Enantioselective radical addition reactions to imines using binaphthol-derived chiral N-triflyl phosphoramides

Modular synthesis of α-branched secondary alkylamines via visible-light-mediated carbonyl alkylative amination

The development of methods for the assembly of secondary α-alkyl amines remains a central challenge to chemical synthesis because of their critical importance in modulating the physical properties of biologically active molecules. Despite decades of intensive research, chemists still rely on selective N-alkylation and carbonyl reductive amination to make most amine products. Here we report the further evolution of a carbonyl alkylative amination process that, for the first time, brings together primary amines, aldehydes and alkyl iodides in a visible-light-mediated multicomponent coupling reaction for the synthesis of a wide range of α-branched secondary alkylamines. In addition to exploring the tolerance and limitations in each reaction component, we also report preliminary applications to the telescoped synthesis of α-branched N-heterocycles and an N-alkylation protocol that is selective for primary over cyclic secondary amines. Our data support a mechanism involving addition of an alkyl radical to an uncharged alkyl imine which, to the best of our knowledge, has not previously been described. We believe that this method will enable practitioners of synthetic chemistry in academic and industrial settings to approach the synthesis of these important molecules in a manner that is streamlined compared to established approaches.

Visible-Light-Promoted Enantioselective Acylation and Alkylation of Aldimines Enabled by 9-Fluorenone Electron-Shuttle Catalysis

Chiral acyclic α-tertiary amino ketones are widely present in various natural products and pharmaceuticals; however, the direct synthesis of this pharmacophore through a robust strategy still presents significant challenges. The emerging photocatalysis provides a powerful approach to construct chemical bonds that are difficult to form via a traditional two-electron pathway. Herein, we developed visible-light-induced chiral Lewis acid-catalyzed highly enantioselective acylation/alkylation of aldimines enabled by cooperative FLN (9-fluorenone) electron-shuttle catalysis via radical addition. An array of α-tertiary amino ketones, β-amino alcohols, and chiral amines were achieved with high yields and good to excellent stereocontrol (87 examples, up to 84% yield, 96% ee). These products can be easily transformed into valuable and bioactive skeletons. Extensive control experiments, detailed mechanism studies, and density functional theory calculations elucidated the reaction process and highlighted the crucial role played by FLN.

Mutual influence of non-covalent interactions formed by imidazole: A systematic quantum-chemical study

Imidazole is a five-membered heterocycle that is part of a number of biologically important molecules such as the amino acid histidine and the hormone histamine. Imidazole has a unique ability to participate in a variety of non-covalent interactions involving the NH group, the pyridine-like nitrogen atom or the π-system. For many biologically active compounds containing the imidazole moiety, its participation in formation of hydrogen bond NH⋯O/N and following proton transfer is the key step of mechanism of their action. In this work a systematic study of the mutual influence of various paired combinations of non-covalent interactions (e.g., hydrogen bonds and π-interactions) involving the imidazole moiety was performed by means of quantum chemistry (PW6B95-GD3/def2-QZVPD) for a series of model systems constructed based on analysis of available x-ray data. It is shown that for considered complexes formation of additional non-covalent interactions can only enhance the proton-donating ability of imidazole. At the same time, its proton-accepting ability can be both enhanced and weakened, depending on what additional interactions are added to a given system. The mutual influence of non-covalent interactions involving imidazole can be classified as weak geometric and strong energetic cooperativity-a small change in the length of non-covalent interaction formed by imidazole can strongly influence its strength. The latter can be used to develop methods for controlling the rate and selectivity of chemical reactions involving the imidazole fragment in larger systems. It is shown that the strong mutual influence of non-covalent interactions involving imidazole is due to the unique ability of the imidazole ring to effectively redistribute electron density in non-covalently bound systems with its participation.

Cobalt-Catalyzed Asymmetric Aza-Nozaki-Hiyama-Kishi (NHK) Reaction of α-Imino Esters with Alkenyl Halides

Chromium-catalyzed enantioselective Nozaki-Hiyama-Kishi (NHK) reaction represents one of the most powerful approaches for the formation of chiral carbon-heteroatom bond. However, the construction of sterically encumbered tetrasubstituted stereocenter through NHK reaction still posts a significant challenge. Herein, we disclose a cobalt-catalyzed aza-NHK reaction of ketimine with alkenyl halide to provide a convenient synthetic approach for the manufacture of enantioenriched tetrasubstituted α-vinylic amino acid. This protocol exhibits excellent functional group tolerance with excellent 99 % ee in most cases. Additionally, this asymmetric reductive method is also applicable to the aldimine to access the trisubstituted stereogenic centers.

Iodoperfluoroalkylation of unactivated alkenes via pyridine-boryl radical initiated atom-transfer radical addition

The pyridine/bis(pinacolate)diboron combination has been found to be able to initiate the iodoperfluoroalkylation of unactivated alkenes with perfluoroalkyl iodides. Theoretical calculations and control experiments indicate that the atom transfer radical addition mechanism is responsible for the formation of iodoperfluoroalkylation products. This metal-free and photo-free strategy is applicable to a wide range of perfluoroalkyl iodides and unactivated alkenes with good functional group tolerance. Further applications in iodoperfluoroalkylation of organic semiconductor-relevant or bioactive molecules demonstrate the synthetic potential of this method.

Visible Light-Induced Pd-Catalyzed Alkyl-Heck Reaction of Oximes

A visible light-induced palladium-catalyzed oxidative C-H alkylation of oximes has been developed. This mild protocol allows for an efficient atom economical C-C bond construction of alkyl-substituted oximes. A broad range of primary, secondary, and tertiary alkyl bromides and iodides, as well as a range of different formaldoximes, can efficiently undergo this transformation. The method features visible light-induced generation of nucleophilic hybrid alkyl Pd radical intermediates, which upon radical addition at the imine moiety and a subsequent β-hydrogen elimination deliver substituted imines.

Chiral Alkyl Amine Synthesis via Catalytic Enantioselective Hydroalkylation of Enecarbamates

Chiral alkyl amines are omnipresent as bioactive molecules and synthetic intermediates. The catalytic and enantioselective synthesis of alkyl amines from readily accessible precursors is challenging. Here we develop a nickel-catalyzed hydroalkylation method to assemble a wide range of chiral alkyl amines from enecarbamates (N-Cbz-protected enamines) and alkyl halides with high regio- and enantioselectivity. The method works for both nonactivated and activated alkyl halides and is able to produce enantiomerically enriched amines with two minimally differentiated α-alkyl substituents. The mild conditions lead to high functional group tolerance, which is demonstrated in the postproduct functionalization of many natural products and drug molecules, as well as the synthesis of chiral building blocks and key intermediates to bioactive compounds.

Et3 B/Et2 AlCl/O2 -Mediated Radical Coupling Reaction between α-Alkoxyacyl Tellurides and 2-Hydroxybenzaldehyde Derivatives

A newly devised radical-based strategy enabled coupling between multiply oxygenated α-alkoxyacyl tellurides and 2-hydroxybenzaldehyde derivatives. A reagent combination of Et₃ B, Et₂ AlCl, and O₂ promoted the formation of the α-alkoxy carbon radical from the α-alkoxyacyl telluride and the addition of the radical to the carbonyl group of 2-hydroxybenzaldehyde. The reaction chemo- and stereoselectively forged the hindered C-C bond between two oxygen-functionalized carbons at ambient temperature. The method was applied to the preparation of 12 coupling adducts with three to six contiguous stereocenters and to the concise synthesis of an antitumor compound, LLY-283.

Iron(II)-Catalyzed Radical Addition to Aldimines with Hantzsch Ester as a Two-Hydrogen Atom Donor

The first Fe(OTf)₂-catalyzed radical addition to aldimines with Hantzsch ester as a two-hydrogen atom donor is reported. The tin-free reaction works well for electron-deficient substrates and provides a potentially useful approach to α-branched amines and α-amino acids.

Mild, Redox-Neutral Alkylation of Imines Enabled by an Organic Photocatalyst

An operationally simple, mild, redox-neutral method for the photoredox alkylation of imines is reported. Utilizing an inexpensive organic photoredox catalyst, alkyl radicals are readily generated from the single-electron oxidation of ammonium alkyl bis(catecholato)silicates and are subsequently engaged in a C–C bond-forming reaction with imines. The process is highly selective, metal-free, and does not require a large excess of the alkylating reagent or the use of acidic additives.

Control of asymmetry in the radical addition approach to chiral amine synthesis

The state-of-the-science in asymmetric free radical additions to imino compounds is presented, beginning with an overview of methods involving stereocontrol by various chiral auxiliary approaches. Chiral N-acylhydrazones are discussed with respect to their use as radical acceptors for Mn-mediated intermolecular additions, from design to scope surveys to applications to biologically active targets. A variety of aldehydes and ketones serve as viable precursors for the chiral hydrazones, and a variety of alkyl iodides may be employed as radical precursors, as discussed in a critical review of the functional group compatibility of the reaction. Applications to amino acid and alkaloid synthesis are presented to illustrate the synthetic potential of these versatile stereocontrolled carbon-carbon bond construction reactions. Asymmetric catalysis is discussed, from seminal work on the stereocontrol of radical addition to imino compounds by non-covalent interactions with stoichiometric amounts of catalysts, to more recent examples demonstrating catalyst turnover.

Intermolecular alkyl radical additions to enantiopure N-tert-butanesulfinyl aldimines

The sulfinyl group in (R)-N-tert-butanesulfinyl aldimines provides efficient control of the stereoselectivity in the intermolecular reactions with alkyl radicals. The methodology is applicable to aryl, heteroaryl, benzyl, and alkynyl imines, even those containing CN, CO2Me, COR, and OH groups. The best results are attained with hindered radicals (tertiary and secondary ones) without C═N bond reduction. This reaction complements the well-established organometallic additions to N-sulfinyl aldimines to obtain enantiomerically pure functionalized α-branched primary amines.

Recent applications of the (TMS)3SiH radical-based reagent

This review article focuses on the recent applications of tris(trimethylsilyl)silane as a radical-based reagent in organic chemistry. Numerous examples of the successful use of (TMS)(3)SiH in radical reductions, hydrosilylation and consecutive radical reactions are given. The use of (TMS)(3)SiH allows reactions to be carried out under mild conditions with excellent yields of products and remarkable chemo-, regio-, and stereoselectivity. The strategic role of (TMS)(3)SiH in polymerization is underlined with emphasis on the photo-induced radical polymerization of olefins and photo-promoted cationic polymerization of epoxides.

Intermolecular radical addition to N-acylhydrazones as a stereocontrol strategy for alkaloid synthesis: formal synthesis of quinine

Stereocontrolled Mn-mediated radical addition of alkyl iodides to chiral N-acylhydrazones enables strategic C-C bond disconnection of chiral amines. This strategy was examined in the context of a total synthesis of quinine, generating new findings of functional group compatibility leading to a revised strategy. Completion of a formal synthesis of quinine is presented, validating the application of Mn-mediated radical addition as a useful new C-C bond construction method for alkaloid synthesis. The Mn-mediated addition generates the chiral amine substructure of quinine with complete stereocontrol. Subsequent elaboration includes two successive ring closures to forge the azabicyclo[2.2.2]octane ring system of quincorine, linked to quinine through two known reactions.