Homogeneous and Supramolecular Catalysis group, Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam.
Dalton Trans. 2011 Jun 7;40(21):5697-705. doi: 10.1039/c1dt10027k. Epub 2011 Apr 11.
The mechanism of cobalt(II) porphyrin-mediated aziridination of styrene with PhSO(2)N(3) was studied by means of DFT calculations. The computations clearly indicate the involvement of a cobalt 'nitrene radical' intermediate in the Co(II)(por)-catalyzed alkene aziridination. The addition of styrene to this species proceeds in a stepwise fashion via radical addition of the 'nitrene radical'C to the C=C double bond of styrene to form a γ-alkyl radical intermediate D. The thus formed tri-radical species D easily collapses in an almost barrierless ring closure reaction (TS3) to form the aziridine, thereby regenerating the cobalt(II) porphyrin catalyst. The radical addition of the 'nitrene radical'C to the olefin (TS2) proceeds with a comparable barrier as its formation (TS1), thus providing a good explanation for the first order kinetics in both substrates and the catalyst observed experimentally. Formation of C is clearly accelerated by stabilization of C and TS1 via hydrogen bonding between the S=O and N-H units. The computed radical-type mechanism agrees well with all available mechanistic and kinetic information. The computed free energy profile readily explains the superior performance of the Co(II)(porAmide) system with H-bond donor functionalities over the non-functionalized Co(TPP).
通过密度泛函理论（DFT）计算研究了钴(II)卟啉介导的苯亚磺酰基叠氮与苯乙烯的环氧化反应的机理。计算清楚地表明，在 Co(II)(por)-催化的烯烃环氧化反应中，涉及钴“氮自由基”中间体。该物种与苯乙烯的加成是分步进行的，通过“氮自由基”C 的自由基加成到苯乙烯的 C=C 双键上，形成γ-烷基自由基中间体 D。由此形成的三自由基物种 D 很容易在几乎无势垒的闭环反应（TS3）中坍塌，形成氮丙啶，从而再生钴(II)卟啉催化剂。“氮自由基”C 与烯烃的自由基加成（TS2）与形成（TS1）的势垒相当，因此很好地解释了实验中观察到的底物和催化剂的一级动力学。C 的形成通过 S=O 和 N-H 单元之间的氢键稳定化加速 C 和 TS1 的形成。计算出的自由基型机理与所有可用的机理和动力学信息非常吻合。计算出的自由能曲线很容易解释了具有氢键供体官能团的 Co(II)(porAmide)体系相对于非功能化的 Co(TPP)的优异性能。
