Transition‐Metal‐Free Formal Sonogashira Coupling and α‐Carbonyl Arylation Reactions

Transition‐metal‐free formal Sonogashira coupling and α‐carbonyl arylation reactions have been developed. These transformations are based on the nucleophilic aromatic substitution (S_NAr) of β‐carbonyl sulfones to electron‐deficient aryl fluorides, producing a key intermediate that, depending on the reaction conditions, gives the aromatic alkynes or α‐aryl carbonyl compounds. The development of these reactions is presented and, based on investigations under basic and acidic conditions, mechanisms have been proposed. To develop the formal Sonogashira coupling further, a milder, two‐step protocol is also disclosed that expands the reaction concept. The scope of these reactions is demonstrated for the synthesis of Sonogashira and α‐carbonyl arylated products from a range of electron‐deficient aryl fluorides with a variety of functional groups and aryl‐, heteroaryl‐, alkyl‐, and alkoxy‐substituted sulfone nucleophiles. These transition‐metal‐free reactions complement the metal‐catalyzed versions in terms of substitution patterns, simplicity, and reaction conditions.     

Umpolung Reactions of α‐Imino Esters: Useful Methods for the Preparation of α‐Amino Acid Frameworks

This paper summarizes our recent efforts toward the development of tandem reactions utilizing umpolung reactions of α‐imino esters. A highly diastereoselective tandem N‐alkylation–Mannich reaction of α‐imino esters was developed. A tandem N‐alkylation–addition reaction of α‐imino esters derived from ethyl glyoxylate with various aldehydes proceeded to give 1,2‐amino alcohols. The same reaction also proceeded efficiently using a novel flow system comprising two connected microreactors. Novel syntheses of α‐quaternary alkynyl amino esters and allenoates were developed through the use of umpolung N‐addition to β,γ‐alkynyl α‐imino esters, followed by regioselective acylation. In addition, a highly regioselective tandem N‐alkylation–vinylogous aldol reaction of β,γ‐alkenyl α‐imino esters was discovered. N‐Alkylation of α‐iminophosphonates followed by a Horner–Wadsworth–Emmons reaction with aldehydes occurred to afford enamines, which can be used in a four‐component coupling reaction with methyl vinyl ketone. α‐N‐Acyloxyimino esters served as highly efficient substrates for the N,N,C‐trialkylation reaction to introduce various nucleophiles at the imino nitrogen and carbon atoms.     

Organocatalyzed Cyclopropanation of α‐Substituted α,β‐Unsaturated Aldehydes: Enantioselective Synthesis of Cyclopropanes Bearing a Chiral Quaternary Center

An enantioselective cyclopropanation of α‐substituted α,β‐unsaturated aldehydes with bromomalonate has been successfully developed through a domino Michael/α‐alkylation strategy. The method allows the efficient formation of cyclopropanes bearing a quaternary carbon stereocenter at the α‐position of the aldehydes by using iminium/enamine catalysis and gives a nice extension on the organocatalytic cyclopropanation of bromomalonate and α,β‐unsaturated aldehydes previously reported by other groups. Very good yields (up to 81 %) and enantioselectivities (up to 97 % ee) have been obtained. The optically active cyclopropane derivatives are of high synthetic interest as useful targets for further elaboration into more complex structures.     

A Photochemical Organocatalytic Strategy for the α‐Alkylation of Ketones by using Radicals

Reported herein is a visible‐light‐mediated radical approach to the α‐alkylation of ketones. This method exploits the ability of a nucleophilic organocatalyst to generate radicals upon S_N2‐based activation of alkyl halides and blue light irradiation. The resulting open‐shell intermediates are then intercepted by weakly nucleophilic silyl enol ethers, which would be unable to directly attack the alkyl halides through a traditional two‐electron path. The mild reaction conditions allowed functionalization of the α position of ketones with functional groups that are not compatible with classical anionic strategies. In addition, the redox‐neutral nature of this process makes it compatible with a cinchona‐based primary amine catalyst, which was used to develop a rare example of enantioselective organocatalytic radical α‐alkylation of ketones.     

Chemoselectivity Control in the Reactions of 1,2‐Cyclic Sulfamidates with Amines

Although 1,2‐cyclic sulfamidates derived from α‐methylisoserine undergo nucleophilic displacement at the quaternary center, to the best of our knowledge their behavior with amines as nucleophiles has never been explored. We have found that a broad range of amines can be used, demonstrating the scope of the reaction, and that excellent control of the chemoselectivity can be achieved. Application of this methodology for the synthesis of a chiral α,β‐diamino acid and an important piperazinone heterocycle is also presented. Additionally, we have found that DMF and DMSO behave not only as polar aprotic solvents but also as O‐nucleophilic reagents, allowing the incorporation of an oxygen atom at a quaternary center of the electrophile, with inversion of configuration.     

Light‐Driven Enantioselective Organocatalytic β‐Benzylation of Enals

A photochemical organocatalytic strategy for the direct enantioselective β‐benzylation of α,β‐unsaturated aldehydes is reported. The chemistry capitalizes upon the light‐triggered enolization of 2‐alkyl‐benzophenones to afford hydroxy‐o ‐quinodinomethanes. These fleeting intermediates are stereoselectively intercepted by chiral iminium ions, transiently formed upon condensation of a secondary amine catalyst with enals. Density functional theory (DFT) studies provided an explanation for why the reaction proceeds through an unconventional Michael‐type addition manifold, instead of a classical cycloaddition mechanism and subsequent ring‐opening.     

Brønsted Acid Catalyzed Oxygenative Bimolecular Friedel–Crafts‐type Coupling of Ynamides

A non‐metal approach for accessing α‐oxo carbene surrogates for a C−C bond‐forming bimolecular coupling between ynamides and nucleophilic arenes was developed. This acid‐catalyzed coupling features mild temperature, which is critical for the required temporal chemoselectivity among nucleophiles. The scope of nucleophiles includes indoles, pyrroles, anilines, phenols and silyl enolethers. Furthermore, a direct test of S_N2′ mechanism has been provided by employing chiral N,N′‐dioxides which also enlightens the nature of the intermediates in related metal‐catalyzed processes.     

The homo‐PADAM Protocol: Stereoselective and Operationally Simple Synthesis of α‐Oxo‐ or α‐Hydroxy‐γ‐acylaminoamides and Chromanes

A straightforward and fully stereoselective synthesis of a new class of peptidomimetics, that is α‐oxo‐γ‐acylaminoamides, was achieved starting from various benzaldehydes by a sequence of 1) an asymmetric organocatalytic Mannich reaction, 2) a Passerini multicomponent reaction, 3) an amine deprotection–acyl migration protocol, and 4) a final oxidation. The whole sequence can be performed without purification of the intermediates and represents the first example of a homo‐Passerini–amine deprotection–acyl migration (PADAM) strategy. Highly stereoselective reduction of the α‐oxo‐γ‐acylaminoamides afforded α‐hydroxy‐γ‐acylaminoamides as well. In some cases both diastereomers were obtained by simply changing the reducing agent. Finally, starting from protected salicylaldehyde, the same sequence, followed by a Mitsunobu cyclization, afforded highly substituted chromanes.     

Preparation of THP‐Ester‐Derived Pyridinium‐Type Salts and their Reactions with Various Nucleophiles

Nucleophilic substitution at the anomeric positions of tetrahydropyranyl (THP) and related carbohydrate‐derived esters that proceeded through pyridinium‐type salt intermediates have been developed. Treatment of the 6‐substituted α‐acetoxy‐tetrahydropyrans with TMSOTf (TMS=trimethylsilyl) and 2‐substitutited pyridines, such as 2‐p‐tolylpyridine and 2‐methoxypyridine, led to the efficient generation of cis‐pyridinium‐type salts. These salts reacted with various nucleophiles, such as alcohols, azides, and organozinc reagents, to form nucleophilic‐substitution products. A characteristic feature of these processes was that they took place under mild conditions, which did not affect acid‐labile protecting groups. Furthermore, the reactions that employed azides and C‐nucleophiles generated 2,6‐trans products with high degrees of stereoselectivity.     

Automated Generation and Reactions of 3‐Hydroxymethylindoles in Continuous‐Flow Microreactors

An automated sequential approach for the generation and reactions of 3‐hydroxymethylindoles in continuous‐flow microreactors is described. Consecutive halogen–magnesium exchanges of four 3‐iodoindoles followed by addition to three aldehydes provided twelve 3‐hydroxymethylindoles in a multi‐microreactor setup. The synthetic flow strategy could be coupled with an in line continuous liquid–liquid extraction workup protocol for each reaction. Further elaboration of each of these indoles within the fluidic setup was achieved by acid‐catalysed nucleophilic substitutions with allyltrimethylsilane and methanol used as nucleophiles. Overall, a set of four 3‐iodoindoles was converted into thirty‐six indole derivatives by a range of transformations including iodo–magnesium exchange/electrophile trapping and acid‐catalysed nucleophilic substitution in a fully automated sequential fashion.     

1‐Methyl‐3‐phenyl‐3‐thiocyanato‐1H,3H‐quinoline‐2,4‐dione: A novel thiocyanating agent

Under mild reaction conditions, the thiocyanato group is selectively transferred from 1‐methyl‐3‐phenyl‐3‐thiocyanato‐1H,3H‐quinoline‐2,4‐dione ( 3 ) to some nucleophiles. Aliphatic primary and secondary amines are converted to S‐cyanothiohydroxylamines, anilines afford p‐thiocyanatoanilines, Wittig reagent is thiocyanated in α‐position, and thiols are oxidized to disulfides.     

Quaternary Chiral β2,2‐Amino Acids with Pyridinium and Imidazolium Substituents

The reactions of cyclic sulfamidates as electrophiles with a variety of nitrogen‐containing aromatic heterocycle nucleophiles, such as pyridines, N‐alkylimidazoles and N‐methylbenzimidazol, was explored. In all cases, although the nucleophilic substitution reactions occurred on quaternary centres, elimination products were not detected. The inversion of configuration at this quaternary centre was determined by X‐ray diffraction analysis and the enantiomeric excess of the reactions was checked by chiral HPLC. This synthetic approach allowed us to obtain a new family of chiral charged β^2,2‐amino acids, including a new bisamino acid that incorporates an imidazolium salt as a cross‐linker. In this context, the treatment of these chiral imidazolium salts with Ag₂O opens the way to new chiral N‐heterocyclic carbenes, which are important substrates in the fields of organometallic and organocatalytic chemistry. Additionally, we have done a thorough conformational analysis of these β‐amino acid derivatives, both in the solid state and in solution. The most important conformational feature of these acyclic systems is the rigidity of the N‐CH₂‐C‐N⁺ dihedral angle, which is essentially due to the gauche effect.     

Copper‐Catalyzed Aerobic Oxidations of 3‐N‐Hydoxyaminoprop‐1‐ynes to Form 3‐Substituted 3‐Amino‐2‐en‐1‐ones: Oxidative Mannich Reactions with a Skeletal Rearrangement

Cu‐catalyzed aerobic oxidations of readily available 3‐N‐hydroxyaminopro‐1‐ynes with water, alcohols, or thiols to form diverse 3‐substituted 3‐amino‐2‐en‐1‐ones are described. The utility of this catalysis is manifested by a wide scope of applicable N‐hydroxyl propargylamines and nucleophiles, thus enabling the design of one‐pot cascade or two‐step sequential reactions. Besides synthetic significances, such oxidative Mannich reactions are mechanistically interesting because structurally reorganized products were obtained. Our mechanistic studies reveal that the aerobic oxidations involve initial formation of nitrone intermediates, followed by the attack of nucleophiles. Herein, water and MeOH implement the conversion of nitrone intermediates to reaction products in two distinct pathways.     

Geometry‐Retentive C‐Alkenylation of Lithiated α‐Aminonitriles: Quaternary α‐Alkenyl Amino Acids and Hydantoins

α‐Amino nitriles tethered to alkenes through a urea linkage undergo intramolecular C‐alkenylation on treatment with base by attack of the lithionitrile derivatives on the N′‐alkenyl group. A geometry‐retentive alkene shift affords stereospecifically the E or Z isomer of the 5‐alkenyl‐4‐iminohydantoin products from the corresponding starting E ‐ or Z ‐N ′‐alkenyl urea, each of which may be formed from the same N ‐allyl precursor by stereodivergent alkene isomerization. The reaction, formally a nucleophilic substitution at an sp² carbon atom, allows the direct regioselective incorporation of mono‐, di‐, tri‐, and tetrasubstituted olefins at the α‐carbon of amino acid derivatives. The initially formed 5‐alkenyl iminohydantoins may be hydrolyzed and oxidatively deprotected to yield hydantoins and unsaturated α‐quaternary amino acids.     

Nickel‐Catalyzed α‐Allylation of Aldehydes and Tandem Aldol Condensation/Allylation Reaction with Allylic Alcohols

An additive‐free nickel‐catalyzed α‐allylation of aldehydes with allyl alcohol is reported. The reaction is promoted by 1 mol % of in situ formed nickel complex in methanol, and water is the sole by‐product of the reaction. The experimental conditions allow the conversion of various α‐branched aldehydes and α,β‐unsaturated aldehydes as nucleophiles. The same catalyst and reaction conditions enabled a tandem aldol condensation of aldehyde/α‐allylation reaction.     

α‐Selective Ring‐Opening Reactions of Bicyclo[1.1.0]butyl Boronic Ester with Nucleophiles

The reaction of bicyclo[1.1.0]butyl pinacol boronic ester (BCB‐Bpin) with nucleophiles has been studied. Unlike BCBs bearing electron‐withdrawing groups, which react with nucleophiles at the β‐position, BCB‐Bpin reacts with a diverse set of heteroatom (O, S, N)‐centred nucleophiles exclusively at the α‐position. Aliphatic alcohols, phenols, carboxylic acids, thiols and sulfonamides were found to be competent nucleophiles, providing ready access to α‐heteroatom‐substituted cyclobutyl boronic esters. In contrast, sterically hindered bis‐sulfonamides and related nucleophiles reacted with BCB‐Bpin at the β′‐position leading to cyclopropanes with high trans‐selectivity. The origin of selectivity is discussed.     

Stereospecific Nucleophilic Substitution with Arylboronic Acids as Nucleophiles in the Presence of a CONH Group

Stereospecific nucleophilic substitution was achieved for the first time with arylboronic acids as nucleophiles. This transition‐metal‐free coupling between chiral α‐aryl‐α‐mesylated acetamides and arylboronic acids provided access to a series of chiral α,α‐diaryl acetamides with excellent enantioselectivity and moderate to good yields. The CONH functionality proved to be crucial for bridging the reactants and promoting the reaction. Efficient syntheses of a cannabinoid CB₁ receptor ligand, the antidepressant (S)‐diclofensine, and a key chiral building block of the inhibitor implitapide were successfully accomplished by using this method.     

Amidation of Aldehydes and Alcohols through α‐Iminonitriles and a Sequential Oxidative Three‐Component Strecker Reaction/Thio‐Michael Addition/Alumina‐Promoted Hydrolysis Process to Access β‐Mercaptoamides from Aldehydes,Amines, and Thiols

Mild and general alumina‐promoted hydrolysis conditions for converting α‐iminonitriles into carboxamides have been developed. In combination with the oxidative three‐component Strecker reaction, the one‐pot direct amidation of aldehydes and alcohols is reported. Subsequently, an Yb(OTf)₃‐catalyzed Michael addition of thiols to α,β‐unsaturated α‐iminonitriles is reported for the synthesis of β‐mercapto‐α‐iminonitriles. The successful integration of an oxidative Strecker reaction, thio‐Michael addition, and neutral‐alumina‐promoted hydrolysis of β‐mercapto‐α‐iminonitriles into a three‐component one‐pot process allowed us to develop the direct conversion of amines, aldehydes, and thiols into β‐mercaptoamides. All of these procedures were applicable to aromatic and aliphatic amines and aldehydes.     

Catalytic Enantioselective Oxidative Cross‐Coupling of Benzylic Ethers with Aldehydes

The first one‐pot enantioselective oxidative coupling of cyclic benzylic ethers with aldehydes has been developed. A variety of benzylic ethers were transformed into the corresponding oxygen heterocycles with high enantioselectivity. Mechanistic experiments were conducted to determine the nature of the reaction intermediates. The application of this strategy to coupling reactions with other nucleophiles besides aldehydes was also explored.     

Organocatalysts Derived from Unnatural α‐Amino Acids: Scope and Applications

The organocatalytic properties of unnatural α‐amino acids are reviewed. Post‐translational derivatives of natural α‐amino acids include 4‐hydroxy‐l ‐proline and 4‐amino‐l ‐proline scaffolds, and also proline homologues. The activity of synthetic unnatural α‐amino acid‐based organocatalysts, such as β‐alkyl alanines, alanine‐based phosphines, and tert‐leucine derivatives, are reviewed herein. The organocatalytic properties of unnatural monocyclic, bicyclic, and tricyclic proline derivatives are also reviewed. Several families of these organocatalysts permit the efficient and stereoselective synthesis of complex natural products. Most of the reviewed organocatalysts accelerate the reported reactions through covalent interactions that raise the HOMO (enamine intermediates) or lower the LUMO (iminium intermediates).