Direct catalytic enantio- and diastereoselective aldol reaction of thioamides

A direct catalytic asymmetric aldol reaction of thioamides using a soft Lewis acid/hard Br?nsted base cooperative catalyst comprising (R,R)-Ph-BPE/[Cu(CH(3)CN)(4)]PF(6)/LiOAr is described. Exclusive enolate generation from thioacetamides through a soft-soft interaction with the soft Lewis acid allowed for a direct aldol reaction to α-nonbranched aliphatic aldehydes, which are usually susceptible to self-condensation under conventional basic conditions. A hard Lewis basic phosphine oxide has emerged as an effective additive to constitute a highly active ternary soft Lewis acid/hard Br?nsted base/hard Lewis base cooperative catalyst, enabling a direct enantio- and diastereoselective aldol reaction of thiopropionamides. Strict control of the amount of the hard Lewis base was essential to drive the catalytic cycle efficiently with a minimized retro-aldol pathway, affording syn-aldol products with high stereoselectivity. Divergent transformation of the thioamide functionality is an obvious merit of the present aldol methodology, allowing for a facile transformation of the aldol product into the corresponding aldehyde, ketone, amide, amine, and ketoester. An aldehyde derived from the direct aldol reaction was subjected to a second direct aldol reaction, which proceeded in a catalyst-controlled manner to provide 1,3-diols with high stereoselectivity.     

Regioselective hydrosilylations of propiolate esters with tris(trimethylsilyl)silane

Lewis acid and substituent dependency on the regioselectivity of hydrosilylation of propiolate esters 1a-c with tris(trimethylsilyl)silane (2a) was found. The reaction of methyl and ethyl propiolate esters and 2a without Lewis acid and in the presence of EtAlCl2 and Et2AlCl gave beta-silicon-substituted Z-alkenes 3 selectively. On the other hand, reaction in the presence of AlCl3 in dichloromethane gave alpha-silicon-substituted alkenes 4. In the case of trifluoroethyl propiolate ester 1c, reaction with aluminum chloride-based Lewis acids gave alpha-silicon-substituted alkenes 4 exclusively. Two competitive mechanisms, free-radical and ionic, are proposed as the source of the complementary regioselectivity displayed in these reactions. A transition state of the radical-forming step was obtained computationally. The reaction of various reactive acetylene substrates and 2a without Lewis acid and without solvent at room temperature gave beta-silicon-substituted Z-alkenes 3 selectively.     

Exploring the synthetic versatility of the Lewis acid induced decomposition reaction of α-diazo-β-hydroxy esters. The case of ethyl diazo(3-hydroxy-2-oxo-2,3-dihydro-1H-indol-3-yl)acetate

Ethyl diazo(3-hydroxy-2-oxo-2,3-dihydro-1H-indol-3-yl)acetate was prepared by aldol-type condensation of ethyl diazoacetate with isatin. A systematic and mechanistic study on the Lewis acid induced decomposition reaction of this valuable diazo precursor was carried out with the aim to gain new insights into the mechanistic aspects of the reaction as well as to further understand the factors and experimental conditions which affect the relative product distribution. The reaction, which may proceed via cationic and noncationic mechanisms, was found to be significantly influenced by the reaction environment determined by the characteristics of the Lewis acid employed, by the ability of the Lewis acid to form a complex with the alcohol functionality of the α-diazo-β-hydroxy ester, and by the polarity and nucleophilicity of the solvent used.     

Enantioselective radical cyclizations: a new approach to stereocontrol of cascade reactions

Stereocontrol in a cascade radical addition-cyclization-trapping reaction was achieved by a new approach, which utilizes a hydroxamate ester moiety as a coordinating chiral Lewis acid tether between two radical acceptors. A remarkable feature of this reaction is the construction of three bonds and tertiary and quaternary stereogenic centers through both inter- and intramolecular carbon-carbon bond-forming processes. The chiral Lewis acid mediated reaction of oxime ethers also proceeded smoothly with good enantio- and diastereoselectivities, indicating the usefulness of the cascade approach for the asymmetric synthesis of various gamma-lactams.     

The one-pot, multi-component construction of highly substituted tetrahydropyran-4-ones using the Maitland-Japp reaction

A one-pot, multi-component reaction for the synthesis of highly substituted tetrahydropyran-4-ones, based on the long forgotten Maitland-Japp reaction has been realised. Two different aldehydes and a derivative of a beta-ketoester can be condensed regioselectively in the presence of a Lewis acid to form tetrahydropyran-4-ones in excellent yields. The diastereoselectively of the reaction was found to be dependant upon the nature of the Lewis acid and the temperature at which the reaction was carried out. This procedure was also extended to the formation of tetrahydropyran-4-ones in greater than 95% enantiomeric excess.     

Synthesis of 3-substituted indole by AlCl3-promoted reaction of β,γ-unsaturated ketone with indole

A one-pot Lewis acid-promoted reaction condition of β,γ-unsaturated ketone with indole was developed for the synthesis of 3-substituted indoles with moderate to good yields. A Lewis acid such as AlCl₃ was shown to be a promising promoter for in situ isomerization of β,γ-unsaturated ketone to its corresponding α,β-unsaturated ketone, then undergoing Friedel–Crafts Michael addition reaction with indole to afford 3-substituted indole.     

Lewis acid-mediated Michaelis-Arbuzov reaction at room temperature: a facile preparation of arylmethyl/heteroarylmethyl phosphonates

A facile preparation of arylmethyl and heteroarylmethyl phosphonate esters was achieved involving a Lewis acid mediated Michaelis-Arbuzov reaction at room temperature. Interaction of arylmethyl halides/alcohols with triethyl phosphite in the presence of Lewis acid at room temperature afforded phosphonate esters in good yields.     

A chiral auxiliary derived from levoglucosenone in asymmetric Diels-Alder transformations

The Diels-Alder reaction of the acrylate derived from levoglucosenone with cyclopentadiene was studied under several conditions, in the presence and absence of a Lewis acid. The results showed satisfactory diastereomeric excess and the ratio of cycloadducts was found to depend on the reaction conditions. A reversal in stereoselectivity was observed when EtAlCl₂ or Et₂AlCl were employed as Lewis acids.     

The effects of Lewis acid on the 1,3-dipolar cycloaddition reaction of C-arylaldonitrones with alkenes

The regio- and stereoselectivity of the 1,3-dipolar cycloaddition reactions of C-aryl-N-alkylaldonitrones (1a-e) with some alkenes were found to be affected significantly by the addition of Lewis acid. The rate of the reaction was also affected by adding the Lewis acid. In the reactions using allyl alcohol as a dipolarophile an addition of Lewis acid caused a remarkable acceleration of the reaction and a great change in the stereoselectivity. In the reactions using ethyl acrylate as a dipolarophile the regioselectivity was reversed whether the reaction was performed in the presence or the absence of Lewis acid; i.e. isoxazolidine-5-carboxylates were obtained mainly in the absence of Lewis acid although isoxazolidine-4-carboxylates were obtained mainly in the presence of Lewis acid. When the reaction of C,N-diarylaldonitrones (1k, 1m, 1n) with ethyl acrylate was carried out in the presence of Lewis acid, the cleavage of the N-O bond of the cycloadducts giving gamma-aminoalcohols was also observed besides a reverse phenomenon of regioselectivity.     

Carbopyronine荧光染料中间体的有效合成方法

在非质子路易斯酸的辅助下高收率的合成了Carbopyronine荧光染料中间体2,7-双二甲氨基-9,10-二氢-9,9-二甲基蒽环。反应关键步骤是路易斯酸辅助下的环化反应,考查了几种路易斯酸的效果。确立了环化反应的最佳条件为： 2.5mol的路易斯酸与1mol的叔醇2在0 ˚C反应6 h,再于室温下反应10 h。     

Lewis碱协助的 Lewis酸催化醇脱水合成烯烃及二氢吡喃开环合成2-肉桂基取代1,3-二羰基化合物

醇脱水是合成烯烃的重要方法之一。全球每年约有15%的苯乙烯是通过1-苯乙醇在酸性条件下脱水反应生产。虽然人们对该反应进行了较为深入的研究,但是当使用活性较高的1-苯乙醇衍生物为底物时,由于得到的取代苯乙烯产物具有较高的反应性,在脱水过程中会发生聚合而导致反应选择性降低,因此有必要探索适宜在高活性1-苯基乙醇脱水反应中应用的催化剂体系。
　　本文借助酸碱协同催化方法考察了1-(4-甲氧基苯基)乙醇制备4-甲氧基苯乙烯的反应。发现三苯基磷与 AlCl3构建的 Lewis碱/Lewis酸协同催化体系在硝基甲烷中可以接近定量的收率得到4-甲氧基苯乙烯。 Lewis碱/Lewis酸协同催化体系有效避免了4-甲氧基苯乙烯的二聚现象。底物拓展研究显示该方法具有很好的底物普适性,对多种取代苯乙烯的收率均超过80%。机理研究表明,1-(4-甲氧基苯基)乙醇在酸作用下先生成碳正离子,三苯基磷作为偶极性的电子给体不但能在一定程度上稳定该苄基碳正离子,而且抑制了其与4-甲氧基苯乙烯之间的亲电反应,进而最大化了脱质子生成4-甲氧基苯乙烯的选择性。
　　将Lewis碱协助的 Lewis酸催化提高反应选择性策略用于2-苯基-3,4-二氢吡喃衍生物合成2-肉桂基-1,3-二羰基化合物的开环反应。该类取代二氢吡喃在酸催化剂作用下也可生成苄基碳正离子,但是该中间体易受分子间和分子内亲电反应影响,反应选择性不高。而当使用单质碘/三苯基磷协同催化体系时,2-苯基-3,4-二氢吡喃衍生物能高选择性地实现开环反应,得到反式2-肉桂基-1,3-二羰基化合物。该类1,3-二羰基化合物具有丰富的反应性,是一类重要的合成子。     

Reactions of ferricinium salts with Lewis bases

A reaction of the ferricinium radical cation with Lewis bases leads to substitution in the cyclopentadienyl ring with the formation of ferrocene derivatives only when the radicals are formed from the Lewis bases in the process of the reaction.     

Multisite catalysis: a mechanistic study of beta-lactone synthesis from epoxides and CO--insights into a difficult case of homogeneous catalysis

Carbonylation of epoxides with a combination of Lewis acids and cobalt carbonyls was studied by both theoretical and experimental methods. Only multisite catalysis opens a low-energy pathway for trans opening of oxirane rings. This ring-opening reaction is not easily achieved with a single-site metal catalyst due to structural and thermodynamic constraints. The overall reaction pathway includes epoxide ring opening, which requires both a Lewis acid and a tetracarbonylcobaltate nucleophile, yielding a cobalt alkyl-alkoxy-Lewis acid moiety. After CO insertion into the Co-C(alkyl) bond, lactone formation results from a nucleophilic attack of the alkoxy Lewis acid entity on the acylium carbon atom. A theoretical study indicates a marked influence of the Lewis acid on both ring-opening and lactone-formation steps, but not on carbonylation. Strong Lewis acids induce fast ring opening, but slow lactone formation, and visa versa: a good balance of Lewis acidity would give the fastest catalytic cycle as all steps have low barriers. Experimentally, carbonylation of propylene oxide to beta-butyrolactone was monitored by online ATR-IR techniques with a mixture of tetracarbonylcobaltate and Lewis acids, namely BF(3), Me(3)Al, Et(2)Al(+).diglyme, and a combination of Me(3)Al/dicobaltoctacarbonyl. We found that the last two mixtures are extremely active in lactone formation.     

Catalyst-free multicomponent Strecker reaction in acetonitrile

The multicomponent Strecker reaction using trimethylsilyl cyanide was accomplished without any type of Lewis acid. The reaction performed in acetonitrile as solvent gave excellent results for any class of aldehydes (aromatic or aliphatic), as well as amines (aromatic or aliphatic). In many cases, α-aminonitrile product was isolated pure after the usual work-up, with quantitative chemical yields. A comparison between different solvents indicated that acetonitrile is the best choice. The rate comparison using different Lewis acids showed that all of them catalyzed the reaction in a similar extent, the difference with the acid Lewis-free being minimal.     

Synthesis of precursors of the agalacto (exo) fragment of the quartromicins via an auxiliary-controlled exo-selective Diels-Alder reaction

[reaction: see text] A direct synthesis of the alpha-hydroxyaldehyde exo-5, a precursor of the exo-spirotetronate subunit of the quartromicins, was achieved through an exo-selective Lewis acid-catalyzed Diels-Alder reaction of dienophile 12a and diene 1.     

Lewis Acid mediated reactions of zirconacyclopentadienes with aldehydes: one-pot synthetic route to indene and cyclopentadiene derivatives from aldehydes and benzyne or alkynes

One-pot procedures for the preparation of highly substituted indenes, tetrahydroindenes, and cyclopentadienes have been developed by using a combination of zirconocene-mediated Cbond;C-bond-forming reactions with Lewis acid mediated activation of carbonyl groups. The carbonyl groups of aldehydes were deoxygenated in the reaction and behaved formally as a one-carbon unit. A variety of Lewis acids were checked and showed different reactivities in this reaction.     

Domino "[3+3]-cyclization-homo-Michael" reactions of 1,3-bissilyl enol ethers with 1,1-diacylcyclopropanes

The Lewis acid mediated domino "[3+3]-cyclization-homo-Michael" reaction of 1,3-bissilyl enol ethers with 1,1-diacylcyclopropanes allows an efficient one-pot synthesis of functionalized salicylates containing a halogenated side chain. A great variety of substitution patterns could be realized by variation of the starting materials and of the Lewis acid. The mechanism of the domino process was studied.     

A new carbon—phosphorous bond forming reaction and synthesis of aminoalkylphosphonic acid derivatives

A new carbon—phosphorous bond forming reaction, that is, the reaction of α-methoxyurethanes with trialkyl phosphites in the presence of Lewis acid catalysts was studied on twelve examples.     

A theoretical study of imine-ene reaction influencing factors

Potential energy surfaces of the imine-ene reaction between methanimine and propene have been calculated using restricted and unrestricted density functional theory at the B3LYP level. The results show that a concerted mechanism with an exo configuration for lone pair electrons transition structure is more favourable for the intermolecular bare imine-ene reaction. In addition, the C-C bond formation and the migration of the propene α-hydrogen occur simultaneously. The promoting effect of seventeen Lewis acids was also studied. When a Lewis acid is used as a promoter, the corresponding activation and reaction energies decline greatly compared to those of the bare imine-ene reaction. A good correlation (R(2) > 0.9) was found between the activation barriers and the polar character of the transition states. Lewis acids and electron-withdrawing substituents on methanimine were proven to facilitate the imine-ene reaction, whereas electron-donating groups, conjugated groups or a bulky group hinders the reaction. The steric factor is the most unfavourable. The reactivity indices defined by the conceptual density functional theory were also studied and there is a good correlation between the activation barrier and electrophilicity. Frontier molecular orbital theory gives a good explanation for the above results.     

Quantum-chemical study of the Lewis acid influence on the cycloaddition of benzonitrile oxide to acetonitrile, propyne and propene

Quantum chemical methods (MP2 and B3LYP) together with a topological analysis of the charge density have been used to study the BH₃- or BF₃-mediated reaction of benzonitrile oxide with acetonitrile, propyne and propene. In the reaction with propene or propyne, addition of Lewis acids has only little influence on the outcome of the reactions. The cycloaddition of nitrile oxides with nitriles, however, is generally promoted by strong Lewis acids. When the Lewis acid coordination takes place at the nitrile oxide the reactant is activated and the product binds weakly to the Lewis acid so that the reaction is expected to be catalytic. In the case of coordination to the nitrile the reaction is Lewis acid mediated. Here the reactant is not much influenced by addition of Lewis acid, but the transition state and the product are stabilised and consequently such processes require a stoichiometric amount of Lewis acid and form a stable Lewis acid-product complex.It has also been demonstrated that the different activation routes for these reactions involve different reaction mechanisms. Whereas the reaction of a Lewis acid coordinated nitrile oxide is of ‘inverse electron demand’, the Lewis acid coordinated nitrile reacts through a ‘normal electron demand’ cycloaddition.