Sterically Hindered Internal Rotation in an Acyclic α,β‐Unsaturated Ketone. 2, 2, 5‐Trimethyl‐4‐isopropylhex‐4‐en‐3‐one

Abstract

An activation barrier of 45 kJ mol−1 follows from the 1H-NMR spectra in the case of the first detected hindered rotation about the sp2—sp2 single bond of an acylic α,β-unsaturated ketone (1). The intensities of the UV bands of the π→π* and n → π* transitions are consistent with a skewed conformation of 1. However, molecular mechanics calculations predict a much higher steric barrier than is observed, which demonstrates the limitations of our current understanding of sterically hindered rotation about sp2—sp2 single bonds in aliphatic systems.