3,3‘-Bis(trisarylsilyl)-Substituted Binaphtholate Rare Earth Metal Catalysts for Asymmetric Hydroamination

Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes: A Mechanistic View

Hydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included.

Current applications of kinetic resolution in the asymmetric synthesis of substituted pyrrolidines

Chiral substituted pyrrolidines are key elements in various biologically active molecules and are therefore valuable synthetic targets. One traditional method towards enantiomerically pure compounds is the application of kinetic resolution. In this review, current KR methodology used in the synthesis of substituted pyrrolidines is surveyed, including enzymatic methods, cycloadditions and reduction of ketones.

Cobalt-Catalyzed Radical Hydroamination of Alkenes with N-Fluorobenzenesulfonimides

An efficient and general radical hydroamination of alkenes using Co(salen) as catalyst, N-fluorobenzenesulfonimide (NFSI) and its analogues as both nitrogen source and oxidant was successfully disclosed. A variety of alkenes, including aliphatic alkenes, styrenes, α, β-unsaturated esters, amides, acids, as well as enones, were all compatible to provide desired amination products. Mechanistic experiments suggest that the reaction underwent a metal-hydride-mediated hydrogen atom transfer (HAT) with alkene, followed by a pivotal catalyst controlled S_N 2-like pathway between in situ generated organocobalt(IV) species and nitrogen-based nucleophiles. Moreover, by virtue of modified chiral cobalt(II)-salen catalyst, an unprecedented asymmetric version was also achieved with good to excellent level of enantiocontrol. This novel asymmetric radical C-N bond construction opens a new door for the challenging asymmetric radical hydrofunctionalization.

Zn(OTf)2-catalyzed hydroamination of ynamides with aromatic amines

The Zn(OTf)₂-catalyzed hydroamination of ynamides 2a-2l with aromatic amines 1a-1r was developed. This protocol features broad substrate scope of aromatic amines, good functional group tolerance for ynamides, and excellent regioselectivities. As a result, a variety of substituted amidine compounds 3aa-3oa, 3ab-3al and 3pa-3rk were prepared in moderate to excellent yields and with high regioselectivities.

Design, scope and mechanism of highly active and selective chiral NHC-iridium catalysts for the intramolecular hydroamination of a variety of unactivated aminoalkenes

Chiral, cationic NHC-iridium complexes are introduced as catalysts for the intramolecular hydroamination reaction of unactivated aminoalkenes. The catalysts show high activity in the construction of a range of 5- and 6-membered N-heterocycles, which are accessed in excellent optical purity, with various functional groups being tolerated with this system. A major deactivation pathway is presented and eliminated by using alternative reaction conditions. A detailed experimental and computational study on the reaction mechanism is performed providing valuable insights into the mode of action of the catalytic system and pointing to future modifications to be made for this catalytic platform.

Photocatalytic intermolecular anti-Markovnikov hydroamination of unactivated alkenes with N-hydroxyphthalimide

A visible-light-induced/phosphite-promoted anti-Markovnikov hydroamination of alkenes with N-hydroxyphthalimide was successfully realized, which was initiated by a proton-coupled electron transfer to enable direct cleavage of its N–O bond.

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.

Catalytic Amination of β-(Hetero)arylethyl Ethers by Phosphazene Base t-Bu-P4

We describe the catalytic amination of β-(hetero)arylethyl ethers with amines using the organic superbase t-Bu-P4 to obtain β-(hetero)arylethylamines. The reaction has a broad substrate scope and allows the transformations of electron-deficient and electron-neutral β-(hetero)arylethyl ethers with various amines including pyrrole, N-alkylaniline, diphenylamine, aniline, indole, and indoline derivatives. Mechanistic studies indicate a two-reaction pathway of MeOH elimination from the substrate to form a (hetero)arylalkene followed by the hydroamination of the alkene.

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.

Highly Active Dinuclear Titanium(IV) Complexes for the Catalytic Formation of a Carbon-Heteroatom Bond

A series of mononuclear titanium(IV) complexes with the general composition κ³-[R{NHPh₂P(X)}₂Ti(NMe₂)₂] [R = C₆H₄, X = Se (3b); R = trans-C₆H₁₀, X = S (4a), Se (4b)] and [{κ²-N(PPh₂Se)₂}₂Ti(NMe₂)₂] (6b) and two dinuclear titanium(IV) complexes, [C₆H₄{(NPh₂PS)(N)}Ti(NMe₂)]₂ (3c) and [{κ²-N(PPh₂Se)}Ti(NMe₂)₂]₂ (6c), are reported. Dinuclear titanium(IV) complex 6c acts as an efficient catalyst for the chemoselective addition of an E-H bond (E = N, O, S, P, C) to heterocumulenes under mild conditions. The catalytic addition of aliphatic and aromatic amines, alcohol, thiol, phosphine oxide, and acetylene to the carbodiimides afforded the corresponding hydroelemented products in high yield at mild conditions with a broader substrate scope. The catalytic efficiency of the dinuclear complex depends on the cooperative effect of the Ti^IV ions, the systematic variation of the intermetallic distance, and the ligand's steric properties of the complex, which enhances the reaction rate. Most interestingly, this is the first example of catalytic insertion of various E-H bonds into the carbodiimides using a single-site catalyst because only the titanium-mediated insertion of E-H into a C═N unsaturated bond is reported to date. The amine and alcohol insertion reaction with the carbodiimides showed first-order kinetics with respect to the titanium(IV) catalyst as well as substrates. A most plausible mechanism for hydroelementation reaction is also proposed, based on the spectroscopic data of the controlled reaction, a time-course study, and the Hammett plot.

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.

Diastereoselective C-H Bond Amination for Disubstituted Pyrrolidines

We report herein the improved diastereoselective synthesis of 2,5-disubstituted pyrrolidines from aliphatic azides. Experimental and theoretical studies of the C-H amination reaction mediated by the iron dipyrrinato complex (Ad L)FeCl(OEt2 ) provided a model for diastereoinduction and allowed for systematic variation of the catalyst to enhance selectivity. Among the iron alkoxide and aryloxide catalysts evaluated, the iron phenoxide complex exhibited superior performance towards the generation of syn 2,5-disubstituted pyrrolidines with high diastereoselectivity.

The 7-aza-norbornane nucleus of epibatidine: 7-aza-bicyclo-[2.2.1]heptan-7-ium chloride

7-Aza-bicyclo-[2.2.1]heptane (7-aza-norbornane) is a bridged heterocyclic nucleus found in epibatidine, the alkaloid isolated from the skin of the tropical poison frog Epipedobates tricolor. Since epibatidine is known as one of the most potent acetyl-choline nicotinic receptor agonists, a plethora of literature has been devoted to this alkaloid. However, there are no structural data on the unsubstituted 7-aza-norbornane, the parent bicyclic ring of epibatidine and its derivatives. We herein present the structural characterization of the 7-aza-bicyclo-[2.2.1]heptane parent ring as its hydro-chloride salt, namely 7-aza-bicyclo-[2.2.1]heptan-7-ium chloride, C6H12N+·Cl-. The compete cation is generated by a crystallographic mirror plane with the N atom lying on the mirror, as does the chloride anion. In the crystal, the cations are linked to the anions by N-H⋯Cl hydrogen bonds, which generate [001] chains.

Synthesis and biological evaluation of largazole zinc-binding group analogs

Histone acetylation is an extensively investigated post-translational modification that plays an important role as an epigenetic regulator. It is controlled by histone acetyl transferases (HATs) and histone deacetylases (HDACs). The overexpression of HDACs and consequent hypoacetylation of histones have been observed in a variety of different diseases, leading to a recent focus of HDACs as attractive drug targets. The natural product largazole is one of the most potent natural HDAC inhibitors discovered so far and a number of largazole analogs have been prepared to define structural requirements for its HDAC inhibitory activity. However, previous structure-activity relationship studies have heavily investigated the macrocycle region of largazole, while there have been only limited efforts to probe the effect of various zinc-binding groups (ZBGs) on HDAC inhibition. Herein, we prepared a series of largazole analogs with various ZBGs and evaluated their HDAC inhibition and cytotoxicity. While none of the analogs tested were as potent or selective as largazole, the Zn²⁺-binding affinity of each ZBG correlated with HDAC inhibition and cytotoxicity. We expect that our findings will aid in building a deeper understanding of the role of ZBGs in HDAC inhibition as well as provide an important basis for the future development of new largazole analogs with non-thiol ZBGs as novel therapeutics for cancer.

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.

Structural and Mechanistic Insights into s-Block Bimetallic Catalysis: Sodium Magnesiate-Catalyzed Guanylation of Amines

To advance the catalytic applications of s-block mixed-metal complexes, sodium magnesiate [NaMg(CH₂ SiMe₃ )₃ ] (1) is reported as an efficient precatalyst for the guanylation of a variety of anilines and secondary amines with carbodiimides. First examples of hydrophosphination of carbodiimides by using a Mg catalyst are also described. The catalytic ability of the mixed-metal system is much greater than that of its homometallic components [NaCH₂ SiMe₃ ] and [Mg(CH₂ SiMe₃ )₂ ]. Stoichiometric studies suggest that magnesiate amido and guanidinate complexes are intermediates in these catalytic routes. Reactivity and kinetic studies imply that these guanylation reactions occur via (tris)amide intermediates that react with carbodiiimides in insertion steps. The rate law for the guanylation of N,N'-diisopropylcarbodiimide with 4-tert-butylaniline catalyzed by 1 is first order with respect to [amine], [carbodiimide], and [catalyst], and the reaction shows a large kinetic isotopic effect, which is consistent with an amine-assisted rate-determining carbodiimide insertion transition state. Studies to assess the effect of sodium in these transformations denote a secondary role with little involvement in the catalytic cycle.

Hydroamination of carbodiimides, isocyanates, and isothiocyanates by a bis(phosphinoselenoic amide) supported titanium(iv) complex

The hydroamination of heterocumulenes such as carbodiimides, isocyanates, and isothiocyanates by a bis(phosphinoselenoic amide) supported titanium(iv) complex as a precatalyst is reported here. The titanium(iv) complex [{Ph₂P(Se)NCH₂CH₂NPPh₂(Se)}Ti(NMe₂)₂] (1) was synthesised by the reaction of tetrakis-(dimethylamido)titanium(iv) [Ti(NMe₂)₄] with [{Ph₂P(Se)NHCH₂CH₂NHPPh₂(Se)}] in toluene at ambient temperature. Titanium complex 1 proved to be a competent pre-catalyst for the addition of an amine N-H bond to carbodiimides, isocyanates, and isothiocyanates. The reaction scope was expanded to reactions of aliphatic and aromatic amines with phenylisocyanates and phenylisothiocyanates in toluene solvents proceeding rapidly at room temperature with 5 mol% catalyst loadings to yield the corresponding urea and thio-urea derivatives up to 99%. However, ambient temperature was needed for hydroamination of 1,3-dicyclohexylcarbodiimide. The amine addition reactions with isocyanates showed first order kinetics with respect to catalyst 1 as well as substrates. The most plausible mechanism for the hydroamination reaction was established by isolating 1,1-dimethylphenyl urea as a side product.

Synthesis of 3D-Rich Heterocycles: Hexahydropyrazolo[1,5-a]pyridin-2(1H)-ones and Octahydro-2H-2a,2a1-diazacyclopenta[cd]inden-2-ones

Two cyclic azomethine imines, 7-methyl- and 7-phenyl-2-oxo-Δ⁷-hexahydropyrazolo[1,5-a]pyridin-8-ium-1-ide, were prepared in seven steps from the respective commercially available δ-keto acids. The addition of Grignard reagents followed by N-alkylation at position 1 afforded the 1,7,7-trisubstituted hexahydropyrazolo[1,5-a]pyridin-2(1H)-ones, whereas 1,3-dipolar cycloadditions of these dipoles to typical acetylenic and olefinic dipolarophiles gave 4a-substituted 2a,2a¹-diazacyclopenta[cd]indene derivatives as the first representatives of a novel heterocyclic system. Regio- and stereoselectivity as well as the mechanism of these [3 + 2]-cycloadditions were evaluated using computational and experimental methods. The data obtained were in agreement with the polar concerted cycloaddition mechanism via the energetically favorable syn/endo-transition states.

A concise synthesis and evaluation of new malonamide derivatives as potential α-glucosidase inhibitors

A series of new malonamide derivatives were synthesized by Michael addition reaction of N(1),N(3)-di(pyridin-2-yl)malonamide into α,β-unsaturated ketones mediated by DBU in DCM at ambient temperature. The inhibitory potential of these compounds in vitro, against α-glucosidase enzyme was evaluated. Result showed that most of malonamide derivatives were identified as a potent inhibitors of α-glucosidase enzyme. Among all the compounds, 4K (IC50=11.7 ± 0.5 μM) was found out as the most active one compared to standard drug acarbose (IC50=840 ± 1.73 μM). Further cytotoxicity of 4a-4m were also evaluated against a number of cancer and normal cell lines and interesting results were obtained.

Imidazolin-2-iminato Ligand-Supported Titanium Complexes as Catalysts for the Synthesis of Urea Derivatives

The reactions of tetrakis(dimethylamido)titanium(IV) [Ti(NMe2)4] with three different imidazolin-2-imines (Im(R)NH; R = tert-butyl (tBu), mesityl (Mes), and 2,6-diisopropylphenyl (Dipp)) afforded the corresponding titanium imidazolin-2-iminato complexes [(Im(R)N)Ti(NMe2)3] (R = tBu, 1a; R = Mes, 1b; R = Dipp, 1c). Treatment of complex 1a with two different carbodiimides [R'N═C═NR'; R' = cyclohexyl (Cy) and isopropyl (iPr)] resulted in the formation of imidazolin-2-iminato titanium mono(guanidinate) complex of the type [(Im(R)N)Ti(R'NC(NMe2)NR') (NMe2)2 (R' = iPr; R = tBu (2a), R = Dipp (2c); R' = Cy, R = tBu (3a)], as yellow solid in 94% yield. However, a similar reaction of 1b and 1c with 2 equiv of phenyl isocyanates at ambient temperature resulted in the formation of corresponding titanium bis(ureate) complexes [(Im(R)N)Ti{κ(2)-OC(NMe2)NPh}2(NMe2)] (R = Mes, 4b and R = Dipp, 4c). Three equivalents of phenyl isothiocyanate reacted with complex 1c to afford respective titanium tris(thioureate) complex [(Im(Dipp)N)Ti{κ(2)-SC(NMe2)NPh}2{κ(1)-SC(NMe2)NPh}] (6c). The molecular structures of 1a-c, 2a, 2c, 3a, 4c, and 6c were established by X-ray diffraction analyses, and, from the solid-state structures of 1a-c, 2a, 2c, 3a, 4c, and 6c, it was confirmed that the imidazolin-2-iminato titanium bond in each case is very short and possesses a multiple-bonding character. The imidazolin-2-iminato titanium complex 1c was utilized as a precatalyst for the addition of amine N-H bond to phenyl isocyanate. High yields of the corresponding urea derivatives were achieved under mild conditions. The mechanistic study of the aforementioned catalytic reaction was performed, and the active catalyst complex 7b was isolated using 2 equiv of iminopyrrole [2-(2,6-iPr2C6H3N═CH)C4H3NH] and the complex 4b. The molecular structure of 7b was thereafter established.

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.