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

✍ Scribed by Balaban, A. T. ;Wray, V. ;Furmanova, N. G. ;Minkin, V. I. ;Minkina, L. S. ;Czernysch, Y. U. E. ;Borodkin, G. S.

Publisher
John Wiley and Sons
Year
1985
Tongue
English
Weight
454 KB
Volume
1985
Category
Article
ISSN
0947-3440

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✦ Synopsis

Abstract

The molecular structure of the vinylogous 4__H__‐pyrone 2,6‐dimethyl‐4‐(2‐oxopropylidene)‐4__H__‐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 4__H__‐pyrone 4, Ar = p‐C~6~H~4~Cl, has an experimental dipole moment of 5.65 D. Corrected ^13^C chemical shifts of the ring β‐ and especially α‐carbons decrease in the series 2,6‐dimethyl‐4__H__‐pyrone (10) to 1 to 4, indicating that the higher vinylogs have decreased contributions of the dipolar canonical structures. Dynamic ^1^H 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 Δ__G__± = 22.4 kcal/mol in [D~6~]DMSO.

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