Diols as hydrogen bond acids: 1H NMR study of the hetero‐association of pyridine with sterically hindered EDOT diols

Abstract

AbstractThe hetero‐association of pyridine with the cis and trans methyl‐3,4‐ethylenedioxy‐2‐[3‐(2,2,4,4‐tetramethylpentan‐ 3‐ol)]thiophenes (Me‐EDOT alcohols), 3,4‐ethylenedioxy‐2,5‐bis[3‐(2,2,4,4‐tetramethylpentan‐3‐ol)]thiophene (EDOT diol) and methyl‐3,4‐ethylenedioxy‐2,5‐bis[3‐(2,2,4,4‐tetramethylpentan‐3‐ol)]thiophene (Me‐EDOT diol) has been studied by 1H nuclear magnetic resonance (NMR) spectroscopy in benzene at 298 K. Self‐consistent association constants for the syn (S), anti‐syn (AS) and syn‐syn (SS) rotamers are determined from the variation of the OH proton chemical shift and of the rotamer composition with the pyridine concentration. The bridge methyl group has no effect upon the association constant of a syn OH group. Association constants are the same for the syn OH groups in the AS and syn‐anti (SA) rotamers of the diols, being slightly higher than for the alcohols. In contrast, values for a single syn OH group in the SS rotamers of the diols are significantly lower. Hydrogen bond acidity A values, determined for some congested alcohols and diols by the NMR method of Abraham, confirm that the syn OH protons of SS rotamers are less acidic than the single syn OH proton of an AS or SA rotamer. A values, with the notable exceptions of those for propan‐1,3‐diol and butan‐1,4‐diol, correlate with pyridine hetero‐association constants. Copyright © 2008 John Wiley & Sons, Ltd.