Tetrahedron
Tetrahedron
5 years ago

Structural and electronic modifications of pyridones and pyrones via regioselective bromination and trifluoromethylation

Structural and electronic modifications of pyridones and pyrones via regioselective bromination and trifluoromethylation
We report regioselective functionalization of pyridones and pyrones via electrophilic bromination (Br2) or radical trifluoromethylation (NaSO2CF3/tBuOOH) at the 3-position. Counter-intuitively, the 3-position EW groups decreased the carbonyl stretching energy by 6–23 cm−1; however, 3,5-dibromination increased the CO frequency by 10–22 cm−1 compared to the 3-Br pyridones. X-ray crystallography revealed pyridone tautomers with contracted CO bond metrics. pK a values and 1H NMR shifts for the series 3-H→Br→CF3 revealed the expected trend of increasing acidity (pKa = 8.85 → 8.33→6.78, MeOH) and increasing chemical shifts (10.97 → 11.42→11.71 DMSO-d 6). We conclude that the paradoxical decrease in CO stretching frequencies by the 3-positoin EW groups is explained by an ‘assistive’ electron-withdrawing effect, whereby the 3-position EW group assists the electronegative oxygen atom in recruiting more electron density, and – as a result – attaining more oxyanion character (decreased the CO bond strength).

Publisher URL: www.sciencedirect.com/science

DOI: S0040402017305914

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