DSpace Angular :: Browsing by Author "Tia, Richard"

Browsing by Author "Tia, Richard"

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1, 3-Dipolar cycloaddition reactions of selected 1,3-dipoles with 7-isopropylidenenorbornadiene and follow-up thermolytic cleavage: Acomputational study
(Elsevier Inc., 2019-08-08) Arhin, Grace; Adams, Anita Houston; Opoku, Ernest; Tia, Richard; Adei, Evans
The mechanism, regio-, stereo-, and enantio-selectivities of the 1,3-dipolar cycloaddition reactions of7-isopropylidenenorbornadiene (DENBD) with nitrones and azides to form pharmaceutically relevantisoxazolidine and triazole analogues have been studied computationally at the M06/6-31G(d), 6-31G(d,p), 6-311G(d,p), 6e311þþG(d,p) and M06-2X/6-31G(d) levels of theory. In the reactions of DENBDwith phenyl nitrones, the cycloaddition steps have low activation barriers, with the highest being 16 kcal/mol; and the Diels-Alder cycloreversion steps have generally high barriers, with the lowest being 20 kcal/mol, suggesting that the isolable products in these reactions are the bicyclic isoxazolidine cycloadductsand not the thermolytic products. This is in contrast to the reactions of DENBD with phenyl azide wherethe isolable products are predicted to be the thermolytic products since the Diels-Alder cycloreversionsteps had relatively lower activation barriers. Electron-donating substituents on the dipolarophile sub-strate favour attack of the nitrone on the least hindered side of the DENBD substrate while electron-withdrawing substituents on the dipolarophile substrate favour attack on the more hindered side ofthe DENBD, indicating that site-selectivity is affected by nature of substituents. Global reactivity indicescalculations are in good agreement with the activation barriers obtained. Analysis of the electrophilic(PþKÞand nucleophilic (P KÞParr functions at the reactive centres reveal that the cycloaddition occursbetween atoms with the largest Mulliken and NBO atomic spin densities which agrees well with theenergetic trends and the experimental product outcomes.
[3 + 2] Versus [2 + 2] addition: A density functional theory study on the mechanistic aspects of transition-metal-assisted formation of 1,2-dinitrosoalkanes
(Hindawi Publishing Corporation, 2016-11-21) Opoku, Ernest; Tia, Richard; Adei, Evans
The pathways for the transition metal-assisted formation of 1,2-dinitrosoalkane complexes of cobalt and its congeners, have beenstudied using DFT/M06 with the LACVP∗basis set. The activation barriers for the one-step [3 + 2] addition pathway for theformation of 1,2-dinitrosoalkanes, proposed by Bergman and Becker, are generally low compared to the activation barriers for the[2 + 2] addition to form an intermediate, which is the first of the two-step pathway proposed by Rapp ́e and Upton, which are veryhigh. The barriers of the rearrangement of the Rapp ́e intermediates to the final products by reductive elimination involving thesecond metal-nitrogen𝜋-bond are also very high. The reactions of the Co complexes have lower activation barriers than Rh andIr complexes. The barriers of the reactions involving olefins with electron-donating groups are generally lower compared to thereactions of the parent (unsubstituted) ethylene while the activation barriers for reactions of olefins with electron-withdrawinggroups are generally higher compared to the parent (unsubstituted) ethylene. The one-step [3 + 2] pathway remains the mostfavoured irrespective of the metal centre or the nature of the olefin. The mechanism of the reaction is therefore settled in favour ofthe [3 + 2] addition pathway.
Ab initio investigation of O2 adsorption on Ca-doped LaMnO3 cathodes in solid oxide fuel cells
(Royal Society of Chemistry, 2018) Aniagyei, Albert; Dzade, Nelson Y.; Tia, Richard; Adei, Evans; Catlow, C. R. A.; et. al
We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces. The effect of oxygen vacancies on the adsorption characteristics and energetics has also been investigated. Our results show that the O2 adsorption/reduction process occurs through the formation of superoxide and peroxide intermediates, with the Mn sites found to be generally more active than the La sites. The LCM25{110} surface is found to be more efficient for O2 reduction than the LCM25{100} surface due to its stronger adsorption of O2, with the superoxide and peroxide intermediates shown to be energetically more favorable at the Mn sites than at the Ca sites. Moreover, oxygen vacancy defect sites on both the {100} and{110} surfaces are shown to be more efficient for O2 reduction, as reflected in the higher adsorption energies calculated on the defective surfaces compared to the perfect surfaces. We show from Lowdin population analysis that the O 2 adsorption on the pure and 25% Ca-doped ̈ LaMnO3 surfaces is characterized by charge transfer from the interacting surface species into the adsorbed oxygen pg orbital, which results in weakening of the O–O bonds and its subsequent reduction. The elongated O–O bonds were confirmed via vibrational frequency analysis.
A comparative studyof the interactionof nickel, titanium,palladium, andgold metalswith single-walled carbonnanotubes:A DFT approach
(Elsevier B.V., 2019-02-20) Kayang, K.W.; Nyankson, E.; Efavi, J.K.; Apalangya, V.A.; Tia, Richard; et. al
Interactions between transition metal atoms, nickel, titanium, palladium and gold and (3,3), (4,2), (6,0) and(5,1) carbon nanotubes were studied using first principles calculations. The Fermi energy levels of the carbonnanotubesstudiedwerefoundtoincreaseduringinteractionswiththetransitionmetalatoms.Amongstthefourmetals, gold atom was found to have an enhanced interaction with the nanotubes transforming from semi-conductingtoaconductingtube.Titaniumwasalsofoundtoshowsimilarcharacteristicstogoldonlywhentheatomwasplacedinthemiddleofthecarbonnanotubes.Nickelandpalladiumatomsinteractionsdidnotaffectmuchtheelectronicpropertiesofthecarbonnanotubes,withsomeslightchangesintheelectronicpropertiesatsomespecificsitesofthenanotubes.Itisproposedfromthisstudythat,thecarbonnanotube-metalinteractionscould be used as a guide to shed light on the electronic properties of such materials which could become pro-mising engineering materialsand revolutionizethe electronic industry.
A density functional theory study of the mechanisms of oxidation of ethylene by rhenium oxide complexes†
(Elsevier, 2013-04-01) Aniagyei, Albert; Tia, Richard; Adei, Evans
The 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
A density functional theory study of the mechanisms of oxidation of ethyleneby technetium oxo complexes
(Elsevier B.V, 2013-01-22) Aniagyei, Albert; Tia, Richard; Adei, Evans
The mechanisms of oxidation of ethylene by transition metal-oxo complexes of the type LTcO3(L = O , Cl,CH3, OCH3, Cp, NPH3) have been explored by computing the activation barriers and reaction energies forthe concerted and stepwise addition pathways at the density functional theory B3LYP/LACVP level oftheory. The results indicate that in the reaction of LTcO3(L = O , Cl, CH3, OCH3, Cp, NPH3) with ethylene,the formation of the dioxylate intermediate through the concerted [3 + 2] addition pathway on the singletpotential energy surface is favored kinetically and thermodynamically over its formation through thetwo-step process via the metallaoxetane intermediate. The activation barrier for the formation of thedioxylate on the singlet PES for the ligands studied is found to follow the order: O >CH3> NPH3>CH3O >Cl > Cp while the reaction energies follow the order: Cl >O >CH3> NPH3>CH3O > Cp. Onthe doublet PES, the [2 + 2] addition leading to the formation of the four-membered metallacycle inter-mediate is favored kinetically and thermodynamically for the ligands when L = NPH3. The direct [2 + 1]addition of ethylene across the oxo- ligand of doublet TcO3(CH3) to form the epoxide precursor is favoredwhen L = CH3. The activation barriers for the formation of the dioxylate intermediate are found to followthe order: Cl
A DFT mechanistic study of the generation of azomethine ylides from thering-opening reactions of stabilized aziridines and follow-up 1,3-dipolarcycloaddition reactions with acetaldehyde
(Elsevier B.V., 2018-10-17) Nantogma, Shiraz; Tia, Richard; Adei, Evans
This work investigated computationally with density functional theory calculations at the M06/6-311G∗level,the ring opening reaction of various methyl-, phenyl- and carbonyl- substituted aziridines to obtain azomethineylides and the subsequent 1,3-dipolar cycloaddition reaction with acetaldehyde leading to 3-methyl and 4-methyl regioisomers andendo- andexo- stereoisomers. The activation barrier for the electrocyclic ring openingof the parent aziridine is very high (51.3 kcal/mol) but is lowered by at least 15.5 kcal/mol upon methyl andester group substitutions. In the reaction of 1,3-diphenyl-2,2-methoxycarbonylaziridineA2with acetaldehyde,the ring opening step is rate-determining with an activation barrier of 28.9 kcal/mol. The activation barrier forthe formation of the 4-methyl isomer from this reaction is at least 7.4 kcal/mol lower than that for the formationof the 3-methyl isomer, which is in accord with the experimentally-observed regioselectivity. Also, the formationof theexoisomer is more favoured than theendoisomer as the barrier of the former is 2.7 kcal/mol compared to6.1 kcal/mol for the latter. There is an inverse correlation between the activation barriers for the electrocycliccleavage of the aziridines and the electrophilicities of the resulting azomethine ylides. The results are rationa-lized in terms of perturbation molecular orbital theory.
DFT Mechanistic Study on the Reaction of Benzenesulfonyl Azideswith Oxabicyclic Alkenes
(Beilstein-Institut, 2020-05-05) Akuamoah, Daniel A.; Tia, Richard; Adei, Evans
The reaction of benzenesulfonyl azides with oxabicyclic alkenes to form aziridines, reported by Chen et al (J. Org. Chem.2019, 84, 18, 11863-11872), couldproceed via initial [3+2] cycloaddition to form triazoline intermediates followed by dinitrogen cleavage or via initial dinitrogen cleavage of the benzenesulfonyl azideto afford a nitrene intermediate followed by insertion of this species into the olefinic bond of the oxabicyclic alkene. Calculations at the DFTM06-2X/6-311G+(d,p) level showthat the initial [3+2] cycloaddition has barriers of 17.3 kcal/mol (endo) and 10.2 kcal/mol (exo) while the initial nitrogen extrusion step has a barrier of 38.9 kcal/mol. The rate-determining step along the former pathway isthe dinitrogen cleavage from triazoline cycloadducts which hasbarriers of 32.3 kcal/mol (endo) and 38.6 kcal/mol (exo) and that along the latter pathway isdinitrogen cleavage from benzenesulfonyl azide with an activation of barrier of 38.9 kcal/mol. The[3+2] addition of benzenesulfonyl azide with oxabicyclic alkene to afford endoandexotriazoline intermediatesis kinetically favoredover the dinitrogen cleavage from benzenesulfonyl azide by 21.6 and 28.1 kcal/mol for endoand exopathway respectively.Thus,the preferred pathway for the reaction of oxabicyclic alkene with benzenesulfonyl azide is via initial [3+2] addition followed by dinitrogen cleavage, contrary to the proposal by Chen et al. The lower activation barrier for the dinitrogen extrusion step leading to endoaziridine compared toexoisomer means that the endoproduct will be formed as the major product, confirming the 2experimental observation.The position of substituents on the benzene group of the benzenesulfonyl azide greatly affectsthe endo/ exodiastereoselectivity. Keywords: Aziridines, oxabicyclic alkene, benzenesulfonyl azides, triazoline, dinitrogen extrusion, cycloaddition
Diels-Alder cycloaddition versus ring-opening esterification: Acomputational study of the mechanism of formation of oxa-norbonenelactones from the reaction of furfuryl alcohol and itaconic anhydride
(Elsevier B.V., 2018) Fosu, Evans; Botchway, Cecil Humphrey; Tia, Richard; Adei, Evans
The reactions of furfuryl alcohol with itaconic anhydride can proceedviafour plausible pathways–two path-ways involving initial Diels-Alder cycloaddition followed by lactonization and two pathways involving initialesterification followed by intramolecular Diels-Alder cycloaddition–to afford two distinct norbornene productsbearing either afive- or six-membered butyrolactone ring respectively. DFT calculations reported herein showthat thefirst-step Diels-Alder cycloaddition reactions have barriers of 8.5 and 11.8 kcal/mol which are far lowerthan the barriers of 29.6 and 36.9 kcal/mol for the esterification pathways. The calculated energies for theexperimentally observedfive- and six membered oxa-norbornene lactone isomers are exergonic with reactionenergies of−17.1 and−12.8 kcal/mol respectively. Thermodynamically thefive-membered isomer is morestable adduct than the six-membered adduct. Thus the most preferred pathway is the one involving initial[4 + 2] cycloaddition followed by lactonization to form thefive-membered butylrolactone ring. Substitution ofmethyl groups on the carbon bearing the hydroxyl group shows that the substituents do not affects the reactionpathways markedly.
Exploring the peri-, chemo-, and regio-selectivity of additionof manganese metal oxides MnO3L(L=Cl ,O , OCH3,CH3)tosubstituted ketenes: A computational study
(Elsevier B.V. I., 2015-11-11) Ahmed, Issahaku; Tia, Richard; Adei, Evans
Ketenes are interesting reactive intermediates that find a wide range of synthetic applications. Densityfunctional theory calculations at the MO6/LACVP⁄and B3LYP/LACVP⁄levels of theory have beenemployed to explore the peri-, chemo-, and regio-selectivity of the addition of manganese oxo complexesMnO3L (L = Cl, O , OCH3,CH3) to substituted ketenes O@C@C(CH3)(X) [X = H, CH3, Cl, CN, Ph] with the aimof elucidating the effects of substituents on the mechanism of the reactions. The results show that theconcerted [3+2] addition of the C@C bond of the ketene across the metal complex is the most preferredpathway in all the reactions studied (with respect to changing ligand L on the metal complex or sub-stituent X on the ketene) except in the reaction of MnO4 (i.e. for L = O ) with dimethyl ketene, which fol-lows only a stepwise addition pathway. [2+2] addition is found to be possible only in the reaction ofMnO3–OCH3with dimethyl ketene where the activation barrier for [2+2]C@Oaddition is 23.79 kcal/mol,which is far greater than the barrier for the [3+2] addition. The reactions of dimethyl ketene withMnO4 will most likely lead to the formation of an ester precursor and the reaction of MnO3Cl with thesubstituted ketenes would lead to the formation of an ester precursor, chlorohydrin precursor, acetalde-hyde and carbon monoxide (for X = H, Cl). Generally, reactions involving an increase in oxidation state ofmetal have higher activation barriers. For both [3+2] and [2+2] addition, low activation barriers areobtained when the substituent on the ketene is electron-donating while high activation barriers areobtained when the substituent is electron-withdrawing. The reactions of ketenes with MnO3L complexeshave lower activation barriers for the preferred [3+2] and [2+2] addition pathways as well as fewer sidereactions than those of the ReO3L complexes reported in the literature, a trend which was seen in our ear-lier work with reactions of group VII metals with olefins, implying that manganese oxo complexes effi-ciently and selectively catalyze specific reactions in oxidation of ketenes and olefins than do Re oxocomplexes and therefore Mn oxo complexes may be better catalysts for specific oxidation reactions ofketenes and olefins than Re complexes are.
Exploring the peri-, chemo-, and regioselectivity of addition oftechnetium metal oxides of the type TcO3L(L=Cl–,O–, OCH3,CH3) to substituted ketenes: a DFT computational study
(NRC Research Press, 2016-02-02) Ahmed, Issahaku; Tia, Richard; Adei, Evans
The addition of TcO3L(L=Cl,O–, OCH3,CH3) to substituted ketenes along various addition pathways was studied withdensity functional theory calculations to explore the peri-, chemo-, and regioselectivity of the reactions. In the reactions of TcO3L withdimethyl ketene, the results show that forL=O–and CH3, [1 + 1] addition to form a triplet zwitterionic intermediate is the preferredfirst step; for L = Cl, the [3 + 2]C=Caddition across the O–Tc–Cl bond is the preferred first step and forL=OCH3the [3 + 2]C=Cadditionacross the O–Tc–OCH3 bond is the preferred first step. In the reactions of TcO3Cl with substituted ketenes, [1 + 1] addition to form atriplet zwitterionic intermediate is the preferred first step for X = Ph, CN, and Cl; the [3 + 2]C=Caddition across the O–Tc–O bond of thecomplex is the preferred first step forX=H,while the [3 + 2]C=Caddition across the O–Tc–CH3 bond is the preferred first step. Reactionsinvolving a change in the oxidation state of metal have high activation barriers, while reactions that do not involve a change inoxidation state have low activation barriers. Reactions of ketenes with TcO3L complexes have lower activation barriers for thepreferred addition pathways than those of the ReO3L complexes reported in the literature. Thus, the TcO3L complexes may be bettercatalysts for the activation of the C=C bonds of substituted ketenes than the reported ReO3L complexes.
Hydrogenation of carbon dioxide to formate bya-diimine RuII,RhIII,IrIIIcomplexes as catalyst precursors
(Elsevier B.V, 2019-08-15) Makuve, Nyasha; Mehlana, Gift; Tia, Richard; Darkwa, James; Makhubela, Banothile C.E.
The conversion of CO2into valuable chemicals has been of major interest because it is cheap and readilyavailable. The concept of reducing CO2pollutionviaits utilization into valuable products has inspired usto synthesise novel 4,4'-((1Z)-butane-2,3-diylidenebis(azanylylidene))dibenzoic acid (L) metal com-plexes {[(L)RuII](C1), [(L)RhIII](C2), [(L]IrIII(C3)} complexes for catalytic hydrogenation of CO2. Thea-diimine metal complexes (C1eC3) were characterised using several analytical techniques, including:NMR spectroscopy and single crystal X-ray crystallography. In a mixture of THF/H2O and a base, all threecatalyst precursors were able to hydrogenate CO2cleanly to formate as a product. However, the bestcombination of catalyst precursor and a base wasC1and DBU that selectively produced formate at amoderate temperature of 120 C and at 60 bar. The best productivity under these conditions is TOF of 35h 1within 2 h and a TON of 322. This work is significant because it provides a one-step synthesis forformate from CO2usinga-diimine-based complexes which can be synthesised in a one-step reaction. Thedensity functional theory calculations onC1supports that RueH is the active species in the process ofCO2hydrogenation to formate with the insertion of the CO2to RueH being the rate determining step.
The 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, Evans
The 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.
Mechanistic 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, Evans
Diels-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.
Mechanistic studies on tandem cascade [4þ2]/ [3þ2] cycloadditionof 1,3,4-oxadiazoles with olefins
(Elsevier Inc, 2019-09-13) Roland, Daniel; Haleegoah, Jamin Nathaniel; Opoku, Ernest; Tia, Richard; Adei, Evans
The mechanism of the reaction of 1,3,4-oxadiazoles with alkenes (ethylene) and cycloalkenes (cyclo-butene, cyclopentene, cyclohexene and cycloocene) have been studied computationally at the DFT M06e2X/6-311G* level. The reaction is found to proceedviaa concerted [4þ2] addition followed by nitrogenextrusion and then [3þ2] addition in a tandem cascade fashion, which in the case of cycloalkenes leadsto exo-fused or endo-fused subframes, the exo of which is kinetically and thermodynamically favored.The [4þ2] step is the rate-determining step of the reaction. CF3as a substituent on the 1,3,4-oxadiazoledecreases the activation barriers of the rate-determining step, while CO2Me on the oxadiazole increasesthe activation barriers of the rate-determining step, markedly in the case of the reaction with cyclo-pentene and only marginally in the reactions with ethylene. Increasing temperature decreases the barrierof the rate-determining step and stability of the products but increases the rate of the nitrogen extrusionstep. The low barriers of the second and third steps of the reaction compared to thefirst step means thatthe intermediates will not be isolated in the reaction, confirming the experimental observations of earlierworkers. Based on calculated activation barriers, the reactivity of the various cycloalkenes considered inthis study follows the order: cyclooctene>cyclopentene>cyclohexene>cyclobutene which isconsistent with the trends in product yields obtained in earlier experimental studies.
Peri-, Chemo-, Regio-, Stereo- and Enantio-Selectivities of 1,3-dipolarcycloaddition reaction of C,N-Disubstituted nitrones withdisubstituted 4-methylene-1,3-oxazol-5(4H)- one: A quantummechanical study
(Elsevier Inc., 2020-01-21) Pipim, George Baffour; Opoku, Ernest; Tia, Richard; Adei, Evans
The peri-, chemo-, regio-, stereo- and enantio-selectivities of 1,3-dipolar cycloaddition reaction of C,N-disubstituted nitrones with disubstituted 4-methylene-1,3-oxazol-5(4H)-one have been studied usingdensity functional theory (DFT) at the M06e2X/6-311G (d,p) level of theory. The 1,3-dipole preferentiallyadds chemo-selectively across the olefinic bond in a (3þ2) fashion forming the corresponding spi-rocycloadduct. The titled reaction occurs with poor enantio- and stereo-selectivities, but a high degree ofregio-selectivity is observed for the addition of the 1,3-dipole across the dipolarophile. Electron-withdrawing groups on the dipolarophile significantly reduce the activation barriers while electron-donating groups on the dipolarophile increase the activation barriers. Analysis of the HOMO andLUMO energies of the two reacting species indicates that the 1,3-dipole reacts as a nucleophile while thedipolarophile reacts as the electrophile. Investigation of the electrophilic Parr function (PþKÞat the variousreaction centers in the dipolarophile indicates that the 1,3-dipole selectively adds across the atomicspecies with the largest electrophilic Mulliken and NBO atomic spin densities which is in accordancewith the energetic trends observed.
Permanganyl chloride-mediated oxidation of tetramethylethylene: Adensity functional theory study
(Elsevier Inc., 2020-04-08) Aniagyei, Albert; Kwawu, Caroline; Tia, Richard; Adei, Evans
The mechanisms of the oxidation of tetramethylethylene (TME) by permanganyl chloride (MnO3Cl) havebeen explored on the singlet and triplet potential energy surfaces at the B3LYP LANL2DZ/6-31G (d) levelof theory. The results show that the pathway leading to the formation of thefive-membered dioxylatethrough concerted [3þ2] addition is favored kinetically and thermodynamically over the three otherpossible pathways, namely the [2þ2] addition via the transient metallaoxetane intermediate, epoxi-dation, and hydrogen transfer pathways. The epoxide precursor that on hydrolysis would yield theepoxide product will most likely arise from a stepwise path through the intermediacy of an organo-metallic intermediate. This pathway affords the product that is more stable (thermodynamically favor-able). However, kinetically, both the stepwise and the concerted [2þ1] addition pathways leading to theepoxide precursors are very competitive (activation barrier difference of<0.7 kcal/mol).
Quantum chemical studies on the mechanistic aspects of tandemsequential cycloaddition reactions of cyclooctatetraene with ester andnitrones
(Elsevier Inc, 2019-07-05) Opoku, Ernest; Tia, Richard; Adei, Evans
The mechanisms of the tandem sequential [4þ2]/[3þ2] and [3þ2]/[4þ2] cycloaddition sequencesinvolving an ester, cyclooctatetraene (COTE), and cyclic and acyclic nitrones for the formation of a diverserange of isoxazolidine derivatives and other synthetic precursors are reported. A thorough exploration ofthe PES has characterized several regio-, stereo- and enantio-selective mechanistic channels involved inthese reactions. A perturbation molecular orbital (PMO) analysis been employed to rationalize the re-sults. It has also been found that the initial electrocyclic ring closure of the COTE is the rate-determiningstep in the tandem sequential [4þ2]/[3þ2] addition sequence. The thermolytic breakdown of thetandem adducts to subsequent monocyclic, bicyclic and tricyclic adducts occurs generally with very highactivation barriers making it an inconvenient synthetic approach. The different reactivity of all the threedouble bonds present in the dipolarophile is reported. Finally, the mechanistic possibilities of [3þ2]/[4þ2] addition sequences involving the same reaction components in the case of cyclic and acyclicnitrones are explored extensively. The results suggest a novel and convenient routes for obtainingproducts of high selectivity with less energetic requirements. In some instances, new cycloadductshitherto unreported are obtained.
A quantum chemical study of the mechanisms of olefin addition to group 9 transition metal dioxo compounds
(Springer International Publishing, 2016-12-01) Ahmed, Issahaku; Tia, Richard; Adei, Evans
The mechanistic aspects of ethylene addition to MO2(CH2)(CH3) (M=Co, Rh, Ir) have been investigated with a Hartree–Fock/DFT hybrid functional at the MO6/LACVP* and B3LYP/LACVP* levels of theory to elucidate the reaction pathways on the singlet, doublet and triplet potential energy surfaces (PES). In the reaction of the IrO2CH2CH3 complex with ethylene, [3+2]C,O addition is the most plausible pathway on the singlet PES, the [3+2]O,O addition is the most favoured pathway on the doublet surface whiles the stepwise [1+1] addition involving the oxygen atom of the com-plex in the first step and the carbon atom of the complex in the second step is the most plausible pathway on the triplet PES. For the reaction of the RhO2(CH2)(CH3) complex, the [2+2]Rh,O addition pathway is the most favoured on the singlet surface, the [2+2]Rh,C is the most plausible pathway on the triplet PES and [3+2]C,O is the most plausible on the doublet surface. For the reactions of the CoO2(CH2)(CH3) complex, the [1+2]O addition is the most plausible on the singlet PES, [3+2]C=Co=O cycloaddition to form the five–membered intermediate is the most preferred path-way on the doublet PES, whiles on the triplet PES the preferred pathway is the [3+2] addition across the O=Co=O bond of the metal complex. The reactions of olefins with the Co dioxo complex have lower activation barriers for the preferred [3+2] and [2+2] addition pathways as well as fewer side reactions than those of the rhodium and iridium systems. This could imply that the cobalt dioxo complexes can efficiently and selectively catalyze specific reactions in oxidation of olefins than Rh and Ir oxo complexes will do and therefore Co oxo complexes may be better catalysts for specific oxidation reactions of olefins than Rh and Ir complexes are. The activation barriers for the formation of the four—or five-membered metallacycle intermediates through [2+2] or [3+2] cyclo-addition are lower on the triplet PES than on the singlet PES for the formation of similar analogues. There are fewer competitive reaction pathways on the triplet surface than on the singlet PES. Also, cycloadditions that seem impossible on the singlet PES seem possible on the doublet and or triplet PESs, this is the case typically for the Rh and Co complexes, illustrating the importance of multiple spin states in organometallic reactions.
Quantum chemical study of the mechanisms of oxidation of ethyleneby Molybdyl and Tungstyl Chloride
(Indian Academy of Sciences., 2016-02-25) Tia, Richard; Adei, Evans; Baidoo, Joseph; Edor, Juliana
The mechanisms of oxidation of olefins with MoO2Cl2and WO2Cl2are studied with DFT. Theformation of epoxide from these reactions is not very feasible by any of the postulated paths. If the epox-ide precursor will form at all, it will ariseviainitial [3+2]O,Claddition of ethene to MoO2Cl2and WO2Cl2to form an intermediate, followed by re-arrangement to form the precursor, from which the epoxide can begenerated by hydrolysis. The chlorohydrin precursor was also found to originate from [3+2]O,Claddition ofethene to MO2Cl2. The results also indicate that a dichloride is not a likely product in the oxidation of ethyleneby molybdyl chloride. However, in the case of WO2Cl2, the formation of a dichloride may not be precluded.The formation of acetaldehyde and vinyl alcohol from the oxidation of ethylene does not appear energeti-cally feasible with MoO2Cl2, but appears thermodynamically plausible with WO2Cl2. Thus, the oxidation ofethylene with MoO2Cl2will most likely lead to the formation of chlorohydrins predominantly via [3+2]O,Claddition; oxidation with WO2Cl2may also form chlorohydrins, but only extremely slowly. The oxyhalidesMO2Cl2become weaker oxidants in the order CrO2Cl2>>MoO2Cl2>WO2Cl2. Corresponding to this, reac-tions involving reduction of the metal [3+2] and [2+1] show a sharp increase in barrier going from Cr to W;reactions without a change in metal oxidation state ([2+2]) show much smaller variations, which are possiblymainly determined by sterics.

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