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Correlation Analysis of the Substituent Electronic Effects on the Allylic H-Abstraction in Cyclohexene by Phthalimide-N-oxyl Radicals: a DFT Study Yong Sun, Wensong Zhang, Xingbang Hu, and Haoran Li* Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, P. R. China ReceiVed: January 11, 2010; ReVised Manuscript ReceiVed: March 10, 2010
The H-abstraction by pthalimide-N-oxyl radicals is an important step in oxidation reactions catalyzed by N-hydroxyphthalimide (NHPI). Herein, substituent electronic effects on the allylic H-abstraction process by phthalimide-N-oxyl radicals are evaluated by a systemically theoretical analysis in the case of cyclohexene. The catalyst with electron-withdrawing substituent possesses larger spin density on the oxygen atom of the N-O section, which results in a larger O-H bond dissociation energy (BDE) and smaller isotropic Fermi contact coupling constants of the nitrogen atom. The BDE of O-H bond plays a very important role, determining the H-abstraction activation energy. The isotropic Fermi contact coupling is closely related to the coupling constant of the EPR spectrogram. The conjugation effect plays an important part in the aryl substituent effect. According to the results above, not all ionic-compound-supported NHPIs are good catalysts. A cation-supported NHPI is better than an anion-supported NHPI. The present theoretical study reveals the relationship between the structure and the catalytic activity of NHPI and its analogues, complementary to the previous work on NHPI, and allows for a reasonable prediction of the catalysis efficiency of NHPI analogues. 1. Introduction Allylic oxidation of olefins is an attractive process for transforming inexpensive and readily available substrates into valuable intermediates for the fine chemicals industry.1 During the past decade, N-hydroxyphthalimide (NHPI) and its analogues have emerged as powerful tools in organic synthesis. This kind of catalyst allows selective and mild oxidation or, more generally, the functionalization of a broad variety of organic compounds, including alkanes, benzylic hydrocarbons, alkenes, alkynes, alcohols, amides, and so on, under environmentally benign conditions.2–12 Moreover, NHPI could be applied as a metal-free catalyst for oxidation of hydrocarbons.13–15 NHPI is an inexpensive, nontoxic compound easily prepared by the reaction of phthalic anhydride with hydroxylamine.16 Due to its idiosyncratic behavior, which is very different from that of any system proposed previously, and its general efficiency, NHPI has attracted increasing attention recently, both in academia and the chemical industries. The first reported oxidation of olefins under NHPI catalysis was an electro-catalytic system developed by Masui et al. in 1985.17 The compounds obtained are the corresponding enones with a product distribution similar to that observed in free radical autoxidation of olefins. Allylic H-abstraction by phthalimide N-oxyl radical (PINO) generated from NHPI was the key step of the reaction mechanism.17 In 1986, Foricher and co-workers patented the oxidation of isoprenoid derivatives with a free allylic group.18,19 The reactions took place by using O2 or air and stoichiometric amounts of NHPI. The NHPI acted as a catalyst and was claimed to be easily recovered in the end of the process. Ishii reported the oxidation of cyclohexene to cyclohexenone and cyclohexenol with O2 and a catalytic amount of NHPI in the absence of metal cocatalyst.6 Ishii and coworkers also showed that the combination of NHPI with a variable valence metal (known as the “Ishii system”), notably * Corresponding author. Fax: +86-571-8795-1895. E-mail: lihr@ zju.edu.cn.
cobalt, afforded an effective catalytic system for the autoxidation of a broad range of organic substrates; for example, alkanes and alkyl aromatic compounds.10–12 Subsequently, various NHPI analogues were prepared, with the objective of trying to tune the catalysis performances or developing asymmetric versions.20–26 To widen its applicability and gain a more mature understanding of NHPI catalysis, we need not only the great efforts made in the past decade by the Ishii group and other prominent researchers in the synthetic aspects of this catalyst system but also a fuller theoretical exploration of its physical chemistry aspects. This is required for more systematic planning of new synthetic processes. It is also necessary to get a better overview of the advantages and limitations of this very important catalysis. Understanding chemistry is for making chemicals.2 Einhorn and co-workers studied the hydrogen abstraction from ethylbenzene by NHPI with or without O2 theoretically.27 Calculations reproduced experimental trends and indicated that O2 did not assist the H migration, but made the whole process exothermic and the hydrogen abstraction irreversible. Hermans and coworkers theoretically investigated the mechanism of the oxidation of hydrocarbons catalyzed by NHPI.28 The NHPI + ROO• f PINO• + ROOH reaction was in near-perfect equilibrium, and the PINO• + RH f R• + NHPI was crucial to enhancement of the catalytic performance. The nonterminating nature of the PINO• radical was highly related to the high catalytic efficiency of NHPI.29 In addition, Hermans studied the pronounced nonArrhenius behavior of hydrogen abstractions from toluene and derivatives by PINO• radicals specifically using the theoretical method, which was in good agreement with experimental results.30 This work, complementary to the experimental rate coefficient data, allowed for a reliable prediction of the rate coefficient at higher temperatures. Furthermore, Bozzelli and co-workers used quantum chemical methods to calculate the molecular geometries, vibrational frequencies, and thermochemical properties of NHPI and the corresponding PINO• radical, providing important thermochemical data required for modeling the catalyzed process by NHPI.31 Computational
10.1021/jp100259v 2010 American Chemical Society Published on Web 03/24/2010
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Figure 1. A series of aryl-substituted N-hydroxyphthalimides and the analogues, compounds 1-11.
chemistry is useful to set forth the intrinsic molecular interactions and provides the possible mechanism of reactions.32–35 As mentioned in the above research, whether the oxidation of hydrocarbons or the halogenations of alkanes was catalyzed by NHPI and its analogues, the hydrogen-abstraction step was always involved. Moreover, the target sites in oxidation reactions catalyzed by NHPI were typically resonantly stabilized benzylic or allylic sites. In addition, the oxidation of cyclohexene was a subject of great interest from both academic and industrial points of view. Among the catalytic oxidation products, the allylic oxidation products of cyclohexene could act as a building block for the synthesis of various functional macromolecules. Thus, we investigated the process of allylic hydrogen abstraction from cyclohexene by the PINO• radical generated from NHPI. In our present paper, we discuss how different aryl substituent groups of NHPI would influence the O-H bond dissociation energy (BDE) and the hydrogen-abstraction process. We try to find the underlying relationship between the structure of NHPI and its catalytic activity. Does the BDE of the O-H bond determine the hydrogen-abstraction process absolutely? Is the O-H BDE in line with the electronic properties of the aryl substituent group? We also try to find out reasonable explanations for some previous experimental results. For example, why does N,N-dihydroxypyromellitimide (NDHPI) have a better catalytic efficiency than that of NHPI?36 Why does 4-carboxyN-hydroxyphthalimide (4-COOH-NHPI, shown in Figure 1) perform poorly in the allylic oxidation of cholesteryl acetate?37 Is any kind of ionic-compound-supported NHPI a good catalyst for allylic H-abstraction?38,39 2. Computational Methods All calculations were performed using the Gaussian03 programs package.40 DFT calculations at the B3LYP/6-311+G* level of theory were carried out on NHPI, PINO, a series of their aryl-substituted compounds, and their analogues, such as NDHPI and 6-hydroxy-pyrrolo [3, 4-b] pyridine-5, 7-dione (NNHPI) (Figure 1). Totally, 11 compounds were studied. Geometry optimizations and frequency calculations were performed for all the compounds under investigation. The O-H BDEs were calculated using nonisodesmic work reactions (e.g., NHPI f PINO + H). Such a reaction did not result in the level of error cancellation found in isodesmic work reactions, but the only reference enthalpy required was that of H, which was very precisely known. Bozzelli et al. indicated in their study that the results of isodesmic work reactions and nonisodesmic work reactions were in excellent agreement.31 NBO and EPR calculations were also carried out to explain how the substituted group
would influence the electronic structures and isotropic Fermi contact couplings, which might provide a hint on the prediction of spin-spin coupling constants of the EPR spectrogram. On the contrary, if the EPR spectrograms of NHPI and its analogues are available, it is possible to predict their O-H bond energies roughly. The hydrogen abstraction processes were also investigated at the B3LYP/6-311+G* level of theory. Zero-point energy (ZPE) corrections were performed at the same theory throughout the calculations. Through diagonalizing the Hessian matrix and analyzing the vibrational normal modes, the computed stationary points were characterized as minima or transition states. In this way, the stationary points could be classified as minima if no imaginary frequencies were shown or as transition states if only one imaginary frequency was obtained.41 The particular nature of the transition states was determined by analyzing the motion described by the eigenvector associated with the imaginary frequency. The geometrical parameters, which were closely related to the H-abstraction activation energy, were analyzed in detail. To get close to the real catalytic system, we extended singlepoint calculations to the ionic derivative systems under study using PCM included in Gaussian 03, where CH3CN was used as solvent. Concerning the solvent effect, we obtain the same conclusion that the largest spin density corresponds to the least H-abstract barrier as in the gas-phase (as shown in Supporting Information Figure S3). Furthermore, to ensure that our calculation results were reliable, we carried out some test calculations. Larger basis sets of 6-311++G** were used to correct the BDEs of all the systems. The calculated data are listed in Table 1. The relationship between the BDEs and the spin density of the O atom is displayed in Supporting Information Figure S4. The qualitative result is the same with that using basis set 6-311+G*. 3. Results and Discussion Molecular Geometries and Electronic Structures of the O-H bond. The molecules of compounds 1-11 are depicted in Figure 1, optimized at the B3LYP/6-311+G* level of theory. The optimized structures are displayed in Figure 2. Compound 1 is normal NHPI. Compounds 2, and 4-8 are derivatives of NHPI with different substituent groups. Some of the substituent groups are electron-withdrawing, and the others are electrondonating. Compound 3 is ionic-compound-supported NHPI. Compound 9 is NDHPI, which is different from NHPI by having an extra imide-N-oxyl group. Compound 10 is NNHPI, which is different from NHPI by replacing the aromatic benzene ring in NHPI with a pyridine ring. Compound 11 is ionic-compound-
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TABLE 1: Key Geometric Features, Electronic Structure, and Bond Dissociation Energies (BDEs) of Compounds 1-11 and the Corresponding Radicals compda
O-H (Å)
N-O (Å)
