Theoretical study of mechanisms of aromatic nucleophilic substitution in the gas phase

Dynamics of a gas-phase SNAr reaction: non-concerted mechanism despite the Meisenheimer complex being a transition state

The commonly accepted mechanism of the nucleophilic aromatic substitution (SNAr) reaction has been found to be governed by the nature of the Meisenheimer structure on the potential energy surface. A stable Meisenheimer intermediate favors a stepwise mechanism, while a Meisenheimer transition state favors a concerted mechanism. Here, we show by using a detailed potential energy map (using the DFT and DLPNO-CCSD(T)/CBS methods) and ab initio classical trajectory simulations that the F- + C6H5NO2 SNAr reaction involves a Meisenheimer transition state and follows a stepwise mechanism in contrast to the expected concerted pathway. The stepwise mechanism observed in the trajectory simulations takes place by the formation of various ion-dipole and σ-complexes. While the majority of the trajectories follow the multi-step mechanism and avoid the minimum energy path, a considerable fraction exhibit a roaming atom mechanism where the F atom hovers around the phenyl ring before the formation of the products.

How Do Aromatic Nitro Compounds React with Nucleophiles? Theoretical Description Using Aromaticity, Nucleophilicity and Electrophilicity Indices

In this study, we present a complete description of the addition of a model nucleophile to the nitroaromatic ring in positions occupied either by hydrogen (the first step of the S_NAr-H reaction) or a leaving group (S_NAr-X reaction) using theoretical parameters including aromaticity (HOMA), electrophilicity and nucleophilicity indices. It was shown both experimentally and by our calculations, including kinetic isotope effect modeling, that the addition of a nucleophile to the electron-deficient aromatic ring is the rate limiting step of both S_NAr-X and S_NAr-H reactions when the fast transformation of σ^H-adduct into the products is possible due to the specific reaction conditions, so this is the most important step of the entire reaction. The results described in this paper are helpful for better understanding of the subtle factors controlling the reaction direction and rate.

How Does Nucleophilic Aromatic Substitution Really Proceed in Nitroarenes? Computational Prediction and Experimental Verification

The aim of this paper is to present a correct and complete mechanistic picture of nucleophilic substitution in nitroarenes based on the results obtained by theoretical calculations and experimental observations coming from numerous publications, reviews, and monographs. This work gives the theoretical background to the very well documented experimentally yet still ignored observations that the addition of nucleophiles to halo nitroarenes resulting in the formation of σ(H) adducts, which under proper reaction conditions can be transformed into the product of the SNArH reaction, is faster than the competing process of addition to the carbon atom bearing a nucleofugal group (usually a halogen atom) resulting in the "classic" SNAr reaction. Only when the σ(H) adduct cannot be transformed into the SNArH reaction product, SNAr reaction is observed.

Negative ion gas-phase chemistry of arenes

Reactions of aromatic and heteroaromatic compounds involving anions are of great importance in organic synthesis. Some of these reactions have been studied in the gas phase and are occasionally mentioned in reviews devoted to gas-phase negative ion chemistry, but no reviews exist that collect all existing information about these reactions. This work is intended to fill this gap. In the first part of this review, methods for generating arene anions in the gas phase and studying their physicochemical properties and fragmentation reactions are presented. The main topics in this part are as follows: processes in which gas-phase arene anions are formed, measurements and calculations of the proton affinities of arene anions, proton exchange reactions, and fragmentation processes of substituted arene anions, especially phenide ions. The second part is devoted to gas-phase reactions of arene anions. The most important of these are reactions with electrophiles such as carbonyl compounds and α,β-unsaturated carbonyl and related compounds (Michael acceptors). Other reactions including oxidation of arene anions and halogenophilic reactions are also presented. In the last part of the review, reactions of electrophilic arenes with nucleophiles are discussed. The best known of these is the aromatic nucleophilic substitution (SN Ar) reaction; however, other processes that lead to the substitution of a hydrogen atom in the aromatic ring are also very important. Aromatic substrates in these reactions are usually but not always nitroarenes bearing other substituents in the ring. The first step in these reactions is the formation of an anionic σ-adduct, which, depending on the substituents in the aromatic ring and the structure of the attacking nucleophile, is either an intermediate or a transition state in the reaction path. In the present review, we attempted to collect the results of both experimental and computational studies of the aforementioned reactions conducted since the very beginning of gas-phase negative ion chemistry.

Rate-determining factors in nucleophilic aromatic substitution reactions

Quantum chemical calculations (OPBE/6-311++G(d,p)) have been performed to uncover the electronic factors that govern reactivity in the prototypical S(N)Ar reaction. It was found that intrinsic nucleophilicity--expressed as the critical energy (the energy required for forming the Meisenheimer structure Ph(X)(2)(-)) in the identity substitution reaction X(-) + PhX --> X(-) + PhX (Ph = phenyl)--shows the following approximate trend: NH(2)(-) approximately OH(-) approximately F(-) >> PH(2)(-) approximately SH(-) approximately Cl(-) > AsH(2)(-) approximately SeH(-) approximately Br(-). The periodic trends are discussed in terms of molecular properties (proton affinity of X(-) expressing Lewis basicity of the nucleophile and C(1s) orbital energy expressing Lewis acidity of the substrate) based on a dative bonding model. Furthermore, the stepwise progress of the reactions and the critical structures are analyzed applying energy decomposition analysis. Increased stability, and thereby increased intrinsic nucleophilicity, correlates with decreasing aromatic character of the Meisenheimer structure. This apparent contradiction is explained in consistency with the other observations using the same model.

Aromatic nucleophilic substitution (SNAr) reactions of 1,2- and 1,4-halonitrobenzenes and 1,4-dinitrobenzene with carbanions in the gas phase

In the gas-phase reactions of halonitro- and dinitrophenide anions with X (X = F, Cl, Br, NO(2)) and NO(2) groups in ortho or para position to each other with selected C-H acids: CH(3)CN, CH(3)COCH(3), and CH(3)NO(2), products of the S(N)Ar-type reaction are formed. Nitrophenide anions are generated by decarboxylation of the respective nitrobenzenecarboxylate anions in ESI ion source and the S(N)Ar reaction takes place either in the medium-pressure zone of the ion source or in the collision chamber of the triple quadrupole mass spectrometer. In the case of F, Cl, and NO(2) derivatives, the main ionic product is the respective [NO(2)-Ph-CHR](-) anion (R = CN, COCH(3), NO(2)). In the case of Br derivatives, the main ionic product is Br(-) ion because it has lower proton affinity than the [NO(2)-Ph-CHR](-) anion (for R = CN, COCH(3)). For some halonitrophenide anion C-H acid pairs of reactants, the S(N)Ar reaction is competed by the formation of halophenolate anions. This reaction can be rationalized by the single electron-transfer mechanism or by homolytic C-H bond cleavage in the proton-bound complex, both resulting in the formation of the halonitrobenzene radical anion, which in turn undergoes -NO(2) to -ONO rearrangement followed by the NO(.) elimination.

Solvent effects and mechanism for a nucleophilic aromatic substitution from QM/MM simulations

The nucleophilic aromatic substitution (SNAr) reaction between azide ion and 4-fluoronitrobenzene has been investigated using QM/MM and DFT/PCM calculations in protic and dipolar aprotic solvents. The effects of solvation on the transition structures, the intermediate Meisenheimer complex, and the rate of reaction are elucidated. The large rate increases in proceeding from protic to dipolar aprotic solvents are only reproduced by the QM/MM methodology.

Meisenheimer complexes bonded at carbon and at oxygen

The carbon-bonded gas-phase Meisenheimer complex of 2,4,6-trinitrotoluene (TNT) and the nitromethyl carbanion CH(2)NO(2)(-) (m/z 60) is generated for the first time by chemical ionization using nitromethane as the reagent gas. Collision-induced dissociation (CID) of the Meisenheimer complex furnishes deprotonated TNT, a result of the higher gas-phase acidity of TNT than nitromethane. The formation of Meisenheimer complexes with CH(2)NO(2)(-) in the gas phase is selective to highly electron-deficient compounds such as dinitrobenzene and trinitrobenzene and does not occur with organic molecules with lower electron-affinity such as methanol, methylamine, propionaldehyde, acetone, ethyl acetate, chloroform, toluene, m-methoxytoluene, and even nitrobenzene and p-fluoronitrobenzene. As such, the reaction allows selective detection of TNT in mixtures. Meisenheimer complexes between CH(2)NO(2)(-) and the three dinitrobenzene isomers display distinctive fragmentations. The oxygen-bonded sigma-complex of TNT with the deprotonated hemiacetal anion CH(3)OCH(2)O(-) (m/z 61), represents a different type of Meisenheimer complex. It displays characteristic fragmentation involving loss of HNO(2) upon CID. The combination of a selective ion/molecule reaction (Meisenheimer complex formation) followed by a characteristic CID process provides a second novel and highly selective approach to the detection of TNT and closely related compounds in mixtures. The assay is readily implemented using neutral loss scans in a triple quadrupole mass spectrometer. Gas-phase reactions of denitrosylated TNT with benzaldehyde produce the corresponding dihydrofuran in an aldol condensation, a result that parallels the corresponding condensed-phase reaction.

Trimethylsilyl- and trimethylstannyldimethylphosphane--convenient and versatile reagents for the synthesis of polyfluoroaryldimethylphosphanes

Trimethylsilyldimethylphosphane (Me3SiPMe2) and the corresponding tin compound (Me3SnPMe2) were used as reagents for the substitution of fluorine by the Me2P group in polyfluoroarenes C6F5X (X = F, H, Cl, CF3) and C5NF5. The reactions occur even under mild conditions (T = 0-20 C), either in benzene or without solvent, to give as a rule 4-X-1-(dimethylphosphano)tetrafluorobenzenes (XC6F4PMe2, 1-4) and 4-(dimethylphosphano)tetrafluoropyridine (C5NF4PMe2, 5), respectively, in yields between 75 and 95%. In the case of C6F6, double substitution is also observed, which affords 1,4-bis(dimethylphosphano)tetrafluorobenzene (6). A very efficient route to the compounds XC6F4PMe2 (X = F, H, Cl, CF3) and C5NF4PMe2 was developed as a one-pot reaction of the corresponding fluoroarenes with tetramethyldiphosphane (P2Me4) and trimethyltin hydride (Me3SnH) at moderate temperatures. This process was tested for C6F6 and perfluorobiphenyl which gave C6F5PMe2 (1) and 4,4'-bis(dimethylphosphano)octafluorobiphenyl (7), respectively. The results, which included kinetic measurements that used the intensities of the 31P signals, revealed the influence of the substrate type on the rate of reaction in the sequence: C5NF5>C6F5CF3> C6F5Cl, C6F5PMe2>C6F5H>C6F6>> C6H5F. Ab initio calculations were carried out on the model reactions of pentafluoropyridine with silylphosphane, phosphane or phosphide to discriminate between possible reaction mechanisms. The novel phosphanes were characterised by spectroscopic investigations (NMR, MS), by preparation of the related thiophosphanes ArFP(=S)Me2 (8-14), their spectroscopic and analytic data and single crystal X-ray diffraction studies on five of these derivatives.