Effect of aza-substitution on the geometry and electrocyclization of pentadienyl anions: an apparent exception to the Woodward-Hoffmann rules?

Two geometrical isomers of 1,3,5-triphenyl-1,3-pentadiene 1 have been prepared and characterized; trans,cis-1 and trans,trans-1.These have been used to prepare and characterize the 1,3,5-triphenylpentadienyl anion by quenching the lithium salt in THF and by following hydrogen–deuterium exchange in methanol-O-d with potassium methoxide. Attempts to induce electrocyclization (130 °C in THF or 200 °C in KOtBu–HOtBu) have proven unsuccessful. A comparison of the behaviour of this anion with the 2-aza and 2,4-diaza analogues has helped in elucidating the mechanisms for their electrocyclization and the apparent exception to the Woodward-Hoffmann rules.