Modular Access to Azepines by Directed Carbonylative C-C Bond Activation of Aminocyclopropanes

Selective Hydroboration of C-C Single Bonds without Transition-Metal Catalysis

Selective hydroboration of C-C single bonds presents a fundamental challenge in the chemical industry. Previously, only catalytic systems utilizing precious metals Ir and Rh, in conjunction with N- and P- ligands, could achieve this, ensuring bond cleavage and selectivity. In sharp contrast, we discovered an unprecedented and general transition-metal-free system for the hydroboration of C-C single bonds. This methodology is transition-metal and ligand-free and surpasses the transition-metal systems regarding chemo- and regioselectivities, substrate versatility, or yields. In addition, our system tolerates various functional groups such as Ar-X (X=halides), heterocyclic rings, ketones, esters, amides, nitro, nitriles, and C=C double bonds, which are typically susceptible to hydroboration in the presence of transition metals. As a result, a diverse range of γ-boronated amines with varied structures and functions has been readily obtained. Experimental mechanistic studies, density functional theory (DFT), and intrinsic bond orbital (IBO) calculations unveiled a hydroborane-promoted C-C bond cleavage and hydride-shift reaction pathway. The carbonyl group of the amide suppresses dehydrogenation between the free N-H and hydroborane. The lone pair on the nitrogen of the amide facilitates the cleavage of C-C bonds in cyclopropanes.

The Synthesis of Benzazepines via Sequential [3 + 2]-Annulation and [3,3]-Sigmatropic Rearrangement

A novel annulation reaction of prop-2-ynylsulfonium salts with sulfur ylides and nitrosobenzenes has been developed, affording various benzazepines in moderate to good yields. Prop-2-ynylsulfonium salts act as C3 synthons in the reactions, providing a promising benzazepine skeleton in a one-pot operation with readily accessible starting materials.

Electrochemical oxidative radical cascade cyclization of dienes and diselenides towards the synthesis of seleno-benzazepines

Selenium-containing compounds are important scaffolds owing to their value in medicinal chemistry, biochemistry and material chemistry. Herein, we report an electrochemical approach to access seleno-benzazepines through an oxidative radical cascade cyclization of dienes with diselenides under metal-free, external oxidant-free and base-free conditions. In a simple undivided cell, various dienes and diselenides were suitable for this transformation, generating the desired products in up to 84% yields. This method provides a green and convenient route for the synthesis of valuable selenium-containing seven-membered N-heterocycles from simple starting materials.

Ruthenium-Catalyzed Carbonylation of α-Aminoaryl-Tethered Alkylidenecyclopropanes: Synthesis of Eight-Membered Benzolactams

The synthesis of medium-sized lactams is a great challenge because of the unfavorable transannular interactions and entropic barriers in the transition state. We have developed a ruthenium-catalyzed carbonylation of α-aminoaryl-tethered alkylidenecyclopropanes (ACPs) that allows for the efficient preparation of valuable eight-membered benzolactams under ligand-free conditions. The amino group served a dual role of both directing group and nucleophile to facilitate the metallacycle formation and the carbonylation.

Photo- or Electrochemical Cyclization of Dienes with Diselenides to Access Seleno-Benzo[b]azepines

A cascade selenylation/cyclization of dienes with diselenides has been realized under visible-light irradiation or electrolysis conditions. Employing O₂ or electricity as a "green" oxidant, this protocol provides a green and efficient method for an array of biologically important seleno-benzo[b]azepine derivatives in moderate to good yields. The direct sunlight irradiation and gram-scale reaction render the approach practical and attractive.

Catalytic enantioselective alkenylation-heteroarylation of olefins: stereoselective syntheses of 5-7 membered azacycles and oxacycles

Catalytic enantioselective domino alkenylation-heteroarylation of nonconjugated iododienes proceeded with excellent stereoselectivity and broad scope of substrates. The reaction enables stereoselective syntheses of substituted azacycles such as piperidine, pyrrolidine azepine and dihydropyrans carrying new quaternary stereocenters. Mechanistically, C-H bonds of heterocycles were activated by lithium alkoxides via reversible deprotonation, rather than conventional palladium(ii)-assisted metalation processes. Many types of heteroarenes can be used, including not only azoles (such as thiazoles, oxazoles, imidazoles and oxadiazoles), but also nonazoles (thiophene, furan and azine N-oxides).

C–C Bond Activations of Minimally Activated Cyclopropanes

Catalytic processes involving oxidative addition of a C–C bond to a transition metal allow the atom economical assembly of complex scaffolds. The focus of this Account is on C–C bond activation-based methodologies that employ minimally activated cyclopropanes.

An endo-Directing-Group Strategy Unlocks Enantioselective (3+1+2) Carbonylative Cycloadditions of Aminocyclopropanes

An endo-directing group strategy enables enantioselective (3+1+2) cycloadditions that are triggered by carbonylative C-C bond activation of cyclopropanes. These processes are rare examples of cycloadditions where C-C bond oxidative addition is enantiodetermining, and the first where this is achieved within the context of a multicomponent (higher order) reaction design.

Carbonylative N-Heterocyclization via Nitrogen-Directed C-C Bond Activation of Nonactivated Cyclopropanes

Under Rh-catalyzed conditions, secondary amines and anilines function as directing groups to facilitate regioselective C–C bond activation of nonactivated cyclopropanes. The resulting amino-stabilized rhodacycles undergo carbonylative C–N bond formation en route to challenging seven- and eight-membered lactams. The processes represent rare examples where C–C bond oxidative addition of nonactivated cyclopropanes is exploited in reaction design.

K2S2O8/I2-Promoted Electrophilic Selenylative Cyclization To Access Seleno-Benzo[b]azepines

A novel and simple organoselenium-involved 7-membered cyclization to access diverse seleno-benzo[b]azepines has been developed. This protocol involves an electrophilic cyclization process and is accomplished under mild conditions. Discussion of the mechanism rationalizes the regioselectivity observed in transformation. The studies of further transformation of seleno-benzo[b]azepines and large-scale experiment reveal the promising utility of this methodology.

Catalytic Net Oxidative C-C Activation and Silylation of Cyclopropanols with a Traceless Acetal Directing Group

Redox-neutral carbon-carbon (C-C) bond activation and functionalization strategies of cyclopropanols that give metallo homoenolate have offered merits to construct a range of useful β-functionalized ketones in an inverse-polarity fashion. Discovery and identification of oxidative C-C activation reactions of cyclopropanols that generate metallo enolate-homoenolate would provide an opportunity to afford α,β-difunctionalized ketones. We report catalytic, net oxidative C-C activation, and silylation of cyclopropanols with traceless acetal directing groups under consecutive Ir and Rh catalysis in regio-, stereo-, and chemo-selective fashion. In detail, Ir-catalyzed hydrosilylation of cyclopropyl acetates provides the acetal directing group in quantitative yield. Rh-catalyzed proximal C-C silylation of the resulting cyclopropyl silyl acetal produces the metallo enolate-homoenolate equivalent, dioxasilepine, which uniquely holds an interconnected β-silyl moiety and Z-vinyl acetal. Upon sequential treatment of a silaphile that removes the acetal directing group and electrophile, the seven-membered silicon-containing heterocycle, serving as the ketone α,β-dianion equivalent, delivers α,β-difunctionalized ketones. Scope of the hitherto unexplored reactivity of cyclopropanols toward net oxidative C-C silylation and the versatility of the resulting dioxasilepines were demonstrated. These include late-stage, net oxidative C-C silylation of biologically relevant molecules and facile production of a range of α,β-difunctionalized ketones. Preliminary mechanistic studies suggest that the C-C activation harnessing the electron-rich Wilkinson-type catalyst is likely the turnover-determining step and a Rh-π interaction is the key to the efficient metal insertion to the proximal C-C bond in cyclopropanols.

BF3·OEt2 catalyzed chemoselective CC bond cleavage of α,β-enones: an unexpected synthesis of 3-alkylated oxindoles and spiro-indolooxiranes

A BF₃·OEt₂ catalyzed highly chemoselective formal CC double bond cleavage reaction of α,β-enones with diazoamides for the synthesis of 3-alkylated oxindoles is developed. Boron trifluoride etherate is found to be an effective catalyst for the chemoselective C_α-C_β cleavage of enones to obtain 3-alkylated oxindoles. The product formation indicates a selective β-carbon elimination pathway of α,β-enones using the inexpensive BF₃·OEt₂ as a catalyst, transition metal-free conditions, an open-air environment, good functional tolerance and broad substrate scope. The synthetic utility of this protocol is highlighted by synthesizing spiro-indolooxiranes.

Recent Advances in the Development of Catalytic Methods that Construct Medium-ring Lactams, Partially Saturated Benzazepines and their Derivatives

Recent catalytic methods to construct medium-sized lactams and partially saturated benzazepines and their derivatives are surveyed. The review is divided into the following sections: 1 Introduction 2 Non-Transition Metal Catalyzed Reactions 2.1 Beckmann Rearrangement 2.2 Non-Beckmann Rearrangement Reactions 2.3 Multi-component reactions 3 Transition Metal-Catalyzed Reactions 3.1 Au-catalyzed reactions to access medium-sized N-heterocycles 3.2 Reactions involving a metal η³-complex catalytic intermediate 3.3 Transition metal-catalyzed reactions of strained cycloalkanes 4 Conclusions.

Modular Entry to Functionalized Tetrahydrobenzo[b]azepines via the Palladium/Norbornene Cooperative Catalysis Enabled by a C7-Modified Norbornene

Tetrahydrobenzo[b]azepines (THBAs) are commonly found in many bioactive compounds; however, the modular preparation of functionalized THBAs remains challenging to date. Here, we report a straightforward method to synthesize THBAs directly from simple aryl iodides via palladium/norbornene (Pd/NBE) cooperative catalysis. Capitalizing on an olefin-tethered electrophilic amine reagent, an ortho amination followed by 7-exo-trig Heck cyclization furnishes the seven-membered heterocycle. To overcome the difficulty with ortho-unsubstituted aryl iodide substrates, we discovered a unique C7-bromo-substituted NBE (N1) to offer the desired reactivity and selectivity. In addition to THBAs, synthesis of other benzo-seven-membered ring compounds can also be promoted by N1. Combined experimental and computational studies show that the C7-bromo group in N1 plays an important and versatile role in this catalysis, including promoting β-carbon elimination, suppressing benzocyclobutene formation, and stabilizing reaction intermediates. The mechanistic insights gained could guide future catalyst design. The synthetic utility has been demonstrated in a streamlined synthesis of tolvaptan and forming diverse pharmaceutically relevant THBA derivatives. Finally, a complementary and general catalytic condition to access C6-substituted THBAs from ortho-substituted aryl iodides has also been developed.

Copper-Catalyzed Cycloisomerization of Unactivated Allene-Tethered O-Propargyl Oximes: A Domino Reaction Sequence toward the Synthesis of Hexahydropyrrolo[3,4-b]azepin-5(4H)-ones

A novel copper-catalyzed cycloisomerization of unactivated allene-tethered O-propargyl oximes has been developed for the synthesis of hexahydropyrrolo[3,4-b]azepin-5(4H)-ones. This one-pot domino reaction proceeds via a [2,3]-sigmatropic rearrangement, a [3 + 2] cycloaddition, and another [3,3]-sigmatropic rearrangement. The methodology offers a practical and straightforward route for the rapid assembly of both ring components of the fused bicyclic motifs from acyclic precursors by simultaneously forming four new bonds (a C═O, a C═N, and two C-C bonds) in a single step.

Palladium-Catalyzed C-C Bond Activation of Cyclopropenone: Modular Access to Trisubstituted α,β-Unsaturated Esters and Amides

Strain-driven palladium/N-heterocyclic carbene-catalyzed C-C bond activation of diphenylcyclopropenone (DPC) has been explored for one-step access to trisubstituted α,β-unsaturated esters and amides. The designed transformation works under mild conditions providing exclusively a single stereoisomer. Mechanistic studies support the oxidative addition of the C-C bond of cyclopropenone to in-situ-generated Pd(0) intermediate. We have proved that vinylic hydrogen in the product is coming from phenol/aniline through deuterium-labeling studies. Late-stage functionalization of bioactive molecules such as procaine, estrone, and hymecromone demonstrates the robustness of this protocol.

Rhodium-catalysed selective C-C bond activation and borylation of cyclopropanes

Transition metal (TM)-catalysed directed hydroboration of aliphatic internal olefins which facilitates the construction of complex alkylboronates is an essential synthetic methodology. Here, an efficient method for the borylation of cyclopropanes involving TM-catalysed directed C-C activation has been developed. Upon exposure to neutral Rh(i)-catalyst systems, N-Piv-substituted cyclopropylamines (CPAs) undergo proximal-selective hydroboration with HBpin to provide valuable γ-amino boronates in one step which are otherwise difficult to synthesize by known methods. The enantioenriched substrates can deliver chiral products without erosion of the enantioselectivities. Versatile synthetic utility of the obtained γ-amino boronates is also demonstrated. Experimental and computational mechanistic studies showed the preferred pathway and the origin of this selectivity. This study will enable the further use of CPAs as valuable building blocks for the tunable generation of C-heteroatom or C-C bonds through selective C-C bond activation.

Pd/LA-catalyzed decarboxylation enabled exclusive [5 + 2] annulation toward N-aryl azepanes and DFT insights

A practical approach towards the synthesis of non-fused N-aryl azepane derivatives with diversity is described. DFT calculations revealed the origins of this unusual exclusive [5 + 2] rather than empirical [3 + 2] annulation process.

Transition-Metal-Catalyzed Cycloaddition Reactions to Access Seven-Membered Rings

The efficient and selective synthesis of functionalized seven-membered rings remains an important pursuit within synthetic organic chemistry, as this motif appears in numerous drug-like molecules and natural products. Use of cycloaddition reactions remains an attractive approach for their construction within the perspective of atom and step economy. Additionally, the ability to combine multiple components in a single reaction has the potential to allow for efficient combinatorial strategies of diversity-oriented synthesis. The inherent entropic penalty associated with achieving these transformations has impressively been overcome with development of catalysis, whereby the reaction components can be pre-organized through activation by transition-metal-catalysis. The fine-tuning of metal/ligand combinations as well as reaction conditions allows for achieving chemo-, regio-, diastereo- and enantioselectivity in these transformations. Herein, we discuss recent advances in transition-metal-catalyzed construction of seven-membered rings via combination of 2-4 components mediated by a variety of metals. An emphasis is placed on the mechanistic aspects of these transformations to both illustrate the state of the science and to highlight the unique application of novel processes of transition-metals in these transformations.

Enantioselective Type II Cycloaddition of Alkynes via C-C Activation of Cyclobutanones: Rapid and Asymmetric Construction of [3.3.1] Bridged Bicycles

Synthesis of bridged scaffolds via Type II cyclization constitutes substantial challenges due to the intrinsic ring strain accumulated in reaction transition states. Catalytic enantioselective Type II-cyclization methods are even rarer. Here, we describe a detailed study of developing a Rh(I)-catalyzed enantioselective intramolecular Type II cyclization of alkynes via C-C activation of cyclobutanones. This method offers a rapid approach to access a wide range of functionalized [3.3.1]-bridged bicycles along with an exocyclic olefin and an all-carbon quaternary stereocenter. Excellent enantioselectivity has been achieved using a combination of cationic rhodium(I) and DTBM-segphos. Attributed to the redox neutral and strong acid/base-free reaction conditions, high chemoselectivity has also been observed. For the oxygen-tethered substrates, the reaction can proceed at room temperature. In addition, partial kinetic resolution has been achieved for substrates with existing stereocenters, forging interesting chiral tricyclic scaffolds. The methylalkyne-derived substrates gave unexpected dimeric structures in good yield with excellent enantioselectivity and complete diastereoselectivity. Furthermore, the bridged bicyclic products can be diversely functionalized through simple transformations. Finally, mechanistic studies reveal a surprising reaction pathway that involves forming a metal-stabilized anti-Bredt olefin intermediate.

Palladium-Catalyzed Enantioselective Heck Carbonylation with a Monodentate Phosphoramidite Ligand: Asymmetric Synthesis of (+)-Physostigmine, (+)-Physovenine, and (+)-Folicanthine

Reported herein is the development of the first enantioselective monodentate ligand assisted Pd-catalyzed domino Heck carbonylation reaction with CO. The highly enantioselective domino Heck carbonylation of N-aryl acrylamides and various nucleophiles, including arylboronic acids, anilines, and alcohols, in the presence of CO was achieved. A novel monodentate phosphoramidite ligand, Xida-Phos, has been developed for this reaction and it displays excellent reactivity and enantioselectivity. The reaction employs readily available starting materials, tolerates a wide range of functional groups, and provides straightforward access to a diverse array of enantioenriched oxindoles having β-carbonyl-substituted all-carbon quaternary stereocenters, thus providing a facile and complementary method for the asymmetric synthesis of bioactive hexahydropyrroloindole and its dimeric alkaloids.

Reaction of Vinyl Aziridines with Arynes: Synthesis of Benzazepines and Branched Allyl Fluorides

We report the cycloaddition between vinyl aziridines and arynes. Depending on the reaction conditions and the choice of the aryne precursor, the aziridinium intermediate can be trapped through two distinct mechanistic pathways. The first one proceeds through a formal [5+2] cycloaddition to furnish valuable multi-substituted benzazepines. In the second pathway, the aziridinium is intercepted by a fluoride ion to afford allylic fluorides in good yields. Both reactions proceed stereospecifically and furnish enantiopure benzazepines and allylic fluorides.

Synthesis and hetero-Diels–Alder reactions of enantiomerically pure dihydro-1H-azepines

Thermolysis of enantiomerically pure 3-substituted 7,7-dihalo-2-azabicyclo[4.1.0]heptanes in the presence of K₂CO₃gives in good yields 2-alkyl-6-halo-1-tosyl-2,3-dihydro-1H-azepines.

Rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes: DFT analysis of origins of regio- and diastereo-selectivities

A unconventional reaction mechanism can be responsible for the regio- and diastereo-selectivities of rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes.