Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis

Bifunctional Chiral Electrocatalysts Enable Enantioselective α-Alkylation of Aldehydes

Herein, we describe an innovative approach to the asymmetric electrochemical α-alkylation of aldehydes facilitated by a newly designed bifunctional chiral electrocatalyst. The highly efficient bifunctional chiral electrocatalyst combines a chiral aminocatalyst with a redox mediator. It plays a dual role as a redox mediator for electrooxidation, while simultaneously providing remarkable asymmetric induction for the stereoselective α-alkylation of aldehydes. Additionally, this novel catalyst exhibits enhanced catalytic activity and excellent stereoselective control comparable to conventional catalytic systems. As a result, this strategy provides a new avenue for versatile asymmetric electrochemistry. The electrooxidation of diverse phenols enables the C-H/C-H oxidative α-alkylation of aldehydes in a highly chemo- and stereoselective fashion. Detailed mechanistic studies by control experiments and cyclic voltammetry analysis demonstrate possible reaction pathways and the origin of enantio-induction.

Regulating iminophosphorane PN bond reactivity through geometric constraints with cage-shaped triarylphosphines

Structure-reactivity investigations and quantum-chemical parametrization of steric and electronic properties of geometrically constrained iminophosphoranes enabled the design of new frustrated Lewis pairs and revealed unusual properties at the phosphonium center embedded in the cage-shaped triptycene tricyclic scaffold.

Efficient and Sustainable Electrosynthesis of N-Sulfonyl Iminophosphoranes by the Dehydrogenative P-N Coupling Reaction

Iminophosphoranes are commonly used reagents in organic synthesis and are, therefore, of great interest. An efficient and sustainable iodide-mediated electrochemical synthesis of N-sulfonyl iminophosphoranes from readily available phosphines and sulfonamides is reported. This method features low amounts of supporting electrolytes, inexpensive electrode materials, a simple galvanostatic setup, and high conversion rates. The broad applicability could be demonstrated by synthesizing 20 examples in yields up to 90%, having diverse functional groups including chiral moieties and biologically relevant species. Furthermore, electrolysis was performed on a 20 g scale and could be run in repetitive mode by recycling the electrolyte, which illustrates the suitability for large-scale production. A reaction mechanism involving electrochemical mediation by the iodide-based supporting electrolyte is proposed, completely agreeing with all of the results.

Bifunctional iminophosphorane organocatalyst with additional hydrogen bonding: Calculations predict enhanced catalytic performance in a michael addition reaction

A new iminophosphorane-thiourea superbase was rationally designed and investigated as an organocatalyst for the enantioselective Michael addition reaction of nitromethane to 4-phenylbut-3-en-2-one. Starting from an iminophosphorane-thiourea organocatalyst structure already known, we have used theoretical calculations to determine the structures of transition states involved in the carbon-carbon bond formation step and carried out structural modifications to accelerate the reaction rate and to increase the enantioselectivity. The effective structural modification was adding a rigid hydroxyl group able to make an additional hydrogen bond to the transition state, producing a substantial decrease of the ΔG‡ by 7 kcal mol-1. The enantiomeric excess is predicted to be above of 97% using the reliable M06-2X and ωB97M - V functionals. The determination of the complete reaction mechanism and free energy profile was followed by a detailed microkinetic analysis. The present study points out a new direction for structural modifications on this kind of organocatalyst.

Scaffold-Oriented Asymmetric Catalysis: Conformational Modulation of Transition State Multivalency during a Catalyst-Controlled Assembly of a Pharmaceutically Relevant Atropisomer

A new class of superbasic, bifunctional peptidyl guanidine catalysts is presented, which enables the organocatalytic, atroposelective synthesis of axially chiral quinazolinediones. Computational modeling unveiled the conformational modulation of the catalyst by a novel phenyl urea N-cap, that preorganizes the structure into the active, folded state. A previously unanticipated noncovalent interaction involving a difluoroacetamide acting as a hybrid mono- or bidentate hydrogen bond donor emerged as a decisive control element inducing atroposelectivity. These discoveries spurred from a scaffold-oriented project inspired from a fascinating investigational BTK inhibitor featuring two stable chiral axes and relies on a mechanistic framework that was foreign to the extant lexicon of asymmetric catalysis.

Strong Bases and beyond: The Prominent Contribution of Neutral Push-Pull Organic Molecules towards Superbases in the Gas Phase

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted-Lowry theory, was constructed in the gas-phase (PA-proton affinity and/or GB-gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push-pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid-base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push-pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.

Electrosynthesis of iminophosphoranes and applications in nickel catalysis

P(v) iminophosphorane compounds are accessed via electrochemical oxidation of commercially available P(iii) phosphines, including mono-, di- and tri-dentate phosphines, as well as chiral phosphines. The reaction uses inexpensive bis(trimethylsilyl)carbodiimide as an efficient and safe aminating reagent. DFT calculations, cyclic voltammetry, and NMR studies provide insight into the reaction mechanism. The proposed mechanism reveals a special case of sequential paired electrolysis. DFT calculations of the frontier orbitals of an iminophosphorane are compared with those of the analogous phosphines and phosphine oxides. X-ray crystallographic studies of the ligands as well as a Ni-coordination complex provide structural insight for these ligands. The utility of these iminophosphoranes as ligands is demonstrated in nickel-catalyzed cross-electrophile couplings including C(sp2)-C(sp3) and C(sp2)-C(sp2) couplings, an electrochemically driven C-N cross-coupling, and a photochemical arylative C(sp3)-H functionalization. In some cases, these new ligands provide improved performance over commonly used sp2-N-based ligands (e.g. 4,4'-di-tert-butyl-2,2'-bipyridine).

Second Generation Catalytic Enantioselective Nucleophilic Desymmetrization at Phosphorus (V): Improved Generality, Efficiency and Modularity

A broadly improved second generation catalytic two-phase strategy for the enantioselective synthesis of stereogenic at phosphorus (V) compounds is described. This protocol, consisting of a bifunctional iminophosphorane (BIMP) catalyzed nucleophilic desymmetrization of prochiral, bench stable P(V) precursors and subsequent enantiospecific substitution allows for divergent access to a wide range of C-, N-, O- and S- substituted P(V) containing compounds from a handful of enantioenriched intermediates. A new ureidopeptide BIMP catalyst/thiaziolidinone leaving group combination allowed for a far wider substrate scope and increased reaction efficiency and practicality over previously established protocols. The resulting enantioenriched intermediates could then be transformed into an even greater range of distinct classes of P(V) compounds by displacement of the remaining leaving group as well as allowing for even further diversification downstream. Density functional theory (DFT) calculations were performed to pinpoint the origin of enantioselectivity for the BIMP-catalyzed desymmetrization, to rationalize how a superior catalyst/leaving group combination leads to increased generality in our second-generation catalytic system, as well as shed light onto observed stereochemical retention and inversion pathways when performing late-stage enantiospecific SN2@P reactions with Grignard reagents.

Diastereoselective (3 + 3)-Annulations of Trisubstituted Michael Acceptors for Access to Polyfunctional Cyclohexanones

Michael-aldol domino reactions are powerful tools for rapidly assembling carbocyclic scaffolds. We herein disclose a base-catalyzed Michael-aldol domino reaction of trisubstituted Michael acceptors with β-keto ester nucleophiles. The cyclohexanone products are obtained in excellent diastereoselectivity (up to >20:1 dr) and good yields (up to 84%). An attractive practical consideration is that pure products are isolated directly via filtration of the unpurified reaction mixtures. Further functionalization of the cyclohexanones is achieved without perturbation of stereocenters installed through the preceding annulation.

Enantio- and Diastereoselective Variations on α-Iminonitriles: Harnessing Chiral Cyclopropenimine-Thiourea Organocatalysts

Chiral 1-pyrrolines containing a nitrile motif serve as crucial structural scaffolds in biologically active molecules and exhibit diversity as building blocks owing to their valuable functional groups; however, the asymmetric synthesis of such compounds remains largely unexplored. Herein, we present an enantio- and diastereoselective method for the synthesis of α-chiral nitrile-containing 1-pyrroline derivatives bearing vicinal stereocenters through the design and introduction of chiral cyclopropenimine-based bifunctional catalysts featuring a thiourea moiety. This synthesis entails a highly stereoselective conjugate addition of α-iminonitriles to a wide array of enones, followed by cyclocondensation, thereby affording a series of cyanopyrroline derivatives, some of which contain all-carbon quaternary centers. Moreover, we demonstrate the synthetic utility of this strategy by performing a gram-scale reaction with 1% catalyst loading, along with a variety of chemoselective transformations of the product, including the synthesis of a vildagliptin analogue. Finally, we showcase the selective synthesis of all four stereoisomers of the cyanopyrroline products through trans-to-cis isomerization, highlighting the versatility of our approach.

Electronic Effects in Phosphino-Iminophosphorane PdII Complexes upon Varying the N Substituent

Three series of palladium(II) complexes supported by a phosphine-iminophosphorane ligand built upon an ortho-phenylene core were investigated to study the influence of the iminophosphorane N substituent. Cis-dichloride palladium(II) complexes 1 in which the N atom bears an isopropyl (iPr, 1 a), a phenyl (Ph, 1 b), a trimethylsilyl (TMS, 1 c) group or an H atom (1 d) were synthesized in high yield. They were characterized by NMR, IR spectroscopy, HR-mass spectrometry, elemental analysis, and X-ray diffraction. A substantial bond length difference between the Pd-Cl bonds was observed in 1. Complexes 1 a-d were converted into [Pd(LR )Cl(CNt Bu)](OTf)] 2 a-d whose isocyanide is located trans to the iminophosphorane. The corresponding dicationic complexes [Pd(LR )(CNt Bu)2 ](OTf)2 3 a-d were also synthesized, however they exhibited lower stability in solution than 2, the isopropyl derivative 3 a being the most stable of the series. Molecular modeling was performed to rationalize the regioselectivity of the substitution of the single chloride by isocyanide (from 1 to 2) and to study the electronic distribution in the complexes. In particular differences between the TMS and H containing complexes vs. the iPr and Ph ones were found. This suggests that the nature of the N substituent is far from innocent and can help tune the reactivity of iminophosphorane complexes.

Amide C-N bonds activation by A new variant of bifunctional N-heterocyclic carbene

We report an organocatalyst that combines a triazolium N-heterocyclic carbene (NHC) with a squaramide as a hydrogen-bonding donor (HBD), which can effectively catalyze the atroposelective ring-opening of biaryl lactams via a unique amide C-N bond cleavage mode. The free carbene species attacks the amide carbonyl, forming an axially chiral acyl-azolium intermediate. Various axially chiral biaryl amines can be accessed by this methodology with up to 99% ee and 99% yield. By using mercaptan as a catalyst turnover agent, the resulting thioester synthon can be transformed into several interesting atropisomers. Both control experiments and theoretical calculations reveal the crucial role of the hybrid NHC-HBD skeleton, which activates the amide via H-bonding and brings it spatially close to the carbene centre. This discovery illustrates the potential of the NHC-HBD chimera and demonstrates a complementary strategy for amide bond activation and manipulation.

Enantioselective Total Synthesis of (+)-Incargranine A Enabled by Bifunctional Iminophosphorane and Iridium Catalysis

Herein we report the first enantioselective total synthesis of (+)-incargranine A, in nine steps. The total synthesis was enabled by an enantioselective intramolecular organocatalysed desymmetrising Michael addition of a malonamate ester to a linked dienone substrate that established pivotal stereocentres with excellent enantio- and complete diastereoselectivity. Furthermore, a key hemiaminal intermediate was accessed by developing an iridium-catalysed reductive cyclisation, and the scope of this transformation was explored to produce a range of bicyclic hemiaminal motifs. Once installed, the hemiaminal motif was used to initiate a biomimetic cascade to access the natural product directly in a single step.

Bifunctional Iminophosphorane Catalyzed Amide Enolization for Enantioselective Cyclohexadienone Desymmetrization

The organocatalytic enolization of 2-arylacetamides, followed by an enantioselective intramolecular conjugate addition to tethered 2,5-cyclohexadienones, yielding 3D fused N-heterocycles, is described. The transformation represents the first strong activating group-free activation of carboxamides via α-C-H deprotonation in a metal-free, catalytic, and enantioselective reaction, and is achieved by employing a bifunctional iminophosphorane (BIMP) superbase.

Catalyst control over pentavalent stereocentres

A monumental diversity of catalytic methods imparts the ability to select one of two configurations of tetravalent stereocentres. Conversely, catalyst control over pentavalent stereocentres, where a fifth moiety bound to the central atom encodes an expanded stereochemical space, remained a challenge to be accomplished. Herein, we report the feasibility of the catalytic tractability of pentavalent stereocentres. A bifunctional iminophosphorane thiourea catalyst enables enantio- and diastereocontrol over pentavalent phosphoranes to differentiate configurationally stable enantiomers and ensembles of diastereomers which emerge together from a single stereocentre. The desired dioxophosphorane stereoisomers are obtained with excellent yield and selectivity (up to 99% yield, 96:4 e.r. and 99:1 d.r.), while stereodivergent catalysis reroutes the reaction for selective access to each of the viable stereoisomeric states of pentavalent phosphoranes. Considering the diversity of high-valent main group species, it is expected that catalyst control over pentavalent stereocentres significantly increases the synthetically addressable stereochemical space.

Brønsted Base-Catalyzed Enantioselective α-Functionalization of Carbonyl Compounds Involving π-Extended Enolates

Chiral Brønsted base (BB) catalyzed asymmetric transformations constitute an important tool for synthesis. A meaningful fraction of these transformations proceeds through transiently generated enolate intermediates, which display quite versatile reactivity against a variety of electrophiles. Some years ago, our group became interested in developing BB-catalyzed asymmetric reactions of enolizable carbonyl substrates that involve π-extended enolates in which, besides control of reaction diastereo and enantioselectivity, the site-selectivity control is an additional issue in most cases. In the examples covered in this account the opportunities deployed, and the challenges posed, by these methods are illustrated, with a focus on the generation of quaternary carbon stereocenters. In the way, new bifunctional BB catalysts as well as achiral templates were developed that may find further applications.

NbCl5-catalyzed sulfa-Michael addition for constructing quaternary centers in enones

A novel NbCl5-catalyzed sulfa-conjugate addition has been developed to construct quaternary centers in various enones. This new method enables a range of functionalized thiols to access different β-sulfido carbonyl compounds bearing a quaternary center. 27 novel β-sulfido ketones have been obtained with moderate to excellent yields. The preparative scale reactions also proceed well, showing no decrease in yield. We further studied the mechanism by DFT calculations. This methodology is significant in sulfur chemistry, especially in sulfa-conjugate addition, giving a new pathway to add thiols to tri-substituted enones.

Development of organic reactions that productively leverage physical properties

Crystallization-induced stereoconvergent Michael additions demonstrate the potential of using physical properties to reveal unique reactivity.

Catalytic, Asymmetric Michael-Aldol Annulations via a Stereodivergent/Stereoconvergent Path Operating under Curtin-Hammett Control

A bifunctional iminophosphorane (BIMP)-catalyzed method for the synthesis of densely functionalized cyclohexanols establishes five contiguous stereocenters (diastereoselection up to >20:1, enantioselectivity up to >99:1) in a Michael/aldol domino reaction between trisubstituted electrophilic alkenes and γ-nitroketones. Mechanistic studies suggest a scenario in which stereoconvergency is achieved by kinetically controlled cyclization after the initial diastereodivergent Michael addition. Diastereoconvergency during cyclization is shown to result from Curtin-Hammett kinetics, a finding that contrasts the crystallization-driven stereoconvergency previously reported in similar systems. Despite the change in the stereocontrol mechanism, the operational attributes remain attractive, with the crystalline products typically isolated in analytically pure form upon filtration of the reaction mixture.

Bifunctional Iminophosphorane-Catalyzed Enantioselective Nitroalkane Addition to Unactivated α,β-Unsaturated Esters

Herein we describe the enantioselective intermolecular conjugate addition of nitroalkanes to unactivated α,β-unsaturated esters, catalyzed by a bifunctional iminophosphorane (BIMP) superbase. The transformation provides the most direct access to pharmaceutically relevant enantioenriched γ-nitroesters, utilizing feedstock chemicals, with unprecedented selectivity. The methodology exhibits a broad substrate scope, including β-(fluoro)alkyl, aryl and heteroaryl substituted electrophiles, and was successfully applied on a gram scale with reduced catalyst loading, and, additionally, catalyst recovery was carried out. The formal synthesis of a range of drug molecules, and an enantioselective synthesis of (S)-rolipram were achieved. Additionally, computational studies revealed key reaction intermediates and transition state structures, and provided rationale for high enantioselectivities, in good agreement with experimental results.

Catalytic enantioselective nucleophilic desymmetrization of phosphonate esters

Molecules that contain a stereogenic phosphorus atom are crucial to medicine, agrochemistry and catalysis. While methods are available for the selective construction of various chiral organophosphorus compounds, catalytic enantioselective approaches for their synthesis are far less common. Given the vastness of possible substituent combinations around a phosphorus atom, protocols for their preparation should also be divergent, providing facile access not only to one but to many classes of phosphorus compounds. Here we introduce a catalytic and enantioselective strategy for the preparation of an enantioenriched phosphorus(V) centre that can be diversified enantiospecifically to a wide range of biologically relevant phosphorus(V) compounds. The process, which involves an enantioselective nucleophilic substitution catalysed by a superbasic bifunctional iminophosphorane catalyst, can accommodate a wide range of carbon substituents at phosphorus. The resulting stable, yet versatile, synthetic intermediates can be combined with a multitude of medicinally relevant O-, N- and S-based nucleophiles.

Azide-Free Synthesis of N-Alkyliminophosphoranes from Phosphines and Hydroxylamine Derivatives

A broadly applicable and efficient method for the synthesis of N-alkyliminophosphoranes from phosphines that does not use potentially hazardous alkyl azides is reported. Under iron catalysis, a hydroxylamine-derived triflic acid salt oxidizes phosphines to a wide range of iminophosphorane triflic acid salts. Diphosphines afford phosphine-iminophosphoranes that can serve as ligands in transition metal complexes. The developed method can be employed in the synthesis of mixed diiminophosphoranes and in a traceless Staudinger ligation.

OSCAR: an extensive repository of chemically and functionally diverse organocatalysts

The automated construction of datasets has become increasingly relevant in computational chemistry. While transition-metal catalysis has greatly benefitted from bottom-up or top-down strategies for the curation of organometallic complexes libraries, the field of organocatalysis is mostly dominated by case-by-case studies, with a lack of transferable data-driven tools that facilitate both the exploration of a wider range of catalyst space and the optimization of reaction properties. For these reasons, we introduce OSCAR, a repository of 4000 experimentally derived organocatalysts along with their corresponding building blocks and combinatorially enriched structures. We outline the fragment-based approach used for database generation and showcase the chemical diversity, in terms of functions and molecular properties, covered in OSCAR. The structures and corresponding stereoelectronic properties are publicly available (https://archive.materialscloud.org/record/2022.106) and constitute the starting point to build generative and predictive models for organocatalyst performance.

Nucleophilicity Prediction Using Graph Neural Networks

The quantitative description between chemical reaction rates and nucleophilicity parameters plays a crucial role in organic chemistry. In this regard, the formula proposed by Mayr et al. and the constructed reactivity database are important representatives. However, the determination of Mayr's nucleophilicity parameter N often requires time-consuming experiments with reference electrophiles in the solvent. Several machine learning (ML)-based models have been proposed to realize the data-driven prediction of N in recent years. However, in addition to DFT-calculated electronic descriptors, most of them also use a set of artificially predefined structural descriptors as input, which may result in a biased representation of the nucleophile's structural information depending on descriptors' definition preference. Compared with traditional ML algorithms, graph neural networks (GNNs) can naturally take the molecule's structural information into account by applying the message passing technique. We herein proposed a SchNet-based GNN model that only takes the molecular conformation and solvent type as input. The model achieves a comparable performance to the previous benchmark study on 10-fold cross-validation of 894 data points (R² = 0.91, RMSE = 2.25). To enhance the model's ability to capture the molecule's electronic information, some DFT-calculated parameters are then incorporated into the model via graph global features, and substantial improvement is achieved in the prediction precision (R² = 0.95, RMSE = 1.63). These results demonstrate that both structural and electronic information are important for the prediction of N, and GNN can integrate these two kinds of information more effectively.

Crystallization-Enabled Henry Reactions: Stereoconvergent Construction of Fully Substituted [N]-Asymmetric Centers

Tetrasubstituted stereogenic carbon centers bearing a nitrogen substituent represent important motifs in medicinal chemistry and natural products; therefore, the development of efficient methods for the stereoselective synthesis of this class of compounds continues to be an important problem. This article describes stereoconvergent Henry reactions of γ,γ-disubstituted nitroalkanes to deliver highly functionalized building blocks containing up to five contiguous stereogenic centers including a fully substituted [N]-asymmetric center. Henry reactions of higher order nitroalkanes are often characterized by their reversibility and minimal accompanying thermodynamic stereocontrol. In contrast, mechanistic studies for the present case suggest a scenario in which reversibility is productively leveraged through crystallization-based stereocontrol, thereby enabling the efficient sequential π-additions of readily accessible starting materials to assemble complex acyclic stereoarrays.

Photoredox Catalytic Phosphine-Mediated Deoxygenation of Hydroxylamines Enables the Construction of N-Acyliminophosphoranes

The chemistry of phosphoranyl radicals has received increasing attention in recent years. Here, we report the generation of amidyl radicals through photocatalytic deoxygenation of hydroxylamines with triphenylphosphine. This methodology offers a novel and convenient route to a diverse range of N-acyliminophosphoranes in moderate to good yields under visible-light photoredox conditions. Fluorescence quenching experiments suggest that the excited-state of the organic photocatalyst (4CzIPN) was oxidatively quenched by a Cu(II) salt.

Reaction of Amide and Sodium Azide for the Synthesis of Acyl Azide, Urea, and Iminophosphorane

Amides reacted with NaN₃ to give the acyl azides in DMF at 25 °C and produce the symmetrical ureas in THF/H₂O at 80 °C via the sequential reaction of acyl substitution and Curtius rearrangement. All acyl azides were also obtained from the secondary amides via sequential reaction of p-toluenesulfonyl chloride and NaN₃. In addition, keto-stabilized iminophosphoranes were prepared from a one-pot reaction of amides, NaN₃, and phosphines.

Doubly stereoconvergent crystallization enabled by asymmetric catalysis

Synthetic methods that enable simultaneous control over multiple stereogenic centers are desirable for the efficient preparation of pharmaceutical compounds. Herein, we report the discovery and development of a catalyst-mediated asymmetric Michael addition/crystallization-induced diastereomer transformation of broad scope. The sequence controls three stereogenic centers, two of which are stereochemically labile. The configurational instability of 1,3-dicarbonyls and nitroalkanes, typically considered a liability in stereoselective synthesis, is productively leveraged by merging enantioselective Brønsted base organocatalysis and thermodynamic stereocontrol using a single convergent crystallization. The synthesis of useful γ-nitro β-keto amides containing three contiguous stereogenic centers is thus achieved from Michael acceptors containing two prochiral centers.

Construction of N-Acyliminophosphoranes via Iron(II)-Catalyzed Imidization of Phosphines with N-Acyloxyamides

Employing FeCl₂ as a cheap and readily available catalyst, a facile imidization of phosphines with N-acyloxyamides is described, affording synthetically useful N-acyliminophosphoranes with high functional group tolerance. The transformation is easily performed under an air atmosphere at room temperature and could be scaled up to gram scale with a catalyst loading of 1 mol %. The iminophosphoranyl moiety in the product was further utilized as an effective directing group for controllable ortho C(sp²)-H bond amidations under Rh(III) catalysis.

Chiral Cyclopropenimine-catalyzed Asymmetric Michael Addition of Bulky Glycine Imine to α,β-Unsaturated Isoxazoles

A highly efficient asymmetric Michael addition of bulky glycine imine to α,β-unsaturated isoxazoles has been achieved by using 5 mol% of chiral cyclopropenimine as a chiral organo-superbase catalyst under mild conditions. Michael adducts were obtained in excellent yields (up to 97%) and stereoselectivities (up to>99 : 1 dr and 98% ee). A significant solvent effect was found in these chiral organosuperbase catalyzed asymmetric Michael reactions. Gram-scale preparation of Michael adducts and their transformations are realized to provide corresponding products without loss of stereoselectivities. The configurations of Michael adduct was determined by single-crystal X-ray diffraction analysis.

Bifunctional Iminophosphorane-Catalyzed Enantioselective Sulfa-Michael Addition to Unactivated α,β-Unsaturated Amides

The first metal-free catalytic intermolecular enantioselective Michael addition to unactivated α,β-unsaturated amides is described. Consistently high enantiomeric excesses and yields were obtained over a wide range of alkyl thiol pronucleophiles and electrophiles under mild reaction conditions, enabled by a novel squaramide-based bifunctional iminophosphorane catalyst. Low catalyst loadings (2.0 mol %) were achieved on a decagram scale, demonstrating the scalability of the reaction. Computational analysis revealed the origin of the high enantiofacial selectivity via analysis of relevant transition structures and provided substantial support for specific noncovalent activation of the carbonyl group of the α,β-unsaturated amide by the catalyst.

Stable Singlet Carbenes as Organic Superbases

A simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol-4-ylidenes, a type of mesoionic carbene, also named cyclic-bentallenes (CBA). They are more basic (pK_aH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P₄ (^t Bu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality

ConspectusOne of the constant challenges of synthetic chemistry is the molecular design and synthesis of nonionic, metal-free superbases as chemically stable neutral organic compounds of moderate molecular weight, intrinsically high thermodynamic basicity, adaptable kinetic basicity, and weak or tunable nucleophilicity at their nitrogen, phosphorus, or carbon basicity centers. Such superbases can catalyze numerous reactions, ranging from C-C bond formation to cycloadditions and polymerization, to name just a few. Additional benefits of organic superbases, as opposed to their inorganic counterparts, are their solubility in organic reaction media, mild reaction conditions, and higher selectivity. Approaching such superbasic compounds remains a continuous challenge. However, recent advances in synthetic methodology and theoretical understanding have resulted in new design principles and synthetic strategies toward superbases. Our computational contributions have demonstrated that the gas-phase basicity region of 350 kcal mol^-1 and even beyond is easily reachable by organosuperbases. However, despite record-high basicities, the physical limitations of many of these compounds become quickly evident. The typically large molecular weight of these molecules and their sensitivity to ordinary reaction conditions prevent them from being practical, even though their preparation is often not too difficult. Thus, obviously structural limitations with respect to molecular weight and structural complexity must be imposed on the design of new synthetically useful organic superbases, but strategies for increasing their basicity remain important.The contemporary design of novel organic superbases is illustrated by phosphazenyl phosphanes displaying gas-phase basicities (GB) above 300 kcal mol^-1 but having molecular weights well below 1000 g·mol^-1. This approach is based on a reconsideration of phosphorus(III) compounds, which goes along with increasing their stability in solution. Another example is the preparation of carbodiphosphoranes incorporating pyrrolidine, tetramethylguanidine, or hexamethylphosphazene as a substituent. With gas-phase proton affinities of up to 300 kcal mol^-1, they are among the top nonionic carbon bases on the basicity scale. Remarkably, the high basicity of these compounds is achieved at molecular weights of around 600 g·mol^-1. Another approach to achieving high basicity through the cooperative effect of multiple intramolecular hydrogen bonding, which increases the stabilization of conjugate acids, has recently been confirmed.This Account focuses on our efforts to produce superbasic molecules that embody many desirable traits, but other groups' approaches will also be discussed. We reveal the crucial structural features of superbases and place them on known basicity scales. We discuss the emerging potential and current limits of their application and give a general outlook into the future.

Phosphorus-Containing Superbases: Recent Progress in the Chemistry of Electron-Abundant Phosphines and Phosphazenes

The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron-rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.

Interweaving Visible-Light and Iron Catalysis for Nitrene Formation and Transformation with Dioxazolones

Herein, visible-light-driven iron-catalyzed nitrene transfer reactions with dioxazolones for intermolecular C(sp³ )-N, N=S, and N=P bond formation are described. These reactions occur with exogenous-ligand-free process and feature satisfactory to excellent yields (up to 99 %), an ample substrate scope (109 examples) under mild reaction conditions. In contrast to intramolecular C-H amidations strategies, an intermolecular regioselective C-H amidation via visible-light-induced nitrene transfer reactions is devised. Mechanistic studies indicate that the reaction proceeds via a radical pathway. Computational studies show that the decarboxylation of dioxazolone depends on the conversion of ground sextet state dioxazolone-bounding iron species to quartet spin state via visible-light irradiation.

A bifunctional iminophosphorane squaramide catalyzed enantioselective synthesis of hydroquinazolines via intramolecular aza-Michael reaction to α,β-unsaturated esters

An efficient synthesis of enantioenriched hydroquinazoline cores via a novel bifunctional iminophosphorane squaramide catalyzed intramolecular aza-Michael reaction of urea-linked α,β-unsaturated esters is described. The methodology exhibits a high degree of functional group tolerance around the forming hydroquinazoline aryl core and wide structural variance on the nucleophilic N atom of the urea moiety. Excellent yields (up to 99%) and high enantioselectivities (up to 97 : 3 er) using both aromatic and less acidic aliphatic ureas were realized. The potential industrial applicability of the transformation was demonstrated in a 20 mmol scale-up experiment using an adjusted catalyst loading of 2 mol%. The origin of enantioselectivity and reactivity enhancement provided by the squaramide motif has been uncovered computationally using density functional theory (DFT) calculations, combined with the activation strain model (ASM) and energy decomposition analysis (EDA).

The activation of both aromatic and aliphatic ureas as N-centered nucleophiles in intramolecular Michael addition reactions to α,β-unsaturated esters was achieved under bifunctional iminophosphorane squaramide superbase catalysis.

Nucleophilicity Prediction via Multivariate Linear Regression Analysis

The concept of nucleophilicity is at the basis of most transformations in chemistry. Understanding and predicting the relative reactivity of different nucleophiles is therefore of paramount importance. Mayr’s nucleophilicity scale likely represents the most complete collection of reactivity data, which currently includes over 1200 nucleophiles. Several attempts have been made to theoretically predict Mayr’s nucleophilicity parameters N based on calculation of molecular properties, but a general model accounting for different classes of nucleophiles could not be obtained so far. We herein show that multivariate linear regression analysis is a suitable tool for obtaining a simple model predicting N for virtually any class of nucleophiles in different solvents for a set of 341 data points. The key descriptors of the model were found to account for the proton affinity, solvation energies, and sterics.

Main Group Redox Catalysis of Organopnictogens: Vertical Periodic Trends and Emerging Opportunities in Group 15

A growing number of organopnictogen redox catalytic methods have emerged-especially within the past 10 years-that leverage the plentiful reversible two-electron redox chemistry within Group 15. The goal of this Perspective is to provide readers the context to understand the dramatic developments in organopnictogen catalysis over the past decade with an eye toward future development. An exposition of the fundamental differences in the atomic structure and bonding of the pnictogens, and thus the molecular electronic structure of organopnictogen compounds, is presented to establish the backdrop against which organopnictogen redox reactivity-and ultimately catalysis-is framed. A deep appreciation of these underlying periodic principles informs an understanding of the differing modes of organopnictogen redox catalysis and evokes the key challenges to the field moving forward. We close by addressing forward-looking directions likely to animate this area in the years to come. What new catalytic manifolds can be developed through creative catalyst and reaction design that take advantage of the intrinsic redox reactivity of the pnictogens to drive new discoveries in catalysis?

Taming the reactivity of alkyl azides by intramolecular hydrogen bonding: site-selective conjugation of unhindered diazides

The intramolecular hydrogen bonding in the α-azido secondary acetamides (α-AzSAs) enabled site-selective integration onto the diazide modular hubs even without steric hindrance.