Cross‐Coupling of Alkyl Redox‐Active Esters with Benzophenone Imines: Tandem Photoredox and Copper Catalysis

Alkyl amines are an important class of organic compounds in medicinal and materials chemistry. Until now very have been very few methods for the synthesis of alkyl amines by metal‐catalyzed cross‐coupling of alkyl electrophiles with nitrogen nucleophiles. Described here is an approach to employ tandem photoredox and copper catalysis to enable the cross‐coupling of alkyl N‐hydroxyphthalimide esters, readily derived from alkyl carboxylic acids, with benzophenone‐derived imines. Hydrolysis of the coupling products furnish alkylated primary amines. Primary, secondary, and tertiary alkyl groups can be transferred, and the coupling tolerates a diverse set of functional groups. The method allows rapid functionalization of natural products and drugs, and can be used to expedite syntheses of pharmaceuticals from readily available chemical feedstocks.     

Disulfonimide‐Catalyzed Asymmetric Reduction of N‐Alkyl Imines

A chiral disulfonimide (DSI)‐catalyzed asymmetric reduction of N‐alkyl imines with Hantzsch esters as a hydrogen source in the presence of Boc₂O has been developed. The reaction delivers Boc‐protected N‐alkyl amines with excellent yields and enantioselectivity. The method tolerates a large variety of alkyl amines, thus illustrating potential for a general reductive cross‐coupling of ketones with diverse amines, and it was applied in the synthesis of the pharmaceuticals (S)‐Rivastigmine, NPS R‐568 Hydrochloride, and (R)‐Fendiline.     

Iron‐Catalyzed Mild and Selective Hydrogenative Cross‐Coupling of Nitriles and Amines To Form Secondary Aldimines

The first example of a base‐metal‐catalyzed homogeneous hydrogenative coupling of nitriles and amines to selectively form secondary cross‐imines is reported. The reaction is catalyzed under mild conditions by a well‐defined (iPr‐PNP)Fe(H)Br(CO) pincer pre‐catalyst and catalytic t BuOK.     

Rhodium‐Catalyzed Asymmetric Hydrogenation of Unprotected NH Imines Assisted by a Thiourea

Asymmetric hydrogenation of unprotected NH imines catalyzed by rhodium/bis(phosphine)‐thiourea provided chiral amines with up to 97 % yield and 95 % ee. ¹H NMR studies, coupled with control experiments, implied that catalytic chloride‐bound intermediates were involved in the mechanism through a dual hydrogen‐bonding interaction. Deuteration experiments proved that the hydrogenation proceeded through a pathway consistent with an imine.     

Highly Stereoselective Cobalt(III)‐Catalyzed Three‐Component C−H Bond Addition Cascade

A highly stereoselective three‐component C(sp²)?H bond addition across alkene and polarized π‐bonds is reported for which Co^III catalysis was shown to be much more effective than Rh^III. The reaction proceeds at ambient temperature with both aryl and alkyl enones employed as efficient coupling partners. Moreover, the reaction exhibits extremely broad scope with respect to the aldehyde input; electron rich and poor aromatic, alkenyl, and branched and unbranched alkyl aldehydes all couple in good yield and with high diastereoselectivity. Multiple directing groups participate in this transformation, including pyrazole, pyridine, and imine functional groups. Both aromatic and alkenyl C(sp²)?H bonds undergo the three‐component addition cascade, and the alkenyl addition product can readily be converted into diastereomerically pure five‐membered lactones. Additionally, the first asymmetric reactions with Co^III‐catalyzed C?H functionalization are demonstrated with three‐component C?H bond addition cascades employing N‐tert‐butanesulfinyl imines. These examples represent the first transition metal catalyzed C?H bond additions to N‐tert‐butanesulfinyl imines, which are versatile and extensively used intermediates for the asymmetric synthesis of amines.     

Lewis Acid Mediated Vinyl‐Transfer Reaction of Alkynes to N‐Alkylimines by Using the N‐Alkyl Residue as a Sacrificial Hydrogen Donor

A variety of N‐alkyl‐α,α‐dichloroaldimines were vinylated by terminal acetylenes in the presence of Lewis acids such as In(OTf)₃ or BF₃ ? OEt₂ and hexafluoroisopropanol (HFIP) as an additive. The reaction proceeds at ambient temperature and leads to geometrically pure allylic β,β‐dichloroamines. This approach is complementary to previously reported transition‐metal‐catalyzed vinyl‐transfer methods, which are not applicable to aliphatic imines and are restricted to imines that contain an electron‐withdrawing nitrogen substituent. In the present approach, terminal alkynes were used as a source of the vinyl residue, and the N‐alkyl moiety of the imine acts as a sacrificial hydrogen donor. The additional advantage of this methodology is the fact that no external toxic or hazardous reducing agents or molecular hydrogen has to be used. This new methodology nicely combines a C(sp²)? C(sp) bond formation, hydride transfer, and an unusual cleavage of an unactivated C? N bond, thereby giving rise to functionalized primary allylic amines. A detailed experimental study supported by DFT calculations of the mechanism has been done.     

Ligand‐Controlled α‐ and β‐Arylation of Acyclic N‐Boc Amines

The palladium‐catalyzed ligand‐controlled arylation of α‐zincated acyclic amines, obtained by directed α‐lithiation and transmetalation, is described. Whereas PtBu₃ gave rise to α‐arylated Boc‐protected amines, more flexible N‐phenylazole‐based phosphine ligands induced major β‐arylation through migrative cross‐coupling.     

Copper‐Catalyzed Decarboxylative Radical Silylation of Redox‐Active Aliphatic Carboxylic Acid Derivatives

A decarboxylative silylation of aliphatic N ‐hydroxyphthalimide (NHPI) esters using Si−B reagents as silicon pronucleophiles is reported. This C(sp³)−Si cross‐coupling is catalyzed by copper(I) and follows a radical mechanism, even with exclusion of light. Both primary and secondary alkyl groups couple effectively, whereas tertiary alkyl groups are probably too sterically hindered. The functional‐group tolerance is generally excellent, and α‐heteroatom‐substituted substrates also participate well. This enables, for example, the synthesis of α‐silylated amines starting from NHPI esters derived from α‐amino acids. The new method extends the still limited number of C(sp³)−Si cross‐couplings of unactivated alkyl electrophiles.     

Lanthanides Mediated Oxidative Cross Coupling of Benzylalcohol and Various Amines to Form Corresponding Imines

Herein, a new and efficient approach towards the oxidative cross‐coupling of benzylalcohol and various aromatic amines to form corresponding imines with high degree conversion (>80 %) and chemo‐selectivity using lanthanide salts as pre‐catalysts is presented. The catalyzed oxidative cross‐coupling reaction using La(NO₃)₃ · 6H₂O as pre‐catalyst displayed a broad substrate scope. The reaction afforded various substituted imines from the reaction of benzylalcohol with ample variety of amines in good yields.     

Continuous‐Flow Oxidative Cyanation of Primary and Secondary Amines Using Singlet Oxygen

Primary and secondary amines can be rapidly and quantitatively oxidized to the corresponding imines by singlet oxygen. This reactive form of oxygen was produced using a variable‐temperature continuous‐flow LED‐photoreactor with a catalytic amount of tetraphenylporphyrin as the sensitizer. α‐Aminonitriles were obtained in good to excellent yields when trimethylsilyl cyanide served as an in situ imine trap. At 25°C, primary amines were found to undergo oxidative coupling prior to cyanide addition and yielded secondary α‐aminonitriles. Primary α‐aminonitriles were synthesized from the corresponding primary amines for the first time, by an oxidative Strecker reaction at –50 °C. This atom‐economic and protecting‐group‐free pathway provides a route to racemic amino acids, which was exemplified by the synthesis of tert‐leucine hydrochloride from neopentylamine.     

Visible‐Light‐Promoted Asymmetric Cross‐Dehydrogenative Coupling of Tertiary Amines to Ketones by Synergistic Multiple Catalysis

Reported herein is an unprecedented photocatalytic asymmetric cross‐dehydrogenative coupling reaction between tertiary amines and simple ketones, and it proceeds by synergistic multiple catalysis with substoichiometric amounts of a hydrogen acceptor. This process is enabled by a simple chiral primary amine catalyst through the coupling of a catalytic enamine intermediate and an iminium cation intermediate in situ generated from tetrahydroisoquinoline derivatives by coupled Ru/Co catalysis.     

3 d Transition Metal‐Catalyzed Hydrogenation of Nitriles and Alkynes

Selective hydrogenation of nitriles and alkynes is crucial considering the vast applications of reduced products in industries and in the synthesis of bioactive compounds. Particularly, the late 3d transition metal catalysts (manganese, iron, cobalt, nickel and copper) have shown promising activity for the hydrogenation of nitriles to primary amines, secondary amines and imines. Similarly, semihydrogenation of alkynes to E‐ and Z‐alkenes by 3d metals is adequately successful both via the transfer hydrogenation and by using molecular hydrogen. The emergence of 3d transition metals in the selective synthesis of industrially relevant amines, imines and alkenes makes this protocol more attractive. Herein, we provide a concise overview on the late 3d transition metal‐catalyzed hydrogenation of nitriles to amines and imines as well as semihydrogenation of alkynes to alkenes.     

Highly Diastereo‐ and Enantioselective Cu‐Catalyzed Borylative Coupling of 1,3‐Dienes and Aldimines

A Cu‐catalyzed diastereo‐ and enantioselective borylative coupling reaction of 1,3‐dienes with imines was realized. Branched homoallylic amines are readily prepared in a syn‐selective manner with high regio‐, diastereo‐ and enantioselectivity. Moreover, these three‐component coupling reactions feature good functional‐group compatibility and easy access to the substrates and catalyst.     

Formation of 1,3‐Thiazol‐5(4H)‐imines and 1,3‐Oxazol‐5(4H)‐imines in Aib‐Containing Thiopeptides

When tripeptides of type Axx^t‐Aib‐Axx‐OH were coupled with amino acid methyl esters by means of commonly used coupling reagents, the formation of 1,3‐thiazol‐5(4H)‐imines and 1,3‐oxazol‐5(4H)‐imines was observed. With the aim of understanding which structure elements are required for this reaction, several model peptides have been prepared according to our recently described methodology, a modification of the ‘azirine/oxazolone method', followed by selective isomerization of the peptide thioamides. In addition, attempts to prepare peptides that contain more than one C=S group by the same methodology also led to the formation of 1,3‐thiazol‐5(4H)‐imine‐containing derivatives. An additional C=S group can be introduced into the peptide, when the 1,3‐thiazol‐5(4H)‐imines were treated with H₂S, although mixtures of epimers were obtained. The structures of an endothiohexapeptide, two 1,3‐thiazol‐5(4H)‐ones, and two peptides containing a 1,3‐thiazol‐5(4H)‐imine moiety have been established by X‐ray crystal‐structure analysis.     

One‐Pot Tandem Photoredox and Cross‐Coupling Catalysis with a Single Palladium Carbodicarbene Complex

The combination of conventional transition‐metal‐catalyzed coupling (2 e^? process) and photoredox catalysis (1 e^? process) has emerged as a powerful approach to catalyze difficult cross‐coupling reactions under mild reaction conditions. Reported is a palladium carbodicarbene (CDC) complex that mediates both a Suzuki–Miyaura coupling and photoredox catalysis for C?N bond formation upon visible‐light irradiation. These two catalytic pathways can be combined to promote both conventional transition‐metal‐catalyzed coupling and photoredox catalysis to mediate C?H arylation under ambient conditions with a single catalyst in an efficient one‐pot process.     

Enantioselective Generation of Adjacent Stereocenters in a Copper‐Catalyzed Three‐Component Coupling of Imines,Allenes, and Diboranes

A highly enantio‐ and diastereoselective copper‐catalyzed three‐component coupling affords the first general synthesis of homoallylic amines bearing adjacent stereocenters from achiral starting materials. The method utilizes a commercially available NHC ligand and copper source, operates at ambient temperature, couples readily available simple imines, allenes, and diboranes, and yields high‐value homoallylic amines that exhibit versatile amino, alkenyl, and boryl units.     

Palladium‐Catalyzed One‐Pot Three‐ or Four‐Component Coupling of Aryl Iodides,Alkynes, and Amines through CN Bond Cleavage: Efficient Synthesis of Indole Derivatives

An efficient synthesis of N‐substituted indole derivatives was realized by combining the Pd‐catalyzed one‐pot multicomponent coupling approach with cleavage of the C(sp³)?N bonds. Three or four components of aryl iodides, alkynes, and amines were involved in this coupling process. The cyclopentadiene–phosphine ligand showed high efficiency. A variety of aryl iodides, including cyclic and acyclic tertiary amino aryl iodides, and substituted 1‐bromo‐2‐iodobenzene derivatives could be used. Both symmetric and unsymmetric alkynes substituted with alkyl, aryl, or trimethylsilyl groups could be applied. Cyclic secondary amines such as piperidine, morpholine, 4‐methylpiperidine, 1‐methylpiperazine, 2‐methylpiperidine, and acyclic amines including secondary and primary amines all showed good reactivity. Further application of the resulting indole derivatives was demonstrated by the synthesis of benzosilolo[2,3‐b]indole.     

Enantioselective Generation of Adjacent Stereocenters in a Copper‐Catalyzed Three‐Component Coupling of Imines,Allenes, and Diboranes

A highly enantio‐ and diastereoselective copper‐catalyzed three‐component coupling affords the first general synthesis of homoallylic amines bearing adjacent stereocenters from achiral starting materials. The method utilizes a commercially available NHC ligand and copper source, operates at ambient temperature, couples readily available simple imines, allenes, and diboranes, and yields high‐value homoallylic amines that exhibit versatile amino, alkenyl, and boryl units.     

Cu(I)‐catalyzed Green Synthesis of Propargyl Amines Decorated with Carbazole Moiety by A3 ‐Coupling

A series of novel N ‐alkylcarbazol–propargylamine hybrids were designed and synthesized by Cu^IBr ‐catalyzed A³ ‐coupling of N ‐octylcarbazol‐3‐carbaldehyde, amines, and alkynes. The tri‐substituted propargyl amines decorated with carbazole moiety were obtained under solvent‐free conditions in good to moderate yields. Furthermore, the scope of the method was studied, which was found to be applicable to primary aliphatic and aromatic amines. Also, a large variety of substituents both on alkynes and anilines are well tolerated.     

Copper‐Catalyzed Borylative Cross‐Coupling of Allenes and Imines: Selective Three‐Component Assembly of Branched Homoallyl Amines

A copper‐catalyzed three‐component coupling of allenes, bis(pinacolato)diboron, and imines allows regio‐, chemo‐, and diastereoselective assembly of branched α,β‐substituted‐γ‐boryl homoallylic amines, that is, products bearing versatile amino, alkenyl, and borane functionality. Alternatively, convenient oxidative workup allows access to α‐substituted‐β‐amino ketones. A computational study has been used to probe the stereochemical course of the cross‐coupling.