ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2007, том 81, № 9, с. 1577-1582
^ CHEMICAL KINETICS ^^^^^^^^^^^^
AND CATALYSIS
УДК 541.124
MECHANISM OF THE KOLBE-SCHMITT REACTION WITH LITHIUM
AND SODIUM PHENOXIDES
© 2007 S. Markovic*, Z. Markovic* *, Ne. Begovic* **, Ne. Manojlovic* ***
* Faculty of Science, University of Kragujevac, 12 Radoja Domanovica, 34000 Kragujevac, Serbia ** Faculty of Agronomy, University of Kragujevac, 34 Cara Dusana, 32000 Cacak, Serbia ***lnstitute of General and Physical Chemistry, 121V Studentski trg, 11000 Belgrade, Serbia ****Faculty of Medicine, University of Kragujevac, 69 SvetozaraMarkovica, 34000 Kragujevac, Serbia
E-mail: mark@kg.ac.yu; mark@tcf.kg.ac.yu
Abstract - The mechanisms of the carboxylation reactions of lithium and sodium phenoxides are investigated by means of DFT method with CEP-31 + G(d) basis set. The introduction of diffusion functions does not affect the outcome of the calculations. As a consequence, the results of this investigation are in good agreement with the findings obtained by means of LANL2DZ basis set. Lithium phenoxide yields only salicylic acid in the Kolbe-Schmitt reaction. The reaction of sodium phenoxide can proceed in the ortho and para positions, but the para substituted product can be expected at a very low concentration in the reaction mixture. The deviation of lithium and sodium phenoxides from the mechanisms of carboxylations of other alkali metals is a consequence of small ionic radii of lithium and sodium.
INTRODUCTION
The Kolbe-Schmitt reaction is a carboxilation reaction of alkali metal phenoxides with carbon dioxide where hydroxybenzoic acids are formed [1-3]. This reaction has been used for more than a century for industrial production of aromatic hydroxy acids, such as salicylic acid, /»-hydroxybenzoic acid, 3-hydroxy-2-naph-toic acid, 6-hydroxy-2-naphtoic acid, etc. These acids play a significant role in the synthesis of numerous products, such as pharmaceuticals, antiseptic, fungicidal, and color-developing agents, textile assistants, polyesters, high-polymeric liquid crystals and dyes [4, 5]. The reaction scheme is presented in Fig. 1.
The mechanism of the Kolbe-Schmitt reaction has been the subject of investigations of many experimental and theoretical studies. The early results on the Kolbe-Schmitt reaction mechanism can be found in the review [6], and references cited there. In this work we refer to the later results concerning the Kolbe-Schmitt reaction mechanism. The work of Kunert et al. [7] provided valuable information on the mechanism of the Kolbe-Schmitt reaction. On the basis of FT-IR spectra and DTA analysis the presence of the intermediate NaOPh-CO2 complex was confirmed. It was also found that the complex changed to a further intermediate at increased temperature (75-80°C). It was concluded that a direct carboxylation could be excluded from the Kolbe-Schmitt reaction mechanism.
Kosugi [8] investigated the Kolbe-Schmitt reaction mechanism of phenol and 2-naphthol. It was concluded, on the basis of C-13 NMR and MOPAC/PM3 calculations, that a direct carboxylation of phenoxide with carbon dioxide took place, and KOPh-CO2 (or NaOPh-CO2) was not an intermediate in the reaction.
This study was focused on the carbonation of potassium phenoxide, and an exact mechanism of the reaction was not put forward.
A DFT study on the mechanism of the carboxylation reaction of sodium phenoxide was performed by means of Gaussian 98 software package at B3LYP level of theory with LANL2DZ basis set [9]. A mechanism including three transition states and three intermediates was proposed. A quantitative explanation for the low yield of/-hydroxybenzoic acid and the equilibrium behavior of the Kolbe-Schmitt reaction was provided. In order to investigate solvent effects on the KolbeSchmitt reaction kinetics Stanescu et al. [10, 11] performed theoretical DFT studies, where Jaguar 4.2 program package at the B3LYP level of theory was used. The investigations of Achenie and Stanescu confirmed the mechanism proposed in [9].
In a recent research [12, 13] the mechanisms of the carboxylation reactions of lithium, potassium, rubidium and cesium phenoxides were investigated at the DFT/LANL2DZ level of theory. It was observed that the mechanism of the carboxylation reaction of lithium phenoxide was significantly different from those of other alkali metal phenoxides, but it was not studied in details. It was also observed that there was a significant resemblance between the mechanisms of the carboxy-lation reactions of lithium and sodium phenoxides. This provoked us to reinvestigate the mechanism of the Kolbe-Schmitt reaction of lithium and sodium phenox-ides using different basis set.
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Fig. 1. Reaction scheme for the Kolbe-Schmitt reaction. M represents alkali metals.
COMPUTATIONAL METHODS
Geometrical parameters of all stationary points and transition states for the reactions of lithium and sodium phenoxides with carbon dioxide are optimized in vacuum, employing analytic energy gradients by means of the Becke-type three-parameter hybrid combined with the gradient-corrected correlation functional of Lee, Yang, and Parr [14, 15]. This functional, commonly known as B3LYP [15, 16], implemented in GAUSSIAN98 program package [17], turned out to be quite reliable for geometrical optimizations [18]. All theoretical calculations are carried out by employing the CEP-31 + G(d) basis set. CEP-31 + G(d) uses Stevens/Basch/Krauss ECP split-valence basis set and includes a set of polarization functions [19]. It also includes corrections for relativistic effects. The vibra-tional analysis and the natural bond orbital (NBO) analysis [20, 21] are performed for all structures at the all B3LYP/CEP-31 + G(d) level. All the fully optimized transition state structures are confirmed by the existence of a sole imaginary frequency, whereas the optimized intermediate structures possess only real frequencies. From the transition state structures, the intrinsic reaction coordinates (IRCs) are obtained, and
Table 1. APT (atomic polar tensors) charge distribution in reactants and first intermediate
PhO-M B- M
Li Na Li Na
M 0.91 0.92 0.83 0.88
O -1.29 -1.23 -1.21 -1.20
o-C -0.19 -0.20 -0.10/-0.12 -0.10/-0.13
m-C 0.11 0.13 0.04/0.02 0.05/0.03
p-c -0.20 -0.24 -0.07 -0.09
Carbon dioxide
C 1.15 1.80 1.82
O -0.57 -1.07 -1.09
O -0.57 -0.80 -0.83
the free energies are maximized along these paths. These paths and free energies maxima are obtained using the IRC routine in GAUSSIAN98. The analysis of the vibrational frequencies is performed by means of the Molden program, version 3.7 [22].
RESULTS AND DISCUSSION
The mechanism of the reaction of carbon dioxide with PhO-M (M stands for Li and Na) is examined. It is found out that the Kolbe-Schmitt reaction of lithium phenoxide and sodium phenoxide proceeds via three transition states and three intermediates. The three transition states are verified by the intrinsic reaction coordinate (IRC) calculations. A general outline of the mechanism is presented in Fig. 2.
Direct carboxylation of benzene ring is investigated by performing a forced attack of carbon dioxide to the ortho, meta and para positions of PhO-M. This attack does not reveal any possible reaction path. On the other hand, a stable intermediate complex B-M is formed by approaching the reactants to each other. This step of the reaction proceeds smoothly, with the stabilization of the system, and without any activation barrier. This is in agreement with the finding that not all chemical reactions proceed via transition states, particularly in the gas phase [23]. The existence of the intermediate B-M in the Kolbe-Schmitt reaction has been confirmed experimentally [7, 8]. This reaction path is supported by theoretical results, as follows.
The APT (atomic polar tensors) charge distribution [24] of the reactants (Table 1) shows that in PhO-M the positive charge is located on the metal, whereas the negative charge is distributed among the oxygen and ortho and para carbons of the benzene ring. The charge distribution analysis undoubtedly indicates that the oxygen of carbon dioxide will bond to the alkali metal, and the carbon of carbon dioxide will bond to the adjacent oxygen of PhO-M, thus forming the intermediate B-M. The optimized geometry of B-M is presented in Fig. 2, whereas the bond lengths in B-Li and B-Na are given in Tables 2 and 3, respectively. It is worth men-
Fig. 2. Mechanism of the Kolbe-Schmitt reaction of lithium and sodium phenoxides. M stands for Li and Na. B-M, C-M and D-M represent the first, second and third intermediates, whereas TS2-M and TS3-M represent the second and third transition states, respectively. The geometry of oTS1-M is not presented because it is very similar to that of C-M. E-M represents alkali metal salt of hydroxybenzoic acid.
tioning that the distances of 1.602 A in B-Li and 1.586 A in B-Na reveal the formation of weak C7-O8 bonds.
Now, the intermediate complex undergoes further conversion. A direct carboxylation with another molecule of carbon dioxide is examined, but this attack does not reveal any possible reaction path. On the other hand, Table 1 clearly reveals that the carbon of the CO2 moiety in B-M is strongly electrophilic, indicating that an electrophilic attack of the CO2 moiety to the benzene ring can be expected as a plausible step of the reaction. For this reason the reaction paths for electrophilic attacks of this carbon to the ortho, meta and para positions of B-M are examined. The meta route is not revealed. This finding is in agreement with Table 1 which shows that the ortho and para carbons in the benzene ring are nucleophilic.
2.232
Fig. 3. Optimized geometry of the first transition state in the carboxylation reaction of sodium phenoxide in the para-position.
Table 2. Bond lengths and free energies (G298) of the intermediates, transition states and product in the carboxilation reaction of lithium p
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