Acta Chimica Sinica ›› 2019, Vol. 77 ›› Issue (3): 278-286.DOI: 10.6023/A18110461 Previous Articles     Next Articles

Article

“受阻路易斯酸碱对”催化的2,3-二取代2H-1,4-苯并噁嗪氢化反应机理研究

魏思敏a, 王英辉b, 赵红梅b   

  1. a 陕西中医药大学 陕西省中药资源产业化协同创新中心 咸阳 712083;
    b 中国科学院化学研究所 北京分子科学国家实验室 北京 100190
  • 收稿日期:2018-11-13 出版日期:2019-03-15 发布日期:2019-01-31
  • 通讯作者: 王英辉, 赵红梅 E-mail:wangyinghui@iccas.ac.cn;hmzhao@iccas.ac.cn
  • 基金资助:

    项目受国家自然科学基金(Nos.21705029,21701131)资助.

Study on the Mechanism of Frustrated Lewis Pairs Catalysed Hydrogenation of 2,3-Disubstituted 2H-1,4-Benzoxazine

Wei Simina, Wang Yinghuib, Zhao Hongmeib   

  1. a Shaanxi Collaborative Innovation Center of Chinese Medicinal Resources Industrialization, Shaanxi University of Chinese Medicine, Xianyang 712083;
    b Beijing National Laboratory for Molecular Science(BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190
  • Received:2018-11-13 Online:2019-03-15 Published:2019-01-31
  • Supported by:

    Project supported by the National Natural Science Foundation of China (Nos. 21705029, 21701131).

Due to the different reactivity of hydrogenation reaction by metal-free FLPs catalyst for 2,3-disubstituted 2H-1,4-benzoxazine, we explored the reaction mechanism by density functional theory calculations. We have chosen three kinds of substrates with different hydrogenation reactivity as the prototype substrates and toluene as the solvent to calculate the potential energy profile for the FLPs-catalysed hydrogenation reaction at M06-2X/6-311++G(d,p) level with polarized continuum model (PCM) to simulate the solvent effect. From the potential energy profile, we found that when B(C6F5)3 encounters with 2,3-diphenyl 2H-1,4-benzoxazine (1o) or 2-methyl-3-phenyl 2H-1,4-benzoxazine (1p) in toluene, it mainly generates the mixture of Lewis acid-base adducts and Frustrated Lewis Pairs, which has almost similar stability suggesting the transformation of each other by intermolecular rearrangement. However, it reveals big difference when the B(C6F5)3 encounters with 2,3-dimethyl 2H-1,4-benzoxazine (1q), where the Lewis acid-base adducts is the preference rather than the mixture of Lewis acid-base adducts and Frustrated Lewis Pairs or Frustrated Lewis Pairs since the lower stability energy. Due to the big energy gap (10.9 kcal/mol) between Lewis acid-base adducts and Frustrated Lewis Pairs, the generated Lewis acid-base adducts could not transform into Frustrated Lewis Pairs in the FLPs-catalysed hydrogenation of 1q at 298 K. That is the main reason why 1q is an inert substrate for the hydrogenation catalysed by FLPs. Natural Bond Orbital, Mulliken charge analysis and the proton affinity energy of N4 site was carried out to assess the electric effect of substituent at C3 on N4 site. It reveals negligible effect of substituent at C3 on N4 charge (basicity) and thus proposes that steric hindrance effect is the major factor affecting the stability energy of Lewis acid-base adducts and Frustrated Lewis Pairs. This is confirmed further by calculative investigation about the substituent effect (-CH2CH3, -CH(CH3)2 and C(CH3)3) on the stability of Lewis acid-base adducts and Frustrated Lewis Pairs in 2-methyl-3-substituted 2H-1,4-benzoxazine, where with the increased steric hindrance effect, Lewis acid-base adducts tend to have similar stability with Frustrated Lewis Pairs even though less stability. These results clearly illustrate the elusive phenomenon in our previous experiment and may provide new insight for the design of another novel FLPs-catalysed hydrogenation reaction.

Key words: Frustrated Lewis Pairs, Lewis acid-base adducts, hydrogenation reaction, potential energy profile