Unprecedented Strong Lewis Bases—Synthesis and Methyl Cation Affinities of Dimethylamino‐Substituted Terpyridines

A versatile method for the synthesis of functionalized 2,2′:6′,2′′‐terpyridines by assembly of the terminal pyridine rings is presented. The cyclization precursors—bis‐β‐ketoenamides—are prepared from 4‐substituted 2,6‐pyridinedicarboxylic acids and acetylacetone or its corresponding enamino ketone. Treatment with trimethylsilyl trifluoromethanesulfonate induces a twofold intramolecular condensation providing an efficient access to 4,4′′‐di‐ and 4,4′,4′′‐trifunctionalized 6,6′′‐dimethyl‐2,2′:6′,2′′‐terpyridines. Using this method, hitherto unknown 4,4′′‐bis(dimethylamino)‐ and 4,4′,4′′‐tris(dimethylamino)terpyridines have been prepared that show remarkably high calculated Lewis basicities.     

The very strong solid Lewis acids aluminium chlorofluoride (ACF) and bromofluoride (ABF)—Synthesis, structure, and Lewis acidity

Aluminium chlorofluoride, ACF, (AlCl_xF_3−x, x ≈ 0.05-0.25) is an amorphous solid Lewis acid with an extraordinary acid strength that was patented by DuPont in 1992. It reaches the acidity of antimony pentafluoride, SbF₅, and, in some cases, even exceeds it.In this review, an overview of known structural data of ACF and the corresponding bromine compound, ABF, is presented, together with additional unpublished data. This includes a discussion of soft chemistry synthesis, structure of bulk and surface, and Lewis acidities of molecular and solid group 13 halides. The order of Lewis acidity of gaseous halides is established to be AlF_3(g) < AlCl_3(g) < AlBr_3(g) ≈ AlI_3(g) and AlX_3(g) > GaX_3(g) > InX_3(g) (X = F, Cl, Br, I).ACF and ABF are highly distorted solids. Different spectroscopic bulk methods show that the heavy halogen, chlorine or bromine, is a necessary perturbation that generates “disorder”. Highly Lewis acidic centres have been detected by several methods on the surface of ACF and ABF, which are responsible for the high catalytic reactivity observed in many reactions.The data gained on ACF and ABF show that many published data about heterogeneously catalysed fluorine-halogen exchange reactions in presence of aluminium chloride and bromide must be treated critically. In general, ACF and ABF are the active species.     

Establishing the Coordination Chemistry of Antimony(V) Cations: Systematic Assessment of Ph4Sb(OTf) and Ph3Sb(OTf)2 as Lewis Acceptors

The coordination chemistry of the stiboranes Ph₄Sb(OTf) ( 1 a , OTf = OSO₂CF₃) and Ph₃Sb(OTf)₂ ( 3 ) with Lewis bases has been investigated. The significant steric encumbrance of the Sb center in 1 a precludes interaction with most ligands, but the relatively low steric demands of 4‐methylpyridine‐N‐oxide (OPyrMe) and OPMe₃ enabled the characterization of [Ph₄Sb(OPyrMe)][OTf] ( 2 a ) and [Ph₄Sb(OPMe₃)][OTf] ( 2 b ), rare examples of structurally characterized complexes of stibonium acceptors. In contrast, 3 was found to engage a variety of Lewis bases, forming stable isolable complexes of the form [Ph₃Sb(donor)₂][OTf]₂ [donor=OPMe₃ ( 6 a ), OPCy₃ ( 6 b , Cy=cyclohexyl), OPPh₃ ( 6 c ), OPyrMe ( 6 d )], [Ph₃Sb(dmap)₂(OTf)][OTf] ( 6 e , dmap=4‐(dimethylamino)pyridine) and [Ph₃Sb(donor)(OTf)][OTf] [donor=1,10‐phenanthroline ( 7 a ) or 2,2′‐bipy ( 7 b , bipy=bipyridine)]. These compounds exhibit significant structural diversity in the solid‐state, and undergo ligand exchange reactions in line with their assignment as coordination complexes. Compound 3 did not form stable complexes with phosphine donors, with reactions instead leading to redox processes yielding SbPh₃ and products of phosphine oxidation.     

Addition of Me3SiCN to trifluoromethyl derivates of N‐(pyridylmethylidene) anilines catalyzed by Lewis acids

A series of novel Shiff bases (1a–h) was synthesized by condensation of pyridinecarboxaldehydes (1–4) with 3‐ and 4‐trifluoromethylanilines (5, 6) in the presence of molecular sieves (4 Å). It was found that AlCl₃ and AlBr₃ catalyzed the addition of Me₃SiCN to the C?N bond of the imines obtained, whereas the other Lewis acids studied (YCl₃, LaCl₃, ZnI₂) were not active. The reactivity of the imines in the title reaction, on the whole, correlated with their basicity. Besides the addition giving the expected α‐amino nitriles (2a,b,d–f,h), an unusual reaction leading to unsaturated nitriles (3a–h) was observed. The structures of saturated and unsaturated products 2d and 3c were determined by X‐ray diffraction. Copyright © 2001 John Wiley & Sons, Ltd.     

On the Lewis Basicity of Phosphoramides: A Critical Examination of Their Donor Number through Comparison of Enthalpies of Adduct Formation with SbCl5 and BF3

The Lewis basicity of a series of phosphoryl compounds was examined using DFT and ab initio methods, including solvation effects. The enthalpies of adduct formation with two archetypal Lewis acids, antimony pentachloride and boron trifluoride, used to define the donor number DN and the BF₃ affinity (BF₃A) respectively, were examined. The BF₃ adducts allow the use of the high-accuracy G4 approach, whereas for SbCl₅ adducts, three different DFT formalisms, including empirical dispersion corrections, were used because the G4 formalism is not available for third-row elements. For a comparison with experimental data, solvation effects were taken into account by using the polarizable continuum model. The experimental BF₃ affinities were well reproduced by G4 calculations when including PCM solvation. Conversely, comparisons of our calculated values and experimental results reported in the literature show that SbCl₅ enthalpies for phosphoramides are in error. In particular the DN for HMPA should be revised.     

Nitrogen‐Based Lewis Acids: Synthesis and Reactivity of a Cyclic (Alkyl)(Amino)Nitrenium Cation

A room‐temperature‐stable crystalline cyclic (alkyl)(amino)nitrenium cation 2 features cationic nitrogen atom with a smaller HOMO–LUMO gap compared to that of a 1,2,3‐triazolium 5 (an N‐heterocyclic nitrenium cation). The low‐lying LUMO of 2 results in an enhanced electrophilicity, which allowed for the formation of Lewis adducts with neutral Lewis bases, such as Me₃P, nBu₃P, and IiPr. The N‐based Lewis acid 2 also forms an FLP with tBu₃P but subsequently reacts with (PrS)₂ to cleave the S?S bond. Both experimental and theoretical results suggest that the Lewis acidity of 2 is stronger than its N₃ analogues.     

路易斯酸碱催化的外消旋体(动态)动力学拆分反应

外消旋体的(动态)动力学拆分反应是制备手性化合物的重要方法之一.反应可以通过酶催化或非酶催化的手段来实现,也可以通过两种方法的有机结合来进行.在非酶催化反应中,路易斯酸碱催化是比较常用的方法,它们被广泛地用于多种外消旋体的(动态)动力学拆分反应中,目前在该领域取得了很大的进展.本文讨论了路易斯酸及路易斯碱催化体系在外消旋体(动态)动力学拆分反应中应用的最新进展.     

A Tris(triazolate) Ligand for a Highly Active and Magnetically Recoverable Palladium Catalyst of Selective Alcohol Oxidation Using Air at Atmospheric Pressure

High efficiency and selectivity, easy magnetic recovery and recycling, and use of air as the oxidant at atmospheric pressure are major objectives for oxidation catalysis in terms of sustainable and green processes. A tris(triazolyl) ligand, so far only used in copper‐catalyzed alkyne azide cycloadditions, was found to be extremely efficient in SiO₂/γ‐Fe₂O₃‐immobilized palladium complexes. It was characterized by inductively coupled plasma (ICP) analysis, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy‐dispersive X‐ray spectroscopy (EDX), and X‐ray photoelectron spectra (XPS) and found to fulfill the combined conditions for the selective oxidation of alcohols to aldehydes and ketones.     

新手性双-(三氟甲磺酰)亚胺基硫代缩酮基硅烷的合成及其不对称催化Diels-Alder反应

合成了数个新手性双-(三氟甲磺酰)亚胺基二硫代缩酮基硅路易斯酸2～4, 并研究了它们对环戊二烯和丙烯酸衍生物11a～11c的Diels-Alder反应的催化活性. 结果表明催化剂活性高, 环加成反应的产率高达89%～96%, 并且获得较高的非对映选择性和中等对映选择性, 当催化剂为2b时, 反应的对映选择性达到最高, 为48% ee.     

Synthesis,Characterisation and Reactivity of Copper(I) Amide Complexes and Studies on Their Role in the Modified Ullmann Amination Reaction

A series of copper(I) alkylamide complexes have been synthesised; copper(I) dicyclohexylamide ( 1 ), copper(I) 2,2,6,6‐tetramethylpiperidide ( 2 ), copper(I) pyrrolidide ( 3 ), copper(I) piperidide ( 4 ), and copper(I) benzylamide ( 5 ). Their solid‐state structures and structures in [D₆]benzene solution are characterised, with the aggregation state in solution determined by a combination of DOSY NMR spectroscopy and DFT calculations. Complexes 1 , 2 and 4 are shown to exist as tetramers in the solid state by X‐ray crystallography. In [D₆]benzene solution, complexes 1 , 2 and 5 were found by using ¹H DOSY NMR to exist in rapid equilibrium between aggregates with average aggregation numbers of 2.5, 2.4 and 3.3, respectively, at 0.05 M concentration. Conversely, distinct trimeric, tetrameric and pentameric forms of 3 and 4 were distinguishable by one‐dimensional ¹H and ¹H DOSY NMR spectroscopy. Complexes 3 – 5 are found to react stoichiometrically with iodobenzene, in the presence or absence of 1,10‐phenanthroline as an ancillary ligand, to give arylamine products indicative of their role as potential intermediates in the modified Ullmann reaction. The role of phenanthroline has also been explored both in the stoichiometric reaction and in the catalytic Ullmann protocol.     

Probing the Lewis acidity and catalytic activity of the metal-organic framework [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate)

An optimized procedure was designed for the preparation of the microporous metal-organic framework (MOF) [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate). The crystalline material was characterized by X-ray diffraction, optical microscopy, SEM, X-ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of alpha-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2-bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re-usability, and heterogeneity are critically assessed.     

Synthesis and Lewis acid catalyzed Claisen rearrangement of 2-(1,3-oxazolin-2-yl)-substituted allyl vinyl ethers

Allyl vinyl ethers containing an acceptor function in the 2-position are useful substrates for the Lewis acid-catalyzed Claisen rearrangement. The first synthesis of acyclic 2-(1,3-oxazolin-2-yl)-substituted allyl vinyl ethers is reported. The Lewis acid catalyzed Claisen rearrangement of these allyl vinyl ethers afforded the rearrangement products with low to moderate diastereo- and enantioselectivity. The catalyzed rearrangement of chiral allyl vinyl ethers was investigated. The combination of substrate- and catalyst-induced diastereoselectivity led to unexpected and unprecedented results.     

Fast,Efficient and Low E‐Factor One‐Pot Palladium‐Catalyzed Cross‐Coupling of (Hetero)Arenes

The homocoupling of aryl halides and the heterocoupling of aryl halides with either aryl bromides or arenes bearing an ortho‐lithiation directing group are presented. The use of a Pd catalyst, in combination with t‐BuLi, allows for the rapid and efficient formation of a wide range of polyaromatic compounds in a one pot procedure bypassing the need for the separate preformation of an organometallic coupling partner. These polyaromatic structures are obtained in high yields, in 10 min at room temperature, with minimal waste generation (E‐factors as low as 1.5) and without the need for strict inert conditions, making this process highly efficient and practical in comparison to classical methods. As illustration, several key intermediates of widely used BINOL‐derived structures are readily prepared.