Base-Catalyzed Asymmetric α-Functionalization of 2-(Cyanomethyl)azaarene N-Oxides Leading to Quaternary Stereocenters

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.

Kinetic Resolution of Azaarylethynyl Tertiary Alcohols by Chiral Brønsted Acid Catalysed Phosphine-Mediated Deoxygenation

A chiral Brønsted acid catalysed phosphine-mediated deoxygenation protocol is reported. This metal-free method provides a precise kinetic resolution platform for azaarylethynyl tertiary alcohols, which are a broad category of biologically and synthetically important azaarene derivatives. In addition to providing an efficient method for the first asymmetric preparation of these tertiary alcohols, the strategy facilitates the construction of azaaryl-functionalized allenes with good to excellent enantioselectivities. The high selectivity factors (s up to 235), broad substrate scope, and ability to convert azaaryl compounds into both chiral tertiary alcohols and allenes robustly underscore the efficiency and promising utility of this method. The practicability is further validated by the successful synthesis of deuterated allenes with high ee values and substantial incorporation of deuterium using inexpensive D₂ O as the deuterium source.

Iridium-Catalyzed Asymmetric Allylic Substitution of Methyl Azaarenes

Herein, an Ir-catalyzed asymmetric allylic substitution reaction of methyl azaarenes is described. Azaarenes such as (benzo)thiazole, oxazole, benzoimidazole, pyridine, and (iso)quinoline are all tolerated. The corresponding chiral azaarene derivatives are obtained in good yields with high enantioselectivity (up to 96 % yield and 99 % ee). The utilization of the Knochel reagent TMPZnBr⋅LiBr warrants the in situ formation of benzylic nucleophiles without additional activating reagents. ¹ H NMR studies suggested a two-fold function of the Knochel reagent in this reaction. The synthetic utility of this method has been showcased by a concise enantioselective synthesis of an allosteric protein kinase modulator.

Asymmetric α-Functionalization of 2-Alkyl Azaarenes: Synthesis of Tertiary Fluorides Having Vicinal Stereogenic Centers

An enantioselective approach for synthesizing fluorinated azaarenes containing vicinal quaternary-tertiary stereocenters is summarized. The chiral copper(I)-phosphine complex binds with the azaarenes followed by Michael addition to unsaturated acyl imidazoles, resulting in α-functionalized products with an excellent level of enantioselectivities (up to 99%), diastereoselectivities (>20:1), and yields (up to 97%). Furthermore, post-functionalization of the acyl imidazole part has also been demonstrated.

Pyridylacetic acids and related systems as alkylheteroarene surrogates in asymmetric decarboxylative Michael addition

In this manuscript, a novel method for the preparation of enantiomerically enriched pyridine derivatives has been described. It is based on the utilization of readily available 2-pyridylacetic acids as valuable synthons for the introduction of a pyridine ring in an asymmetric fashion. They have been used as pronucleophiles in asymmetric decarboxylative Michael addition to α,β-unsaturated aldehydes. The synthesis based on iminium activation using a chiral aminocatalyst that controlled the stereochemical outcome of the transformation has been successfully accomplished.

Cooperative photoredox and chiral hydrogen-bonding catalysis

Chiral hydrogen-bonding catalysis is a classic strategy in asymmetric organocatalysis. Recently, it has been used to cooperate with photoredox catalysis, becoming a powerful tool to access optical pure compounds via radical-based transformations.

Enantioselective Alkylation of 2-Alkylpyridines Controlled by Organolithium Aggregation

Direct enantioselective α-alkylation of 2-alkylpyridines provides access to chiral pyridines via an operationally simple protocol that obviates the need for prefunctionalization or preactivation of the substrate. The alkylation is accomplished using chiral lithium amides as noncovalent stereodirecting auxiliaries. Crystallographic and solution NMR studies provide insight into the structure of well-defined chiral aggregates in which a lithium amide reagent directs asymmetric alkylation.

Asymmetric Synthesis of Adjacent Tri- and Tetrasubstituted Carbon Stereocenters: Organocatalytic Aldol Reaction of an Hydantoin Surrogate with Azaarene 2-Carbaldehydes

A bifunctional amine/squaramide catalyst promoted direct aldol addition of an hydantoin surrogate to pyridine 2-carbaldehyde N-oxides to afford adducts bearing two vicinal tertiary/quaternary carbons in high diastereo- and enantioselectivity (d.r. up to >20:1; ee up to 98 %) is reported. Acid hydrolysis of adducts followed by reduction of the N-oxide group yields enantiopure carbinol-tethered quaternary hydantoin-azaarene conjugates with densely functionalized skeletons. DFT studies of the potential energy surface (B3LYP/6-31+G(d)+CPCM (dichloromethane)) of the reaction correlate the activity of different catalysts and support an intramolecular hydrogen-bond-assisted activation of the squaramide moiety in the transition state of the catalytic reaction.

Site-Selective C-H Alkylation of Diazine N-Oxides Enabled by Phosphonium Ylides

The synthesis of alkylated diazine derivatives is important for their practical utilization as pharmaceuticals and for other purposes. Herein, we describe the metal-free site-selective C-H alkylation of diazine N-oxides using phosphonium ylides that affords a variety of alkylated diazine derivatives with broad functional group tolerance. The utility of this method is showcased by the late-stage functionalization of a commercially available drug such as varenicline. Notably, the sequential C-H alkylation of pyrazine N-oxides for the total synthesis of a pyrazine-containing natural product, paenibacillin A, highlights the importance of this method.

Diastereo- and Enantioselective 1,6-Conjugate Addition of 2-Azaarylacetamides to para-Quinone Methides

A chiral Mg(II)/N,N'-dioxide complex was found to be highly effective in promoting the diastereo- and enantioselective 1,6-conjugate addition of various azaarylacetamides to p-quinone methides under mild reaction conditions. Various chiral azaarene derivatives containing gem(1,1)-diaryl skeletons were obtained in good yields, dr, and excellent ee values. Meanwhile, on the basis of insight gained from the control experiments as well as the single-crystal structure of the Mg(II)/N,N'-dioxide complex and product, a possible transition state was proposed to explain the origin of coordination and stereoinduction.

Photoredox-catalysed formal [3+2] cycloaddition of N-aryl α-amino acids with isoquinoline N-oxides

This work realizes a new synthetic utility of N-aryl α-amino acids as a 1,2-synthon, a new strategy to achieve dearomatization of isoquinolines, and the synthesis of valuable diazabicyclo[3.2.1]octane-based compounds.

Copper-catalyzed regioselective cyclization of vinyl azides by gem-difluoromethylene for trisubstituted pyridines

A novel copper-catalyzed cyclization of readily available vinyl azides with gem-difluoromethylene is described.

Chiral phosphoric acid-catalyzed asymmetric C(sp3)–H functionalization of biomass-derived 2,5-dimethylfuran via two sequential Cope-type rearrangements

The first organocatalytic asymmetric C(sp³)–H functionalization of biomass-derived 2,5-dimethylfuran with alcohols to afford enantioenriched furan-derived 3,3′-disubstituted oxindoles in high enantioselectivity.

Enantioselective reduction of azaarene-based ketones via visible light-driven photoredox asymmetric catalysis

An enantioselective reduction of azaarene-based ketones through photoredox asymmetric catalysis is reported.

Diastereoselective Rh-catalyzed decarboxylative allylation to form quaternary stereocenters using sulfinimine as the directing group

In this paper, we reported for the first time that the diastereoselective Rh-catalyzed decarboxylative allylation of chiral sulfinimines could be used to form quaternary stereocenters.

Enantioselective Reaction between 2-(Cyanomethyl)azaarenes and N-Boc-amino Sulfones

A series of 2-(cyanomethyl)azaarenes containing benzothiazole or benzoxazole were designed and synthesized for asymmetric α-functionalization with N-Boc-amino sulfones. The Mannich adducts were obtained in high yields with good diastereo- and enantioselectivities. Aryl-substituted amino sulfones were tolerated under the current conditions, and the reaction can be performed on gram scale in good results.

Enantioselective direct Mannich-type reactions of 2-benzylpyridine N-oxides catalyzed by chiral bis(guanidino)iminophosphorane organosuperbase

2-Benzylpyridine N-oxides possessing less acidic α-protons were utilized as pronucleophiles for the first time in enantioselective addition reactions under Brønsted base catalysis.

2-Benzylpyridine N-oxides possessing less acidic α-protons were utilized as pronucleophiles for the first time in enantioselective addition reactions under Brønsted base catalysis. A chiral bis(guanidino)iminophosphorane was able to overcome the inherent issue of low acidity of the pronucleophiles, establishing the diastereo- and enantioselective direct Mannich-type reaction with N-Boc imines. The control experiments indicated that the N-oxide moiety of the substrates played a critical role in achieving the high stereoselectivity.

Conjugate Addition-Enantioselective Protonation of N-Aryl Glycines to α-Branched 2-Vinylazaarenes via Cooperative Photoredox and Asymmetric Catalysis

An enantioselective protonation strategy has been successfully applied to the synthesis of chiral α-tertiary azaarenes. With a dual catalytic system involving a chiral phosphoric acid and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer that is mediated by visible light, a variety of α-branched 2-vinylpyridines and 2-vinylquinolines with N-aryl glycines underwent a redox-neutral, radical conjugate addition-protonation process and provided valuable chiral 3-(2-pyridine/quinoline)-3-substituted amines in high yields with good to excellent enantioselectivities (up to >99% ee). An application of this methodology to a two-step synthesis of the enantiomerically pure medicinal compound pheniramine (Avil) is also presented.

α-Alkylation of Chiral Sulfinimines for Constructing Quaternary Chiral Carbons by Introducing Removable Directing Groups

This study developed a facile and efficient synthetic strategy to construct quaternary chiral centers at the α-position of imines and ketones. High regioselectivity and diastereoselectivity were achieved through the synergetic effect of electron-withdrawing directing groups and N-tert-butyl sulfinamide as chiral auxiliaries. Either of them could be removed under the optimized conditions without any epimerization.

Stereodivergent Allylation of Azaaryl Acetamides and Acetates by Synergistic Iridium and Copper Catalysis

We report stereodivergent allylic substitution reactions of allylic esters with prochiral enolates derived from azaaryl acetamides and acetates to form products from addition of the enolates at the most substituted carbon of an allyl moiety with two catalysts, a chiral metallacyclic iridium complex and a chiral bisphosphine-ligated copper(I) complex, which individually control the configuration of the electrophilic and nucleophilic carbon atoms, respectively. By simple permutations of enantiomers of the two catalysts, all four stereoisomers of products containing two stereogenic centers were synthesized individually with high diastereoselectivity and enantioselectivity. A variety of azaaryl acetamides and acetates bearing pyridyl, benzothiazolyl, benzoxazolyl, pyrazinyl, quinolinyl and isoquinolinyl moieties were all found to be suitable for this transformation.

Strategy for Stereoselective Metal-free α-Functionalization of 2-Azaaryl Acetates with N-Boc Imines

We report the first diastereo- and enantioselective formal Mannich reaction of 2-pyridyl acetates which gives rise to α- and β-functionalized 2-substituted pyridines. Key for success is the previous azaarene N-oxide formation enabling α-carbon deprotonation by a mild bifunctional Brønsted base and subsequent reaction with N-Boc imines under almost perfect stereocontrol.

Highly enantioselective metallation-substitution alpha to a chiral nitrile

Nitrile anions do not necessarily lack stereochemical integrity and we show good results for stereospecific reaction with a simple magnesium base.

We report the deprotonation of a chiral nitrile and reaction of the resulting chiral organometallic species with a variety of electrophiles to give highly enantiomerically enriched 2-substituted nitrile products. The nitrile was treated with TMPMgCl and the resulting anion, an asymmetric alpha cyano Grignard species, was found to be configurationally stable at low temperature for a short time (half-life several minutes at –104 °C).