Barriers to intramolecular hydride transfers in some polycyclic hydroxyketones

Abstract

The hydroxyketones (1) and (2) have been prepared from the Diels–Alder adducts of p-benzoquinone with cyclopentadiene and cyclohexa-1,3-diene. The boron trifluoride–ether catalysed reaction of cyclohepta-1,3-diene with p-benzoquinone gave good yields of the Diels–Alder adduct which was converted into hydroxyketone (3). Experiments with cyclohexanol and admantan-2-ol established characteristic ¹³C n.m.r. chemical shifts associated with deprotonation of alcohols by dimsyl sodium in dimethyl sulphoxide. Solutions of the sodium salts of (1)–(3) were prepared and dynamic ¹³C n.m.r. spectroscopy gave barriers for their degenerate rearrangement of >21.7 (100 °C), 19.0 (100 °C), and 17.3 (72 °C) kcal mol^–1, respectively. The relationship between variation of barrier and molecular geometry is discussed with the aid of empirical force field calculations.