Theoretical Study On The Reactivities Of Heterocyclic Silylenes

Divalent silicon species (silylenes), corresponding to carbenes in organic chemistry, are key intermediates in numerous thermal and photochemical reactions of organosilicon compounds. Considering the electronic characteristics of this class of compounds, the higher homologues of the nitro-heterocyclic silylenes are also of interest and have been studied experimentally and theoretically. They can react with various species by insertion to polar bonds, addition to multiple bonds, polymerization, and so on. These reactions were recongnized as important and effective methods for preparation of the new silicon-bonded and heterocyclic silicon compounds. Since the first stable silylene was synthesized and isolated as an N-heterocyclic silylene in1994by Denk, some new heterocyclic silylenes were synthesized and studied experimentally and theoretically in the following years. This prompted us to take a closer look at the chemical behavior of nitro-heterocyclic silylenes on a theoretical basis. This work can be expected to present deep understanding for N-heterocyclic stable silylenes, and we hope these can be helpful for experimental studies.In contrast to extensive experimental results, the theoretical studies are only a few reports on N-heterocyclic silylene and reactions of heterocyclic silylene are still quite limited. This dissertation is mainly composed of the following parts based on the above the research background.In the chapter1, the historical background and current states on N-heterocyclic silylenes have been briefly illustrated, and then the theoretical methods of studying have been simply represented. In the last, the primary research viewpoints and the innovations of this paper are expounded.In the chapter2, the insertion reactions of N-heterocyclic unsaturated stable silylenes (1,3,4) and N-heterocyclic saturated silylenes (2,5) into HX molecules are performed using density functional theory (DFT) at the B3LYP/6-311++G(d, p) level. The insertion mechanisms are investigated and results from various reactions were compared. The results indicated that the insertion reaction mechanisms of heterocyclic silylenes and HX are similar to those of simple silylene. From the reaction energy barrier and the reaction enthalpy, the reaction activities of saturated silylenes are stronger than those of unsaturated silylenes.In the chapter3, the cycloaddition reactions of N-heterocyclic silylenes1-4with ethylene (C2H4) and formaldehyde (CH2O) molecules are performed at the MP2/6-31G(d, p) level of theory, respectively. The possible cycloaddition mechanisms ware investigated and results from various reactions were compared in detail. The theoretical results indicated that the cycloaddition reactions of N-heterocyclic silylenes with C2H4and CH2O proceed through a concerted mechanism to form a three-membered ring containing Cl, C2(O), and Si atoms in the products, which is similar to those of simple silylene H2Si. Silylenes1-4exhibit some electrophilicity toward C2H4whereas nucleophilicity toward CH2O leading to the reaction process.In the chapter4, the oligomer of N-heterocyclic silylenes are studied using density func tional theory (DFT) at the B3LYP/6-31G(d, p). and the time dependent density functional theory (TDDFT) are performed at the same level. The results showed that the energy gap and the excitation energy decreasing with the π-conjugation system components increasing, which lead to the molecular excitation wavelength of the maximum shifted to the long-wave, that is red shift. It is implied that the polysilanes can serve as a good optical material. The heterocyclic silylene undergoes further coupling reaction to produce large cycles or linear polysilanes.In the chapter5, the solvent effects are modeled using the self-consistent reaction filed (SCRF) method with Tomasi’s polarized continuum model. The synthetic work of heterocyclic silylene is usually carried out in solvents. To our knowledge, few theoretical calculations for the solvent effects on N-heterocyclic silylenes, so the solvent effects should be considered. The THF is chosen for this study, our objective is to analyze the influence of THF solvent on the reaction activities of N-heterocyclic silylenes.