Browsing by Author "Opoku, Ernest"

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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.
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    [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.
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    Investigating customer attitude towards Automated Teller Machine; (a case study at GCB Bank Limited)
    (July, 2015) Opoku, Ernest
    ATMs serve the primary purpose of allowing people to withdraw money from their bank accounts from virtually anywhere in the world. The machines work by reading the magnetic strip of an ATM or debit card. This study seeks to examine customers’ attitude towards ATM services at Ghana Commercial Bank (GCB) Limited. The study was quantitative in nature and adopted descriptive and explanatory research approach. The population for the study consisted of all customers of GCB in Ashanti region of Ghana. Using convenience sampling technique, 30 customers each were selected from the five branches used for the purpose of this study (Bekwai, Bantama, KNUST, Suame and Adum). Structured questionnaires were used to collect primary data and analysis was done using frequencies, percentages, mean, standard deviation, one sample t-test, and binary logistic regression. The study revealed that four demographic variables of customers significantly affected the use of ATMs at GCB. These were gender, education, occupation, and level of income. The type of account operated (be it savings or current), did not significantly influence the usage of ATM services. The study indicated that, the majority of the customers had GCB’s ATM card. And they mostly used either once or twice a month. The customers of GCB were satisfied with the existing ATM locations, speed of response, convenience of service, 24/7 service accessibility, the process of money withdrawn under ATM scheme, and the easy of login. The major challenges faced by customers when using GCB’s ATM were, unreliable network system, machine out of order, valid limit on amount of cash withdrawn, wrong debits, reduction in balance without cash payment, card gets blocked or locked up, and machine out of cash. The study found out that males were more likely to use ATM services more than females. The researcher recommends that, for banks to fully realize the potential benefits of customers using its ATM facilities, it must have special promotional packages to attract females unto the ATM service.
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    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.
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    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.
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    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.
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    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.
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    Site‑, enantio‑ and stereo‑selectivities of the 1,3‑dipolar cycloaddition reactions of oxanorbornadiene with C,N‑disubstituted nitrones and dimethyl nitrilimines: a DFT mechanistic study
    (Springer-Verlag GmbH Germany, 2020-01-01) Opoku, Ernest; Arhin, Grace; Pipim, George Baffour; Adams, Anita Houston; Tia, Richard; et. al
    1,3-Dipolar cycloaddition of nitrones to oxanorbornadienes is an important method for the enantioselective synthesis of highly substituted 5-membered heterocycles such as furans and isoxazolidines, which have high utility in the chemical and pharmaceutical industries. The mechanism of the reaction and the effects of substituents on the (3 + 2) cycloaddition reactions (32CA) of C,N-dialkyl nitrones with a series of substituted oxanorbornadienes have been studied with focus on the site-selectivity (attack on the more substituted double bond of the oxanorbornadiene derivatives versus attack on the less substituted double bond), enantioselectivity and stereo-selectivity using density functional theory calculations at the M06/6-311++G(d,p) of theory. The results showed that the addition step to form the bicyclic isoxazolidines cycloadducts has generally low barriers compared to the cycloreversion step which converts the cycloadducts into furans and monocyclic isoxazolidines. Generally, electron-withdrawing substituents favour the nitrone attack on the highly substituted double bond, while electron-donating substituents favour the attack on less substituted double bond. The R enantiomers are generally favoured over the S enantiomers, and exo stereo-isomers are generally favoured over the endo stereo-isomers, irrespective of substituents.