Reactivity of Diphenylberyllium as a Brønsted Base and Its Synthetic Application

Diphenylberyllium [Be₃Ph₆] is shown here to react cleanly as a Brønsted base with a vast variety of protic compounds. Through the addition of the simple molecules tBuOH, HNPh₂ and HPPh₂, as well as the more complex 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride, one or two phenyl groups in diphenylberyllium were protonated. As a result, the long-postulated structures of [Be₃(OtBu)₆] and [Be(μ-NPh₂)Ph]₂ have finally been verified and shown to be static in solution. Additionally [Be(μ-PPh₂)(HPPh₂)Ph]₂ was generated, which is only the second beryllium-phospanide to be prepared; the stark differences between its behaviour and that of the analogous amide were also examined. The first crystalline example of a beryllium Grignard reagent with a non-bulky aryl group has also been prepared; it is stabilised with an N-heterocyclic carbene.     

Halogen Bonding of N-Halosuccinimides with Amines and Effects of Brønsted Acids in Quinuclidine-Catalyzed Halocyclizations

An arguable expectation in halogen chemistry is that an amine will react oxidatively with an N-halosuccinimide (NXS) to form an N-halogenated species bearing a covalent N−X bond. While likely for NCS under most conditions, we find this expectation simply not true for NIS and largely inaccurate for NBS. Herein, we disclose evidence through systematic NMR and X-ray studies that non-covalent halogen bonded amine complexes of NIS predominate over covalent N-halogenated species, even with primary and secondary amines. For example, during the catalytic electrophilic halocyclization of gem-disubstituted alkenes by cinchona-like amines, the quinuclidine complexes of NIS and NBS display lower reactivity than their parent N-halosuccinamides and require the presence of an appropriate Brønsted acid. Specifically, a Brønsted acid and quinuclidine jointly catalyze the halo-cycloetherification of γ-alkenyl alcohols with NIS or NBS, while only quinuclidine acts as a catalyst in the halolactonization of γ-alkenoic acids. Although our evidence confirms a transient N-halogenated quaternary ammonium salt as the halonium species, it is important to note that NIS predominantly forms ‘off-cycle’ halogen bonded amine complexes in solution.     

Brønsted acid-promoted cyclizations of siloxyalkynes with arenes and alkenes

We have described the first Br?nsted acid-mediated cyclizations of siloxyalkynes with simple arenes and alkenes to afford substituted tetralone and cyclohexenone derivatives. The most notable aspect of the carbocyclizations involving siloxyalkynes is the ability to employ a range of substrates that are not restricted to those containing electron-rich arenes and alkenes. The key mechanistic feature of the reaction is the generation of a highly reactive ketenium ion upon protonation of siloxyalkyne. We believe that the low nucleophilicty of the counteranion is crucial for enabling the formation and effective interception of this highly reactive intermediate.     

Synergy between Brønsted acid sites and Lewis acid sites

Synergy between Br?nsted acid sites and Lewis acid sites in mesoporous Al-Zr-TUD-1 was demonstrated to exist in Br?nsted acid catalysed reactions, but not in Lewis acid catalysed reactions.     

Complex containing a Lewis acid and Brønsted acid for the catalytic reactions of aza-Michael addition

The novel efficient complex catalyst containing a Lewis acid and a Brønsted acid have been prepared by the reaction of proline ion liquid and cuprous iodide. The catalyst was characterized by FT-IR techniques using pyridine as probe molecule. A fast, mild, and quantitative procedure for aza-Michael addition reactions between various amines and α,β-unsaturated carbonyl compounds and nitriles has been developed using the novel complex catalyst. The results showed that the novel catalyst owned high activities for the reactions with excellent yields within 1 min.     

Brønsted acid-catalyzed intramolecular hydroamination of protected alkenylamines. Synthesis of pyrrolidines and piperidines

[reaction: see text]. The cyclization of aminoalkenes bearing an electron-withdrawing group on the nitrogen atom was catalyzed by triflic or sulfuric acid in toluene. Pyrrolidines and piperidines were formed in excellent yields. N-phenylanilides also underwent cyclization to form gamma-lactams.     

Synthesis of 3,4-disubstituted piperidines by carbonyl ene and Prins cyclizations: a switch in diastereoselectivity between Lewis and Brønsted acid catalysts

[reaction: see text] A novel diastereoselective approach to cis and trans 3,4-disubstituted piperidines is described. Carbonyl ene cyclization of aldehydes 6a-e catalyzed by the Lewis acid methyl aluminum dichloride in refluxing chloroform affords trans piperidines 8a-e with diastereomeric ratios of up to 93:7. By contrast, Prins cyclization of 6a-e catalyzed by hydrochloric acid at low temperatures affords cis products 7a-e with diastereomeric ratios of up to 98:2.     

Preparation and characterization of composite membranes with Brønsted acidic ionic liquid

Poly-(vinyl alcohol) (PVA) proton-conducting composite membranes were prepared using succinic acid (SA) as a cross-linking agent and Brønsted acidic ionic liquid (BAIL) as a proton source. The incorporated BAILs resulted in a relatively high proton conductivity compared with PVA/SA membrane without BAILs. The proton conductivities of PVA/SA/BAIL composite membranes increased versus the BAIL content. In addition, the optimal resultant proton conductivity of PVA/SA/BAIL composite membrane under dry condition could reach 0.4 mS/cm at 140 °C, which was higher than that of PVA/sulfosuccinic acid (SSA) composite membrane (0.032 mS/cm), PVA/SSA/5-aminotetrazole membrane (0.022 mS/cm at 130 °C), and PVA/chlorosulfonic acid/glutaraldehyde membrane (0.0585 mS/cm at 90 °C) measured at the same condition. It was notable that the PVA/SA/BAIL composite membranes could reach high thermal stability up to 150 °C, which was higher than that of traditional PVA membranes (below 80 °C).     

Synthesis of 3,4-disubstituted piperidines by carbonyl ene and prins cyclizations: switching between kinetic and thermodynamic control with Brønsted and Lewis acid catalysts

A novel approach to cis and trans 3,4-disubstituted piperidines is described. Carbonyl ene cyclization of aldehydes 4a-e catalyzed by MeAlCl(2) in refluxing chloroform afforded the trans piperidines 7a-e with diastereomeric ratios of up to 93:7, while aldehyde 4f afforded solely the cis product 6f, which was resistant to isomerization to the trans isomer. It was demonstrated for 4a that the cyclization catalyzed by a variety of Lewis acids at low temperature proceeded under kinetic control to afford predominantly the cis piperidine 6a, and this isomerized to the thermodynamically more stable trans piperidine 7a on warming. In contrast, Prins cyclization of 4a-e catalyzed by concentrated hydrochloric acid in CH2Cl2 at low temperature afforded cis piperidines 6a-e with diastereomeric ratios of up to >98:2. The yield and diastereoselectivity of these cyclizations could be improved by using HCl-saturated CH2Cl2 to form the corresponding chloride, followed by elimination of HCl effected by ammonia. Aldehydes 4f and 4galso cyclized in good yield under the latter conditions. Mechanistic studies supported by DFT calculations (B3LYP/6-31G(d)) suggest that the cyclizations proceed via a mechanism with significant carbocationic character, with the cis carbocation being more stable than the trans carbocation. DFT calculations (B3LYP/6-31G(d)) of the transition state energies for concerted cyclization show that the cis piperidine is also the favored product from cyclization through a more concerted mechanism.     

Synthesis and structural aspects of N-triflylphosphoramides and their calcium salts--highly acidic and effective Brønsted acids

Recently, 1,1′‐bi‐2‐naphthol (BINOL)‐based N‐triflylphosphoramides emerged as a new class of potent Brønsted acid catalysts. In this paper we describe the efficient synthesis of various BINOL‐based N‐triflylphosphoramides and their calcium salts. Furthermore, X‐ray crystal structure analysis combined with energy‐dispersive X‐ray spectroscopy (EDX) measurements confirmed that the synthesised chiral N‐triflylphosphoramides are highly acidic metal‐free catalysts.     

Enantioselective Synthesis of Tropanes: Brønsted Acid Catalyzed Pseudotransannular Desymmetrization

The enantioselective synthesis of tropanols has been accomplished through chiral phosphoric acid catalyzed pseudotransannular ring opening of 1-aminocyclohept-4-ene-derived epoxides. The reaction proceeds together with the desymmetrization of the starting material and leads to the direct formation of the 8-azabicyclo[3.2.1]octane scaffold with excellent stereoselectivity. The synthetic applicability of the reaction was demonstrated by the enantioselective synthesis of the two natural products (−)-α-tropanol and (+)-ferruginine.     

Novel Brønsted acid catalyzed three-component alkylations of indoles with N-phenylselenophthalimide and styrenes

Novel and efficient Br?nsted acid (p-TsOH) catalyzed inter- and intramolecular Friedel-Crafts alkylations have been developed to synthesize selenated three-component coupling and selenation-cyclization indole derivatives. Chemical removal of the phenylseleno moiety was investigated, and the reaction mechanisms were discussed.     

Unprecedented synergistic effects between weak Lewis and Brønsted acids in Prins cyclization

Novel synergistic effects between Lewis and Br?nsted acids in Prins cyclization are reported. Non-reactive Lewis acids and non-reactive Br?nsted acids, which failed to perform Prins cyclization when used alone, have shown remarkable synergistic effects when used in combination to perform the reaction successfully.     

Generation of a solid Brønsted acid site in a chiral framework

Protonation of chiral porous materials introduces a Br?nsted acid centre, the structure of which is unique to the heterogeneous phase requiring pore wall confinement for stable isolation.     

Asymmetric cross aldol addition of isatins with α,β-unsaturated ketones catalyzed by a bifunctional Brønsted acid–Brønsted base organocatalyst

The asymmetric cross-aldol reaction of isatins with α,β-unsaturated ketones has been developed under catalysis by a Cinchona alkaloid-derivated bifunctional Brønsted acid–Brønsted base catalyst, affording the aldol adducts in moderate to good yields (18–98%) with moderate to good enantioselectivities (30–97%). The noncovalent organo-catalyzed asymmetric cross-aldol reaction displays a broad substrate scope and wide functional-group tolerability, albeit the electronic and steric properties of both reaction partners have considerable and regular effects on the reactivity and stereocontrol.     

Oxidation of Condensed Thiophene Derivatives with Brønsted Acidic Ionic Liquid

Moscow University Chemistry Bulletin - Abstract—A catalyst based on ionic liquid consisting of a pyridinium cation having Brønsted acidity and a molybdenum anion...