Professor Sun Jianwei of Hong Kong University of science and technology and Academician Wu Yundong of Shenzhen Research Institute of Peking University worked together to discover a new mode of partial hydrogenation of silyne

Partial hydrogenation of alkynes to olefins is an important reaction in organic synthesis The hydrogenation of internal alkynes involves selectivity In recent years, a large number of metal catalysts have been developed to realize CIS addition and trans addition of internal alkynes to form (z) - and (E) - alkenes respectively However, another new mode of gem addition, that is, two hydrogen atoms are added to the same carbon atom of alkyne, and at the same time, the shift of the original substituent group occurs to form the end alkene, has not been reported (Fig 1) Fig 1 Selectivity of partial hydrogenation of internal alkynes (source: J am Chem SOC.) recently, Professor Sun Jianwei's research group of Hong Kong University of science and technology, Academician Wu Yundong of Shenzhen Graduate School of Peking University, Professor Zhang Xinhao's research group and Professor Zhong Longhua's research group of South University of science and technology cooperated to realize the internal silyne hydrogenation for the first time 。 In their previous studies and related literature reports (J am Chem SOC 2013, 135, 13835; angelw Chem., int ed 2013, 52, 14050; J am Chem SOC 2015, 137, 5506), [Cp * Ru (MeCN) 3] pf 6 catalyst can be used in hydrosilylation of alkynes, borohydride and other hydrogen functionalization reactions and obtain high trans selectivity, but the hydrogenation activity and selectivity are very poor In order to consider the effect of steric hindrance, a small volume of [cpru (MeCN) 3] pf 6 catalyst was used in this experiment It was found that the catalyst had high activity in the internal alkynes substituted by TMS, and two products, gem addition and trans addition, were produced, but the selectivity was very low (3.3:1) When the silicon base is replaced with phmesi, the selectivity can be as high as 30:1 (Fig 2) When the pressure of H2 increases, the ratio of gem to product decreases (from 1 to 10, the yield increases, but the ratio of gem to product does not decrease) The reactions have good functional group compatibility, including alcohols, ethers, esters, methanesulfonate esters, methylsilyl ethers, acetals, halides and phthalimide, as well as electron rich heterocyclic aromatic hydrocarbons, such as furan and thiophene When using aryl substituents which are easy to combine with catalysts, high gem selectivity can also be obtained by adding NABAR F4 (Fig 3) Fig 2 Optimization of reaction conditions (source: J am Chem SOC.) Fig 3 Optimization of aryl substitution reaction conditions (source: J am Chem SOC.) interestingly, when the additive - triethylenediamine (DABCO) is added to the reaction, the selectivity is reversed, and trans addition is dominant (Fig 4) Figure 4 Trans addition (source: J am Chem SOC.) Professor Wu Yundong and Professor Zhong Longhua's research group have carried out theoretical calculation research on this (Figure 4) The traditional mechanism of vinylidene group was excluded because of the high reaction energy barrier The first step of the reaction is similar to hydrosilylation (j.am.chem.soc 2013, 135, 13835) After oxidative hydrogen metallization, the ternary ring intermediate is formed However, the second step is not the direct transfer of reducing hydrogen to another carbon atom, but the transfer of hydrogen to the silicon substituted carbon atom to form a stable Ru carbene intermediate C2 This intermediate has been confirmed by the f ü rstner team by NMR (angelw Chem., int ed 2015, 54, 12431) Carbene intermediate C2 may follow two reaction paths, one is the occurrence of silicon group 1, When phmesi is used to replace alkynes, benzene ring can combine with Ru center to reduce the energy barrier of gem addition and obtain higher selectivity (C3 TSA (phsime2)); the other is to combine with another molecule H2 to generate oxidative addition, hydrogen migration and elimination to obtain trans addition product (mechanism a) This is consistent with the high concentration of H 2 in the experiment When the crowded DABCO is added, the steric hindrance leads to the increase of the energy barrier (C3 TSA (DABCO)), which is beneficial to trans addition Figure 5 DFT calculated reaction mechanism (source: J am Chem SOC.) in order to verify the calculated reaction mechanism, they conducted a series of mechanism experiments Through the reasonable design of the substrate, the experiment excluded the vinyl mechanism (Fig 6 (a)) If it is the vinyl mechanism, the two substrates 4 and 5 respectively pass through the intermediates vd-4 and vd-5 to obtain a single product 6 and 7 The results show that the two products are more likely to pass through Ru carbene intermediate Fig 6 (b) no cross product (2h-d 1) was observed in gem addition experiments, indicating that the added H2 came from the same molecule, while trans addition had cross product (3y-d1), indicating that the added H2 might come from different molecules This experiment supports the mechanism of Ru carbene reaction Figure 6 Mechanism verification experiment (source: J am Chem SOC.) Summary: this work has realized the first mobile double bond half hydrogenation of alkynes for the first time This method not only provides a new way to obtain terminal vinyl silane, but also provides a new reactive mode for alkylalkynes, which complements the established CIS and trans hemihydrogenation With [cpru] as catalyst, by selecting proper alkyl group and adjusting the cation property of catalyst, both aryl and alkyl substituted alkynes can achieve high efficiency and gem selectivity Appropriate additives (DABCO) can also change selectivity and promote trans addition DFT calculation provides an important theoretical basis for understanding gem selectivity and trans selectivity The addition of gem in carbene will open up a new way for the conversion of alkynes Relevant research results were recently published in Journal of the American Chemical Society (DOI: 10.1021 / JACS 9b09658) The experimental part of the work is mainly completed by Feng Qiang, a doctoral student of Professor Sun Jianwei's research group, and the theoretical research part is completed by Dr Song Lijuan, a doctoral student of Professor Wu Yundong's research group Both of them work together Professor Sun Jianwei and Professor Zhong Longhua are co authors of this paper The work was supported by NSFC, Shenzhen Science and technology innovation Commission, Shenzhen Nobel Prize scientists laboratory project and Shenzhen Sanming project