Brønsted-acid-catalyzed coupling of electron-rich arenes with substituted allylic and secondary benzylic alcohols

p-Toluenesulfonic acid and triflic acid catalyze efficiently the coupling of electron-rich arenes with allylic and benzylic alcohols. Reactions are conducted under mild conditions, in air, and in the absence of solvent.     

Brønsted acid-assisted N-alkylation of sulfonamides using ethers as the alkylation reagents

N-Alkylation of sulfonamides using cyclic ethers as alkylation reagents and Brønsted acid as a catalyst produced pyrrolidine and piperidine derivatives in good yields. When using symmetrical and unsymmetrical ethers as alkylation reagents, mono-N-alkylation of sulfonamides took place to afford the corresponding products.     

Novel Brønsted acidic deep eutectic solvent as reaction media for esterification of carboxylic acid with alcohols

New halogen-free Brønsted acidic deep eutectic solvents (DES) have been prepared by mixing new quaternary ammonium methanesulfonate salts with p-toluenesulfonic acid (PTSA). They have been used as dual solvent-catalyst for esterification of several carboxylic acids with different alcohols with a reagent molar ratio of 1:1. The method is mild, safe, and simple. Ease of recovery and reusability of DES with high activity makes this method efficient and eco-friendly. The tunability of DES properties, attained by changes in the cation, was performed in order to achieve various esters in good yields.     

Palladium-catalyzed asymmetric hydrogenation of simple ketones activated by Brønsted acids

Homogeneous Pd(OCOCF₃)₂/(R)-C₄-TunePhos has been successfully applied in the asymmetric hydrogenation of simple ketones activated by catalytic amount of Brønsted acid with up to 88% ee.     

Brønsted acid-catalyzed C-C bond forming reaction for one step synthesis of oxazinanones

An efficient Brønsted acid-catalyzed synthesis of oxazinanones (2) from corresponding Boc-imines (1) in one-step has been developed. Oxazinanones are obtained via an unprecedented tandem elimination-cycloaddition reaction. The reaction is remarkably significant given its novelty, mild reaction conditions, simplicity and low cost of the catalyst system.     

Brønsted acid TfOH-mediated reactions of 2-(arylmethylene)cyclopropylcarbinols with acetonitrile

We report herein Brønsted acid TfOH-mediated reactions of 2-(arylmethylene)cyclopropylcarbinols with acetonitrile to give the corresponding ring-enlarged N-(3-arylmethylidenecyclobutyl)acetamides in moderate to good yields under mild conditions. A plausible mechanism is proposed on the basis of previous investigations.     

A convenient protocol for the synthesis of axially chiral Brønsted acids

Partially hydrogenated binaphthol monophosphoric acids were prepared via an improved four-step sequence. It is demonstrated that typical protection and deprotection of the phenolic groups are dispensable. The vinyl-substituted derivative has been polymerized to give a novel polymerized organocatalyst.     

Chiral Brønsted acid-catalyzed hydrophosphonylation of imines—DFT study on the effect of substituents of phosphoric acid

The enantioselective hydrophosphonylation reaction of diisopropyl phosphite with aldimine furnished α-amino phosphonates with high enantioselectivities by means of a chiral phosphoric acid. DFT calculation of the effect of 3,3′-substituents of the phosphoric acid revealed the reason for the high enantioselectivities.     

Enantioselective synthesis of the bicyclo[4.3.0]nonane ring system of the pinguisane-type sesquiterpenoids via a Brønsted acid promoted transannular enol alkylation

The bicyclo[4.3.0]nonane ring system of the pinguisane-type sesquiterpenoid natural products, including the vicinal quaternary stereocentres, has been synthesised as a single enantiomer via a novel Brønsted acid promoted transannular alkylation of an enol with an unactivated alkene.     

Asymmetric synthesis of activated cyclopropanes catalyzed by cinchonidine as a chiral Brønsted base

Cinchonidine catalyzed the cyclopropanation reaction between chloromethyl ketones and β-substituted methylidenemalononitriles to give trans-cyclopropanes with enantioselectivity up to 82% ee. Experimental evidence suggests that cinchonidine functions as a chiral Brønsted base catalyst in the reaction and hydrogen bonding is essential for inducing high enantioselectivity.     

Brønsted acid-mediated opening of nitroso cycloadducts under anhydrous conditions

An unusual bicyclic hydroxamate resulted from C-O bond cleavage of acylnitroso hetero-Diels-Alder cycloadducts when treated with catalytic Brønsted acids under anhydrous conditions. Similarly, the formation of a nitrone was observed using one equivalent of triflic acid.     

Brønsted acidic imidazolium salts containing perfluoroalkyl tails catalyzed one-pot synthesis of 1,8-dioxo-decahydroacridines in water

One-pot three-component synthesis of 1,8-dioxo-9,10-diaryl-decahydroacridines in water was efficiently realized in the presence of the Brønsted acidic imidazolium salts containing perfluoroalkyl tails in good yields. The method provided several advantages such as low catalyst loading; recycle of the catalyst and simple work procedure.     

Synthesis of tetrahydroisoquinolines and isochromans via Pictet-Spengler reactions catalyzed by Brønsted acid-surfactant-combined catalyst in aqueous media

Perfluorooctanesulfonic acid (PFOSA), Brønsted acid-surfactant-combined catalyst, efficiently catalyzes the Pictet-Spengler reactions of β-arylethyl carbamate derivatives with aldehydes in water. The present reaction is accelerated by the addition of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). PFOSA in HFIP-water (10 v/v %) is also successfully applied to the oxa-Pictet-Spengler reactions of β-arylethyl alcohol compounds.     

Effect of Brønsted acid co-catalyst in asymmetric conjugate addition of 3-aryloxindoles to maleimide under base-free phase-transfer conditions

Highly diastereo- and enantioselective conjugate addition of 3-aryloxindoles to maleimide was accomplished under base-free phase-transfer conditions. The effect of Brønsted acid co-catalyst was examined, and we found that the use of a phenol derivative as co-catalyst was effective for obtaining the target conjugate adduct in good yield with high stereoselectivity.     

Theory-guided design of Brønsted acid-assisted phosphine catalysis: synthesis of dihydropyrones from aldehydes and allenoates

The phosphine-catalyzed addition of 2,3-butadienoates to aldehydes has been extended to the formation of disubstituted dihydro-2-pyrones. The requisite shift in equilibrium of the intermediate zwitterionic β-phosphonium dienolates toward the s-cis intermediate was accomplished through the use of a Brønsted acid additive, which disrupts the favorable Coulombic interaction present in the s-trans intermediate. The detailed nature of the synergistic interactions involving the Brønsted acid additives and phosphine involved in the formation of s-cis β-phosphonium dienolates was analyzed through a series of DFT calculations. Unlike previously reported annulations of aldehydes with allenoates, where trialkylphosphines are optimal catalysts, in this study triphenylphosphine was also found for the first time to be a suitable catalyst for the synthesis of dihydropyrones. This method provides a one-step route toward functionalized dihydropyrones from simple, stable starting materials. In addition, new reaction pathways of phosphine-catalyzed allene annulations are unveiled, with the formation of dihydropyrones being the first example of dual activation in this sphere.     

Asymmetric sp C-H functionalization via a chiral Brønsted acid-catalyzed redox reaction for the synthesis of cyclic aminals

An organocatalytic asymmetric tandem 1,5-hydride transfer/ring closing reaction of o-aminobenzoketones with anilines to give cyclic aminals in fairly good diastereo- and enantioselectivities.     

Brønsted acid-mediated ring-opening reactions of methylenecyclopropanes: a dramatic counter ion effect

We report herein two different ring-opening patterns of methylenecyclopropanes (MCPs) in the presence of two Brønsted acids heptadecafluorooctane-1-sulfonic acid (C₈F₁₇SO₃H) and toluene p-sulfonic acid (TsOH) under mild conditions: (a) the ring-opening of MCPs by H₂O and subsequent etherification give the corresponding homoallylic ethers in the presence of heptadecafluorooctane-1-sulfonic acid; (b) the direct ring-opening of MCPs by the Brønsted acid gives the corresponding homoallylic alcohol derivatives in the presence of toluene p-sulfonic acid.     

o-Benzenedisulfonimide as a reusable Brønsted acid catalyst for an efficient and facile synthesis of quinolines via Friedländer annulation

The synthesis of quinolines via Friedländer annulation was carried out in the presence of catalytic amount of o-benzenedisulfonimide as Brønsted acid organocatalyst; the reaction conditions were mild and the yields of the target products were good. The organocatalyst was easily recovered and purified, ready to be used in further reactions.     

Development of N,N-bis(perfluoroalkanesulfonyl)squaramides as new strong Brønsted acids and their application to organic reactions

New strong Brønsted acids derived from a squaric acid scaffold bearing different perfluoroalkanesulfonyl groups have been developed and applied to several organic reactions. These squaramides are bench-stable and exhibit much higher reactivities in several organic reactions than squaric acid itself. N,N-Bis(trifluoromethanesulfonyl)squaramide 2a was applied to the Mukaiyama aldol reaction and Mukaiyama Michael reaction. Mechanistic studies revealed that the Brønsted acid might be the predominant catalyst through direct protonation of carbonyl compound by the acid itself rather than the silylated Brønsted acid. The utility of this acid 2a was further extended to Hosomi-Sakurai allylation of aldehydes and a carbonyl-ene reaction. Furthermore, other squaramides 2b and c bearing longer perfluoroalkyl chains have been developed, which are also bench-stable and displayed similar reactivities with squaramide 2a in several organic reactions.     

Synthesis of 1-C-alkyl-α-d-glucopyranosides by Lewis acid- or Brønsted acid-catalyzed O-glycosidation

We prepared several kinds of 1-C-alkyl-2,3,4,6-tetra-O-benzyl-α-d-glucopyranose derivatives containing methyl, ethyl, n-butyl, and benzyl groups as the alkyl groups at their anomeric positions. The Lewis acid- or Brønsted acid-catalyzed O-glycosidations using them as the glycosyl donors to synthesize 1-C-alkyl-d-glucopyranosides were investigated. Using 10 mol % of triphenylmethyl perchlorate efficiently catalyzed the glycosidation of 2,3,4,6-tetra-O-benzyl-1-C-methyl-α-d-glucopyranosyl dimethylphosphinothioate. The glycosidation using the 1-C-alkyl-2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl acetates smoothly proceeded in the presence of only 5 mol % of scandium(III) trifluoromethanesulfonate. The dehydration-condensation type glycosidation using the 1-C-alkyl-2,3,4,6-tetra-O-benzyl-α-d-glucopyranoses was significantly promoted using 5 mol % of bis(trifluoromethane)sulfonimide. These glycosidations successfully afforded various 1-C-alkyl-α-d-glucopyranosides in good yields with high α-stereoselectivities.