Catalytic, enantioselective intramolecular hydroamination of primary amines tethered to di- and trisubstituted alkenes

Jumping in the Chiral Pool: Asymmetric Hydroaminations with Early Metals

The application of early-metal-based catalysts featuring natural chiral pool motifs, such as amino acids, terpenes and alkaloids, in hydroamination reactions is discussed and compared to those beyond the chiral pool. In particular, alkaline (Li), alkaline earth (Mg, Ca), rare earth (Y, La, Nd, Sm, Lu), group IV (Ti, Zr, Hf) metal-, and tantalum-based catalytic systems are described, which in recent years improved considerably and have become more practical in their usability. Additional emphasis is directed towards their catalytic performance including yields and regio- as well as stereoselectivity in comparison with the group IV and V transition metals and more widely used rare earth metal-based catalysts.

Recent Advances in the Green Synthesis of Heterocycles: From Building Blocks to Biologically Active Compounds

Recent advances in the environmentally benign synthesis of common heterocycles are described. This account features three main parts; the preparation of non-aromatic heterocycles, one-ring aromatic heterocycles and their condensed analogs. Due to the great variety of and high interest in these compounds, this work focuses on providing representative examples of the preparation of the target compounds.

Catalysis and regioselectivity in hydrofunctionalization reactions of unsaturated carbon bonds. Part II. Hydroamination

This review continues consideration of the regioselectivity problem in the catalyzed hydrofunctionalization of unsaturated organic compounds and addresses hydroamination of unsaturated hydrocarbons. Particular parts of the review deal with reactions of alkenes, alkynes, allenes and dienes. It is shown that the selectivity of hydroamination depends on the natures of the reactants and the catalyst. Conditions of the reactions are described; in some cases, reaction mechanisms are discussed. Reactions for which divergent regioselectivity is possible are noted.

The bibliography includes 249 references.

Dedicated to the memory of V.V.Markovnikov.

Desymmetrization of unactivated bis-alkenes via chiral Brønsted acid-catalysed hydroamination

Although great success has been achieved in catalytic asymmetric hydroamination of unactivated alkenes using transition metal catalysis and organocatalysis, the development of catalytic desymmetrising hydroamination of such alkenes remains a tough challenge in terms of attaining a high level of stereocontrol over both remote sites and reaction centers at the same time. To address this problem, here we report a highly efficient and practical desymmetrising hydroamination of unactivated alkenes catalysed by chiral Brønsted acids with both high diastereoselectivity and enantioselectivity. This method features a remarkably broad alkene scope, ranging from mono-substituted and gem-/1,2-disubstituted to the challenging tri- and tetra-substituted alkenes, to provide access to a variety of diversely functionalized chiral pyrrolidines bearing two congested tertiary or quaternary stereocenters with excellent efficiency under mild and user-friendly synthetic conditions. The key to success is indirect activation of unactivated alkenes by chiral Brønsted acids via a concerted hydroamination mechanism.

3-Aryl-2,5-Dihydropyrroles via Catalytic Carbonyl-Olefin Metathesis

Herein, we describe the development of a synthetic strategy towards chiral 3-pyrrolines based on the design principle of iron(III)-catalyzed carbonyl-olefin metathesis. This approach takes advantage of commercially available amino acids as chiral pool reagents and FeCl3 as a Lewis acid catalyst. Our strategy is characterized by its operational simplicity, mild reaction conditions and functional group tolerance. Investigations show that an electron-deficient nitrogen protecting group overcomes limitations arising from competitive binding of the Lewis acid catalyst to unfavorable Lewis basic sites, which ultimately enables catalytic turnover.

Half-sandwich chiral rare-earth metal complexes with linked tridentate amido-indenyl ligand: synthesis, characterization, and catalytic properties for intramolecular hydroamination

Novel rare-earth metal complexes featuring a tridentate carbon-linked amido-indenyl ligand have been synthesized and used to catalyze the intramolecular olefin hydroamination.

Novel yttrium and zirconium catalysts featuring reduced Ar-BIANH2 ligands for olefin hydroamination (Ar-BIANH2 = bis-arylaminoacenaphthylene)

Even demanding aminoalkenes can be cyclized by Ar-BIANH₂ yttrium complexes; a related zirconium complex has been crystallographically characterized.

Rare-earth metal diisopropylamide-catalyzed intramolecular hydroamination

Well-defined rare-earth metal diisopropylamide complexes provide an exemplary case study to investigate the effect of donor solvent, alkali metal, chloro co-ligands, and in situ catalyst formation.

Aluminium-catalysed intramolecular hydroamination of aminoalkenes: computational perusal of alternative pathways for aminoalkene activation

A comprehensive computational examination of alternatively plausible mechanistic pathways for the intramolecular hydroamination (HA) of aminoalkenes utilising a recently reported novel phenylene-diamine aluminium amido compound is presented. On the one hand, a proton-assisted concerted N-C/C-H bond-forming pathway to afford the cycloamine in a single step can be invoked, and, on the other, a stepwise σ-insertive pathway that involves a relatively fast, reversible migratory olefin 1,2-insertion step linked to a less rapid, irreversible Al-C alkyl bond protonolysis. The present study, which employs a sophisticated and reliable computational methodology, supports the prevailing mechanism to be a stepwise σ-insertive pathway. The predicted effective barrier for turnover-limiting aminolysis compares favourably with reported catalytic performance data. Non-competitive kinetic demands militates against the operation of the concerted proton-assisted pathway, which describes N-C bond-forming ring closure triggered by concomitant amino proton delivery at the C[double bond, length as m-dash]C linkage evolving through a six-centre transition state structure. The valuable insights into mechanistic intricacies of aluminium-mediated intramolecular HA reported herein will help guide the rational design of group 13 metal-based HA catalysts.

Characteristic conformation of Mosher's amide elucidated using the cambridge structural database

Conformations of the crystalline 3,3,3-trifluoro-2-methoxy-2-phenylpropanamide derivatives (MTPA amides) deposited in the Cambridge Structural Database (CSD) were examined statistically as R_acid-enantiomers. The majority of dihedral angles (48/58, ca. 83%) of the amide carbonyl groups and the trifluoromethyl groups ranged from –30° to 0° with an average angle θ¹ of −13°. The other conformational properties were also clarified: (1) one of the fluorine atoms was antiperiplanar (ap) to the amide carbonyl group, forming a staggered conformation; (2) the MTPA amides prepared from primary amines showed a Z form in amide moieties; (3) in the case of the MTPA amide prepared from a primary amine possessing secondary alkyl groups (i.e., Mosher-type MTPA amide), the dihedral angles between the methine groups and the carbonyl groups were syn and indicative of a moderate conformational flexibility; (4) the phenyl plane was inclined from the O–C_chiral bond of the methoxy moiety with an average dihedral angle θ² of +21°; (5) the methyl group of the methoxy moiety was ap to the ipso-carbon atom of the phenyl group.

Asymmetric hydroamination catalyzed by a new chiral zirconium system: reaction scope and mechanism

A new class of chiral zirconium complexes supported by chiral tridentate [O(-)NO(-)]-type of ligands derived from amino acids were synthesized and structurally characterized. They catalyzed asymmetric hydroamination/cyclization of primary aminoalkenes to give five- and six-membered N-heterocyclic amines with up to 94% ee.

Synthesis of axially chiral gold complexes and their applications in asymmetric catalyses

Several novel chiral N-heterocyclic carbene and phosphine ligands were prepared from (S)-BINOL. Moreover, their ligated Au complexes were also successfully synthesized and characterized by X-ray crystal diffraction. A weak gold-π interaction between the Au atom and the aromatic ring in these gold complexes was identified. Furthermore, we confirmed the formation of a pair of diastereomeric isomers in NHC gold complexes bearing an axially chiral binaphthyl moiety derived from the hindered rotation around C–C and C–N bonds. In the asymmetric intramolecular hydroamination reaction most of these chiral Au(I) complexes showed good catalytic activities towards olefins tethered with a NHTs functional group to give the corresponding product in moderate yields and up to 29% ee.

Anti-Markovnikov hydroamination of alkenes catalyzed by an organic photoredox system

Herein we report a metal-free method for the direct anti-Markovnikov hydroamination of unsaturated amines. Irradiation of the amine substrates with visible light in the presence of catalytic quantities of easily synthesized 9-mesityl-10-methylacridinium tetrafluoroborate and thiophenol as a hydrogen-atom donor furnished the nitrogen-containing heterocycles with complete regiocontrol. Two examples of intermolecular anti-Markovnikov alkene hydroamination are also disclosed.

Enantioselective thiourea-catalyzed intramolecular cope-type hydroamination

Catalysis of Cope-type rearrangements of bis-homoallylic hydroxylamines is demonstrated using chiral thiourea derivatives. This formal intramolecular hydroamination reaction provides access to highly enantioenriched α-substituted pyrrolidine products and represents a complementary approach to metal-catalyzed methods.

Highly enantioselective zirconium-catalyzed cyclization of aminoalkenes

Aminoalkenes are catalytically cyclized in the presence of cyclopentadienylbis(oxazolinyl)borato group 4 complexes {PhB(C5H4)(Ox(R))2}M(NMe2)2 (M = Ti, Zr, Hf; Ox(R) = 4,4-dimethyl-2-oxazoline, 4S-isopropyl-5,5-dimethyl-2-oxazoline, 4S-tert-butyl-2-oxazoline) at room temperature and below, affording five-, six-, and seven-membered N-heterocyclic amines with enantiomeric excesses of >90% in many cases and up to 99%. Mechanistic investigations of this highly selective system employed synthetic tests, kinetics, and stereochemistry. Secondary aminopentene cyclizations require a primary amine (1-2 equiv vs catalyst). Aminoalkenes are unchanged in the presence of a zirconium monoamido complex {PhB(C5H4)(Ox(4S-iPr,Me2))2}Zr(NMe2)Cl or a cyclopentadienylmono(oxazolinyl)borato zirconium diamide {Ph2B(C5H4)(Ox(4S-iPr,Me2))}Zr(NMe2)2. Plots of initial rate versus [substrate] show a rate dependence that evolves from first-order at low concentration to zero-order at high concentration, and this is consistent with a reversible substrate-catalyst interaction preceding an irreversible step. Primary kinetic isotope effects from substrate conversion measurements (k'obs((H))/k'obs((D)) = 3.3 ± 0.3) and from initial rate analysis (k2((H))/k2((D)) = 2.3 ± 0.4) indicate that a N-H bond is broken in the turnover-limiting and irreversible step of the catalytic cycle. Asymmetric hydroamination/cyclization of N-deutero-aminoalkenes provides products with higher optical purities than obtained with N-proteo-aminoalkenes. Transition state theory, applied to the rate constant k2 that characterizes the irreversible step, provides activation parameters consistent with a highly organized transition state (ΔS(++) = -43(7) cal·mol(-1) K(-1)) and a remarkably low enthalpic barrier (ΔH(++) = 6.7(2) kcal·mol(-1)). A six-centered, concerted transition state for C-N and C-H bond formation and N-H bond cleavage involving two amidoalkene ligands is proposed as most consistent with the current data.