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The Theory Of The Ring-opening Reaction Mechanism Of The Four-membered Ring. Silicon Transition Metal

Posted on:2009-08-16Degree:MasterType:Thesis
Country:ChinaCandidate:Y Y ZhaoFull Text:PDF
GTID:2191360245462572Subject:Inorganic Chemistry
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Over the past few decades, considerable attention has been focused on the chemistry of silylene complexes that could participate in various transformation reactions of organosilicon compounds, such as dehydrogenative coupling of hydrosilanes, redistribution of substituents on silicon atoms and silylene transfer to unsaturated organic compounds. Since silylene complexes can provide valuable information for these transformation reactions, preparation of transition-metal silylene complexes have proven to be an important synthetic target. Among the various silylene complexes, silyl(silylene) complexes occupy a unique position. These complexes undergo intramolecular 1,3-migration, which is postulated as a key step in the metal-mediated redistribution of substituents on organosilicon compounds. Ogino group have successfully synthesized and characterized a series donor-bridged bis(silylene) transition-metal complexes in decades.Among these donor-bridged complexes, alkoxy-bridged bis(silylene) transition-metal complexes with peculiar bond modes and novel reactivity, occupy a unique position in these systems. To our knowledge, transition-metal complexes with silylene or silyl ligands have been analyzed in a variety of theoretical studies, but few detailed theoretical studies are focused on donor-bridged bis(silylene) transition-metal complexes. The silicon atom is expected to be vulnerable to various nucleophiles such as ROH, RCN, H2O, etc. This kind of nucleophilic reactions should be one of the most common reactions of silylene complexes.Thus, it will be very interesting and of guiding meaning to make theoretical study on silametallacycles complexes.In this paper, Density functional theory (DFT) calculations at the B3LYP level are carried out to study and analyze the following two projects. Our major intention is to investigate the molecular structures, bonding, and mechanisms on the reactions of the silylene complexes1. The model reaction mechanism on the reaction of Cp(OC)Ru(H)(SiMe2OMe)2 derived from CpRu(CO)SiMe2SiMe2OCH3 with MeOH, was investigated with the aid of density functional theory (DFT). The structures and bonding involved in the reaction mechanism were analyzed. Ogino proposed the mechanism of such reactions that may be initiated by nucleaophilic attack of MeOH from the side at the electronphilic silylene ligand. By our calculation, a more stable intermediate with the hydrogen bonding between the Ru and the MeOH proton is formed. The cis- Cp(CO)RuH(SiMe2OMe)2 could be obtained with the ring opening of the four-membered ring and the hydrogen migrating to the Ru center. The cis-product is -8.1 kcal/mol but less stable than the intermediate. The activation energy is 24.2 kcal/mol. The mechanism is calculated to be unfavorable on thermodynamics and kinetics We proposed another mechanism that the MeOH attacks oxygen atom of RuSiSiO four-member ring. Four steps are involved in the reaction (formation of hydrogen bonding between CpRu(CO)SiMe2SiMe2OCH3 and a MeOH, ring-opening of the Ru-Si1-O1-Si2 four-membered ring, formation of the six-membered ring, and the hydroxyl hydrogen migration to the metal center). The activation energy of rate-determining step is 23.5 kcal/mol. The trans-Cp(CO) RuH (SiMe2OMe)2 is more stable than the intermediate with six-member ring by 5.0 kcal/mol, and is the most stable among all the species throughout the reaction mechanism. For the hydrogen migration process, the hydrogen atom lying in between the two SiMe2 moiety is theoretically predicted to be favored than the hydrogen atom outside of the Ru-Si bond.2. The cleavage of C-Cσbonds by transition-metal complexes represents a fundamental challenge in the field of organometallic chemistry. The transition- metal-mediated C(R)-CN cleavage reaction attracted many interests because the alkyl(aryl) metal-cyanide complexes can afford import intermediates on some important metal catalyze reactions.To our known, it is usually difficult that the C-C bond activation of unstrained nitriles by using transition metal complexes, but some interesting C-C bond cleavages of unstrained nitriles have been reported in reactions of some organosilicon metal compounds of Mo, Fe, Rh, Ir, Ni, Pt and Pd. Recently, silicon transition-metal complexes play a role of the cleavage of C-C bond of nitrile. We investigated different mechanisms of the reaction of Cp*Fe(CO)(=SiMes2)SiMe3 with MeCN, and found a more reasonable mechanism. The path I is the C-CN bond cleavage via the CN bond insertion into the FeSi bond. The barrier energy of rate-determining step of the mechanism is 17.2 kcal/mol. The mechanism is calculated to be favorable both thermodynamically and kinetically. The mechanism of path II is the oxidative addition of a C-CN bond to low-valent metal center to afford an alky-cyano complex. The barrier energy of rate-determining step of the mechanism is 50.7 kcal/mol. It is not favorable kinetically. In other word, the silicon assists the mechanism of the C-C cleavage reaction. We also calculated the mechanisms of the reaction of Cp*Ru(CO)(=SiMes2)SiMe3 with MeCN and found that the favorable mechanism is similar to the Path I of reaction of Cp*Fe(CO)(=SiMes2)SiMe3 with MeCN. The barrier energy of rate-determining step of the mechanism is 22.0 kcal/mol, which is higher than that of the path II. It is indicated that the catalic effect of Cp*Fe(CO)(=SiMes2)SiMe3 is better than Cp*Ru(CO)(=SiMes2)SiMe3.
Keywords/Search Tags:Four-membered ring, nucleophiles, mechanism, transition metall, silicon, density functional theory (DFT)
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