Rhodium(II)-catalyzed carbocyclization reaction of alpha-diazo carbonyls with tethered unsaturation

o-Alkynyl-substituted alpha-diazoketones undergo internal cyclization to produce indenone derivatives upon treatment with catalytic quantities of Rh(II)-carboxylates. A variety of structural influences were encountered by varying the nature of the substituent group attached to the diazo center. The cyclization reaction involves addition of a rhodium-stabilized carbenoid onto the acetylenic pi-bond to generate a cycloalkenone carbenoid. The cyclized carbenoid was found to undergo both aromatic and aliphatic C-H insertion as well as cyclopropanation across a tethered pi-bond. Subjection of diazo phenyl acetic acid 3-phenylprop-2-ynyl ester to Rh(II) catalysis furnished 8-phenyl-1, 8-dihydro-2-oxacyclopenta[a]indenone in high yield. The formation of this compound involves cyclization of the initially formed carbenoid onto the alkyne to produce a butenolide which then undergoes C-H insertion into the neighboring aromatic system. When a vinyl ether is added, the initially formed rhodium carbenoid intermediate can be intercepted by the electron-rich pi-bond prior to cyclization. Different rhodium catalysts were shown to result in significant variation in the product ratios. The competition between bimolecular cyclopropanation, 1,2-hydrogen migration, and internal cyclization was probed using several enol ethers as well as diazoesters which possess different substituent groups on the ester backbone. The specific path followed was found to depend on electronic, steric, and conformational factors.     

Rhodium(II) mediated cyclizations of diazo alkynyl ketones

The rhodium(II)-catalyzed reaction of -diazo ketones bearing tethered alkyne units represents a new and useful method for the construction of a variety of substituted cyclopentenones. The process proceeds by addition of the rhodium-stabilized carbenoid onto the acetylenic π-bond to give a vinyl carbenoid intermediate. The resulting rhodium complex undergoes a wide assortment of reactions including cyclopropanation, 1,2-hydrogen migration, CH-insertion, addition to tethered alkynes and ylide formation. The exact pathway followed is dependent on the specific metal/ligand employed and is also influenced by the nature of the solvent. Sulfonium ylide formation occurred both intra and intermolecularly when the reaction was carried out in the presence of a sulfide. In the case where an ether oxygen was present on the backbone of the vinyl carbenoid, cyclization afforded an oxonium ylide which underwent a [1,2] or [2,3]-sigmatropic shift to give a rearranged product. These cyclic metallocarbenoids were also found to interact with a neighboring carbonyl π-bond to produce carbonyl ylide dipoles that could be trapped with added dipolarophiles. The domino transformation was also performed intramolecularly by attaching an alkene directly to the carbonyl group. When 2-alkynyl-2-diazo-3-oxobutanoates were treated with a Rh(II)-catalyst, furo[3,4-c]furans were formed in excellent yield. The 1,5-electrocyclization process involved in furan formation has also been utilized to produce indeno[1,2-c]furans. Rotamer population was found to play a significant role in the cyclization of -diazo amide systems containing tethered alkynes. In this account, an overview of our work in this area is presented.     

The nature of the monomer insertion step in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene: sigma-allyl-insertion mechanism versus pi-allyl-insertion mechanism

We present a theoretical investigation on the nature of the monomer insertion step in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene that employed a gradient-corrected DFT method. We have explored critical elementary steps of the whole polymerization cycle for the trans-1,4 regulating cationic allylnickel(II) [RC3H4NiII(C4H6)L]+ catalyst. These steps are i) cis-butadiene insertion into either the eta 1-sigma-butenyl-NiII bond (sigma-allyl insertion mechanism) or the eta 3-pi-butenyl-NiII bond (pi-allyl insertion mechanism) along with competing pathways for generation of trans-1,4 and cis-1,4 polymer units, and ii) anti-syn isomerization. Based on the analysis of geometric and electronic structures of key species involved and the energetics, we present a detailed insight into the different nature of the monomer insertion step according to the two mechanistic alternatives. An understanding of why the pi-allyl insertion mechanism is favored over the sigma-allyl insertion mechanism is provided. eta 1-sigma-butenyl-NiII Species are predicted to be sparsely populated and also distinctly less reactive than eta 3-pi-butenyl-NiII species. Although they are commonly believed to be reactive intermediates, eta 1-sigma-butenyl-NiII species are, therefore, not likely to be involved along viable pathways for cis-butadiene insertion into the butenyl-NiII bond. The present investigation corroborates our previous conclusion that the pi-allyl insertion mechanism represents the preferred mechanism for the monomer insertion step in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene. On the other hand, the suggested alternative sigma-allyl insertion mechanism has to be considered to be not operative, for both thermodynamic and kinetic reasons. Furthermore, the sigma-allyl insertion mechanism would neither provide a rationalization of cis-trans selectivity nor of chemoselectivity in the allylnickel(II)-catalyzed 1,4-polymerization of 1,3-butadiene.     

Syntheses, structures, and electrochemical properties of platinum(II) complexes containing di-tert-butylbipyridine and crown ether annelated dithiolate ligands

A series of new platinum(II) complexes containing both 4,4'-di-tert-butyl-2,2'-bipyridine (dbbpy) and the extended tetrathiafulvalenedithiolate ligands have been prepared and characterized. These complexes include [Pt(dbbpy)(C8H4S8)] (1; C8H4S82- = 2-{(4,5-ethylenedithio)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate), [Pt(dbbpy)(ptdt)] (2; ptdt = 2-{(4,5-cyclopentodithio)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate), [Pt(dbbpy)(mtdt)] (3; mtdt = 2-{(4,5-methylethylenedithio)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate), [Pt(dbbpy)(btdt)] (4; btdt = benzotetrathiafulvalenedithiolate), [Pt(dbbpy)(C8H6S8)] (5; C8H6S82- = 2-{4,5-bis(methylthio)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate), [Pt(dbbpy)(3O-C6S8)] (6; 3O-C6S82- = 2-{4,5-dithia-(3',6',9'-trioxaundecyl)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate), and [Pt(dbbpy)(4O-C6S8)] (7; 4O-C6S82- = 2-{4,5-dithia-(3',6',9',12'-tetraoxatetradecyl)-1,3-dithiol-2-ylidene}-1,3-dithiol-4,5-dithiolate). The crystal structures of a new ligand precursor (2-[4,5-dithia-(3',6',9',12'-tetraoxatetradecyl)-1,3-dithiol-2-ylidene]-4,5-bis(2-cyanoethylsulfanyl)-1,3-dithiole, IIIc) and complexes 5-7 have been determined by X-ray crystallography. Complexes 1-7 show intense electronic absorption bands in the UV-vis region due to the intramolecular mixed metal/ligand-to-ligand charge-transfer transition, and they display significant solvatochromic behavior. Redox properties of these compounds have been investigated by cyclic voltammetry, and complex 7 shows a significant response for Na+ ions with a large positive shift of ca. 45 mV.     

The bis(trimethylsilyl)methyl group as an effective N-protecting group and site-selective control element in rhodium(II)-catalyzed reaction of diazoamides

The intramolecular rhodium(II)-carbenoid-mediated C-H insertion reaction of structurally varied N-bis(trimethylsilyl)methyl,N-substituted diazoamides is studied. It has been found that in tertiary diazoamides the N-bis(trimethylsilyl)methyl (N-BTMSM) group is effective for conformational control about the amide N-C(O) bond; C-H insertion occurs at the other N-substituent. In C(alpha)-branched diazoamides, the N-BTMSM is found also to exert its influence on the conformational preference about the N-C(alpha) bond, which affects the regioselectivity of the C-H insertion in these systems. In unbranched diazoamides, inherent electronic effects of the N-substituent affect the regio- and chemoselectivity of the reaction; however, in branched diazoamides, electronic effects of the N-substituent and the alpha-substituent at the carbenoid carbon are subtle, but important in the deciding the eventual outcome of the reaction.     

A Linear Free Energy Correlation Study of Rh(Ⅱ)-Mediated Carbenoid Intramolecular C-H Insertion

The reaction mechanism of Rh(Ⅱ)-mediated carbenoid intramolecular C-H insertion have been intensively investigated. The reaction has been observed to be affected by the electrophilicity of the Rh(Ⅱ)-carbene intermediate, the substituents on the carbon at which the C-H insertion occurs, and steric and conformational factors.¹ It has been well documented that the electronic property of the ligands of the Rh(Ⅱ) catalyst has marked influence over the electrophylicity of Rh(Ⅱ)-carbene intermediate. In addition, the α-substituent on the carbenoid carbon are expected to exert similar affect on the reactivity of Rh(Ⅱ)-carbene. According to the reaction mechanism proposed by Doyle, an electron-donating α-substituent decreases the electrophilicity of the Rh(Ⅱ)-carbene complex and causes the C-H insertion to occur with a later transition state,while an electron withdrawing α-substituent operates in the opposite way. However, this prediction is still lack of solid support by experimental data. Most of the studies on the electronic effects have so far been concentrated on electron-withdrawing α-substituent, such as ester or acetyl group. The α-substituent effect, although important, is generally subtle and may be readily overridden by other effects, such as steric and conformational effects. This makes it difficult to accurately evaluate the α-substituenteffect.     

Effects of Alloyed Metal on the Catalysis Activity of Pt for Ethanol Partial Oxidation: Adsorption and Dehydrogenation on Pt(3)M (M=Pt, Ru, Sn, Re, Rh, and Pd)

The adsorption and dehydrogenation reactions of ethanol over bimetallic clusters, Pt(3)M (M = Pt, Ru, Sn, Re, Rh, and Pd), have been extensively investigated with density functional theory. Both the α-hydrogen and hydroxyl adsorptions on Pt as well as on the alloyed transition metal M sites of PtM were considered as initial reaction steps. The adsorptions of ethanol on Pt and M sites of some PtM via the α-hydrogen were well established. Although the α-hydrogen adsorption on Pt site is weaker than the hydroxyl, the potential energy profiles show that the dehydrogenation via the α-hydrogen path has much lower energy barrier than that via the hydroxyl path. Generally for the α-hydrogen path the adsorption is a rate-determining-step because of rather low dehydrogenation barrier for the α-hydrogen adsorption complex (thermodynamic control), while the hydroxyl path is determined by its dehydrogenation step (kinetic control). The effects of alloyed metal on the catalysis activity of Pt for ethanol partial oxidation, including adsorption energy, energy barrier, electronic structure, and eventually rate constant were discussed. Among all of the alloyed metals only Sn enhances the rate constant of the dehydrogenation via the α-hydrogen path on the Pt site of Pt(3)Sn as compared with Pt alone, which interprets why the PtSn is the most active to the oxidation of ethanol.     

Cyclometallated Pt(II) and Pd(II) complexes with a trithiacrown ligand

We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(II) and Pd(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3). Reaction of [M(C insertion mark N)(micro-Cl)]2 (M = Pt(II), Pd(II); C insertion mark N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS3 followed by metathesis with NH4PF6 yields [M(C insertion mark N)([9]aneS3)](PF6). The complexes [M(C insertion mark P)([9]aneS3)](PF6) (M = Pt(II), Pd(II); Cinsertion markP = [CH2C6H4P(o-tolyl)2-C,P]-) were synthesized from their respective [Pt(C insertion mark P)(micro-Cl)]2 or [Pd(C insertion mark P)(micro-O2CCH3)]2 (C insertion mark P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(II) complexes, but unusual two-electron reversible oxidations for the Pd(II) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The structure of [Pd(bzq)([9]aneS3)](PF6) exhibits disorder in the [9]aneS3 conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS3)(ppy)](PF6) and [Pd([9]aneS3)(ppy)](PF6) indicate the HOMO for both complexes is primarily dz2 in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy pi* in character for [Pt([9]aneS3)(ppy)](PF6), but dx2-y2 in character for [Pd([9]aneS3)(ppy)](PF6).     

Reactions of macrocyclic rhodium carbenoids: regioselective synthesis of indol-3-yl macrocyclic lactones and cryptands

A wide variety of new macrocyclic diazocarbonyl compounds with various spacers was synthesized. Macrocyclic rhodium(II) carbenoid insertion with various substituted indoles was performed to afford regioselectively, indol-3-yl macrocyclic di- or tetralactones (C3-alkylation). Double carbenoid insertion was also performed to afford indolyl cryptand molecules.     

Highly regioselective rhodium(II)-catalysed carbenoid insertion reaction into sp CH bond: a general method for the synthesis of 3,3a-dihydro-2H,5H-pyrrolo[1,2-a]quinoline-1,4-dione ring system

A simple and high yielding general method for the synthesis of 3,3a-dihydro-2H,5H-pyrrolo[1,2-a]quinoline-1,4-dione derivatives through a highly regioselective rhodium(II)-catalysed carbenoid sp² C-H insertion reaction on suitably substituted γ-lactam diazocarbonyl compounds is described.     

Gold(I)-Catalyzed Intramolecular C(sp3)−H Insertion by Decarbenation of Cycloheptatrienes

A novel synthesis of indanes and dihydronaphtalenes based on the intramolecular insertion into C(sp³)−H bonds of gold(I) carbenes generated by retro-Buchner reaction (decarbenation) has been developed. Deuterium-labeling and kinetic isotope effect experiments, DFT calculations, and generation of the proposed carbene intermediate from a well-characterized gold(I) carbenoid support the involvement of a three-center concerted mechanism for the C(sp³)−H functionalization process.     

Theoretical analysis of the conversion mechanism of acetylene to ethylidyne on Pt(111)

The conversion of acetylene to ethylidyne on Pt(111) has been comprehensively investigated using self-consistent periodic density functional theory. Geometries and energies for all of the intermediates involved as well as the conversion mechanism were analyzed. On Pt(111), the carbon atoms in the majority of stable C(2)H(x) (x = 1-4) intermediates prefer saturated sp(3) configurations with the missing H atoms substituted by the adjacent metal atoms. The most favorable conversion pathway for acetylene to ethylidyne is via a three-step reaction mechanism, acetylene → vinyl → vinylidene → ethylidyne. The first step, acetylene → vinyl, depends on the availability of surface H atoms: without preadsorbed H the reaction occurs via the initial disproportionation of acetylene, which resulted in adsorbed vinyl; with an abundance of preadsorbed H, acetylene could transform to vinyl via both the disproportionation and hydrogenation reactions. Conversions through initial dehydrogenation of acetylene and isomerizations of acetylene and vinyl are unfavorable due to high energy barriers along the relevant pathways. The conversion rate involving vinylidene as an intermediate is at least 100 times larger than that involving ethylidene.     

Reaction pathways to dimer in polystyrene pyrolysis: A mechanistic modeling study

A detailed mechanistic model for polystyrene pyrolysis was created that built on a modeling framework developed in our previous work and was used to probe three competing pathways to dimer formation: benzyl radical addition, 1,3-hydrogen shift, and 7,3-hydrogen shift, based on recent literature reports. To incorporate the chemistry involved in the 7,3-hydrogen shift pathway, the 1,7- and 7,3-hydrogen shift reaction families were added to the model. The updated version of the model tracks 75 species and over 3500 reactions. Rate parameters for all families were specified based on our previous work, more recent literature reports, and regression against limited experimental data. The model was able to accurately predict the experimental results for polystyrene pyrolysis for different reactor configurations for a temperature range of 100 °C and two orders of magnitude of initial molecular weight for experimental data collected in our own lab and from Bouster and coworkers and Bockhorn and coworkers. The results from our model were studied using net rate analysis to gain insight into the competitiveness of the various reaction pathways to dimer formation. The net rate analysis demonstrated that 7,3-hydrogen shift is the dominant reaction pathway to dimer formation at the temperatures studied. Benzyl radical addition becomes a more competitive reaction pathway as the temperature increases, which is caused predominantly by an increase in the benzyl radical concentration with increasing temperature. Overall, it is quantitatively shown that both 7,3-hydrogen shift and benzyl radical addition are important pathways for dimer formation, with their relative competitiveness influenced by temperature.     

Theoretical study on intramolecular allene-diene cycloadditions catalyzed by PtCl2 and Au(I) complexes

The intramolecular [4C+3C] cycloaddition reaction of allenedienes catalysed by PtCl(2) and several Au(I) complexes has been studied by means of DFT calculations. Overall, the reaction mechanism comprises three main steps: (i) the formation of a metal allyl cation intermediate, (ii) a [4C(4π)+3C(2π)] cycloaddition that produces a seven-membered ring and (iii) a 1,2-hydrogen migration process on these intermediates. The reaction proceeds with complete diastereochemical control resulting from a favoured exo-like cycloaddition. Allene substituents have a critical influence in the reaction outcome and mechanism. The experimental observation of [4C+2C] cycloadducts in the reaction of substrates lacking substituents at the allene terminus can be explained through a mechanism involving Pt(IV)-metallacycles. With gold catalysts it is also possible to obtain [4C+2C] cycloaddition products, but only with substrates featuring terminally disubstituted allenes, and employing π-acceptor ligands at gold. However the mechanism for the formation of these adducts is completely different to that proposed with PtCl(2), and consists of the formation of a metal allyl cation, subsequent [4C+3C] cycloaddition and a 1,2-alkyl shift (ring contraction). Electronic analysis indicates that the divergent pathways are mainly controlled by the electronic properties of the gold heptacyclic species (L-Au-C(2)), in particular, the backdonation capacity of the metal center to the unoccupied C(2) (pπ-orbital) of the intermediate resulting from the [4C+3C] cycloaddition. The less backdonation, (i.e. using P(OR)(3)Au(+) complexes), the more favoured is the 1,2-alkyl shift.     

Synthesis, spectroscopic characterization, redox properties and catalytic activity of some ruthenium(II) complexes containing aromatic aldehyde and triphenylphosphine or triphenylarsine

A series of new mixed ligand penta-coordinated square pyramidal ruthenium(II) complexes containing benzaldehyde or its substituents and triphenylphosphine or triphenylarsine have been synthesized and characterized. In the electronic spectra, three well-defined peaks in the visible region were observed and assigned to d-d transitions in D(4h) and low spin axially distortion from O(h) symmetry. The spectrochemical parameters of the complexes were calculated and placed the ligands in the middle of the spectrochemical series. The redox properties and stability of the complexes toward oxidation were related to the electron-withdrawing or releasing ability of the substituent in the phenyl ring of the benzaldehyde. The electron-withdrawing substituents stabilized Ru(2+) complexes, while electron-donating groups favored oxidation to Ru(3+). The mechanism and kinetics of the catalytic oxidation of benzyl alcohol by the complex [RuCl(2)(Pph(3))(C(6)H(5)CHO)(2)] in the presence of N-methylmorpholine-N-oxide have also been studied.     

Rh(II)-catalyzed intramolecular C-H insertion of diazo substrates in water: scope and limitations

Preferential Rh(II) carbenoid intramolecular C-H versus O-H insertion derived from alpha-diazo-acetamides can be achieved in water by using an appropriate combination of the catalyst and amide groups, which creates a larger hydrophobic environment around the reactive carbenoid center.     

Total synthesis of (-)-virginiamycin M2: application of crotylsilanes accessed by enantioselective Rh(II) or Cu(I) promoted carbenoid Si-H insertion

A stereoselective synthesis of the antibiotic (-)-virginiamycin M(2) is detailed. A convergent strategy was utilized that proceeded in 10 steps (longest linear sequence) from enantioenriched silane (S)-15. This reagent, which was prepared via a Rh(II)- or Cu(I)-catalyzed carbenoid Si-H insertion, was used to introduce the desired olefin geometry and stereocenters of the C1-C5 propionate subunit. A modified Negishi cross-coupling or an efficient alkoxide-directed titanium-mediated alkyne-alkyne reductive coupling strategy was utilized to assemble the trisubstituted (E,E)-diene. An underutilized late-stage SmI(2)-mediated macrocyclization was employed to construct the 23-membered macrocycle scaffold of the natural product.     

Density functional calculations on CO attached to PtnRu(10−n) (n = 6–10) clusters

B3LYP and SCF‐Xα calculations have been performed on Pt_nRu_(10−n)CO (n = 6–10) clusters. The work aims to simulate the adsorption of CO on the (111) surface of platinum metal and to examine the electronic effects that arise when some Pt atoms are replaced with Ru. Adsorption energies and Pt C and C O stretching frequencies have been calculated for each cluster. Ru does affect the electronic structure of the clusters, the calculated adsorption energies, and frequencies, the Pt C frequency more than the C O. The donation‐backbonding mechanism that accompanies the shift in CO stretching frequency that occurs when CO adsorbs on platinum does not explain the differences in frequency shift observed in CO on various Pt/Ru surfaces. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 589–598, 2000     

Synthesis and antitumor activities of alkyl-1,4-butanediamine Pt(II) complexes having seven-membered ring structure

Novel alkyl-1,4-butanediamine Pt(II) complexes having a seven-membered ring structure were synthesized and characterized by fast atom bombardment mass and infrared spectra and elemental analysis. Their antitumor activities in vivo toward lymphoid leukemia L1210 and Lewis lung carcinoma LL were studied in the case where the leaving group was either dichloride or cyclobutane-1,1-dicarboxylate. 1,4-Butanediamine Pt(II) complexes (seven-membered ring) showed higher antitumor activities than those of ethylenediamine Pt(II) (five-membered ring) and 1,3-propanediamine Pt(II) (six-membered ring) complexes toward L1210 for both leaving groups. Alkyl-1,4-butanediamine Pt(II) complexes showed high antitumor activities toward L1210, except for 1,1-dimethyl-1,4-butanediamine Pt(II) complexes. In particular, 2,2-dimethyl-1,4-butanediamine and 2,3-dimethyl-1,4-butanediamine Pt(II) complexes exhibited excellent antitumor activities with T/C% values higher than 300. None of the dichloro Pt(II) complexes showed antitumor activities toward LL, but the cyclobutane-1,1-dicarboxylato Pt(II) complexes, which were moderately active toward L1210 with T/C% values around 200, also showed high antitumor activities toward LL with T/C% values of more than 200. Alkyl-1,4-butanediamine Pt(II) complexes with a seven-membered ring structure were found to be stable and to have antitumor activities in vivo.     

Theoretical study of the mechanism for the gas‐phase pyrolysis kinetics of 2‐methylbenzyl chloride

The study of the kinetics and mechanism of dehydrochlorination reaction of 2‐methyl benzyl chloride in the gas phase was carried out by means of electronic structure calculations using ab initio Móller‐Plesset MP2/6‐31G(d,p), and Density Functional Theory (DFT) methods: B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p)], PBE/6‐31G(d,p), PBE/6‐31++G(d,p). Investigated reaction pathways comprise: Mechanism I, a concerted reaction through a six‐centered cyclic transition state (TS) geometry; Mechanism II, a 1,3‐chlorine shift followed by beta‐elimination and Mechanism III, a single‐step elimination with simultaneous HCl and benzocyclobutene formation through a bicyclic type of TS. Calculated parameters ruled out Mechanism III and suggest the elimination reaction may occur by either unimolecular Mechanism I or Mechanism II. However, the TS of the former is 20 kJ/mole more stable than the TS of the latter. Consequently, the Mechanism I seem to be more probable to occur. The rate‐determining process is the breaking of C‐Cl bond. The involvement of π‐electrons of the aromatic system was demonstrated by NBO charges and bond order calculations. The reaction is moderately polar in nature. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 537–546, 2011