Synthesis of quinoline liquid crystals by inverse electron demand Diels-Alder cycloaddition

Tetrahydroquinolines (THQ) cycloadducts, as well as their oxidation to the corresponding quinolines are reported. THQs were achieved through the Inverse Electron Demand [4 + 2]-Diels–Alder (IEDDA) cycloaddition. The [4 + 2]-IEDDA cycloadducts were obtained through a three-component reaction involving aromatic aldehydes, amines and pyrano dienophile, in the presence of TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperydine-1-oxyl), trifluoroacetic acid and acetonitrile, in low yields <40 %. Theoretical studies were complimentary performed to analyze the chemical reactivity, regio- and stereoselectivity of the IEDDA cycloaddition. The preference for the formation of the THQs cycloadducts was dictated by frontier orbital molecular (FOMs) LUMO of 2-azadieno and HOMO of the dienophile. Oxidation of the THQs with DDQ yielded quinolines in high yields >90 %. The thermal behavior of the THQs and quinolines was studied using DSC and POM techniques. The final quinolines exhibited an amorphous appearance with vitreous, glossy, and brittle characteristics. Through POM studies only one quinoline displayed stable liquid crystal behavior with a glass transition temperature (Tg) and glassy nematic mesophase at room temperature was assigned, without no thermal decomposition detected after cycles of heating and cooling.