Anion–π catalysis on carbon allotropes

• M. Ángeles Gutiérrez López,
• Mei-Ling Tan,
• Giacomo Renno,
• Augustina Jozeliūnaitė,
• J. Jonathan Nué-Martinez,
• Javier Lopez-Andarias,
• Naomi Sakai and
• Stefan Matile

Beilstein J. Org. Chem. 2023, 19, 1881–1894, doi:10.3762/bjoc.19.140

• because it took some time to find the benchmark reaction needed to develop the catalysts (Figure 2) [2]. With this operational enolate chemistry in hand, it quickly became clear that increasing π acidity at the same time decreases the stability of the catalyst [3][4][5]. This suggested that induced rather

• below that of unsubstituted NDIs despite stronger π acidity. Supported by experimental and computational data, this poor performance of dyad 46 was explained by lone-pair π interactions of the fullerene with the donating oxygens. These lone-pair π interactions were even more impactful on the sulfone

• level. Compared to the sulfoxides in 46, the catalytic activity of dyad 47 decreased rather than increased despite stronger π acidity. Supported by computational predictions [80][81], record activities in anion–π catalysis with fullerene dimers called for higher oligomers. However, synthetic efforts

Effects of the aldehyde-derived ring substituent on the properties of two new bioinspired trimethoxybenzoylhydrazones: methyl vs nitro groups

• Dayanne Martins,
• Roberta Lamosa,
• Talis Uelisson da Silva,
• Carolina B. P. Ligiero,
• Sérgio de Paula Machado,
• Daphne S. Cukierman and
• Nicolás A. Rey

Beilstein J. Org. Chem. 2023, 19, 1713–1727, doi:10.3762/bjoc.19.125

• ; phenol acidity; ring substituents; XRD; Introduction N-Acylhydrazones are a class of compounds that contain the hydrazonic functional group (–NH–N=C–) attached to an acyl group, which can be modified to generate a range of different structures with varying properties [1]. The versatility of this class

• (hdz-NO2). It is expected that those substituents impact the chemical (e.g., acidity and hydrolysis susceptibility) as well as the structural and spectroscopic properties of the compounds. Results and Discussion The methyl-substituted hdz-CH3 and its nitro-containing analogue hdz-NO2 were isolated as

• the phenol: an apparent value of 5.68 ± 0.02 was obtained. In spite of this difference in acidity, both hydrazones are stable at physiological-like conditions, especially hdz-CH3, as deprotonation of the phenol group also impacts the basicity of N1, increasing it and thus turning hdz-NO2 more

Synthesis of 5-arylidenerhodanines in L-proline-based deep eutectic solvent

• Stéphanie Hesse

Beilstein J. Org. Chem. 2023, 19, 1537–1544, doi:10.3762/bjoc.19.110

• obstacle to their use as solvent in syntheses. Acidity and alkalinity of DES may also have a significant impact in designing organic reactions. In 2003, Abbott described a DES formed by combination of choline chloride (ChCl) and urea in a 1:2 ratio with a melting point of 12 °C [15]. This DES was further

Complexes of resorcin[4]arene with secondary amines: synthesis, solvent influence on “in-out” structure, and theoretical calculations of non-covalent interactions

• Waldemar Iwanek

Beilstein J. Org. Chem. 2023, 19, 1525–1536, doi:10.3762/bjoc.19.109

• detached, with the proton of the hydroxy group in the resorcinol unit of the anion R[4]A− located opposite to the unit from which a proton had previously been detached exhibiting the highest acidity (lowest value pKa2-2 = 9.86). The next two protons of the hydroxy groups in the anion R[4]A− are “less

• acidic”, which seems to explain the position of the amine molecules in the 1:2 complex. Interestingly, the “proton acidity” in the anions R[4]A2− (pKa3 = 11.28) and R[4]A3− (pKa4 = 11.45) is more than two orders of magnitude lower than the first proton of the hydroxy group in R[4]A (pKa1 = 9.23). This

• chains, complexes with 1:2 stoichiometry are formed. This stoichiometry is justified by calculations of the acidity of protons of hydroxy groups in the R[4]A molecule. These complexes are highly ionic because of their very low solubility in non-polar solvents. Complexes with higher stoichiometry are not

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp³)–H to construct C–C bonds

• Hui Yu and
• Feng Xu

Beilstein J. Org. Chem. 2023, 19, 1259–1288, doi:10.3762/bjoc.19.94

• carbocation D. Finally, the nucleophile attacks the carbocation D, to obtain the final coupled product. The deprotonation of the nucleophile occurs before or after the attack on the carbocation intermediate, depending on the acidity of the nucleophile. In 2008, Li et al. reported that Fe2(CO)9 as a catalyst

Exploring the role of halogen bonding in iodonium ylides: insights into unexpected reactivity and reaction control

• Carlee A. Montgomery and
• Graham K. Murphy

Beilstein J. Org. Chem. 2023, 19, 1171–1190, doi:10.3762/bjoc.19.86

• ≈ 8; 50e, 64–74% yield) were suitable, they were unreactive in the presence of a carboxylic acid (50c). If these insertions proceed via initial protonation of the iodonium ylide to produce 51 (Figure 11), its high acidity (pKa ≈ 0) [131][132] would suggest that only strong acids should undergo this

• acidity and the Lewis basicity of the X–H molecule were important. This led the authors to propose an initial halogen-bonded complex 52 (consistent with the rate acceleration seen for more Lewis basic reactants), which would weaken the X–H bond and simultaneously strengthen the basicity of the ylidic

• direct cycloaddition between the ylide and alkene. Murphy’s report of formal X–H insertions with iodonium ylides was similarly proposed to initiate upon complex formation with a Lewis base. Adduct formation was believed to both increase the acidity of halogen bond acceptor’s attached protons, as well as

Aromatic C–H bond functionalization through organocatalyzed asymmetric intermolecular aza-Friedel–Crafts reaction: a recent update

• Anup Biswas

Beilstein J. Org. Chem. 2023, 19, 956–981, doi:10.3762/bjoc.19.72

• various asymmetric chemical transformations. These compounds play a dual role in the catalytic cycle due to their intrinsic Brønsted acidity and the ability to H-bond formation. Organophosphoric acids can perform as both H-bond acceptors and donors. 1,1’-Bi-2-naphthol (BINOL) and 1,1’-spirobiindane-7,7

•  7b) [31]. In 2018, Ishihara and co-workers developed a novel C2 and C1-symmetric bisphosphoric acid-catalyzed asymmetric aza-Friedel–Crafts reaction. Both catalysts showed intramolecular H-bonding causing a sharp increase in Brønsted acidity of free OH groups and prevention of catalyst dimerization

Clauson–Kaas pyrrole synthesis using diverse catalysts: a transition from conventional to greener approach

• Dileep Kumar Singh and
• Rajesh Kumar

Beilstein J. Org. Chem. 2023, 19, 928–955, doi:10.3762/bjoc.19.71

• ). The function of squaric acid as a catalyst was not clear, but the authors suggested that the Brønsted acidity of squaric acid affects the reactivity and selectivity of this process. The tentative mechanism of this protocol was proposed (Scheme 13b), in which a reversible acid–base reaction of aniline

• a mechanism for the synthesis of nano-sulfated titania-catalyzed N-substituted pyrroles as shown in Figure 5b. In general, both Lewis and Brønsted acid sites are present in sulfated metallic oxides, as shown in Figure 5a. The acidity of these Brønsted acid sites is increased by the presence of

• adjacent strong Lewis acid sites, and the acidity of these Lewis acid sites is due to the inductive effect of sulfate on the metallic cation. Therefore, this nano-sulfated titanium dioxide acts as a new type of Lewis acid catalyst. Intermediate A was first formed by reaction of the catalyst with 2,5-DMTHF

Pyridine C(sp²)–H bond functionalization under transition-metal and rare earth metal catalysis

• Haritha Sindhe,
• Malladi Mounika Reddy,
• Karthikeyan Rajkumar,
• Akshay Kamble,
• Amardeep Singh,
• Anand Kumar and
• Satyasheel Sharma

Beilstein J. Org. Chem. 2023, 19, 820–863, doi:10.3762/bjoc.19.62

• cationic Zr complexes provided good transformations, probably due to good accessibility of the coordination site and an increased Lewis acidity of the metal center. The authors also demonstrated that this catalytic system also catalyzes the alkylation of benzylic C–H bonds (C(sp3)–H) of various

Eschenmoser coupling reactions starting from primary thioamides. When do they work and when not?

• Lukáš Marek,
• Jiří Váňa,
• Jan Svoboda and
• Jiří Hanusek

Beilstein J. Org. Chem. 2023, 19, 808–819, doi:10.3762/bjoc.19.61

• thiazole synthesis and elimination to nitriles) were identified. The key factor that enables the successful Eschenmoser coupling reaction involves the optimum balance in acidity of nitrogen and carbon atoms of the intermediary α-thioiminium salts. Keywords: Eschenmoser coupling reaction; Hantzsch thiazole

• polar aprotic solvent. Acetonitrile is the best solvent for the reaction if the intermediary salt (e.g., 6a) may be isolated, since it is only sparingly soluble in it. Since the pKa values of known primary aliphatic/aromatic α-thioiminium salts (N–H acidity) in water [10][32][33] are less than 7 and 6

• leaving aniline moiety, prefers cyclization to give 13 (Scheme 6). Another key factor for successful ECR concerns the acidity of C–H in particular α-thioiminium salts 6a,b, 10a,b, 12a, 15 (pKaC) or the ease of proton transfer between the carbon and nitrogen in imidothioate followed by formation of the

A new oxidatively stable ligand for the chiral functionalization of amino acids in Ni(II)–Schiff base complexes

• Alena V. Dmitrieva,
• Oleg A. Levitskiy,
• Yuri K. Grishin and
• Tatiana V. Magdesieva

Beilstein J. Org. Chem. 2023, 19, 566–574, doi:10.3762/bjoc.19.41

• )) and includes a chiral auxiliary, an amino acid, and a bifunctional linker capable to arrange the components in the Schiff base complex. Such templates provide a significant C–H acidity at the α-amino acid carbon and a possibility for recycling of the chiral auxiliaries (for reviews see [5][14][15][16

Design, synthesis, and evaluation of chiral thiophosphorus acids as organocatalysts

• Karen R. Winters and
• Jean-Luc Montchamp

Beilstein J. Org. Chem. 2022, 18, 1471–1478, doi:10.3762/bjoc.18.154

• exploratory work, thiophosphorus acids were chosen due to their appropriate acidity and intrinsic chirality. Thiophosphorus acids undergo a tautomeric equilibrium between the thiolic and the thionic forms [22] (Scheme 1). If the substituents R1 and R2 are different, the phosphorus atom is always chiral

Facile and diastereoselective arylation of the privileged 1,4-dihydroisoquinolin-3(2H)-one scaffold

• Dmitry Dar’in,
• Grigory Kantin,
• Alexander Bunev and
• Mikhail Krasavin

Beilstein J. Org. Chem. 2022, 18, 1070–1078, doi:10.3762/bjoc.18.109

• investigate their suitability as substrates in the Regitz diazo transfer. We reasoned that if the C–H acidity of the methylene group in 11 would turn out to be insufficient for the Regitz protocol to be directly applied, these substrates could have been additionally activated by trifluoroacetylation

Post-synthesis from Lewis acid–base interaction: an alternative way to generate light and harvest triplet excitons

• Hengjia Liu and
• Guohua Xie

Beilstein J. Org. Chem. 2022, 18, 825–836, doi:10.3762/bjoc.18.83

• . In 2002, Monkman reported the addition of camphor sulfonic acid (CSA) to the fluorescent polymer poly{2,5-pyridylene-co-1,4-[2,5-bis(2-ethylhexyloxy)]phenylene} (compound 1 in Figure 2) containing pyridine groups led to the protonation effect [26]. CSA has strong acidity and low volatility, which is

• acids B(C6F5)3 and B(C6H5)3 as electron acceptors, respectively [29]. B(C6F5)3 displays high chemical stability and Lewis acidity [30]. Moreover, its good solubility endows the possibility to form Lewis acid–base adducts in films by solution processing. The strong electron attraction of the fluorine

• substituents on the benzene rings of B(C6F5)3 is responsible for its stronger Lewis acidity compared to B(C6H5)3, and reacted efficiently with the basic fluorescent materials. In 2011, Hayashi investigated the modification of pyridyl-conjugated polymer films with the Lewis acid BF3 [31]. Through repeated acid

Inductive heating and flow chemistry – a perfect synergy of emerging enabling technologies

• Conrad Kuhwald,
• Sibel Türkhan and
• Andreas Kirschning

Beilstein J. Org. Chem. 2022, 18, 688–706, doi:10.3762/bjoc.18.70

• results due to its high Brønsted acidity [26]. Using inductive heating resulted in a highly improved catalytic system that showed long-term stability. This example is of relevance for the fragrance and flavour industries, as isopulegol (2) can be transformed into menthol in one step by catalytic

Bioinspired tetraamino-bisthiourea chiral macrocycles in catalyzing decarboxylative Mannich reactions

• Hao Guo,
• Yu-Fei Ao,
• De-Xian Wang and
• Qi-Qiang Wang

Beilstein J. Org. Chem. 2022, 18, 486–496, doi:10.3762/bjoc.18.51

• -derived bisisothiocyanate fragments 5e,f afforded the desired hetero-combination macrocycles M7–M12 without additional difficulties. It should be noted that the incorporation of CF3 groups on the aryl moieties was to increase the acidity of thiourea so as to provide better hydrogen-bonding complexation

A resorcin[4]arene hexameric capsule as a supramolecular catalyst in elimination and isomerization reactions

• Tommaso Lorenzetto,
• Fabrizio Fabris and
• Alessandro Scarso

Beilstein J. Org. Chem. 2022, 18, 337–349, doi:10.3762/bjoc.18.38

• all these reactions is the combination of its weak Brønsted acidity [42] that, by protonation of the substrate, leads to the formation of cationic species [43] and the stabilization of the latter through cation–π interactions [44] within the electron-rich aromatic cavity of the capsule, thus providing

• excess of tetrabutylammonium bromide (3) as a competitive guest for the capsule, to demonstrate the importance of the presence of an accessible cavity, and iii) acetic acid (4) in order to mimic only the Brønsted acidity of the capsule without providing the stabilization properties related to the

• with water content lower than 50 mM, while in the presence of increasing amounts of water a second capsule B is also present comprising overall 15 water molecules, 6−7 of which spontaneously incorporated into a single edge of the cubic suprastructure leading to increased Brønsted acidity. Therefore, we

Asymmetric organocatalytic Michael addition of cyclopentane-1,2-dione to alkylidene oxindole

• Estelle Silm,
• Ivar Järving and
• Tõnis Kanger

Beilstein J. Org. Chem. 2022, 18, 167–173, doi:10.3762/bjoc.18.18

• . Also, a higher C–H acidity of the proton at the stereogenic centre and possible racemisation can’t be excluded. A heteroaromatic oxindole derivative afforded the product 3j in lower yield and high ee values. 4- and 5-bromo oxindole derivatives (2l and 2k, respectively) were also used as starting

Mechanistic studies of the solvolysis of alkanesulfonyl and arenesulfonyl halides

• Malcolm J. D’Souza and
• Dennis N. Kevill

Beilstein J. Org. Chem. 2022, 18, 120–132, doi:10.3762/bjoc.18.13

• show a component with decreased electron density at the oxygen, which is relayed to give increased acidity for the hydrogen of the OH group and increased electrophilicity (solvent ionizing power) for a solvent in which it is a major component. Maskill and co-workers [26][27] investigated the solvolyses

• [32]. Probably the best known LFER is the Hammett equation which presents a way of correlating the behavior of reactants with a substituent present in an aromatic ring with the effect of that substituent on the acidity of benzoic acids in water at 25 °C. As one would expect as once moves away from the

1,2-Naphthoquinone-4-sulfonic acid salts in organic synthesis

• Ruan Carlos B. Ribeiro,
• Patricia G. Ferreira,
• Amanda de A. Borges,
• Luana da S. M. Forezi,
• Fernando de Carvalho da Silva and
• Vitor F. Ferreira

Beilstein J. Org. Chem. 2022, 18, 53–69, doi:10.3762/bjoc.18.5

• naphthoquinones A prevail in all pH regions except for extreme acidity, where there is a shift to the form of 2-hydroxy-1,4-naphthoquinone-4-arylimines [73][74]. However, in weakly acidic or alkaline solutions, A is the most stable tautomer (Scheme 3C) [75]. Fragoso and co-workers [76] studied the tautomeric

Recent advances and perspectives in ruthenium-catalyzed cyanation reactions

• Thaipparambil Aneeja,
• Cheriya Mukkolakkal Abdulla Afsina,
• Padinjare Veetil Saranya and
• Gopinathan Anilkumar

Beilstein J. Org. Chem. 2022, 18, 37–52, doi:10.3762/bjoc.18.4

• metal. Ruthenium complexes have astonishing characteristics such as high electron transfer ability, low redox potentials, high Lewis acidity, and greater stabilities of the reactive metallic species like oxometals, metallacycles, and metal carbene complexes [27]. The wide availability of highly reactive

Recent advances in the asymmetric phosphoric acid-catalyzed synthesis of axially chiral compounds

• Alemayehu Gashaw Woldegiorgis and
• Xufeng Lin

Beilstein J. Org. Chem. 2021, 17, 2729–2764, doi:10.3762/bjoc.17.185

• (Brønsted acidity/basicity, hydrogen-bonding units, and counter-anions toward metals) [48][49]. In 2019, Shi, Lin, and co-workers achieved an enantioselective synthesis of axially chiral quinoline-derived biaryl atropisomers via Pd-catalyzed C–H olefination of 8-phenylquinoline (11) using a novel chiral

N-Sulfinylpyrrolidine-containing ureas and thioureas as bifunctional organocatalysts

• Viera Poláčková,
• Dominika Krištofíková,
• Boglárka Némethová,
• Renata Górová,
• Mária Mečiarová and
• Radovan Šebesta

Beilstein J. Org. Chem. 2021, 17, 2629–2641, doi:10.3762/bjoc.17.176

• acidity of the hydrogen-bond donating unit. This notion has often been realized with substituted aryls such as 3,5-bis(trifluoromethyl)phenyl. Ellman introduced a different approach and developed N-sulfinylureas. An additional potentially useful feature is the stereogenic center on sulfur. tert

Cryogels: recent applications in 3D-bioprinting, injectable cryogels, drug delivery, and wound healing

• Luke O. Jones,
• Leah Williams,
• Tasmin Boam,
• Martin Kalmet,
• Chidubem Oguike and
• Fiona L. Hatton

Beilstein J. Org. Chem. 2021, 17, 2553–2569, doi:10.3762/bjoc.17.171

• value (index to express acidity of weak acids) of the polymer [35]. Polymers responsive to pH can be classified by the functional groups present within their polymerised structure [35][41][42]. Including (i) polyacids with weakly acidic groups (i.e., -COOH or -SO3H), (ii) polybases with weakly basic

• . Furthermore, a response to pH can be utilised to release the therapeutic agent at a site of specific acidity, thus ensuring targeted delivery. Poly(acrylic acid)-based cryogels have been investigated as a pH oscillator in oscillatory bromate-sulphite-ferrocyanide reactions as potential soft materials for

Allylic alcohols and amines by carbenoid eliminative cross-coupling using epoxides or aziridines

• Matthew J. Fleming and
• David M. Hodgson

Beilstein J. Org. Chem. 2021, 17, 2385–2389, doi:10.3762/bjoc.17.155

• with cross-coupling using the same carbenoid and epoxide 5 (Scheme 5), where the presence of LTMP also proved necessary. A cinnamylamine 23 could be obtained in a tin-free process (Scheme 10), which utilises the increased acidity of a benzylic ether 22. In this case, the presence of LTMP was necessary