Online ATR-IR investigations and mechanistic understanding of the carbonylation of epoxides - the selective synthesis of lactones or polyesters from epoxides and CO

In situ ATR-IR spectroscopy is applied as a powerful tool to study the factors that control the reaction of epoxides with carbon monoxide in the presence of [Lewis acid]⁺[Co(CO)₄]⁻ salts. Based on these investigations, a consistent mechanistic scheme is presented, comprising the main lactone and polyester products and minor components, e.g., acetone and crotonic acid derivatives. β-Alkoxy-acyl-cobalttetracarbonyl species are shown to be key intermediates from which two reaction routes start in dependence of the applied Lewis acid (LA). Labile LA-alkoxy combinations favor a backbiting process of the oxygen function on the Co-acyl bond, primarily producing lactone products. More stable LA-alkoxy entities are unreactive toward PO conversion and afford a polymerization reaction after the addition of a nucleophile. In that case, the Lewis acid remains bonded to the chain end.     

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.     

Ru–η6‐Arene Cations [{(Ph2PC6H4)2B(η6‐Ph)}RuX]+ (X=Cl,H) as Lewis Acids

The reactivity of [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuCl][B(C₆F₅)₄] ( 1 ) as a Lewis acid was investigated. Treatment of 1 with mono and multidentate phosphorus Lewis bases afforded the Lewis acid–base adducts with the ortho‐carbon atom of the coordinated arene ring. Similar reactivity was observed upon treatment with N‐heterocyclic carbenes; however, adduct formation occurred at both ortho‐ and para‐carbon atoms of the bound arene with the para‐position being favoured by increased steric demands. Interestingly treatment with isocyanides resulted in adduct formation with the B‐centre of the ligand framework. The hydride‐cation [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuH] [B(C₆F₅)₄] was prepared via reaction of 1 with silane. This species in the presence of a bulky phosphine behaves as a frustrated Lewis pair (FLP) to activate H₂ between the phosphorus centre and the ortho‐carbon atom of the η⁶‐arene ring.     

Protonation of phenylboronic acid: Comparison of G3B3 and G2MP2 methods

G3B3 and G2MP2 calculations using Gaussian 03 have been carried out to investigate the protonation preferences for phenylboronic acid. All nine heavy atoms have been protonated in turn. With both methodologies, the two lowest protonation energies are obtained with the proton located either at the ipso carbon atom or at a hydroxyl oxygen atom. Within the G3B3 formalism, the lowest‐energy configuration by 4.3 kcal · mol^?1 is found when the proton is located at the ipso carbon, rather than at the electronegative oxygen atom. In the resulting structure, the phenyl ring has lost a significant amount of aromaticity. By contrast, calculations with G2MP2 show that protonation at the hydroxyl oxygen atom is favored by 7.7 kcal · mol^?1. Calculations using the polarizable continuum model (PCM) solvent method also give preference to protonation at the oxygen atom when water is used as the solvent. The preference for protonation at the ipso carbon found by the more accurate G3B3 method is unexpected and its implications in Suzuki coupling are discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Alternating copolymerization of carbon dioxide and propylene oxide under bifunctional cobalt salen complexes: Role of Lewis base substituent covalent bonded on salen ligand

Alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO₂) was realized under mild conditions with a moderate turnover frequency (TOF), employing sole bifunctional cobalt salen complexes containing Lewis acid metal center and covalent bonded Lewis base on the ligand. Variation of the covalent bonded Lewis base substituents on the salen ligands could tailor the catalytic activity with TOF changing from 19.3 to 34.9 h^?1, polymeric/cyclic carbonate selectivity from 95.3 to 72.8%, and the head‐to‐tail structure in the polymer from 72.2 to 86.0%. The IR analysis confirmed that the Lewis base moiety on one molecule could coordinate with cobalt center of adjacent molecule, playing similar role to the Salen metal complex/Lewis base binary catalytic system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 359–365, 2010     

From fullerene-mixed peroxide to open-cage oxafulleroid C(59)(O)(3)(OH)(2)(OO(t)()Bu)(2) embedded with furan and lactone motifs

[reaction: see text] Removal of one carbon atom from the C60 cage is achieved under mild conditions. The process involves the formation of fullerene-mixed peroxide, subsequent Lewis acid induced cleavage of O-O and C-O bonds, and thermolysis at 75 degrees C. In the proposed mechanism, the carbon atom is deleted as CO and an oxygen atom occupies the vacancy to form a furan ring. Single-crystal X-ray analysis confirmed the results.     

Superelectrophilicity of 1,2‐Azaborine: Formation of Xenon and Carbon Monoxide Adducts

The BN analogue of ortho‐benzyne, 1,2‐azaborine, is shown to bind carbon monoxide and a xenon atom under matrix isolation conditions, demonstrating its strongly Lewis acidic superelectrophilic nature. The Lewis acid–base complexes involving CO and Xe can be cleaved photochemically and reformed by mildly annealing the matrices. The interaction energy of 1,2‐azaborine with Xe is 3 kcal mol^?1 according to quantum chemical computations, and is similar to that of the superelectrophilic carbene difluorovinylidene.     

Activity of catalysts in thiophene synthesis from furan and hydrogen sulfide

Thiophene synthesis from furan and H₂S over acid catalysts is reported. Proton donor catalysts are low-active, nonselective, and prone to deactivation. Alumina-supported catalysts having Lewis acid sites, such as alumina-supported catalysts, are more efficient. With these catalysts, the thiophene formation rate per Lewis acid site increases with increasing site strength. It is assumed that the reaction proceeds via the formation of a surface intermediate consisting of an acid site bonded to an α carbon atom of the furan ring and an H₂S molecule nondissociatively adsorbed on a basic site. At atmospheric pressure, T = 250–450°C, initial furan concentrations of 1–20 vol %, and H₂S/furan = 0.4–20 (mol/mol), the thiophene formation reaction is first-order with respect to both reactants and its rate constant increases with increasing temperature. The thiophene formation rate depends on the H₂S/furan molar ratio. Under optimal conditions, the thiophene yield is 95–98 mol % and the thiophene formation rate is high.     

Reaction of an “Invisible” Frustrated N/B Lewis Pair with Dihydrogen

D ‐(+)‐Camphor forms the enamine 2 with piperidine. Compound 2 adds HB(C₆F₅)₂ at the enamine carbon atom C3 to form a Lewis acid/Lewis base adduct (exo‐/endo‐isomers of 3 ). Exposure of 3 to dihydrogen (2.5 bar, room temperature) leads to heterolytic splitting of H₂ to form the H⁺/H^? addition products ( 4 , two diastereoisomers) of the “invisible” frustrated Lewis pairs ( 5 , two diastereoisomers) that were apparently generated in situ by enamine hydroboration under equilibrium conditions.     

Structure of the CO bond on supported Pd particles: influence of size and surface state

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumi na (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd _n CO⁺ (n=1, 2) for molecular adsorption and Pd _n C⁺ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO⁺/σ _n Pd _n CO⁺ signal ratio over various size particles (mean diameter in the 2–9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO+O₂ (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO⁺ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.     

The investigation of acid effect on chemical polymerization of indole

The present study investigates the effects of Lewis acid and protonic acid on the chemical polymerization of indole using electrical conductivity measurements and nuclear magnetic resonance, UV–visible, and Fourier transform infrared spectroscopy techniques. These effects are explained by theoretical calculations on the basis of molecular mechanic (MM+) and semi-empirical Austin Model 1 methods. As a result, it has been shown that indole interacts with proton and Lewis acids by the way of different mechanisms. Theoretical research has demonstrated that while BF₃, a Lewis acid, adds to the N atom in the indole, which has a basic character due to its lone-pair electrons, proton, H⁺ adds to the indole ring on C₃ atom. These additions affect both the polymerization of indole and the conductivity of polyindole. Polyindole conductivity is increased by BF₃ addition and decreased by H⁺ addition.     

Quantum chemical study of the arylacyl azide complexes with Lewis acids and their Curtius rearrangement to isocyanates

The structures of donor-acceptor complexes of syn-benzoyl azide, its 2-methyl- and 2,6-dimethyl-substituted derivatives with BF₃, AlCl₃, and SbCl₅, and the corresponding transition states of the rearrangement into isocyanates were studied by the PBE/TZ2P method in the framework of the density functional theory (DFT). The complexes are formed at the oxygen and nitrogen atoms of the acyl azide group and have the composition 1: 1 or 1: 2 depending on the Lewis acid (L) structure. The complexes at the oxygen atom are more stable; the most stable complexes are formed by the reactions of acyl azides with AlCl₃. Complex formation with Lewis acids decreases the activation energy of the transformation of acyl azides into isocyanates owing to the +M effect and stabilization of the Ar-C(O-L^(1?))=N(1)-N(2)⁽¹⁺⁾≡N(3) mesomeric form. The activation energy decreases with an increase in the number of ortho-methyl substituents in benzoyl azide due to the +I effect of the phenyl group. The turn of the phenyl ring at almost 90° with respect to the CON₃ group is needed for the rearrangement to occur, and the energy necessary for this process is ～8 kcal mol^?1.     

The 19F chemical shift in oxygen containing carbon fluorine products

We have examined the ¹⁹F NMR spectra of a number of oxygen-containing fluorocarbon products and obtained a comprehensive set of ¹⁹F chemical shift values, which enabled us to determine the influence of an oxygen atom bonded to a fluorocarbon group on the ¹⁹F chemical shift. The influence of neighbouring fluorocarbon groups, either directly connected or separated by an oxygen atom, was also considered. Our results may be summarized as follows. An oxygen atom bonded by a single bond (ether type bond) to a fluorine substituted carbon atom decreases the ¹⁹F chemical shift, as does the introduction of a further fluorine atom. Considering two adjacent fluorocarbon groups, a variation of x ppm in the ¹⁹F chemical shift of one of the two groups gives a variation of 0·12 x ppm in the opposite sense on the ¹⁹F chemical shift of the other group. If the two groups are connected by an ether oxygen atom, the effect is only about 0·06 x ppm.     

Promoting the Hydrosilylation of Benzaldehyde by Using a Dicationic Antimony‐Based Lewis Acid: Evidence for the Double Electrophilic Activation of the Carbonyl Substrate

The concomitant activation of carbonyl substrates by two Lewis acids has been investigated by using [1,2‐(Ph₂MeSb)₂C₆H₄]²⁺ ([ 1 ]²⁺), an antimony‐based bidentate Lewis acid obtained by methylation of the corresponding distibine. Unlike the simple stibonium cation [Ph₃MeSb]⁺, dication [ 1 ]²⁺ efficiently catalyzes the hydrosilylation of benzaldehyde under mild conditions. The catalytic activity of this dication is correlated to its ability to doubly activate the carbonyl functionality of the organic substrate. This view is supported by the isolation of [ 1 ‐μ₂‐DMF][OTf]₂, an adduct, in which the DMF oxygen atom bridges the two antimony centers.     

Ring cleavage reactions of methyl α-D-allopyranoside derivatives with phenylboron dichloride and triethylsilane

In the course of our studies on the regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent, we found that a combination of a Lewis acid and a reducing agent triggered a ring-opening reaction of the pyranose ring of methyl α-D-allopyranosides. The formation of an acyclic boronate ester by the attachment of a hydride ion at C-1 indicated that the unexpected endocyclic cleavage of the bond between the anomeric carbon atom and the pyranose ring oxygen atom proceeded via an oxacarbenium ion intermediate produced by the chelation between O5/O6 of the pyranoside and the Lewis acid, followed by nucleophile substitution with a hydride ion at C1.     

Acidic and basic sites in NaX and NaY faujasites investigated by NH3, CO2 and CO molecular probes

The acid-base properties of samples of NaY and NaX faujasites have been investigated by adsorbing different probe molecules and measuring IR spectra. In the case of the NaY sample, only Na⁺ ions were found to be involved in the adsorption of CO₂ and CO, confirming the overwhelming Lewis acid character of this material. In contrast, carbonate-like species were formed by adsorbing carbon dioxide on the NaX sample, due to the reaction of basic framework oxygen atoms with CO₂ molecules polarised on neighbour Na⁺ ions. The spectroscopic analysis of NaX also showed evidence of Brønsted acid hydroxyls and OH groups bonded to extra-framework Al atoms. Ammonia adsorption revealed that the amount of Brønsted acid hydroxyls is significantly lower than the Lewis acid Na⁺ countercations. Moreover, small oxide particles, carrying carbonate-like species on their surface, are present in the zeolitic cavities. These particles could be responsible for the basic reactivity towards CO observed after outgassing the NaX sample at high temperature.     

Synthesis of γ-keto carboxylic acids from γ-keto-α-chloro carboxylic acids via carbonylation—decarboxylation reactions catalysed by a palladium system

A palladium-based catalytic system is highly active in the synthesis of γ-keto acids of type ArCOCH₂CH₂COOH via carbonylation-decarboxylation of the corresponding α-chloride. Typical reaction conditions are: P(CO) = 20–30 atm; substrate/H₂O/Pd = 100–400/800–1000/1 (mol); temperature: 100–110 °C; [Pd]=0.25 × 10⁻²−1 × 1O⁻² M; solvent: acetone; reaction time: 1–2 h. A palladium(II) complex can be used as catalyst precursor. Under the reaction conditions above, reduction of the precursor to palladium metal occurs to a variable extent. High catalytic activity is observed when the precursor undergoes extensive decomposition to the metal. Pd/C is also highly active. Slightly higher yields are obtainable when the catalytic system is used in combination with a ligand such as PPh₃. A mechanism for the catalytic cycle is proposed: (i) The starting keto chloride undergoes oxidative addition to reduced palladium with formation of a catalytic intermediate having a Pd-[CH(COOH)CH₂COPh] moiety. The reduced palladium may be the metal coordinated by other atoms of palladium and/or by carbon monoxide and/or by a PPh₃ ligand when catalysis is carried out in the presence of this ligand. It is also proposed that the keto group in the β-position with respect to the carbon atom bonded to chlorine weakens the CCl bond, easing the oxidative addition step and enhancing the activity of the catalyst. (ii) Carbon monoxide ‘inserts’ into the PdC bond of the above intermediate to give an acyl catalytic intermediate having a Pd-[COCH(COOH)CH₂COPh] moiety. (iii) Nucleophilic attack of H₂O to the carbon atom of the carbonyl group bonded to the metal of the acyl intermediate yields a malonic acid derivative as product intermediate. This, upon decarboxylation, gives the final product. Alternatively, the desired product may form without the malonic acid derivative intermediate, through the following reaction pathway: the acyl intermediate undergoes decarboxylation with formation of a different acyl intermediate, having a Pd-[CO-CH₂CH₂COPh] moiety, which, upon nucleophilic attack of H₂O on the carbon atom of the carbonyl group bonded to the metal, yields the final product.     

非对称环氧乙烷的区域选择性亲核开环反应

本文总结了常用亲核试剂对非对称环氧乙烷的亲核开环反应及其区域选择性。强亲核性的亲核试剂通常只受空间效应影响,进攻非对称环氧乙烷位阻小的碳原子,对于烯基取代环氧乙烷还可以进攻烯基的β-碳原子发生S_N2'开环反应,其他亲核试剂同时受空间效应和电子效应的影响,对于烷基环氧乙烷通常进攻其取代少的碳原子, 空间效应起主导作用,而对芳基和烯基取代环氧乙烷开环反应通常发生在环氧乙烷芳甲位和烯丙位的碳原子上, 电子效应起主导作用。在质子酸或强Lewis酸存在下,虽然单烷基环氧乙烷的开环仍然发生在其取代少的碳原子上,但对于芳基、烯基和同碳双取代环氧乙烷,亲核开环反应将主要受电子效应控制,一般亲核试剂倾向于进攻环氧乙烷的芳甲位、烯丙位或多取代的碳原子。分子内的亲核开环反应主要受成环时环大小的控制, 成环时的倾向是五元环> 六元环> 七元环。环氧乙烷亲核开环的区域选择性是环氧乙烷和亲核试剂空间效应和电子效应平衡的结果。     

Synthesis and spectroscopic study of 2-butenedioic acid (Z)-monophenyl ester and its ethereal oxygen coordinated complexes with Th(IV) and Ce(IV)

2-Butenedioic acid (Z)-monophenyl ester and its complexes with tetravalent thorium and cerium have been synthesized and characterized by means of elemental analysis, TG-DTA, IR, ¹H NMR and ¹³C NMR spectra. Chemical analysis and TG-DTA analysis indicated that the complexes possess the formula: ML₂(OH)₂ (M?=?Th or Ce; L?=?(Z)-monophenyl ester-2-butenedioate ion). The spectroscopic studies showed that besides the bidentate carboxylate bonded to the central tetravalent metal ions, the ethereal oxygen also coordinated. This was indicated by a higher wave number shift of ν_C=O in the complexes than that of free ligand and its sodium salt, and a remarkable upfield chemical shift of carbon atom (C-8) in the p-position of ethereal oxygen substituted phenyl.     

Structural and conformational analysis of 1‐oxaspiro[2.5]octane and 1‐oxa‐2‐azaspiro[2.5]octane derivatives by 1H, 13C,and 15N NMR

A structural and conformational analysis of 1‐oxaspiro[2.5]octane and 1‐oxa‐2‐azaspiro[2.5]octane derivatives was performed using ¹H, ¹³ C, and ¹⁵ N NMR spectroscopy. The relative configuration and preferred conformations were determined by analyzing the homonuclear coupling constants and chemical shifts of the protons and carbon atoms in the aliphatic rings. These parameters directly reflected the steric and electronic effects of the substituent bonded to the aliphatic six‐membered ring or to C3 or N2. The parameters also were sensitive to the anisotropic positions of these atoms in the three‐atom ring. The preferred orientation of the exocyclic substituents directed the oxidative attack. Copyright © 2012 John Wiley & Sons, Ltd.