Merging Aerobic Oxidation and Enamine Catalysis in the Asymmetric α-Amination of β-Ketocarbonyls Using N -Hydroxycarbamates as Nitrogen Sources

Generation and Trapping of Nitrosocarbonyl Intermediates

The nitrosocarbonyls (R-CONO) are highly reactive species and remarkable intermediates toward different synthetic targets. This review will cover a research area whose impact in current organic synthesis is constantly increasing in the chemical community. This review represents the first and comprehensive picture on the generation and trapping of nitrosocarbonyls and is solidly built on more than 380 papers. Six different classes of key starting materials such as hydroxamic acids, N-hydroxy carbamates, N-hydroxyureas, nitrile oxides, and 1,2,4-oxadiazole-4-oxides were highlighted. The content of the review surveys all the methods to generate the nitrosocarbonyls through different approaches (oxidative, thermal, photochemical, catalytic, aerobic, and the less common ones) in the light of efficiency, yields, and mildness. The most successful trapping agents employed to catch these fleeting intermediates are reviewed, exploiting their superior dienophilic, enophilic, and electrophilic power. The work is completed by paragraphs dedicated to the detection of the intermediates, theoretical studies, and insights about the challenges and future directions for the field.

Copper-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines

A Cu-catalyzed asymmetric alkynylation of cyclic N-sulfonyl ketimines was developed, providing the corresponding chiral α-tertiary amines with up to 98% ee.

Enantioselective Synthesis of β-Hydrazino Alcohols Using Alcohols and N-Boc-Hydrazine as Substrates

An enantioselective approach for the synthesis of α-hydrazino aldehydes is described that utilizes alcohols and N-Boc hydrazine instead of the conventional combination of aldehydes with azodicarboxylates. This protocol is enabled by merging in situ aerobic dual oxidation with asymmetric organocatalysis. This reaction also exhibits a high tolerance for varieties of substituents on the alcohol component. This approach features excellent enantiocontrol, cheap starting materials, operational simplicity, and scalability. The corresponding chiral β-hydrazino alcohols were obtained by sequential reduction with excellent enantioselectivity (up to 98% ee).

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The transition metal catalyst complements by activating a variety of substrates through a range of interactions (e.g., electrophilic π-allyl complex formation, Lewis acid activation, allenylidene complex formation, photoredox activation, C-H activation, etc.), and thereby novel concepts within catalysis are created. The inclusion of heterogeneous catalysis strategies allows for "green" chemistry development, catalyst recyclability, and the more eco-friendly synthesis of valuable compounds.

Development of N-Substituted Hydroxamic Acids with Pyrazolone Leaving Groups as Nitrosocarbonyl Precursors

A novel class of nitrosocarbonyl precursors, N-substituted hydroxamic acids with pyrazolone leaving groups (NHPY), has been synthesized. Under physiological conditions, these compounds generate nitrosocarbonyl intermediates, which upon hydrolysis release nitroxyl (azanone, HNO) in excellent yields. The amount and rate of nitrosocarbonyl generation are dependent on the nature of the pyrazolone leaving groups and significantly on the structural properties of the NHPY donors. Pyrazolones have been found to be efficient nitrosocarbonyl traps, undergoing an N-selective nitrosocarbonyl aldol reaction. This trapping reaction has been used to confirm the involvement of nitrosocarbonyl intermediates in NHPY aqueous decomposition. In addition, NHPY compounds are shown to generate nitrosocarbonyls efficiently under mild basic conditions in organic solvent and may therefore also enjoy synthetic utility.

Enantioselective Organocatalyzed Transformations of β-Ketoesters

The β-ketoester structural motif continues to intrigue chemists with its electrophilic and nucleophilic sites. Proven to be a valuable tool within organic synthesis, natural product, and medicinal chemistry, reports on chiral β-ketoester molecular skeletons display a steady increase. With the reignition of organocatalysis in the past decade, asymmetric methods available for the synthesis of this structural unit has significantly expanded, making it one of the most exploited substrates for organocatalytic transformations. This review provides comprehensive information on the plethora of organocatalysts used in stereoselective organocatalyzed construction of β-ketoester-containing compounds.

Brønsted acid mediated N–O bond cleavage for α-amination of ketones through the aromatic nitroso aldol reaction

A Brønsted acid mediated N–O bond cleavage for α-amination of ketones has been developed through the nitroso aldol reaction of less-reactive aromatic nitroso compounds and silyl enol ethers having a disilane (–SiMe₂TMS) backbone.

The Mukaiyama aldol reaction of in situ generated nitrosocarbonyl compounds: selective C-N bond formation and N-O bond cleavage in one-pot for α-amination of ketones

A practical protocol for the α-amination of ketones (up to 99% yield) has been developed via the Mukaiyama aldol reaction of in situ generated nitrosocarbonyl compounds. The reaction with silyl enol ethers having a disilane (-SiMe2TMS) backbone proceeded not only with perfect N-selectivity but concomitant N-O bond cleavage was also accomplished. Such a cascade of C-N bond formation and N-O bond cleavage in a single step was heretofore unknown in the field of nitrosocarbonyl chemistry. A very high diastereoselectivity (dr = 19 : 1) was accomplished using (-)-menthol derived chiral nitrosocarbonyl compounds.

A Nucleophilic Strategy for Enantioselective Intermolecular α-Amination: Access to Enantioenriched α-Arylamino Ketones

The enantioselective addition of anilines to azoalkenes was accomplished through the use of a chiral phosphoric acid catalyst. The resulting α-arylamino hydrazones were obtained in good yields and excellent enantioselectivities and provide access to enantioenriched α-arylamino ketones. A serendipitous kinetic resolution of racemic α-arylamino hydrazones is also described.

Pushing the limits of aminocatalysis: enantioselective transformations of α-branched β-ketocarbonyls and vinyl ketones by chiral primary amines

Enantioselective α-functionalizations of carbonyl compounds are fundamental transformations for the asymmetric synthesis of organic compounds. One of the more recent developments along this line is in aminocatalysis, which leads to the direct α-functionalization of simple aldehydes and ketones. However, most of the advances have been achieved with linear aldehydes and ketones as substrates. Effective aminocatalysis with α-branched carbonyls, particularly α-branched ketones, has remained elusive. The primary difficulty arises from the space-demanding α-substituent, which impedes iminium/enamine formation. In 2005, synthetic organic chemists revived catalysis using primary amines, which brought new attention to these challenges, because of the conformational flexibility of primary amines. On the basis of early biomimetic studies by Hine, in 2007 we developed the bioinspired chiral primary amine catalysts featuring primary-tertiary diamines. This type of catalyst involves enamine/iminium catalysis, and we could apply this chemistry to all of the major types of ketones and aldehydes. In this Account, we present research from our laboratory that significantly expands aminocatalysis to include α-branched ketones such as β-ketocarbonyls and α-substituted vinyl ketones. Our primary amine catalysis methodology, when used alone or in conjunction with metal catalysts, provides convenient access to both enantiopure α-tertiary and quaternary ketones, structures that are not available via other approaches. Our mechanistic studies showed that acidic additives play the critical role in facilitating catalytic turnover, most likely by shuttling protons during the enamine/iminium tautomerizations. These additives are also critical to induce the desired stereochemistry via ammonium N-H hydrogen bonding. Proton transfer by shuttling is also stereoselective, resulting in enantioselective enamine protonation as observed in the reactions of α-substituted vinyl ketones. In addition, we have carried out density functional theory studies that help to delineate the origins of the stereoselectivity in these reactions.

Direct asymmetric amination of α-branched cyclic ketones catalyzed by a chiral phosphoric acid

Here we report the direct asymmetric amination of α-substituted cyclic ketones catalyzed by a chiral phosphoric acid, yielding products with a N-containing quaternary stereocenter in high yields and excellent enantioselectivities. Kinetic resolution of the starting ketone was also found to occur on some of the substrates under milder conditions, providing enantioenriched α-branched ketones, another important building block in organic synthesis. The utility of this methodology was demonstrated in the short synthesis of (S)-ketamine, the more active enantiomer of this versatile pharmaceutical.

Copper-catalyzed aerobic oxidative amination of C(sp3)–H bonds: synthesis of imidazo[1,5-a]pyridines

Copper-catalyzed synthesis of imidazo[1,5-a]pyridine-1-carboxylates through oxidative amination of C(sp³)–H bonds under mild aerobic conditions with broad substrate scope has been achieved.

Asymmetric enamine catalysis with β-ketoesters by chiral primary amine: divergent stereocontrol modes

α-Branched ketones remain a challenging type of substrates in aminocatalysis due to their congested structures as well as the associated difficulties in controlling chemo- and stereoselectivity. In this work, we have explored asymmetric aminocatalysis with α-substituted β-ketoesters. A simple chiral primary amine catalyst was identified to enable unprecedentedly effective catalysis of β-ketoesters in α-hydrazination and Robinson annulation reaction with good yields and high enantioselectivities. Stoichiometric experiments with preformed enamine ester intermediates revealed their enamine-catalytic nature as well as the critical roles of acidic additives in facilitating catalytic turnovers and in tuning the chemo- and stereoselectivity. With the identical catalytic system, the two reactions demonstrated opposite chiral inductions in terms of the absolute configurations of the newly formed stereogenic centers. Investigations into this intriguing issue by DFT have revealed divergent stereocontrol modes. For α-hydrazination, H-bonding-directed facial attack determines the stereoselectivity, whereas a steric model is applied to the Robinson annulation where dual activations of both β-ketoester and vinyl ketone/aldehyde are involved.

Origins of the enantio- and N/O selectivity in the primary-amine-catalyzed hydroxyamination of 1,3-dicarbonyl compounds with in-situ-formed nitrosocarbonyl compounds: a theoretical study

Chemoselective control over N/O selectivity is an intriguing issue in nitroso chemistry. Recently, we reported an unprecedented asymmetric α-amination reaction of β-ketocarbonyl compounds that proceeded through the catalytic coupling of enamine carbonyl groups with in-situ-generated carbonyl nitroso moieties. This process was facilitated by a simple chiral primary and tertiary diamine that was derived from tert-leucine. This reaction featured high chemoselectivity and excellent enantioselectivity for a broad range of substrates. Herein, a computational study was performed to elucidate the origins of the enantioselectivity and N/O regioselectivity. We found that a bidentate hydrogen-bonding interaction between the tertiary N(+)-H and nitrosocarbonyl groups accounted for the high N selectivity, whilst the enantioselectivity was determined by Si-facial attack on the (E)- and (Z)-enamines in a Curtin-Hammett-type manner. The bidentate hydrogen-bonding interaction with the nitrosocarbonyl moieties reinforced the facial selectivity in this process.

Asymmetric construction of quaternary stereocenters by magnesium catalysed direct amination of β-ketoesters using in situ generated nitrosocarbonyl compounds as nitrogen sources

An oxophilic magnesium salt in combination with a chiral N,N′-dioxide ligand switches the chemoselectivity of an ambident nitrosocarbonyl species to nitrogen for the highly enantioselective α-amination of β-ketoesters.