The comprehensive understanding of the intrinsic mechanisms of chemical reactions plays an extremely important role in optimizing synthetic routes and realizing significant cost savings in laboratory preparation and industrial production.In particular, deciphering the rate-determining step (RDS)is necessary to provide the scheme of increasing the yield and efficiency. The kinetic isotope effect (KIE) has been applied as an important tool to determine the RDS and demonstrate the configuration of the transition state (TS) in many reactions. However, due to the limitation by the naturally low abundance of isotopes and the extent of the reaction, which result in low sensitivity and poor accuracy, it is hard to measure KIEs precisely (especially the secondary H/D KIE).Therefore, circumventing the inherent limitation of conventional ensemble KIE to realize the visualization of intrinsic reaction pathways is an urgent and key challenge that needs to be solved. Recently, Prof. Xuefeng Guo group and Prof. Fanyang Mo group from Peking University and his collaborates developed a new method based on the combination of single molecule detection and kinetic isotope effect—the single-molecule kinetic isotope effect (sm-KIE), which abandons the defects of macro-KIE and has the capability of obtaining the KIE value of each elementary reaction accurately and conveniently, thus leading to the methodological iteration of KIE, of great significance to analyze the structure of transition states (Fig. 1).
Fig.1Single-molecule kinetic isotope effect in the Claisen rearrangement
Xuefeng Guo group from Peking University focuses the study on single-molecule reaction dynamics for a long time. Together with their collaborators, they have revealed new mechanisms and novel phenomena that are hidden by ensemble averages. Recently, they used Claisen rearrangement as a model reaction, and measure KIEs by monitoring the I−t curves of different types of deuterated substrates (allyl phenyl ethers sp2-D (deuterium connected to sp2 C) and sp3-D (deuterium connected to sp3 C)) to obtain the rates of the rearrangement and the proton shuttle processes (the Butterworth filtering method was used to distinguish signals), which indicates the concerted but asynchronous mechanism of 3,3 σ-migration (Fig. 2).
Fig. 2Characterization of sm-KIEs in different deuterated substrates
In addition to the determination of the rate-determining step, the degrees of the C−C bond formation and the C−O bond breaking of the transition states were able to be quantitively calculated. The greater degree of C−O bond cleavage indicates the asynchrony of this pericyclic reaction. Interestingly, they found that the structure of the transition state has the significant dependence on electric fields—the degree of bond breaking and formation of the transition state gradually increases with the increasing electric field, implying that the structure of the transition state is more similar to the product. The More O’Ferrall-Jencks diagram was used to orientate the position of the transition state, which demonstrated an early transition stateand its shift along the reaction coordinate with increasing the bias.Therefore, they found thediversification of the EEFregulation: the molecular configuration, the energy barrier (orthogonal direction), and the TS position (horizontal direction) (Fig. 3). From another perspective, this sm-KIE approach offers the multi-dimensional description of the TS, the Holy Grail of chemical research, which forms the foundation of direct detection of the TS.
Fig. 3The More O’Ferrall-Jencks diagram for the Claisen rearrangement under the different bias voltages
With the advantages of high detection sensitivity and accuracy, this sm-KIE approach is ready to be appliedto a variety of single-molecule dynamics research, which is crucial to the development of fundamental chemical reactions and understanding of basic life sciences.This work was published online in Journal of the American Chemical Society on January 17, 2022 with the title "Accurate single-molecule kinetic isotope effects" (https://pubs.acs.org/doi/10.1021/jacs.1c12490). The co-first authors are Yilin Guo, Chen Yang and Lei Zhang from Peking University, and Huiping Li from University of Science and Technology of China. The co-corresponding authors are Prof. Xuefeng Guo and Prof. Fanyang Mo from Peking University, Prof. Deqing Zhang from Institute of Chemistry, Chinese Academy of Sciences, Prof. Wenguang Zhu from University of Science and Technology of China, and Prof. Zitong Liufrom Lanzhou University. The research was supported by the National Key R&D Program of China, the National Natural Science Foundation of China and Beijing National Laboratory for Molecular Sciences.
Paper link：https://pubs.acs.org/doi/10.1021/jacs.1c12490
