Brønsted acid-promoted cyclizations of siloxy alkynes with unactivated arenes, alkenes, and alkynes

In this article, we describe the development of a general concept for the development of new carbon-carbon bond-forming processes, which is based on Brønsted acid-mediated activation of a siloxy alkyne, followed by efficient interception of the resulting highly reactive ketenium ion by unactivated arenes, alkenes or alkynes. We found that trifluoromethane sulfonimide (HNTf₂) proved to be a superior promoter of these reactions compared to a range of other Brønsted acids. This finding could be attributed to a high acidity of HNTf₂ in aprotic organic solvents combined with a low nucleophilicity of the NTf₂⁻ anion. Depending on the nature of the nucleophile, the carbocyclizations proceeded either via 6-endo-dig or 5-endo-dig manifolds. In the case of 1-siloxy-1,5-diynes, the cyclizations occurred with a concomitant halide abstraction or arylation.     

6-endo-dig Cyclization of heteroarylesters to alkynes promoted by Lewis acid catalyst in the presence of Brønsted acid

We report a regiocontrolled 6-endo-dig cyclization of 2-(2-arylethynyl)heteroaryl esters occurred under Brønsted acidic conditions and in the presence of a catalytic amount of Lewis acids such as Cu(OTf)₂, AuCl₃, or (CF₃CO₂)Ag. A variety of heterocyclic lactones are readily prepared in excellent yields.     

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.     

Chiral bis-trifluoromethanesulfonylamide as a chiral Brønsted acid catalyst for the asymmetric hetero Diels-Alder reaction with Danishefsky’s diene

Bis-trifluoromethanesulfonylamide (bis-triflylamide) itself works as an efficient chiral Brønsted acid catalyst for the hetero Diels-Alder reactions between Danishefsky’s diene and glyoxylate or phenylglyoxal presumably through double hydrogen bonding.     

Brønsted酸性离子液体催化合成聚甲醛二甲醚的研究

合成并表征了两种Brønsted酸性离子液体N-甲基-2吡咯烷酮硫酸氢盐([Hnmp]HSO₄)和N-甲基-2吡咯烷酮对甲苯磺酸盐([Hnmp]PTSA),对两种离子液体在由甲缩醛和多聚甲醛缩合制备聚甲醛二甲醚(DMM_n,n > 1)反应中的催化性能进行了研究.结果显示,离子液体的催化活性与其酸性相关,离子液体[Hnmp]HSO₄具有较高的催化活性;当离子液体[Hnmp]HSO₄ 的用量为2.0%(质量分数)、m(甲缩醛)/m(多聚甲醛)= 2.00、反应温度110℃、反应时间6 h时,甲缩醛的转化率和DMM_3~8 的选择性分别为52.28%和49.18%.反应结束后,离子液体[Hnmp]HSO₄与产物自动分成两相,且该离子液体的稳定性好,重复使用五次后仍有较高的催化活性.     

Solvent-free Brønsted acid catalysed alkylation of arenes and heteroarenes with benzylic alcohols

A simple and efficient alkylation of aromatic and heteroaromatic compounds via the direct S_N1-type nucleophilic substitution of benzylic alcohols in the presence of catalytic amounts of the strong Brønsted acid o-benzenedisulfonimide under neat conditions is herein reported. A library of di- and triaryl (and heteroaryl) methanes was prepared in good yields and high regioselectivity. The observed reactivity was shown to be in agreement with Mayr's nucleophilicity and electrophilicity scales.     

Efficient silver-free gold(I)-catalyzed hydration of alkynes at low catalyst loading

The use of [(IPr)AuOH] as versatile, air- and moisture-stable pre-catalyst permits the in situ generation of the cationic gold(I) species [(IPr)Au]X after reaction with a Brønsted acid. This catalytic system presents as a main advantage the lack of use of a silver salt activator or co-catalyst which is often air-, light- and moisture-sensitive. A general gold(I)-catalyzed procedure using this in situ activation at very low catalyst loading is reported for the hydration of a broad range of internal and terminal alkynes.     

Heteropoly acids: a green and efficient heterogeneous Brønsted acidic catalyst for the intermolecular hydroamination of olefins

Intermolecular hydroamination of non-activated olefins with amides and benzyl carbamate proceeds efficiently in the presence of environmentally benign silicotungstic acid (HSiW) catalyst under mild conditions in air to afford addition products in good to excellent yields.     

1-(1-Alkylsulfonic)-3-methylimidazolium chloride as a reusable Brønsted acid catalyst for the regioselective azidolysis of epoxides under solvent-free conditions

In this study, 1-(1-alkylsulfonic)-3-methylimidazolium chloride as a new, green, and reusable Brønsted acid catalyst was prepared. In this protocol, we used for the regioselective ring-opening reactions of various epoxiodes with sodium azide to afford the corresponding β-azido alcohols in excellent yields and short reaction time under mild and neutral reaction conditions. This method offers several advantages including excellent regioselectivity, clean reactions, simple work-up procedure, a recyclable catalyst, and use of an eco-friendly catalyst.     

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.     

Lipase-catalyzed transformation of poly(lactic acid) into cyclic oligomers

Enzymatic transformations into cyclic oligomers were carried out with the objective of developing chemical recycling of poly(lactic acid)s, such as poly(D,L-lactic acid) (PDLLA), poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA), which are typical biodegradable polymers. They were degraded by lipase in an organic solvent to produce the corresponding cyclic oligomer with a molecular weight of several hundreds. PDLLA (with a Mw of 84,000) was quantitatively transformed into cyclic oligomers by lipase RM (Lipozyme RM IM) in chloroform/hexane at 60 degrees C. PLLA (with a Mw of 120,000) was transformed into cyclic oligomer by lipase CA (Novozym 435) at a higher temperature of 100 degrees C in o-xylene. The oligomer structure was identified by 1H and 13C NMR spectroscopy and MALDI-TOF (matrix assisted laser desorption/ionization-time-of-flight) mass spectrometry.     

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.     

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.     

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.     

Brønsted acid-base polymer electrolyte membrane based on sulfonated poly(phenylene oxide) and imidazole

The Brønsted acid-base polymer electrolyte membrane was prepared by entrapping imidazole in sulfonated poly(phenylene oxide) at the molar ratio of Im/SPPO = 2:1. The hybrid showed a high thermal stability up to 200 °C and peroxide tolerance. Differential scanning calorimetry shows that glass transition temperature is 232 °C. The conductivity increases with temperature exceeding 10⁻³ S/cm above 120 °C and a high conductivity of 6.9 × 10⁻³ S/cm was obtained at 200 °C under 33% RH conditions.     

Syntheses of vinylindoles via a Brønsted acid catalyzed highly regio- and stereoselective cis-hydroarylation of ynamides

A highly regio- and stereoselective Brønsted acid-catalyzed coupling of ynamides and indoles is described. This process is the equivalent of hydroarylation of ynamides and leads to the efficient syntheses of vinylindoles. Diels-Alder reaction between the vinylindoles and DMAD afforded carbazole derivatives in good yields.     

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.     

Synthesis of furo[2,3-b]pyridin-4(7H)-ones and related quinolinone via Brønsted acid-promoted cyclisation of alkynes

N-Alkyl-4-alkoxy-3-alkynylpyridin-2(1H)-ones readily undergo acid-promoted 5-endo-heteroannulation to furopyridinium intermediates that are dealkylated in situ to provide the corresponding furo[2,3-b]pyridin-4(7H)-ones. The same strategy applies to the formation of furo[2,3-b]quinolin-4(9H)-ones. In the case of Me₃Si-substituted alkynes, hydration of the triple bond was observed.