Permanganyl chloride-mediated oxidation of tetramethylethylene: Adensity functional theory study

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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).
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contrary to the observed trends in Tc and Re systems. They reportedthat for all the systems studied, the best epoxidation catalyst wouldbe the Cl ligand on the singlet surface, whiles the probable diox-ylating catalyst would be Cp ligand for both Tc and Re systems. Forthe Mn system, MnO3(OCH3) catalyst was reported to yield onlydioxylate.For permanganyl chloride catalyzed oxidation of tetramethyl-ethylene (TME), Wistuba et al. [20] reported the direct [3þ2]pathway leading to the formation of the dioxylate intermediate tobe the most favored thermodynamically, when compared to theepoxidation product from the direct [2þ1] addition pathway.However, at the B3LYP LANL2DZ/6-311G (d) level of theory, Wis-tuba reported both reactions to be of equal kinetic heights.Nevertheless, in an experiment to study TMEeMnO3Cl system bylow temperature photolytic matrix isolation technique, the epoxi-dation product ClO2MnO[C(CH3)2]2was only formed. The epoxideproduct was characterized by infra-red spectroscopy coupled withisotopic-enrichment experiment. The effects of permethylation onthese barriers were not accurately calculated and were ignored byway of assumptions based on results obtained for an analogousethylene system partly due to the fact that detailed investigation ofthe TME system is computationally expensive.The main aim of the paper is to extend the earlier theoreticalwork of Wistuba et al. [20] by exploring the potential energy sur-faces for the permanganyl chloride catalyzed oxidation of tetra-methylethylene at the B3LYP LANL2DZ/6-31G (d) level of theory atT¼193.15 K and P¼0.00001 atm. The initialfirst step leading tothe formation of the four- andfive-membered metallacycles, theinterconversion of the four-tofive-membered metallacyclicintermediate and their subsequent rearrangement to the epoxideprecursors (Scheme 1) are investigated. Several organometallicreactions have been reported to exist in several spin states andsurfaces [5,12e14,21]. The bond lengths, angular distortions andmolecular geometry are affected by change of spin states. Fororganometallic transformations, cross effects of the spin can impartthe reaction mechanisms [22]. As a result, the singlet and tripletstates in C1symmetry which are possible have been considered inall the calculations reported in this paper.2. Details of calculationThe density functional/Hartree-Fock hybrid model B3LYP[23e26] as implemented in Gaussian 09 [27] has been usedthroughout this study together with the basis set of LanL2DZ formanganese atoms whiles the split valence double-x(DZ) [28] 6-31G(d) for the non-metal atoms (H, C, O and Cl). Molecular structureswere generated with Avogadro software [29]. All structural opti-mizations were done without symmetry restrictions. Normal modeanalysis was performed to verify the nature of the stationary pointslocated. Minima, representing reactants, intermediates and prod-ucts were shown to have no imaginary frequencies. Guess struc-tures for transition state calculations were obtained byfirstconstraining specific bonds along the reaction coordinates atfixedlengths while the remaining internal coordinates were fully opti-mized. This procedure gives an approximate transition state guesswhich is then submitted for transition state calculation using thestandard transition state optimization procedure in Gaussian 09. Alltransition state structures were subjected to full normal modeScheme 1.Proposed pathway for the reaction of MnO3Cl with ethylene and tetramethylethylene.A. Aniagyei et al. / Journal of Molecular Graphics and Modelling 98 (2020) 107616.