Browsing by Author "Tia, Richard"
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- ItemA density functional theory study of the mechanisms of oxidation of ethylene by rhenium oxide complexes†(Elsevier, 2013-04-01) Aniagyei, Albert; Tia, Richard; Adei, EvansThe oxo complexes of group VII B are of great interest for their potential toward epoxidation and dihy-droxylation. In this work, the mechanisms of oxidation of ethylene by rhenium-oxo complexes of thetype LReO3(L = O−, Cl, CH3, OCH3, Cp, NPH3) have been explored at the B3LYP/LACVP* level of theory.The activation barriers and reaction energies for the stepwise and concerted addition pathways involvingmultiple spin states have been computed. In the reaction of LReO3(L = O−, Cl, CH3, OCH3, Cp, NPH3) withethylene, it was found that the concerted [3 + 2] addition pathway on the singlet potential energy sur-faces leading to the formation of a dioxylate intermediate is favored over the [2 + 2] addition pathwayleading to the formation of a metallaoxetane intermediate and its re-arrangement to form the dioxylate.The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied isfound to follow the order O−>CH3> NPH3>CH3O−>Cl−> Cp and the reaction energies follow theorder CH3>O−> NPH3>CH3O−>Cl−> Cp. On the doublet PES, the [2 + 2] addition leading to the for-mation the metallaoxetane intermediate is favored over dioxylate formation for the ligands L = CH3,CH3O−,Cl−. The activation barriers for the formation of the metallaoxetane intermediate are found toincrease for the ligands in the order CH3
- ItemThe mechanisms of gallium-catalysed skeletal rearrangement of1,6-enyneseInsights from quantum mechanical computations(Elsevier Inc, 2019-10-10) Borketey, Joseph Bortey; Opoku, Ernest; Tia, Richard; Adei, EvansThe transition metal-catalysed skeletal reorganization of 1,6-enynes can lead to three types of productsea typeIproduct occurring via the cleavage of the alkene CeC bonds and the migration of the terminalalkene carbon to the terminus of the alkyne; a typeIIproduct arising from cleavage of both the doubleand the triple bonds followed by insertion of the terminal alkene carbon into the alkyne CeC triple bond;and a typeIIIproduct which is obtained when there is a cleavage of the olefinic bond followed byformation of two new bonds from each carbon to each of the acetylenic carbons. The course of thesereactions is highly dependent on the metal catalyst used and type of substitution at the alkene andalkyne moieties of the enyne. In this mechanistic study of the re-organization of 1,6-enynes catalysed byGaCl3, performed at the DFT M06/6-311G(d,p) level of theory, the parent reaction selectively leads to theformation of the typeIproduct through the formation of the open cyclopropane ring. The presence ofsubstituents at the acetylenic moiety governs the preferred position of the metal along the alkyne bondwithin the pi-complex: with electron-withdrawing groups (EWGs), the metal prefers the terminal carbonwhile electron-donating groups (EDGs) lead to the metal preferring the internal carbon. EWGs at thealkyne moiety efficiently favour the formation of the typeIproduct. Substituents at the olefin moietyalter the mechanism of the reaction which may favour the selective formation of the typeIorIIIproductdepending on the type of substituent. EWGs at the olefinic moiety favour formation of the typeIIIproduct when the alkyne moiety is unsubstituted whiles EDGs forms the typeIproduct selectively.Solvent and temperature have no substantial effects on the energetic trends and product distribution.Hence, gas-phase calculations are deemed adequate for the problem at hand.
- ItemMechanistic studies on Diels-Alder [4þ2] cycloaddition reactions ofa,b-substituted cyclobutenones: Role of substituents in regio- andstereoselectivity(Elsevier Ltd., 216-11-03) Fosu, Evans; Tia, Richard; Adei, EvansDiels-Alder reactions of substituted cyclobutenones with 6-methoxy-1-vinyl-3,4-dihydronaphthaleneand methoxy-substituted-1,3-butadiene have been studied with DFT. In the reactions of 6-methoxy-1-vinyl-3,4-dihydronaphthalene with cyclobutenone anda-bromocyclobutenone, the formation of themetaandorthoisomers have the same barriers, indicating that the two isomers might be formed in equalproportions, contrary to earlier reports. The regiochemistry of the reaction is mainly controlled by theketone functionality at C1 on the dienophiles. In the reactions of methoxy-substituted-1,3-butadienewith cyclobutenone anda,b-substituted cyclobutenones theortho/endoandpara/endostereo-isomericpathways are the most favorable pathways, changing toexoselectivity when OH, Br, CH3are placedon theb-carbon of the cyclobutenone, but still withorthoandpararegioselectivity. The stereoselectivityis independent of the bulkiness of substituents. The stability of substituted cycloadducts are lowercompared to unsubstituted adducts and this explains why thea-cyanoketones anda-bromoketoneproducts readily undergotrans-methylation and angular-alkylation as electrophiles.
- ItemQuantum Mechanical Studies of the Mechanisms of Some Transition Metal Organometallic Reactions(2009-07-18) Tia, RichardThe mechanistic pathways of three organometallic reactions, namely the oxidation of ethylene by chromyl chloride leading to the formation of epoxide, 1,2-dichloroethane, 1,2-chlorohydrin, acetaldehyde, and vinyl alcohol precursors; the olefin metathesis reaction involving ethylene and 20 metal methylidene Cl4MCH2(M=Cr, Mo, W, Ru, Re) and metal oxo-methylidene Cl2(O)MCH2(M=Cr, Mo, W, Ru, Re) complexes; and the transition-metal-assisted formation of 1,2-dinitroso complexes of cobalt and its congeners have been studied using hybrid density functional theory at the B3LYP/LACVP* level of theory. The formation of the epoxide precursor (Cl2(O)Cr-OC2H4) was found to take place via initial [2+2] addition of ethylene across the Cr=O bond of CrO2Cl2 to form a chromaoxetane intermediate, as opposed to [3+2] addition across the two Cr=O bonds of CrO2Cl2 as suggested in earlier studies. The hitherto unexplored pathway involving initial [3+2] addition of ethylene across the Cr=O and Cr-Cl bonds of CrO2Cl2 was found to be more favorable than the [3+2] addition across the two Cr-O bonds of CrO2Cl2. The formation of the 1,2-dichloroethane precursor was found to take place via [3+2] addition of ethylene across the two Cr-Cl bonds of CrO2Cl2. The 1,2-chlorohydrin precursor was also found to originate from [3+2] addition of ethylene across the Cr-O and Cr-Cl bonds of CrO2Cl2 as opposed to [2+2] addition of ethylene across the Cr-Cl bond. Also the vinyl alcohol and acetaldehyde precursors were found to arise from a direct attack of one of the carbon atoms of ethylene on an oxygen atom of CrO2Cl2 through a triplet intermediate. In the reactions of Cl4MCH2(M=Cr, Mo, W, Ru, Re) with ethylene it was found that the formation of the metallacyclobutane through formal [2+2] cycloaddition, a key step in the olefin metathesis reaction according to the Herrison-Chauvin mechanism, is a low-barrier process in each of the complexes studied. It was also found that the active species for the formation of the metallacyclobutane is a carbene complex and not a carbenoid complex. One key factor was found to be responsible for the difference in metathesis activity in these complexes: the stability of the carbenoid complexes relative to the carbenes. In Cr and Ru, the carbenoid complexes are more stable than the carbenes and thus Cl4CrCH2 and Cl4RuCH2 are likely to exist in the lower- 21 energy carbenoid Cl3MCH2Cl form as opposed to the carbene Cl4M=CH2 form. This is likely to deplete the reaction surface of the active species of the process, making Cl4MCH2(M=Cr, Ru) not suitable for olefin metathesis. This suggests that whereas Cl4MCH2 (M = Mo, W, Re) may catalyze olefin metathesis, Cl4MCH2 (M = Cr, Ru) may not. The W and Re complexes have been found to have greater metathesis activity than the Mo complex. In the Cl2(O)MCH2 (M=Mo, W, Re) complexes the metathesis reaction has favorable energetics and is found to be more feasible than the side-reactions studied while in the Cl2(O)MCH2(M=Cr, Ru) complexes, the olefin metathesis is found to be less favorable than the side reactions. In the transition-metal-assisted formation of 1,2-dinitrosoalkanes, it was found that the activation barriers for the one-step [3+2] addition pathway for the formation 1,2-dinitrosoalkanes are generally very low while the activation barriers for the [2+2] addition of the C=C bond of the olefins across the M-N bonds of CpM(NO)2 (M=Co, Rh, Ir) to form an intermediate are generally very high. A transition state for the re-arrangement of the products of [2+2] addition to the products of [3+2] addition could not be located, indicating that the re-arrangement of the products of [2+2] addition by reductive elimination involving the second metal-nitrogen π-bond to form the observed 1,2-dintrosoalkanes as suggested in the work of Rappé and Upton may not be possible. Therefore it is concluded that the direct one-step [3+2] addition pathway proposed by Bergman and Becker for the formation of 1,2-dinitrosoalkanes is a more plausible pathway.