Nickel‐Catalyzed Synthesis of N‐Aryl‐1,2‐dihydropyridines by [2+2+2] Cycloaddition of Imines with Alkynes through T‐Shaped 14‐Electron Aza‐Nickelacycle Key Intermediates

Despite there being a straightforward approach for the synthesis of 1,2‐dihydropyridines, the transition‐metal‐catalyzed [2+2+2] cycloaddition reaction of imines with alkynes has been achieved only with imines containing an N‐sulfonyl or ‐pyridyl group. Considering the importance of 1,2‐dihydropyridines as useful intermediates in the preparation of a wide range of valuable organic molecules, it would be very worthwhile to provide novel strategies to expand the scope of imines. Herein we report a successful expansion of the scope of imines in nickel‐catalyzed [2+2+2] cycloaddition reactions with alkynes. In the presence of a nickel(0)/PCy₃ catalyst, a reaction with N‐benzylidene‐P,P‐diphenylphosphinic amide was developed. Moreover, an application of N‐aryl imines to the reaction was also achieved by adopting N‐heterocyclic carbene ligands. The isolation of an (η²‐N‐aryl imine)nickel(0) complex containing a 14‐electron nickel(0) center and a T‐shaped 14‐electron five‐membered aza‐nickelacycle is shown. These would be considered as key intermediates of the reaction. The structure of these complexes was unambiguously determined by NMR spectroscopy and X‐ray analyses.     

A Transient‐Directing‐Group Strategy Enables Enantioselective Reductive Heck Hydroarylation of Alkenes

Metal‐coordinating directing groups have seen extensive use in the field of transition‐metal‐catalyzed alkene functionalization; however, their waste‐generating installation and removal steps limit the efficiency and practicality of reactions that rely on their use. Inspired by developments in asymmetric organocatalysis, where reactions rely on reversible covalent interactions between an organic substrate and a chiral mediator, we have developed a transient‐directing‐group approach to reductive Heck hydroarylation of alkenyl benzaldehyde substrates that proceeds under mild conditions. Highly stereoselective migratory insertion is facilitated by in situ formation of an imine from catalytic amounts of a commercially available amino acid additive. Computational studies reveal an unusual mode of enantioinduction by the remote chiral center in the transient directing group.     

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.     

Chiral Cp‐Rhodium(III)‐Catalyzed Asymmetric Hydroarylations of 1,1‐Disubstituted Alkenes

Metal‐catalyzed functionalizations at the ortho position of a directing group have become an efficient bond‐forming strategy. A wide range of transformations that employ Cp*Rh^III catalysts have been described, but despite their synthetic potential, enantioselective variants that use chiral versions of the Cp* ligand remain scarce (Cp*=pentamethyl cyclopentadienyl). Cyclopentadienyl compounds with an atropchiral biaryl backbone are shown to be suitable ligands for the efficient intramolecular enantioselective hydroarylation of aryl hydroxamates. Dihydrofurans that bear methyl‐substituted quaternary stereocenters are thus obtained by C? H functionalization under mild conditions.     

Pd(0)‐Catalyzed Tandem One‐Pot Reaction of Biphenyl Ketones/Aldehydes to the Corresponding Di‐substituted Aryl Olefins

Synthesis of di‐substituted aryl olefins via a Pd(0)‐catalyzed cross‐coupling reaction of biphenyl ketones/aldehydes, tosylhydrazide, and aryl bromides (or benzyl halides) was developed. This methodology was achieved by one‐pot two‐step reactions involving the preparation of N ‐tosylhydrazones by reacting tosylhydrazide with biphenyl ketones/aldehydes, followed by coupling with aryl bromides (or benzyl halides) in the presence of Pd(PPh₃ )₄ and lithium t ‐butoxide to produce various di‐substituted aryl olefins in moderate to good yields.     

Mechanistic investigation of imine formation in ruthenium‐catalyzed N‐alkylation of amines with alcohols

Imines are observed frequently in ruthenium‐catalyzed N‐alkylation of amines with alcohols. Herein, nitrogen–phosphine functionalized carbene ligands were developed and used in ruthenium‐catalyzed N‐alkylation to explore the mechanism of imine formation. The results showed that strongly electron‐donating ligands were beneficial for imine formation and alcohol dehydrogenation to generate acid. In addition, with an increase of electron density of nitrogen atom in substituted amines, the yield of imines in N‐alkylation was improved. At the same time, with electron‐rich imines as substrates, the transfer hydrogenation of imines became difficult. It is suggested that strongly electron‐donating ligands and substrates caused an increase of electron density on the ruthenium center, which resulted in the elimination of hydrogen atoms in active species [LRuH₂] as hydrogen gas rather than transfer onto the imine coordinated with the ruthenium center.     

Heck‐Type Reactions of Imine Derivatives: A DFT Study

The mechanism of a recently discovered intramolecular Heck‐type coupling of oximes with aryl halides (Angew. Chem. Int. Ed. 2007 , 46, 6325) was systematically studied by using density functional methods enhanced with a polarized continuum solvation model. The overall catalytic cycle of the reaction was found to consist of four steps: oxidative addition, migratory insertion, β‐H elimination, and catalyst regeneration, whereas an alternative base‐promoted C? H activation pathway was determined to be less favorable. Migratory insertion was found to be the rate determining step in the catalytic cycle. The apparent activation barrier of migratory insertion of the (E)‐oxime was +20.5 kcal mol^?1, whereas the barrier of (Z)‐oxime was as high as +32.7 kcal mol^?1. However, (Z)‐oxime could isomerize to form the more active (E)‐oxime with the assistance of K₂CO₃, so that both the (E)‐ and (Z)‐oxime substrates could be transformed to the desired product. Our calculations also indicated that the Z product was predominant in the equilibrium of the isomerization of the imine double bond, which constituted the reason for the good Z‐selectivity observed for the reaction. Furthermore, we examined the difference between the intermolecular Heck‐type reactions of imines and of olefins. It was found that in the intermolecular Heck‐type coupling of imines, the apparent activation barrier of migratory insertion was as high as +35 kcal mol^?1, which should be the main obstacle of the reaction. The analysis also revealed the main problem for the intermolecular Heck‐type reactions of imines, which was that the breaking of a C?N π bond was much more difficult than the breaking of a C?C π bond. After systematic examination of a series of substituted imines, (Z)‐N‐amino imine and N‐acetyl imine were found to have relatively low barriers of migratory insertion, so that they might be possible substrates for intermolecular Heck‐type coupling.     

Cobalt‐Catalyzed C(sp2)−H Methylation by using Dicumyl Peroxide as both the Methylating Reagent and Hydrogen Acceptor

The first cobalt‐catalyzed direct methylation of a C(sp²)?H bond using dicumyl peroxide (DCP) as both the methylating reagent and hydrogen acceptor has been established. The reaction proceeded without the use of any additives, and was proven to be applicable to various amides bearing a 2‐pyridinylisopropyl (PIP) directing group, providing an efficient access to o‐methyl aryl amides with high functional‐group tolerance. Preliminary mechanistic studies suggest a radical process would be involved in the catalytic process.     

Catalytic Asymmetric Synthesis of 3‐Indolyl Methanamines Using Unprotected Indoles and N‐Boc Imines under Basic Conditions

A chiral imidazolidine‐containing NCN/Pd‐OTf catalyst ( C4 ) promoted the nucleophilic addition of unprotected indoles to N‐Boc imines. Using sulfinyl amines as the N‐Boc imine precursors, the combined use of C4 with K₂CO₃ activated the NH indoles to give chiral 3‐indolyl methanamines with up to 98 % ee. Compared with conventional acid‐catalyzed Friedel–Crafts reactions, this reaction proceeds under mildly basic conditions and is advantageous for the use of acid‐sensitive substrates.     

Intramolecular Aza‐Diels–Alder Reactions of ortho‐Quinone Methide Imines: Rapid,Catalytic, and Enantioselective Assembly of Benzannulated Quinolizidines

Aza‐Diels–Alder reactions (ADARs) are powerful processes that furnish N‐heterocycles in a straightforward fashion. Intramolecular variants offer the additional possibility of generating bi‐ and polycyclic systems with high stereoselectivity. We report herein a novel Brønsted acid catalyzed process in which ortho‐quinone methide imines tethered to the dienophile via the N substituent react in an intramolecular ADAR to form complex quinolizidines and oxazinoquinolines in a one‐step process. The reactions proceed under very mild conditions, with very good yields and good to very good diastereo‐ and enantioselectivities. Furthermore, the process was extended to a domino reaction that efficiently combines substrate synthesis, ortho‐quinone methide imine formation, and ADAR.     

Manganese‐Catalyzed C−H Functionalizations: Hydroarylations and Alkenylations Involving an Unexpected Heteroaryl Shift

A manganese‐catalyzed regio‐ and stereoselective hydroarylation of allenes is reported. The C−H functionalization method provides access to various alkenylated indoles in excellent yields. Moreover, a hydroarylation/cyclization cascade involving an unexpected C−N bond cleavage and aryl shift has been developed, which provides a new synthetic approach to substituted pyrroloindolones.     

Pd‐Catalyzed C−H Alkylation of Arenes Using PyrDipSi,a Transformable and Removable Silicon‐Tethered Directing Group

An efficient Pd‐catalyzed ortho‐C?H alkylation reaction of arenes using a transformable and removable Si‐tethered pyridyldiisopropylsilyl (PyrDipSi) directing group has been developed. In addition, the PyrDipSi directing group allows for an efficient sequential double‐fold C?H alkylation/oxygenation of arenes to produce meta‐alkylated phenols. This directing group can easily be removed or converted into valuable functionalities, such as aryl, iodo, boronic ester, or phenol.     

Rhodium‐Catalyzed Addition of Aryl Boronic Acids to 2,2‐Disubstituted Malononitriles

β‐Ketonitriles bearing a quaternary carbon at the 2‐position were prepared through Rh‐catalyzed addition of aryl boronic acids to 2,2‐disubstituted malononitriles. In contrast to the previously described transnitrilative cyanation of aryl boronic acids with dialkylmalononitriles, the present reaction avoids retro‐Thorpe collapse of the intermediate addition product through the use of a milder base. The reaction was amenable to a variety of aryl boronic acids and disubstituted malononitriles, providing a diverse array of β‐ketonitriles. The products could be further derivatized to valuable chiral α,α‐disubstituted‐β‐aminonitriles through addition reactions to the corresponding N ‐tert ‐butanesulfinyl imines.     

Tandem Gold/Silver‐Catalyzed Cycloaddition/Hydroarylation of 7‐Aryl‐1,6‐enynes to Form 6,6‐Diarylbicyclo[3.2.0]heptanes

Mixtures of [{PCy₂(o‐biphenyl)}AuCl] and AgSbF₆ catalyze the tandem cycloaddition/hydroarylation of 7‐aryl‐1,6‐enynes with electron‐rich arenes to form 6,6‐diarylbicyclo[3.2.0]heptanes in good yield under mild conditions. Experimental observations point to a mechanism involving gold‐catalyzed cycloaddition followed by silver‐catalyzed hydroarylation of a bicyclo[3.2.0]hept‐1(7)‐ene intermediate.     

Rh‐catalyzed Transient Directing Group Promoted C—H Amidation of Benzaldehydes Utilizing Dioxazolones

Transition‐metal catalyzed C—H functionalization of benzaldehydes is of great interest in organic synthesis. Herein, we developed a transient directing group assisted amidation of benzaldehydes catalyzed by rhodium catalyst. With the employment of 10 mol% of 4‐trifluoromethyl aniline, the in situ generated imine groups as the directing group efficiently enable this transformation. By using this protocol, a wide range of benzaldehydes were efficiently converted into the corresponding N‐(2‐formylphenyl)benzamides utilizing dioxazolones as the nitrogen source.     

Cobalt(II)‐Catalyzed CH Alkynylation/Annulation with Terminal Alkynes: Selective Access to 3‐Methyleneisoindolin‐1‐one

A highly efficient cobalt(II)‐catalyzed alkynylation/annulation of terminal alkynes assisted by an N,O‐bidentate directing group is described. This protocol is characterized by wide substrate scope utilizing cheap cobalt catalysts, and offers a new approach to 3‐methyleneisoindolin‐1‐one, which can be converted into an oxadiazine salt in one step. Moreover, the directing group could be removed in three steps.     

Ruthenium(II)‐Catalyzed Asymmetric Inert C−H Bond Activation Assisted by a Chiral Transient Directing Group

A ruthenium(II)‐catalyzed asymmetric intramolecular hydroarylation assisted by a chiral transient directing group has been developed. A series of 2,3‐dihydrobenzofurans bearing chiral all‐carbon quaternary stereocenters have been prepared in remarkably high yields (up to 98 %) and enantioselectivities (up to >99 % ee). By this methodology, a novel asymmetric total synthesis of CB2 receptor agonist MDA7 has been successfully developed.     

Ligand‐Promoted RhIII‐Catalyzed Thiolation of Benzamides with a Broad Disulfide Scope

A ligand‐promoted Rh^III‐catalyzed C(sp²)?H activation/thiolation of benzamides has been developed. Using bidentate mono‐N‐protected amino acid ligands led to the first example of Rh^III‐catalyzed aryl thiolation reactions directed by weakly coordinating directing amide groups. The reaction tolerates a broad range of amides and disulfide reagents.     

Metal‐Catalyzed Cyclization Reactions of 2,3,4‐Trien‐1‐ols: A Joint Experimental–Computational Study

Controlled preparation of tri‐ and tetrasubstituted furans, as well as carbazoles has been achieved through chemo‐ and regioselective metal‐catalyzed cyclization reactions of cumulenic alcohols. The gold‐ and palladium‐catalyzed cycloisomerization reactions of cumulenols, including indole‐tethered 2,3,4‐trien‐1‐ols, to trisubstituted furans was effective, due to a 5‐endo‐dig oxycyclization by attack of the hydroxy group onto the central cumulene double bond. In contrast, palladium‐catalyzed heterocyclization/coupling reactions with 3‐bromoprop‐1‐enes furnished tetrasubstituted furans. Also studied was the palladium‐catalyzed cyclization/coupling sequence involving protected indole‐tethered 2,3,4‐trien‐1‐ols and 3‐bromoprop‐1‐enes that exclusively generated trisubstituted carbazole derivatives. These results could be explained through a selective 6‐endo‐dig cumulenic hydroarylation, followed by aromatization. DFT calculations were carried out to understand this difference in reactivity.     

Ligand‐Enabled Nickel‐Catalyzed Redox‐Relay Migratory Hydroarylation of Alkenes with Arylborons

A redox‐relay migratory hydroarylation of isomeric mixtures of olefins with arylboronic acids catalyzed by nickel complexes bearing diamine ligands is described. A range of structurally diverse 1,1‐diarylalkanes, including those containing a 1,1‐diarylated quaternary carbon, were obtained in excellent yields and with high regioselectivity. Preliminary experimental evidence supports the proposed non‐dissociated chainwalking of aryl‐nickel(II)‐hydride species along the alkyl chain of alkenes before selective reductive elimination at a benzylic position. A catalyst loading as low as 0.5 mol % proved to be sufficient in large‐scale synthesis while retaining high reactivity, highlighting the practical value of this transformation.