QUANTUM COMPUTATIONAL DESIGN OF A PYROLIDINE-BASED ORGANOCATALYST WITH ENHANCED SELECTIVITY AND ACTIVITY TOWARD ASYMMETRIC ALDOL REACTION
QUANTUM COMPUTATIONAL DESIGN OF A PYROLIDINE-BASED ORGANOCATALYST WITH ENHANCED SELECTIVITY AND ACTIVITY TOWARD ASYMMETRIC ALDOL REACTION
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Date
2019-03
Authors
IBRAHIM, Usman
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Abstract
In this study, a data set of 34 pyrrolidine-based organocatalysts applicable on asymmetric aldol reaction were considered. The data set was subjected to Density Functional Theory(DFT) optimization using BLY3P/6-31G* modeland Quantitative Structure-Property Relatisonship(QSPR) calculation. From the optimization and calculation, several electronic, topological and stereochemical-based descriptors were generated which led to the development of five regression models. The best model (Model one), with R2= 0.84 and Rext=0.88, was used to adjust theprolinestructure for finding new catalyst candidates
A simulation reaction was carried out in which the candidate catalyst was applied over asymmetric aldol reaction between acetone and 4-nitrobenzaldehyde in an acetone medium. A plausible mechanism of the reaction was developed using HOMO-LUMO energies calculated using density functional theory at 6-31G*/B3LYP level of theory. From the itemized elementary steps, the fourth step was identified as the rate-determining step, with the highest activation energy of 54.20 kJmol-1.The mechanism was used to derive the rate law from which the overall rate constant was calculated and found to be 0.787 M-1 s-1. The simulation reaction was also conducted on the same reaction catalyzed by proline and themechanism was developed. New mechanistic steps that followed the previously reported iminium-enamine route of typical class 1 aldolase enzymes were proposed in greater detail. From the elementary steps, the first step which involves a bimolecular collision of acetone and proline was considered as the rate-determining step, having the highest activation energy of 59.07 kJmol-1. The mechanism was used to develop the rate law from which the overall rate constant was calculated and found to be 4.04×10−8𝑑𝑚3𝑀𝑜𝑙−1𝑠−1. The new mechanistic insights and the explicit computation of the rate constant further improved the kinetic knowledge of the reaction.
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A DISSERTATION SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES, AHMADU BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF MASTER DEGREE IN PHYSICAL CHEMISTRY. DEPARTMENT OF CHEMISTRY, FACULTY OF PHYSICAL SCIENCES AHMADU BELLO UNIVERSITY, ZARIA NIGERIA.
Keywords
QUANTUM COMPUTATIONAL DESIGN,, PYROLIDINE-BASED,, ORGANOCATALYST,, ENHANCED SELECTIVITY,, ASYMMETRIC ALDOL REACTION,