S. Catak

The efficacy of organophosphorus radicals as anticoking agents was subjected to a computational study in which a representative set of radicals derived from industrially relevant organophosphorus additives was used to explore competitive reaction pathways on the graphene-like coke surface formed during thermal cracking. The aim was to investigate the nature of the competing reactions of different organophosphorus radicals on coke surfaces, and elucidate their mode of attack and inhibiting effect on the forming coke layer by use of contemporary computational methods. Density functional calculations on benzene and a larger polyaromatic hydrocarbon, namely, ovalene, showed that organophosphorus radicals have a high propensity to add to the periphery of the coke surface, inhibiting methyl radical induced hydrogen abstraction, which is known to be a key step in coke growth. Low addition barriers reported for a phosphatidyl radical suggest competitive aptitude against coke formation. Moreover, organophosphorus additives bearing aromatic substituents, which were shown to interact with the coke surface through dispersive π–π stacking interactions, are suggested to play a nontrivial role in hindering further stacking among coke surfaces. This may be the underlying rationale behind experimental observation of softer coke in the presence of organophosphorus radicals. The ultimate goal is to provide information that will be useful in building single-event microkinetic models. This study presents pertinent information on potential reactions that could be taken up in these models.

The ring opening of 2-substituted N,N-dibenzylaziridinium ions by bromide is known to occur exclusively at the substituted aziridine carbon atom via an SN2 mechanism, whereas the opposite regioselectivity has been observed as the main pathway for ring opening by fluoride. Similarly, the hydride-induced ring opening of 2-substituted N,N-dibenzylaziridinium ions has been shown to take place solely at the less hindered position. To gain insight into the main factors causing this difference in regioselectivity, a thorough and detailed computational analysis was performed on the hydride- and halide-induced ring openings of 1-benzyl-1-(α-(R)-methylbenzyl)-2(S)-(phenoxymethyl)aziridinium bromide. Intramolecular π−π stacking interactions in the aziridinium system were investigated at a range of levels that enable a proper description of dispersive interactions; a T-stacking conformer was found to be the most stable. Ring-opening mechanisms were investigated with a variety of DFT and high level ab initio methods to test the robustness of the energetics along the pathway in terms of the electronic level of theory. The necessity to utilize explicit solvent molecules to solvate halide ions was clearly shown; the potential energy surfaces for nonsolvated and solvated cases differed dramatically. It was shown that in the presence of a kinetically viable route, product distribution will be dictated by the energetically preferred pathway; this was observed in the case of hard nucleophiles (both hydride donors and fluoride). However, for the highly polarizable soft nucleophile (bromide), it was shown that in the absence of a large energy difference between transition states leading to competing pathways, the formation of the thermodynamic product is likely to be the driving force. Distortion/interaction analysis on the transition states has shown a considerable difference in interaction energies for the solvated fluoride case, pointing to the fact that sterics plays a major role in the outcome, whereas for the bromide this difference was insignificant, suggesting bromide is less influenced by the difference in sterics.

The alcoholysis of cyclic meso-anhydrides catalyzed by β-amino alcohols has been investigated with DFT quantum mechanics to determine the mechanism of this reaction. Both nucleophilic catalysis and general base catalysis pathways are explored for methanol-induced ring opening of an anhydride catalyzed by a chiral amino alcohol. The nucleophilic pathway involves a late transition state with a high energy barrier. In this mechanism, methanolysis is expected to take place following the amine-induced ring opening of the anhydride. In the base-catalyzed mechanism, methanol attack on one carbonyl group of the meso-anhydride is assisted by the β-amino alcohol; the amine functionality abstracts the methanol proton. The chiral amino alcohol also catalyzes the reaction by stabilizing the oxyanion that forms upon ring opening of the anhydride by hydrogen bonding with its alcoholic moiety. Both stepwise and concerted pathways have been studied for the general base catalysis route. Transition structures for both are found to be lower in energy than in the nucleophilic mechanism. Overall this study has shed light on the mechanism of the β-amino alcohol-catalyzed alcoholysis of cyclic meso-anhydrides, showing that the nucleophilic pathway is approximately 100 kJ mol−1 higher in energy than the general base pathway.

The synthetic utility of N-alkylidene-(2,3-dibromo-2-methylpropyl)amines and N-(2,3-dibromo-2-methylpropylidene)benzylamines was demonstrated by the unexpected synthesis of 3-methoxy-3-methylazetidines upon treatment with sodium borohydride in methanol under reflux through a rare aziridine to azetidine rearrangement. These findings stand in contrast to the known reactivity of the closely related N-alkylidene-(2,3-dibromopropyl)amines, which are easily converted into 2-(bromomethyl)aziridines under the same reaction conditions. A thorough insight into the reaction mechanism was provided by both experimental study and theoretical rationalization.