Studies On Mechanisms Of Several Novel Reactions For Iron,Manganese And Tungsten Complexes

Organometallic compounds, especially the traditional organometallic compounds, are widely used as reagents or catalysts in fields of organic synthesis chemistry, pharmaceutical industry, life sciences, and so on. Therefore, studies on synthesis of organometallic compounds with high application value are always a hot topic in the scope of modern international chemistry. In recent years, experimentalists made many important improvements in designing and synthesizing some new-shape transitiona-organometallic compounds. However, relevant theoretical investigations on such kind of reactions are developed slowly. With the help of progress on quantum chemistry and computational chemistry, plenty of studies indicate that B3LYP method of density functional theory (DFT) could be well employed in theoretically dealing with organometallic reactions. In this article, we chose some controversial new-shape organometallic reactions on Iron, Manganese, and Tungsten compounds as our objects of study, and explored their detailed reaction mechanisms with the help of density functional theory and molecular orbital theory. Meanwhile, combining the experimental facts, we analyzed the rationality of all possible pathways, and sought the factors affecting the reaction process from dynamic and thermaldynamic points of view. Results of our studies supply some necessary information for experimentalist undertaking such kind of organometallic synthesis. The primary innovations in this article were depicted as below:(1) Mechanisms on arylation of styrene using aryliron complexes [CpFe(CO)2Ar]Complexes [CpFe(CO)2Ar] have been widely utilized as a kind of arylmetal reagents in organic synthesis. Compared with other arylmetal complexes, complexes [CpFe(CO)2Ar] are much easier to be handled in organic reactions, and have higher yields in the arylation reactions. Therefore, they have been used as primary reagents in arylation of styrene. Recently, Yasuda and co-workers reported an experimental observation regarding reactions of [CpFe(CO)2Ar] with styrenes to give the corresponding stilbenes. But they can not give an explanation on this reaction mechanism. We were thus promoted to explore the reaction mechanisms using density functional theory (DFT) calculations.Our calculations reveal that mechanisms of the reactions studied here mainly involves three steps:(1) a ligand exchange process (dissociation of one carbonyl ligand and subsequent addition of styrene or the styrene derivative to the Fe center);(2) migration of Ar from Fe to β-C of styrene;(3) β-H or β-F elimination and dissociation of the stilbene derivative from the CpFeHCO moiety. In arylation of styrene, the insertion step of the C=C bond of styrene and the β-H elimination step both need transition states with a four-member ring (Fe C1C2Ar and Fe C1C2H). Insertion of the C=C bond of styrene into the aryl-metal bond originates from the interaction between Fe-C3a orbital and the π orbital of the C=C bond of styrene, and further dπ-pπ interaction makes the C=C π coordinated complex stable. The agostic interaction plays an important role in stabilizing intermediate and makes the β-H elimination step easy to proceed.(2) Two competitive arylation mechanisms for reaction of a-CF3styrene with complex [CpFe(CO)2Ar]Complexes [CpFe(CO)2Ar] have been widely utilized as primary reagents in arylation of styrene because they are much easier to be handled and have high selectivity. We have investigated the detailed mechanisms for arylation of styrene with [CpFe(CO)2Ar] before. Calculational results indicate that β-H elimination is the key step for the whole arylation process. Yorimitsu and co-workers found in their experiment that β-F elimination not P-H elimination occurred when the reagents are α-CF3and [CpFe(CO)2Ar]. To explore the reason for this interesting phenomenon, we investigated the reaction mechanisms for α-CF3with [CpFe(CO)2Ar] with the aid of DFT calculations. Our calculational results show that both of β-F elimination and β-H elimination are thermodynamically favorable. There are two main reasons for the final product generated from β-F elimination:(1) The intermediate IntlH generated in PathH is very stable, which makes the subsequent processes of β-F elimination and product formation quite difficult to proceed. Therefore, β-F elimination is the dominant step and give the final product PF. To better understand the mechanism of β-F elimination, we compared the arylation process of α-CH3styrene with α-CF3styrene. Calculations indicate that intrusion of CH3group does not change mechanism of styrene with [CpFe(CO)2Ar]. We have confirmed the rationality of our proposed mechanisms by exclusion of other mechanisms of β-F elimination.(3) Mechanism on production of disulfides from catalytic reaction of catalyst CpMn(CO)3with thiolsDisulfides can be used widely as useful reagents in organic synthesis, and in sulphenylation of enolates and other anions. Furthermore, disulfides are essential moieties of biological active compounds for peptides and protein stabilization. Therefore, studies on compounds containing "-S-S-" group are of great importance in chemistry and life sciences. Oxidation of thiols is the most used method for disulfide synthesis mainly because a great deal of thiols are commercially available and/or are easily synthesized. Oxidation of thiols is easy to make thiols to be overoxidized, generating many unwanted byproducts. Selective metal-catalyzed dehydrogenation of thiols into diulfides is a possible alternative to avoid overoxidation. However, these reactions remain largely unexplored up to the present. Recently, Kheng Yee Desmond Tan and co-workers found a greener method to convert thiols into disulfides. In their experiments, an organometallic manganese complex CpMn(CO)3was used to catalyze thiols into disulfides, with dihydrogen as the only side-product. On basis of their experimental results, the experimentalists proposed a possible reaction mechanism for the Mn-catalyzed reactions in their work. Checking their proposed mechanism, we think it not reasonable enough. Therefore, we investigated the detailed mechanism for conversion of thiols into disulfides and dihydrogen, catalyzed by CpMn(CO)3complex with the aid of DFT calculations. Our computational results showed that this Mn-catalyzed reaction of thiols into disulfides contains two possible pathways:path1and path2. Four major steps are involved in both of the two pathways:(1) a ligand substitution process (dissociation of one carbonyl ligand and subsequent addition of SRH to the metal center);(2) intermolecular hydride migration;(3) addition of RSH and release of dihydrogen;(4) production of S2R2and regeneration of the catalyst. Patha is the favorable pathway for substitution of CO with RSH as a result of the inaccessible barrier for pathb. Substitution process is the rate-determined step. The compound CpMnH(CO)2SR (Int4) is probably existing as a key intermediate, which brings two possible pathways:pathl and path2. Large steric repulsion between S2H2R with C4=C5makes Int6a16-electron complex. Pathl was found the dominant pathway compared with path2, which results from difficult formation of the five-member ring of TS8in path2. Ligand substitution process is the rate-determined reaction step. Our computational results are in consistent with the experimental observations of Kheng and co-workers. We hope our work could supply some useful information for the experimentalist undertaking studies on production of disulfides with effective and green methods.(4) Interconversion between transition-metal silyl and silylene in tungsten-complex Cp*(CO)2WSi R2CH3Plenty of attention has been focused on synthesis and chemical behavior of the silylene-transition metal complexes. Among them, study on interconversion between transition-metal silyl and silylene complexes has attracted more attention. One important reaction that enables such interconversion is1,2-group-migration between a silyl ligand and a metal center. The1,-2-migration on silyl complexes has been known for several kinds of substituents such as silyl groups, hydrogen, halogen, and hydrocaryl groups on a silyl ligand, however, examples of1,2-migration of alkyl groups involving the cleavage of a relatively inert Si-C(sp3) bond are rare. Until recently, Eiji Suzuki, Hiromi Tobita and co-workers made a breakthrough on1,2-alkyl-migration in a Cp*(CO)2W-silyl system. They reported that reactions of the methyl-tungsten complex Cp*(CO)2W(DMAP)CH3with trialkylsilanes HSiR2Et(R=Me, Et)(eql) and dimethoxy(methyl)silane (eq2), could afford a (dimethoxysilylene)tungsten complex through1,2-methyl migration for eq2, while not for eql. This is the first example of isolation and characterization of a transition-metal silylene complex formed by1,2-alkyl-migration. Based on the results of their experiments, they proposed a possible mechanism for this process. After investigating their proposed mechanism, we found that there are still many questions are unclear. Based on experimental fact, we employed DFT calculations to investigate reaction mechanisms between Cp*(CO)2W(DMAP)CH3with HSiMe2Et and HSi(OMe)2Me. Moreover, we explored factors affecting1,2-methyl migration. Calculational results demonstrate that oxidative addition can hardly occur directly when HS1R3reacts with intermediate Cp*(CO)2WMe, Si-H a bond could coordinate to metal center firstly to form intermediate2with similar agostic interaction. Three possible pathways are involved in methane elimination process, in which pathc(2-4) is the most favorable pathway dynamically. In other words, oxidative process is not the necessary one. Elimination of methane is favorable thermodynamically. The six-coordinated18-electron intermediate Cp*WR5is thermaldynamically inclined to eliminate one "R-R" group to form a four-coordinated16-electron tungsten complex Cp*WR3.1,2-methyl migration process adopts DMAP-synergic mechanism. We can see that DMAP is not only a stabilization, but also participates in1,2-methyl migration. Methoxyl groups on silicon atom reduce the whole reaction energy, meanwhile, DMAP could stabilize the final product trans-7, which make1,2-methyl migration of Cp*(CO)2W(OMe)2Me accessible. However,1,2-methyl migration for Cp*(CO)2W(OMe)2Me is unfavorable thermaldynamically. Therefore, we can get the conclusion that participation of stabilization and existence of electron-donating groups on silicon atoms are two important factors that affects1,2-methyl migration. Products trans-7' and5' could co-exist in solution because5'is thermaldynamically very stable.