Embracing heterogeneous photocatalysis: evolution of photocatalysts in annulation of dimethylanilines and maleimides

Recent advances in visible-light-promoted construction of tetrahydroquinolines from dimethylanilines and maleimides are documented. Homogeneous and heterogeneous photocatalytic systems, as well as the reaction mechanism, are emphasized. The mechanism of this photocatalytic annulation reaction is quite clear, i.e., dimethylanilines and maleimides serve as the radical precursors and radical acceptors, respectively. This annulation reaction could serve as an excellent platform for evaluating novel oxidative heterogeneous photocatalytic systems, which could further inspire chemists in this field to develop more efficient photocatalytic systems. Significant opportunities are expected in the future for heterogeneous photocatalysis strategies.

Regioselective α-Phosphonylation of Unprotected Alicyclic Amines

Unprotected alicyclic amines undergo α-C-H bond phosphonylation via a two-stage one-pot process involving the oxidation of amine-derived lithium amides with simple ketone oxidants, generating transient imines which are then captured with phosphites or phosphine oxides. Amines with an existing α-substituent undergo regioselective α'-phosphonylation. Amine α-arylation and α'-phosphonylation can be combined, generating a difunctionalized product in a single operation.

TEMPO/PhI(OAc)2 promotes the α-aminophosphinoylation of alcohols with amines and H-phosphine oxides in aqueous medium

A novel method for synthesizing α-aminoalkyl phosphine oxides in aqueous medium, using Ar2P(O)-H reagents, alcohols and amines, is described. This method: (i) allows for the smooth aminophosphinoylation of alcohols with amines and H-phosphine oxides under mild conditions; (ii) provides an efficient and alternative approach to access various α-aminoalkylphosphine oxides. Although various amines exhibited remarkable versatility and tolerance for functional groups in this reaction, alcohols and H-phosphine oxides demonstrated limited applicability as reactants. Hence, further investigation using a wider range of substrates is crucial. The postulated mechanism indicated that the three-component reaction followed the imine pathway due to the in situ oxidation of alcohol to aldehyde.

Novel bidentate N-coordinated alkylaluminum complexes: synthesis, characterization, and efficient catalysis for hydrophosphonylation

Five new alkylaluminum complexes with different pyridinyl-substituted imines or cyclohexyl-substituted imines were synthesized and characterized successfully. The aluminum complex [FlCHNCH(CH3)Py]AlMe2(Py = 2-pyridyl) (1) was obtained by reacting 9-[2-pyridyl-CH(CH3)-NCH]Fl (Fl = fluorenyl) (L1) and equimolar AlMe3. The reactions of 9-(2-pyridyl-NCH)Fl (L2) and 9-[2-N(CH3)2-cyclohexyl-NCH]Fl (L3) with equimolar AlMe3 or AlEt3 afforded other alkylaluminum complexes [FlCHNPy]AlMe2(Py = 2-pyridyl) (2), [FlCHNPy]AlEt2 (Py = 2-pyridyl) (3), [FlCHNCyN(CH3)2]AlMe2 (Cy = 2-cyclohexyl) (4) and [FlCHNCyN(CH3)2]AlEt2 (Cy = 2-cyclohexyl) (5). All these complexes (1-5) were characterized using NMR spectroscopy, elemental analysis, and X-ray crystal structure analysis. The catalytic properties of these new alkylaluminum complexes for the hydrophosphonylation of aldimines were examined. Complex 5 showed the best catalytic performance under mild reaction conditions with a low catalyst loading (1 mol%), and 20 different substituents of aldimines were isolated with more than 90% yields.

Oxidative α-Functionalization of 1,2,3,4-Tetrahydroisoquinolines Catalyzed by a Magnetically Recoverable Copper Nanocatalyst. Application in the Aza-Henry Reaction and the Synthesis of 3,4-Dihydroisoquinolones

The oxidative α-functionalization of 2-aryl-1,2,3,4-tetrahydroisoquinolines (THIQs) promoted by a versatile heterogeneous nanocatalyst consisting of copper nanoparticles immobilized on silica-coated maghemite (CuNPs/MagSilica) has been accomplished. The methodology was successfully applied in the cross-dehydrogenative coupling (CDC) reaction of N-aryl THIQs and other tertiary amines with nitromethane as a pro-nucleophile (aza-Henry reaction) and the α-oxidation of THIQs with O₂ as a green oxidant. Phosphite, alkyne, or indole derivatives were also shown to be suitable candidates for their use as pro-nucleophiles in the CDC reaction with THIQs. The catalyst, with very low copper loading (0.4-1.0 mol % Cu), could be easily recovered by means of an external magnet and reused in four cycles without significant loss of activity.

Visible Light Driven Metal-Free Photoredox Catalyzed α-benzylation and α-oxygenation of N-substituted tetrahydroisoquinolines: Applications to Synthesis of Natural Products

Herein, visible light mediated organophoto redox catalysed simple and convenient method for the a-benzylation and a-oxygenation of tertiary amines is demonstrated. Synthesis of novel thiophenol based donor acceptor organophotoredox catalysts 4a - 4d were investigated along with commercial catalyst 4-CzIPN ( 4e ). A diverse biologically active a-benzylated tetrahydroisoquinolines and natural products such as (±)-Norlaudanosine, (±)-laudanosine and (±)-xylopinine have been synthesized under the optimized conditions in yields ranging from from 62-91%. Exploitation of synthesized a-benzylated compound using present phtoredox catalyzed conditions gave rise to dehydyrogenative benzylic oxidation product under oxygen atmosphere which is known to display biologically and structurally important properties. Also, various N-protected tertiary amines were found to be suitable for the a-oxygenation reactions using catalyst 4e and resulted in good yields (61-85%).

TEMPO-Promoted Mono- and Bisimidation of Tertiary Anilines: Synthesis of Symmetric and Unsymmetric N-Mannich Bases

A TEMPO-promoted method was developed for the synthesis of symmetric bis-N-Mannich bases via sequential activation of two α,α'-amino C(sp³)-H bonds of N,N-dimethylanilines under mild conditions. This methodology was further extended for monoimidation of α-amino-functionalized methylanilines to give unsymmetric N-Mannich bases in good to high yields. Several control experiments were performed, and the coupling reaction outcomes indicated that the oxoammonium (TEMPO⁺) species is involved in the reaction.

Direct phosphorylation of benzylic C-H bonds under transition metal-free conditions forming sp3C-P bonds

Direct phosphorylation of benzylic C-H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp3C-H/P(O)-H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp3C-P bonds.

Fiaud's Acid, a novel organocatalyst for diastereoselective bis α-aminophosphonates synthesis with in-vitro biological evaluation of antifungal, antioxidant and enzymes inhibition potential

(S, S)-1-hydroxy-1-oxo-2-c,5-t-diphenylphospholane or Fiaud's acid is used as a novel and effective chiral organocatalyst for bis α-aminophosphonates synthesis with excellent diastereoselectivity and yields within short reaction time. All synthesized bis α-aminophosphonates revealed a good to excellent antifungal capacity, where the six compounds 4a, 4b, 4e, 4h, 4k and 4l are the best fungicide inhibiting the growth of Fusarium oxysporumandBotrytis cinereaby 65% to 84% with IC₅₀ values <0.02 mg/mL. Similarly, these six products exhibited a strong antioxidant effect, whereas a low inhibition activity was obtained with both AChE and BChE. Furthermore, they displayed a very weak inhibitory activity against tyrosinase except for the compound4l.These findings suggest a possible use of these compounds as synthetic pesticides with less hazardous effects with antioxidant, and anti-tyrosinase properties.

Novel α-Aminophosphonates of imatinib Intermediate: Synthesis, anticancer Activity, human Abl tyrosine kinase Inhibition, ADME and toxicity prediction

An efficient method for the synthesis of a new class of α-aminophosphonates of imatinib derivative has been developed in one-pot Kabachnik-Fields reaction of N-(5-amino-2-methyl phenyl)-4-(3-pyridyl)-2-pyrimidine amine with various aldehydes and diethyl phosphite under microwave irradiation and neat conditions using NiO nanoparticles as an reusable and heterogeneous catalyst, with 96% yield at 450 W within 15 min. All the compounds were evaluated for their in vitro cytotoxicity with various cancer cell lines by MTT assay method. Compounds with halo (4f, -4Br, IC₅₀ = 1.068 ± 0.88 µM to 2.033 ± 0.97 µM), nitro substitution (4 h, -3NO₂, IC₅₀ = 1.380 ± 0.94 µM to 2.213 ± 0.64 µM), (4 g, -4NO2, IC₅₀ = 1.402 ± 0.79 µM to 2.335 ± 0.73 µM) and (4i, 4-Cl, 3-NO₂, IC₅₀ = 1.437 ± 0.92 µM to 2.558 ± 0.76 µM) were showed better anticancer activity when compared with standard drugs Doxorubicin and Imatinib using MTT assay method. Further in silico target hunting reveals the anticancer activity of the designed compounds by inhibiting human ABL tyrosine kinase and all the designed compounds have shown significant drug-like characteristics.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Redox-Selective Iron Catalysis for α-Amino C-H Bond Functionalization via Aerobic Oxidation

Single-electron oxidation and α-deprotonation of tertiary anilines using Fe(phen)₃(PF₆)₃ afford α-aminoalkyl radicals, which can be coupled with electrophilic partners to afford various tetrahydroquinolines. Mechanistically, the Fe(phen)_n^2+/3+ catalytic cycle is maintained by O₂ or a TBHP oxidant, and the presence of the oxygen bound iron complex, Fe(III)-OO(H), was elucidated by electron paramagnetic resonance and electrospray ionization mass spectrometry. This redox-selective nonheme iron catalyst behaves similarly to bioinspired heme iron catalysts.

Visible-light-mediated α-phosphorylation of N-aryl tertiary amines through the formation of electron-donor–acceptor complexes: synthetic and mechanistic studies

A visible light-mediated photocatalyst-free approach for the oxidative α-CH functionalization of N-aryl tertiary amines with secondary phosphine oxides has been developed.

DDQ-mediated direct C(sp3)–H phosphorylation of xanthene derivatives

A direct route to alkylphosphonates has been achievedviaDDQ-mediated cross-dehydrogenative coupling between diarylphosphine oxides and xanthene derivatives.

Oxidative C(sp3)–H amidation of tertiary arylamines with nitriles

The first amidation of tertiary arylamines with nitriles via a cascade dehydrogenation/heterocoupling/addition reaction has been developed.

Copper-catalyzed oxidative phosphonation of 3,4-dihydro-1,4-benzoxazin-2-ones

A straight-forward and atom-economical Cu(ii)/PBQ-catalyzed oxidative Pudovik reaction of 3,4-dihydro-1,4-benzoxazin-2-ones has been developed to construct new C_sp3–P bonds.

Visible light-mediated oxidative C(sp3)–H phosphonylation for α-aminophosphonates under oxidant-free conditions

An external oxidant-free synthesis of α-aminophosphonates by synergistically combining photocatalysis and proton-reduction catalysis is developed herein.

Catalyst-Free Cross-Dehydrogenative Coupling Strategy Using Air as an Oxidant: Synthesis of α-Aminophosphonates

α-Aminophosphonates are synthesized by employing unfunctionalized starting materials using cross-dehydrogenative coupling strategy. This method does not require any catalyst and proceeds in the presence of open air as the only oxidant. Mechanistic studies revealed that the reaction is nucleophile dependent and specific to dialkyl phosphite and N-aryl tetrahydroisoquinoline derivatives.

Introducing tetramethylurea as a new methylene precursor: a microwave-assisted RuCl3-catalyzed cross dehydrogenative coupling approach to bis(indolyl)methanes

The microwave-assisted synthesis of symmetrical/unsymmetrical bis(indolyl)methanes using tetramethylurea as a methylene source.

Practical and Efficient Synthesis of α-Aminophosphonic Acids Containing 1,2,3,4-Tetrahydroquinoline or 1,2,3,4-Tetrahydroisoquinoline Heterocycles

We report here a practical and efficient synthesis of α-aminophosphonic acid incorporated into 1,2,3,4-tetrahydroquinoline and 1,2,3,4-tetrahydroisoquinoline heterocycles, which could be considered to be conformationally constrained analogues of pipecolic acid. The principal contribution of this synthesis is the introduction of the phosphonate group in the N-acyliminium ion intermediates, obtained from activation of the quinoline and isoquinoline heterocycles or from the appropriate δ-lactam with benzyl chloroformate. Finally, the hydrolysis of phosphonate moiety with simultaneous cleavage of the carbamate afforded the target compounds.