Theoretical Studies On The Structures And Reactivities Of Silylenoids, Silylenes

Silylenoids and silylenes are important intermediates in some organosilicon reactions and their reactions were recognized as important and effective methods for the preparation of new silicon-bonded and heterocyclic silicon compounds.Both experimental and theoretical results show that silylenoids and silylenes have ambiphilic character,nucleophilicity and electrophilicity,and can take part in many reactions,such as insertion,addition,rearrangement,and polymerization.Based on the present research, this dissertation is mainly composed of the:following parts.Chapter 1 introduces the characters and the previous researches of silylenoids and silylenes.Chapter 2 mainly illustrates the ab initio and density functional theory.In chapter 3,the DFT investigation on the insertion and substitution reactions of H₂SiLiF and CH₃XH_n-1(X=F,Cl,Br,O,N;n=1,1,1,2,3) is performed.1) The theoretical results indicate that the substitution reactions of H₂SiLiF with CH₃XH_n-1 occur in a concerted manner via two reaction paths,â… andâ…¡,forming same products,H₂SiFCH₃ and LiXH_n-1.For both pathways,the substitution barriers decrease with the increase of the atomic number of the element X for the same family systems or for the same row systems.Pathâ… is more favorable than pathâ…¡.2) H₂SiLiF inserts into a C-X bond via a concerted manner,forming H₂SiXH_n-1CH₃ and LiF.For C-X bonds,the order of reactivity by A insertion indicates the reaction barriers increase for the same-row element X from right to left in the periodic table,whereas change very little for the insertion into X-C bonds of the same-family element X.3) The total energies of the insertion transition states are higher than those of the corresponding substitution states.Thus,the substitution reactions are more favorable than the insertion reactions for the H₂SiLiF and CH₃XH_n-1 systems.It is reasonable to expect that the amount of substitute product H₂SiFCH₃ is more than that of insertion product H₂SiXH_n-1CH₃ among the final products.The research in chapter 3 is about the reaction of(TSi)BrSiLi₂ and Me₃SiCl. 1) The calculations indicate that CH₃BrSiLi₂,C(SiH₃)₃BrSiLi₂ and(TSi)BrSiLi₂ have three probably structures,p complex structure,the plain structures and the folded structures.The energy of the p complex is highest and the energy of the plain structure nearly equals to that of the folded structure.2) The insertion of CH₃BrSiLi₂,C(SiH₃)₃BrSiLi₂ and(TSi)BrSiLi₂ into Si-Cl bond is a concerted reaction.The insertion barriers is in the order of H₂SiLiBr＞C(SiH₃)₃BrSiLi₂＞CH₃BrSiLi₂.The big group C(SiH₃)₃ make the insertion more difficult.3) The negative silicon in silylenoids can nucleophilicly substitute with Me3SiCl. The energy barriers are in the order C(SiH₃)₃BrSiLi₂＞H₂SiLiBr＞CH₃BrSiLi₂.4) On the view of energy,substitution barriers are lower that those of insertion. The electron-donating group,C(SiH₃)₃,makes the barrier difference(between the substitution and the insertion) for the case of C(SiH₃)₃BrSiLi₂ relative to the case CH₃BrSiLi₂.5) Due to the lower barriers of the substitutions,the substitution products are dominant in the reaction system.In chapter 5,the addition reactions of H₂SiLiX(X=F,Cl) and H₂CO are discussed.1) The three-member ring and the p complex structures of H₂SiLiX(X=F,Cl) are adopted in this study.The addition of the three member ring structure to H₂CO is easy to occur,and the barrier of H₂SiLiF is lower that of H₂SiLiCl.2) The structures of the precursor complexes in the asymmetric additions of H₂SiLiX(X=F,Cl) to H₂CO are different from those in the symmetric additions of H₂SiLiX(X=F,Cl) to C₂H₄ and C₂H₂.3) The calculations at the B3LYP/6-31G(d) level indicate that the addition reactions of H₂SiLiX(X=F,Cl) to H₂CO easily occur in the polar solvent.The higher the solvent polarity is,the lower the barriers of the addition reactions are.The final complexes also easily dissociate into oxasilacyclopropane and LiX in the polar solvent.In chapter 6,the theoretical investigation on the structures and reactions of silylenoid PhCH₂(NH₂)CH₃SiLiF is performed. 1) Silylenoid PhCH₂(NH₂)CH₃SiLiF has three isomers,1(three member ring structure),2(p complex structure) and 3(σcomplex structure).The structures of silylenoid PhCH₂(NH₂)CH₃SiLiF in THF are similar to those in vacuum.Their energies are in the order of 2 > 1 > 3 both in vacuum and in THF.2) The insertion of Silylenoid PhCH₂(NH₂)CH₃SiLiF into the C-F bond is similar to that of silylene into the C-F bond.The activation barrier for the former is 62.9 kJ/mol higher than that for the latter.Silylenoid PhCH2(NH2)CH3SiLiF is more stable than silylene PhCH₂(NH₂)CH₃Si.3)The energy barrier of PhCH₂(NH₂)CH₃SiLiF insertion into C-F is higher than that of PhCH₃CH₃SiLiF,showing that the N atom plays an important role on the stability of silylenoid PhCH₂(NH₂)CH₃SiLiF.In chapter 7,the insertion reaction of silylene into H₂B-X(X=F,Cl,Br) is considered.The reaction is in a concert manner and barriers increase from F to Br.The valuable results in this dissertation can be summarized as follows:1)Silylenes and silylenoids have similar characters in reactions.In chapter 3 and chapter,this dissertation investigates the reactions of silylenoids with CH₃XH_n-1(X=F, Cl,Br,O,N;n=1,1,1,2,3) and Me₃SiCl.It is found that the barriers of substitutions are lower than those of corresponding insertions,and that the substitutions are dominant. But insertions usually occur in the silylene with CH₃XH_n-1(X=F,Cl,Br,O,N;n=1,1,1, 2,3) and Me₃SiCl systems.So silylenes and silylenoids are different in this point.This calculated results are in agreement with experiments and can provide theoretical guide for the design of silylenoid experiments.2) Relative to the tetracoordinate silylenoids,the researches on the pentcoordinate silylenoids are rare.The chapter 5 theoretically studies the structures for the first time, and the effect of the coordinate atom,N,on the character of silylenoid are also discussed.This may support guides for the further study of pentcoordinate silylenoids.3)The addition reactions are very.important ones for silylenoids.Previous researches are all concerned about the symmetric additions.This paper investigate the asymmetric additions of silylenoids and a contrast is made between the asymmetric and symmetric additions.4) Borosilicate compounds are important in chemical reactions,but syntheses are rather difficult.The chapter 7 discusses the character and laws for the insertion of silylene into C-X(X=F,Cl and Br).This will be useful to the syntheses of borosilicate compounds.