Hydrogen bonding and solvent effects in heteroaryldi(1-adamantyl)methanols: an NMR and IR spectroscopic study

Abstract

Reaction of the organolithium derivatives of certain heteroaromatics [2-furanyl, 2-thienyl, 2-thiazolyl, 2-pyridyl and 2-(3-methylpyridyl)] with di(1-adamantyl) ketone gives potentially rotameric tertiary alcohols. With 2-pyridyl- and 2-(3-methylpyridyl)lithium only the syn isomer is obtained. The syn isomer makes up 85–100% of (2-furanyl)diadamantylmethanol and 80–90% of the 2-thienyl derivative, depending on the NMR solvent. In chloroform or benzene the 2-thiazolyl derivative is a 2∶1 mixture, the isomer with the sulfur atom syn to the OH group predominating; in DMSO or in the solid state this is the sole species. The IR absorption frequency for OH stretching correlates with the corresponding proton NMR shift in chloroform and with its temperature dependence, Δδ/ΔT. In pyridine Δδ/ΔT is either large (–20 ppb/°C) or small (–1 to –2 ppb/°C) for intermolecular and intramolecular hydrogen-bonded species, respectively. Semi-empirical calculations (AM1 and PM3) suggest that the anti alcohols should be the more stable in the gas phase, but solvent effects and hydrogen bonding, in the case of the pyridyl derivatives, appear to reverse this situation, making the isomer with OH syn to the heteroatom the principal, and sometimes the only, species observed in solution.