Face-selective addition to a cation-pi complex of a pyridinium salt: synthesis of chiral 1,4-dihydropyridines

Grignard Reagent Addition to Pyridinium Salts: A Catalytic Approach to Chiral 1,4-Dihydropyridines

Catalytic dearomatization of pyridinium salts is a powerful technique for constructing chiral N-heterocycles, which are crucial in alkaloid natural products and drugs. Despite its potential, progress in metal-catalyzed asymmetric dearomatization of pyridinium derivatives has been limited. Here, we present the enantioselective 1,4-dearomatization of pyridinium salts using Grignard reagents and chiral copper catalysis. This approach yields enantioenriched functionalized 1,4-dihydropyridines. Experimental kinetic isotope effects and density functional theory calculations provide insights into the reaction mechanism, regio- and enantioselectivity, and the rate-limiting step.

Catalytic Access to Chiral δ-Lactams via Nucleophilic Dearomatization of Pyridine Derivatives

Nitrogen-bearing rings are common features in the molecular structures of modern drugs, with chiral δ-lactams being an important subclass due to their known pharmacological properties. Catalytic dearomatization of preactivated pyridinium ion derivatives emerged as a powerful method for the rapid construction of chiral N-heterocycles. However, direct catalytic dearomatization of simple pyridine derivatives are scarce and methodologies yielding chiral δ-lactams are yet to be developed. Herein, we describe an enantioselective C4-dearomatization of methoxypyridine derivatives for the preparation of functionalised enantioenriched δ-lactams using chiral copper catalysis. Experimental ¹³ C kinetic isotope effects and density functional theory calculations shed light on the reaction mechanism and the origin of enantioselectivity.

Direct C-H Sulfonylimination of Pyridinium Salts

A direct pyridinium C-H sulfonylimination has been developed for the synthesis of sulfonyl iminopyridine derivatives with high efficiency. This transformation features the direct and efficient formation of a C═N bond with a high functional group tolerance under metal-free conditions. The spectroscopic properties potentially enable these sulfonyl iminopyridine compounds to be useful new emitting materials.

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts

The regio- and stereoselective addition of C(1)-ammonium enolates - generated in situ from aryl esters and the isothiourea catalyst (R)-BTM - to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

Dearomative Synthetic Entry into the Altemicidin Alkaloids

Altemicidin and related Streptomyces-derived monoterpene alkaloids possess dense, highly polar azaindane cores as well as potent cytotoxic and tRNA synthetase inhibitory properties. The congested α-amino acid motif decorating their presumed iridoid-like core structure has proven to be both a synthetic challenge and a biosynthetic mystery to date. Herein, we report a distinct, abiotic strategy to these alkaloids resulting in a concise synthesis of altemicidin from simple chemical feedstocks. Key chemical findings include the exploitation of a dearomative pyridinium addition and dipolar cycloaddition sequence to stereospecifically install the quaternary amine moiety, and a chemoselective molybdenum-mediated double reduction to establish the fully functionalized azaindane nucleus with minimal redox manipulations.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Understanding the Reactivity and Selectivity of Fluxional Chiral DMAP-Catalyzed Kinetic Resolutions of Axially Chiral Biaryls

Fluxional chiral DMAP-catalyzed kinetic resolutions of axially chiral biaryls were examined using density functional theory. Computational analyses lead to a revised understanding of this reaction in which the interplay of numerous non-covalent interactions control the conformation and flexibility of the active catalyst, the preferred mechanism, and the stereoselectivity. Notably, while the DMAP catalyst itself is confirmed to be highly fluxional, electrostatically driven π⋅⋅⋅π⁺ interactions render the active, acylated form of the catalyst highly rigid, explaining its pronounced stereoselectivity.

Synthesis of 3,3-Diarylazetidines by Calcium(II)-Catalyzed Friedel-Crafts Reaction of Azetidinols with Unexpected Cbz Enhanced Reactivity

Azetidines are valuable motifs that readily access under explored chemical space for drug discovery. 3,3-Diarylazetidines are prepared in high yield from N-Cbz azetidinols in a calcium(II)-catalyzed Friedel-Crafts alkylation of (hetero)aromatics and phenols, including complex phenols such as β-estradiol. Electron poor phenols undergo O-alkylation. The product azetidines can be derivatized to drug-like compounds through the azetidine nitrogen and the aromatic groups. The N-Cbz group is crucial to reactivity by providing stabilization of an intermediate carbocation on the four-membered ring.

Asymmetric Cu-Catalyzed 1,4-Dearomatization of Pyridines and Pyridazines without Preactivation of the Heterocycle or Nucleophile

We show that a chiral copper hydride (CuH) complex catalyzes C-C bond-forming dearomatization of pyridines and pyridazines at room temperature. The catalytic reaction operates directly on free heterocycles and generates the nucleophiles in situ, eliminating the need for stoichiometric preactivation of either reaction partner; further, it is one of very few methods available for the enantioselective 1,4-dearomatization of heteroarenes. Combining the dearomatization with facile derivatization steps enables one-pot syntheses of enantioenriched pyridines and piperidines.

Why Does Alkylation of the N-H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?

Molecular metal/NH bifunctional Noyori-type catalysts are remarkable in that they are among the most efficient artificial catalysts developed to date for the hydrogenation of carbonyl functionalities (loadings up to ∼10^-5 mol %). In addition, these catalysts typically exhibit high C═O/C═C chemo- and enantioselectivities. This unique set of properties is traditionally associated with the operation of an unconventional mechanism for homogeneous catalysts in which the chelating ligand plays a key role in facilitating the catalytic reaction and enabling the aforementioned selectivities by delivering/accepting a proton (H⁺) via its N-H bond cleavage/formation. A recently revised mechanism of the Noyori hydrogenation reaction (Dub, P. A. et al. J. Am. Chem. Soc. 2014, 136, 3505) suggests that the N-H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N-H···O hydrogen-bonding interactions (HBIs). The present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N-H functionality within M/NH bifunctional Noyori-type catalysts leads to detrimental catalytic activity. The purpose of this work is to demonstrate that decreasing the strength of this HBI, ultimately to the limit of its complete absence, are conditions under which the same alkylation may lead to beneficial catalytic activity.

Aggregation phenomena in photobicyclised pyridinium salts

Absorption and emission spectra of PQPBF₄ and BPDTQClO₄ in water provide evidence, supported by theoretical calculations, of H-aggregation in this solvent.

Enantioselective [3+2] cycloaddition of azomethine ylides and aldehydes via Ni/bis(oxazoline)-catalyzed ring opening of N-tosylaziridines through a chirality transfer approach

An enantioselective [3+2] cycloaddition of N-tosylaziridines and aldehydes catalyzed by a Ni(ii)-bisoxazoline complex has been accomplished through a chirality transfer approach.

Ammonium catalyzed cyclitive additions: evidence for a cation-π interaction with alkynes

The addition of carbamate nitrogen to a non-conjugated carbon-carbon triple bond is catalyzed by an ammonium salt leading to a cyclic product. Studies in homogeneous systems suggest that the ammonium agent facilitates nitrogen-carbon bond formation through a cation-π interaction with the alkyne unit that, for the first time, is directly observed by Raman spectroscopy.

Activation of Benzyl Aryl Carbonates: The Role of Cation-π Interactions

Benzyl aryl carbonates can react with a nucleophile to yield an activated electrophile and an aryloxide anion. Previously, we had utilized this in the synthesis of α-nitro esters from nitroalkanes. To further understand the process of activation of these carbonates by nucleophiles, we have performed kinetic studies on the hydrolysis of carbonates using nucleophiles. Rate constants for the hydrolysis were obtained under pseudo-first-order conditions with DABCO as the nucleophile. A comparison of rate constant for hydrolysis of isobutyl phenyl carbonate with benzyl phenyl carbonate shows that the presence of benzyl group results in a 16-fold acceleration of hydrolysis rate. This indicates that the transition state for activation of carbonate is stabilized by cation-π interactions. A comparison of the rate constant for various aromatic rings indicates that electron-donating substituents on the benzyl groups accelerate the rate of hydrolysis. Studies were also carried out with DMAP as nucleophile and the results are presented. Our studies show that stable carbonates can be activated using nucleophiles. Activated acyl groups generated from acid anhydrides have been used in several enantioselective reactions. Our studies show that carbonates can be stable alternatives to acid anhydrides.

Alstoscholarisines H-J, Indole Alkaloids from Alstonia scholaris: Structural Evaluation and Bioinspired Synthesis of Alstoscholarisine H

Alstoscholarisines H-J (1-3), new monoterpenoid indole alkaloids with an unprecedented skeleton created via the formation of a C-3/N-1 bond, were isolated from Alstonia scholaris. Their structures were established by extensive spectroscopic analyses and the assessment of single-crystal X-ray diffraction data. The total synthesis of alstoscholarisine H was achieved via the regioselective nucleophilic addition of pyridinium through a bioinspired iminium ion intermediate followed by Pictet-Spengler-like cyclization.

Platinum nanoparticles supported on polymeric ionic liquid functionalized magnetic silica: effective and reusable heterogeneous catalysts for the selective oxidation of alcohols in water

In this manuscript, we demonstrate that functionalized magnetic silica nanoparticles are efficient and recyclable catalysts for the selective, aerobic oxidation of various primary and secondary alcohols.

Vapochromism associated with the changes in the molecular arrangement of organic crystals

Exposure of yellow anhydrate organic crystals to water vapor gave red hydrate crystals with significant changes in the molecular arrangement.

Unravelling the mechanism of the asymmetric hydrogenation of acetophenone by [RuX2(diphosphine)(1,2-diamine)] catalysts

The mechanism of catalytic hydrogenation of acetophenone by the chiral complex trans-[RuCl2{(S)-binap}{(S,S)-dpen}] and KO-t-C4H9 in propan-2-ol is revised on the basis of DFT computations carried out in dielectric continuum and the most recent experimental observations. The results of these collective studies suggest that neither a six-membered pericyclic transition state nor any multibond concerted transition states are involved. Instead, a hydride moiety is transferred in an outer-sphere manner to afford an ion-pair, and the corresponding transition state is both enantio- and rate-determining. Heterolytic dihydrogen cleavage proceeds neither by a (two-bond) concerted, four-membered transition state, nor by a (three-bond) concerted, six-membered transition state mediated by a solvent molecule. Instead, cleavage of the H-H bond is achieved via deprotonation of the η(2)-H2 ligand within a cationic Ru complex by the chiral conjugate base of (R)-1-phenylethanol. Thus, protonation of the generated (R)-1-phenylethoxide anion originates from the η(2)-H2 ligand of the cationic Ru complex and not from NH protons of a neutral Ru trans-dihydride complex, as initially suggested within the framework of a metal-ligand bifunctional mechanism. Detailed computational analysis reveals that the 16e(-) Ru amido complex [RuH{(S)-binap}{(S,S)-HN(CHPh)2NH2}] and the 18e(-) Ru alkoxo complex trans-[RuH{OCH(CH3)(R)}{(S)-binap}{(S,S)-dpen}] (R = CH3 or C6H5) are not intermediates within the catalytic cycle, but rather are off-loop species. The accelerative effect of KO-t-C4H9 is explained by the reversible formation of the potassium amidato complexes trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH2}] or trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH(K)}]. The three-dimensional (3D) cavity observed within these molecules results in a chiral pocket stabilized via several different noncovalent interactions, including neutral and ionic hydrogen bonding, cation-π interactions, and π-π stacking interactions. Cooperatively, these interactions modify the catalyst structure, in turn lowering the relative activation barrier of hydride transfer by ~1-2 kcal mol(-1) and the following H-H bond cleavage by ~10 kcal mol(-1), respectively. A combined computational study and analysis of recent experimental data of the reaction pool results in new mechanistic insight into the catalytic cycle for hydrogenation of acetophenone by Noyori's catalyst, in the presence or absence of KO-t-C4H9.

Copper-catalyzed Perkin-acyl-Mannich reaction of acetic anhydride with pyridine: expeditious entry to unconventional piperidines

A regioselective introduction of a methoxycarbonyl methyl group at the C(2) position of unsubstituted pyridine has been accomplished with catalytic amounts of copper(II) triflate in mild reaction conditions. The N-acetyl-1,2-dihydropyridyl acetic acid methyl ester obtained is a valuable building block for the synthesis of new polyfunctionalized piperidine derivatives bearing unconventional substitution patterns.

Amine, alcohol and phosphine catalysts for acyl transfer reactions

An overview of the area of organocatalytic asymmetric acyl transfer processes is presented including O- and N-acylation. The material has been ordered according to the structural class of catalyst employed rather than reaction type with the intention to draw mechanistic parallels between the manner in which the various reactions are accelerated by the catalysts and the concepts employed to control transfer of chiral information from the catalyst to the substrates.

Rhodium-catalyzed enantioselective addition of boronic acids to N-benzylnicotinate salts

The highly enantioselective catalytic asymmetric addition of aryl and alkenylboronic acids to N-benzylnicotinate salt 1 is described. The dihydropyridine 2 reaction products can be converted to synthetically useful piperidines. Application of the methodology to the preparation of enantioenriched quaternary chiral centers is also discussed.