Electrocyclic reactions. Part VI. Thermal reactions of 1,5-diphenylpentadienide ion and the anions derived from cis- and trans-3,4-diphenylcyclopentene

Abstract

Thermal cyclisation of the anion derived from the stereoisomeric 1,5-diphenylpenta-1,4-dienes could not be effected in 2,5,8-trioxanonane (diglyme) at 160° or in decalin at 190°, but has been achieved by Wolff–Kishner reaction of the semicarbazone of trans,trans-dibenzylideneacetone with powdered potassium t-butoxide at 225°, to give cis-3,4-diphenylcyclopentene. In all such Wolff–Kishner reactions trans-3,4-diphenylcyclopentene was also formed, and was the major product when solid sodium methoxide and potassium hydroxide were used as reagents; 1,5- and 1,2-diphenylcyclopentene were also produced. Thermal cycloreversion of the anions derived from cis- and trans-3,4-diphenylcyclopentene could not be effected in diglyme at 160° or in decalin at 190°; instead, the cis-material was isomerised to 1,5-diphenylcyclopentene (89% in diglyme, 56% in decalin), but the trans-substance was essentially not isomerised (2–3% in diglyme, 0% in decalin). With powdered potassium t-butoxide at 225° both cis- and trans-materials were isomerised to a mixture of 1,5- and 1,2-diphenylcyclopentene (ca. 1 : 5).