Arenium Research Articles

ChemInform Abstract: Aromatic Alkylation by Gaseous Me3C+ Ions. Kinetic Role of Deprotonation of Intermediate Arenium Ions.

ChemInform Abstract: Aromatic Alkylation by Gaseous Me3C+ Ions. Kinetic Role of Deprotonation of Intermediate Arenium Ions.

Electrophile Affinity: Quantifying Reactivity for the Bromination of Arenes

Electrophile affinity (Ealpha), a recently proposed theoretical construct based on computed energies of arenium ion formation, rationalizes the substrate reactivity and regioselectivity of S(E)Ar bromination of three sets of available experimental arene data where closely related conditions had been employed uniformly. The Ealpha parameters (computed at B3LYP/6-311+G(2d,2p)) correlated very well (r = 0.987) with the partial rate factors (log f) for 18 regiospecific brominations of benzene and various methyl benzenes. Analysis of the bromination reactivities of 32 mono- and polysubstituted benzenes including various polar groups gave similar results (r = 0.982). The electrophile affinity treatment also accounted satisfactorily (r = 0.957) for bromination reactivities of polybenzenoid hydrocarbons. Conversely, comparisons with NBO-based charges and the electrostatic potential at nuclei (EPN) were not generally successful. The uniform effectiveness of Ealpha treatments for the cases analyzed with regard both to relative substrate reactivity (e.g., benzene vs toluene) and to regiospecificity (e.g., the positional reactivity of toluene) supports the "limiting case" conventional interpretation of the electrophilic aromatic substitution mechanism as being governed by the energy of sigma-complex formation. Although other mechanisms are possible under different conditions, the computed energies of arene-dibromine pi-complex formation for the polysubstituted benzene set examined correlated poorly with experimental reactivity data (r = 0.714) and only varied from 1.8 (for benzene) to 3.5 kcal/mol, in contrast to the 10(12) range in reactivity measured experimentally.

Electrophilic Aromatic Substitution: The Role of Electronically Excited States

Electrophilic aromatic substitutions (EAS), one of the most extensively studied organic reactions, can be considered under certain circumstances as a photochemical reaction without light. Thermochemical considerations show that in the gas phase, the reaction system (electrophile plus aromatic neutral) is often found initially in an electronically excited state, whereas the reaction products are formed on the ground state potential energy surface (PES). The crossing to the ground state is usually very rapid, so that the rate-determining steps take place on the ground state surface. It is shown that after the crossing (through a conical intersection (CI)), the system can be found on different parts of the ground state potential surface. In particular, the CI is connected without a barrier to all moieties assumed to be important in the reaction (pi complex, radical pair, and sigma complex). In some cases, due to a relatively low electron affinity of the electrophile and bond reorganization, the reaction starts on the ground state PES; a conical intersection exists in these cases, but is not accessed by the reactants. The topology of the reaction surface due to the avoided crossing is reminiscent of that in which an actual crossing takes place. The paper provides a comprehensive model for several EAS reactions. The CIs are located computationally, and an energy level diagram is proposed for some representative EAS reactions.

Silaimidazolium and silaimidazolidinium ions

2-Silaimidazolium ions and 2-silaimidazolidinium ions were synthesized by Lewis acid-base adduct formation between N-heterocyclic silylenes and silyl arenium ions. They were isolated in high yields as their [B(C(6)F(5))(4)](-) salts. These salts are stable at room temperature and were characterized by NMR spectroscopy supported by the results of density functional computations of molecular structures and NMR chemical shifts. NICS calculations suggest for the imidazolium ions only a modest degree of aromaticity. Despite the bulkiness of the used substituents R(1) and R(2), silylium ions and behave as classical Lewis pairs and show no unexpected reactivity.

Electrophile Affinity: A Reactivity Measure for Aromatic Substitution

The reactivity and regioselectivity of the electrophilic chlorination, nitration, and alkylation of benzene derivatives were rationalized by comparing literature data for the partial rate factors (ln f) for these S(E)Ar processes with theoretical reactivity parameters. The Electrophile Affinity (Ealpha), a new quantity, is introduced to characterize reactivity and positional selectivity. Ealpha is evaluated theoretically by the energy change associated with formation of an arenium ion by attachment of a model electrophile to the aromatic ring. The dependence between Ealpha and ln f values for chlorination for 11 substitutions of benzene and methyl benzenes had a high correlation coefficient (r = 0.992). Quite satisfactory correlations between Ealpha values and partial rate factors also were obtained for the nitration of substituted benzenes (r = 0.971 for 12 processes) and benzylation of benzene and halobenzenes (r = 0.973 for 13 processes). These results provide clear evidence for the usefulness of the electrophile affinity in quantifying reactivity and regiochemistry. Satisfactory relationships (r >0.97) also were found between EPN (electrostatic potential at nuclei) values, which reflect the variations of electron density at the different arene ring positions, and the experimental partial rate factors (ln f) for the chlorination and nitration reactions, but not for the benzylation. This disaccord is attributed to strong steric influences on the reaction rates for substitutions involving the bulky benzyl moiety.

Mechanism of Dimethylallyltryptophan Synthase: Evidence for a Dimethylallyl Cation Intermediate in an Aromatic Prenyltransferase Reaction

Dimethylallyltryptophan synthase is an aromatic prenyltransferase that catalyzes an electrophilic aromatic substitution reaction between dimethylallyl diphosphate (DMAPP) and L-tryptophan. The synthase is found in a variety of fungi, where it catalyzes the first committed step in the biosynthesis of the ergot alkaloids. The enzymatic reaction could follow either a dissociative mechanism involving a discrete dimethylallyl cation intermediate or an associative mechanism in which the indole ring directly displaces diphosphate in a single step. In this work, positional isotope exchange (PIX) experiments are presented in support of a dissociative mechanism. When [1-(18)O]-DMAPP is subjected to the synthase reaction and recovered starting material is analyzed, 15% of the (18)O-label is found to have scrambled from a bridging to a nonbridging position on the alpha-phosphorus. Kinetic isotope effect studies show that steps involved in the formation of the arenium ion intermediate are rate-determining, and therefore the scrambling occurs during the lifetime of the dimethylallyl cation/diphosphate ion pair. Similarly, when the unreactive substrate analogue, 6-fluorotryptophan, was employed, complete scrambling of the (18)O-label in DMAPP was observed. To our knowledge, this is the first observation of PIX in any prenyltransferase reaction, and it provides strong evidence supporting the existence of a carbocation intermediate.

Computational Studies on the Reactivity of Substituted 1,2-Dihydro-1,2-azaborines

We have investigated important intermediates of electrophilic aromatic substitution reactions and one-electron oxidation of substituted 1,2-dihydro-1,2-azaborines with density-functional theory. The results show that electrophilic substitution reactions and one-electron oxidation of substituted 1,2-azaborines are generally much more favorable than those of the corresponding benzene derivatives. Both chlorination and nitration of several boron-unsubstituted 1,2-azaborines are expected to break the boron-hydrogen bond, yielding boron-chlorinated 1,2-azaborines and a novel class of boron-bound 1,2-azaborinyl nitrites, respectively. Comparison between the relative stabilities of C(3)-bound and C(5)-bound Wheland intermediates of different electrophilic substitution reactions of 1,2-azaborines further suggests that the preference of the C(3)- over C(5)-substitution decreases with decreasing electrophilicity of the attacking group.

Superacidity of boron acids H2(B12X12) (X = Cl, Br).

Acid remarks: The anhydrous diprotic boron acids H(2)(B(12)X(12)) (X = Cl, Br; see picture, B orange, X green) are the first examples of diprotic superacids and may be the strongest acids yet isolated. Both protons protonate benzene to give benzenium ion salts that are stable at room temperature. These acids owe their existence to the stability of the icosahedral B(12) cluster with its dinegative charge buried beneath a layer of halide substituents.

Discrimination Among Geometrical Isomers of α-Linolenic Acid Methyl Ester Using Low Energy Electron Ionization Mass Spectrometry

There is a consensus that electron impact ionization mass spectrometry is not capable of discriminating among geometrical isomers of unsaturated fatty acid methyl esters (and in general olefinic compounds). In this paper, we report the identification of all eight geometrical isomers of alpha-linolenic acid, one of the few essential omega-3 fatty acids that has attracted great attention, using low-energy electron ionization mass spectrometry. Three electron energies 70, 50, and 30 eV were studied and the mass spectrum of each isomer was obtained from the analysis of different concentrations of a standard mixture of alpha-linolenic acid methyl ester geometrical isomers to ensure the robustness of the method. Principal component analysis was employed to model the complex variation of m/z intensities across the isomers. Only using the data of 30 eV energy was complete differentiation among geometrical isomers observed. The unique cleavage pattern of the alpha-linolenic acid methyl ester isomers leading to a benzenium ion structure is discussed and general fragmentation rules are derived using the mass spectra of over 300 compounds with different kinds and levels of unsaturation. Application of the proposed method is not limited to alpha-linolenic acid. It can potentially be used to identify the geometrical isomers of any compounds with an olefinic chain.

On reversible bonding of hydrogen molecules on platinum clusters

The local reactivity of hydrogenated platinum clusters (Pt clusters) has been studied using the regional density functional theory method. We observed that antibond orbitals constitute the preferable binding site for hydrogen molecules H(2). Those sites are characterized by lowered electronic chemical potential and strong directionality and exhibit electrophilic nature. The platinum-dihydrogen (Pt-H(2)) sigma complexes were formed only by occupation of the lowest electronic chemical potential sites associated with Pt-H antibonds (sigma(PtH) ( *)) in saturated platinum clusters. The formation of sigma complexes caused mutual stabilization with the trans Pt-H bond. Such activated H(2) molecules on Pt clusters in a sense resemble heme-oxygen (heme-O(2)) complex with interaction strength greater than physisorption or hydrogen bonding but below chemisorption strength.

Methanol to Olefin Conversion on HSAPO-34 Zeolite from Periodic Density Functional Theory Calculations: A Complete Cycle of Side Chain Hydrocarbon Pool Mechanism

For its unique position in the coal chemical industry, the methanol to olefin (MTO) reaction has been a hot topic in zeolite catalysis. Due to the complexities of catalyst structure and reaction networks, many questions such as how the olefin chain is built from methanol remain elusive. On the basis of periodic density functional theory calculations, this work establishes the first complete catalytic cycle for MTO reaction via hexamethylbenzene (HMB) trapped in HSAPO-34 zeolite based on the so-called side chain hydrocarbon pool mechanism. The cycle starts from the methylation of HMB that leads to heptamethylbenzenium ion (heptaMB+) intermediate. This is then followed by the growth of side chain via repeated deprotonation of benzenium ions and methylation of the exocyclic double bond. Ethene and propene can finally be released from the side ethyl and isopropyl groups of benzenium ions by deprotonation and subsequent protonation steps. We demonstrate that (i) HMB/HSAPO-34 only yields propene as the primary ...

Theoretical Study of Ethylbenzenium Ions: The Mechanism for Splitting Off Ethene, and the Formation of a π Complex of Ethene and the Benzenium Ion

Theoretical Study of Ethylbenzenium Ions: The Mechanism for Splitting Off Ethene, and the Formation of a π Complex of Ethene and the Benzenium Ion

Reactivity of Substituted Chlorines and Ensuing Dechlorination Pathways of Select PCB Congeners with Pd/Mg Bimetallics

Conflicting accounts occur on the reactivity of substituted chlorines and the ensuing dechlorination pathway of PCBs undergoing catalytic hydrodechlorination (HDCl). In order to understand these relationships, intermediates and dechlorination pathways of carefully selected 17 congeners were investigated with reactive Pd/Mg systems that bring about their rapid and complete dechlorination. The preferential site of electrophilic attack and its mechanistic aspects were interpreted in terms of steric, inductive, and resonance stabilization. The trends for electrophilic substitution were consistently p- > m- > o- positions indicating that more toxic "coplanar" PCB congeners were easily reduced. The dechlorination rates and pathways were influenced both by inductive effect of Cl that likely governs the stability of the intermediate arenium ion and by steric effects primarily effecting the adsorption step (especially for the o-congeners). Electrophilic attack occurred preferentially on the less substituted phenyl ring in absence of steric effects. A distinct correlation between rate of HDCl and the degree of chlorination was not observed, rather it depended on positions of Cl with respecttothe biphenyl bond, and the dominance between counteracting factors of deactivation by subsequent chlorinations and improvement in probability of dechlorination through increased number of Cls.

Theoretical Study of Ethylbenzenium Ions: The Mechanism for Splitting Off Ethene, and the Formation of a π Complex of Ethene and the Benzenium Ion

Structures and energies of protonated ethylbenzene and 2,6-dimethylethylbenzene have been studied by quantum chemical calculations. The main goal is to study the mechanism for splitting off ethene from protonated ethylbenzene. Data are reported for the four ethylbenzenium isomers arising depending on the position of the proton on the benzene ring: a pi complex where ethene is weakly bonded to a benzenium ion and two transition states connected with the cleavage of the ethylbenzenium ion. The larger part of the data that are reported has been obtained at the B3LYP/cc-pVTZ level of theory. Energies obtained by the Gaussian-3 G3B3 composite method are also given. Computations have also been carried out at the MP2/6-311++G(d,p) level of theory. The calculated results can be reconciled with published experimental observations, and they give information about the reaction system that is not obtained by experimental techniques.

Hydrogen Tunneling in Protonolysis of Platinum(II) and Palladium(II) Methyl Complexes: Mechanistic Implications

Platinum(II) and palladium(II) complexes are well-known to catalyze the partial oxidation of alkanes. Herein, we present experimental evidence that tunneling occurs in the protonolysis of M(II)-CH(3) (M = Pt, Pd) model systems. We propose that there may be a connection between the observation of tunneling and a protonolysis mechanism involving direct protonation of the M-C bond and that tunneling may also be expected for electrophilic C-H activation of methane by Pt(II) and Pd(II) that proceeds via direct proton loss from a sigma complex.

Radical cation/radical reactions: A Fourier transform ion cyclotron resonance study of allyl radical reacting with aromatic radical cations

A method for the study of reactions of open-shell neutrals (radicals) and radical cations is described. Pyrolysis (25–1500 °C) of thermally labile compounds, such as, 1,5-hexadiene via a Chen nozzle yields a seeded beam of reactive species in helium. The pyrolysis products are then analyzed by electron ionization (EI) or reacted with stored ions. Electron ionization of the pyrolysis products of 1,5-hexadiene shows that both the allyl radical and allene are generated. Reactions of benzene radical cations and the pyrolysis products of 1,5-hexadiene result in carbon–carbon bond formation. Those reactions of allyl radical with the benzene radical cation yield the C 7H 7 + ion of m/ z 91, permitting an unusual entry into arenium ions. The reaction of allene with benzene radical cation in contrast yields C 9H 10 + and C 9H 9 +.

Deprotonation of 3(2H)-Furanone and 3(2H)-Thiophenone by Carbanions in the Gas Phase: Disproportionately High Aromaticity of the Transition State: An Ab Initio Study

The reversible deprotonation of 3(2H)-furanone (3H-O) and 3(2H)-thiophenone (3H-S) by a series of delocalized carbanions and by CN(-), and the identity proton transfer of 3H-O to its conjugate base (3(-)-O) and of 3H-S to 3(-)-S have been studied at the MP2//6-31+G** level. The main objective has been to examine to what extent the aromaticity of 3(-)-O and 3(-)-S is expressed at the transition state of these reactions and how the intrinsic barriers are affected by the transition state aromaticity. Aromaticity parameters such as NICS values, HOMA and Bird Indices indicate a disproportionately high degree of aromatic stabilization of the transition state. This stabilization results in a reduction of the intrinsic barriers which is most clearly manifested in the identity reactions. However, these reductions are relatively modest compared to those reported previously for the identity proton transfers from the benzenium ion to benzene and of cyclopentadiene to its conjugate base, reflecting the smaller aromatic stabilization of 3(-)-O and 3(-)-S compared to those of benzene and cyclopentadienyl anion.

DFT study of the biphenylene–NO complexes formed in nitration mechanism

AbstractThe most probable complexes formed in biphenylene (BP) nitration pathway have been investigated at B3LYP/6‐31+G(d,p) level of theory in the gas phase. To obtain more accurate energies, single point calculations were carried out at B3LYP/6‐31++G(2d,2p), B3PW91/6‐31+G(d,p), and B3PW91/6‐31++G(2d,2p) levels using B3LYP/6‐31+G(d,p) optimized geometry. The six intermediates and one transition state were found before the subsequent formation of the arenium ion on the potential energy surface of the electrophilic nitration of BP. It was also shown that the position β in the BP is much more susceptible to electrophilic attack than the competing position α. The Natural Bond Orbital (NBO), Charges from Electrostatic Potentials using a Grid based method (CHelpG), and Merz–Singh–Kollman (MK) charges and s‐characters of atoms involved in the reaction mechanism were calculated. Inspection of charges in the moieties indicates that the positive charge in all complexes is chiefly located on the BP, which means that theNO2 moiety received the electron from the BP. To investigate the nature of BP–${\rm NO}_{2}^{\rm + } {\rm }$ interaction in the five π‐complexes, atoms in molecules (AIM) analysis was performed. The AIM results suggested that the BP–${\rm NO}_{2}^{+ } {\rm }$ interactions have an electrostatic characteristic. In addition, high electrostatic interactions were predicted in π‐complexes in which one of the oxygen atoms of ${\rm NO}_{2}^{ + } {\rm }$ interacts with the BP. Nucleus‐independent chemical shift (NICS) methodology has been applied to study the change of antiaromaticity in four‐membered ring of BP upon complexation with ${\rm NO}_{2}^{+ } {\rm }$. The results based on NICS calculations show that antiaromaticity of four‐membered ring decreases upon complexation. Copyright © 2008 John Wiley & Sons, Ltd.

Does an Ethene/Benzenium Ion Complex Exist? A Discrepancy between B3LYP and MP2 Predictions

To investigate the possible existence of a complex between the benzenium ion and ethene, computations employing B3LYP and MP2 were carried out. The two methodologies gave conflicting answers; B3LYP confirmed the existence, but according to MP2, the structure found by B3LYP transforms into an ethylbenzenium ion. Computations utilizing the CCSD and QCISD methods showed the B3LYP result to be correct; 21 kJ/mol is needed to separate the two moieties.

Reaction of 6-Methyl-2,2′-bipyridine with 1,2-Os3(CO)10(MeCN)2: Syntheses, Reductive Elimination/Ligand Displacement Kinetics, and X-ray Diffraction Structures of the Isomeric Clusters HOs3(CO)9(μ2-N2C11H9) and H2Os3(CO)8(μ3-N2C11H8)

Treatment of Os3(CO)10(MeCN)2 (1) with the heterocyclic ligand 6-methyl-2,2′-bipyridine (6-Me-2,2′-bpy) at room temperature leads to the formation of the isomeric hydride-bridged clusters HOs3(CO)9(μ2-CH2N2C10H7) (2) and HOs3(CO)9(μ2-N2C11H9) (3). The cyclometalation of the ancillary 6-Me group in 2 and the ortho metalation of the nonsubstituted pyridyl ring in 3 have been confirmed by spectroscopic and crystallographic methods. Thermolysis of 2 leads to the formation of 3 and the dihydride cluster H2Os3(CO)8(μ3-N2C11H8) (4); the latter cluster, whose structure has been crystallographically determined, derives from a formal loss of CO and C−H bond activation of the methylene moiety in 2. Heating 2 in the presence of ligand-trapping agents proceeds with the release of the 6-Me-2,2′-bpy ligand and formation of Os3(CO)9L3 [where L = CO, P(OMe)3]. The kinetics for the reaction between 2 and added ligand have been investigated by UV−vis and NMR spectroscopies and found to be first-order in starting cluster and independent of the incoming ligand. Parallel kinetic experiments employing the deuterated cluster DOs3(CO)9(μ2-CD2N2C10H7) (2-d3), which was prepared from cluster 1 and 6-Me-d3-2,2′-bpy, confirm the existence of a primary kinetic isotope effect (KIE) of 1.78 at 323 K. The KIE data and the calculated activation parameters [ΔH⧧ = 21.7(4) kcal/mol; ΔS⧧ = −13(1) eu] are strongly suggestive of a reaction scheme involving a rate-limiting reductive coupling of the bridging hydride ligand and cyclometalated alkyl moiety in 2 to furnish a putative sigma complex containing an intact methyl group bound to the Os3 cluster, prior to the generation of the unsaturated cluster Os3(CO)9(μ-N2C11H10). Thermolysis of 3 in the presence of added P(OMe)3 does not furnish free 6-Me-2,2′-bpy but proceeds by a ligand-induced displacement of the methyl-substituted pyridyl ring and formation of the cluster compound HOs3(CO)9[P(OMe)3](μ2-N2C11H9) (5). The kinetics for the reaction between 3 and P(OMe)3 have been studied over the temperature range 333−356 K, and on the basis of the observed activation parameters [ΔH⧧ = 13.0(3) kcal/mol; ΔS⧧ = −30(1) eu] and the first-order dependence on the cluster and ligand, an associative process that involves P(OMe)3 ligand attack on the cluster and release of the methyl-substituted pyridyl ring in the rate-limiting step is proposed.