Recent developments in Cope-type hydroamination reactions of hydroxylamine and hydrazine derivatives

Oxidative Syntheses of N,N-Dialkylhydroxylamines

Despite the wide utility of hydroxylamines in organic synthesis, relatively few are commercially available, and there is a need for direct, efficient, and selective methods for their synthesis. Herein, we report two complementary methods to accomplish direct oxidation of secondary amines using UHP as an oxidant. The first method uses 2,2,2-trifluoroethanol (TFE) and a large excess of amine. Isolation of hydroxylamine products is enabled by selective salt formation, and recovery of excess amine is demonstrated. The second method uses hexafluoroacetone as an additive and is highlighted by the 1:1 stoichiometry between the oxidant and amine.

Enamine N-oxides: Design, Synthesis, and Function in Bioorthogonal Reactions

Enamine N-oxides act as a chemical linchpin bridging two bioorthogonal associative and dissociative reactions. This article describes the design of enamine N-oxides; their synthesis through the retro-Cope elimination reaction; the use of solvent, hyperconjugation, strain, and rehybridization effects to achieve bioorthogonal reactivity; and their rapid reductive cleavage with diboron reagents. The coordinated assembly and disassembly of the enamine N-oxide motif constitutes a powerful chemical operation that enables the attachment and detachment of small molecules from biomacromolecules in a biological setting.

Intermolecular 1,3-Dipolar Cycloaddition Reaction of N-Carbamoyl Nitrones Generated by N-Selective Carbamoylation of Oximes with Isocyanates

N-Selective carbamoylation reaction of oximes with isocyanates generates nitrones, which undergo 1,3-dipolar cycloaddition with various dipolarophiles to afford diverse isoxazolidines. Notably, combinations of highly electron-rich oxime and highly electron-deficient dipolarophile exhibited high reactivity, with product yields of up to 94 %. The substituent on the isoxazolidine-nitrogen atom could be successfully removed without loss of the cyclic structure. Computational studies have also elucidated the mechanism of the reaction and origin of stereoselectivity.

Accessing Bioactive Hydrazones by the Hydrohydrazination of Terminal Alkynes Catalyzed by Gold(I) Acyclic Aminooxy Carbene Complexes and Their Gold(I) Arylthiolato and Gold(III) Tribromo Derivatives: A Combined Experimental and Computational Study

Hydrohydrazination of terminal alkynes with hydrazides yielding hydrazones 5-14 were successfully catalyzed by a series of gold(I) acyclic aminooxy carbene complexes of the type [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuCl, where R² = H, R¹ = Me (1b); R² = H, R¹ = Cy (2b); R² = t-Bu, R¹ = Me (3b); R² = t-Bu, R¹ = Cy (4b). The mass spectrometric evidence corroborated the existence of the catalytically active solvent-coordinated [(AAOC)Au(CH₃CN)]SbF₆ (1-4)A species and the acetylene-bound [(AAOC)Au(HC≡CPhMe)]SbF₆ (3B) species of the proposed catalysis cycle. The hydrohydrazination reaction was successfully employed in synthesizing several bioactive hydrazone compounds (15-18) with anticonvulsant properties using a representative precatalyst (2b). The DFT studies favored the 4-ethynyltoluene (HC≡CPhMe) coordination pathway over the p-toluenesulfonyl hydrazide (NH₂NHSO₂C₆H₄CH₃) coordination pathway, and that proceeded by a crucial intermolecular hydrazide-assisted proton transfer step. The gold(I) complexes (1-4)b were synthesized from the {[(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)]CH}⁺OTf^- (1-4)a by treatment with (Me₂S)AuCl in the presence of NaH as a base. The reactivity studies of (1-4)b yielded the gold(III) [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuBr₃ (1-4)c complexes upon reaction with molecular bromine and the gold(I) perfluorophenylthiolato derivatives, [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuSC₆F₅ (1-4)d, upon treatment with C₆F₅SH.

Cope-Type Hydroamination of Vinylboronates

Aminoboronic acid derivatives can serve as versatile synthetic intermediates and pharmacophores but remain difficult to synthesize. Herein we report a synthesis of the β-aminoboronic acid motif via anti-Markovnikov hydroamination of vinylboronates. This reaction benefits from the activating effect of the boronate substituent and forms novel BON-containing heterocycles, oxazaborolidine zwitterions. A computational study is included to help determine the effects of alkene boron substitution. Derivatization reactions also support the synthetic utility of the oxazaborolidine adducts.

Recent Advances in the Synthesis of Di- and Trisubstituted Hydroxylamines

As an underrepresented functional group in bioorganic and medicinal chemistry, the hydroxylamine unit has historically received little attention from the synthetic community. Recent developments, however, suggest that hydroxylamines may have broader applications such that a review covering recent developments in the synthesis of this functional group is timely. With this in mind, this review primarily covers developments in the past 15 years in the preparation of di- and trisubstituted hydroxylamines. The mechanism of the reactions and key features and shortcomings are discussed throughout the review.

A redox-enabled strategy for intramolecular hydroamination

Metal- or acid-catalyzed intramolecular hydroamination and Cope-type intramolecular hydroamination, a distinct concerted approach using hydroxylamines, typically suffer from significant synthetic limitations. Herein we report a process for intramolecular hydroamination that uses a redox-enabled strategy relying on efficient in situ generation of hydroxylamines by oxidation, followed by Cope-type hydroamination, then reduction of the resulting pyrrolidine N-oxide. The steps are performed sequentially in a single pot, no catalyst is required, the conditions are mild, the process is highly functional group tolerant, and no chromatography is generally required for isolation. A robustness screen and a gram-scale example further support the practicality of this approach.

Computational study on N-triflylphosphoramide-catalyzed enantioselective hydroamination of alkenyl thiourea

The mechanism of the enantioselective intramolecular hydroamination of alkenyl thiourea catalyzed by chiral binaphthol N-triflylphosphoramide (NPTA) was investigated using density functional theory calculations. This study reveals the details of the hydrogen bonding mode between NPTA and the substrate and indicates the importance of the dual hydrogen binding properties of the thiourea moiety for the reactivity and stereoselectivity of the hydroamination.

Bioorthogonal Hydroamination of Push-Pull-Activated Linear Alkynes

A bioorthogonal reaction between N,N-dialkylhydroxylamines and push-pull-activated halogenated alkynes is described. We explore the use of rehybridization effects in activating alkynes, and we show that electronic effects, when competing stereoelectronic and inductive factors are properly balanced, sufficiently activate a linear alkyne in the uncatalyzed conjugative retro-Cope elimination reaction while adequately protecting it against cellular nucleophiles. This design preserves the low steric profile of an alkyne and pairs it with a comparably unobtrusive hydroxylamine. The kinetics are on par with those of the fastest strain-promoted azide-alkyne cycloaddition reactions, the products regioselectively formed, the components sufficiently stable and easily installed, and the reaction suitable for cellular labeling.

Enamine N-Oxides: Synthesis and Application to Hypoxia-Responsive Prodrugs and Imaging Agents

Tumor hypoxia induces the large-scale adaptive reprogramming of cancer cells, promoting their transformation into highly invasive and metastatic species that lead to highly negative prognoses for cancer patients. We describe the synthesis and application of a hypoxia-responsive trigger derived from previously inaccessible enamine N-oxide structures. Hypoxia-dependent reduction of this motif by hemeproteins results in the concomitant activation of a caged molecule and a latent electrophile. We exploit the former in a hypoxia-activated prodrug application using a caged staurosporine molecule as a proof-of-principle. We demonstrate the latter in in vivo tumor labeling applications with enamine-N-oxide-modified near-infrared probes. Hypoxia-activated prodrug development has long been complicated by the heterogeneity of tumor hypoxia in patients. The dual drug release and imaging modalities of the highly versatile enamine N-oxide motif present an attractive opportunity for theranostic development that can address the need not only for new therapeutics but paired methods for patient stratification.

Bioorthogonal Retro-Cope Elimination Reaction of N,N-Dialkylhydroxylamines and Strained Alkynes

A bioorthogonal reaction between N,N-dialkylhydroxylamines and cyclooctynes is described. This reaction features a highly regioselective transformation between small, easily functionalizable reaction components with second-order rate constants reaching 84 M^-1 s^-1. The reaction is orthogonal to the inverse-electron demand Diels-Alder reactions between tetrazine and strained alkenes, and its components exhibit exquisite stability and chemoselectivity in cell lysate. This retro-Cope elimination reaction introduces a new member to the bioorthogonal reaction compendium outside the prolific class of cycloaddition reactions.

Hydrazine derivative synthesis by trifluoroacetyl hydrazide alkylation

N′-Alkyl hydrazides were effectively synthesized by routes featuring installation, alkylation, and removal of a trifluoroacetyl group. A set of amino acid derived hydrazides were acylated using trifluoroacetic anhydride, and the resulting trifluoroacetyl hydrazides were alkylated with alcohols in Mitsunobu reactions and with alkyl halides under alkaline conditions. Removal of the trifluoroacetyl group was affected under reductive and hydrolytic conditions to provide the respective N′-alkyl hydrazides. This three-step process may be performed without isolation of intermediates to yield N′-alkyl hydrazide after a single chromatographic purification.

Aminimide Synthesis Using Concerted Amination Reactions of Alkenes: Scope and Mechanistic Information

Aminimides are key intermediates in the thermal cycloadditions of suitable alkenyl-hydrazine derivatives. Substrate modifications (β-N,N-dialkyl) allowed the isolation of these reactive intermediates, and the analysis of their stereochemistry provided support for concerted (Cope-type) hydroamination and concerted [3 + 2] aminocarbonylation reaction pathways. This work also establishes the applicability of these approaches to form complex aminimides in moderate to excellent yields.

Rare-Earth-Metal-Catalyzed Kinetic Resolution of Chiral Aminoalkenes via Hydroamination: The Effect of the Silyl Substituent of the Binaphtholate Ligand on Resolution Efficiency

The kinetic resolution of α-substituted aminopentenes via intramolecular hydroamination was investigated using various 3,3'-silyl-substituted binaphtholate yttrium catalysts. High efficiencies in the kinetic resolution were observed for methyl-, benzyl-, and phenyl-substituted substrates utilizing the cyclohexyldiphenylsilyl-substituted catalyst 2c with resolution factors reaching as high as 90(5) for hex-5-en-2-amine (3a). Kinetic analysis of the enantioenriched substrates with the matching and mismatching catalyst revealed that the efficiency of catalyst 2c benefits significantly from a favorable Curtin-Hammett pre-equilibrium and by a large kfast/kslow ratio. Other binaphtholate catalysts were less efficient due to a less favorable Curtin-Hammett pre-equilibrium, which often favored the mismatching substrate-catalyst combination. Cyclization of the matched substrate proceeds generally with large trans-selectivity, whereas the trans/cis-ratio for mismatched substrates is significantly diminished, favoring the cis-cyclization product isomer in some instances.

Continuous-flow synthesis of 3,5-disubstituted pyrazoles via sequential alkyne homocoupling and Cope-type hydroamination

A flow chemistry-based approach is presented for the synthesis of 3,5-disubstituted pyrazoles via sequential copper-mediated alkyne homocoupling and Cope-type hydroamination of the intermediary 1,3-diynes in the presence of hydrazine as nucleophilic reaction partner. The proposed multistep methodology offers an easy and direct access to valuable pyrazoles from cheap and readily available starting materials and without the need for the isolation of any intermediates.

A telescoped continuous-flow method is presented for the synthesis of 3,5-disubstituted pyrazoles via copper-mediated alkyne homocoupling and Cope-type hydroamination of the intermediary 1,3-dialkynes.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Synthesis of N-Oxyureas by Substitution and Cope-Type Hydroamination Reactions Using O-Isocyanate Precursors

Oxy-carbamate O-isocyanate precursors facilitate access to synthetically valuable N-oxyureas via substitution with amines. This work exploits the reactivity of suitable O-isocyanate precursors, identified by a thorough study highlighting the different reactivity of isocyanate masking groups. This led to bench-stable O-isocyanate precursors, offering improved versatility in the synthesis of N-oxyureas, and demonstrates the controlled reactivity of masked O-isocyanates. Suitable precursors also enabled the first example of Cope-type hydroamination of unsaturated hydroxyureas.

Catalytic substitution/cyclization sequences of O-substituted Isocyanates: synthesis of 1-alkoxybenzimidazolones and 1-alkoxy-3,4-dihydroquinazolin-2(1H)-ones

O-Substituted isocyanates (O-isocyanates) have rarely been used in organic synthesis, given their tendency to undergo side reactions (e.g., trimerization). Herein, we show that masked (blocked) O-isocyanate precursors allow one-pot or cascade reaction sequences featuring base-catalyzed substitution with 2-iodoanilines and 2-iodobenzylamines followed by copper-catalyzed cyclization, to form benzimidazolones and 3,4-dihydroquinazolin-2(1H)-ones. This work shows that O-isocyanates can serve as efficient building blocks for the synthesis of hydroxylamine-containing heterocycles.

Base-Mediated Hydroamination of Alkynes

Inter- or intramolecular hydroamination reactions are a paradigmatic example of modern sustainable organic chemistry, as they are a catalytic, 100% atom-economical, and waste-free process of fundamental simplicity in which an amine is added to an alkyne substrate. Many enamines are found in many natural and synthetic compounds possessing interesting physiological and biological activities. The development of synthetic protocols for such molecules and their transformation is a persistent research topic in pharmaceutical and organic chemistry. Hydroamination is conspicuously superior to the other accessible methods, such as the imination of ketones or the aminomercuration/demercuration of alkynes, that involve the stoichiometric use of toxic reagents. Additionally, the hydroamination of alkyne substrates has been successfully employed as a key step in synthesizing target molecules through total syntheses containing substituted indoles, pyrroles, imidazoles, and other heterocycles as core moieties. Many research groups have explored inter- or intramolecular hydroamination of alkynes for the synthesis of diversely substituted nitrogen heterocycles using expensive metal catalysts. However, in contrast to metal-catalyzed hydroamination, the base-mediated hydroamination of alkynes has not been extensively studied. Various inorganic (such as hydroxides, phosphates, and carbonates) and organic bases have been proven to be valuable reagents for achieving the hydroamination process. This method represents an attractive strategy for the construction of a broad range of nitrogen-containing compounds that prevents the formation of byproducts in the creation of a C-N linkage. The presence of a base is thought to facilitate the attack of nitrogen nucleophiles, such as indoles, pyrroles, and imidazoles, on unsaturated carbon substrates through the activation of the triple bond and thus transforming the electron-rich alkyne into an electrophile. In the past few years, we have been involved in the development of methods for the nucleophilic addition of N-heterocycles onto terminal and internal alkynes using alkali base catalysts to achieve new carbon-nitrogen bond-forming reactions. During our study, we discovered the regioselective preferential nucleophilic addition of N-heterocycles onto the haloarylalkyne over N-arylation of the aryl halide. In this Account, we summarize our latest achievements in regio-, stereo-, and chemoselective hydroamination chemistry of N-nucleophiles with alkynes using a superbasic medium to produce a broad range of highly functionalized vinyl and styryl enamines, which are valuable and versatile synthetic intermediates for the synthesis of bioactive compounds. Interestingly, the stereoselectivity of the addition products (kinetically stable Z and thermodynamically stable E isomers) was found to be dependent upon time. It is worthwhile to note that hydroaminated products formed by the addition reaction can further be utilized for the synthesis of indolo-/pyrrolo[2,1-a]isoquinolines, naphthyridines, and bisindolo/pyrrolo[2,1-a]isoquinolinesvia tandem cyclization. This chemistry is expected to find application in organic synthesis for constructing a diverse variety of fused π-conjugated compounds, enaminones, and C-C coupled products.

Organocatalysis using aldehydes: the development and improvement of catalytic hydroaminations, hydrations and hydrolyses

Aldehydes as organocatalysts? Simple aldehydes achieve difficult intermolecular reactions by exploiting temporary intramolecularity and inducing electrophilic activation.

Recent developments in alkene hydro-functionalisation promoted by homogeneous catalysts based on earth abundant elements: formation of C-N, C-O and C-P bond

This Perspective article provides an overview of the recent advancements in the field of intra- and inter-molecular C-N, C-O and C-P bond formation by hydroamination, hydroalkoxylation, hydrophosphination, hydrophosphonylation or hydrophosphinylation of unactivated alkenes, including allenes, 1,3-dienes and strained alkenes, promoted by (chiral) homogeneous catalysts based on earth abundant elements of the s and p blocks, the first row transition metals and the rare-earth metals. The relevant literature from 2009 until late 2014 has been covered.

Hydroamination of alkenyl N-arylhydrazones mediated by t-BuOK for the synthesis of nitrogen heterocycles

The t-BuOK-mediated reactions of γ,δ-alkenyl hydrazones enabled intramolecular hydroamination with the outer nitrogen, affording tetrahydropyridazine derivatives.

Cascade reactions of nitrogen-substituted isocyanates: a new tool in heterocyclic chemistry

In contrast to normal C-substituted isocyanates, nitrogen-substituted isocyanates (N-isocyanates) are rare. Their high reactivity and amphoteric/ambident nature has prevented the scientific community from exploiting their synthetic potential. Recently, we have developed an in situ formation approach using a reversible equilibrium, which allows controlled generation and reactivity of N-isocyanates and prevents the dimerization that is typically observed with these intermediates. This blocked (masked) N-isocyanate approach enables the use of various N-isocyanate precursors to assemble heterocycles possessing the N-N-C[double bond, length as m-dash]O motif, which is often found in agrochemicals and pharmaceuticals. Cascade reactions for the rapid assembly of several valuable 5- and 6-membered heterocycles are reported, including amino-hydantoins, acyl-pyrazoles, acyl-phthalazinones and azauracils. Over 100 different compounds were synthesized using amino-, imino- and amido-substituted N-isocyanates, demonstrating their potential as powerful intermediates in heterocyclic synthesis. Their reactivity also enables access to unprecedented bicyclic derivatives and to substitution patterns of azauracils that are difficult to access using known methods, illustrating that controlled reactivity of N-isocyanates provides new disconnections, and a new tool to assemble complex N-N-C[double bond, length as m-dash]O containing motifs.

Formaldehyde as Tethering Organocatalyst: Highly Diastereoselective Hydroaminations of Allylic Amines

Catalysts possessing sufficient activity to achieve intermolecular alkene hydroaminations under mild conditions are rare, and this likely accounts for the scarcity of asymmetric variants of this reaction. Herein, highly diastereoselective hydroaminations of allylic amines utilizing hydroxylamines as reagents and formaldehyde as catalyst are reported. This catalyst induces temporary intramolecularity, which results in high rate accelerations, and high diastereocontrol with either chiral allylic amines or chiral hydroxylamines. The reaction scope includes internal alkenes. Overall this work provides a new, stereocontrolled route to form complex vicinal diamines.

Direct access to pyrido/pyrrolo[2,1-b]quinazolin-9(1H)-ones through silver-mediated intramolecular alkyne hydroamination reactions

We report a synthetic methodology for the construction of the fused heterocyclic compounds pyrido[2,1-b]quinazolin-9(1H)-ones and pyrrolo[2,1-b]quinazolin-9(1H)-ones through an AgOTf-catalyzed intramolecular alkyne hydroamination reaction. The methodology is applicable to a wide scope of substrates and produces a series of fused quinazolinone heterocycles in good to excellent yields.

Recent advances in metal free- and late transition metal-catalysed hydroamination of unactivated alkenes

This Perspective article outlines some of the recent advancements in the development of (chiral) metal-free and late transition metal catalysts for hydroamination of unactivated alkenes.

Regio- and chemoselective intermolecular hydroamination of allyl imines for the synthesis of 1,2-diamines

The synthesis of 1,2-diamines via a Rh-catalyzed intermolecular hydroamination of N-allyl imines with cyclic amines is presented. Coordinating groups proximal to the olefin bind to the catalyst and promote the transformation. The reaction affords 1,2-diamines in very good yields and is functional-group-tolerant and highly diastereoselective.