Lewis base activation of Lewis acids: development of a Lewis base catalyzed selenolactonization

The concept of Lewis base activation of Lewis acids has been applied to the selenolactonization reaction. Through the use of substoichiometric amounts of Lewis bases with "soft" donor atoms (S, Se, P) significant rate enhancements over the background reaction are seen. Preliminary mechanistic investigations have revealed the resting state of the catalyst as well as the significance of a weak Br?nsted acid promoter.     

Developing carbon Lewis base/boron Lewis acid frustrated Lewis pair chemistry derived from conjugated dienamines

Several dienamines R₂N-CH=CH-CH=CMe₂ react with Piers' borane by HB(C₆F₅)₂ addition to the terminal -CH=CMe₂ unit to form the isopropyl substituted products 14. The strongly electrophilic -B(C₆F₅)₂ functionality at the C3 position of the chain shows a marked contact to the adjacent enamine β-carbon atom C2. This generates a pronounced C/B FLP character which chemically comes to bear in the 1,2-addition reactions to the C=O bond of benzaldehyde or the -C≡N bonds of a series of alkylcyanides. In some cases, the formation of uncommon rearrangement products is observed.     

Lewis base activation of Lewis acids. Vinylogous aldol reactions

A highly regioselective vinylogous aldol reaction catalyzed by SiCl4 and a chiral phosphoramide (R,R)-5, providing delta-hydroxy enones for a variety of aldehyde and dienol ether structures, has been developed. Low catalyst loadings (1 mol %) can be employed, giving the products in good yields, excellent enantioselectivities, and in some cases excellent anti diastereoselectivities. Both simple ester-derived dienol ethers as well as dioxanone-derived dienol ethers are employed. The observed regioselectivity is rationalized in terms of the sensitivity of the catalyst to the steric demands of the nucleophile.     

The Behavior of Acid Halides Toward Lewis Acids and Lewis Bases

Reactions of some typical acid halides of carbonic and trithiocarbonic acids and of orthophosphoric and sulfuric acids with Lewis acids and Lewis bases are compared. Acylium, perfluoroacylium, thioacylium, and even sulfonylium ions are obtainable with Lewis acids. It is possible by conductivity measurements and by electronic and above all IR spectroscopic investigations to determine whether the 1:1 adducts of acid halides and Lewis compounds are acylium or sulfonylium salts or donor-acceptor complexes. In the reaction with Lewis bases, the halogen atom in the acid halide is replaced by the electron donor, generally with formation of nonpolar molecular compounds or complexes.     

Cooperative effects in the complexation of anions and Lewis bases by a heteronuclear bifunctional Lewis acid

As shown by the complexation of electron-rich substrates to the heteronuclear bifunctional Lewis acid bis(mu-1,8-naphthalenediyl)(mu-chloride)methyltin-chlorogallium (1), the primary Lewis acidic site of this molecule is the triorganotin chloride moiety whose electron deficiency is enhanced through partial abstraction of its chloride ligand by the neighbouring gallium centre.     

Lewis酸碱对在聚合中的应用

自从Stephan和Erker两位科学家提出“位阻型(Frustrated)Lewis酸碱对”概念以来,Lewis酸碱对的催化化学得到极大的关注。近年来,人们也发现Lewis酸碱对在催化极性乙烯基单体和内酯单体聚合中有着重要的应用。Lewis酸碱对催化极性乙烯基单体聚合可形成具有高分子量和窄分子量分布的聚合物,而催化活性与所使用的Lewis酸碱对关系密切,最有效的Lewis酸是Al(C₆F₅)₃和B(C₆F₅)₃,Lewis碱是有机磷、氮杂环卡宾和氮杂环卡宾烯和膦腈超强碱,可聚合的单体包括甲基丙烯酸甲酯、γ-甲基-α-亚甲基-γ-丁内酯、α-亚甲基-γ-丁内酯、 丙烯酸正丁酯、 N,N-二甲基丙烯酰胺、 N,N-二苯基丙烯酰胺、 乙烯基磷酸二乙酯、2-乙烯基吡啶、2-异丙烯基-2-氧 NFDA1 唑林以及非对称的极性二乙烯基单体。聚合过程包括:链引发、链增长和链终止。链引发过程是通过Lewis酸、Lewis碱和单体相互作用形成两性离子,链增长是通过双金属活化单体加成方式进行,链终止通过两种途径:1)增长聚合物链中活化的酯氧负离子对相邻酯中羰基碳原子的亲核进攻形成δ-戊内酯;2)增长聚合物链中活化的烯酯碳负离子对倒数第三个酯中羰基碳原子的亲核进攻形成β-酮酸酯。Lewis酸碱对催化内酯单体聚合可形成线形和环状聚合物,所使用的Lewis酸为Zn(C₆F₅)₃、有机铝、氯化铟,Lewis碱为有机胺。     

Investigation of Lewis Acid versus Lewis Base Catalysis in Asymmetric Cyanohydrin Synthesis

The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}₂] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO₃ or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}₂O] and triphenylphosphine oxide, a non‐linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron‐deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case.     

Lewis base ionic liquids

Ionic liquids which are (weak) Lewis bases have a number of interesting and useful properties different to those of traditional ionic liquids, including volatility and the possibility of being distillable in some cases, a base catalysis effect in others and enhancement of the acidity of dissolved acids.     

Fullerene-carbene Lewis acid-base adducts

The reaction between a bulky N-heterocylic carbene (NHC) and C(60) leads to the formation of a thermally stable zwitterionic Lewis acid-base adduct that is connected via a C-C single bond. Low-energy absorption bands with weak oscillator strengths similar to those of n-doped fullerenes were observed for the product, consistent with a net transfer of electron density to the C(60) core. Corroborating information was obtained using UV photoelectron spectroscopy, which revealed that the adduct has an ionization potential ～1.5 eV lower than that of C(60). Density functional theory calculations showed that the C-C bond is polarized, with a total charge of +0.84e located on the NHC framework and -0.84e delocalized on the C(60) cage. The combination of reactivity, characterization, and theoretical studies demonstrates that fullerenes can behave as Lewis acids that react with C-based Lewis bases and that the overall process describes n-doping via C-C bond formation.     

Bifunctional Gyroidal MOFs: Highly Efficient Lewis Base and Lewis Acid Catalysts

A family of gyroidal metal–organic frameworks (STUs) composited with transition metal ions and bi‐imidazolate ligands (BIm) were prepared and applied as both Lewis base and acid catalysts. Benefiting from the intrinsic basicity of the ligands and the Lewis acidic sites of the open metal centres, the STUs materials show excellent catalytic activities as Lewis base for the Knoevenagel condensation reaction between various aldehydes and malononitrile, and as Lewis acid for cyanosilylation reactions. Among these STUs, STU‐4 (Ni(BIm)) shows the best catalytic efficiency (conversions >99 %) in both Knoevenagel condensation and cyanosilylation reactions under mild conditions, providing thus an advanced material for both Lewis base and Lewis acid catalysis.     

Lewis acidic organoboron polymers

We have developed a highly efficient new method for the introduction of Lewis acidic boron centers into the side chains of organic polymers. Our methodology involves three steps: (i) the controlled polymerization of a functional monomer, (ii) the exchange of the functional group for Lewis acidic boron centers, and (iii) the fine-tuning of the Lewis acidity of the individual centers through substituent exchange reactions with nucleophiles. This approach gives access to a family of new well-defined organoboron polymers including moderately Lewis acidic poly(arylboronates) and the first examples of highly Lewis acidic fluorinated triarylborane polymers.     

Lewis base stabilized phosphanylborane

The abstraction of the Lewis acid from [W(CO)(5)(PH(2)BH(2)NMe(3))] (1) by an excess of P(OMe(3))(3) leads to the quantitative formation of the first Lewis base stabilized monomeric parent compound of phosphanylborane [H(2)PBH(2)NMe(3)] 2. Density functional theory (DFT) calculations have shown a low energetic difference between the crystallographically determined antiperiplanar arrangement of the lone pair and the trimethylamine group relative to the P-B core and the synperiplanar conformation. Subsequent reactions with the main-group Lewis acid BH(3) as well as with an [Fe(CO)(4)] unit as a transition-metal Lewis acid led to the formation of [(BH(3))PH(2)BH(2)NMe(3)] (3), containing a central H(3)B-PH(2)-BH(2) unit, and [Fe(CO)(4)(PH(2)BH(2)NMe(3))] (4), respectively. In oxidation processes with O(2), Me(3)NO, elemental sulfur, and selenium, the boranylphosphine chalcogenides [H(2)P(Q)BH(2)NMe(3)] (Q = S 5 b; Se 5 c) as well as the novel boranyl phosphonic acid [(HO)(2)P(O)BH(2)NMe(3)] (6 a) are formed. All products have been characterized by spectroscopic as well as by single-crystal X-ray structure analysis.     

Preorganized frustrated Lewis pairs

Geminal frustrated Lewis pairs (FLPs) are expected to exhibit increased reactivity when the donor and acceptor sites are perfectly aligned. This is shown for reactions of the nonfluorinated FLP tBu(2)PCH(2)BPh(2) with H(2), CO(2), and isocyanates and supported computationally.     

Lewis acid - assisted high speed living anionic polymerization by the aluminum porphyrin - Lewis acid systems

Living anionic polymerization of methacrylic esters initiated with aluminum porphyrins ( 1 ) was found to be dramatically accelerated (e.g., 46 000 times) upon addition of bulky Lewis acids such as alkylaluminum diphenolates ( 3 ). Under appropriate conditions, a narrow MWD poly(methyl methacrylate) with Mn exceeding a million could be synthesized. The key conception of this high speed living polymerization is the coexistence of nucleophile ( 2 ) and Lewis acid ( 3 ) realized by steric repulsion.     

Synthesis of Lewis a and Lewis X Pentasaccharides Based on N-Trichloroethoxycarbonyl Protection 1

Abstract

Thexyldimethylsilyl 4,6-O-benzylidene-2-deoxy-2-trichloroethoxycarbonylamino-β-D- glucopyranoside (4), having the 3-hydroxy group unprotected, is a versatile starting material for the synthesis of glucosamine containing oligosaccharides. Thus, reaction with galactosyl donor 5 or fucosyl donor 6 afforded the desired β(1-3)- and α(1-3)-linked disaccharides 7 and 8, respectively, in high yields. Reductive opening of the benzylidene moieties in 7 and 8 gave access to the 4-hydroxy groups in 9 and 10. Ensuing fucosylation of 9 or galactosylation of 10 led to Lewis A (Le^a) and Lewis X (Le^x) trisaccharide building blocks 13 and 14, respectively. Their transformation into glycosyl donors 19 and 20 and subsequent reaction with 3b-O-unprotected lactose derivative 23 as acceptor furnished the Le^a? and Le^x pentasaccharide precursors 24 and 25. Exchange of the N-trichloroethoxycarbonyl group for an N-acetyl group and removal of the O-benzyl and O-acetyl protective groups afforded the desired Le^a? and Le^x? pentasaccharides 1 and 2.