7‐Arylnorbornanes: Model Compounds for the Study of CH···π and OH···π Interactions

Preorganised 7-arylnorbornanes were selected as suitable model compounds for the study of CH...p and OH...p interactions. The most stable conformation of these derivatives favours such interactions by avoiding secondary hydrogen bonds between the oxygen atoms and the ortho-H of the aryl rings in the planar conformation as well as by increasing the gauche/trans ratio for steric reasons. The rotational barriers of the studied compounds are higher when electron-donor substituents are placed at the para position of the aryl ring. The trend of the electronic effects exerted by the substituents is in agreement with the existence of CH...p and OH...p interactions, which are favoured in electron-rich aromatic systems. However, interestingly, DFT calculations show that the rotational barriers in these species are mainly governed by the occurrence of p(Cipso=C)?s*(CO) donations instead of the expected CH...p and OH...p hydrogen bonds. Both NMR and IR data support this hypothesis. Therefore, the assertion that the predominance of the gauche conformation is a proof of the presence of weak hydrogen bonds in benzyl alcohols and benzyl methyl ethers should be made with caution.