Silyl ketene imines: highly versatile nucleophiles for catalytic, asymmetric synthesis

Direct Catalytic Enantioselective Conjugate Addition of α-Substituted Benzyl Nitriles to Alkyl Acrylates

The first enantioselective copper-catalyzed conjugate addition of α-substituted benzyl nitriles to alkyl acrylates is described. This protocol allows the direct and 100% atom-economic generation of a nitrile-containing quaternary stereogenic center in a highly enantioselective manner. The practical application of our methodology was demonstrated through the concise formal synthesis of (-)-aphanorphine.

Asymmetric formal sp2-hydrocarbonations of dienes and alkynes via palladium hydride catalysis

Transition metal-catalyzed asymmetric hydrofunctionalizations of unsaturated bonds via π-ƞ3 substitution have emerged as a reliable method to construct stereogenic centers, and mainly rely on the use of heteroatom-based or carbon nucleophiles bearing acidic C-H bonds. In comparison, sp2 carbon nucleophiles are generally not under consideration because of enormous challenges in cleaving corresponding inert sp2 C-H bonds. Here, we report a protocol to achieve asymmetric formal sp2 hydrocarbonations, including hydroalkenylation, hydroallenylation and hydroketenimination of both 1,3-dienes and alkynes via hydroalkylation and Wittig reaction cascade. A series of unachievable motifs via hydrofunctionalizations, such as di-, tri- and tetra-substituted alkenes, di-, tri- and tetra-substituted allenes, and tri-substituted ketenimines in allyl skeletons are all facilely constructed in high regio-, diastereo- and enantioselectivities with this cascade design. Stereodivergent synthesis of all four stereoisomers of 1,4-diene bearing a stereocenter and Z/E-controllable olefin unit highlights the power of present protocol. An interesting mechanistic feature is revealed that alkyne actually undergoes hydrocarbonation via the formation of conjugated diene intermediate, different from conventional viewpoint that the hydrofunctionalization of alkynes only involves allene species.

Regio- and Enantioselective Allylic Cyanomethylation by Synergistic Rhodium and Silane Catalysis

Rh/silane-cocatalyzed regio- and enantioselctive allylic cyanomethylation with inert acetonitrile directly has been developed. Addition of a catalytic amount neutral silane reagent as an acetonitrile anion carrier is essential for the success of this reaction. The synthesis of mono- and bis-allylation products can be switched by adjusting the size of substituents on the silane, ligands, and temperature. Chiral homoallylic nitriles could be synthesized in above 20:1 branch/linear ratio, up to 98% yield and >99% ee.

Divergent Synthesis of 2-Cyanoaryl Carbamate and 2-Cyanoaryl Urea Derivatives via Hypervalent Iodine-Induced C-C Bond Cleavage

Divergent synthetic methods for transforming isatins to 2-cyanoaryl carbamate and 2-cyanoaryl urea derivatives were developed using ammonium carbamate as the nitrogen source and iodobenzene diacetate as the oxidant. This reaction features mild conditions, broad substrate scope, and moreover, the use of toxic cyano-containing compounds is avoided.

Enantioselective Synthesis of Nitriles Containing a Quaternary Carbon Center by Michael Reactions of Silyl Ketene Imines with 1-Acrylpyrazoles

The enantioselective construction of quaternary carbon centers is a marked challenge in asymmetric catalysis research. It is extremely difficult when a chiral catalyst can not distinguish the facial selectivity of the substrate through bond interactions. Here we realized an enantioselective Michael reaction of silyl ketene imines to 1-acrylpyrazoles using a chiral N,N'-dioxide-Co(II) complex. The protocol is highly efficient for the construction of nitrile-, aryl-, and dialkyl-bearing carbon centers and has been successful applied in the divergent synthesis of pharmaceuticals and natural products. The through-space dispersion interactions between unbound silyl ketene imines and the 1-acrylpyrazole-bonded catalyst play a key role in facilitating the reactivity and the enantioselectivity of this process.

Asymmetric Rhodium-Catalyzed Allylic Substitution Reactions with Nitrile-Stabilized Carbanions

This Account summarizes our recent work on rhodium-catalyzed allylic alkylation reactions with nitrile-stabilized carbanions. Despite the challenges associated with employing nitrile stabilized nucleophiles in transition-metal-catalyzed reactions, we recently developed both enantiospecific and enantioselective allylic alkylation reactions. Notably, these novel reactions permit the expedient and selective access to an array of acyclic ternary and quaternary stereogenic centers that are present in important biologically active molecules. 1 Introduction 2 Enantiospecific Reactions of Nitrile-Stabilized Anions 3 Enantioselective Reactions of Nitrile-Stabilized Anions 4 Conclusion

Azine Activation via Silylium Catalysis

Practical, efficient, and general methods for the diversification of N-heterocycles have been a recurrent goal in chemical synthesis due to the ubiquitous influence of these motifs within bioactive frameworks. Here, we describe a direct, catalytic, and selective functionalization of azines via silylium activation. Our catalyst design enables mild conditions and a remarkable functional group tolerance in a one-pot setup.

Main Group Metal-Mediated Transformations of Imines

Main-group-metal-mediated transformations of imines have earned a valued place in the synthetic chemist's toolbox. Their versatility allows the simple preparation of various nitrogen containing compounds. This review will outline the early discoveries including metallation, addition/cyclisation and metathesis pathways, followed by the modern-day use of imines in synthetic methodology. Recent advances in imine C-F activation protocols are discussed, alongside revisiting "classic" imine reactivity from a sustainable perspective. Developments in catalytic methods for hydroelementation of imines have been reviewed, highlighting the importance of s-block metals in the catalytic arena. Whilst stoichiometric transformations in alternative reaction media such as deep eutectic solvents or water have been summarised. The incorporation of imines into flow chemistry has received recent attention and is summarised within.

Synthesis and Use of Trifluoromethylthiolated Ketenimines

The synthesis of trifluoromethylthiolated ketenimines is herein described. They are easily synthesized from the corresponding α-trifluoromethylthiolated oximes upon activation with triflic anhydride and a base. The presumed nitrilium ion resulting from the Beckmann rearrangement is deprotonated to lead to the key intermediate, whose stability brought by the fluorinated substituent was unforeseeable. The reaction of these new building blocks with a variety of nucleophiles affords a vast array of cyclic and acyclic products bearing the valuable SCF₃ moiety.

Catalytic Claisen Rearrangement by Intercepting Ketenimines with Propargylic Alcohols: A Strategy to Generate and Transform Ketenimines from Radicals

An efficient strategy for facilitating the cross-coupling of two radicals has been established via the coordination of a radical with a metal catalyst. This strategy provides a remarkable ability to harness the reactivity of nitrile-containing azoalkanes and enables a novel cascade reaction with nitrile-containing azoalkanes and propargylic alcohols to be established. By using this reaction, a range of acetylenic and allenic amides were obtained that provides a versatile platform for further derivatizations.

Cyanide-Free Enantioselective Catalytic Strategies for the Synthesis of Chiral Nitriles

The development of enantioselective syntheses of nitriles gained increasing interest due to, e.g., an increasing demand for chiral nitriles for drug synthesis. Complementing existing routes, recently catalytic processes enabling an enantioselective formation of the chiral nitrile moiety without the need to utilize cyanide were accomplished. It is noteworthy that these processes are complementary to each other as they are based on different types of substrates, catalytic methods (utilizing chemo- and biocatalysts), and stereochemical reaction concepts (asymmetric synthesis versus resolution).

Aminophosphonates and aminophosphonic acids with tetrasubstituted stereogenic center: diastereoselective synthesis from cyclic ketimines

New chiral tetrasubstituted aminophosphonic acid derivatives of hexahydroquinoxalin-2(1H)-one were synthesised via highly diastereoselective hydrophosphonylation of the corresponding imines with tris(trimethylsilyl) phosphite as phosphorus nucleophile. High asymmetric induction, good yields, mild reaction conditions, and ease of purification of the final products are the key advantages of the presented protocol.

TetraPh-Tol-BITIOPO: a new atropisomeric 3,3'-bithiophene based phosphine oxide as an organocatalyst in Lewis base-catalyzed Lewis acid mediated reactions

A new chiral phosphine oxide based on a 3,3'-bithiophene scaffold (TetraPh-Tol-BITIOPO) was synthesized, fully characterized and separated into antipodes through chiral HPLC. This new compound was successfully employed as an organocatalyst in Lewis base-catalyzed Lewis acid mediated reactions involving trichlorosilyl compounds. The new atropisomeric catalyst was able to promote the allylation of aldehydes with allyltrichlorosilane in up to 98% yield and up to 96% enantiomeric excess (ee), and the direct aldol reaction to afford β-hydroxy ketones and β-hydroxy thioesters, with good chemical yields and modest stereochemical efficiency. Computational studies helped to elucidate and to rationalize the stereochemical outcome of the reactions catalyzed by TetraPh-Tol-BITIOPO that was found to favour the formation of the isomer with an opposite absolute configuration in comparison with the products obtained with the previously reported 3,3'-bithiophene-based catalyst.

Reductive C-C Coupling from α,β-Unsaturated Nitriles by Intercepting Keteniminates

We present an atom-economic strategy to catalytically generate and intercept nitrile anion equivalents using hydrogen transfer catalysis. Addition of α,β-unsaturated nitriles to a pincer-based Ru-H complex affords structurally characterized κ-N-coordinated keteniminates by selective 1,4-hydride transfer. When generated in situ under catalytic hydrogenation conditions, electrophilic addition to the keteniminate was achieved using anhydrides to provide α-cyanoacetates in high yields. This work represents a new application of hydrogen transfer catalysis using α,β-unsaturated nitriles for reductive C-C coupling reactions.

Catalytic Asymmetric Mannich Reaction of α-Fluoronitriles with Ketimines: Enantioselective and Diastereodivergent Construction of Vicinal Tetrasubstituted Stereocenters

Diastereodivergent and enantioselective conversion of isatin ketimines to α-fluoro-β-aminonitriles with vicinal tetrasubstituted stereocenters is achieved by a chiral copper complex/guanidine base catalyzed Mannich reaction with proper choice of the bisphosphine ligand. The reaction is broad in scope, scalable, and provides efficient access to a series of 3-aminoindolinones exhibiting a quaternary carbon-fluorine stereocenter with high yields and stereoselectivities. Selective transformations of the Mannich reaction products into multifunctional 3-aminooxindoles without erosion of enantiomeric and diastereomeric purity highlight the synthetic utility.

Anti-Carboalumination of Alkynes Using Aluminum Trihalide and Silyl Ketene Imines: Stereo- and Regioselective Synthesis of Alkenylaluminum Compounds Bearing a Cyano Group

An organoaluminum-free and catalyst-free anti-carboalumination of alkynes using aluminum trihalides and silyl ketene imines was developed. Three components, an alkyne, AlX₃, and a silyl ketene imine, were simply mixed to give the alkenylaluminum bearing a cyano group with regioselectivity. Theoretical calculations revealed the effective activation of the alkyne by AlX₃ to enhance the regioselective carboalumination. The synthesized alkenylaluminums were applicable to many organic transformations.

DBU-Mediated Diastereoselective Aldol-Type Cyanomethylation of Isatins

An efficient, metal-free approach to 3-substituted 3-hydroxyoxindole by DBU-mediated highly diastereoselective addition of aryl acetonitrile to N-protected isatin under mild conditions has been developed. The reaction proceeds smoothly to produce respective cyanomethylated adducts in good yield and excellent diastereoselectivity. Further transformation of the cyanide group allowed the synthesis of an advance intermediate of corresponding (±) CPC analogue. The mechanistic insight toward the aldol-type cyanomethylation of N-tritylisatin with benzyl cyanide was obtained by DFT calculations.

Enantioselective Hydrophosphonylation of in Situ Generated N-Acyl Ketimines Catalyzed by BINOL-Derived Phosphoric Acid

An efficient route to pharmacologically interesting isoindolinone-based α-amino phosphonates is described via asymmetric hydrophosphonylation of in situ generated ketimines catalyzed by BINOL-derived phosphoric acid. The reaction proceeds smoothly at ambient temperature affording a variety of α-amino phosphonates with a quaternary stereogenic center embedded in isoindolinone motif in high yields with excellent enantiomeric ratios (up to 98.5:1.5 er). Several interesting transformations of the products into valuable synthetic intermediates are also depicted.

Advances in chemoselective intermolecular cross-benzoin-type condensation reactions

An exhaustive and critical overview of synthetically relevant methods for intermolecular cross-acyloin condensation reactions is reported.

Second Generation of Aldol Reaction

Since the discovery of the Mukaiyama aldol reaction more than 40 years ago, several landmark publications have inspired researchers in the field. The Mukaiyama AR is one of the most significant named reactions in organic synthesis. In the past few decades, development of the modern AR has been at the forefront in addressing the challenges of regio-, chemo-, diastereo-, and enantioselectivity in organic synthesis. All of these selectivity challenges maybe present in a single pair of reactants, thus controlling the outcome of such a process has great practical value. More than 10 years ago, our group became involved in this iconic carbon-carbon bond-forming process and attempted to very closely investigate all possible features of the AR to solve several issues still encountered by chemists, most notably the selectivity challenges mentioned above. In this context, our group initiated the second generation of the AR based on a Lewis or Brønsted acid-catalyzed process in conjunction with the use of a "super silyl" (tris(trimethylsilyl)silyl) directing group, which has demonstrated unrivalled properties in controlling the outcome of the AR. Using the extraordinary power of the super silyl group, we were able to develop new methods and concepts that broadly impacted the ability to control the selectivity attributes and thus allowed for a highly stereoselective construction of polyketide, halogenated polyketide, polypropionate, and polyol scaffolds through inter- and/or intramolecular aldolization protocols. Our diastereoselective ARs of super silyl enol ethers and aldehydes have shown great efficiency and modularity in producing exclusively and preferentially syn- or anti-adducts, creating up to four new adjacent stereocenters in a one-pot sequential manner and under mild reaction conditions. The super silyl-directed AR does not only provide a solution to stereochemistry control challenges, but also offers an efficient, modular and high yielding technique toward nontrivial construction of complex architectures with unprecedented ease. We believe that the new Lewis- or Brønsted-acid-catalyzed super-silyl-directed AR processes chronicled in our laboratories have come to maturity and now offer a "road map" for strategic stereoselective synthesis of polyketide-like units. Herein we report our recent achievements in the diastereoselective C-C bond formation, through the super-silyl-directed AR, toward the synthesis of complex and sophisticated hydroxy aldehydes. We would like to note that due to the extremely broad range of work reported in this field, only stereoselective AR involving aldehyde-derived super SEEs will be discussed in this Account.

Spontaneous Oxygenation of Siloxy-N-silylketenimines to α-Ketoamides

Siloxy-N-silylketenimines generated in situ from O-silyl cyanohydrins were converted to α-ketoamides by brief exposure to air or oxygen. Oxidation under extremely mild conditions can be explained by assuming the intermediacy of a 3-imino-1,2-dioxetane derivative generated via triplet-singlet intersystem crossing after the reaction of siloxy-N-silylketenimines with triplet oxygen.

Indium Tribromide Catalyzed Coupling Reaction of Enol Ethers with Silyl Ketene Imines toward the Synthesis of β,γ-Unsaturated Nitriles

Herein, a coupling reaction of enol ethers with silyl ketene imines in the presence of catalytic amounts of InBr3 and Me3 SiBr is described. Kinetic studies have revealed that an indium catalyst and Me3 SiBr accelerated the coupling process and the regeneration of the catalyst, respectively. Various types of enol ethers and silyl ketene imines are applicable. In addition, a formal synthesis of verapamil was achieved by using this novel coupling reaction.

Hydroalumination of Ketenimines and Subsequent Reactions with Heterocumulenes: Synthesis of Unsaturated Amide Derivatives and 1,3-Diimines

The series of differently substituted ketenimines 1 was hydroluminated using di-iso-butyl aluminum hydride. For the sterically congested ketenimine 1a, preferred hydroalumination of the C═N-bond was proven by X-ray crystallography (compound 5a). In situ treatment of the hydroaluminated ketenimines 5 with various heterocumulenes like carbodiimides, isocycanates, isothiocyanates and ketenimines as electrophiles and subsequent hydrolytic workup resulted in novel enamine derived amide species in case of N-attack (sterically less hindered ketenimines) under formation of a new C-N-bond or in 1,3-diimines by C-C-bond-formation in case of bulky substituents at the ketenimine-nitrogen atom. Furthermore, domino reactions with more than 1 equiv of the electrophile or by subsequent addition of two different electrophiles are possible and lead to polyfunctional amide derivatives of the biuret type which are otherwise not easily accessible.