- Email: [email protected]
Methyl-to-double bond transfer in the gas phase: A theoretical study
Computational and Theoretical Chemistry 1020 (2013) 7–13
Contents lists available at SciVerse ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Methyl-to-double bond transfer in the gas phase: A theoretical study Zhang Xiang Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, China
a r t i c l e
i n f o
Article history: Received 4 June 2013 Received in revised form 12 July 2013 Accepted 13 July 2013 Available online 24 July 2013 Keywords: Methyl-to-double bond transfer Alkene Substituent effect Methylation site Theoretical calculation
a b s t r a c t A novel methyl-to-double bond transfer between the methylsulfonium ion and alkene has been given a systematic theoretical study with density functional theory (DFT) method. Two possible pathways have been proposed: (i) methyl attack on the terminal carbon of the double bond and (ii) formation of the bridged methyl-onium ion. The concrete pathway is mainly associated with the type of the alkene rather than that of the methyl donor. The symmetrical aliphatic alkene favors pathway ii, while aromatic alkene and asymmetrical aliphatic one favor pathway i. In pathway i, the methylation site is affected by the substituent. Moreover, for the S-substituted methylsulfonium ion, the electron-withdrawing group can promote the methyl transfer, while the electron-donating group has the opposite effect. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Methyl transfer from an electrophile to a nucleophile by an SN2 mechanism is a significant reaction in chemistry and biochemistry. Up to now, it is still the subject in many experimental and theoretical studies [1–10]. In the organometallic chemistry, the methyl transfer plays an important role because of its involvement in the homogeneous and heterogeneous catalysis and organic synthesis [11–13]. In the gas-phase chemical reactions, the methyl transfer has always been a popular study. For example, O’Hair’s group has reported the methylation of some pyridine derivatives and amino acids in the gas phase and studied the differences in the methylated loci [14–16]. Brodbelt’s group has investigated the methylation of the nitrogen atom in different hybridization states [17]. In biochemistry, transmethylation is an important process and the mechanism of its enzymic catalysis is of great interest, such as the conversion of homocysteine to methionine and the methylation of glutamate residues in chemoreceptors and gene regulation [18–20]. Most of the methyl transfers occur in the enzyme-catalyzed or transition-metal-catalyzed reactions, while the methyl transfer in the metal-free small molecules has received considerably less attention. Besides the above-mentioned gas-phase methyl transfers, Wolfenden’s group has also reported the migration of the methyl group between the aliphatic amines in water [21]. Our group has investigated the methylation of the nitrogen-containing compounds in the gas phase. In that work, the protonated N,N-dimethylisopropylamine and N,N-dimethylbutylamine were selected E-mail address: [email protected] 2210-271X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2013.07.015
as the methyl donors [22]. In addition, the theoretical analyses of the intramolecular methyl transfers in the N-oxide radical cations and 2-methoxy-pyridine derivative radical cations have been reported [23]. Recently, methyl-to-double bond transfer in the methylene arenium rhodium complexes has been reported by Milstein’s group [24]. In more specific terms, the methyl transfer from the rhodium atom to the double bond was assumed. Inspired by this report, the possibility of the gas-phase methyl-to-double bond transfer in the metal-free small molecules will be discussed in this work. For this purpose, all kinds of potential methyl donors, such as (CH3)2Cl+, + ðCH3 Þ2 NHþ 2 , (CH3)3S [25,26], have been tested, as shown in Scheme 1. The alkene series were appointed the methyl acceptors. Due to the weak cation-p interaction [27,28], the complex might be formed first of the methyl donor and the alkene. The methyl transfer occurs subsequently in the complex. For the methyl-to-double bond transfer, two possible pathways are assumed: (i) methyl attack on the terminal carbon of the double bond and (ii) formation of the bridged methyl-onium ion (see Scheme 1). The first pathway is easy to comprehend, which is similar to the proton addition. It should be noted that there are two methylation sites, namely two sides of the double bond. For the second pathway, the methyl is proposed to combine with the double bond to form the three-membered ring structure. This hypothesis is founded on the reaction of Br2 with alkene, in which a three-membered ring bromonium ion has been presented [29,30]. Here, the study on the substituent effect and the regioselectivity is the focus of this work. We expect that this work may offer some new views and methods to the synthesis.
8
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
R2
R2 R1 R3
(CH3)3S+ (CH3)2Cl
R2
+
(CH3)2NH2+ (CH3)2SH+
R1 R4
R3
R4
(CH3)2S CH3Cl +
COMPLEX
CH3SH2+ methyl donor
R4
i)
R1
+
R3 or
ii)
double bond
H3C
R3
R2
CH3NH2 CH3SH H2S
R1 R4
Scheme 1. Possible methyl-to-double bond transfers between methyl donors and alkenes.
2. Computational methods All structures were computed on the basis of the hybrid density functional theory (B3LYP) [31–34] and the 6-31+G(d, p) basis set which were implemented in Gaussian 09 program package [35]. All the gas phase minima and transition structures (here also referred to as transition states) were characterized by frequency analysis. Frequency calculations identified minimum structures with all real frequencies, while transition states with only one imaginary frequency. Zero point energy (ZPE) corrections were applied at the same level of theory [36]. To confirm the transition states connecting the designated intermediates, intrinsic reaction coordinate (IRC) calculation was carried out. Moreover, some transition structures were reoptimized with B3LYP/6-311+G(3df, 2p) and MP2(full)/6-311+G(3df, 2p) methods. Charge distribution analysis was calculated by using the NBO 3.1 program [37] implemented in Gaussian 09. Furthermore, to obtain more reliable energetic data, higher-level single-point energy calculations were performed at B3LYP/6-311++G(3df, 2p), CCSD(T) [38] and G3B3 [39] levels respectively by using the B3LYP/6-31+G(d, p) optimized geometries. Computed molecular structures were drawn with the CYLview program [40].
3. Results and discussion Simple alkenes, such as ethylene, propylene and so on, were examined first. All the gas phase minima and transition states were optimized with the B3LYP/6-31+G(d, p) method, because it has been proved that B3LYP is the most practical method for the structural optimization [41,42]. Besides the complexes formed of the methyl donors and the simple alkenes (see Supporting information), all the transition states for the methyl transfers have been successfully found, as shown in Fig. 1. As for the energy calculations, we have temporarily used the B3LYP/6-311++G(3df, 2p)// B3LYP/6-31+G(d, p) method. According to the transition state structures, it is found that the mechanism for the methyl transfer is mainly determined by the structure of the alkene. For the symmetrical alkenes, including ethylene, cis-2-butylene and trans2-butylene, we only found the transition states for the formation of the bridged methylonium ions (pathway ii in Scheme 1). The corresponding activation free energies are 36.4, 30.0, 18.2, 42.3, 14.6 and 13.4 kcal/mol, respectively. On the contrary, for propylene, no matter what type of the methyl donor is used, the methyl transfer proceeds via pathway i. The corresponding activation free energies are 26.2, 14.4, 39.3 and 1.4 kcal/mol, respectively. In the meantime, the methylation site is sole, for the transition state only leads to the more stable 2°-carbocation. Despite repeated attempts, we failed to find the transition state leading to the 1°-carbocation.
The reason might be that the 1°-carbocation is very unstable and it is easy to undergo the adjacent methyl transfer to form the 2°-carbocation. The transition states above were also reoptimized with more advanced methods, such as B3LYP/6-311+G(3df, 2p) and MP2(full)/6-311+G(3df, 2p), shown in Fig. 1. For B3LYP method, it is found that larger basis set has little effect on the structural optimization. On the other hand, for MP2 method, the structures of several transition states have changed to some extent. To further confirm the validity of the B3LYP/6-31+G(d, p) method, the reactants have been optimized separately with different methods. The data of the pivotal bond lengths are listed in Table 1. According to the experimental data and computational efficiency, it is found that the B3LYP/6-31+G(d, p) method can be competent with the structural optimization. It should be noted that there exists underestimation of the barriers when the DFT methods (such as B3LYP) are used to calculate the free energy barriers [43,44]. At this point we decided to use higher level ab initio methods (e.g., CCSD(T) and G3B3 by using the B3LYP/6-31+G(d, p) optimized geometries). The corresponding data about the barriers are also depicted in Fig. 1 (activation free energies in red and blue). It is found that the activation free energies calculated by the CCSD(T)/6-311+G(d, p) method are systematically higher than the values predicted by the B3LYP/ 6-311++G(3df, 2p) method by ca. 4–6 kcal/mol. Nonetheless, when the CCSD(T) results are plotted against the B3LYP data, we obtain an excellent regression line with a very high correlation coefficient of 0.9970, as shown in Fig. 2a. Thus, although the B3LYP method cannot accurately predict the absolute activation free energies, it can reliably predict the relative activation free energies in the methyl-to-double bond transfer. The same conclusion can also be made by comparing the G3B3 results and the B3LYP data and the corresponding correlation coefficient is 0.9983, as shown in Fig. 2b. In addition, when the transition states for the methyl transfers were reoptimized with B3LYP/6-311+G(3df, 2p) and MP2(full)/6311+G(3df, 2p) methods, the corresponding energy calculations by using higher level ab initio methods (e.g., G3B3) have been performed (see Fig. 1, activation free energies in brown and pink). Analogously, when the G3B3 results (activation free energies calculated at G3B3//B3LYP/6-311+G(3df, 2p) and G3B3//B3LYP/6311+G(3df, 2p) levels) are plotted against the B3LYP data (activation free energies calculated at B3LYP/6-311++G(3df, 2p)//B3LYP/ 6-31+G(d, p) level) respectively, two excellent regression lines can be obtained with very high correlation coefficients (0.9979 and 0.9934), as shown in Fig. 3a and b. As a result, the B3LYP/6311++G(3df, 2p)//B3LYP/6-31+G(d, p) method can be fully competent with the calculations of the free energies in this work. Besides the simple alkenes, more complex systems, namely the styrene series, have been selected as the methyl acceptors. As the
9
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Fig. 1. Transition states for the methyl-to-double bond transfers with B3LYP/6-31+G(d, p) method. Relevant distances optimized by B3LYP/6-311+G(3df, 2p) (in parentheses) and MP2(full)/6-311+G(3df, 2p) (in brackets) methods. Black: the activation free energies calculated by the B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) method; Red: the activation free energies calculated by the CCSD(T)/6-31+G(d, p)//B3LYP/6-31+G(d, p) method; Blue: the activation free energies calculated by the G3B3//B3LYP/6-31+G(d, p) method; Brown: the activation free energies calculated by the G3B3//B3LYP/6-311+G(3df, 2p) method; Pink: the activation free energies calculated by the G3B3//MP2(full)/6311+G(3df, 2p) method (distances in Å) (energies in kcal/mol). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 The data of the pivotal bond lengths from the experiments and calculations with different methods. Bond length Method
B3LYP/6-31+G(d, p) B3LYP/6-311+G(3df, 2p) MP2(full)/6-311+G(3df, 2p) CCSD/6-311+G(d, p) Experiment a b
1.33 1.32 1.33 1.34 1.34a
1.34 1.33 1.33 1.34 1.34a
1.34 1.33 1.33 1.34 1.34a
1.82 1.81 1.78 1.81 1.82a
1.51 1.50 1.49 1.50 1.48a
1.85 1.84 1.80 1.83 1.80b
J.G. Speight, Lange’s Handbook of Chemistry, sixteenth ed., CD&W Inc., Laramie, Wyoming. E.S. Stoyanov, I.V. Stoyanova, F.S. Tham, C.A. Reed, Dialkyl chloronium ions, J. Am. Chem. Soc. 132 (2010) 4062–4063.
transfer pathway has little relationship with the methyl donor, in the following work, trimethylsulfonium (CH3)3S+ was chosen as the sole methyl donor. The methyl transfer between trimethylsulfonium and unsubstituted styrene was investigated. Only one transition state leading to the stable benzyl cation has been successfully located (Fig. 4), which indicates that this transfer proceeds via pathway i (Scheme 1) and there is only one methylation site (C2 position). The corresponding activation free energy is 27.1 kcal/mol. Since substitution on an olefinic bond has a great influence on the reaction [45,46], thus the substituent effects in the methylto-double bond transfers have been taken into account. The substi-
tuent at the C1-position was considered first, as is shown in Fig. 5. The substituents mainly include Me (to represent alkyl groups), iPr, t-Bu, OMe, NO2 and Cl (to represent halogens). The detailed activation free energies are also shown in Fig. 5. Similar to the unsubstituted styrene, the methyl transfer takes place via pathway i (Scheme 1). The methylation only occurs at the C2-position. Obviously C1-substituent cannot change the regioselectivity in the methyl transfer. It is found that the electron-donating group can decrease the activation free energy, while the electron-withdrawing group has the opposite effect. The reason is that the electrondonating group can promote the stability of the product cation. t-Bu is an exception. It also increases the activation free energy
10
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Fig. 2. (a) Correlation between the CCSD(T)/6-311+G(d, p)//B3LYP/6-31+G(d, p) and B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) results; (b) correlation between the G3B3//B3LYP/6-31+G(d, p) and B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) results.
slightly, for the steric hindrance effect of t-Bu might block the intermolecular methyl transfer. Besides the C1-substituents, C2-substituents have also been examined. The detailed activation free energies are shown in Fig. 6. According to the calculations, four items can be summarized as follows: (i) the methyl-to-double bond transfer proceeds via the pathway i (see Scheme 1); (ii) For the electron-donating groups (such as i-Pr, t-Bu and OMe), the methylation occurs at the C1position exclusively (C1-methylation); (iii) For the electron-withdrawing group (such as NO2), it is found that the methylation only occurs at the C2-position (C2-methylation); (iv) For Me and Cl, two sides of the double bond are both the methylation sites. The corresponding regioselectivity of C1-methylation: C2-methylation is 74%: 26% (for Me) and 54%: 46% (for Cl), respectively. It is obvious that the introduction of the C2-substituents can change the regioselectivity in the methyl transfer. Compared with the linear alkenes, the methyl transfer between trimethylsulfonium and cyclic alkenes deserves our attention as well. The detailed activation free energies and the optimized structures of the transition states for the methyl transfer are all shown in Fig. 7. Obviously the methyl transfer also proceeds via the pathway i (Scheme 1) and C2-position is the sole methylation site. It is found that the methyl transfer between trimethylsulfonium and 1-phenyl-cyclopentene (n = 3) is the least in the activation free
Fig. 3. (a) Correlation between the G3B3//B3LYP/6-311+G(3df, 2p) and B3LYP/6311++G(3df, 2p)///B3LYP/6-31+G(d, p) results; (b) correlation between the G3B3// MP2(full)/6-311+G(3df, 2p) and B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) results.
energy and the corresponding value is 24.2 kcal/mol. The reason might be that the producing five-membered ring cation is the most stable due to the strain. According to the calculations above, when the asymmetric styrene series were chosen as the methyl acceptors, the methyl transfers proceed exclusively via the pathway i (Scheme 1). In this condition, the bridged methyl-onium ion cannot be formed (see the pathway ii in Scheme 1). On the other hand, for the symmetric styrene series, such as cis-stilbene and trans-stilbene, unfortunately the transition states leading to the bridged methyl-onium ions cannot be found either, as is shown in Fig. 8. In fact, cis-stilbene is not really symmetric owing to the repulsion effect of the benzene ring (Fig. 8a). Trans-stilbene has symmetric planar structure due to the hyperconjugation effect. Transition state with three-membered ring cannot be formed, maybe because it is difficult to bend the p-orbit perpendicular to the plane (Fig. 8c). Therefore, whether for cis- or trans-stilbene, the methyl-to-double bond transfer proceeds via the pathway i, namely the methyl attack on the terminal carbon of the double bond. The next section will focus on the effect of the substituent at the sulfur atom (methyl donor) in the methyl transfer, as is shown in Fig. 9. Electron-donation groups, such as i-Pr, t-Bu and OMe, can increase the activation free energy for the methyl transfer. On the contrary, electron-withdrawing groups, including NO2 and Cl, can
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
11
Fig. 4. Methyl transfer between trimethylsulfonium and unsubstituted styrene (kcal/mol) (Å).
Fig. 5. Activation free energies (DG–) for the methyl transfer between trimethylsulfonium and C1-substituted styrene (kcal/mol).
Fig. 6. Activation free energies (DG–) for the methyl transfer between trimethylsulfonium and C2-substituted styrenes (kcal/mol).
Fig. 7. Activation free energies (DG–) in the methyl transfer between trimethylsulfonium and cyclic alkenes (kcal/mol) (Å).
decrease the corresponding activation free energy. Similar to the electron- withdrawing group, the phenyl group also promotes the methyl transfer slightly. These phenomena can be explained by the electrophilicity of the methyl moiety in the methylsulfonium ion, for generally the activation free energy for the methyl
transfer decreases with the increasing of the electrophilicity of the methyl moiety. On the basis of NBO charge analyses, electron-donation group can obviously decrease the positive charge on the methyl moiety of the methylsulfonium ion (from 0.05 to 0.02). In other words, electron-donation group decreases the
12
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Fig. 8. (a) Transition state for the methyl transfer between trimethylsulfonium cation and cis-stilbene; (b) transition state for the methyl transfer between trimethylsulfonium cation and trans-stilbene; (c) orbital interaction between trimethylsulfonium cation and trans-stilbene.
Fig. 9. Activation free energies (DG–) and NBO charge on the methyl moiety of the methyl donor in the methyl transfer between S-substituted methylsulfonium cations and styrene (kcal/mol).
electrophilicity of the methyl moiety in the methylsulfonium ion. On the other hand, electron-withdrawing group can increase the positive charge on the methyl moiety (from 0.05 to 0.1), which means that electron-withdrawing group can increase electrophilicity of the methyl moiety. The methyl transfer between the cyclic methylsulfonium and styrene was also examined, as is shown in Fig. 10. It is found that five- and six-membered ring can hinder the methyl transfer (n = 2 and 3, Fig. 10a), for the corresponding activation free energies increase by at least 0.5 kcal/mol. As for the four-membered ring, namely S-methyl thio-cyclobutane cation, the activation free energy for the methyl transfer is 26.0 kcal/mol, which is 1.1 kcal/ mol less than that for the methyl transfer between trimethylsulfonium and styrene. In other words, four-membered ring can pro-
mote the methyl transfer, which is probably due to the strain of the ring. Moreover, the ability of S-methyl bisbenzo-thio-cyclopentane cation to give methyl has been tested (Fig. 10b). Compared with S-methyl thio-cyclopentane cation, the corresponding activation free energy for the methyl transfer decreases from 27.6 kcal/ mol to 21.2 kcal/mol. NBO charge analyses show that the methyl moiety in the S-methyl bisbenzo-thio-cyclopentane cation carries the most positive charge (0.08), while that in the S-methyl thiocyclohexane cation carries the least positive charge (0.03). In other words, the electrophilicity of the former is bigger than that of the latter. As a result, the ability of S-methyl bisbenzo-thio-cyclopentane cation giving methyl is much stronger than that of S-methyl thio-cyclohexane cation.
4. Conclusion In this work, we have conducted the systematic theoretical study on the intermolecular methyl-to-double bond transfer between the methylsulfonium ion and alkene series. The following conclusions can be made from our results:
Fig. 10. Activation free energies (DG–) and NBO charge on the methyl moiety of the methyl donor in the methyl transfer between cyclic methylsulfonium cations and styrene (kcal/mol).
1. For the methyl-to-double bond transfer, there exist two possible pathways: (i) methyl attack on the terminal carbon of the double bond and (ii) formation of the bridged methyl-onium ion. The pathway is mainly determined by the types of the alkenes. 2. For the symmetrical aliphatic alkene, the methyl transfer generally proceeds via the pathway ii, which leads to the bridged methyl-onium ion. For the asymmetrical one, the methyl transfer proceeds via the pathway i. 3. For the aromatic alkene, whether symmetrical or not, the methyl transfer always proceed via the pathway i. The regioselectivity (methylation site) is affected by the substituent. 4. For the S-substituted methylsulfonium ions, the electron-withdrawing group can promote the methyl transfer, while the electron-donating group has the opposite effect.
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Acknowledgement The author gratefully acknowledges financial support from the start-up costs to introduce research personnel of Zhejiang Gongshang University (1110XJ2010044, and 1110XJ2110044). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2013.07 .015. References [1] Y. Suzuki, T. Yasumoto, K. Mashima, J. Okuda, Hafnocene catalysts for selective propylene oligomerization: efficient synthesis of 4-methyl-1-pentene by bmethyl transfer, J. Am. Chem. Soc. 128 (2006) 13017–13025. [2] E. Khaskin, P.Y. Zavalij, A.N. Vedernikov, Bidirectional transfer of phenyl and methyl groups between PtIV and boron in platinum dipyridylborato complexes, J. Am. Chem. Soc. 130 (2008) 10088–10089. [3] H. Castejon, K.B. Wiberg, S. Sklenak, W. Hinz, Solvent effects on methyl transfer reactions. 2: The reaction of amines with trimethylsulfonium salts, J. Am. Chem. Soc. 123 (2001) 6092–6097. [4] L.G. Arnaut, S.J. Formosinho, Understanding chemical reactivity: the case for atom, proton and methyl transfers, Chem. Eur. J. 14 (2008) 6578–6587. [5] V. Moliner, I.H. Williams, Influence of compression upon kinetic isotope effects for SN2 methyl transfer: a computational reappraisal, J. Am. Chem. Soc. 122 (2000) 10895–10902. [6] P. Wang, J.D. Atwood, Methyl transfer reactions to tetracarbonylferrate (2): rate and mechanistic studies, Organometallics 12 (1993) 4241–4249. [7] T.D. Fridgen, T.B. McMahon, The reaction of protonated dimethyl ether with dimethyl ether: temperature and isotope effects on the methyl cation transfer reaction forming trimethyloxonium cation and methanol, J. Am. Chem. Soc. 123 (2001) 3980–3985. [8] J.J. Christie, E.S. Lewis, E.F. Casserly, Methyl-transfer reactions.7: System with CHOSO+ intermediate, J. Org. Chem. 48 (1983) 2531–2534. [9] G.D. Ruggiero, I.H. Williams, QM/MM determination of kinetic isotope effects for COMT-catalyzed methyl transfer does not support compression hypothesis, J. Am. Chem. Soc. 126 (2004) 8634–8635. [10] I. Lee, C.K. Kim, C.K. Sohn, H.G. Li, H.W. Lee, A high-level theoretical study on the gas-phase identity methyl transfer reactions, J. Phys. Chem. A 106 (2002) 1081–1087. [11] W. Galezowski, Methyl transfer from CH3CoIIIPc to thiophenoxides revisited: remote substituent effect on the rates, Inorg. Chem. 44 (2005) 5483–5494. [12] Q.B. Bao, T.B. Brill, Methyl-group transfer involving transition-metal complexes by the Michaelis-Arbuzov mechanism, Organometallics 6 (1987) 2588–2589. [13] G.S. Hill, G.P.A. Yap, R.J. Puddephatt, Electrophilic platinum complexes: methyl transfer reactions and catalytic reductive elimination of ethane from a tetramethylplatinum (IV) complex, Organometallics 18 (1999) 1408–1418. [14] R.A.J. O’Hair, M.A. Freitas, J.A.R. Schmidt, Gas-phase methylation of the 2hydroxypyridine 2-pyridone system by the dimethylchlorinium ion, Eur. Mass Spectrom. 1 (1995) 457–463. [15] R.A.J. O’Hair, M.A. Freitas, S. Gronert, J.A.R. Schmidt, T.D. Williams, Concerning the regioselectivity of gas phase reactions of glycine with electrophiles. The cases of the dimethylchlorinium ion and the methoxymethyl cation, J. Org. Chem. 60 (1995) 1990–1998. [16] M.A. Freitas, R.A.J. O’Hair, Gas phase reactions of cysteine with charged electrophiles:regioselectivities of the dimethylchlorinium ion and the methoxymethyl cation, J. Org. Chem. 62 (1997) 6112–6120. [17] J.S. Brodbelt, J. Isbell, J.M. Goodman, J.I. Seeman, Gas phase versus solution chemistry: on the reversal of regiochemistry of methylation of sp2- and sp3nitrogens, Tetrahedron Lett. 42 (2001) 6949–6952. [18] M.F. Hegazi, R.T. Borchardt, R.L. Schowen, SN2-like transition state for methyl transfer catalyzed by catechol-O-methyltransferase, J. Am. Chem. Soc. 98 (1976) 3048–3049. [19] F.C. Cui, X.L. Pan, W. Liu, J.Y. Liu, Elucidation of the methyl transfer mechanism catalyzed by chalcone O-methyltransferase: a density functional study, J. Comput. Chem. 32 (2011) 3068–3074. [20] I. Graham, H. Refsum, I.H. Rosenberg, P.M. Ueland, Homocysteine Metabolism: From Basic Science to Clinical Medicine, Kluwer Academic Publishers, Boston, 1997.
13
[21] B.P. Callahan, R. Wolfenden, Migration of methyl groups between aliphatic amines in water, J. Am. Chem. Soc. 125 (2003) 310–311. [22] X. Zhang, S.Y. Yao, Y.L. Guo, Intramolecular methyl migration in protonated N, N0 -dimethylpropane-1, 3-diamine and N, N0 -dimethylethane-1, 2-diamine, Int. J. Mass Spectrom. 270 (2008) 31–38. [23] X. Zhang, Theoretical studies on intramolecular methyl migration in N-oxide radical cations, J. Mol. Struct.: THEOCHEM. 955 (2010) 91–96. [24] A. Vigalok, D. Milstein, Methyl-to-double bond migration in methylene arenium rhodium complexes, Organometallics 19 (2000) 2341–2345. [25] R.T. Borchardt, C.R. Creveling, E. Usdin (Eds.), Biochemistry of SAdenosylmethionine and Related Compounds, MacMillan, New York, 1982. [26] E.S. Lewis, S. Kukes, C.D. Slaterl, Reactivity in methyl transfer reactions. 5. Relation between rates and equilibria, J. Am. Chem. Soc. 102 (1980) 1619– 1623. [27] J.C. Ma, D.A. Dougherty, The cation-p interaction, Chem. Rev. 97 (1997) 1303– 1324. [28] D.A. Dougherty, Cation-p interactions in chemistry and biology: a new view of benzene, Phe, Tyr, and Trp, Science 271 (1996) 163–168. [29] G. Bellucci, C. Chiappe, R. Bianchini, D. Lenoir, R. Herges, Nature of the interaction of olefin-bromine complexes. Inference from (E)-2,2,5,5tetramethyl -3,4-diphenylhex-3-ene, the first example of an olefin whose reaction with bromine stops at the stage of pi-complex formation, J. Am. Chem. Soc. 117 (1995) 12001–12002. [30] G. Bellucci, R. Bianchini, C. Chiappe, F. Marioni, R. Ambrosetti, R.S. Brown, H. Slebocka-Tilk, The solution behavior of the adamantylideneadamantane– bromine system: existence of equilibrium mixtures of bromonium– polybromide salts and a strong 1:1 molecular charge-transfer complex, J. Am. Chem. Soc. 111 (1989) 2640–2647. [31] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648–5652. [32] C. Lee, W. Yang, R.G. Parr, Development of the colle–salvetti conelation energy formula into a functional of the electron density, Phys. Rev. B 37 (1988) 785– 789. [33] S.H. Vosko, L. Wilk, M. Nusair, Accurate spin-dependent electron-liquid correlation energies for local spin density calculations: a critical analysis, Can. J. Phys. 58 (1980) 1200–1211. [34] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields, J. Phys. Chem. 98 (1994) 11623–11627. [35] M.J. Frisch, et al., Gaussian 09, Revision A.1, Gaussian, Inc., Wallingford CT, 2009. [36] A.P. Scott, L. Radom, Harmonic vibrational frequencies: an evaluation of Hartree–Fock, Møller–Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors, J. Phys. Chem. 100 (1996) 16502–16513. [37] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold. NBO 3.0; Department of Chemistry, University of Wisconsin, Madinson, WI. [38] J.A. Pople, M. Head-Gordon, K. Raghavachari, Quadratic configuration interaction – a general technique for determining electron correlation energies, J. Chem. Phys. 87 (1987) 5968–5975. [39] A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghavachari, Gaussian-3 theory using density functional geometries and zero-point energies, J. Chem. Phys. 110 (1999) 7650–7657. [40] C.Y. Legault. CYLview, version 1.0b; Universitede Sherbrooke, Sherbrooke, QC, 2009;. [41] E.H. Krenske, S. Agopcan, V. Aviyente, K.N. Houk, B.A. Johnson, A.B. Holmes, Causation in a cascade: the origins of selectivities in intramolecular nitrone cycloadditions, J. Am. Chem. Soc. 134 (2012) 12010–12015. [42] S.M. Bronner, J.L. Mackey, K.N. Houk, N.K. Garg, Steric effects compete with aryne distortion to control regioselectivities of nucleophilic additions to 3silylarynes, J. Am. Chem. Soc. 134 (2012) 13966–13969. [43] A. Kinal, P. Piecuch, Computational investigation of the conrotatory and disrotatory isomerization channels of bicyclo[1.1.0]butane to buta-1,3-diene: a completely renormalized coupled-cluster study, J. Phys. Chem. A 111 (2007) 734–742. [44] J. Poater, M. Sola, M. Duran, J. Robles, Analysis of the effect of changing the a0 parameter of the Becke3-LYP hybrid functional on the transition state geometries and energy barriers in a series of prototypical reactions, Phys. Chem. Chem. Phys. 4 (2002) 722–731. [45] N.A. Bokacha, M.L. Kuznetsovb, V.Y. Kukushkina, 1, 3-Dipolar cycloaddition of nitrone-type dipoles to uncomplexed and metal-bound substrates bearing the C„N triple bond, Coord. Chem. Rev. 255 (2011) 2946–2967. [46] N. Acharjee, A. Banerji, DFT interpretation of 1, 3-dipolar cycloaddition reaction of C, N-diphenyl nitrone to methyl crotonate in terms of reactivity indices, interaction energy and activation parameters, Comput. Theor. Chem. 967 (2011) 50–58.
Contents lists available at SciVerse ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Methyl-to-double bond transfer in the gas phase: A theoretical study Zhang Xiang Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310035, China
a r t i c l e
i n f o
Article history: Received 4 June 2013 Received in revised form 12 July 2013 Accepted 13 July 2013 Available online 24 July 2013 Keywords: Methyl-to-double bond transfer Alkene Substituent effect Methylation site Theoretical calculation
a b s t r a c t A novel methyl-to-double bond transfer between the methylsulfonium ion and alkene has been given a systematic theoretical study with density functional theory (DFT) method. Two possible pathways have been proposed: (i) methyl attack on the terminal carbon of the double bond and (ii) formation of the bridged methyl-onium ion. The concrete pathway is mainly associated with the type of the alkene rather than that of the methyl donor. The symmetrical aliphatic alkene favors pathway ii, while aromatic alkene and asymmetrical aliphatic one favor pathway i. In pathway i, the methylation site is affected by the substituent. Moreover, for the S-substituted methylsulfonium ion, the electron-withdrawing group can promote the methyl transfer, while the electron-donating group has the opposite effect. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Methyl transfer from an electrophile to a nucleophile by an SN2 mechanism is a significant reaction in chemistry and biochemistry. Up to now, it is still the subject in many experimental and theoretical studies [1–10]. In the organometallic chemistry, the methyl transfer plays an important role because of its involvement in the homogeneous and heterogeneous catalysis and organic synthesis [11–13]. In the gas-phase chemical reactions, the methyl transfer has always been a popular study. For example, O’Hair’s group has reported the methylation of some pyridine derivatives and amino acids in the gas phase and studied the differences in the methylated loci [14–16]. Brodbelt’s group has investigated the methylation of the nitrogen atom in different hybridization states [17]. In biochemistry, transmethylation is an important process and the mechanism of its enzymic catalysis is of great interest, such as the conversion of homocysteine to methionine and the methylation of glutamate residues in chemoreceptors and gene regulation [18–20]. Most of the methyl transfers occur in the enzyme-catalyzed or transition-metal-catalyzed reactions, while the methyl transfer in the metal-free small molecules has received considerably less attention. Besides the above-mentioned gas-phase methyl transfers, Wolfenden’s group has also reported the migration of the methyl group between the aliphatic amines in water [21]. Our group has investigated the methylation of the nitrogen-containing compounds in the gas phase. In that work, the protonated N,N-dimethylisopropylamine and N,N-dimethylbutylamine were selected E-mail address: [email protected] 2210-271X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2013.07.015
as the methyl donors [22]. In addition, the theoretical analyses of the intramolecular methyl transfers in the N-oxide radical cations and 2-methoxy-pyridine derivative radical cations have been reported [23]. Recently, methyl-to-double bond transfer in the methylene arenium rhodium complexes has been reported by Milstein’s group [24]. In more specific terms, the methyl transfer from the rhodium atom to the double bond was assumed. Inspired by this report, the possibility of the gas-phase methyl-to-double bond transfer in the metal-free small molecules will be discussed in this work. For this purpose, all kinds of potential methyl donors, such as (CH3)2Cl+, + ðCH3 Þ2 NHþ 2 , (CH3)3S [25,26], have been tested, as shown in Scheme 1. The alkene series were appointed the methyl acceptors. Due to the weak cation-p interaction [27,28], the complex might be formed first of the methyl donor and the alkene. The methyl transfer occurs subsequently in the complex. For the methyl-to-double bond transfer, two possible pathways are assumed: (i) methyl attack on the terminal carbon of the double bond and (ii) formation of the bridged methyl-onium ion (see Scheme 1). The first pathway is easy to comprehend, which is similar to the proton addition. It should be noted that there are two methylation sites, namely two sides of the double bond. For the second pathway, the methyl is proposed to combine with the double bond to form the three-membered ring structure. This hypothesis is founded on the reaction of Br2 with alkene, in which a three-membered ring bromonium ion has been presented [29,30]. Here, the study on the substituent effect and the regioselectivity is the focus of this work. We expect that this work may offer some new views and methods to the synthesis.
8
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
R2
R2 R1 R3
(CH3)3S+ (CH3)2Cl
R2
+
(CH3)2NH2+ (CH3)2SH+
R1 R4
R3
R4
(CH3)2S CH3Cl +
COMPLEX
CH3SH2+ methyl donor
R4
i)
R1
+
R3 or
ii)
double bond
H3C
R3
R2
CH3NH2 CH3SH H2S
R1 R4
Scheme 1. Possible methyl-to-double bond transfers between methyl donors and alkenes.
2. Computational methods All structures were computed on the basis of the hybrid density functional theory (B3LYP) [31–34] and the 6-31+G(d, p) basis set which were implemented in Gaussian 09 program package [35]. All the gas phase minima and transition structures (here also referred to as transition states) were characterized by frequency analysis. Frequency calculations identified minimum structures with all real frequencies, while transition states with only one imaginary frequency. Zero point energy (ZPE) corrections were applied at the same level of theory [36]. To confirm the transition states connecting the designated intermediates, intrinsic reaction coordinate (IRC) calculation was carried out. Moreover, some transition structures were reoptimized with B3LYP/6-311+G(3df, 2p) and MP2(full)/6-311+G(3df, 2p) methods. Charge distribution analysis was calculated by using the NBO 3.1 program [37] implemented in Gaussian 09. Furthermore, to obtain more reliable energetic data, higher-level single-point energy calculations were performed at B3LYP/6-311++G(3df, 2p), CCSD(T) [38] and G3B3 [39] levels respectively by using the B3LYP/6-31+G(d, p) optimized geometries. Computed molecular structures were drawn with the CYLview program [40].
3. Results and discussion Simple alkenes, such as ethylene, propylene and so on, were examined first. All the gas phase minima and transition states were optimized with the B3LYP/6-31+G(d, p) method, because it has been proved that B3LYP is the most practical method for the structural optimization [41,42]. Besides the complexes formed of the methyl donors and the simple alkenes (see Supporting information), all the transition states for the methyl transfers have been successfully found, as shown in Fig. 1. As for the energy calculations, we have temporarily used the B3LYP/6-311++G(3df, 2p)// B3LYP/6-31+G(d, p) method. According to the transition state structures, it is found that the mechanism for the methyl transfer is mainly determined by the structure of the alkene. For the symmetrical alkenes, including ethylene, cis-2-butylene and trans2-butylene, we only found the transition states for the formation of the bridged methylonium ions (pathway ii in Scheme 1). The corresponding activation free energies are 36.4, 30.0, 18.2, 42.3, 14.6 and 13.4 kcal/mol, respectively. On the contrary, for propylene, no matter what type of the methyl donor is used, the methyl transfer proceeds via pathway i. The corresponding activation free energies are 26.2, 14.4, 39.3 and 1.4 kcal/mol, respectively. In the meantime, the methylation site is sole, for the transition state only leads to the more stable 2°-carbocation. Despite repeated attempts, we failed to find the transition state leading to the 1°-carbocation.
The reason might be that the 1°-carbocation is very unstable and it is easy to undergo the adjacent methyl transfer to form the 2°-carbocation. The transition states above were also reoptimized with more advanced methods, such as B3LYP/6-311+G(3df, 2p) and MP2(full)/6-311+G(3df, 2p), shown in Fig. 1. For B3LYP method, it is found that larger basis set has little effect on the structural optimization. On the other hand, for MP2 method, the structures of several transition states have changed to some extent. To further confirm the validity of the B3LYP/6-31+G(d, p) method, the reactants have been optimized separately with different methods. The data of the pivotal bond lengths are listed in Table 1. According to the experimental data and computational efficiency, it is found that the B3LYP/6-31+G(d, p) method can be competent with the structural optimization. It should be noted that there exists underestimation of the barriers when the DFT methods (such as B3LYP) are used to calculate the free energy barriers [43,44]. At this point we decided to use higher level ab initio methods (e.g., CCSD(T) and G3B3 by using the B3LYP/6-31+G(d, p) optimized geometries). The corresponding data about the barriers are also depicted in Fig. 1 (activation free energies in red and blue). It is found that the activation free energies calculated by the CCSD(T)/6-311+G(d, p) method are systematically higher than the values predicted by the B3LYP/ 6-311++G(3df, 2p) method by ca. 4–6 kcal/mol. Nonetheless, when the CCSD(T) results are plotted against the B3LYP data, we obtain an excellent regression line with a very high correlation coefficient of 0.9970, as shown in Fig. 2a. Thus, although the B3LYP method cannot accurately predict the absolute activation free energies, it can reliably predict the relative activation free energies in the methyl-to-double bond transfer. The same conclusion can also be made by comparing the G3B3 results and the B3LYP data and the corresponding correlation coefficient is 0.9983, as shown in Fig. 2b. In addition, when the transition states for the methyl transfers were reoptimized with B3LYP/6-311+G(3df, 2p) and MP2(full)/6311+G(3df, 2p) methods, the corresponding energy calculations by using higher level ab initio methods (e.g., G3B3) have been performed (see Fig. 1, activation free energies in brown and pink). Analogously, when the G3B3 results (activation free energies calculated at G3B3//B3LYP/6-311+G(3df, 2p) and G3B3//B3LYP/6311+G(3df, 2p) levels) are plotted against the B3LYP data (activation free energies calculated at B3LYP/6-311++G(3df, 2p)//B3LYP/ 6-31+G(d, p) level) respectively, two excellent regression lines can be obtained with very high correlation coefficients (0.9979 and 0.9934), as shown in Fig. 3a and b. As a result, the B3LYP/6311++G(3df, 2p)//B3LYP/6-31+G(d, p) method can be fully competent with the calculations of the free energies in this work. Besides the simple alkenes, more complex systems, namely the styrene series, have been selected as the methyl acceptors. As the
9
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Fig. 1. Transition states for the methyl-to-double bond transfers with B3LYP/6-31+G(d, p) method. Relevant distances optimized by B3LYP/6-311+G(3df, 2p) (in parentheses) and MP2(full)/6-311+G(3df, 2p) (in brackets) methods. Black: the activation free energies calculated by the B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) method; Red: the activation free energies calculated by the CCSD(T)/6-31+G(d, p)//B3LYP/6-31+G(d, p) method; Blue: the activation free energies calculated by the G3B3//B3LYP/6-31+G(d, p) method; Brown: the activation free energies calculated by the G3B3//B3LYP/6-311+G(3df, 2p) method; Pink: the activation free energies calculated by the G3B3//MP2(full)/6311+G(3df, 2p) method (distances in Å) (energies in kcal/mol). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 The data of the pivotal bond lengths from the experiments and calculations with different methods. Bond length Method
B3LYP/6-31+G(d, p) B3LYP/6-311+G(3df, 2p) MP2(full)/6-311+G(3df, 2p) CCSD/6-311+G(d, p) Experiment a b
1.33 1.32 1.33 1.34 1.34a
1.34 1.33 1.33 1.34 1.34a
1.34 1.33 1.33 1.34 1.34a
1.82 1.81 1.78 1.81 1.82a
1.51 1.50 1.49 1.50 1.48a
1.85 1.84 1.80 1.83 1.80b
J.G. Speight, Lange’s Handbook of Chemistry, sixteenth ed., CD&W Inc., Laramie, Wyoming. E.S. Stoyanov, I.V. Stoyanova, F.S. Tham, C.A. Reed, Dialkyl chloronium ions, J. Am. Chem. Soc. 132 (2010) 4062–4063.
transfer pathway has little relationship with the methyl donor, in the following work, trimethylsulfonium (CH3)3S+ was chosen as the sole methyl donor. The methyl transfer between trimethylsulfonium and unsubstituted styrene was investigated. Only one transition state leading to the stable benzyl cation has been successfully located (Fig. 4), which indicates that this transfer proceeds via pathway i (Scheme 1) and there is only one methylation site (C2 position). The corresponding activation free energy is 27.1 kcal/mol. Since substitution on an olefinic bond has a great influence on the reaction [45,46], thus the substituent effects in the methylto-double bond transfers have been taken into account. The substi-
tuent at the C1-position was considered first, as is shown in Fig. 5. The substituents mainly include Me (to represent alkyl groups), iPr, t-Bu, OMe, NO2 and Cl (to represent halogens). The detailed activation free energies are also shown in Fig. 5. Similar to the unsubstituted styrene, the methyl transfer takes place via pathway i (Scheme 1). The methylation only occurs at the C2-position. Obviously C1-substituent cannot change the regioselectivity in the methyl transfer. It is found that the electron-donating group can decrease the activation free energy, while the electron-withdrawing group has the opposite effect. The reason is that the electrondonating group can promote the stability of the product cation. t-Bu is an exception. It also increases the activation free energy
10
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Fig. 2. (a) Correlation between the CCSD(T)/6-311+G(d, p)//B3LYP/6-31+G(d, p) and B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) results; (b) correlation between the G3B3//B3LYP/6-31+G(d, p) and B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) results.
slightly, for the steric hindrance effect of t-Bu might block the intermolecular methyl transfer. Besides the C1-substituents, C2-substituents have also been examined. The detailed activation free energies are shown in Fig. 6. According to the calculations, four items can be summarized as follows: (i) the methyl-to-double bond transfer proceeds via the pathway i (see Scheme 1); (ii) For the electron-donating groups (such as i-Pr, t-Bu and OMe), the methylation occurs at the C1position exclusively (C1-methylation); (iii) For the electron-withdrawing group (such as NO2), it is found that the methylation only occurs at the C2-position (C2-methylation); (iv) For Me and Cl, two sides of the double bond are both the methylation sites. The corresponding regioselectivity of C1-methylation: C2-methylation is 74%: 26% (for Me) and 54%: 46% (for Cl), respectively. It is obvious that the introduction of the C2-substituents can change the regioselectivity in the methyl transfer. Compared with the linear alkenes, the methyl transfer between trimethylsulfonium and cyclic alkenes deserves our attention as well. The detailed activation free energies and the optimized structures of the transition states for the methyl transfer are all shown in Fig. 7. Obviously the methyl transfer also proceeds via the pathway i (Scheme 1) and C2-position is the sole methylation site. It is found that the methyl transfer between trimethylsulfonium and 1-phenyl-cyclopentene (n = 3) is the least in the activation free
Fig. 3. (a) Correlation between the G3B3//B3LYP/6-311+G(3df, 2p) and B3LYP/6311++G(3df, 2p)///B3LYP/6-31+G(d, p) results; (b) correlation between the G3B3// MP2(full)/6-311+G(3df, 2p) and B3LYP/6-311++G(3df, 2p)//B3LYP/6-31+G(d, p) results.
energy and the corresponding value is 24.2 kcal/mol. The reason might be that the producing five-membered ring cation is the most stable due to the strain. According to the calculations above, when the asymmetric styrene series were chosen as the methyl acceptors, the methyl transfers proceed exclusively via the pathway i (Scheme 1). In this condition, the bridged methyl-onium ion cannot be formed (see the pathway ii in Scheme 1). On the other hand, for the symmetric styrene series, such as cis-stilbene and trans-stilbene, unfortunately the transition states leading to the bridged methyl-onium ions cannot be found either, as is shown in Fig. 8. In fact, cis-stilbene is not really symmetric owing to the repulsion effect of the benzene ring (Fig. 8a). Trans-stilbene has symmetric planar structure due to the hyperconjugation effect. Transition state with three-membered ring cannot be formed, maybe because it is difficult to bend the p-orbit perpendicular to the plane (Fig. 8c). Therefore, whether for cis- or trans-stilbene, the methyl-to-double bond transfer proceeds via the pathway i, namely the methyl attack on the terminal carbon of the double bond. The next section will focus on the effect of the substituent at the sulfur atom (methyl donor) in the methyl transfer, as is shown in Fig. 9. Electron-donation groups, such as i-Pr, t-Bu and OMe, can increase the activation free energy for the methyl transfer. On the contrary, electron-withdrawing groups, including NO2 and Cl, can
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
11
Fig. 4. Methyl transfer between trimethylsulfonium and unsubstituted styrene (kcal/mol) (Å).
Fig. 5. Activation free energies (DG–) for the methyl transfer between trimethylsulfonium and C1-substituted styrene (kcal/mol).
Fig. 6. Activation free energies (DG–) for the methyl transfer between trimethylsulfonium and C2-substituted styrenes (kcal/mol).
Fig. 7. Activation free energies (DG–) in the methyl transfer between trimethylsulfonium and cyclic alkenes (kcal/mol) (Å).
decrease the corresponding activation free energy. Similar to the electron- withdrawing group, the phenyl group also promotes the methyl transfer slightly. These phenomena can be explained by the electrophilicity of the methyl moiety in the methylsulfonium ion, for generally the activation free energy for the methyl
transfer decreases with the increasing of the electrophilicity of the methyl moiety. On the basis of NBO charge analyses, electron-donation group can obviously decrease the positive charge on the methyl moiety of the methylsulfonium ion (from 0.05 to 0.02). In other words, electron-donation group decreases the
12
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Fig. 8. (a) Transition state for the methyl transfer between trimethylsulfonium cation and cis-stilbene; (b) transition state for the methyl transfer between trimethylsulfonium cation and trans-stilbene; (c) orbital interaction between trimethylsulfonium cation and trans-stilbene.
Fig. 9. Activation free energies (DG–) and NBO charge on the methyl moiety of the methyl donor in the methyl transfer between S-substituted methylsulfonium cations and styrene (kcal/mol).
electrophilicity of the methyl moiety in the methylsulfonium ion. On the other hand, electron-withdrawing group can increase the positive charge on the methyl moiety (from 0.05 to 0.1), which means that electron-withdrawing group can increase electrophilicity of the methyl moiety. The methyl transfer between the cyclic methylsulfonium and styrene was also examined, as is shown in Fig. 10. It is found that five- and six-membered ring can hinder the methyl transfer (n = 2 and 3, Fig. 10a), for the corresponding activation free energies increase by at least 0.5 kcal/mol. As for the four-membered ring, namely S-methyl thio-cyclobutane cation, the activation free energy for the methyl transfer is 26.0 kcal/mol, which is 1.1 kcal/ mol less than that for the methyl transfer between trimethylsulfonium and styrene. In other words, four-membered ring can pro-
mote the methyl transfer, which is probably due to the strain of the ring. Moreover, the ability of S-methyl bisbenzo-thio-cyclopentane cation to give methyl has been tested (Fig. 10b). Compared with S-methyl thio-cyclopentane cation, the corresponding activation free energy for the methyl transfer decreases from 27.6 kcal/ mol to 21.2 kcal/mol. NBO charge analyses show that the methyl moiety in the S-methyl bisbenzo-thio-cyclopentane cation carries the most positive charge (0.08), while that in the S-methyl thiocyclohexane cation carries the least positive charge (0.03). In other words, the electrophilicity of the former is bigger than that of the latter. As a result, the ability of S-methyl bisbenzo-thio-cyclopentane cation giving methyl is much stronger than that of S-methyl thio-cyclohexane cation.
4. Conclusion In this work, we have conducted the systematic theoretical study on the intermolecular methyl-to-double bond transfer between the methylsulfonium ion and alkene series. The following conclusions can be made from our results:
Fig. 10. Activation free energies (DG–) and NBO charge on the methyl moiety of the methyl donor in the methyl transfer between cyclic methylsulfonium cations and styrene (kcal/mol).
1. For the methyl-to-double bond transfer, there exist two possible pathways: (i) methyl attack on the terminal carbon of the double bond and (ii) formation of the bridged methyl-onium ion. The pathway is mainly determined by the types of the alkenes. 2. For the symmetrical aliphatic alkene, the methyl transfer generally proceeds via the pathway ii, which leads to the bridged methyl-onium ion. For the asymmetrical one, the methyl transfer proceeds via the pathway i. 3. For the aromatic alkene, whether symmetrical or not, the methyl transfer always proceed via the pathway i. The regioselectivity (methylation site) is affected by the substituent. 4. For the S-substituted methylsulfonium ions, the electron-withdrawing group can promote the methyl transfer, while the electron-donating group has the opposite effect.
Z. Xiang / Computational and Theoretical Chemistry 1020 (2013) 7–13
Acknowledgement The author gratefully acknowledges financial support from the start-up costs to introduce research personnel of Zhejiang Gongshang University (1110XJ2010044, and 1110XJ2110044). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.comptc.2013.07 .015. References [1] Y. Suzuki, T. Yasumoto, K. Mashima, J. Okuda, Hafnocene catalysts for selective propylene oligomerization: efficient synthesis of 4-methyl-1-pentene by bmethyl transfer, J. Am. Chem. Soc. 128 (2006) 13017–13025. [2] E. Khaskin, P.Y. Zavalij, A.N. Vedernikov, Bidirectional transfer of phenyl and methyl groups between PtIV and boron in platinum dipyridylborato complexes, J. Am. Chem. Soc. 130 (2008) 10088–10089. [3] H. Castejon, K.B. Wiberg, S. Sklenak, W. Hinz, Solvent effects on methyl transfer reactions. 2: The reaction of amines with trimethylsulfonium salts, J. Am. Chem. Soc. 123 (2001) 6092–6097. [4] L.G. Arnaut, S.J. Formosinho, Understanding chemical reactivity: the case for atom, proton and methyl transfers, Chem. Eur. J. 14 (2008) 6578–6587. [5] V. Moliner, I.H. Williams, Influence of compression upon kinetic isotope effects for SN2 methyl transfer: a computational reappraisal, J. Am. Chem. Soc. 122 (2000) 10895–10902. [6] P. Wang, J.D. Atwood, Methyl transfer reactions to tetracarbonylferrate (2): rate and mechanistic studies, Organometallics 12 (1993) 4241–4249. [7] T.D. Fridgen, T.B. McMahon, The reaction of protonated dimethyl ether with dimethyl ether: temperature and isotope effects on the methyl cation transfer reaction forming trimethyloxonium cation and methanol, J. Am. Chem. Soc. 123 (2001) 3980–3985. [8] J.J. Christie, E.S. Lewis, E.F. Casserly, Methyl-transfer reactions.7: System with CHOSO+ intermediate, J. Org. Chem. 48 (1983) 2531–2534. [9] G.D. Ruggiero, I.H. Williams, QM/MM determination of kinetic isotope effects for COMT-catalyzed methyl transfer does not support compression hypothesis, J. Am. Chem. Soc. 126 (2004) 8634–8635. [10] I. Lee, C.K. Kim, C.K. Sohn, H.G. Li, H.W. Lee, A high-level theoretical study on the gas-phase identity methyl transfer reactions, J. Phys. Chem. A 106 (2002) 1081–1087. [11] W. Galezowski, Methyl transfer from CH3CoIIIPc to thiophenoxides revisited: remote substituent effect on the rates, Inorg. Chem. 44 (2005) 5483–5494. [12] Q.B. Bao, T.B. Brill, Methyl-group transfer involving transition-metal complexes by the Michaelis-Arbuzov mechanism, Organometallics 6 (1987) 2588–2589. [13] G.S. Hill, G.P.A. Yap, R.J. Puddephatt, Electrophilic platinum complexes: methyl transfer reactions and catalytic reductive elimination of ethane from a tetramethylplatinum (IV) complex, Organometallics 18 (1999) 1408–1418. [14] R.A.J. O’Hair, M.A. Freitas, J.A.R. Schmidt, Gas-phase methylation of the 2hydroxypyridine 2-pyridone system by the dimethylchlorinium ion, Eur. Mass Spectrom. 1 (1995) 457–463. [15] R.A.J. O’Hair, M.A. Freitas, S. Gronert, J.A.R. Schmidt, T.D. Williams, Concerning the regioselectivity of gas phase reactions of glycine with electrophiles. The cases of the dimethylchlorinium ion and the methoxymethyl cation, J. Org. Chem. 60 (1995) 1990–1998. [16] M.A. Freitas, R.A.J. O’Hair, Gas phase reactions of cysteine with charged electrophiles:regioselectivities of the dimethylchlorinium ion and the methoxymethyl cation, J. Org. Chem. 62 (1997) 6112–6120. [17] J.S. Brodbelt, J. Isbell, J.M. Goodman, J.I. Seeman, Gas phase versus solution chemistry: on the reversal of regiochemistry of methylation of sp2- and sp3nitrogens, Tetrahedron Lett. 42 (2001) 6949–6952. [18] M.F. Hegazi, R.T. Borchardt, R.L. Schowen, SN2-like transition state for methyl transfer catalyzed by catechol-O-methyltransferase, J. Am. Chem. Soc. 98 (1976) 3048–3049. [19] F.C. Cui, X.L. Pan, W. Liu, J.Y. Liu, Elucidation of the methyl transfer mechanism catalyzed by chalcone O-methyltransferase: a density functional study, J. Comput. Chem. 32 (2011) 3068–3074. [20] I. Graham, H. Refsum, I.H. Rosenberg, P.M. Ueland, Homocysteine Metabolism: From Basic Science to Clinical Medicine, Kluwer Academic Publishers, Boston, 1997.
13
[21] B.P. Callahan, R. Wolfenden, Migration of methyl groups between aliphatic amines in water, J. Am. Chem. Soc. 125 (2003) 310–311. [22] X. Zhang, S.Y. Yao, Y.L. Guo, Intramolecular methyl migration in protonated N, N0 -dimethylpropane-1, 3-diamine and N, N0 -dimethylethane-1, 2-diamine, Int. J. Mass Spectrom. 270 (2008) 31–38. [23] X. Zhang, Theoretical studies on intramolecular methyl migration in N-oxide radical cations, J. Mol. Struct.: THEOCHEM. 955 (2010) 91–96. [24] A. Vigalok, D. Milstein, Methyl-to-double bond migration in methylene arenium rhodium complexes, Organometallics 19 (2000) 2341–2345. [25] R.T. Borchardt, C.R. Creveling, E. Usdin (Eds.), Biochemistry of SAdenosylmethionine and Related Compounds, MacMillan, New York, 1982. [26] E.S. Lewis, S. Kukes, C.D. Slaterl, Reactivity in methyl transfer reactions. 5. Relation between rates and equilibria, J. Am. Chem. Soc. 102 (1980) 1619– 1623. [27] J.C. Ma, D.A. Dougherty, The cation-p interaction, Chem. Rev. 97 (1997) 1303– 1324. [28] D.A. Dougherty, Cation-p interactions in chemistry and biology: a new view of benzene, Phe, Tyr, and Trp, Science 271 (1996) 163–168. [29] G. Bellucci, C. Chiappe, R. Bianchini, D. Lenoir, R. Herges, Nature of the interaction of olefin-bromine complexes. Inference from (E)-2,2,5,5tetramethyl -3,4-diphenylhex-3-ene, the first example of an olefin whose reaction with bromine stops at the stage of pi-complex formation, J. Am. Chem. Soc. 117 (1995) 12001–12002. [30] G. Bellucci, R. Bianchini, C. Chiappe, F. Marioni, R. Ambrosetti, R.S. Brown, H. Slebocka-Tilk, The solution behavior of the adamantylideneadamantane– bromine system: existence of equilibrium mixtures of bromonium– polybromide salts and a strong 1:1 molecular charge-transfer complex, J. Am. Chem. Soc. 111 (1989) 2640–2647. [31] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648–5652. [32] C. Lee, W. Yang, R.G. Parr, Development of the colle–salvetti conelation energy formula into a functional of the electron density, Phys. Rev. B 37 (1988) 785– 789. [33] S.H. Vosko, L. Wilk, M. Nusair, Accurate spin-dependent electron-liquid correlation energies for local spin density calculations: a critical analysis, Can. J. Phys. 58 (1980) 1200–1211. [34] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields, J. Phys. Chem. 98 (1994) 11623–11627. [35] M.J. Frisch, et al., Gaussian 09, Revision A.1, Gaussian, Inc., Wallingford CT, 2009. [36] A.P. Scott, L. Radom, Harmonic vibrational frequencies: an evaluation of Hartree–Fock, Møller–Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors, J. Phys. Chem. 100 (1996) 16502–16513. [37] E.D. Glendening, A.E. Reed, J.E. Carpenter, F. Weinhold. NBO 3.0; Department of Chemistry, University of Wisconsin, Madinson, WI. [38] J.A. Pople, M. Head-Gordon, K. Raghavachari, Quadratic configuration interaction – a general technique for determining electron correlation energies, J. Chem. Phys. 87 (1987) 5968–5975. [39] A.G. Baboul, L.A. Curtiss, P.C. Redfern, K. Raghavachari, Gaussian-3 theory using density functional geometries and zero-point energies, J. Chem. Phys. 110 (1999) 7650–7657. [40] C.Y. Legault. CYLview, version 1.0b; Universitede Sherbrooke, Sherbrooke, QC, 2009;