Rotational isomerism involving an acetylenic carbon IV: synthesis and structure of bis(1,1 ';3 ',1 ''-terphenyl-2 '-yl)ethynes: molecular design of sterically congested alkynes toward restricted rotation about acetylenic axis

The title diphenylethyne derivative with 4-methylphenyl ( tolyl) groups at all the ortho positions was synthesized by the Stille or Sonogashira coupling from the corresponding iodide. The X-ray structure revealed that the two terminal phenyl groups at the sp carbons are twisted by 63degrees out of the coplanar conformation to avoid steric interactions between the tolyl groups. The relative stabilities of possible conformers were analyzed by the PM3 calculations. The axially chiral derivative with two methoxymethyl groups showed no evidence of restricted rotation about the acetylenic axis by VT NMR measurements, its barrier being less than 35 kJ mol(-1). The spectroscopic features and reactivities of this sterically congested alkyne are also described.