Arenium Research Articles

Thermodynamic versus kinetic control in substituent redistribution reactions of silylium ions steered by the counteranion.

An in-depth experimental and theoretical study of the substituent exchange reaction of silylium ions is presented. Apart from the substitution pattern at the silicon atom, the selectivity of this process is predominantly influenced by the counteranion, which is introduced with the trityl salt in the silylium ion generation. In contrast to Müller's protocol for the synthesis of triarylsilylium ions under kinetic control, the use of Reed's carborane anions leads to contact ion pairs, allowing selective formation of trialkylsilylium ions under thermodynamic control. DFT calculations finally revealed an unexpected mechanism for the rate-determining alkyl exchange step, which is initiated by an unusual 1,2-silyl migration in the intermediate ipso-disilylated arenium ion. The resulting ortho-disilylated arenium ion can then undergo an alkyl transfer via a low-barrier five-centered transition state.

Aromatic Bromination of N-Phenylacetamide Inside CNTs. Are CNTs Real Nanoreactors Controlling Regioselectivity and Kinetics? A QM/MM Investigation

We carried out a computational investigation on the mechanism of the bromination reaction of N-phenylacetamide inside CNTs, in water, and in an aprotic solvent (ethylbenzene). A full QM and a QM/MM approach was used. In the aprotic solvent, a Wheland intermediate (ion pair formed by arenium ion and chloride) exists only for the attack in the ortho position, while the para attack proceeds in a concerted manner (concerted direct substitution). The reaction is catalyzed by the HCl byproduct, which lowers significantly the activation barriers. The ortho product is favored, in contrast to the common belief based on simple steric effects. In water solution a Wheland intermediate was located for both ortho and para attacks (the ion pair is stabilized by the polar protic solvent). The formation of the para product is favored with respect to the ortho product: 9.0 and 9.9 kcal mol–1 are the corresponding activation barriers. Inside CNTs, as found in aprotic solvent, the Wheland-type arenium ion exists only along t...

Reactive pathways in the bromobenzene-ammonia dimer cation radical: Evidence for a roaming halogen radical

Photoinitiated reactions in van der Waals complexes provide a means to examine reactive pathways from well-defined initial geometries. In recent work, we re-examined reactive pathways following resonant two-photon ionization (R2PI) of the chlorobenzene-ammonia (PhCl⋯NH3) dimer. The dimer cation radical reacts primarily via Cl atom loss, and additional channels corresponding to HCl and H atom loss were identified. The structure of the reactive complex was confirmed as an in-plane σ-type, and computational studies of the dimer cation radical potential energy landscape revealed two nearly isoenergetic arenium ion intermediates (or Wheland intermediates). The intermediate produced from ipso addition was not stable with respect to either Cl or HCl loss, and the relative branching observed in experiment was well reproduced by microcanonical transition state theory (TST) calculations. Here, we report experimental and computational studies of the related PhBr⋯NH3 dimer, examined for the first time. We present evidence that the dimer structure is also an in-plane σ-type. However, in contrast to the PhCl⋯NH3 system, calculations predict that the structure of the reactive intermediate corresponds to a distonic ion-radical complex, where the radical density is largely localized on the bromine atom. The calculated barrier to HBr loss is sufficiently high to render this channel nearly insignificant (less than 1% branching) in TST calculations, yet experiment shows a sizable (37%) branching into this channel. We rationalize these results in terms of a roaming Br radical mechanism for HBr formation.

Alkylation of toluene with cyclohexene over phosphotungstic acid: A combined experimental and computational study

Abstract C C bond formation between an aromatic ring and cyclohexene is of great importance for the preparation of bicyclic hydrocarbons. In this work, alkylation of toluene with cyclohexene over phosphotungstic acid (HPW) has been investigated using both experimental reaction kinetics and ab initio density functional theory (DFT) methods. Kinetic models for the formations of o-cyclohexyltoluene, m-cyclohexyltoluene and p-cyclohexyltoluene have been established and used to fit the experimental kinetic data. The reaction rate constants and apparent activation energies for the formation of the three products have been calculated and compared. Based on DFT calculation, the energy profiles with relevant equilibrium and transition states have been determined for the adsorption of reactants, formation & deprotonation of the metastable states and desorption of the products. A metastable state of an adsorbed arenium ion on HPW has been demonstrated. The rate constants (k) for the major elementary steps at different temperatures have been calculated using DFT data. The apparent rate constants are further determined theoretically based on kinetic analysis. Results show that the theoretical calculation fits well with experimental data, both of which have similar tendency changing with temperature. Combined experimental kinetic results and DFT calculations suggest that both the formation and deprotonation of the metastable state play key roles in affecting the product selectivity.

An alternative description of aromaticity in metallabenzenes

Experimental and theoretical studies of metallabenzenes have revealed important differences in their aromatic properties when compared with their organic analogs. In this work, an alternative explanation for this unconventional behavior is presented based on the idea that metallabenzenes can be conceived as an intermediate structure between the benzene and the benzenium ion; an idea derived from a critical analysis of the isolobal analogies in metallabenzenes.

Approaches to Sigma Complexes via Displacement of Agostic Interactions: An Experimental and Theoretical Investigation

A series of coordinatively unsaturated, 16-electron, cationic ruthenium complexes bearing PNP pincer ligands of the type [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) has been prepared and characterized. These complexes exhibit agostic interaction in the sixth coordination site. The binding of various X–H (X = H, Si, B, and C) bonds in small molecules such as H2, silanes, tetracoordinate boranes, and CH4 to the ruthenium center via displacement of the relatively weak agostic interaction in these complexes has been studied. Structures of [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) complexes and the sigma-borane complex trans-[RuH(CO)(η1-H-BH2·NMe3)(PNRP)]+ (R = PhCH2 (15)) have been established by X-ray crystallography. The relative binding strengths of the X–H bonds to ruthenium center in these complexes has been studied using computational methods.

Nucleophile/Electrophile Combinations in Aromatic Substitution: From Wheland to Wheland–Meisenheimer Intermediates Using Strongly Activated Arenes

This short review provides an overview on the interaction between 1,3,5-triaminobenzene derivatives and different kinds of electrophiles. Due to the ambident reactivity of these nucleophiles (i.e., at the nitrogen atom of the substituents and at the aromatic carbon atom) different compounds can be obtained. Particular attention is devoted to the detection, isolation, and characterization of covalent intermediates of aromatic substitution, starting from Wheland intermediates until the first detection and characterization of Wheland–Meisenheimer intermediates.1 Introduction2 Reactions between 1,3,5-Triaminobenzene Derivatives and Charged Electrophiles2.1 The Proton as an Electrophile2.2 Arenediazonium Salts as Electrophiles3 Reactions between 1,3,5-Triaminobenzene Derivatives and Neutral­ Electrophiles3.1 Alkyl Halides as Electrophiles3.2 Acyl Halides and Sulfonyl Chlorides as Electrophiles3.3 Aryl Halides and Heteroaryl Halides as Electrophiles3.4 Polynitroheteroaromatics as Electrophiles4 Conclusion

Solvated proton as the main reagent and a catalyst in the single-stage aromatic sulfonation and protodesulfonation of sulfonic acids

This review is devoted to the critical analysis of the existing contradictory ideas about kinetic description and the mechanisms of electrophilic aromatic substitution (EAS), ie, reversible arene sulfonation and hydrolysis (protodesulfonation) of arenesulfonic acids. Contemporary scientific and educational literature contains prevailing, widespread points of view on the multistage nature processes of the EAS that proceed via π- and σ-complexes and are accompanied by arenium ion formation. This interpretation does not account for determining influence of the medium, and the process accompanies the loss of aromaticity. The review presents an alternative point of view: These processes proceed via single-stage and the same transition state. To substantiate this position, we reviewed in detail the contemporary ideas on the solvated proton structure, the limits of applicability of acidity function Но and excess acidity Х scales, and the experimental solubility data of arenesulfonic acid hydrates in sulfuric acid. This solubility depends on the medium acidity (ho), which provides the formation of 3 conjugated types of hydrates: anionic (АrSO3−·H2O), hydroxonium-arene sulfonated (АrSO3−·H3O+), and the hydroxonium complex with the H-form of АrSO3Н·H3O+ acid. In sulfuric acid solutions with an excess of water, irreversible hydrolysis proceeds with the participation of ArSO3−·H3O+. However, protodesulfonation becomes reversible and proceeds with the participation of ArSO3Н·H3O+in solutions with an excess of sulfuric acid. A conclusion has been made about determining the role of the hydrated proton environment by means the formation of a cyclic transition state. An alternative 2-stage mechanism of EAS is logically and energetically unsubstantiated and, therefore, should be rejected.

Unsymmetric Twofold Scholl Cyclization of a 5,11-Dinaphthyltetracene: Selective Formation of Pentagonal and Hexagonal Rings via Dicationic Intermediates.

Even though the Scholl reaction is one of the most powerful processes for the synthesis of polycyclic aromatic hydrocarbons (PAHs), its mechanism still remains a subject of discussion. Herein, we report a unique twofold Scholl cyclization of a 5,11-dinaphthyltetracene. Single-crystal X-ray diffraction analysis of the cyclization product revealed that unsymmetric cyclizations of the two naphthyl groups resulted in the formation of fully unsaturated pentagonal and hexagonal rings. The thus obtained product exhibits a twisted π-surface and an absorption band that reaches up to 950 nm. A combined experimental and theoretical study showed that such unsymmetric Scholl cyclizations can be rationalized in terms of a mechanism that involves dicationic intermediates, which stands in contrast to previously reported pathways based on radical cations and arenium ions.

Unsymmetric Twofold Scholl Cyclization of a 5,11‐Dinaphthyltetracene: Selective Formation of Pentagonal and Hexagonal Rings via Dicationic Intermediates

AbstractEven though the Scholl reaction is one of the most powerful processes for the synthesis of polycyclic aromatic hydrocarbons (PAHs), its mechanism still remains a subject of discussion. Herein, we report a unique twofold Scholl cyclization of a 5,11‐dinaphthyltetracene. Single‐crystal X‐ray diffraction analysis of the cyclization product revealed that unsymmetric cyclizations of the two naphthyl groups resulted in the formation of fully unsaturated pentagonal and hexagonal rings. The thus obtained product exhibits a twisted π‐surface and an absorption band that reaches up to 950 nm. A combined experimental and theoretical study showed that such unsymmetric Scholl cyclizations can be rationalized in terms of a mechanism that involves dicationic intermediates, which stands in contrast to previously reported pathways based on radical cations and arenium ions.

Transition‐Metal Complexes with (C–C)→M Agostic Interactions

The coordination of sigma bonding electron density to a Lewis acid was for some time largely the domain of boron hydride chemistry. Subsequently it was found that C–H bonds could undergo intramolecular coordination to metal centers, forming what are now known as “agostic bonds” and “agostic complexes”. Thereafter, even the H–H bond in molecular hydrogen was found to be capable of coordination, leading to what are commonly referred to as “sigma complexes”. Eventually, coordination by much more shielded C–C bonds was established, and is of particular significance with regard to to the interest in C–C bond activation. This review focuses on the key developments in this field, including the nature of the bonding interactions in these and some related species, as revealed by spectroscopic, structural, and theoretical studies, their involvement in important chemical reactions such as alkene polymerizations and metathesis, and comparisons with the other types of electron‐deficient bridge bonding.

Reversible C-C bond formation at a triply cyclometallated platinum(iv) centre.

The oxidation of the tribenzylphosphine derivative of the doubly cylcometallated platinum(ii) complex of diphenylpyridine, 1, with PhICl2 led, as a first step, to the formation of a highly electrophilic metal centre which attacked the benzyl phosphine to give a triply cyclometallated species as the arenium ion. The highly acidic arenium ion protonated unreacted starting 1, a reaction that could be supressed by the addition of water, and gave the neutral species 2(t). Octahedral complex 2(t) was induced to reductively couple, with two five-membered rings coupling to give square planar complex 5 containing a nine-membered ring. The crystal structure of 5 showed the nine-membered ring to span trans across the square planar metal accompanied by considerable distortion: the P-Pt-N bond angle is 155.48(5)°. Oxidation of 5 with PhICl2 resulted in the addition of two chlorides and a change of the nine-membered ring ligand coordination to cis at an octahedral centre, still with considerable distortions: the P-Pt-N bond angle in the crystal structure of 6 is 99.46(5)°. Treatment of 2(t) with AgBF4 also induced a coupling to give a nine-membered ring, and the fluxional three coordinate complex 7. A mono-methylated version of 1, Me-1, was prepared and similar reactions were observed. The presence of the methyl group allowed us to observe selectivity in the coupling reaction to give the nine-membered ring, with two products (a-Me-7 and b-Me7) being initially formed in the ratio 7 : 1. The concentrations of two products changed with time giving a final ratio of 1 : 8 at room temperature (half-life 48 hours), the equilibration being made possible by a reversible C-C bond forming reaction. Reaction of complexes 7 with CO or hydrogen left the nine-membered ring intact, though oxidative degradation resulted in decomplexation of the phosphine donor, accompanied by formation of a P[double bond, length as m-dash]O group.

NHC-Catalyzed Enantioselective Dearomatizing Hydroacylation of Benzofurans and Benzothiophenes for the Synthesis of Spirocycles

Benzofurans and benzothiophenes have been efficiently employed as substrates in an enantioselective intramolecular hydroacylation. Breaking aromaticity in a 5-exo-trig cyclization of easily accessible heteroarenes by NHC-catalyzed hydroacylation gives access to a simple class of mainly unexplored spirocycles with up to 99% enantiomeric excess (ee). The formed product class bears interesting three-dimensional pseudoaxial chirality and shows typical ketone reactivity. Density functional theory (DFT) calculations reveal erosion of the ee via a stepwise mechanism bearing a hetero Wheland intermediate. Theoretical data are in agreement with deuterium-labeling studies and a linear correlation between electronics (σ) and the ee.

ChemInform Abstract: Electrophilic Aromatic Substitution: New Insights into an Old Class of Reactions

AbstractReview: 68 refs.

ChemInform Abstract: Cine Substitution with Arylzinc Reagents: Scope and Mechanistic Studies.

The unexpected ability of arylzinc reagents bearing electron-donating substituents to react in a Friedel-Crafts fashion (cine) with electrophiles like perpivaloylated glucoside bromide and benzhydryl bromides in competition with organometallic coupling (ipso) is shown. The stereoelectronic factors required to promote the cine reactivity versus the classical ipso, and the mechanism of this alternative pathway, have been investigated. The Wheland intermediate is deprotonated intramolecularly in a 1,2-shift but also in a longer-range shift, leaving in this case the C-Zn untouched. In the latter case, it is possible to take advantage of this result for further functionalization.

Hydrogenation and Transfer Hydrogenation Promoted by Tethered Ru−S Complexes: From Cooperative Dihydrogen Activation to Hydride Abstraction/Proton Release from Dihydrogen Surrogates

Hydrogenation and transfer hydrogenation of imines with cyclohexa-1,4-dienes, as well as with a representative Hantzsch ester dihydrogen surrogate, are reported. Both processes are catalyzed by tethered Ru-S complexes but differ in the activation mode of the dihydrogen source: cooperative activation of the H-H bond at the Ru-S bond leads to the corresponding Ru-H complex and protonation of the sulfur atom, whereas the same cationic Ru-S catalyst abstracts a hydride from a donor-substituted cyclohexa-1,4-diene to form the neutral Ru-H complex and a low-energy Wheland intermediate. A sequence of proton and hydride transfers on the imine substrate then yields an amine. The reaction pathways are analyzed computationally, and the established mechanistic pictures are in agreement with the experimental observations.

Electrophilic Aromatic Substitution: New Insights into an Old Class of Reactions.

The classic SEAr mechanism of electrophilic aromatic substitution (EAS) reactions described in textbooks, monographs, and reviews comprises the obligatory formation of arenium ion intermediates (σ complexes) in a two-stage process. Our findings from several studies of EAS reactions challenge the generality of this mechanistic paradigm. This Account focuses on recent computational and experimental results for three types of EAS reactions: halogenation with molecular chlorine and bromine, nitration by mixed acid (mixture of nitric and sulfuric acids), and sulfonation with SO3. Our combined computational and experimental investigation of the chlorination of anisole with molecular chlorine in CCl4 found that addition-elimination pathways compete with the direct substitution processes. Detailed NMR investigation of the course of experimental anisole chlorination at varying temperatures revealed the formation of addition byproducts. Moreover, in the absence of Lewis acid catalysis, the direct halogenation processes do not involve arenium ion intermediates but instead proceed via concerted single transition states. We also obtained analogous results for the chlorination and bromination of several arenes in nonpolar solvents. We explored by theoretical computations and experimental spectroscopic studies the classic reaction of benzene nitration by mixed acid. The structure of the first intermediate in this process has been a subject of contradicting views. We have reported clear experimental UV/vis spectroscopic evidence for the formation of the first intermediate in this reaction. Our broader theoretical modeling of the process considers the effects of the medium as a bulk solvent but also the specific interactions of a H2SO4 solvent molecule with intermediates and transition states along the reaction path. In harmony with the obtained spectroscopic data, our computational results reveal that the structure of the initial π complex precludes the possibility of electronic charge transfer from the benzene π system to the nitronium unit. In contrast to usual interpretations, our computational results provide compelling evidence that in nonpolar, noncomplexing media and in the absence of catalysts, the mechanism of aromatic sulfonation with sulfur trioxide is concerted and does not involve the conventional σ-complex (Wheland) intermediates. Stable under such conditions, (SO3)2 dimers react with benzene much more readily than monomeric sulfur trioxide. In polar (complexing) media, the reaction follows the classic two-stage SEAr mechanism. Still, the rate-controlling transition state involves two SO3 molecules. The reactivity and regioselectivity in EAS reactions that follow the classic mechanistic scheme are quantified using a theoretically evaluated quantity, the electrophile affinity (Eα), which measures the stabilization energy associated with the formation of arenium ions. Examples of applications are provided.

Cine Substitution with Arylzinc Reagents: Scope and Mechanistic Studies.

The unexpected ability of arylzinc reagents bearing electron-donating substituents to react in a Friedel-Crafts fashion (cine) with electrophiles like perpivaloylated glucoside bromide and benzhydryl bromides in competition with organometallic coupling (ipso) is shown. The stereoelectronic factors required to promote the cine reactivity versus the classical ipso, and the mechanism of this alternative pathway, have been investigated. The Wheland intermediate is deprotonated intramolecularly in a 1,2-shift but also in a longer-range shift, leaving in this case the C-Zn untouched. In the latter case, it is possible to take advantage of this result for further functionalization.

New electron-donor and -acceptor architectures from benzofurazans and sym-triaminobenzenes: intermediates, products and an unusual nitro group shift.

New all-conjugated C-C coupling products bearing both an electron-poor and an electron-rich aromatic moiety have been obtained from the reaction between sym-triaminobenzene derivatives and a series of isomeric chloro-nitrobenzofurazans. The reactions occur under mild reaction conditions, and in some cases a different behaviour depending on the presence, or not, of triethylamine was observed. From 1,3,5-tris(N-morpholinyl)benzene and 5-chloro-4-nitrobenzofurazan in the presence of triethylamine an unexpected product derived from the shift of the nitro group from C-4 to C-5 of the electrophile and bearing the nucleophile at position 4 was obtained. Moreover, from the coupling between 1,3,5-tris(N-pyrrolidinyl)benzene and 4-chloro-7-nitrobenzofurazan a highly stable Wheland intermediate was isolated.

Iron‐Promoted CC Bond Formation in the Gas Phase

AbstractAn unusual iron transfer and carbon–carbon coupling take place in gas‐phase ionized mixtures containing ferrocene and dichloromethane. Ferrous chloride and the protonated benzenium ion are eventually formed by a thermal and efficient reaction, through stable intermediates that undergo a remarkable reorganization. The mechanism of the concerted iron extrusion, carbon–chlorine bond activation and carbon–carbon bond formation is elucidated by electronic structure calculations that show the crucial role of iron.