The Scope of Catalytic Asymmetric Hydroboration/Oxidation with Rhodium Complexes of 1,1′‐(2‐Diarylphosphino‐1‐naphthyl)isoquinolines

Enantioselective Synthesis of P-Chirogenic 1,2,3-Triazolobenzophospholes

An enantioselective synthesis of a new class of benzophosphole-based heterocycles bearing a fused triazole ring with enantioselectivities of ≤99% is reported. The key steps of the synthesis are based on an innovative stereospecific phosphinyl N → O migration of aminophosphine-boranes into phosphinites, followed by an intramolecular cyclization. Five X-ray structures of P-chirogenic triazolobenzophospholes and a gold(I) complex were established, for assigning absolute configurations, the stereochemistry of the reactions, and the placement of the triazole substituent at the syn position of the P center.

Catalytic Enantioselective Alkyne Addition to Nitrones Enabled by Tunable Axially Chiral Imidazole-Based P,N-Ligands

Although catalytic enantioselective alkyne addition is an established method for the synthesis of chiral propargylic alcohols and amines, addition to nitrones presents unique challenges, and no general chiral catalyst system has been developed. In this manuscript, we report the first Cu-catalyzed enantioselective alkyne addition to nitrones utilizing tunable axially chiral imidazole-based P,N-ligands. Our approach effectively overcomes difficulties in both reactivity and selectivity, resulting in a simple Cu-catalyzed protocol. The reaction accommodates a wide range of nitrones and alkynes, enabling the streamlined synthesis of chiral propargyl N-hydroxylamines via the enantioselective C-C bond formation. A diverse array of optically active nitrogen-containing compounds, including chiral hydroxylamines, can be accessed directly through facile transformations of the reaction products.

Scalemic natural products

Covering: up to the end of 2022The area of scalemic natural products is often enigmatic from a mechanistic standpoint, since low optical purity is observed in compounds having multiple contiguous stereogenic centers resulting from mechanistically distinct biogenetic steps. A scalemic state is rarely the result of a sloppy enzymatic activity, rather resulting from the expression of antipodal enzymes/directing proteins or from the erosion of optical purity by enzymatic or spontaneous reactions. Evidence for these processes is critically reviewed, identifying the mechanisms most often associated to the enzymatic generation of scalemic natural products and also discussing analytical exploitations of natural products' scalemicity.

Double asymmetric synthesis: faster reactions are more selective and a model to estimate relative rate

The catalysed reaction of an enantiopure substrate with formation of a new chirality element may result in higher diastereoselectivity with one enantiomer of a catalyst (matched pair) than with the other (mismatched pair). The hypothesis that the matched reaction is faster was investigated using literature examples of kinetic resolution procedures that result in the formation of a new stereogenic centre. With one exception from fifteen examples, the selectivity factor (s = kfast/kslow) = kmatched/kmismatched. A model to estimate the relative rate of a fast-matched reaction vs. the corresponding slow-mismatched reaction is proposed. This model also provides insight into the basis of the selectivity displayed in the kinetic resolution procedures studied.

The transition metal-catalysed hydroboration reaction

This review covers the development of the transition metal-catalysed hydroboration reaction, from its beginnings in the 1980s to more recent developments including earth-abundant catalysts and an ever-expanding array of substrates.

Recent advances in asymmetric borylation by transition metal catalysis

Chiral organoboronates have played a critical role in organic chemistry and in the development of materials science and pharmaceuticals. Much effort has been devoted to exploring synthetic methodologies for the preparation of these compounds during the past few decades. Among the known methods, asymmetric catalysis has emerged as a practical and highly efficient strategy for their straightforward preparation, and recent years have witnessed remarkable advances in this respect. Approaches such as asymmetric borylative addition, asymmetric allylic borylation and stereospecific cross-coupling borylation, have been extensively explored and well established employing transition-metal catalysis with a chiral ligand. This review provides a comprehensive overview of transition metal-catalysed asymmetric borylation processes to construct carbon-boron, carbon-carbon, and other carbon-heteroatom bonds. It summarises a range of recent achievements in this area of research, with considerable attention devoted to the reaction modes and the mechanisms involved.

Enantioselective Construction of Axially Chiral Azepine-Containing P,N-Ligands from l-Alanine

A family of axially chiral azepine-containing seven-membered cyclic P,N-ligands (named Indole-azepinap) have been prepared by using l-alanine as an original chirality source. The direct chromatographic separation of two diastereomeric phosphine oxides on silica gel enabled these ligands to be easy available, allowing further structural and electronic modifications. Preliminary application of these Indole-azepinaps has been demonstrated in a Pd-catalyzed asymmetric allylic alkylation with high yields and moderate enantioselectivities.

Recommended Tests for the Self-Disproportionation of Enantiomers (SDE) to Ensure Accurate Reporting of the Stereochemical Outcome of Enantioselective Reactions

The purpose of this review is to highlight the necessity of conducting tests to gauge the magnitude of the self-disproportionation of enantiomers (SDE) phenomenon to ensure the veracity of reported enantiomeric excess (ee) values for scalemic samples obtained from enantioselective reactions, natural products isolation, etc. The SDE always occurs to some degree whenever any scalemic sample is subjected to physicochemical processes concomitant with the fractionation of the sample, thus leading to erroneous reporting of the true ee of the sample if due care is not taken to either preclude the effects of the SDE by measurement of the ee prior to the application of physicochemical processes, suppressing the SDE, or evaluating all obtained fractions of the sample. Or even avoiding fractionation altogether if possible. There is a clear necessity to conduct tests to assess the magnitude of the SDE for the processes applied to samples and the updated and improved recommendations described herein cover chromatography and processes involving gas-phase transformations such as evaporation or sublimation.

Flurbiprofen: A Study of the Behavior of the Scalemate by Chromatography, Sublimation, and NMR

2-(2-Fluoro-4-biphenyl) propionic acid (flurbiprofen), from the phenylalkanoic acid family of nonsteroidal anti-inflammatory drugs (NSAID’s), is currently on the pharmaceutical market as a racemate. This racemic compound was tested for its propensity to undergo the self-disproportionation of enantiomers (SDE) phenomenon by various forms of chromatography (SDEvC), such as routine gravity-driven column chromatography, medium-pressure liquid chromatography (MPLC), preparative thin-layer chromatography (PTLC), and size-exclusion chromatography (SEC), as well as by sublimation (SDEvS). Furthermore, examination by nuclear magnetic resonance (NMR) in various solvents found that flurbiprofen exhibited the phenomenon of self-induced diastereomeric anisochronism (SIDA). By measurement of the diffusion coefficient (D), the longitudinal relaxation time (T1), and the transverse relaxation time (T2) using NMR, as well as by electrospray ionization-mass spectrometry (ESI-MS) examinations, the preferred intermolecular association was found to be solvent dependent, e.g., heterochiral association was preferred in toluene, while homochiral association was preferred in more polar solvents. This study also attempted, unsuccessfully, to correlate the NMR measurements of flurbiprofen with chromatographic outcomes for the rationalization and prediction of chromatographic results based on NMR measurements. Because the intermolecular hydrogen bonding of the acid groups in flurbiprofen overwhelmingly predominates over other intermolecular interactions, flurbiprofen seemed to represent a good test case for this idea. The behavior of scalemic samples of flurbiprofen is important, as, although it is currently dispensed as a racemate, clinical applications of the R enantiomer have been investigated. SDEvC and SDEvS both have ramifications for the preparation, handling, and storage of enantioenriched flurbiprofen, and this concern applies to other chiral drugs as well.

Enantioselective Construction of C-C Axially Chiral Quinazolinones via Chirality Exchange and Phase-Transfer Catalysis

A family of axially chiral quinazolinone-based heterobiaryls were constructed with high levels of enantiocontrol (up to 94% ee). Convergently, three different synthetic methods have been realized to prepare these valuable compounds including central-to-axial chirality transfer, dynamic kinetic resolution, and phase-transfer catalysis. Importantly, novel P,N-ligands with a π-π stacking can be derived from heterobiaryls by chirality exchange strategy or synthesized directly from complementary phase-transfer catalysis by using the inexpensive chiral quaternary ammonium salt.

Enantioselective Construction of Quinoxaline-Based Heterobiaryls and P,N-Ligands via Chirality Transfer Strategy

Central-to-axial chirality transfer via C-N single bond oxidation was first achieved as a versatile and conceptually distinct strategy to prepare a new family of axially chiral heteroaromatic biaryl backbones and P,N-ligands (named as Quinoxalinaps) in gram scale. Two atropisomers of Quinoxalinaps (ee >99%) were readily accessed from the same precursor enantiomer by a simple dehydrogenative oxidation with MnO₂ and t-BuOOH under mild conditions. Phosphine could be introduced into the ligands before or after the chirality control process. Moreover, these Quinoxalinap P,N-ligands performed well for both asymmetric reactions of the CuBr-catalyzed alkyne conjugate addition with up to -94% ee and AgOAc-catalyzed glycinate imine [3 + 2] annulation with 90% ee, respectively.

Synthesis of Axially Chiral Olefin-Oxazoline Ligands via Pd-Catalyzed Multiple C-H Functionalization

We report herein the Pd-catalyzed oxazoline-directed C-H olefination of the N-arylindole skeleton, affording two diastereomers of axially chiral olefin-oxazoline ligands in a one-step procedure. Modifications at the 3- and 3'-positions were facilely achieved via electrophilic substitution of the indole fragment and subsequent oxazoline-directed C-H amidation or olefination of the arene fragment.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

Catalytic Approach toward Chiral P,N-Chelate Complexes Utilizing the Asymmetric Hydrophosphination Protocol

A synthetic procedure for obtaining a chiral P,N ligand was developed by exploiting the versatility of the asymmetric hydrophosphination protocol catalyzed by a phosphapalladacycle complex. The addition of the synthesized ligand to various metal sources led to the generation of chiral and enantioenriched chelate complexes, which can be useful prototypes for catalyst design in the future. The resulting coordination compounds were comprehensively characterized by solid-state (X-ray crystallography) and solution-based (one- and two-dimensional NMR spectroscopy) techniques and natural bond orbital (density functional theory) analysis to determine their structural and key electronic features.

Applications of fluorine-containing amino acids for drug design

Fluorine-containing amino acids are becoming increasingly prominent in new drugs due to two general trends in the modern pharmaceutical industry. Firstly, the growing acceptance of peptides and modified peptides as drugs; and secondly, fluorine editing has become a prevalent protocol in drug-candidate optimization. Accordingly, fluorine-containing amino acids represent one of the more promising and rapidly developing areas of research in organic, bio-organic and medicinal chemistry. The goal of this Review article is to highlight the current state-of-the-art in this area by profiling 42 selected compounds that combine fluorine and amino acid structural elements. The compounds under discussion represent pharmaceutical drugs currently on the market, or in clinical trials as well as examples of drug-candidates that although withdrawn from development had a significant impact on the progress of medicinal chemistry and/or provided a deeper understanding of the nature and mechanism of biological action. For each compound, we present features of biological activity, a brief history of the design principles and the development of the synthetic approach, focusing on the source of tailor-made amino acid structures and fluorination methods. General aspects of the medicinal chemistry of fluorine-containing amino acids and synthetic methodology are briefly discussed.

Branched-Selective Alkene Hydroboration Catalyzed by Earth-Abundant Metals

Catalytic hydroboration of alkenes is a well-established method to access borane-functionalized hydrocarbons. While linear-selective hydroboration was predominantly reported, catalysts enabling opposite selectivity (branched-selective) are attracting considerable interest, especially when Earth-abundant metals are utilized. This Synopsis summarizes recent progress in Earth-abundant-metal-catalyzed, branched-selective hydroboration of alkenes while overviewing the historical contributions to this reaction using precious metals. Lessons learned from the current state of this topic that can guide future catalyst design are presented, along with challenging issues that remain to be addressed.

The self-disproportionation of enantiomers (SDE) of amino acids and their derivatives

This review covers the phenomenon of the self-disproportionation of enantiomers (SDE) of amino acids and their derivatives in all its guises from phase transformations (recrystallization, sublimation, and distillation), to the application of force fields, through to chromatography including HPLC, MPLC, gravity-driven column chromatography, and SEC. The relevance of the SDE phenomenon to amino acid research and to marketed pharmaceuticals is clear given the potential for alteration of the enantiomeric excess of a portion of a scalemic sample. In addition, the possible contribution of the SDE phenomenon to the genesis of prebiotic homochirality is considered.

The self-disproportionation of enantiomers (SDE): a menace or an opportunity?

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied.

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied. The SDE has implications ranging from the origins of prebiotic homochirality to unconventional enantiopurification methods, though the risks of altering the enantiomeric excess (ee) unintentionally, regrettably, remain greatly unappreciated. While recrystallization is well known as an SDE process, occurrences of the SDE in other processes are much less recognized, e.g. sublimation and even distillation. But the most common process that many workers seem to be completely ignorant of is SDE via chromatography and reports have included all manner of structures, all types of interactions, and all forms of chromatography, including GC. The SDE can be either a blessing – as a means to obtain enantiopure samples from scalemates – or a curse, as unwitting alteration of the ee leads to errors in the reporting of results and/or misinterpretation of the system under study. Thus the ramifications of the SDE are relevant to any area involving chirality – natural products, asymmetric synthesis, etc. Moreover, there is grave concern regarding errors in the literature, in addition to the possible occurrence of valid results which may have been overlooked and thus remain unreported, as well as the potential for the SDE to alter the ee, particularly via chromatography, and the following concepts will be conveyed: (1) the SDE occurs under totally achiral conditions of (a) precipitation, (b) centrifugation, (c) evaporation, (d) distillation, (e) crystallization, (f) sublimation, and (g) achiral chromatography (e.g. column, flash, MPLC, HPLC, SEC, GC, etc.). (2) The SDE cannot be controlled simply by experimental accuracy and ignorance of the SDE unavoidably leads to mistakes in the recorded and reported stereochemical outcome of enantioselective transformations. (3) The magnitude of the SDE (the difference between the extremes of enantioenrichment and -depletion) can be controlled and used to: (a) minimize mistakes in the recorded experimental values and (b) to develop unconventional and preparatively superior methods for enantiopurification. (4) The magnitude of the SDE cannot be predicted but can be expected for compounds possessing SDE-phoric groups or which have a general tendency for strong hydrogen or halogen bonds or dipole–dipole or aromatic π–π interactions. (5) An SDE test and the rigorous reporting and description of applied physicochemical processes should become part of standard experimental practice to prevent the erroneous reporting of the stereochemical outcome of enantioselective catalytic reactions and the chirooptical properties of scalemates. New directions in the study of the SDE, including halogen bonding-based interactions and novel, unconventional enantiopurification methods such as pseudo-SDE (chiral selector-assisted SDE resolution of racemates), are also reported.

Asymmetric hydrofunctionalization of minimally functionalized alkenes via earth abundant transition metal catalysis

The development of earth-abundant transition metal-catalysed highly enantioselective hydrofunctionalization of minimally functionalized alkenes was summarized.

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.

Possible Case of Halogen Bond-Driven Self-Disproportionation of Enantiomers (SDE) via Achiral Chromatography

The major breakthrough reported in this work is the discovery of likely halogen bond-driven self-disproportionation of enantiomers (SDE). Taking into account that the halogen-bonding interactions can be rationally designed and can match, or even exceed, the strength of the more familiar hydrogen bond, this discovery clearly opens an unexpected new direction of research in the areas of molecular chirality and the SDE phenomenon.

Partially Saturated Bicyclic Heteroaromatics as an sp(3) -Enriched Fragment Collection

Fragment‐based lead generation has proven to be an effective means of identifying high‐quality lead compounds for drug discovery programs. However, the fragment screening sets often used are principally comprised of sp²‐rich aromatic compounds, which limits the structural (and hence biological) diversity of the library. Herein, we describe strategies for the synthesis of a series of partially saturated bicyclic heteroaromatic scaffolds with enhanced sp³ character. Subsequent derivatization led to a fragment collection featuring regio‐ and stereo‐controlled introduction of substituents on the saturated ring system, often with formation of new stereocenters.

[N,P]-pyrrole PdCl2complexes catalyzed the formation of dibenzo-α-pyrone and lactam analogues

Catalytic applications on direct intramolecular arylation was developed using a new family of palladium complexes that includes [N,P] ligands based on the pyrrole ring with α-phosphine and phosphole units.

Phosphole-based ligands in catalysis

This review provides an overview of phosphole-based ligand families (monophospholes, multidentate hybrid phosphole ligands, diphosphole and 2,2′-biphosphole-based ligands) and their potential in metal- and organo-catalyzed reactions (asymmetric reactions included).

Iminopyridine oxazoline iron catalyst for asymmetric hydroboration of 1,1-disubtituted aryl alkenes

The highly regio- and enantioselective iron-catalyzed anti-Markovnikov hydroboration of 1,1-disubstituted aryl alkenes is reported by using a novel chiral iminopyridine oxazoline (IPO) ligand, in which the iminopyridine group is proposed to stabilize the iron and chiral oxazoline group to control enantioselectivity. This distinct class of reactive IPO ligands will likely be of high value for a large variety of asymmetric transformations using first-row transition metals.

Cobalt-catalyzed enantioselective hydroboration of 1,1-disubstituted aryl alkenes

We report the synthesis of cobalt complexes of novel iminopyridine-oxazoline (IPO) ligands and their application to the asymmetric hydroboration of 1,1-disubstituted aryl alkenes. The new catalysts afforded α-alkyl-β-pinacolatoboranes with exclusive regioselectivity in high yields with up to 99.5% ee. Furthermore, we have applied this method to an efficient synthesis of naproxen.

Asymmetric cobalt catalysts for hydroboration of 1,1-disubstituted alkenes

Chiral iminopyridine oxazoline (IPO) ligands were designed, synthesized and utilized for the first cobalt-catalyzed highly regio- and enantioselective anti-Markovnikov hydroboration of 1,1-disubstituted aryl alkenes. These novel IPO ligands will likely be of high value for asymmetric transformations with first-row transition metals.

Design, preparation, and implementation of an imidazole-based chiral biaryl P,N-ligand for asymmetric catalysis

A new strategy for increasing the barrier to rotation in biaryls has been developed that allows for the incorporation of 5-membered aromatic heterocycles into chiral atropisomers. Using this concept, an imidazole-based biaryl P,N-ligand has been designed and prepared as a single enantiomer. This ligand performs exceptionally well in the enantioselective A(3)-coupling, demonstrating the potential of this new design element.

Using physical organic parameters to correlate asymmetric catalyst performance

A classic strategy of physical organic chemists is to probe reaction mechanisms using linear free energy relationships. Identifying such relationships in asymmetric catalytic reactions provides substantial insight into the key factors controlling enantioselectivity, which in turn increases the predictability and applicability of these reactions. The focus of this JOCSynopsis is to highlight several recent examples in which various parameters were identified and applied to the elucidation of LFERs.