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The Mechanism of the Baeyer–Villiger Rearrangement: Quantum Chemistry and TST Study Supported by Experimental Kinetic Data†

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The Mechanism of the Baeyer–Villiger Rearrangement: Quantum Chemistry and TST Study Supported by Experimental Kinetic Data† PAPER www.rsc.org/obc | Organic & Biomolecular Chemistry The mechanism of the Baeyer–Villiger rearrangement: quantum chemistry and TST study supported by experimental kinetic data† J. Raul Alvarez-Idaboy,*a Lino Reyesa and Nelaine Mora-Diezb Received 15th August 2007, Accepted 19th September 2007 First published as an Advance Article on the web 2nd October 2007 DOI: 10.1039/b712608e The mechanism of the Baeyer–Villiger rearrangement is modelled for the reaction of propanone with trifluoroperacetic acid, catalyzed by trifluoroacetic acid in dichloromethane, using three DFT methods (B3LYP, BH&HLYP and MPWB1K) and MP2. These results are refined and used to calculate the overall reaction rate coefficient using conventional Transition State Theory. The excellent agreement between the calculated (1.00 × 10−3 L mol−1 s−1) and the experimental (1.8 × 10−3 L mol−1 s−1)rate coefficients at the MPWB1K level strongly supports the mechanism recently proposed by our group. This DFT method is then used to study the mechanism of a larger system: cyclohexanone + trifluoroperacetic acid, for which a very good agreement between the calculated and the experimental rate coefficients is also found (1.37 and 0.32 L mol−1 s−1, respectively). The modelled mechanism is not ionic but neutral, and consists of two concerted steps. The first one is strongly catalyzed while the second one, the migration step, seems not to be catalyzed for the systems under study. The results of this work could be of interest for understanding other reactions in non-polar solvents for which ionic mechanisms have been assumed. 1. Introduction (TS) for the addition step was recently reported by our group.6 This TS leads to a Gibbs free-energy of activation that is 12.7 kcal 1 The Baeyer–Villiger (BV) rearrangement which involves the mol−1 lower than the previously reported concerted TS.7 This result oxidation of a ketone to an ester or lactone, is a powerful synthetic confirmed the idea that the first step of the BV reaction in non- tool frequently employed due to its excellent regioselective and polar solvents is termolecular (although effectively bimolecular) stereoselective control. Therefore the reaction has been the subject and acid-catalyzed. of a large number of investigations for more than a century In the second part of this reaction, the adduct undergoes the (for reviews on this topic see references 2, 3 and 4). It is well intramolecular migration of an alkyl or aryl group from the ketone known that the BV oxidation initially involves the carbonyl moiety to the nearest peroxide oxygen with the simultaneous addition of a peroxyacid to a ketone (or an aldehyde) to produce dissociation of the O–O bond.8 Nevertheless, it is not totally clear 5 the tetrahedral Criegee intermediate. However, there is still no if this reaction is acid-catalyzed, or if it occurs in a concerted agreement on the mechanism of the acid catalysis for this step. (neutral) or stepwise (ionic) manner. A recent publication from The most controversial part is whether the protonation of the our group showed that the second part of the BV reaction of carbonyl oxygen and the carbonyl addition occur in a stepwise propanone and cyclohexanone with performic acid, in a non- or concerted manner. This distinction implies that in the former polar solvent and in the presence of formic acid, occurs in a case the reaction would occur through an ionic mechanism while concerted manner and appears not to be acid catalyzed.9 The in the latter it would be neutral. A new concerted transition state lack of experimental data on this reaction limited us from going any further. Even though the migration process has been postulated to be the aFacultad de Qu´ımica, Universidad Nacional Autonoma´ de Mexico,´ Ciudad Universitaria, 04510, Mexico´ D.F., Mexico.´ E-mail: jidaboy@ rate-determining step (RDS), it has also been shown that a rate- servidor.unam.mx determining addition can be feasible depending on the reaction bDepartment of Chemistry, Thompson Rivers University, Kamloops, BC, conditions and the reactants.2,10–19 A recent experimental study V2C 5N3, Canada of the reaction of cyclohexanone with m-chloroperbenzoic acid † Electronic supplementary information (ESI) available: Preliminary cal- showed that the first (addition) step is the RDS.16 Our calculations culations of the BV reaction of propanone at four levels of theory (Table are in agreement with this finding.9 S1); calculated K eq(1), j(2), k(2) and k (for the catalyzed and uncatalyzed second step) values at different temperatures in the range of 280 to The currently accepted mechanism for the BV rearrangement 320 K for the reactions of propanone and cyclohexanone with the in organic chemistry books is an ionic mechanism (see Fig. 1).20 corresponding Arrhenius plots (Tables S2 and S3); comparison of the effect of diffuse functions on the thermodynamic calculations of the reaction The generalization of the idea that in solution reactions should be of propanone (Table S4); Mulliken charge distribution of the stationary ionic in spite of the solvent polarity21 is in clear contradiction with points of the reaction of propanone (Fig. S1); plot of the energy change the work of Benson,22 who proposed that the change in reaction along the relaxed scan of the forced protonation of propanone by TFAA rates when performing a reaction in the gas phase and in non-polar (Fig. S2); MPWB1K/6-311G(d,p)–Onsager Cartesian coordinates of the important stationary points of the calculated kinetic mechanisms. See solvents is usually due to liquid-phase effects since the mechanism DOI: 10.1039/b712608e is most likely unchanged. This difference in reaction rates is due to 3682 | Org. Biomol. Chem., 2007, 5, 3682–3689 This journal is © The Royal Society of Chemistry 2007 Fig. 1 Currently accepted mechanism for the BV rearrangement.20 the lost of translational degrees of freedom in Gibbs free-energy 2. Computational methodology barriers when going from the gas to the solvent phase. Since ions are not stabilized in the gas phase and in non-polar solvents as The calculations were performed with the Gaussian 0332 program much as they are in polar environments, the assumption of their package. For electronic energies, solvent effects and thermody- existence when proposing a mechanism for a reaction that takes namic corrections, we have tested four methods: MP2, B3LYP, place in non-polar solvents seems unjustified. BH&HLYP, and MPWB1K, a functional that was recently In a recent review,4 a possible general mechanism for the BV especially developed for kinetic calculations.33 We have performed reaction is proposed that depends on the acidity of the reaction gas-phase geometry optimizations with each method employing medium but the polarity of the solvent in which this reaction the 6-311G(d,p) basis set, which were confirmed by frequency cal- takes place is not taken into account. This review focuses on culations at the same level of theory. In all cases IRC calculations the reactants, the Criegee intermediate and the ester product, were performed to test that the calculated TSs connect with the but it does not provide details on how the actual transformation proper reactants and products. The energy results were improved from one species to another takes place. The ketone protonation by single-point energy calculations with the same method and the is assumed to occur in high-acidity media. In addition, ionic 6-311++G(d,p) basis set including solvent effects (the IEF-PCM Criegee intermediates are proposed in both high- and low-acidity continuum solvation model using dichloromethane as solvent (basic) media. The general mechanism proposed resembles more and the UFF radii). Using this procedure the method that a reaction scheme than a mechanism itself, probably because the best reproduced the experimental rate coefficient of the reaction details of the reaction mechanism are still unclear. of propanone was chosen to further refine the calculations. Since the BV reaction is one of the most well known and widely Geometries and frequencies were then calculated with the Onsager applied methods in synthetic organic chemistry, several computa- solvent model and the 6-311G(d,p) basis set, followed again by tional studies of its mechanism have been published.6,7,9,19,21,23–31 single-point energy calculation with the IEF-PCM method and Although most of these studies have modelled concerted TSs the 6-311++G(d,p) basis set. This methodology was also used for and have assumed a neutral mechanism in non-polar solvents, the study of the reaction of cyclohexanone. For modelling the these findings have not yet been compiled in organic chemistry ionic system the relaxed scan was performed at the MPWB1K/6- textbooks or review papers.4,20 To the best of our knowledge there 311G(d,p) level of theory including the IEF-PCM solvation is no experimental evidence in favour of an ionic mechanism for model. the BV reaction in non-polar solvents such as dichloromethane— In order to make appropriate comparisons between the calcu- one of the most frequently used solvents for this reaction. The lated and experimental data, several thermodynamic corrections complete reaction mechanism of the BV reaction, including the were applied on the calculated DG values. The standard state addition and migration steps, and the Brønsted acid catalysis, for the DG values was changed from 1 atm to 1 M, in order has only been studied by Okuno,18 Grein et al.,7 and our to relate the calculated Gibbs free energies of activation (DG=) group.9 with experimental rate coefficients that are reported in concentra- In this paper we aim at modelling the complete BV reaction tion units.
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