Mild and efficient boronic acid catalysis of Diels-Alder cycloadditions to 2-alkynoic acids

The concept of boronic acid catalysis (BAC) for the activation of unsaturated carboxylic acids is applied to the Diels-Alder cycloaddition between 2-alkynoic acids as dienophiles and various dienes. These [4+2] cycloadditions produce cyclohexadienyl carboxylic acids, which can be oxidized in situ to produce polysubstituted aromatic carboxylic acids. The boronic acid catalyst is suspected to provide activation by a LUMO-lowering effect of the unsaturated carboxylic acid likely via a covalent, monoacylated hemiboronic ester intermediate.     

Mechanism of cis-dihydroxylation and epoxidation of alkenes by highly H(2)O(2) efficient dinuclear manganese catalysts

In the presence of carboxylic acids the complex [Mn(IV)2(micro-O)3(tmtacn)2]2+ (1, where tmtacn = N,N',N'-trimethyl-1,4,7-triazacyclononane) is shown to be highly efficient in catalyzing the oxidation of alkenes to the corresponding cis-diol and epoxide with H2O2 as terminal oxidant. The selectivity of the catalytic system with respect to (w.r.t.) either cis-dihydroxylation or epoxidation of alkenes is shown to be dependent on the carboxylic acid employed. High turnover numbers (t.o.n. > 2000) can be achieved especially w.r.t. cis-dihydroxylation for which the use of 2,6-dichlorobenzoic acid allows for the highest t.o.n. reported thus far for cis-dihydroxylation of alkenes catalyzed by a first-row transition metal and high efficiency w.r.t. the terminal oxidant (H2O2). The high activity and selectivity is due to the in situ formation of bis(micro-carboxylato)-bridged dinuclear manganese(III) complexes. Tuning of the activity of the catalyst by variation in the carboxylate ligands is dependent on both the electron-withdrawing nature of the ligand and on steric effects. By contrast, the cis-diol/epoxide selectivity is dominated by steric factors. The role of solvent, catalyst oxidation state, H2O, and carboxylic acid concentration and the nature of the carboxylic acid employed on both the activity and the selectivity of the catalysis are explored together with speciation analysis and isotope labeling studies. The results confirm that the complexes of the type [Mn2(micro-O)(micro-R-CO2)2(tmtacn)2]2+, which show remarkable redox and solvent-dependent coordination chemistry, are the resting state of the catalytic system and that they retain a dinuclear structure throughout the catalytic cycle. The mechanistic understanding obtained from these studies holds considerable implications for both homogeneous manganese oxidation catalysis and in understanding related biological systems such as dinuclear catalase and arginase enzymes.     

Boronic Acid Catalysis for Mild and Selective [3+2] Dipolar Cycloadditions to Unsaturated Carboxylic Acids

Herein, the concept of boronic acid catalysis (BAC) for the activation of unsaturated carboxylic acids is applied in several classic dipolar [3+2] cycloadditions involving azides, nitrile oxides, and nitrones as partners. These cycloadditions can be used to produce pharmaceutically interesting, small heterocyclic products, such as triazoles, isoxazoles, and isoxazolidines. These cycloadducts are formed directly and include a free carboxylic acid functionality that can be employed for further transformations, thereby avoiding prior masking or functionalization. In all cases, BAC provides faster reactions, under milder conditions, with much improved product yields and regioselectivities. In some instances, such as triazole formation from the reaction of azides with 2‐alkynoic acids, catalysis with ortho‐nitrophenylboronic acid circumvents the undesirable product decarboxylation observed when using thermal activation. By using NMR spectroscopic studies, the boronic acid catalyst was shown to provide activation by a LUMO‐lowering effect in the unsaturated carboxylic acid, likely via a monoacylated hemiboronic ester intermediate.     

Modeling acid and cationic catalysis on the reactivity of duocarmycins

Several catalyzed alkylation reactions of 9-methyladenine by a model [CPI, cyclopropa[c]pyrrolo[3,2-e]indol-4(5H)-one (1)] of duocarmycin anticancer drugs have been compared to the uncatalyzed reaction in gas phase and in water solvent bulk, using density functional theory at the B3LYP level with the 6-31+G(d,p) basis set and C-PCM solvation model. The effect on the CPI reactivity induced by water, formic and phosphoric acids (general acid catalysis), H3O+ (specific acid catalysis), sodium, and ammonium cation complexation (cationic catalysis) has been investigated. The calculations indicate that the specific acid catalysis and the catalysis induced by sodium cation complexation are strong in the gas phase, but solvation reduces them dramatically by electrostatic effects. The specific acid catalysis is still operative, but strongly reduced in water solution, where the reaction barrier is reduced by 8.6 kcal mol(-1) in comparison to the uncatalyzed reaction. The general acid catalysis induced by phosphoric acid (-7.3 kcal mol(-1)) and the catalysis induced by Na+ and NH4+ complexation become competitive, with a catalytic effect of -3.6 and -4.1 kcal mol(-1) in water, respectively. With the specific acid catalysis, the high acidity (low pK(a) value) of the conjugated acid of CPI (CPIH+), computed in water solution using both C-PCM (pK(a) = +2.6) and PCM-B3LYP/6-31+G(d,p) (pK(a) = +2.4) solvation models, suggests that the catalytic effects induced by NH4+ complexation could become more important than the specific acid catalysis and the general catalysis by H3PO4 under physiological conditions, due to concentration effects of the catalysts.     

Activation of Carboxylic Acids in Asymmetric Organocatalysis

Organocatalysis, catalysis using small organic molecules, has recently evolved into a general approach for asymmetric synthesis, complementing both metal catalysis and biocatalysis. 1 Its success relies to a large extent upon the introduction of novel and generic activation modes. 2 Remarkably though, while carboxylic acids have been used as catalyst directing groups in supramolecular transition‐metal catalysis, 3 a general and well‐defined activation mode for this useful and abundant substance class is still lacking. Herein we propose the heterodimeric association of carboxylic acids with chiral phosphoric acid catalysts as a new activation principle for organocatalysis. This self‐assembly increases both the acidity of the phosphoric acid catalyst and the reactivity of the carboxylic acid. To illustrate this principle, we apply our concept in a general and highly enantioselective catalytic aziridine‐opening reaction with carboxylic acids as nucleophiles.     

cis-Dihydroxylation and epoxidation of alkenes by [Mn(2)O(RCO(2))(2)(tmtacn)(2)]: tailoring the selectivity of a highly H(2)O(2)-efficient catalyst

The carboxylic acid promoted cis-dihydroxylation and epoxidation of alkenes catalyzed by [MnIV2O3(tmtacn)2]2+ 1 employing H2O2 as oxidant is described. The use of carboxylic acids at cocatalytic levels not only is effective in suppressing the inherent catalase activity of 1, but also enables the tuning of the catalyst's selectivity. Spectroscopic studies and X-ray analysis confirm that the control arises from the in situ formation of carboxylate-bridged dinuclear complexes, for example, 2 {[MnIII2O(CCl3CO2)2(tmtacn)2]2+} and 3 {[MnII2(OH)(CCl3CO2)2(tmtacn)2]+}, during catalysis. For the first time, the possibility to tune, through the carboxylate ligands employed, both the selectivity and activity of dinuclear Mn-based catalysts is demonstrated. To our knowledge, the system 1/2,6-dichlorobenzoic acid (up to 2000 turnover numbers for cis-cyclooctanediol) is the most active Os-free cis-dihydroxylation catalyst reported to date.     

Kinetic mechanism of acetyl-CoA synthase: steady-state synthesis at variable Co/Co2 pressures.

Steady-state initial rates of acetyl-CoA synthesis (upsilon/[E(tot)]) catalyzed by acetyl-CoA synthase from Clostridium thermoaceticum (ACS) were determined at various partial pressures of CO and CO2. When [CO] was varied from 0 to 100 microM in a balance of Ar, rates increased sharply from 0.3 to 100 min(-1). At [CO] > 100 microM, rates declined sharply and eventually stabilized at 10 min(-1) at 980 microM CO. Equivalent experiments carried out in CO2 revealed similar inhibitory behavior and residual activity under saturating [CO]. Plots of upsilon/[E(tot)] vs [CO2] at different fixed inhibitory [CO] revealed that Vmax/[E(tot)] (kcat) decreased with increasing [CO]. Plots of upsilon/[E(tot)] vs [CO2] at different fixed noninhibitory [CO] showed that Vmax/[E(tot)] was insensitive to changes in [CO]. Of eleven candidate mechanisms, the simplest one that fit the data best had the following key features: (a) either CO or CO2 (at a designated reductant level and pH) activate the enzyme (E' + CO right arrow over left arrow E, E' + CO2/2e-/2H+ right arrow over left arrow E); (b) CO and CO2 are both substrates that compete for the same enzyme form (E + CO right arrow over left arrow ECO, E + CO2/2e-/2H+ right arrow over left arrow ECO, and ECO --> E + P); (c) between 3 and 5 molecules of CO bind cooperatively to an enzyme form different from that to which CO2 and substrate CO bind (nCO + ECO right arrow over left arrow (CO)nECO), and this inhibits catalysis; and (d) the residual activity arises from either the (CO)nECO state or a heterogeneous form of the enzyme. Implications of these results, focusing on the roles of CO and CO2 in catalysis, are discussed.     

Design of highly enantioselective organocatalysts based on molecular recognition

[reaction: see text] Various organocatalysts have been designed based on molecular recognition to catalyze the asymmetric direct aldol reaction of ketones with aryl and alkyl alpha-keto acids, affording beta-hydroxyl carboxylic acids with a tertiary stereogenic center with excellent enantioselectivities of up to 98% ee. A linear effect was observed for the reaction, demonstrating a single molecule of the catalyst involved in the catalysis.     

Electrolytic Fixation of CO2 by Electrocarboxylation of RX on Nanocrystalline TiO2-Pt Cathode

Electrolytic fixation of CO2 was investigated by electrocarboxylation of organic halides (RX), and four esters Ⅰ,Ⅱ Ⅲ, Ⅳ) were obtained in moderate yields. Electrochemical reduction esterifications of RX in the presence of CO2 were carried out on nanocry‘stalline TiO2-Pt electrode. The electrochemical behavior of RX in the presence of CO2 was investigated by the technique of cyclic voltammetry, and the probable reaction mechanism w,as proposed.     

钛铝载体的合成及负载CuO对NO催化性能研究

以TiCl4为原料合成了TiO2/[[alpha]]-Al2O3载体. 在色谱-微反流动法反应装置上考察了CuO/15%(w, 下同)TiO2/[alpha]-Al2O3系列催化剂对NO+CO 的反应性能. 结果表明上述催化剂对NO+CO 反应表现出较好的活性, 其中12%CuO/15%TiO2/[alpha]-Al2O3反应活性最佳. 空气和H2 预处理后, NO 完全转化的温度分别为300C[[deg]]和275C[deg].通过H2-TPR、XRD 和FT-IR 等技术表征, 发现适量TiO2能促进CuO 在钛铝载体上的分散. TPR 结果显示12%CuO/15%TiO2/[alpha]-Al2O3在整个TPR 过程中出现四个还原峰, 琢和酌还原峰分别是钛铝载体表面裸露的TiO2上高度分散的CuO 和晶相CuO 的还原；茁和啄还原峰为钛铝载体上高度分散的CuO 和晶相CuO 的还原. FT-IR实验表明NO和CO 在经H2气氛预处理的催化剂表面上吸附较强, 且生成了N2O 和NO2等物种；NO+CO混合气在经空气和H2预处理的催化剂表面吸附时, 出现了N2O吸收峰, 峰温分别为200C[deg]和150C[deg].     

4,5,6,7-Tetrachlorobenzo[d][1,3,2]dioxaborol- 2-ol as an effective catalyst for the amide condensation of sterically demanding carboxylic acids

[reaction: see text] 4,5,6,7-Tetrachlorobenzo[d][1,3,2]dioxaborole (4a) and 4,5,6,7-tetrachlorobenzo[d][1,3,2]dioxaborol-2-ol (4b) are effective catalysts for the dehydrative amide condensation between an equimolar mixture of carboxylic acids and amines. In particular, these catalysts are greatly superior to 3,5-bis(trifluoromethyl)phenylboronic acid (1) for the amide condensation of sterically demanding carboxylic acids. In contrast, 4c, which is prepared from a 1:2 molar mixture of B(OH)(3) and tetrachlorocatechol, is effective as a Lewis acid-assisted Br?nsted acid (LBA) catalyst for Ritter reaction.     

Boric Acid‐Catalyzed Direct Condensation of Carboxylic Acids with Benzene‐1,2‐diamine into Benzimidazoles

The applicability of boric acid catalysis for the direct condensation of carboxylic acids with benzene‐1,2‐diamine to give 2‐substituted benzimidazoles was investigated. It was found that catalytic amounts (5–10 mol‐%) of boric acid efficiently promote the cyclocondensation of aliphatic carboxylic acids in refluxing toluene. In addition, the relatively neutral conditions allow the use of acid‐sensitive substrates and give rise to specific transformations and selectivities that are not observed with some classical methods. Benzoic acids were found to be less reactive than aliphatic acids and thus require refluxing xylene for better efficiency. Phenylboronic acid was found to be inactive as a catalyst due to its rapid consumption by condensation with benzene‐1,2‐diamine to give a 2‐phenylbenzodiazaborole.     

Enantioselective conjugate addition of donor-acceptor hydrazones to α,β-unsaturated aldehydes through formal diaza-ene reaction: access to 1,4-dicarbonyl compounds

Donor-acceptor monosubstituted hydrazones participate as suitable reagents able to undergo an enantioselective formal diaza-ene reaction with α,β-unsaturated aldehydes under chiral secondary amine catalysis. This constitutes a new approach for the enantioselective conjugate addition of hydrazones to enals under metal-free conditions and leads to the formation of γ-hydrazono carboxylic acids after oxidation/[1,3]-H shift. The methodology is also useful for the synthesis of enantioenriched β-substituted α-keto-1,5-diesters by using the hydrazone moiety as a masked carbonyl group.     

[C14MIM]Br离子液体对聚丙烯超临界CO2发泡性能的影响

本文对PP/[C₁₄MIM]Br体系的熔融过程和对CO₂的吸收, 以及[C₁₄MIM]Br在PP基体中的分散状态进行了研究, 并初步考察了[C₁₄MIM]Br对PP发泡性能的影响.     

甲基丙烯酸甲酯的原子转移自由基悬浮聚合

以 1 苯基氯乙烷为引发剂 ,氯化亚铜为催化剂 ,2 ,2 联吡啶为配体 ,外加搅拌 ,氮气保护下进行了甲基丙烯酸甲酯 (MMA)在 80℃下的原子转移悬浮聚合 .结果表明 ,聚合反应符合对单体浓度为一级的动力学关系 .经计算聚合体系的增长自由基浓度为 5 .74× 10 - 8mol L .聚合物分子量随转化率呈线性增加 ,分子量分布较窄 ,Mw Mn 在 1.37～ 1.40之间 .还以AIBN为引发剂 ,在三氯化铁和三苯基膦存在下进行了MMA的反向原子转移本体和悬浮聚合研究 .结果证明本体聚合具有好的可控特征 ,分子量随转化率呈线性增长 ,分子量分布指数在 1.2 7～ 1.31之间 .聚合反应速率较快 ,聚合体系中的增长自由基浓度较高 ,为 1.6 4× 10 - 7mol L .而在此催化体系下的悬浮聚合则完全失去了活性特征     

The role of salicylic acid, L-ascorbic acid and oxalic acid in promoting the oxidation of alkenes with H2O2 catalysed by [MnIV2(O)3(tmtacn)2]2+

The role played by the additives salicylic acid, L-ascorbic acid and oxalic acid in promoting the catalytic activity of [MnIV2(O)3(tmtacn)2](PF6)2 (1(PF6)2, where tmtacn = N,N',N'-trimethyl-1,4,7-triazacyclononane) in the epoxidation and cis-dihydroxylation of alkenes with H2O2 and in suppressing the catalysed decomposition of H2O2 is examined. Whereas aliphatic and aromatic carboxylic acids effect enhancement of the catalytic activity of 1 through the in situ formation dinuclear carboxylato bridged complexes of the type [MnIII2(mu-O)(mu-RCO2)2(tmtacn)2]2+, for L-ascorbic acid and oxalic acid notable differences in reactivity are observed. Although for L-ascorbic acid key differences in the spectroscopic properties of the reaction mixtures are observed compared with carboxylic acids, the involvement of carboxylic acids formed in situ is apparent. For oxalic acid the situation is more complex with two distinct catalyst systems in operation; the first, which engages in epoxidation only, is dominant until the oxalic acid additive is consumed completely at which point carboxylic acids formed in situ take on the role of additives to form a second distinct catalyst system, i.e. that which was observed for alkyl and aromatic carboxylic acids, which yield both cis-diol and epoxide products.     

Functional group requirements within the peptide H-Pro-Pro-Asp-NH2 as a catalyst for aldol reactions

H-Pro-Pro-Asp-NH₂1 is a versatile catalyst for asymmetric aldol reactions. In this work, the functional group tolerance within the catalyst structure has been examined. Several analogs of H-Pro-Pro-Asp-NH₂ in which the N-terminal secondary amine or the carboxylic acid in the side chain of the aspartic acid residue is replaced by different functional groups were prepared. Evaluation of their catalytic properties revealed that both the N-terminal secondary amine and the carboxylic acid are important for catalysis. The implications for the reaction mechanism are discussed.     

Palladium-catalyzed decarboxylation and decarbonylation under hydrothermal conditions: decarboxylative deuteration

[reaction: see text] Decarboxylation of free carboxylic acid was performed by Pd/C catalyst under hydrothermal water (250 degrees C/4 MPa). Under the hydrothermal conditions of deuterium oxide, decarbonylative deuteration was observed to give fully deuterated hydrocarbons from carboxylic acids or aldehydes.     

Nucleophilic catalysis with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) for the esterification of carboxylic acids with dimethyl carbonate

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) is an effective nucleophilic catalyst for carboxylic acid esterification with dimethyl carbonate (DMC). The reaction pathway of this new class of nucleophilic catalysis has been studied. A plausible, multistep mechanism is proposed, which involves an initial N-acylation of DBU with DMC to form a carbamate intermediate. Subsequent O-alkylation of the carboxylate with this intermediate generates the corresponding methyl ester in excellent yield. In the absence of DBU or in the presence of other bases, such as ammonium hydroxide or N-methylmorpholine, the same reaction affords no desired product. This method is particularly valuable for the synthesis of methyl esters that contain acid-sensitive functionality.     

Highly Acidic Conjugate-Base-Stabilized Carboxylic Acids Catalyze Enantioselective oxa-Pictet–Spengler Reactions with Ketals

Acyclic ketone-derived oxocarbenium ions are involved as intermediates in numerous reactions that provide valuable products, however, they have thus far eluded efforts aimed at asymmetric catalysis. We report that a readily accessible chiral carboxylic acid catalyst exerts control over asymmetric cyclizations of acyclic ketone-derived trisubstituted oxocarbenium ions, thereby providing access to highly enantioenriched dihydropyran products containing a tetrasubstituted stereogenic center. The high acidity of the carboxylic acid catalyst, which exceeds that of the well-known chiral phosphoric acid catalyst TRIP, is largely derived from stabilization of the carboxylate conjugate base through intramolecular anion-binding to a thiourea site.