Study of reactive intermediates: vinyl cations and aminium radical cations

As important reactive species, both vinyl cations and aminium radical cations play an important role in organic synthesis. However, due to their high activity, instability, short survival time, and lack of general and effective method for their generation, their applications are still very limited. The continuous search for more convenient and efficient generation methods and more powerful catalytic systems still have important research significance. Recently, the Niggemann group have developed a series of electrophilic cyclization reactions of alkynes via vinyl cations with an eco-friendly calcium catalyst under mild reaction conditions. Therefore, as the first part of my work, I focused on the application of the calcium catalyzed transformations via vinyl cations. First, a calcium-catalyzed multicomponent reaction was developed through a bicyclic non-classical carbon cation intermediate which is generated directly from a vinyl cation intermediate. A variety of potential biological activity 3-dimensional bicyclo[3.2.1]octane amides were obtained with good yields and excellent diastereoselectivity. In this reaction, a stable delocalized non-classical carbon cation embedded in a rigid frame was formed. And due to the cation-π interaction between the empty p orbital and the double bond fixed in the rigid geometry of the bicyclic nucleus, excellent diastereoselectively was achieved. In the second project, a straightforward and efficient approach of structurally diverse multi-substituted 4H-chromenes was developed through a calcium catalyzed [4+2] cycloaddition reaction from readily available benzylic alcohols and alkynes. Two possible pathways may involved in this procedure, including a concerted inverse electron demand hetero-Diels-Alder (IED/HDA) process with the in situ generated ortho-quinone methides intermediates or a stepwise process with a highly reactive vinyl cation. Free radical chemistry has important and extensive applications in organic synthesis. Compared with carbon-centered radicals, the development of nitrogen-centered radicals still leaves much space for exploration, mainly owing to the lack of general and convenient approaches to the generation of nitrogen-centered radicals. As the third part of my work, the development of a novel procedure for the intramolecular olefin oxyamination via aminium radical cation was investigated for the first time. This work provides a new method for the oxidation of simple secondary anilines to aminium radical cations. With this methodology, cheap and abundant manganese or just a transition-metal-free condition with oxoammonium salt can be used as oxidant to replace expensive photocatalysts