Enantio- and Regioselective Epoxidation of Olefinic Double Bonds in Quinolones, Pyridones, and Amides Catalyzed by a Ruthenium Porphyrin Catalyst with a Hydrogen Bonding Site

In Silico Study in MPO and Molecular Docking of the Synthetic Drynaran Analogues Against the Chronic Tinnitus: Modulation of the M1 Muscarinic Acetylcholine Receptor

Tinnitus is a syndrome that affects the human auditory system and is characterized by a perception of sounds in the absence of acoustic stimuli, or in total silence. Research indicates that muscarinic acetylcholine receptors (mAChRs), especially the M1 type, have a fundamental role in the alterations of auditory perceptions of tinnitus. Here, a series of computer-aided tools were used, from molecular surface analysis software to services available on the web for estimating pharmacokinetics and pharmacodynamics. The results infer that the low lipophilicity ligands, that is, the 1a-d alkyl furans, present the best pharmacokinetic profile, as compounds with an optimal alignment between permeability and clearance. However, only ligands 1a and 1b have properties that are safe for the central nervous system, the site of cholinergic modulation. These ligands showed similarity with compounds deposited in the European Molecular Biology Laboratory chemical (ChEMBL) database acting on the mAChRs M1 type, the target selected for the molecular docking test. The simulations suggest that the 1 g ligand can form the ligand-receptor complex with the best affinity energy order and that, together with the 1b ligand, they are competitive agonists in relation to the antagonist Tiotropium, in addition to acting in synergism with the drug Bromazepam in the treatment of chronic tinnitus.

Catalytic Photochemical Deracemization via Short-Lived Intermediates

Upon irradiation in the presence of a suitable chiral catalyst, racemic compound mixtures can be converted into enantiomerically pure compounds with the same constitution. The process is called photochemical deracemization and involves the formation of short-lived intermediates. By opening different reaction channels for the forward reaction to the intermediate and for the re-constitution of the chiral molecule, the entropically disfavored process becomes feasible. Since the discovery of the first photochemical deracemization in 2018, the field has been growing rapidly. This review comprehensively covers the research performed in the area and discusses current developments. It is subdivided according to the mode of action and the respective substrate classes. The focus of this review is on the scope of the individual reactions and on a discussion of the mechanistic details underlying the presented reaction.

Ultrasound-Assisted Wittig Reaction for the Synthesis of 3-Substituted 4-Chloroquinolines and Quinolin-4(1H)-ones with Extended π-Conjugated Systems

3-(Vinyl-/buta-1,3-dien-1-yl/4-phenylbuta-1,3-dien-1-yl)-4-chloro quinolines and quinolin-4(1H)-ones were synthesized by ultrasound-assisted Wittig reaction of the corresponding 4-chloro-3-formylquinoline and 3-formylquinolin-4(1H)-ones with nonstabilized ylides. Ease execution, mild conditions, and high yields make this method exploitable for the generation of libraries of 3-substituted 4-chloroquinolines and quinolin-4(1H)-ones with extended π-conjugated systems. To demonstrate the usefulness of these compounds as precursors for the synthesis of more complex structures, 3-vinylquinolin-4(1H)-ones were used as dienes in the Diels–Alder reaction with N-methylmaleimide to produce novel acridone derivatives. The attempted Diels–Alder reaction with 3-(buta-1,3-dien-1-yl)quinolin-4(1H)-one did not afford the expected cycloadduct; instead, 2-methyl-2H-pyrano[3,2-c]quinoline was obtained. The structures and stereochemistry of the new compounds were established by NMR studies.

Photochemical Deracemization of Primary Allene Amides by Triplet Energy Transfer: A Combined Synthetic and Theoretical Study

The photochemical deracemization of 2,4-disubstituted 2,3-butadienamides (allene amides) was investigated both experimentally and theoretically. The reaction was catalyzed by a thioxanthone which is covalently linked to a chiral 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton providing a U-shaped arrangement of the sensitizing unit relative to a potential hydrogen-bonding site. Upon irradiation at λ = 420 nm in the presence of the sensitizer (2.5 mol %), the amides reached at -10 °C a photostationary state in which one enantiomer prevailed. The enantioenriched allene amides (70-93% ee) were isolated in 74% to quantitative yield (19 examples). Based on luminescence data and DFT calculations, energy transfer from the thioxanthone to the allene amides is thermodynamically feasible, and the achiral triplet allene intermediate was structurally characterized. Hydrogen bonding of the amide enantiomers to the sensitizer was monitored by NMR titration. The experimental association constants (K_a) were similar (59.8 vs 25.7 L·mol^-1). DFT calculations, however, revealed a significant difference in the binding properties of the two enantiomers. The major product enantiomer exhibits a noncovalent dispersion interaction of its arylmethyl group to the external benzene ring of the thioxanthone, thus moving away the allene from the carbonyl chromophore. The minor enantiomer displays a CH-π interaction of the hydrogen atom at the terminal allene carbon atom to the same benzene ring, thus forcing the allene into close proximity to the chromophore. The binding behavior explains the observed enantioselectivity which, as corroborated by additional calculations, is due to a rapid triplet energy transfer within the substrate-catalyst complex of the minor enantiomer.

Enantioselective Synthesis of Diaryl Sulfoxides Enabled by Molecular Recognition

The enantioselective sulfoxidation of diaryl-type sulfides was accomplished using a chiral manganese porphyrin complex equipped with a remote molecular recognition site. Despite the marginal size difference between the two substituents at the prostereogenic sulfur center, hydrogen bonding enabled the formation of chiral sulfoxides with exquisite enantioselectivities (16 examples, up to 99% ee). Aside from the precise orientation of a distinct substrate, the quinolone lactam offers an excellent entry point for further derivatization.

Half-Sandwich Ruthenium Complexes of Amide-Phosphine Based Ligands: H-Bonding Cavity Assisted Binding and Reduction of Nitro-substrates

We present synthesis and characterization of two half-sandwich Ru(II) complexes supported with amide-phosphine based ligands. These complexes presented a pyridine-2,6-dicarboxamide based pincer cavity, decorated with hydrogen bonds, that participated in the binding of nitro-substrates closer to the Ru(II) centers, which is further supported with binding and docking studies. These ruthenium complexes functioned as the noteworthy catalysts for the borohydride mediated reduction of assorted nitro-substrates. Mechanistic studies not only confirmed the intermediacy of [Ru-H] in the reduction but also asserted the involvement of several organic intermediates during the course of the catalysis. A similar Ru(II) complex that lacked pyridine-2,6-dicarboxamide based pincer cavity substantiated its unique role both in the substrate binding and the subsequent catalysis.

Recent Developments in Enantioselective Transition Metal Catalysis Featuring Attractive Noncovalent Interactions between Ligand and Substrate

Enantioselective transition metal catalysis is an area very much at the forefront of contemporary synthetic research. The development of processes that enable the efficient synthesis of enantiopure compounds is of unquestionable importance to chemists working within the many diverse fields of the central science. Traditional approaches to solving this challenge have typically relied on leveraging repulsive steric interactions between chiral ligands and substrates in order to raise the energy of one of the diastereomeric transition states over the other. By contrast, this Review examines an alternative tactic in which a set of attractive noncovalent interactions operating between transition metal ligands and substrates are used to control enantioselectivity. Examples where this creative approach has been successfully applied to render fundamental synthetic processes enantioselective are presented and discussed. In many of the cases examined, the ligand scaffold has been carefully designed to accommodate these attractive interactions, while in others, the importance of the critical interactions was only elucidated in subsequent computational and mechanistic studies. Through an exploration and discussion of recent reports encompassing a wide range of reaction classes, we hope to inspire synthetic chemists to continue to develop asymmetric transformations based on this powerful concept.

Multifunctional chiral aminophosphines for enantiodivergent catalysis in a palladium-catalyzed allylic alkylation reaction

Trifunctional MAP-based chiral phosphines were tested as new ligands in a Pd-catalyzed asymmetric allylic alkylation, demonstrating fast and enantiodivergent catalysis. The palladium complexes of representative ligands by X-ray analysis revealed a novel mode of P,N-coordination of the ligand to the palladium center, which may contribute to switching the sense of the asymmetric induction via combined steric and tunable H-bonding interactions between the metal complex and the substrates.

Enantioselective oxygenation of exocyclic methylene groups by a manganese porphyrin catalyst with a chiral recognition site

The natural enzyme cytochrome P450 is widely recognised for its unique ability to catalyse highly selective oxygen insertion reactions into unactivated C–H bonds under mild conditions. Its exceptional potential for organic synthesis served as an inspiration for the presented biomimetic hydroxylation approach. Via a remote hydrogen bonding motif a high enantioselectivity in the manganese-catalysed oxygenation of quinolone analogues (27 examples, 18–64% yield, 80–99% ee) was achieved. The site-selectivity was completely altered in favour of a less reactive but more accessible position.

A Mn porphyrin complex with a remote hydrogen bonding motif induces a high enantioselectivity in the oxygenation of 3-alkylquinolones. Compared to an achiral Mn complex, the site-selectivity was completely altered in favour of less reactive methylene groups.

A domino reaction for generating β-aryl aldehydes from alkynes by substrate recognition catalysis

The development of universal catalyst systems that enable efficient, selective, and straightforward chemical transformations is of immense scientific importance. Here we develop a domino process comprising three consecutive reaction steps based on the strategy of supramolecular substrate recognition. This approach provides valuable β-aryl aldehydes from readily accessible α-alkynoic acids and arenes under mild reaction conditions, employing a supramolecular Rh catalyst containing an acylguanidine-bearing phosphine ligand. Furthermore, the synthesis of a key intermediate of Avitriptan using this protocol is accomplished. The first step of the reaction sequence is proved to be the regioselective hydroformylation of α-alkynoic acids. Remarkably, molecular recognition of the ligand and the substrate via hydrogen bonding plays a key role in this step. Control experiments indicate that the reaction further proceeds via 1,4-addition of an arene nucleophile to the unsaturated aldehyde intermediate and subsequent decarboxylation.

Access to β2-Amino Acids via Enantioselective 1,4-Arylation of β-Nitroacrylates Catalyzed by Chiral Rhodium Catalysts

The highly enantioselective conjugate addition of a variety of arylboronic acids to β-nitroacrylates is reported to provide optically active α-aryl β-nitropropionates in up to 70% yields and >99.5% ee's, which are useful building blocks for preparing chiral β²-amino acids. The applicability of this transformation is demonstrated by converting 3aa into the β²-amino acid 5 and transforming 3ap to β-amino ester 7 via reduction and reductive N-alkylation. The latter compound is a precursor for preparing ent-ipatasertib.

Origins of the enantioselectivity of a palladium catalyst with BINOL–phosphoric acid ligands

The enantioselectivity of the studied C–H activation is related to the Brønsted acidity and isopropyl groups of the effective catalysts.

Rational Optimization of Supramolecular Catalysts for the Rhodium-Catalyzed Asymmetric Hydrogenation Reaction

Rational design of catalysts for asymmetric transformations is a longstanding challenge in the field of catalysis. In the current contribution we report a catalyst in which a hydrogen bond between the substrate and the catalyst plays a crucial role in determining the selectivity and the rate of the catalytic hydrogenation reaction, as is evident from a combination of experiments and DFT calculations. Detailed insight allowed in silico mutation of the catalyst such that only this hydrogen bond interaction is stronger, predicting that the new catalyst is faster. Indeed, we experimentally confirmed that optimization of the catalyst can be realized by increasing the hydrogen bond strength of this interaction by going from a urea to phosphine oxide H-bond acceptor on the ligand.

Catalyst-controlled site-selective asymmetric epoxidation of nerylamine and geranylamine derivatives

Novel catalysts for site- and enantioselective epoxidation of nerylamine and geranylamine derivatives have been developed. Although mCPBA oxidation took place selectively at the more electron-rich double bond to give the 6,7-epoxides, these catalysts provide the 2,3-epoxides in moderate to high enantioselectivity via the oxidation of the relatively electron-deficient double bond.

Unexpected formation of a μ-carbido diruthenium(iv) complex during the metalation of phthalocyanine with Ru3(CO)12 and its catalytic activity in carbene transfer reactions

Catalytic activity of novel μ-carbido Ru(iv) bisphthalocyaninate was firstly demonstrated by the olefin cyclopropanation and carbene insertion into N–H bonds.

Partially Saturated Bicyclic Heteroaromatics as an sp(3) -Enriched Fragment Collection

Fragment‐based lead generation has proven to be an effective means of identifying high‐quality lead compounds for drug discovery programs. However, the fragment screening sets often used are principally comprised of sp²‐rich aromatic compounds, which limits the structural (and hence biological) diversity of the library. Herein, we describe strategies for the synthesis of a series of partially saturated bicyclic heteroaromatic scaffolds with enhanced sp³ character. Subsequent derivatization led to a fragment collection featuring regio‐ and stereo‐controlled introduction of substituents on the saturated ring system, often with formation of new stereocenters.

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2'-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(ii) complex by PhI(OAc)2 generates the Ru(iv)[double bond, length as m-dash]O species, which is very sluggish for olefin epoxidation. When Ru(bpy)2Cl2 was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(ii) by PhI(OAc)2 and the reduction of Ru(iv)[double bond, length as m-dash]O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(iii) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(iv)[double bond, length as m-dash]O with Al(iii), Ru(iv)[double bond, length as m-dash]O/Al(iii), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(iii)-O-Ru(iii) dimer as the reduced form. In particular, both the oxygen transfer from Ru(iv)[double bond, length as m-dash]O/Al(iii) to olefin and the oxidation of Ru(iii)-O-Ru(iii) back to the active Ru(iv)[double bond, length as m-dash]O/Al(iii) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(iii). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.

Polypyrrole-functionalized ruthenium carbene catalysts as efficient heterogeneous systems for olefin epoxidation

New Ru complexes containing the bpea-pyr ligand (bpea-pyr stands for N,N-bis(pyridin-2-ylmethyl)-3-(1H-pyrrol-1-yl)propan-1-amine), with the formula [RuCl2(bpea-pyr)(dmso)] (isomeric complexes 2a and 2b) or [Ru(CN-Me)(bpea-pyr)X)](n+) (CN-Me = 3-methyl-1-(pyridin-2-yl)-1H-imidazol-3-ium-2-ide; X = Cl, 3, or X = H2O, 4), have been prepared and fully characterized. Complexes 3 and 4 have been anchored onto an electrode surface through electropolymerization of the attached pyrrole group, yielding stable polypyrrole films. The electrochemical behaviour of 4, which displays a bielectronic Ru(IV/II) redox pair in solution, is dramatically affected by the electropolymerization process leading to the occurrence of two monoelectronic Ru(IV/III) and Ru(III/II) redox pairs in the heterogeneous system. A carbon felt modified electrode containing complex 4 (C-felt/poly-4) has been evaluated as a heterogeneous catalyst in the epoxidation of various olefin substrates using PhI(OAc)2 as an oxidant, displaying TON values of several thousands in all cases and good selectivity for the epoxide product.

Beyond classical reactivity patterns: hydroformylation of vinyl and allyl arenes to valuable β- and γ-aldehyde intermediates using supramolecular catalysis

In this study, we report on properties of a series of rhodium complexes of bisphosphine and bisphosphite L1-L7 ligands, which are equipped with an integral anion binding site (the DIM pocket), and their application in the regioselective hydroformylation of vinyl and allyl arenes bearing an anionic group. In principle, the binding site of the ligand is used to preorganize a substrate molecule through noncovalent interactions with its anionic group to promote otherwise unfavorable reaction pathways. We demonstrate that this strategy allows for unprecedented reversal of selectivity to form otherwise disfavored β-aldehyde products in the hydroformylation of vinyl 2- and 3-carboxyarenes, with chemo- and regioselectivity up to 100%. The catalyst has a wide substrate scope, including the most challenging substrates with internal double bonds. Coordination studies of the catalysts under catalytically relevant conditions reveal the formation of the hydridobiscarbonyl rhodium complexes [Rh(Ln)(CO)2H]. The titration studies confirm that the rhodium complexes can bind anionic species in the DIM binding site of the ligand. Furthermore, kinetic studies and in situ spectroscopic investigations for the most active catalyst give insight into the operational mode of the system, and reveal that the catalytically active species are involved in complex equilibria with unusual dormant (reversibly inactivated) species. In principle, this involves the competitive inhibition of the recognition center by product binding, as well as the inhibition of the metal center via reversible coordination of either a substrate or a product molecule. Despite the inhibition effects, the substrate preorganization gives rise to very high activities and efficiencies (TON > 18,000 and TOF > 6000 mol mol(-1) h(-1)), which are adequate for commercial applications.

Supramolecular control of selectivity in transition-metal catalysis through substrate preorganization

The Perspective highlights possibilities to use supramolecular interactions between a substrate molecule and a (bifunctional) catalyst as a powerful tool to control the selectivity in transition-metal catalysis.