Dipole Moment, Dynamic NMR, and Molecular Structure of Vinylogous 4H‐Pyrones

Abstract

AbstractThe molecular structure of the vinylogous 4H‐pyrone 2,6‐dimethyl‐4‐(2‐oxopropylidene)‐4H‐pyran (1) was determined by X‐ray diffraction. The molecule is practically planar and has the s‐cis conformation. The experimental dipole moment (3.71 D) of 1 agrees with the CNDO/2 calculated value (3.54 D) for the s‐cis conformation 1 and rules out the s‐trans conformation 2. The doubly vinylogous 4H‐pyrone 4, Ar = p‐C6H4Cl, has an experimental dipole moment of 5.65 D. Corrected 13C chemical shifts of the ring β‐ and especially α‐carbons decrease in the series 2,6‐dimethyl‐4H‐pyrone (10) to 1 to 4, indicating that the higher vinylogs have decreased contributions of the dipolar canonical structures. Dynamic 1H NMR data, obtained using the two‐dimensional accordion technique, indicate that 1 undergoes rotation around the exocyclic C = C bond with a free energy of activation ΔG± = 22.4 kcal/mol in [D6]DMSO.