate an artificial metalloenzyme (ArM) has been explored since the late 1970's. Progress in the ensuing years was limited by the tools available for both organometallic synthesis and protein engineering. Advances in both of these areas, combined with increased appreciation of the potential benefits of combining attractive features of both homogeneous catalysis and enzymatic catalysis, led to a resurgence of interest in ArMs starting in the early 2000's. Perhaps the most intriguing of potential ArM properties is their ability to endow homogeneous catalysts with a genetic memory. Indeed, incorporating a homogeneous catalyst into a genetically encoded scaffold offers the opportunity to improve ArM performance by directed evolution. This capability could, in turn, lead to improvements in ArM efficiency similar to those obtained for natural enzymes, providing systems suitable for practical applications and greater insight into the role of second coordination sphere interactions in organometallic catalysis. Since its renaissance in the early 2000's, different aspects of artificial metalloenzymes have been extensively reviewed and highlighted. Our intent is to provide a comprehensive overview of all work in the field up to December 2016, organized according to reaction class. Because of the wide range of non-natural reactions catalyzed by ArMs, this was done using a functional-group transformation classification. The review begins with a summary of the proteins and the anchoring strategies used to date for the creation of ArMs, followed by a historical perspective. Then follows a summary of the reactions catalyzed by ArMs and a concluding critical outlook. This analysis allows for comparison of similar reactions catalyzed by ArMs constructed using different metallocofactor anchoring strategies, cofactors, protein scaffolds, and mutagenesis strategies. These data will be used to construct a searchable Web site on ArMs that will be updated regularly by the authors.

Palladium-Catalyzed Cascade sp2 C-H Functionalization/Intramolecular Asymmetric Allylation: From Aryl Ureas and 1,3-Dienes to Chiral Indolines

A chiral Pd^II -catalyzed cascade sp² C-H functionalization/intramolecular asymmetric allylation reaction is reported. A new chiral sulfoxide-oxazoline (SOX) ligand bearing single chiral center on the sulfur was identified as the optimal ligand for the reaction, being efficient both in the C-H cleavage step and the stereocontrol of the allylation step. The broad scope of this method with respect to aryl ureas and 1,3-dienes enables the rapid construction of valuable chiral indoline derivatives with high yields and enantioselectivities (up to 99 % yield, up to 95:5 e.r.).

A one-pot route to thioamides discovered by gas-phase studies: palladium-mediated CO2 extrusion followed by insertion of isothiocyanates

A new palladium mediated "one pot" synthesis of thioamides from aromatic carboxylic acids (ArCO₂H + RNCS → ArC(S)NHR + CO₂) was discovered by gas-phase experiments and theoretical studies. Palladium-mediated decarboxylation of aromatic carboxylic acids followed by addition of substituted isothiocyanates leads to the corresponding thioamide derivatives.

Absolute Stereochemical Determination of Asymmetric Sulfoxides via Central to Axial Induction of Chirality

The absolute configuration of chiral sulfoxides is determined by means of host-guest complexation that leads to the induction of axial chirality in an achiral host. The central to axial induction of helicity is rationalized by a simple recognition of the relative length and size of the substituents attached to the S-center. This technique is used to determine the absolute configuration of chiral sulfoxides, requiring micrograms of sample, without the need for prefunctionalization.

Cooperativity of axial and centre chirality in the biaryl disulfoxide/Rh(i)-catalysed asymmetric 1,4-addition of arylboronic aids to 2-cyclohexenone: a DFT study

Atropisomeric biaryl disulfoxides contain two independent chiral elements. Previously, the (M,S,S)-diastereomer showed very high catalytic activity and selectivity in the Rh-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-enones whereas the (M,R,R) counterpart - none. Herein, DFT computations on the key transmetallation (turnover-determining) and carborhodation (enantioselectivity-determining) steps of the catalytic cycle show that the (M,S,S)-ligand gives rise to lower reaction barriers for these elementary steps. However, the barriers for the (M,R,R)-ligand are not sufficiently high to explain the lack of reactivity. Hence, this phenomenon is most likely due to the failure of catalyst formation from the ligand and the dimeric Rh precatalyst complex. The hitherto unknown (M,S,R)-ligand shows predicted enantioselectivity similar to the (M,S,S)-ligand as a consequence of lower reaction barriers associated with those isomers whose key features resemble the (M,S,S)-ligand.

Chiral Organosulfur Ligands/Catalysts with a Stereogenic Sulfur Atom: Applications in Asymmetric Synthesis

Asymmetric synthesis, in which chiral organocatalysts or metal complexes with chiral ligands are used, has become the most valuable methodology for the preparation of enantiomerically pure organic compounds. Among such catalysts/ligands, a growing number constitute various organosulfur compounds. This Review provides comprehensive and critical information on the plethora of sulfur-based chiral ligands and organocatalysts used in asymmetric synthesis, which have been published within the last 15 years. However, it is confined to the presentation of only those chiral catalysts/ligands that possess a stereogenic sulfur atom and includes sulfoxides, sulfinamides, N-sulfinyl ureas, sulfoximines, and some related S-chiral derivatives.

Metal-free O-H/C-H difunctionalization of phenols by o-hydroxyarylsulfonium salts in water

An environmentally benign method for C-H/O-H difunctionalization of phenols with sulfoxides under mild conditions has been developed. The reaction process is mediated by an electrophilic aromatic substitution and subsequent selective aryl or alkyl migration, involving C-S and C-O bond formations with broad substrate scope.