RadicalâMediated Catalytic Reaction
23rd IUPAC Conference on Physical Organic Chemistry (ICPOC23) 3rd – 8th July 2016 • Sydney • Australia
Theoretical Study on Organocatalytic Diboration of Pyrazines: Radical–Mediated Catalytic Reaction T. Ichino,a,* T. Taketsugu,a S. Maeda,a T. Ohmurab and M. Suginomeb a
Department of Chemistry, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 060-0810, Japan b Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan *[email protected]
Diboration of unsaturated compounds has been carried out with the help of transition metal catalysts. In contrast, Suginome et al. have established a transition-metal-free diboration of sterically hindered pyrazines.1 They found that addition of bis(pinacolato)diboron (1) into pyrazines proceeds efficiently in the presence of 2,6-dichloro-4,4’-bipyridine as a catalyst. However, the reaction mechanisms are still unclear. Artificial force induced reaction (AFIR) method developed by Maeda et al. is a powerful tool for analysis of organic synthetic reaction mechanisms.2,3 In this work, reaction mechanisms in diboration of 2,3-dimethylpyrazine (2) catalyzed by 4,4’-bipyridine (3) have been investigated by the AFIR method. Herein, computational results in B–B bond activation of 1 at the early reaction stage are shown. Possible mechanism involves ligation of two 2 (3) to B atoms and following homolytic B–B bond cleavage. 2 and 3 are finally transformed to boryl radicals with N–B bond (4 and 5), respectively. A similar boryl radical of pyridine derivatives has been already observed by ESR mesurement.4 Figure 1 shows Gibbs free energy profiles in B–B bond cleavage at the UM06L/6-31+G* level of theory. Comparisons between 2 and 3 in the free energy changes and barriers indicate that the reaction step of 3 (solid line) is energetically preferable than that of 2 (dashed line). Therefore, the activation process by two 3 is an initiate step in this diboration. After the formation of radical species 5, there are nine possible elementary steps in radical chain reaction mediated by 4 and 5. Based on free energy profiles in the elementary steps, radical–mediated catalytic cycle has been proposed. Furthermore, substituent effects of chloride for the catalyst efficiency of 3 have been studied. Computational results rationalize experimental ones. These results will be shown in the poster session. Figure 1. Gibbs free energy profiles in B–B bond cleavage Intermediates are represented.
References 1. 2. 3. 4.
Ohmura, T.; Morimasa, Y.; Suginome, M. J. Am. Chem. Soc. 2015, 137, 2852-2855. Maeda, S.; Morokuma, K. J. Chem. Phys. 2010, 132, 241102/1-4. Sameera, W. M. C.; Maeda, S.; Morokuma, K. Acc. Chem. Res. 2016, 49, 763-773. Schlüter, K.; Berndt, A. Angew. Chem. Int. Ed. 1980, 19, 57-58.
TU29
(at
333.15 K). schematically
Theoretical Study on Organocatalytic Diboration of Pyrazines: Radical–Mediated Catalytic Reaction T. Ichino,a,* T. Taketsugu,a S. Maeda,a T. Ohmurab and M. Suginomeb a
Department of Chemistry, Faculty of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 060-0810, Japan b Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan *[email protected]
Diboration of unsaturated compounds has been carried out with the help of transition metal catalysts. In contrast, Suginome et al. have established a transition-metal-free diboration of sterically hindered pyrazines.1 They found that addition of bis(pinacolato)diboron (1) into pyrazines proceeds efficiently in the presence of 2,6-dichloro-4,4’-bipyridine as a catalyst. However, the reaction mechanisms are still unclear. Artificial force induced reaction (AFIR) method developed by Maeda et al. is a powerful tool for analysis of organic synthetic reaction mechanisms.2,3 In this work, reaction mechanisms in diboration of 2,3-dimethylpyrazine (2) catalyzed by 4,4’-bipyridine (3) have been investigated by the AFIR method. Herein, computational results in B–B bond activation of 1 at the early reaction stage are shown. Possible mechanism involves ligation of two 2 (3) to B atoms and following homolytic B–B bond cleavage. 2 and 3 are finally transformed to boryl radicals with N–B bond (4 and 5), respectively. A similar boryl radical of pyridine derivatives has been already observed by ESR mesurement.4 Figure 1 shows Gibbs free energy profiles in B–B bond cleavage at the UM06L/6-31+G* level of theory. Comparisons between 2 and 3 in the free energy changes and barriers indicate that the reaction step of 3 (solid line) is energetically preferable than that of 2 (dashed line). Therefore, the activation process by two 3 is an initiate step in this diboration. After the formation of radical species 5, there are nine possible elementary steps in radical chain reaction mediated by 4 and 5. Based on free energy profiles in the elementary steps, radical–mediated catalytic cycle has been proposed. Furthermore, substituent effects of chloride for the catalyst efficiency of 3 have been studied. Computational results rationalize experimental ones. These results will be shown in the poster session. Figure 1. Gibbs free energy profiles in B–B bond cleavage Intermediates are represented.
References 1. 2. 3. 4.
Ohmura, T.; Morimasa, Y.; Suginome, M. J. Am. Chem. Soc. 2015, 137, 2852-2855. Maeda, S.; Morokuma, K. J. Chem. Phys. 2010, 132, 241102/1-4. Sameera, W. M. C.; Maeda, S.; Morokuma, K. Acc. Chem. Res. 2016, 49, 763-773. Schlüter, K.; Berndt, A. Angew. Chem. Int. Ed. 1980, 19, 57-58.
TU29
(at
333.15 K). schematically
