Surfactant-Type Brønsted Acid Catalyzed Dehydrative Nucleophilic Substitutions of Alcohols in Water

α/β-Stereo- and diastereoselective glycosylation with n-pentenyl glycoside donors, promoted by N-iodosuccinimide and catalyzed by chiral Brønsted acid

A method involving stereoselective glycosylation catalyzed by (+)-isomenthol ester of pentacarbomethoxycyclopentadiene as a chiral Brønsted acid, with n-pentenyl glycosides in the presence of N-iodosuccinimide as the promoter is described; this method offered a chiral recognition of racemic substrates.

Methyltrifluoromethanesulfonate Catalyst in Direct Nucleophilic Substitution of Alcohols; One-Pot Synthesis of 4H-Chromene Derivatives

The catalytic activity of methyltrifluoromethanesulfonate (MeOTf) has been explored toward direct nucleophilic substitution of the hydroxyl group of nonmanipulated alcohols such as benzylic, allylic, propargylic, and tertiary alcohols with a wide range of uncharged nucleophiles such as 1,3-dicarbonyl compounds, amides, alkynes, and indoles to generate functionalized 1,3-dicarbonyl compounds, amides, alkynes, and indoles, respectively. Thus, the present protocol defines an alternate pathway to construct new C-C, C-N, and C-O bonds with the formation of water as the byproduct under mild conditions without any acids or metals. A completely different mechanism was established through several control experiments to explain the reaction methodology. As an application of the reported protocol, 1H-indene derivatives have been synthesized in one pot when benzylic alcohols were subjected to react with internal alkynes. The scope of the reaction has been further extended toward a tandem benzylation-cyclization-dehydration of 1,3-dicarbonyl compounds with 2-hydroxybenzyl alcohols, which furnish biologically important 4H-chromene derivatives.

Metal-free thiolation of sulfonyl hydrazone with thiophenol: synthesis of 4-thio-chroman and diarylmethyl thioethers

A simple, efficient, and transition metal-free approach was developed for accessing 4-thio-substituted chroman and diarylmethyl thioethers from sulfonyl hydrazones. This protocol provides straightforward access to a class of diarylmethane derivatives with good to excellent yields. This operationally simple protocol exhibited good tolerance for labile functional groups, providing biologically relevant chemical libraries. This safe and feasible route is applicable to the large-scale synthesis of 4-thio-substituted chromans, which are of great synthetic interest because of their further reaction potential.

Metal-Free Construction of Multisubstituted Indolizines via Intramolecular Amination of Allylic Alcohols

An intramolecular amination of allylic alcohols is developed as an efficient and general access to biologically important multisubstituted indolizines and their variants. Two metal-free synthetic platforms including using aqueous hydrochloric acid solution as the solvent and p-toluenesulfonic acid as the catalyst have been established, enabling the divergent synthesis of these valuable compounds in high yields.

Efficient access to general α-tertiary amines via water-accelerated organocatalytic multicomponent allylation

A tetrasubstituted carbon atom connected by three sp³ or sp²-carbons with single nitrogen, i.e., the α-tertiary amine (ATA) functional group, is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. The synthetic approach toward ATA structures is intricate, therefore, a straightforward catalytic method has remained a substantial challenge. Here we show an efficient water-accelerated organocatalytic method to directly access ATA incorporating homoallylic amine structures by exploiting readily accessible general ketones as useful starting material. The synergistic action of a hydrophobic Brønsted acid in combination with a squaramide hydrogen-bonding donor under aqueous condition enabled the facile formation of the desired moiety. The developed exceptionally mild but powerful system facilitated a broad substrate scope, and enabled efficient multi-gram scalability.

The α-tertiary amine functional group is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. Here the authors show an efficient water-accelerated organocatalytic method to access α-tertiary amines incorporating homoallylic amine structures by exploiting ketones as useful starting material.

An expedient, efficient and solvent-free synthesis of T3P®-mediated amidation of benzhydrols with poorly reactive N-nucleophiles under MW irradiation

An expedient, efficient, economical, environmentally benign, and solvent free amidation protocol of benzhydrols with less reactive nitrogen nucleophiles assisted by propylphosphonic anhydride (T3P®) under microwave irradiation has been developed.

Tropylium Salt-Promoted Vinylogous Aza-Michael Addition of Carbamates to para-Quinone Methides: Elaboration to Diastereomerically Pure α,α'-Diarylmethyl Carbamates

Carbocation catalysis is emerging as an important subarea of Lewis acid catalysis. Some stable and isolable carbocations have been successfully utilized as Lewis acid catalysts and promoters in many synthetic transformations. In this manuscript, we report a tropylium cation-promoted vinylogous aza-Michael addition of carbamates to para-quinone methides (QMs) to access a wide range of unsymmetrical α,α'-diarylmethyl carbamates. This mild protocol was effective for the vinylogous conjugate addition of (-)-menthyl carbamate to p-QMs, and the respective diastereomerically pure α,α'-diarylmethyl carbamate derivatives could be obtained in excellent yields and diastereoselectivities (up to >20:1 de).

Bronsted acidic surfactants: efficient organocatalysts for diverse organic transformations

Organic transformations using efficient, atom-economical, cost-effective and environmentally benign strategies for the construction of diversified molecules have attracted synthetic chemists worldwide in recent years. These processes often minimize the waste production and avoid the use of hazardous flammable organic solvents. Among various green protocols, the procedures using surfactant-based catalytic systems have received a considerable attention in organic synthesis. In this context, Bronsted acidic surfactants have emerged as efficient catalysts for various C–C, C–O, C–N and C–S bond forming reactions. Many of these reactions occur in water, as Bronsted acidic surfactants have a unique ability of creating hydrophobic pocket through micelle formation in aqueous medium and the substrate molecules react efficiently to afford the targeted products in good yields. In the past, Bronsted acidic surfactant combined catalysts successfully displayed their potential to accelerate the reaction rates of diverse organic transformations. This chapter presents a complete overview on Bronsted acidic surfactants catalyzed organic reactions to construct a variety of aromatic and heteroaromatic molecular frameworks.

Surfactant-mediated thioglycosylation of 1-hydroxy sugars in water

Thioglycosides are an important class of sugars, since they can be used as non-ionic biosurfactants, biomimetic glycosides, and building blocks for carbohydrate synthesis. Previously, Brønsted- or Lewis-acid-catalyzed dehydrative glycosylations between a 1-hydroxy sugar and a thiol have been reported to yield open-chain dithioacetal sugars as the major products instead of the desired thioglycosides. These dithioacetal sugars are by-products derived from the endocyclic bond cleavage of the thioglycosides. Herein, we report dehydrative glycosylation in water mediated by a Brønsted acid-surfactant combined catalyst (BASC). Glycosylations between 1-hydroxy furanosyl/pyranosyl sugars and primary, secondary, and tertiary aliphatic/aromatic thiols in the presence of dodecyl benzenesulfonic acid (DBSA) provided the thioglycoside products in moderate to good yields. Microwave irradiation led to improvements in the yields and a shortening of the reaction time. Remarkably, open-chain dithioacetal sugars were not detected in the DBSA-mediated glycosylations in water. This method is a simple, convenient, and rapid approach to produce a library of thioglycosides without the requirement of anhydrous conditions. Moreover, this work also provides an excellent example of complementary reactivity profiles of glycosylation in organic solvents and water.

Lewis acid-catalyzed double addition of indoles to ketones: synthesis of bis(indolyl)methanes with all-carbon quaternary centers

We report herein a Lewis acid-catalyzed nucleophilic double-addition of indoles to ketones under mild conditions. This process occurs with various ketones ranging from dialkyl ketones to diaryl ketones, thereby providing access to an array of bis(indolyl)methanes bearing all-carbon quaternary centers, including tetra-aryl carbon centers. The products can be transformed into bis(indole)-fused polycyclics and bis(indolyl)alkenes.

Biocompatible ionic liquid [Betaine][H2PO4] as a reusable catalyst for the substitution of xanthen-9-ol under solvent-free conditions

An ionic liquid, namely [Betaine][H2PO4], was found to be an efficient catalyst for the direct substitution reaction of xanthen-9-ol with different nucleophiles under solvent-free conditions. This catalytic system is easy to be operated and the following work-up procedure is simple, with the ionic liquid catalyst reusable for at least five cycles at a high catalytic activity level. In addition, the ionic liquid is easy to prepare and its raw materials are inexpensive and have good biocompatibility. Therefore, our study presents an intriguing and sustainable protocol for the direct substitution of alcohol.

A continuous-flow approach for the multi-gram scale synthesis of C2-alkyl- or β-amino functionalized 1,3-dicarbonyl derivatives and ondansetron drug using 1,3-dicarbonyls

Continuous flow application has been developed for the gram-scale synthesis of C2-alkyl- or β-amino functionalized 1,3-dicarbonyl derivatives and ondansetron drug.

Gold(III)-Catalyzed Decarboxylative C3-Benzylation of Indole-3-carboxylic Acids with Benzylic Alcohols in Water

A strategy for the gold(III)-catalyzed decarboxylative coupling reaction of indole-3-carboxylic acids with benzylic alcohols has been developed. This cascade reaction is devised as a straightforward and efficient synthetic route for 3-benzylindoles in moderate to excellent yields (50-93%). A Hammett study of the protodecarboxylation gives a negative ρ value, suggesting that there is a buildup of positive charge on the indole ring in the transition state. Furthermore, comparison of initial rates in H₂O and in D₂O reveals an observed kinetic solvent isotope effect (KSIE = 2.7). This simple protocol, which affords the desired products with CO₂ and water as the coproducts, can be achieved under mild conditions without the need for base or other additives in water.

Supramolecular Chemistry and Self-Organization: A Veritable Playground for Catalysis

The directed assembly of molecular building blocks into discrete supermolecules or extended supramolecular networks through noncovalent intermolecular interactions is an ongoing challenge in chemistry. This challenge may be overcome by establishing a hierarchy of intermolecular interactions that, in turn, may facilitate the edification of supramolecular assemblies. As noncovalent interactions can be used to accelerate the reaction rates and/or to increase their selectivity, the development of efficient and practical catalytic systems, using supramolecular chemistry, has been achieved during the last few decades. However, between discrete and extended supramolecular assemblies, the newly developed “colloidal tectonics” concept allows us to link the molecular and macroscopic scales through the structured engineering of colloidal structures that can be applied to the design of predictable, versatile, and switchable catalytic systems. The main cutting-edge strategies involving supramolecular chemistry and self-organization in catalysis will be discussed and compared in this review.

Quantitative Evaluation of the Effect of the Hydrophobicity of the Environment Surrounding Brønsted Acid Sites on Their Catalytic Activity for the Hydrolysis of Organic Molecules

Sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were fabricated to elucidate the effect of the environment surrounding the Brønsted acid site on their catalytic activity for the hydrolysis of organic molecules. A detailed structural analysis of these siloxane gels by elemental analysis, X-ray photoelectron spectroscopy, Fourier-transformed infrared (FT-IR), and ²⁹Si MAS NMR revealed the formation of gel catalysts with a highly condensed siloxane network, which enabled us to quantitatively evaluate the hydrophobicity of the environment surrounding the catalytically active sulfo-functionality. A sulfo group in a highly hydrophobic environment exhibited excellent catalytic turnover frequency for the hydrolysis of acetate esters with a long alkyl chain, whereas not only conventional solid acid catalysts but also liquid acids showed quite low catalytic activity. Detailed kinetic studies corroborated that the adsorption of oleophilic esters at the Brønsted acid site was facilitated by the surrounding hydrophobic environment, thus significantly promoting hydrolysis under aqueous conditions. Furthermore, sulfo-functionalized siloxane gels with a highly hydrophobic surface showed excellent catalytic activity for the hydrolytic deprotection of silyl ethers.

A Sc(OTf)3 catalyzed dehydrogenative reaction of electron-rich (hetero)aryl nucleophiles with 9-aryl-fluoren-9-ols

New pyrenyl substituted 9,9-diarylfluorene with nonplanar conformations, high fluorescence quantum yield and excellent thermal and morphological stability was synthesized by our current protocol.

Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

Bioconjugation with Thiols by Benzylic Substitution

A benzylic substitution of 3-indolyl(hydroxyl)acetate derivatives with thiols proceeded specifically in the presence of amino, carboxy, and phosphate groups in weakly acidic aqueous solutions under nearly physiological condition, while no reaction occurred at pH over 7. Kinetic studies revealed that the reaction followed second-order kinetics (first-order in the reactant and first-order in thiol) in contrast with the S_N 1 mechanism of common benzylic substitution of alcohols. The utility of the present method for functionalization of biomacromolecules was demonstrated using several model proteins, such as lysozyme, insulin, trypsin, and serum albumin. The catalytic bioactivity of lysozyme in lysis of Micrococcus lysodeikticus cells was completely retained after the modification.

B(C6F5)3 catalyzed direct nucleophilic substitution of benzylic alcohols: an effective method of constructing C–O, C–S and C–C bonds from benzylic alcohols

An efficient and general method of nucleophilic substitution of benzylic alcohols catalyzed by non-metallic Lewis acid B(C₆F₅)₃ was developed. The reaction could be carried out under mild conditions and more than 35 examples of ethers, thioethers and triarylmethanes were constructed in high yields. Some bioactive organic molecules were synthesized directly using the methods.

An efficient and general method of nucleophilic substitution of benzylic alcohols catalyzed by non-metallic Lewis acid B(C₆F₅)₃ was developed.

Direct Substitution of Alcohols in Pure Water by Brønsted Acid Catalysis

With the increasing concern for sustainability, the use of environmentally friendly media to perform chemical processes has attracted the attention of many research groups. Among them, the use of water, as the unique solvent for reactions, is currently an active area of research. One process of particular interest is the direct nucleophilic substitution of an alcohol avoiding its preliminary transformation into a good leaving group, since one of the by-products in this approach would be water. The direct substitution of allylic, benzylic, and tertiary alcohols has been achieved through S_N1-type reactions with catalytic amounts of Brønsted or Lewis acids; however, organic solvents are often required. In this review, the pioneering S_N1 approaches performed in pure water and in the absence of a metal based Lewis acid are compiled and discussed.