Polymeric catalysts for chemo- and enantioselective epoxidation of olefins: New crosslinked chiral transition metal complexing polymers

Polymeric analogs of well-known chiral Mn(III)-salen complexes were synthesized and were used as recyclable catalysts for asymmetric epoxidation of olefins. For this purpose two different monomers, 2 and 3 , bearing chiral Mn(III)-salen moieties were synthesized. The monomer 3 carries a bulky substituent closer to the Schiff base moiety, while monomer 2 lacks such a substituent. These metal complexed chiral monomers were subsequently copolymerized with ethylene glycol dimethacrylate producing insoluble crosslinked functional matrices that possess macroporous morphology. Chemo- and enantioselective catalytic activities of these two polymers were evaluated for epoxidation of olefins. Both polymers catalyzed the epoxidation of a variety of olefins at room temperature in the presence of iodosylbenzene (PhIO) as the terminal oxidant with yields comparable to the homogenous system. In terms of their enantioselective catalytic activity, polymer P-2 (obtained from 3 ) performed better than polymer P-1 (obtained from 2 ). Unfortunately, while the homogeneous systems are reported to offer over 80% enantioselectivity, with the present polymeric catalysts, enantioselectivity to a maximum of 30% were observed. Unlike the homogeneous system, use of an external nitrogenous donor played a very insignificant role in influencing enantioselectivity. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1809–1818, 1997     

A theoretical kinetic study of the reactions of NH2 radicals with methanol and ethanol and their implications in kinetic modeling

Amino (NH₂) radicals play a central role in the pyrolysis and oxidation of ammonia. Several reports in the literature highlight the importance of the reactions of NH₂ radicals with fuel in NH₃-dual-fuel combustion. Therefore, we investigated the reactions of NH₂ radicals with methanol (CH₃OH) and ethanol (C₂H₅OH) theoretically. We explored the various reaction pathways by exploiting CCSD(T)/cc-pV(T, Q)Z//M06-2X/aug-cc-pVTZ level of theory. The reaction proceeds via complex formation at the entrance and exit channels in an overall exothermic process. We used canonical transition state theory to obtain the high-pressure limiting rate coefficients for various channels over the temperature range of 300–2000 K. We discerned the role of various channels in the potential energy surface (PES) of NH₂ + CH₃OH/C₂H₅OH reactions. For both reactions, the hydrogen abstraction pathway at the OH-site of alcohols plays a minor role in the entire T-range investigated. By including the title reactions into an extensive kinetic model, we demonstrated that the reaction of NH₂ radicals with alcohols plays a paramount role in accurately predicting the low-temperature oxidation kinetics of NH₃-alcohols dual fuel systems (e.g., shortening the ignition delay time). On the contrary, these reactions have negligible importance for high-temperature oxidation kinetics of NH₃-alcohol blends (e.g., not affecting the laminar flame speed). In addition, we calculated the rate coefficients for NH₂ + CH₄ = CH₃ + NH₃ reaction that are in excellent agreement with the experimental data.