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Download Author Version (PDF) RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 11 RSC Advances Journal Name RSC Publishing ARTICLE Theoretical Study on Mechanism of Selective Cite this: DOI: 10.1039/x0xx00000x Fluorination of Aromatic Compounds with Selectfluor Cuihuan Geng, a Likai Du, b Fang Liu, a Rongxiu Zhu, a,* Chengbu Liu, a,* Received 00th January 2012, Accepted 00th January 2012 The selective fluorination of aromatic compounds with Selectfluor has been studied theoretically. The π DOI: 10.1039/x0xx00000x structural and energetic features of complexes of substituted benzenes with Selectfluor are investigated, and the fluorine bond (F ···π) has been found to make an important contribution to the www.rsc.org/ stabilization of the π complexes. Our calculations indicate that the SET mechanism, which involves one electron transfer from the aromatic substrate (D) to Selectfluor (A), is preferred over the S N2. The analysis of the minimum energy path (MEP) suggests that the DABCO moiety of Selectfluor seems to take an active role in the fluorination of aromatic compounds with Selectfluor. In addition, a two-state model analysis, as well as the characters of avoid crossing between DA and D +A- states of Manuscript benzene/Selectfluor are addressed to get deep insight into the feature of SET mechanism. 1. Introduction electron transfer (SET) is the dominant process in the functionalization of methyl-substituted benzene derivatives The fluorinated aromatic compounds have attracted growing with Selectfluor. Complementally, the possibility of SET interests due to their extensive application as pesticides, mechanism for some electrophilic aromatic substitutions has Accepted medicinal agents, and various functional materials.1-3 Thus, the also been discussed previously. 18-20 Recently, more clear development of mild and selective synthetic methods for evidences for SET mechanism between Selectfluor and chloride incorporating fluorine into organic aromatic compounds is of were reported. 21 However, no definitive mechanism of great importance. Recently, the N–F reagents with common electrophilic aromatic fluorination reaction with Selectfluor has structure of R 2N–F or R 3N–F are increasingly applied to been identified at present. selective fluorination of aromatic compounds, 4-10 since they are stable, easy to handle. Among many electrophilic N–F reagents, In this work, a systematic theoretical investigation of the the commercially available 1-chloromethyl-4-fluoro-1,4- fluorination reaction for a few aromatic compounds with iazoniabicyclo[2.2.2]octanebis-tetra-fluoroborate (F-TEDA- Selectfluor was performed by computational analysis. The spin 11,12 BF 4, Selectfluor) is the most popular one (Scheme 1). density maps (SDM) was used to predict sites of higher reactivity and interpret the observed regiochemistry for the Advances Selectfluor is usually considered as sources of fluorination reactions. The molecular electrostatic potential ‘pseudopositive’ or ‘electrophilic’ fluorine involved in the (ESP) of Selectfluor was displayed to characterize the nature of fluorination process of aromatic compounds (Scheme 2). 13 Two non-covalent interactions between Selectfluor and the aromatic mechanisms, the nucleophilic substitution (polar mechanism, compounds. The Bader’s atoms in molecules (AIM) approach, RSC SN2) and single-electron transfer (SET) mechanism, have been as well as natural bond orbital (NBO) analysis were also proposed to interpret this electrophilic fluorination reaction. 12,14 applied to understand the non-covalent interactions of the π Differding and coworkers 15,16 have reported several studies complexes. Moreover, the exploration of the minimum energy concerning the mechanism of fluorination using N-F reagents to path (MEP) provides more details about the fluorination of support the S N2 mechanism. Later, based on the observed ESI- aromatic compounds with Selectfluor. In order to get a direct MS and ESI-MS/MS spectra data, Zhang 17 suggested that the view of the inner-sphere SET process, the characters of the fluorination reactions of triphenylethylene and charge localized diabatic states along the fluorine transfer tetraphenylethylene with Selectfluor proceed via the SET pathway are also discussed. mechanism. Stavber and coworkers 6 also proposed that single This journal is © The Royal Society of Chemistry 2013 J. Name ., 2013, 00 , 1-3 | 1 RSC Advances Page 2 of 11 ARTICLE Journal Name 2. Theoretical Methods 3.1 Evaluation of density functionals Considering M06-2X provides reasonable energetics of non-bonded interactions, 22,23 the geometries of all minima and It was reported that M06-2X performs well for main-group transition states for the fluorination of aromatic compounds thermochemistry, barrier heights, and noncovalent 22,23 with Selectfluor were computed at the M06-2X/6-311++G** interactions. In order to examine the effects of different level. The frequency calculations were carried out at the same density functionals on the fluorination reaction, we select M06- level to identify all of the stationary points as minima (zero 2X and other seven well-known and/or recently developed 36 37 38 imaginary frequencies) or first-order saddle points (one density functionals including B3LYP, B3PW91, B97D, 39 23 40 41 imaginary frequency). The intrinsic reaction coordinate (IRC) 24 X3LYP, M05-2X, B2PLYP, , and LC-ωPBE, to calculate calculations were performed to confirm that the optimized the energy barriers of the fluorination of aniline starting with transition states correctly connect the relevant reactants and the structures optimized at the M06-2X/6-311++G(d,p) level of products. All the variances of the geometrical parameters, theory in gas phase. The results are shown in Table 1. Clearly, Mulliken charge and vibrational frequencies along the IRC for M06-2X is satisfactory in predicting the electronic energy the aromatic fluorination were analysed. Natural bond orbital barriers of the fluorination because its deviation of activation (NBO) analysis 25 was carried out to study the nature of bonding energy from the average value is very small. It can also be seen SET SN2 in the π complexes. And binding energies of π complexes were that in each case, TS1 is more stable than TS1 by over 3 calculated by taking into account of the basis set superposition kcal/mol, suggesting that the SET mechanism is more favorable 26 error (BSSE) and thermal corrections. The spin density maps than the S N2 mechanism. In addition, the optimized structure of (SDM) of the aromatic ring radical cations were used to Selectfluor using the M06-2X/6-311++G** method is very interpret the observed regioselectivity in the fluorination close to the MP2/cc-pVTZ geometry as well as its X-ray crystallographic structure (See Table S1), 42 which indicates that reactions. Solvent effects (acetonitrile) have been considered at Manuscript the M06-2X/6-311++G** level using the polarizable continuum the M06-2X with the 6-311++G** basis set was a suitable model (PCM), 27 and single-point energies in acetonitrile were method for the following investigation on the mechanism of combined with gas phase thermal correction computed at the the fluorination with Selectfluor. same level of theory. Unless specifically mentioned, the reported energies are Gibbs free energies with thermal 3.2 π Complexes corrections. Free energies were evaluated based on the The reaction between Selectfluor and aromatic compounds molecular partition functions derived from the rigid-rotor and begins with the formation of π complexes. 43 In this section, harmonic oscillator approximations. Then, the relative free several aromatic compounds (including fluorobenzene, energy is given by the following equation: benzene, toluene, aniline) have been selected to interact with Accepted Selectfluor, aiming to characterize the structural, energetic and ∆G = ∆U + ∆Htr + ∆Hrot + ∆Hvib – T∆S electron features of π complexes. ∆S = ∆Str + ∆Srot + ∆Svib 3.2.1 The Electrostatic potential of Selectfluor All calculations were carried out using the GAUSSIAN 09 program. 28 To investigate how Selectfluor interacts with aromatic compounds, the molecular ESP iso-surface of Selectfluor has The atoms in molecules (QTAIM) analysis 29 was performed been extracted (Fig. 1). We focus on the N1-F region of at M06-2X/6-311++G** level, using AIMALL program.30 The Selectfluor, where the electrostatic potentials around F are all energy decomposition analysis (EDA) is a powerful method for positive with a slightly more positive cap ( σ-hole) which is Advances quantitative
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