Theoretical Studies On Several Organic Reactions Catalyzed By Transition Metals

Organometallic chemistry, as an emerging cross-discipline, has increasingly become a frontier field of chemistry. In the new century, it has been given a new vitality and has greatly promoted the development process of the modernization. Transition metal complexes-the core of organometallic chemistry-possess extensive application prospects in environmental protection, new energy, materials and human health due to their specific properties including high selectivity, high reactivity as well as high stability. So far, however, the general chemical laws can not reasonably elucidate some experimental observations, and the elementary mechanism involved in these reactions is not clear yet. This, to a certain extent, limited the exploitation of the new transition-metal catalysts and the application of the organometallic complexes. Alternatively, theoretical researches are of significance on exploring the nature behind the reactions.In this dissertation, a series of theoretical studies have been performed for several significant organic reactions catalyzed by Fe(Ⅱ), Au, Pt(Ⅱ) transition-metal complexes, respectively, along with the reaction of the W-based complex W(PMe3)4(η2-CH2PMe2)H cleaving the aromatic C-C bond in quinoxaline. Our main aim is to rationalize the experimental findings and to show the mechanism details at the atomic level so as to better understand the intrinsic properties of the important reactions. It is expected that the calculated results will inspire further study for designing novel catalysts and developing new organometallic reactions.The significant and valuable innovations obtained from this dissertation are listed as follows:1. Density functional theory (DFT) calculations have been carried out to study the detailed mechanism of CO inserting into the N-H bond rather than the common Fe-N bond of the iron (Ⅱ) amido complex (dmpe)2Fe(H)(NH2)(dmpe=1,2-bis(dimethylphosphino)ethane). Three mechanisms proposed in previous literature are computationally examined and all of them are found to involve high barriers and thus can not explain the observed N-H insertion product. Alternatively, based on the calculated results, a novel reactant-assisted (self-promoted) mechanism is presented, which provides the most efficient access to the insertion reaction via the assistance of a second reactant molecule. In details, the reaction starts from direct attack of CO on the amide nitrogen atom of (dmpe)2Fe(H)(NH2), followed by the second reactant assisted H-abstracti on/donation processes to afford the trans-product of CO inserting into the N-H bond of the amido complex. The present theoretical results provide a new insight into the mechanism of the unusual insertion reaction and rationalize the experimental findings well.2. The Au(I)-catalyzed cycloisomerization reactions of cycloalkyl-substituted (cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, respectively)1,5-enynes have been investigated by performing density functional theory (DFT) calculations. Theoretical calculations suggest that the reaction proceeds via a stepwise mechanism by the initial formation of a Au(I)-carbene intermediate, followed by a1,2-alkyl shift or C-H insertion reaction to form the ring-expanded three-cyclic product or ring-closed four-cyclic product depending on the size of cycloalkyl substitutions. It is found that the smaller substitutions (cyclobutyl, cyclopentyl) favor the former and the relatively larger one (cycloheptyl) prefers to form the latter from the kinetic point of view, although in all cases the ring-expanded products are calculated to be thermodynamically more favorable than the C-H bond insertion products. Combining the present theoretical results with the recent experimental findings, we conclude that the cycloisomerizations under consideration are kinetically controlled but not thermodynamically controlled.3. By carrying out density functional theory calculations, we have performed a detailed mechanism study for the cycloisomerization reaction of4-phenyl-hexa-l,5-enyne catalyzed by homogeneous gold to better understand the observed different catalytic activity of several catalysts, including (PPh3)AuBF4,(PPh3)AuCl, AuCl3and AuCl. In all situations, the reaction is found to involve two major steps, the initial nucleophilic addition of the alkynyl onto the alkene group and the subsequent1,2-H migration. It is found that the potential energy surface profiles of systems are very different when different catalysts are used. For (PMe3)AuBF4-and (PMe3)AuCl-mediated systems, the nucleophilic addition is the rate-determining step and the calculated free energy barriers are15.2and41.9kcal/mol, respectively. In contrast, for AUCl3-and AuCl-mediated systems, the reactions are controlled by the dissociations of catalysts from the catalyst-product adducts, and the calculated dissociation energies are18.1and21.7kcal/mol, respectively, which are larger than both the corresponding free energy barriers of the nucleophilic addition and the H-migration processes (8.5and7.3kcal/mol for the AuCl3-mediated reaction, and16.9and11.3kcal/mol for the AuCl-mediated reaction). These results can rationalize the early experimental observations well that the reactant conversion rates are100%, zero, and50%when using (PPh3)AuBF4,(PPh3)AuCl, and AuCl3as catalysts, respectively. The present study indicates that both the ligand and counterion of homogeneous Au catalysts importantly influence their catalytic activities, while the oxidation state of Au is not a crucial factor controlling the reactivity.4. With the aid of density functional theory (DFT) calculations, we have performed a detailed mechanism study for the catalytic cycloisomerization reactions of propargylic3-indoleacetate to better understand the observed divergent reactivities of two catalysts, PtCl2and (PPh3)AuSbF6, which afford [3+2]-and [2+2]-cycloaddition products, respectively. The calculated results confirm that the lactone intermediates are common and necessary species for the formations of the two products and that PtCl2and (PH3)AuSbF6respectively favor the formation of the [3+2]-and [2+2]-products. The intrinsic reasons that result in the divergent reactivities of two catalysts have been analyzed in detail. We believe that the essentially different metal-ligand interactions in PtCl2and (PPh3)AuSbF6are mainly responsible for their divergent regioselectivities, while the solvent effects have little impact on the catalyst reactivities. PtCl2induces the intramolecular nucleophilic addition to give the [3+2] cycloisomerization product due to the strong π-electron-donating capability of chlorine ligands, while (PPh3)AuSbF6results in the intramolecular nucleophilic addition reaction to form the [2+2] cycloisomerization product because of the strong a-electron-donating capability of phosphine ligand.5. To understand the formation mechanism of allenes through the rearrangement of cyclopropenes catalyzed by PtCl2, we have performed a detailed density functional theory (DFT) calculation study on a representative substrate,1-trimethylsilyl-2-phenylethyl cyclopropene. Three reaction pathways proposed in the previous experimental work have been examined, and the calculated results seem not to completely rationalize the experimental findings. Alternatively, by performing an exhaustive search on the potential energy surface, we present a novel catalytic mechanism of PtCl2, which is fixed appropriately on the cyclopropene/allene to form the linear Cl-Pt-Cl disposition, a vital configuration for catalyzing the rearrangement of cyclopropenes. The newly proposed mechanism involves a SN2-tye C-C bond activation of the cyclopropene by the PtCl2fixed on a cyclopropene molecule via the d-π interaction between the metal center and the substrate to form the product precursor PtCl2-allene with the metal center coordination to the external not internal C=C bond in the allene framework. Once formed, the PtCl2-allene immediately serves as a new active center to catalyze the rearrangement reaction rather than directly dissociate into the allene product and the PtCl2catalyst due to its high stability. During the catalytic cycle, an allene-PtCl2-allene sandwich compound is identified as the most stable structure on the potential energy surface, and its direct dissociation results in the formation of the product allene and the regeneration of the catalytic active center Pt-allene with an energy demand of24.4kcal/mol. This process is found to be the rate-determining step of the catalytic cycle. In addition, to understand the experimental finding that the unsilylated cyclopropenes do not provide any allenes, we have also performed calculations on the unsilylated cyclopropene system, and found that the highest barrier to be overcome during the catalytic cycle amounts to35.2kcal/mol. The reason is attributed to a lack of filled pπ orbital in H atoms. The theoretical results not only rationalize well the experimental observations but show a new mechanism of the important rearrangement reaction.6. The detailed mechanisms for the formation of the aromatic C-C bond cleavage complex [κ2-C2-C6H4(NC)2]W(PMe3)4from the insertion reaction of W-based complex W(PMe3)4(η2-CH2PMe2)H with quinoxaline have been investigated with the aid of DFT calculations. The computational results have improved Sattler and Parkin's mechanism, which involves a barrier of as high as42.0kcal/mol and thus is unfavorable in energy. Alternatively, our extensive theoretical calculations propose a new mechanism, which features the removal of a second PMe3ligand from the metal center and the C-C cleavage prior to the second C-H bond addition, in contrast to Sattler and Parkin's mechanism where the C-C bond cleavage occurs at the last step after the second C-H bond addition. The rate-determining step for the whole reaction is the ring-opening process of the tungsten complex with an activation barrier of28.5kcal/mol after dissociating the first PMe3group. The mono-hydrido species is located as the global minimum on the potential energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide in-depth insight into the mechanism details of the aromatic C-C bond cleavage and will inspire further study for designing novel catalysts activating C-C bonds in heteroaromatic compounds.