Improving the Capture of CO2 by Substituted Monoethanolamines: Electronic Effects of Fluorine and Methyl Substituents

The influence of electronic and steric effects on the reaction between CO₂ and monoethanolamine (MEA) absorbents is investigated using computational methods. The pK_a of the alkanolamine, the reaction enthalpy for carbamate formation, and the hydrolytic carbamate stability are important factors for the efficiency of CO₂ capture. The steric and electronic effects of CH₃, CH₂F, CHF₂, CF₃, F, dimethyl, difluoro, and bis(2‐trifluoromethyl) substituents at the α carbon of MEA on this reaction are investigated. Density functional theory (DFT) (B3LYP, M06‐2X, M08‐HX and M11‐L) and ab initio methods [spin component‐scaled second‐order Møller‐Plesset theory (SCS‐MP2), G3], each coupled with solvent models [conductor‐like polarizable continuum model (CPCM) and universal solvation models (SM8 and SMD)], are shown to yield accurately calculated pK_a values of the substituted MEAs. Specifically, G3, SCS‐MP2, and M11‐L methods coupled with the SMD and SM8 solvation models perform well with a mean unsigned error (MUE) of only 0.15, 0.24 and 0.25 pK_a units, respectively. SCS‐MP2 is used to calculate the reaction enthalpy for carbamate formation and the carbamate stability towards hydrolysis. With the introduction of β‐fluoro substituents (especially the CH₂F moiety) the reaction enthalpy for the formation of carbamates can be fine‐tuned to be less exothermic than that using the unsubstituted MEA. This implies a reduced energy requirement for the solvent‐regeneration step in the post‐combustion carbon‐capture method, which is currently the energy‐limiting step in efficient CO₂ capture. β‐Fluoro‐substituted MEAs are also shown to form less stable carbamates than MEA. Thus, β‐fluoro‐substituted MEAs display a great potential for the use in the post‐combustion carbon‐capture process. Finally, a clear correlation is observed between the gas‐phase basicity and the tendency to form carbamates. This allows for the rapid prediction of which species will be formed experimentally, and thus the CO₂‐absorbing capacities of alkanolamines can be estimated.     

An FTIR Spectroscopic Study on the Effect of Molecular Structural Variations on the CO2 Absorption Characteristics of Heterocyclic Amines

Herein, the reaction between CO₂ and piperidine, as well as commercially available functionalised piperidine derivatives, for example, those with methyl‐, hydroxyl‐ and hydroxyalkyl substituents, has been investigated. The chemical reactions between CO₂ and the functionalised piperidines were followed in situ by using attenuated total reflectance (ATR) FTIR spectroscopy. The effect of structural variations on CO₂ absorption was assessed in relation to the ionic reaction products identifiable by IR spectroscopy, that is, carbamate versus bicarbonate absorbance, CO₂ absorption capacity and the mass‐transfer coefficient at zero loading. On absorption of CO₂, the formation of the carbamate derivatives of the 3‐ and 4‐hydroxyl‐, 3‐ and 4‐hydroxymethyl‐, and 4‐hydroxyethyl‐substituted piperidines were found to be kinetically less favourable than the carbamate derivatives of piperidine and the 3‐ and 4‐methyl‐substituted piperidines. As the CO₂ loading of piperidine and the 3‐ and 4‐methyl‐ and hydroxyalkyl‐substituted piperidines exceeded 0.5 moles of CO₂ per mole of amine, the hydrolysis of the carbamate derivative of these amines was observed in the IR spectra collected. From the subset of amines analysed, the 2‐alkyl‐ and 2‐hydroxyalkyl‐substituted piperidines were found to favour bicarbonate formation in the reaction with CO₂. Based on IR spectral data, the ability of these amines to form the carbamate derivatives was also established. Computational calculations at the B3LYP/6‐31+G** and MP2/6‐31+G** levels of theory were also performed to investigate the electronic/steric effects of the substituents on the reactivity (CO₂ capture performance) of different amines, as well as their carbamate structures. The theoretical results obtained for the 2‐alkyl‐ and 2‐hydroxyalkyl‐substituted piperidines suggest that a combination of both the electronic effect exerted by the substituent and a reduction in the exposed area of the nitrogen atom play a role in destabilising the carbamate derivative and increasing its susceptibility to hydrolysis. A theoretical investigation into the structure of the carbamate derivatives of these amines revealed shorter N? C bond lengths and a less‐delocalised electron distribution in the carboxylate moiety.     

Carbamate Stabilities of Sterically Hindered Amines from Quantum Chemical Methods: Relevance for CO2 Capture

The influence of electronic and steric effects on the stabilities of carbamates formed from the reaction of CO₂ with a wide range of alkanolamines was investigated by quantum chemical methods. For the calculations, B3LYP, M11‐L, MP2, and spin‐component‐scaled MP2 (SCS‐MP2) methods were used, coupled with SMD and SM8 solvation models. A reduction in carbamate stability leads to an increased CO₂ absorption capacity of the amine and a reduction of the energy required for solvent regeneration. Important factors for the reduction of the carbamate stability were an increase in steric hindrance around the nitrogen atom, charge on the N atom and intramolecular hydrogen bond strength. The present study indicates that secondary ethanolamines with sterically hindering groups near the N atom show significant potential as candidates for industrial CO₂‐capture solvents.     

An ATR‐FTIR Study on the Effect of Molecular Structural Variations on the CO2 Absorption Characteristics of Heterocyclic Amines,Part II

This paper reports on an ATR‐FTIR spectroscopic investigation of the CO₂ absorption characteristics of a series of heterocyclic diamines: hexahydropyrimidine (HHPY), 2‐methyl and 2,2‐dimethylhexahydropyrimidine (MHHPY and DMHHPY), hexahydropyridazine (HHPZ), piperazine (PZ) and 2,5‐ and 2,6‐dimethylpiperazine (2,6‐DMPZ and 2,5‐DMPZ). By using in situ ATR‐FTIR the structure–activity relationship of the reaction between heterocyclic diamines and CO₂ is probed. PZ forms a hydrolysis‐resistant carbamate derivative, while HHPY forms a more labile carbamate species with increased susceptibility to hydrolysis, particularly at higher CO₂ loadings (>0.5 mol CO₂/mol amine). HHPY exhibits similar reactivity toward CO₂ to PZ, but with improved aqueous solubility. The α‐methyl‐substituted MHHPY favours HCO₃^? formation, but MHHPY exhibits comparable CO₂ absorption capacity to conventional amines MEA and DEA. MHHPY show improved reactivity compared to the conventional α‐methyl‐ substituted primary amine 2‐amino‐2‐methyl‐1‐propanol. DMHHPY is representative of blended amine systems, and its reactivity highlights the advantages of such systems. HHPZ is relatively unreactive towards CO₂. The CO₂ absorption capacity C_A (mol CO₂/mol amine) and initial rates of absorption R_IA (mol CO₂/mol amine min^?1) for each reactive diamine are determined: PZ: C_A=0.92, R_IA=0.045; 2,6‐DMPZ: C_A=0.86, R_IA=0.025; 2,5‐DMPZ: C_A=0.88, R_IA=0.018; HHPY: C_A=0.85, R_IA=0.032; MHHPY: C_A=0.86, R_IA=0.018; DMHHPY: C_A=1.1, R_IA=0.032; and HHPZ: no reaction. Calculations at the B3LYP/6‐31+G** and MP2/6‐31+G** calculations show that the substitution patterns of the heterocyclic diamines affect carbamate stability, which influences hydrolysis rates.     

Insights into amine-based CO<Subscript>2</Subscript> capture: an ab initio self-consistent reaction field investigation

Ab initio many-body perturbation theory (MP2/6-311++G(,dp)), density functional theory (B3LYP/6-31++G(d,p)) and self-consistent reaction field (IEF-PCM UA HF/6-31G(d)) calculations have been used to study the CO₂ capture reagents NH₃, 2-hydroxyethylamine (MEA), diaminoethane (EN), 2-amino-1-propanol (2A1P), diethanolamine (DEA), N-methyl-2-hydroxyethylamine (N-methylMEA), 2-amino-2-methyl-1-propanol (AMP), trishydroxymethylaminomethane (tris), piperazine (PZ) and piperidine (PD). This study involved full conformational searches of the capture amines in their native and protonated forms, and their carbamic acid and carbamate derivatives. Using this data, we were able to compute Boltzmann-averaged thermodynamic values for the amines, carbamates and carbamic acid derivatives, as well as equilibrium constants for a series of ‘universal’ aqueous capture reactions. Important findings include (i) relative pK _a values for the carbamic acid derivatives are a useful measure of carbamate stability, due to a particular chemical resonance which is also manifest in short computed N–CO₂H bonds at both levels of theory, (ii) the computational results for sterically hindered amines such as AMP and tris are consistent with these species forming carbamates which readily hydrolyse and (iii) the amine-catalysed reaction between OH⁻ and CO₂ to generate bicarbonate correlates with amine pK _a. Thermodynamic data from the ab initio computations predicts that the heterocycles PD and PZ and the acyclic sorbent EN are good choices for a capture solvent. AMP and tris perform poorly in comparison.     

Structure‐Reactivity Relationships as Probes for the Inhibition Mechanism of Cholesterol Esterase by Aryl Carbamates. I. Steady‐State Kinetics

For substituted phenyl‐N‐butyl carbamates (1) and 4‐nitrophenyl‐N‐substituted carbamates (2), linear relationships between values of NH proton chemical shift (δ_NH), pKa, and logk_[OH] and Hammett substituent constant (σ) or Taft substituent constant (σ*) are observed. Carbamates 1 and 2 are pseudo‐substrate inhibitors of porcine pancreatic cholesterol esterase. Thus, the mechanism of the reaction necessitates that the inhibitor molecule and the enzyme form the enzyme‐inhibitor tetrahedral species at the K_i step of the reaction and then form the carbamyl enzyme at the k_c step of the reaction. Linear relationships between the logarithms of K_i and k_c for cholesterol esterase by carbamates 1 and σ are observed, and the reaction constants (ρs) are ?3.4 and ?0.13, respectively. Therefore, the above reaction forms the negative‐charge tetrahedral species and follows the formation of the relatively neutral carbamyl enzymes. For the inhibition of cholesterol esterase by carbamates 2 except 4‐nitrophenyl‐N‐phenyl carbamate and 4‐nitrophenyl‐N‐t‐butyl carbamate, linear relationships of ‐logK_i and logk_c with σ* are observed and the ρ* values are ?0.50 and 1.03, respectively. Since the above reaction also forms the negative‐charge tetrahedral intermediate, it is possible that the K_i step of this reaction is further divided into two steps. The first K_i step is the development of the positive‐charge at the carbamate nitrogen from the protonation of the carbamate nitrogen. The second K_i step is the formation of the tetrahedral intermediate with the negative‐charge at the carbonyl oxygen. From Arrhenius plots of a series of inhibition reactions by carbamates 1 and 2, the isokinetic and isoequilibrium temperatures are different from the reaction temperature (25°C). Therefore, the observed ρ and ρ* values only depend upon the electronic effects of the substituents. Taken together, the cholesterol esterase inhibition mechanism by carbamates 1 and 2 is proposed.     

Oxidation of ethanolamines by sodium N‐bromobenzenesulfonamide in alkaline buffer medium: A kinetic and mechanistic study

The kinetics of oxidation of ethanolamines, monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA), by sodium N‐bromobenzenesulfonamide or bromamine‐B (BAB) in alkaline buffer medium (pH 8.7–12.2) has been studied at 40°C. The three reactions follow identical kinetics with first‐order in [oxidant] and fractional‐order each in [substrate] and [OH^?]. Under comparable experimental conditions, the rate of oxidation increases in the order: DEA > TEA > MEA. The added reaction product, benzenesulfonamide, retards the reaction rate. The addition of halide ions and the variation of ionic strength of the medium have no significant effect on the rate. The dielectric effect is negative. The solvent isotope effect k′(H₂O)/k′(D₂O) ≈ 0.92. Activation parameters for the composite reaction and for the rate‐limiting step were computed from the Eyring plots. Michaelis‐Menten type of kinetics is observed. The formation and decomposition constants of ethanolamine‐BAB complexes are evaluated. An isokinetic relationship is observed with β = 430 K indicating that enthalpy factors control the rate. For each substrate, a mechanism consistent with the kinetic data has been proposed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 480–490, 2001     

Effects of introduction of α‐carboxylate,N‐methyl,and N‐formyl groups on intramolecular cyclization of o‐quinone amines: Density functional theory‐based study

o‐Quinone amines, which are relevant to various biological processes, can undergo spontaneous intramolecular cyclization (ring closure reaction by amino‐terminated hydrocarbon side chain) that deactivates them toward another possible reactions, that is, thiol binding. Density functional theory‐based calculation is employed for obtaining the potential energy curves along the C? N bond formation in the intramolecular cyclization of various o‐quinone amines, viz., dopaminequinone, dopaquinone, N‐methyl‐dopaminequinone, N‐formyl‐dopaminequinone, and the corresponding methylene‐inserted analogues. The activation barrier is decreased by introduction of α‐carboxylate and N‐methyl group whereas increased by introduction of N‐formyl group. A negative correlation between the activation barriers and the level of highest occupied molecular orbital is pointed out. Furthermore, the methylene‐inserted analogues show decreased activation barriers. This is explained by reduction of steric repulsion in the transition state.     

Towards the design of novel boron‐ and nitrogen‐substituted ammonia‐borane and bifunctional arene ruthenium catalysts for hydrogen storage

Electronic‐structure density functional theory calculations have been performed to construct the potential energy surface for H₂ release from ammonia‐borane, with a novel bifunctional cationic ruthenium catalyst based on the sterically bulky β‐diketiminato ligand (Schreiber et al., ACS Catal. 2012, 2, 2505). The focus is on identifying both a suitable substitution pattern for ammonia‐borane optimized for chemical hydrogen storage and allowing for low‐energy dehydrogenation. The interaction of ammonia‐borane, and related substituted ammonia‐boranes, with a bifunctional η⁶‐arene ruthenium catalyst and associated variants is investigated for dehydrogenation. Interestingly, in a number of cases, hydride‐proton transfer from the substituted ammonia‐borane to the catalyst undergoes a barrier‐less process in the gas phase, with rapid formation of hydrogenated catalyst in the gas phase. Amongst the catalysts considered, N,N‐difluoro ammonia‐borane and N‐phenyl ammonia‐borane systems resulted in negative activation energy barriers. However, these types of ammonia‐boranes are inherently thermodynamically unstable and undergo barrierless decay in the gas phase. Apart from N,N‐difluoro ammonia‐borane, the interaction between different types of catalyst and ammonia borane was modeled in the solvent phase, revealing free‐energy barriers slightly higher than those in the gas phase. Amongst the various potential candidate Ru‐complexes screened, few are found to differ in terms of efficiency for the dehydrogenation (rate‐limiting) step. To model dehydrogenation more accurately, a selection of explicit protic solvent molecules was considered, with the goal of lowering energy barriers for H‐H recombination. It was found that primary (1°), 2°, and 3° alcohols are the most suitable to enhance reaction rate. © 2014 Wiley Periodicals, Inc.     

Mechanistic Insight into Peroxo‐Shunt Formation of Biomimetic Models for Compound II,Their Reactivity toward Organic Substrates,and the Influence of N‐Methylimidazole Axial Ligation

High‐valent iron‐oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe^III(OH) porphyrin ([meso‐tetrakis(2,4,6‐trimethyl‐3‐sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H₂O₂ with (TMPS)Fe^III(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe^IV?O(OH) point to the overall associative nature of the process. A pH‐dependence study on the formation of (TMPS)Fe^IV?O(OH) revealed a very high reactivity of OOH^? toward (TMPS)Fe^III(OH) in comparison to H₂O₂. The influence of N‐methylimidazole (N‐MeIm) ligation on both the formation of iron(IV)‐oxo species and their oxidising properties in the reactions with 4‐methoxybenzyl alcohol or 4‐methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe^III(H₂O)(N‐MeIm) is highly reactive toward H₂O₂ to form the iron(IV)‐oxo species, (TMPS)Fe^IV?O(N‐MeIm). The latter species can also be formed in the reaction of (TMPS)Fe^III(N‐MeIm)₂ with H₂O₂ or in the direct reaction of (TMPS)Fe^IV?O(OH) with N‐MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe^IV?O(OH) and (TMPS)Fe^IV?O(N‐MeIm) do not display a pronounced effect of the N‐MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH^? substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH^? or N‐MeIm) in the trans position to the oxo group in the iron(IV)‐oxo species does not significantly affect the activation barriers calculated for C?H dehydrogenation of the selected organic substrates.     

The “Borrowing Hydrogen Strategy” by Supported Ruthenium Hydroxide Catalysts: Synthetic Scope of Symmetrically and Unsymmetrically Substituted Amines

The N‐alkylation of ammonia (or its surrogates, such as urea, NH₄HCO₃, and (NH₄)₂CO₃) and amines with alcohols, including primary and secondary alcohols, was efficiently promoted under anaerobic conditions by the easily prepared and inexpensive supported ruthenium hydroxide catalyst Ru(OH)_x/TiO₂. Various types of symmetrically and unsymmetrically substituted “tertiary” amines could be synthesized by the N‐alkylation of ammonia (or its surrogates) and amines with “primary” alcohols. On the other hand, the N‐alkylation of ammonia surrogates (i.e., urea and NH₄HCO₃) with “secondary” alcohols selectively produced the corresponding symmetrically substituted “secondary” amines, even in the presence of excess amounts of alcohols, which is likely due to the steric hindrance of the secondary alcohols and/or secondary amines produced. Under aerobic conditions, nitriles could be synthesized directly from alcohols and ammonia surrogates. The observed catalysis for the present N‐alkylation reactions was intrinsically heterogeneous, and the retrieved catalyst could be reused without any significant loss of catalytic performance. The present catalytic transformation would proceed through consecutive N‐alkylation reactions, in which alcohols act as alkylating reagents. On the basis of deuterium‐labeling experiments, the formation of the ruthenium dihydride species is suggested during the N‐alkylation reactions.     

Nitrogen–Carbon Bond Formation by Reactions of a Titanium–Potassium Dinitrogen Complex with Carbon Dioxide,tert‐Butyl Isocyanate,and Phenylallene

Nitrogen–carbon bond‐forming reactions at coordinated dinitrogen in a bifunctional titanium–potassium system are reported. A titanium atrane complex with a tris(aryloxide)methyl ligand ( 1 ) was treated with two equivalents of potassium naphthalenide under N₂ atmosphere to generate a bifunctional complex ( 2 ) in which N₂ binds end‐on to two titanium centers and side‐on to three potassium cations. Dinitrogen complex 2 reacted with carbon dioxide, tert ‐butyl isocyanate, and phenylallene, forming nitrogen–carbon bonds and affording diverse N‐functionalized products. The reaction of 2 with CO₂ followed by addition of Me₃SiCl resulted in the formation of the starting complex 1 with concomitant release of silylated carboxyl hydrazines while the reaction with two equivalents of tert ‐butyl isocyanate proceeded by insertion into the Ti−N bonds. Treatment of 2 with phenylallene afforded vinyl‐substituted hydrazido complexes.     

Palladium‐Catalyzed Cyclization Reaction of o‐Haloanilines,CO2 and Isocyanides: Access to Quinazoline‐2,4(1H,3H)‐diones

Quinazoline‐2,4(1H,3H)‐diones are core structural subunits frequently found in many biologically important compounds. The reaction of 2‐​aminobenzonitrile and CO₂, which was frequently studied, only provided N3‐unsubstituted quinazoline‐2,4(1H,3H)‐dione compounds. Herein we report palladium‐catalyzed cyclization reactions of o‐haloanilines, CO₂ and isocyanides to prepare N3‐substituted quinazoline‐2,4(1H,3H)‐diones. Electron‐rich o‐bromoanilines participated in the cyclization reaction using Cs₂CO₃ at high temperature, and electron‐deficient o‐bromoaniline or o‐iodoaniline substrates conducted the reaction using CsF as base to deliver corresponding quinazoline‐2,4(1H,3H)‐dione products in good yields.     

1,2‐Azaborine: The Boron‐Nitrogen Derivative of ortho‐Benzyne

The BN analogue of ortho‐benzyne, 1,2‐azaborine, is generated by flash vacuum pyrolysis, trapped under cryogenic conditions, and studied by direct spectroscopic techniques. The parent BN aryne spontaneously binds N₂ and CO₂, thus demonstrating its highly reactive nature. The interaction with N₂ is photochemically reversible. The CO₂ adduct of 1,2‐azaborine is a cyclic carbamate which undergoes photocleavage, thus resulting in overall CO₂ splitting.     

A Facile Synthesis of New L‐Proline‐Based Trifluoromethyl Oxazole Derivatives Using Microwave Irradiation and Conventional Method

Cyclization reaction of l ‐proline with trifluoroacetimidoyl chlorides has been developed as an efficient strategy for the synthesis of trifluoromethyl oxazole derivatives by two methods: (a) in the presence of K₂CO₃ as a base in acetonitrile at room temperature and (b) in the presence of K₂CO₃ as a base in acetonitrile using microwave irradiation, in one pot reaction. The microwave irradiation has been found to be the most efficient method. High yields and short reaction times were obtained for both electron‐releasing and electron‐withdrawing substituted N‐aryltrifluoroacetimidoyl chloride derivatives by microwave irradiation.     

Quenching of Singlet Oxygen by Tertiary Aliphatic Amines. Structural Effects on Rates and Products

A kinetic and product study of the reaction of a series of α‐methyl‐substituted N‐methylpiperidines with thermally generated ¹O₂ in MeCN was carried out. It was found that as the number of α‐methyl groups (Me in α‐position relative to the N‐atom) increases, the rate of ¹O₂ quenching (physical plus chemical) slightly decreases. This finding shows that, with respect to the reaction rate, steric effects are much more important than electronic effects as the latter should have produced the opposite result. The opposite outcome was instead found for the chemical quenching that leads to the N‐demethylation products and N‐formyl derivatives. The same trend was observed for the ratio between N‐demethylation and formation of the N‐formyl derivatives (NH/NCHO ratio). All these results are consistent with the mechanism reported in Scheme 1 where an exciplex is first formed that by a H‐atom transfer process produces an α‐amino‐substituted C‐radical. The latter forms the product of N‐demethylation by one electron oxidation, or affords the N‐formyl derivative by radical coupling (Scheme 1). Similar results were obtained with N,N‐dimethylcyclohexanamine. However, this ‘acyclic’ amine exhibited behaviors quite distinct from those of the N‐methylpiperidines series, with respect to reaction rate, extent of chemical quenching, and NH/NCHO ratio.     

An Exceedingly Mild,Green Synthesis of Substituted N‐3‐diaryl‐1,8‐naphthyridin‐2‐amine Derivatives and Their Antimicrobial Activity

An exceedingly and highly efficient procedure has been described for the synthesis of substituted N‐3‐diaryl‐1,8‐naphthyridin‐2‐amines by the reaction of 2‐chloro‐3‐aryl‐1,8‐naphthyridines with various anilines in the presence of N‐methyl‐2‐pyrrolidone and K₂CO₃ under thermal green solvent‐free conditions. The significant features of this green reaction include very good yields in purity, simple experimental, short reaction time, easy workability, and avoidance of toxic solvents. All synthesized compounds have been evaluated for their antibacterial activity.     

Synthesis of Asymmetrically Substituted Terpyridines by Palladium‐Catalyzed Direct C?H Arylation of Pyridine N‐Oxides

The synthesis of asymmetrically substituted 2,2′:6′,2′′‐terpyridines is reported. First, palladium‐catalyzed C? H arylation of pyridine N‐oxides with substituted bromopyridines gave 2,2′‐bipyridine N‐oxides, which were further arylated in a second step to form 2,2′:6′,2′′‐terpyridine N‐oxides. Yields of up to 77 % were obtained with N‐oxides bearing an electron‐withdrawing ethoxycarbonyl substituent in the 4‐position. Pd(OAc)₂ with either P(tBu)₃ or P(o‐tolyl)₃ was used as the catalyst. Cyclometalated complexes derived from Pd(OAc)₂ and these phosphines were also effective. K₃PO₄ as the base gave better results than K₂CO₃. Subsequent deoxygenation with H₂ and Pd/C as the catalyst gave the asymmetrically substituted 2,2′:6′,2′′‐terpyridines in near quantitative yield. This reaction sequence significantly reduces the number of steps required in comparison with known cross‐coupling methods and therefore allows convenient and scalable access to substituted terpyridines.     

Chiral N,N′‐Dioxide–Scandium(III) Complex‐Catalyzed Asymmetric Friedel–Crafts Alkylation Reaction of ortho‐Hydroxybenzyl Alcohols with C3‐Substituted N‐Protected Indoles

The first Lewis acid catalyzed asymmetric Friedel–Crafts alkylation reaction of ortho‐hydroxybenzyl alcohols with C3‐substituted indoles is described. A chiral N,N′‐dioxide Sc(OTf)₃ complex served not only to promote formation of ortho‐quinone methides (o‐QMs) in situ but also induced the asymmetry of the reaction. This methodology enables a novel activation of ortho‐hydroxybenzyl alcohols, thus affording the desired chiral diarylindol‐2‐ylmethanes in up to 99 % yield and 99 % ee. A range of functional groups were also tolerated under the mild reaction conditions. Moreover, this strategy gives concise access to enantioenriched indole‐fused benzoxocines.     

Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzsch’s ester analogues 1 a – d with the Lewis acidic species B(C₆F₅)₃ results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a – d , which are obtained in high to moderate yields. The N‐substituted substrates (N‐Me, N‐Ph) reacted in high yield 90–98 % and the connectivity of the products were confirmed by an X‐ray crystallographic analysis of the N‐Me borohydride salt 2 a . Unsubstituted Hanztsch’s ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester‐bound Lewis acid–base adduct 3 a . Formation of the Lewis acid–base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d . The connectivity of the carbonyl‐bound adduct was confirmed by an X‐ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C₆F₅)₃. We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C₆F₅)₃ with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid–base adduct of the starting material and B(C₆F₅)₃. The 1,2‐dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid–base adduct.