NMR and Theoretical Study on Conformation of Methacrolein Dimer, 2,5-Dimethyl-3,4-dihydro-2H-pyran-2-carboxyaldehyde

Abstract

ABSTRACT:Detailed structures of methacrolein dimer (MAD) were analyzed by nuclear magnetic resonance (NMR) and theoretical calculations. Two dimensional 1 H and 13 C NMR peaks were assigned. Relative stability of conformations and interconversion barrier height were studied by semi-empirical MNDO and ab initio OFT calculations. Barrier height between pseudoaxial and pseudoequatorial half-chair conformers was 5.8 kcal mol-1 . Separations of chemical shifts of two protons bonded to the same methylene carbons became larger as measurement temperature decreased, due to reduced mobility of the ring framework.KEY WORDS [IliA]As MAD became available from several sources at present, polymerization mechanism and conditions of such a unique polymer were studied again in this paper.MAD has an asymmetric carbon in the ring to which the aldehyde group is bonded. Therefore, MAD has two stereo isomers (enantiomers S and R) and each isomer takes several conformations arising from ring puckering deformation and rotation of the aldehyde group around the C*-CHO axis.' To whom correspondence should be addressed.As a result of torsional constraint imposed by the double bond in the ring framework, conformational variation in the 3,4-dihydropyran ring is rather limited compared to the oxycyclohexane ring such as pyranose. MAD is also important in oligo saccharides synthesis, 3 • 4 but information about the structures and reactivities is surprisingly scarce. In early works, Bushweller and O'Neil assumed the predominant existence of a halfchair conformer for 3,4-dihydropyran based on nuclear magnetic resonance (NMR) spectra 5 In another NMR measurement, Eliel et a/. 6 • 7 reported analysis of MAD structures by use of 13 C and 1 H NMR, but did not give detailed assignments of CH 2 and CH 3 .3,4-Dihydropyran has been analyzed theoretically only by the force field approach but there is no study on MAD. Force field studies are appropriate for hydrocarbons such as cyclohexane and cyclohexene, but simple force field potential does not well account for oxygen lone pair and double bond n-electron effects. Highly accurate ab initio calculation is indispensable for determination of the conformer geometries and precise prediction of the barrier height. This paper focuses on conformational preference and interconversion of MAD molecule after successful assignment of 1 H and 13 C NMR peaks of MAD. Various conformers of MAD and the interconversion barrier height were determined by electronic structure calculations. NMR and computational results herein set foundations for the reactivity of MAD in cationic polymerization and are discussed in our sequel papers. EXPERIMENTAL Preparation of Methacrolein DimerMAD was prepared by thermal dimerization of dry methacrolein in the presence of hydroquinone (I% by weight in methacrolein) at 160°C for 3 h under nitrogen 891

Quantum-Chemical Analysis of α-Alkylacroleins Structure

“…Two AA molecules react with each other, whereby one molecule acts as heterodiene and another acts as dienophile. During this reaction 2,5-dialkyl-3,4-dihydro-2H-pyran-2-carbaldehyde -dimers of AA -are formed, where carbaldehyde group can be oriented equatorial or axial to the pyran cycle [4,5]. The important peculiarity of α-alkylacrolein dimerization via Diels-Alder reaction is its regioselectivity, cis-principle of addition and endo-and exo-orientation of its substituents.…”