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Position-dependent fragmentation mechanism for radical anions of fluorinated benzoates
Journal of Fluorine Chemistry 188 (2016) 171–176
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Journal of Fluorine Chemistry j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u o r
Position-dependent fragmentation mechanism for radical anions of fluorinated benzoates Dmitry E. Mashkantseva,b , Irina V. Beregovayaa,* , Lyudmila N. Shchegolevaa a N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, 9 Lavrent’ev Avenue, Novosibirsk 630090, Russia b Novosibirsk State University, 2 Pirogov Street, Novosibirsk 630090, Russia
A R T I C L E I N F O
Article history: Received 25 May 2016 Received in revised form 30 June 2016 Accepted 1 July 2016 Available online 2 July 2016 Keywords: Fluoroarene radical anions Potential energy surfaces Pseudorotation C–F bond cleavage Regioselectivity
A B S T R A C T
DFT calculations were used to study the potential energy surfaces (PESes) of a full series of fluorinated benzoate radical anions (RAs). The sections of PESes along the C–F bond cleavage coordinates in polar media were built, and the transition states for RA fragmentation with fluoride anion elimination were located. The estimated reaction barrier heights let us interpret the experimental regularities of the RA decay including the process regioselectivity. The fragmentation mechanism was shown to depend on the position of the leaving fluorine atom. When defluorination occurs at an ortho- or meta- position to the CO2 group, the reaction coordinate involves pseudorotation as a way for odd electron density transfer to the breaking C–F bond. Additional gas phase calculations were performed to confirm the pseudorotational architecture of the PESes of polyfluorinated benzoate RAs. The results obtained clearly demonstrated that the multihole PES structure gives rise to the multichannel mechanism of RA cleavage. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Radical anions (RAs) of polyfluorinated arenes play an important role as intermediates of reductive defluorination of their neutral precursors [1,2], which yields partially fluorinated compounds that are potentially valuable starting materials for synthesis [3]. The regiodefining stage of the reaction is the RA unimolecular decay through fluoride ion elimination [1], which is the main channel of RA decay in polar media. Experimental investigations of polyfluorinated RAs are complicated by the extremely short lifetime of these particles. Currently, such investigations are possible owing to advanced high-sensitive methods for radical-ionic pairs detection such as Optical Detection (OD) EPR and Time-Resolved Magnetic Field Effect (TR MFE) [4]. While the electronic and spatial structure of fluoroarene RAs have been thoroughly studied [5] by now, the reactivity of these RAs remains unexplored. In a gas phase, fluoroarene RAs are usually stable to fragmentation. Their decay becomes possible in polar media due to the large solvation energy of the fluoride ion. For planar p-type
* Corresponding author. E-mail addresses: [email protected] (D.E. Mashkantsev), [email protected] (I.V. Beregovaya), [email protected] (L.N. Shchegoleva). http://dx.doi.org/10.1016/j.jfluchem.2016.07.002 0022-1139/ã 2016 Elsevier B.V. All rights reserved.
RAs the reaction is symmetry forbidden [6] because of the RA p and s term crossing in the course of reaction. The crossing avoidance is realized by the out-of-plane shift of the leaving fluorine atom. The transition state (TS) for fluoride ion elimination has a noticeably nonplanar structure [7]. It is essential that RA stability decreases drastically under fluorine accumulation in the aromatic nuclei [8] while C–F bond energy increases [9]. As has been shown earlier [7,10], the reaction mechanism depends on the electronic state of the breaking RA. When the halide ion eliminates from the ring position bearing zero or low density of an odd electron, the reaction coordinate includes not only C–Hal bond stretching and out-of-plane deviation but also an aromatic ring in-plane distortion (pseudorotation) resulting from the avoided crossing of p terms. More detailed information about pseudorotation and usage of this term for low-symmetric molecular systems may be found in recent review [5] and references herein. Pseudorotation serves as a mechanism of electron density transfer between the ring positions. These concepts allowed us to interpret the experimental data on relative stability of chlorobenzonitrile RAs [11] and on regioselectivity of octafluoronaphthalene RA fragmentation [12]. However, there is still neither systematic theoretical research of polyhaloarene RA cleavage nor the analysis of reaction position selectivity. Corresponding experimental investigations of precursor defluorination are few and far between [1–3,13,14].
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Consequently, the detailed experimental study of a full set of fluorinated benzoate RAs [8] is still relevant today. However, the interpretation of the decay rate constant variation with the number and relative position of fluorine atoms in [8] was based on semi-empirical INDO calculations without taking into account the solvation effects, except for the strongly required CO2 group local solvation. It was the only accessible level of calculations for such systems at that time. Since the s term is not dissociative in the gas phase, the reaction TS does not exist in this phase; it could only be modeled. Regioselectivity of C–F bond cleavage in polyfluoroarene RAs was not considered theoretically at all. This work presents a new insight into the problem of fluorinated benzoate RAs reactivity based on the up-to-date level of experience. We have located the true TSes for fluoride ion elimination from various ring positions and have built respective sections of RA potential energy surfaces (PESs). The calculated barriers for RA fragmentation are in line with the experimental trends in the rate constant behaviour within the series of RAs and account for the regioselectivity of reductive defluorination for their precursors. 2. Calculation details
We performed additional calculations of the gas phase PESes for a set of polyfluorinated benzoate RAs, for whose precursors there are available experimental data on monodefluorination. 3. Results and discussion 3.1. Spatial and electronic structure of initial radical anions Geometry optimization of the fluorobenzoate RAs studied within the solvation model described above showed the RAs of this family to differ noticeably in their spatial structure. Mono- and difluorinated benzoate RAs that contain no fluorine atom in paraposition to the CO2 group (except for 2,3-difluorobenzoate) are p-type radicals possessing a nearly planar structure (Fig. 1). This is also true for 2,3,5-trifluorobenzoate RA. The rest of the polyfluorinated RAs display out-of-plane distortions that are mostly C– F bond deviations from the ring plane. When ortho-F atoms are present, the planes of the ring and CO2 group become non coplanar. An example of such a nonplanar RA is shown in Fig. 1a. The singly occupied MO (SOMO) structure for the RAs considered is determined by the CO2 group, the major SOMO density located in para- and ipso-positions to the group (Fig. 1b).
Calculations of the electron structure and PES sections for fluorinated benzoate RAs were performed within spin-unrestricted DFT theory with the CAM-B3LYP functional and 6-31 + G(d) basis set. Stationary PES points were located and their types and interrelations were determined by the normal vibrations analysis accompanied by intrinsic reaction coordinate (IRC) calculations. All calculations were done with the GAMESS package [15]. Molecular orbital (MO) images were constructed by MOLDEN [16] program, using the isosurface contour value that corresponds to electron density of 0.05 a.u. The polar media influence was taken into account within the polarizable continuum model (PCM) using built-in parameters for H2O. The CO2 group local solvation was modeled by the direct inclusion of two H2O molecules into calculations (the so-called supermolecular approach). Note that H2O molecules simulating local solvation of the CO2 group form a hydrogen bond between them as well (see Fig. 1 in Results and discussion). The heights of energy barriers (Ea) for the studied RA fragmentation were estimated as total energy differences between the TSes for the C–Fi bond cleavage and the PES minimum corresponding to the initial RA.
3.2. Unimolecular fragmentation of monofluorinated benzoate RAs The PES sections of isomeric mono-fluorobenzoate RAs along the reaction coordinates corresponding to fluoride ion elimination are shown in Figs. 2 and 3. In the case of para-isomer the reaction coordinate is a superposition of the C–F bond stretching and its out-of-plane deviation. The respective TS structure is given in Fig. 2 together with the SOMO image that shows an increase in the antibonding s*C–F orbital contribution during the p-s crossing avoidance. The reaction barrier height is estimated to be 4.1 kcal/mol. For ortho- and meta-fluorobenzoate RAs the reaction coordinate becomes more complicated (Fig. 3). The C–F bond stretching is preceded by odd electron density transfer to the bond, which is realized through moving along the pseudorotation way. The contribution of pseudorotation to the reaction coordinates comes to light as a result of comparison of SOMO evolution (Fig. 3b) when moving along these coordinates with the SOMOs of the benzene RA structures involved in pseudorotation (see [17] for details). To
Fig. 1. Geometrical structures of the benzoate and 2,3,4,6-tetrafluorobenzoate RAs adjusted for the CO2 group local solvation (a) and SOMO images for RAs of monofluorinated benzoates and 2,3,4,6-tetrafluorobenzoate (b).
Fig. 2. PES section of the para-fluorobenzoate RA along the C–F bond cleavage coordinate. The IRC points accompanied by respective SOMO images are marked by their numbers. The TS structure (point 32) is shown in a frame.
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Fig. 3. a) PES sections of the ortho- and meta-fluorobenzoate RAs along the C–F bond cleavage coordinates. The IRC points accompanied by respective SOMO images are marked by their numbers. Black circles indicate the points corresponding to reaction TSes. The TS structures are shown in frames. The C–F bond lengths in the respective IRC points are given in italics under the abscissa axes. b) The RA SOMO images for the IRC points marked by numbers in comparison with those of the benzene RA structures involved in the pseudorotation circle.
match these data with literature we use here the stationary structure notations introduced in [18]. We use them for SOMOs identification as well. Both PES sections in the vicinity of the points corresponding to the initial RAs are extremely flat. The C–F bond length and SOMO shape remain practically unchanged for rather long portions of the way. Thus, in the case of the ortho-isomer RA a hardly noticeable SOMO change arises approximately from the 57-th IRC point when the SOMO approaches B(0). At this point RA becomes slightly nonplanar, and motion along the pseudorotation coordinate starts. C–F bond stretching contributes to reaction coordinates approximately at point 60 for the ortho-isomer and at point 35 for the meta-isomer. At these portions of the reaction ways the RA total energy increases rather sharply until the respective TS structures are attained. The located TS structures (B(p/3) and B (-p/3) ones for ortho- and meta-isomer, respectively) are essentially nonplanar (Fig. 3). The corresponding barrier heights for fragmentation are found to be 8.0 and 12.9 kcal/mol. Thus, the relative barrier heights for mono-fluorinated benzoate RAs comply with their experimentally estimated ability to eliminate a fluoride ion: para- > ortho- > meta [8]. Note that despite the highest barrier for cleavage, the metaisomer displays minimal structure changes in going from the initial RA structure to TS (Fig. 3). This situation differs radically from that of isomeric chlorobenzonitrile RA cleavage [11] when the odd electron density location at the meta- C–Cl bond demands the greatest structure modification. 3.3. Ways of unimolecular fragmentation for polyfluorinated benzoate RAs. An example of the 2,3,4,6-Tetrafluorobenzoate RA We will consider the regularities of polyfluorinated benzoate RAs fragmentation on the basis of 2,3,4,6-tetrafluorobenzoate RA. The PES sections of this RA along the coordinates for each of the four C–F bonds cleavage are shown in Fig. 4 together with SOMO images for the structures corresponding to reaction TSes. As Fig. 4 shows, the ways of fluoride ion elimination from positions 2 and 4 display the lowest energy barriers for RA fragmentation. The mechanism of C4–F bond cleavage is the same as the one depicted in Fig. 2 for parafluorobezoate RA. C2–F bond cleavage is realized by a two-stage mechanism. At the first stage the odd electron density transfer to the C4–F bond occurs by moving along the pseudorotation coordinate. It is clearly
Fig. 4. The 2,3,4,6-tetrafluorobenzoate RA PES sections along the coordinates of fluoride ion elimination from various ring positions, and SOMO images for respective TSes. The sections are marked by the standard number of ring position in a circle. The crosses stand for the final points obtained from IRC calculations in paths 3 and 6. Further curve extrapolations are tentative. At the bottom of the figure, the breaking C–F bond lengths are given for the selected IRC points at each reaction path.
illustrated by the SOMO structure in the first TS. The SOMO shape is similar to the benzene RA A(-p/3) MO in Fig. 3. The second barrier on the reaction path is due to the p-s crossing avoidance. Note that the existence of two activation barriers resulting from p-p and p-s crossing avoidance was predicted in [7].
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It should be emphasized that the barriers for pseudorotation and for C2–F bond cleavage in this case are separated. The fact reflects the complex multihole structure of RA PES. The minimum between the two TSes (Fig. 4) is a minimum on the pseudorotation path. In contrast, the PES of the ortho-fluorobenzoate RA is a onewell PES. The only minimum at this PES corresponds to the initial RA structure. Consequently, C–F bond cleavage in the case of the ortho-fluorobenzoate RA is realized by a one-stage mechanism, as we have seen earlier. The 2,3,4,6-tetrafluorobenzoate RA fragmentation is energy favourable irrespective of which Ci–F bond is breaking. However, the C3–F bond cleavage gives the most gain in energy. Thus, the weakest C–F bond is not the one which is broken, i.e. regioselectivity of the RA fragmentation is controlled kinetically. Calculations similar to those described above were performed for a full series of fluorinated benzoate RAs. The estimated barrier heights for fluoride ion elimination from all possible ring positions are given in Table 1. The data obtained show that in most cases RA fragmentation occurs in one stage. A two-stage mechanism takes place only when F- eliminates from position 2 of the RA when there are fluorine atoms in positions 2 and 4 simultaneously. An exception is the pentafluorobenzoate RA that eliminates F- from position 2 in one stage. Besides 2,3,4,6-F4-C6HCO22 considered above, the twostage fragmentation mechanism was revealed in the following RAs: 2,4-F2-C6H3CO22., 2,3,4-F3-C6H2CO22 , 2,4,5-F3-C6H2CO22 , 2,4,6-F3-C6H2CO22 , 2,3,4,5-F4-C6HCO22 . In most cases the first stage of the reaction, namely the odd electron density transfer to the breaking C–F bond is a limiting stage. The exceptions are 2,4,5F3-C6H2CO22 and 2,3,4,5-F4-C6HCO22 . 3.4. Reaction regioselectivity analysis and relationship with pseudorotation The data in Table 1 may be compared with the results of polyfluorinated benzoic acids reduction by metals in liquid ammonia and in water media [8,19]. The reduction is considered to pass through the benzoate RAs intermediate formation [8], so the regioselectivity of acid defluorination is determined by the
regioselectivity of RA fragmentation. Generally, the estimated activation energies (Table 1) correspond to the relative ability of the acid to eliminate F- from various ring positions, para- > ortho> meta-. Under the polyfluorinated benzoic acids reduction by sodium in liquid ammonia, deep (up to full) defluorination is observed in the majority of cases [8,19]. The most interesting are the results obtained for 2,3,4-F3-C6H2COOH. There are mono-defluorination products that correspond to all three channels. The acids 2,3-F2C6H3COOH, 2,4-F2-C6H3COOH, and 3,4-F2-C6H3COOH are formed at a ratio of 11:1:2. The preference for para-defluorination is not forcefully expressed in the calculations. However, this is the only case when all the three products are observed experimentally, and the corresponding activation energies are small enough to be able to create the conditions for forming all these products. Calculations predict close activation energy values for defluorination from positions 2 and 4 in the case of 2,4-F2-C6H3CO22 , 2,4,6-F3-C6H2CO22 , and 2,3,4,6-F4-C6HCO22 . Unfortunately, the experimental data on respective acid monodefluorination are available only for 2,3,4,6-F4-C6HCOOH. The 2,3,4,6-F4-C6HCOOH reduction by zinc in liquid ammonia yields 2,3,6- and 2,4,5trifluorobenzoic acids in approximately equal amounts. The single product formation at defluorination of 3,4,5-F3C6H2COOH, 2,3,4,5-F4-C6HCOOH, and C6F5COOH can be explained by the low barrier heights for the C4–F bond cleavage found for respective benzoate RAs (Table 1). The presence of several products of polyfluorinated benzoic acid reduction defluorination might be suggested to come from the complex multihole PES structure of respective benzoate RAs. So, we performed the gas phase CAM-B3LYP/6–31+G(d) calculations for some fluorobenzoate RAs that are of interest from the viewpoint of interpreting the available data on the precursor reductive mono-defluorination. The calculation results are listed in Table 2. All the PESes considered were shown to be surfaces of pseudorotation. Nevertheless, the minima of different types were found only for 2,3,4-F3-C6H2CO22 (Fig. 5) and for 2,3,4,6-F4C6HCO22 . It is for these RAs that reduction defluorination of the respective acids yields more than one mono-defluorination product.
Table 1 Experimentally determined fragmentation rate constant (kc) for fluorinated benzoate RAs [8] and calculated (CAM-B3LYP/6-31 + G(d)) total energies of initial RAs (E) and activation barriers (Ea ) for fluoride ion elimination from various ring positions. Ea values used for correlation (Section 3.5) are given in bold. kc, mol
Compound
1
s
1
[8]
E, a.u.
Ea , kcal/mol 2a
2
2-F-C6H4CO2 3-F-C6H4CO22 4-F-C6H4CO22 2,3-F2-C6H3CO22 2,4-F2-C6H3CO22 2,5-F2-C6H3CO22 2,6-F2-C6H3CO22 3,4-F2-C6H3CO22 3,5-F2-C6H3CO22 2,3,5-F3-C6H2CO22 2,3,6-F3-C6H2CO22 2,4,5-F3-C6H2CO22 2,4,6-F3-C6H2CO22 3,4,5-F3-C6H2CO22 2,3,4-F3-C6H2CO22 2,3,4,5-F4-C6HCO22 2,3,4,6-F4-C6HCO22 2,3,5,6-F4-C6HCO22 C6F5CO22 a b c d
4
(1.5 0.8) 10 3 103 (5 2) 104 (1.6 0.8) 107 (1.5 0.7) 107 (1.4 0.6) 105 (1.9 0.8) 106 (7 3) 107 (1 0.8) 104 (6.5 2.5) 105 (1.1 0.5) 108 (1.1 0.8) 104 (6.5 2.5) 108 (2.0 1.0) 104 – (1.7 0.6) 108 (1.2 0.8) 109 (2.1 0.8) 107 (1.2 0.8) 109
627.31002 627.31874 672.30979 771.53395 771.53440 771.54240 771.53224 771.53632 771.54930 870.76450 870.75709 870.76079 870.75614 870.76553 870.75539 969.98325 969.97884 969.97831 1069.19992
For the cases of two-stage mechanism, both barrier heights are given. Nonequivalence of Ea values obtained for equivalent positions will be discussed in 3.5. The value was considered as close to one obtained for position 2 (see Section 3.5). The value was considered as close to one obtained for position 3 (see Section 3.5).
3
4
5
6
8.0 12.9 4.1 3.1 3.3, 2.5 11.5 4.8
7.0 1.6 7.7, 8.6 1.5, 0.8 1.8, 0.6 7.2, 8.6 1.2, 0.8 4.0 7.6
6.1 3.2 15.2 5.1b 5.1 11.4 5.8 8.9
6.7 1.5 3.0 3.7 6.7 4.5
1.1 11.5b 13.6 8.2 1.3 1.5 0.4 1.2 1.1 1.5 1.2
8.5 c
4.1 8.4 d
c
d
c
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Table 2 Stationary structures of polyfluorinated benzoate RA PES on the basis of the data of gas phase CAM-B3LYP/6-31 + G(d) calculations. Total energy values (a.u.) are given for the global PES minima (B(0) structures); the rest of the structures are provided with relative energy values (kcal/mol). F positions
A(0)
B(p/3)
A(-p/3)
2,3,4 3,4,5 2,3,4,5 2,3,4,6 2,3,4,5,6
5.6 9.8 9.9 4.9 6.3
5.4 – – 4.3 –
5.7 – – 5.0 –
B(0) 717.62781 717.62790 816.86332 816.85522 916.08435
A(p/3)
B(-p/3)
1.5 – – 2.0 –
0.8 – – 1.0 –
Note that Fig. 5 demands some elucidation related with CO2 group rotation with respect to the ring plane when the RA contains one or two ortho-fluorine atoms. For such RAs inversion of the outof-plane C–F bond deviations during a motion along the pseudorotation coordinate occurs simultaneously with the change of the angle of CO2 group rotation. As a result, the structures B (-p/3) arising at the joint of the pseudorotation cycle halves differ in the sign of the CO2 group rotation angle. The energy difference between the B(-p/3) structures is only 0.6 kcal mol, and their linking is realized by CO2 group turning. We should note that the estimated values of activation energies also reflect other regularities revealed in experiments [8,19]. So, if there are two nonequivalent ortho-F atoms, the atom that is adjacent to another fluorine atom will be eliminated (see the data for 2,3,6-F3-C6H2CO22 and for 2,3,4,6-F4-C6HCO22 ). The relationships found for the barrier heights for fragmentation at the ortho-position, Ea(2,3) < Ea(2,3,5) and Ea(2,3,6) < Ea(2,3,5,6), comply with the conclusion that mono-fluorination at the expelled meta-position stabilizes the RA towards the channel. At the same time, preferability for the 2,3,5-F3-C6H2CO22 RA fragmentation at position 3, predicted by calculations, contradicts the general regularity para- > ortho- > meta- revealed experimentally. Unfortunately, there are no data on the reductive defluorination of the respective acid, 2,3,5-F3-C6H2COOH, which could confirm or refute the prediction.
Fig. 6. Relationship between the reaction energy barrier Ea for the fragmentation of fluorinated benzoate RAs, and the respective experimental rate constant kc [8], R2 = 0.91. Digits near the points identify fluorine atom positions. ~ C4–F bond is broken; * C2–F bond is broken; - C3–F bond is broken and – more than one decay channel is observed.
3.5. Comparison with the electron photoinjection (EPI) data [8] The comparison of the calculated activation energies for fluoride ion elimination with the experimentally measured rate constants of RA decay is shown in Fig. 6. When RA fragmentation was a complex consecutive reaction with two energy barriers, in order to estimate Ea we used the total energy value of the limiting stage TS. In case the barrier heights for F elimination from different ring positions were close (E1a E2a), we estimated effective activation energy (Eaeff) of the parallel reactions, assuming the pre-exponent values for different decay channels to be approximately equal (A1 A2 A); then
Fig. 5. PES scheme for the 2,3,4-trifluorobenzoate RA in a gas phase. The relative energies (kcal/mol) of stationary structures are given (grey numbers stand for TSes).
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keff = k1 + k2 = A1exp(Ea1/RT )+ A2exp(Ea2/RT )= A(exp(Ea1/RT) + exp (Ea2/RT) )= Aexp(Eaeff/RT) It should be noted that the supermolecular solvation model used (Section 2) has a disadvantage. The associate built from two H2O molecules has no symmetry elements inherent to the benzoate ion and its symmetrically substituted analogues. As a result, the symmetry of the RAs considered in the supermolecular model is lower than that in the gas phase or in the PCM model. So, the calculated activation energies somewhat depend on mutual orientation of the H2O molecules and substituents in the ring. However, the difference is not significant as the examples of 2,6-F2C6H3CO22 and 3,5-F2-C6H3CO22 show (see Table 1). Despite the inaccuracy mentioned, the minimal for each RA activation barrier heights correlate with the experimentally determined fragmentation rate constants. 4. Conclusions The mechanism of fluoroarene RA fragmentation through fluoride ion elimination was investigated for a full series of fluorinated benzoate RAs by DFT method. Adiabatic PES sections along the fragmentation coordinates were built taking into account the solvation effects, that created an opportunity for the reaction transition states to be located. The estimated barrier heights are in line with the rate constants measured experimentally in [8]. The fragmentation mechanism was found to depend on the leaving fluorine atom position. When defluorination occurs from ortho- or meta- position to the CO2 group, the reaction coordinate involves pseudorotation as a way for the odd electron density transfer to the breaking C–F bond. Pseudorotational architecture of the PESes confirmed by additional gas phase calculations clearly demonstrates that the multihole PES structure gives rise to the multichannel mechanism of RA cleavage. The results obtained show that a barrier for fluoroarene RA fragmentation can be due not only to RA p and s term crossing but to p-p crossing as well. Acknowledgements The work was supported by FASO Russia on the project 03022014-0001.
References [1] V.D. Shteingarts, Recent advances in practice and theory of polyfluoroarene hydrodehalogenation, J. Fluorine Chem. 128 (2007) 797–805. [2] V.E. Platonov, A.S. Vinogradov, V.I. Krasnov, Polyfluoroarenes and Zn: hydrogenolysis reactions, synthesis and chemical properties of polyfluoroaromatic organozinc compounds, Fluorine Notes 66 (2009). [3] M.F. Kuehnel, D. Lentz, T. Braun, Synthesis of fluorinated building blocks by transition-metal-mediated hydrodefluorination reactions, Angew. Chem. Int. Ed. 52 (2013) 3328–3348. [4] V. Borovkov, D. Stass, V. Bagryansky, Y. Molin, Study of spin-correlated radical ion pairs in irradiated solutions by optically detected EPR and related techniques, in: A. Lund, M. Shiotani (Eds.), Applications of EPR in Radiation Research, Springer International Publishing, 2014, pp. 629–663. [5] L.N. Shchegoleva, I.V. Beregovaya, Potential energy surface as a key to understanding the structure and properties of short-living radical ions of cyclic organic molecules, Int. J. Quantum Chem. 116 (2016) 161–173. [6] D.D. Clarke, C.A. Coulson, The dissociative breakdown of negative ions, J. Chem. Soc. A 2 (1969) 169–172. [7] I.V. Beregovaya, L.N. Shchegoleva, Potential energy surface and dissociative cleavage of chlorobenzene radical anion, Chem. Phys. Lett. 348 (2001) 501– 506. [8] V.V. Konovalov, S.S. Laev, I.V. Beregovaya, L.N. Shchegoleva, V.D. Steingarts, Yu. D. Tsvetkov, I. Bilkis, Fragmentation of radical anions of polyfluorinated benzoates, J. Phys. Chem. A 104 (2000) 352–361. [9] W.A. Steppard, C.M. Sharts, Organic Fluorine Chemistry, W. A. Benjamin Inc., New York, 1969. [10] A.B. Pierini, D.M. Vera, Ab initio evaluation of intramolecular electron transfer reactions in halobenzenes and stabilized derivatives, J. Org. Chem. 68 (2003) 9191–9199. [11] I.V. Beregovaya, V.P. Vysotskii, L.N. Shchegoleva, Monomolecular decay of anion-radicals of isomeric chlorobenzonitriles. Quantum chemical analysis, J. Struct. Chem. 47 (2006) 211–219. [12] I.V. Beregovaya, L.N. Shchegoleva, Pseudorotation as a mechanism for intramolecular electron density transfer. Fragmentation of the octafluoronaphthalene radical anion, J. Fluorine Chem. 163 (2014) 1–6. [13] S.M. Senaweera, A. Singh, J.D. Weaver, Photocatalytic hydrodefluorination: facile access to partially fluorinated aromatics, J. Am. Chem. Soc. 136 (2014) 3002–3005. [14] P.K. Freeman, R. Srinivasa, Photochemistry of polyhaloarenes. 6. Fragmentation of polyfluoroarene radical anions, J. Org. Chem. 52 (1987) 252– 256. [15] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Cordon, J.J. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, General atomic and molecular electronic structure system, J. Comput. Chem. 14 (1993) 1347–1363. [16] G. Schaftenaar, J.H. Noordik, Molden: a pre- and post-processing program for molecular and electronic structures, J. Comput. Aided Mol. Des. 14 (2000) 123– 134. [17] L.N. Shchegoleva, I.V. Beregovaya, P.V. Schastnev, Potential energy surface of C6F6 radical anion, Chem. Phys. Lett. 312 (1999) 325–332. [18] I.V. Beregovaya, L.N. Shchegoleva, Potential energy surfaces of fluorobenzene radical anions, Int. J. Quantum Chem. 88 (2002) 481–488. [19] S.S. Laev, Reductive Defluorination of Fluorinated Benzoic Acids, PhD Thesis, Novosibirsk, Novosibirsk Institute of Organic Chemistry, SB, RAS, Russia, 1999.
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Journal of Fluorine Chemistry j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u o r
Position-dependent fragmentation mechanism for radical anions of fluorinated benzoates Dmitry E. Mashkantseva,b , Irina V. Beregovayaa,* , Lyudmila N. Shchegolevaa a N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, 9 Lavrent’ev Avenue, Novosibirsk 630090, Russia b Novosibirsk State University, 2 Pirogov Street, Novosibirsk 630090, Russia
A R T I C L E I N F O
Article history: Received 25 May 2016 Received in revised form 30 June 2016 Accepted 1 July 2016 Available online 2 July 2016 Keywords: Fluoroarene radical anions Potential energy surfaces Pseudorotation C–F bond cleavage Regioselectivity
A B S T R A C T
DFT calculations were used to study the potential energy surfaces (PESes) of a full series of fluorinated benzoate radical anions (RAs). The sections of PESes along the C–F bond cleavage coordinates in polar media were built, and the transition states for RA fragmentation with fluoride anion elimination were located. The estimated reaction barrier heights let us interpret the experimental regularities of the RA decay including the process regioselectivity. The fragmentation mechanism was shown to depend on the position of the leaving fluorine atom. When defluorination occurs at an ortho- or meta- position to the CO2 group, the reaction coordinate involves pseudorotation as a way for odd electron density transfer to the breaking C–F bond. Additional gas phase calculations were performed to confirm the pseudorotational architecture of the PESes of polyfluorinated benzoate RAs. The results obtained clearly demonstrated that the multihole PES structure gives rise to the multichannel mechanism of RA cleavage. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Radical anions (RAs) of polyfluorinated arenes play an important role as intermediates of reductive defluorination of their neutral precursors [1,2], which yields partially fluorinated compounds that are potentially valuable starting materials for synthesis [3]. The regiodefining stage of the reaction is the RA unimolecular decay through fluoride ion elimination [1], which is the main channel of RA decay in polar media. Experimental investigations of polyfluorinated RAs are complicated by the extremely short lifetime of these particles. Currently, such investigations are possible owing to advanced high-sensitive methods for radical-ionic pairs detection such as Optical Detection (OD) EPR and Time-Resolved Magnetic Field Effect (TR MFE) [4]. While the electronic and spatial structure of fluoroarene RAs have been thoroughly studied [5] by now, the reactivity of these RAs remains unexplored. In a gas phase, fluoroarene RAs are usually stable to fragmentation. Their decay becomes possible in polar media due to the large solvation energy of the fluoride ion. For planar p-type
* Corresponding author. E-mail addresses: [email protected] (D.E. Mashkantsev), [email protected] (I.V. Beregovaya), [email protected] (L.N. Shchegoleva). http://dx.doi.org/10.1016/j.jfluchem.2016.07.002 0022-1139/ã 2016 Elsevier B.V. All rights reserved.
RAs the reaction is symmetry forbidden [6] because of the RA p and s term crossing in the course of reaction. The crossing avoidance is realized by the out-of-plane shift of the leaving fluorine atom. The transition state (TS) for fluoride ion elimination has a noticeably nonplanar structure [7]. It is essential that RA stability decreases drastically under fluorine accumulation in the aromatic nuclei [8] while C–F bond energy increases [9]. As has been shown earlier [7,10], the reaction mechanism depends on the electronic state of the breaking RA. When the halide ion eliminates from the ring position bearing zero or low density of an odd electron, the reaction coordinate includes not only C–Hal bond stretching and out-of-plane deviation but also an aromatic ring in-plane distortion (pseudorotation) resulting from the avoided crossing of p terms. More detailed information about pseudorotation and usage of this term for low-symmetric molecular systems may be found in recent review [5] and references herein. Pseudorotation serves as a mechanism of electron density transfer between the ring positions. These concepts allowed us to interpret the experimental data on relative stability of chlorobenzonitrile RAs [11] and on regioselectivity of octafluoronaphthalene RA fragmentation [12]. However, there is still neither systematic theoretical research of polyhaloarene RA cleavage nor the analysis of reaction position selectivity. Corresponding experimental investigations of precursor defluorination are few and far between [1–3,13,14].
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Consequently, the detailed experimental study of a full set of fluorinated benzoate RAs [8] is still relevant today. However, the interpretation of the decay rate constant variation with the number and relative position of fluorine atoms in [8] was based on semi-empirical INDO calculations without taking into account the solvation effects, except for the strongly required CO2 group local solvation. It was the only accessible level of calculations for such systems at that time. Since the s term is not dissociative in the gas phase, the reaction TS does not exist in this phase; it could only be modeled. Regioselectivity of C–F bond cleavage in polyfluoroarene RAs was not considered theoretically at all. This work presents a new insight into the problem of fluorinated benzoate RAs reactivity based on the up-to-date level of experience. We have located the true TSes for fluoride ion elimination from various ring positions and have built respective sections of RA potential energy surfaces (PESs). The calculated barriers for RA fragmentation are in line with the experimental trends in the rate constant behaviour within the series of RAs and account for the regioselectivity of reductive defluorination for their precursors. 2. Calculation details
We performed additional calculations of the gas phase PESes for a set of polyfluorinated benzoate RAs, for whose precursors there are available experimental data on monodefluorination. 3. Results and discussion 3.1. Spatial and electronic structure of initial radical anions Geometry optimization of the fluorobenzoate RAs studied within the solvation model described above showed the RAs of this family to differ noticeably in their spatial structure. Mono- and difluorinated benzoate RAs that contain no fluorine atom in paraposition to the CO2 group (except for 2,3-difluorobenzoate) are p-type radicals possessing a nearly planar structure (Fig. 1). This is also true for 2,3,5-trifluorobenzoate RA. The rest of the polyfluorinated RAs display out-of-plane distortions that are mostly C– F bond deviations from the ring plane. When ortho-F atoms are present, the planes of the ring and CO2 group become non coplanar. An example of such a nonplanar RA is shown in Fig. 1a. The singly occupied MO (SOMO) structure for the RAs considered is determined by the CO2 group, the major SOMO density located in para- and ipso-positions to the group (Fig. 1b).
Calculations of the electron structure and PES sections for fluorinated benzoate RAs were performed within spin-unrestricted DFT theory with the CAM-B3LYP functional and 6-31 + G(d) basis set. Stationary PES points were located and their types and interrelations were determined by the normal vibrations analysis accompanied by intrinsic reaction coordinate (IRC) calculations. All calculations were done with the GAMESS package [15]. Molecular orbital (MO) images were constructed by MOLDEN [16] program, using the isosurface contour value that corresponds to electron density of 0.05 a.u. The polar media influence was taken into account within the polarizable continuum model (PCM) using built-in parameters for H2O. The CO2 group local solvation was modeled by the direct inclusion of two H2O molecules into calculations (the so-called supermolecular approach). Note that H2O molecules simulating local solvation of the CO2 group form a hydrogen bond between them as well (see Fig. 1 in Results and discussion). The heights of energy barriers (Ea) for the studied RA fragmentation were estimated as total energy differences between the TSes for the C–Fi bond cleavage and the PES minimum corresponding to the initial RA.
3.2. Unimolecular fragmentation of monofluorinated benzoate RAs The PES sections of isomeric mono-fluorobenzoate RAs along the reaction coordinates corresponding to fluoride ion elimination are shown in Figs. 2 and 3. In the case of para-isomer the reaction coordinate is a superposition of the C–F bond stretching and its out-of-plane deviation. The respective TS structure is given in Fig. 2 together with the SOMO image that shows an increase in the antibonding s*C–F orbital contribution during the p-s crossing avoidance. The reaction barrier height is estimated to be 4.1 kcal/mol. For ortho- and meta-fluorobenzoate RAs the reaction coordinate becomes more complicated (Fig. 3). The C–F bond stretching is preceded by odd electron density transfer to the bond, which is realized through moving along the pseudorotation way. The contribution of pseudorotation to the reaction coordinates comes to light as a result of comparison of SOMO evolution (Fig. 3b) when moving along these coordinates with the SOMOs of the benzene RA structures involved in pseudorotation (see [17] for details). To
Fig. 1. Geometrical structures of the benzoate and 2,3,4,6-tetrafluorobenzoate RAs adjusted for the CO2 group local solvation (a) and SOMO images for RAs of monofluorinated benzoates and 2,3,4,6-tetrafluorobenzoate (b).
Fig. 2. PES section of the para-fluorobenzoate RA along the C–F bond cleavage coordinate. The IRC points accompanied by respective SOMO images are marked by their numbers. The TS structure (point 32) is shown in a frame.
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Fig. 3. a) PES sections of the ortho- and meta-fluorobenzoate RAs along the C–F bond cleavage coordinates. The IRC points accompanied by respective SOMO images are marked by their numbers. Black circles indicate the points corresponding to reaction TSes. The TS structures are shown in frames. The C–F bond lengths in the respective IRC points are given in italics under the abscissa axes. b) The RA SOMO images for the IRC points marked by numbers in comparison with those of the benzene RA structures involved in the pseudorotation circle.
match these data with literature we use here the stationary structure notations introduced in [18]. We use them for SOMOs identification as well. Both PES sections in the vicinity of the points corresponding to the initial RAs are extremely flat. The C–F bond length and SOMO shape remain practically unchanged for rather long portions of the way. Thus, in the case of the ortho-isomer RA a hardly noticeable SOMO change arises approximately from the 57-th IRC point when the SOMO approaches B(0). At this point RA becomes slightly nonplanar, and motion along the pseudorotation coordinate starts. C–F bond stretching contributes to reaction coordinates approximately at point 60 for the ortho-isomer and at point 35 for the meta-isomer. At these portions of the reaction ways the RA total energy increases rather sharply until the respective TS structures are attained. The located TS structures (B(p/3) and B (-p/3) ones for ortho- and meta-isomer, respectively) are essentially nonplanar (Fig. 3). The corresponding barrier heights for fragmentation are found to be 8.0 and 12.9 kcal/mol. Thus, the relative barrier heights for mono-fluorinated benzoate RAs comply with their experimentally estimated ability to eliminate a fluoride ion: para- > ortho- > meta [8]. Note that despite the highest barrier for cleavage, the metaisomer displays minimal structure changes in going from the initial RA structure to TS (Fig. 3). This situation differs radically from that of isomeric chlorobenzonitrile RA cleavage [11] when the odd electron density location at the meta- C–Cl bond demands the greatest structure modification. 3.3. Ways of unimolecular fragmentation for polyfluorinated benzoate RAs. An example of the 2,3,4,6-Tetrafluorobenzoate RA We will consider the regularities of polyfluorinated benzoate RAs fragmentation on the basis of 2,3,4,6-tetrafluorobenzoate RA. The PES sections of this RA along the coordinates for each of the four C–F bonds cleavage are shown in Fig. 4 together with SOMO images for the structures corresponding to reaction TSes. As Fig. 4 shows, the ways of fluoride ion elimination from positions 2 and 4 display the lowest energy barriers for RA fragmentation. The mechanism of C4–F bond cleavage is the same as the one depicted in Fig. 2 for parafluorobezoate RA. C2–F bond cleavage is realized by a two-stage mechanism. At the first stage the odd electron density transfer to the C4–F bond occurs by moving along the pseudorotation coordinate. It is clearly
Fig. 4. The 2,3,4,6-tetrafluorobenzoate RA PES sections along the coordinates of fluoride ion elimination from various ring positions, and SOMO images for respective TSes. The sections are marked by the standard number of ring position in a circle. The crosses stand for the final points obtained from IRC calculations in paths 3 and 6. Further curve extrapolations are tentative. At the bottom of the figure, the breaking C–F bond lengths are given for the selected IRC points at each reaction path.
illustrated by the SOMO structure in the first TS. The SOMO shape is similar to the benzene RA A(-p/3) MO in Fig. 3. The second barrier on the reaction path is due to the p-s crossing avoidance. Note that the existence of two activation barriers resulting from p-p and p-s crossing avoidance was predicted in [7].
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It should be emphasized that the barriers for pseudorotation and for C2–F bond cleavage in this case are separated. The fact reflects the complex multihole structure of RA PES. The minimum between the two TSes (Fig. 4) is a minimum on the pseudorotation path. In contrast, the PES of the ortho-fluorobenzoate RA is a onewell PES. The only minimum at this PES corresponds to the initial RA structure. Consequently, C–F bond cleavage in the case of the ortho-fluorobenzoate RA is realized by a one-stage mechanism, as we have seen earlier. The 2,3,4,6-tetrafluorobenzoate RA fragmentation is energy favourable irrespective of which Ci–F bond is breaking. However, the C3–F bond cleavage gives the most gain in energy. Thus, the weakest C–F bond is not the one which is broken, i.e. regioselectivity of the RA fragmentation is controlled kinetically. Calculations similar to those described above were performed for a full series of fluorinated benzoate RAs. The estimated barrier heights for fluoride ion elimination from all possible ring positions are given in Table 1. The data obtained show that in most cases RA fragmentation occurs in one stage. A two-stage mechanism takes place only when F- eliminates from position 2 of the RA when there are fluorine atoms in positions 2 and 4 simultaneously. An exception is the pentafluorobenzoate RA that eliminates F- from position 2 in one stage. Besides 2,3,4,6-F4-C6HCO22 considered above, the twostage fragmentation mechanism was revealed in the following RAs: 2,4-F2-C6H3CO22., 2,3,4-F3-C6H2CO22 , 2,4,5-F3-C6H2CO22 , 2,4,6-F3-C6H2CO22 , 2,3,4,5-F4-C6HCO22 . In most cases the first stage of the reaction, namely the odd electron density transfer to the breaking C–F bond is a limiting stage. The exceptions are 2,4,5F3-C6H2CO22 and 2,3,4,5-F4-C6HCO22 . 3.4. Reaction regioselectivity analysis and relationship with pseudorotation The data in Table 1 may be compared with the results of polyfluorinated benzoic acids reduction by metals in liquid ammonia and in water media [8,19]. The reduction is considered to pass through the benzoate RAs intermediate formation [8], so the regioselectivity of acid defluorination is determined by the
regioselectivity of RA fragmentation. Generally, the estimated activation energies (Table 1) correspond to the relative ability of the acid to eliminate F- from various ring positions, para- > ortho> meta-. Under the polyfluorinated benzoic acids reduction by sodium in liquid ammonia, deep (up to full) defluorination is observed in the majority of cases [8,19]. The most interesting are the results obtained for 2,3,4-F3-C6H2COOH. There are mono-defluorination products that correspond to all three channels. The acids 2,3-F2C6H3COOH, 2,4-F2-C6H3COOH, and 3,4-F2-C6H3COOH are formed at a ratio of 11:1:2. The preference for para-defluorination is not forcefully expressed in the calculations. However, this is the only case when all the three products are observed experimentally, and the corresponding activation energies are small enough to be able to create the conditions for forming all these products. Calculations predict close activation energy values for defluorination from positions 2 and 4 in the case of 2,4-F2-C6H3CO22 , 2,4,6-F3-C6H2CO22 , and 2,3,4,6-F4-C6HCO22 . Unfortunately, the experimental data on respective acid monodefluorination are available only for 2,3,4,6-F4-C6HCOOH. The 2,3,4,6-F4-C6HCOOH reduction by zinc in liquid ammonia yields 2,3,6- and 2,4,5trifluorobenzoic acids in approximately equal amounts. The single product formation at defluorination of 3,4,5-F3C6H2COOH, 2,3,4,5-F4-C6HCOOH, and C6F5COOH can be explained by the low barrier heights for the C4–F bond cleavage found for respective benzoate RAs (Table 1). The presence of several products of polyfluorinated benzoic acid reduction defluorination might be suggested to come from the complex multihole PES structure of respective benzoate RAs. So, we performed the gas phase CAM-B3LYP/6–31+G(d) calculations for some fluorobenzoate RAs that are of interest from the viewpoint of interpreting the available data on the precursor reductive mono-defluorination. The calculation results are listed in Table 2. All the PESes considered were shown to be surfaces of pseudorotation. Nevertheless, the minima of different types were found only for 2,3,4-F3-C6H2CO22 (Fig. 5) and for 2,3,4,6-F4C6HCO22 . It is for these RAs that reduction defluorination of the respective acids yields more than one mono-defluorination product.
Table 1 Experimentally determined fragmentation rate constant (kc) for fluorinated benzoate RAs [8] and calculated (CAM-B3LYP/6-31 + G(d)) total energies of initial RAs (E) and activation barriers (Ea ) for fluoride ion elimination from various ring positions. Ea values used for correlation (Section 3.5) are given in bold. kc, mol
Compound
1
s
1
[8]
E, a.u.
Ea , kcal/mol 2a
2
2-F-C6H4CO2 3-F-C6H4CO22 4-F-C6H4CO22 2,3-F2-C6H3CO22 2,4-F2-C6H3CO22 2,5-F2-C6H3CO22 2,6-F2-C6H3CO22 3,4-F2-C6H3CO22 3,5-F2-C6H3CO22 2,3,5-F3-C6H2CO22 2,3,6-F3-C6H2CO22 2,4,5-F3-C6H2CO22 2,4,6-F3-C6H2CO22 3,4,5-F3-C6H2CO22 2,3,4-F3-C6H2CO22 2,3,4,5-F4-C6HCO22 2,3,4,6-F4-C6HCO22 2,3,5,6-F4-C6HCO22 C6F5CO22 a b c d
4
(1.5 0.8) 10 3 103 (5 2) 104 (1.6 0.8) 107 (1.5 0.7) 107 (1.4 0.6) 105 (1.9 0.8) 106 (7 3) 107 (1 0.8) 104 (6.5 2.5) 105 (1.1 0.5) 108 (1.1 0.8) 104 (6.5 2.5) 108 (2.0 1.0) 104 – (1.7 0.6) 108 (1.2 0.8) 109 (2.1 0.8) 107 (1.2 0.8) 109
627.31002 627.31874 672.30979 771.53395 771.53440 771.54240 771.53224 771.53632 771.54930 870.76450 870.75709 870.76079 870.75614 870.76553 870.75539 969.98325 969.97884 969.97831 1069.19992
For the cases of two-stage mechanism, both barrier heights are given. Nonequivalence of Ea values obtained for equivalent positions will be discussed in 3.5. The value was considered as close to one obtained for position 2 (see Section 3.5). The value was considered as close to one obtained for position 3 (see Section 3.5).
3
4
5
6
8.0 12.9 4.1 3.1 3.3, 2.5 11.5 4.8
7.0 1.6 7.7, 8.6 1.5, 0.8 1.8, 0.6 7.2, 8.6 1.2, 0.8 4.0 7.6
6.1 3.2 15.2 5.1b 5.1 11.4 5.8 8.9
6.7 1.5 3.0 3.7 6.7 4.5
1.1 11.5b 13.6 8.2 1.3 1.5 0.4 1.2 1.1 1.5 1.2
8.5 c
4.1 8.4 d
c
d
c
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Table 2 Stationary structures of polyfluorinated benzoate RA PES on the basis of the data of gas phase CAM-B3LYP/6-31 + G(d) calculations. Total energy values (a.u.) are given for the global PES minima (B(0) structures); the rest of the structures are provided with relative energy values (kcal/mol). F positions
A(0)
B(p/3)
A(-p/3)
2,3,4 3,4,5 2,3,4,5 2,3,4,6 2,3,4,5,6
5.6 9.8 9.9 4.9 6.3
5.4 – – 4.3 –
5.7 – – 5.0 –
B(0) 717.62781 717.62790 816.86332 816.85522 916.08435
A(p/3)
B(-p/3)
1.5 – – 2.0 –
0.8 – – 1.0 –
Note that Fig. 5 demands some elucidation related with CO2 group rotation with respect to the ring plane when the RA contains one or two ortho-fluorine atoms. For such RAs inversion of the outof-plane C–F bond deviations during a motion along the pseudorotation coordinate occurs simultaneously with the change of the angle of CO2 group rotation. As a result, the structures B (-p/3) arising at the joint of the pseudorotation cycle halves differ in the sign of the CO2 group rotation angle. The energy difference between the B(-p/3) structures is only 0.6 kcal mol, and their linking is realized by CO2 group turning. We should note that the estimated values of activation energies also reflect other regularities revealed in experiments [8,19]. So, if there are two nonequivalent ortho-F atoms, the atom that is adjacent to another fluorine atom will be eliminated (see the data for 2,3,6-F3-C6H2CO22 and for 2,3,4,6-F4-C6HCO22 ). The relationships found for the barrier heights for fragmentation at the ortho-position, Ea(2,3) < Ea(2,3,5) and Ea(2,3,6) < Ea(2,3,5,6), comply with the conclusion that mono-fluorination at the expelled meta-position stabilizes the RA towards the channel. At the same time, preferability for the 2,3,5-F3-C6H2CO22 RA fragmentation at position 3, predicted by calculations, contradicts the general regularity para- > ortho- > meta- revealed experimentally. Unfortunately, there are no data on the reductive defluorination of the respective acid, 2,3,5-F3-C6H2COOH, which could confirm or refute the prediction.
Fig. 6. Relationship between the reaction energy barrier Ea for the fragmentation of fluorinated benzoate RAs, and the respective experimental rate constant kc [8], R2 = 0.91. Digits near the points identify fluorine atom positions. ~ C4–F bond is broken; * C2–F bond is broken; - C3–F bond is broken and – more than one decay channel is observed.
3.5. Comparison with the electron photoinjection (EPI) data [8] The comparison of the calculated activation energies for fluoride ion elimination with the experimentally measured rate constants of RA decay is shown in Fig. 6. When RA fragmentation was a complex consecutive reaction with two energy barriers, in order to estimate Ea we used the total energy value of the limiting stage TS. In case the barrier heights for F elimination from different ring positions were close (E1a E2a), we estimated effective activation energy (Eaeff) of the parallel reactions, assuming the pre-exponent values for different decay channels to be approximately equal (A1 A2 A); then
Fig. 5. PES scheme for the 2,3,4-trifluorobenzoate RA in a gas phase. The relative energies (kcal/mol) of stationary structures are given (grey numbers stand for TSes).
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keff = k1 + k2 = A1exp(Ea1/RT )+ A2exp(Ea2/RT )= A(exp(Ea1/RT) + exp (Ea2/RT) )= Aexp(Eaeff/RT) It should be noted that the supermolecular solvation model used (Section 2) has a disadvantage. The associate built from two H2O molecules has no symmetry elements inherent to the benzoate ion and its symmetrically substituted analogues. As a result, the symmetry of the RAs considered in the supermolecular model is lower than that in the gas phase or in the PCM model. So, the calculated activation energies somewhat depend on mutual orientation of the H2O molecules and substituents in the ring. However, the difference is not significant as the examples of 2,6-F2C6H3CO22 and 3,5-F2-C6H3CO22 show (see Table 1). Despite the inaccuracy mentioned, the minimal for each RA activation barrier heights correlate with the experimentally determined fragmentation rate constants. 4. Conclusions The mechanism of fluoroarene RA fragmentation through fluoride ion elimination was investigated for a full series of fluorinated benzoate RAs by DFT method. Adiabatic PES sections along the fragmentation coordinates were built taking into account the solvation effects, that created an opportunity for the reaction transition states to be located. The estimated barrier heights are in line with the rate constants measured experimentally in [8]. The fragmentation mechanism was found to depend on the leaving fluorine atom position. When defluorination occurs from ortho- or meta- position to the CO2 group, the reaction coordinate involves pseudorotation as a way for the odd electron density transfer to the breaking C–F bond. Pseudorotational architecture of the PESes confirmed by additional gas phase calculations clearly demonstrates that the multihole PES structure gives rise to the multichannel mechanism of RA cleavage. The results obtained show that a barrier for fluoroarene RA fragmentation can be due not only to RA p and s term crossing but to p-p crossing as well. Acknowledgements The work was supported by FASO Russia on the project 03022014-0001.
References [1] V.D. Shteingarts, Recent advances in practice and theory of polyfluoroarene hydrodehalogenation, J. Fluorine Chem. 128 (2007) 797–805. [2] V.E. Platonov, A.S. Vinogradov, V.I. Krasnov, Polyfluoroarenes and Zn: hydrogenolysis reactions, synthesis and chemical properties of polyfluoroaromatic organozinc compounds, Fluorine Notes 66 (2009). [3] M.F. Kuehnel, D. Lentz, T. Braun, Synthesis of fluorinated building blocks by transition-metal-mediated hydrodefluorination reactions, Angew. Chem. Int. Ed. 52 (2013) 3328–3348. [4] V. Borovkov, D. Stass, V. Bagryansky, Y. Molin, Study of spin-correlated radical ion pairs in irradiated solutions by optically detected EPR and related techniques, in: A. Lund, M. Shiotani (Eds.), Applications of EPR in Radiation Research, Springer International Publishing, 2014, pp. 629–663. [5] L.N. Shchegoleva, I.V. Beregovaya, Potential energy surface as a key to understanding the structure and properties of short-living radical ions of cyclic organic molecules, Int. J. Quantum Chem. 116 (2016) 161–173. [6] D.D. Clarke, C.A. Coulson, The dissociative breakdown of negative ions, J. Chem. Soc. A 2 (1969) 169–172. [7] I.V. Beregovaya, L.N. Shchegoleva, Potential energy surface and dissociative cleavage of chlorobenzene radical anion, Chem. Phys. Lett. 348 (2001) 501– 506. [8] V.V. Konovalov, S.S. Laev, I.V. Beregovaya, L.N. Shchegoleva, V.D. Steingarts, Yu. D. Tsvetkov, I. Bilkis, Fragmentation of radical anions of polyfluorinated benzoates, J. Phys. Chem. A 104 (2000) 352–361. [9] W.A. Steppard, C.M. Sharts, Organic Fluorine Chemistry, W. A. Benjamin Inc., New York, 1969. [10] A.B. Pierini, D.M. Vera, Ab initio evaluation of intramolecular electron transfer reactions in halobenzenes and stabilized derivatives, J. Org. Chem. 68 (2003) 9191–9199. [11] I.V. Beregovaya, V.P. Vysotskii, L.N. Shchegoleva, Monomolecular decay of anion-radicals of isomeric chlorobenzonitriles. Quantum chemical analysis, J. Struct. Chem. 47 (2006) 211–219. [12] I.V. Beregovaya, L.N. Shchegoleva, Pseudorotation as a mechanism for intramolecular electron density transfer. Fragmentation of the octafluoronaphthalene radical anion, J. Fluorine Chem. 163 (2014) 1–6. [13] S.M. Senaweera, A. Singh, J.D. Weaver, Photocatalytic hydrodefluorination: facile access to partially fluorinated aromatics, J. Am. Chem. Soc. 136 (2014) 3002–3005. [14] P.K. Freeman, R. Srinivasa, Photochemistry of polyhaloarenes. 6. Fragmentation of polyfluoroarene radical anions, J. Org. Chem. 52 (1987) 252– 256. [15] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Cordon, J.J. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, General atomic and molecular electronic structure system, J. Comput. Chem. 14 (1993) 1347–1363. [16] G. Schaftenaar, J.H. Noordik, Molden: a pre- and post-processing program for molecular and electronic structures, J. Comput. Aided Mol. Des. 14 (2000) 123– 134. [17] L.N. Shchegoleva, I.V. Beregovaya, P.V. Schastnev, Potential energy surface of C6F6 radical anion, Chem. Phys. Lett. 312 (1999) 325–332. [18] I.V. Beregovaya, L.N. Shchegoleva, Potential energy surfaces of fluorobenzene radical anions, Int. J. Quantum Chem. 88 (2002) 481–488. [19] S.S. Laev, Reductive Defluorination of Fluorinated Benzoic Acids, PhD Thesis, Novosibirsk, Novosibirsk Institute of Organic Chemistry, SB, RAS, Russia, 1999.