Extremely Crowded Biaryls

Highly congested derivatives of biphenyl were prepared by double Diels‐Alder reactions of cyclopentadienones with substituted butadiynes. The reaction of 2,3,5‐tri( tert ‐butyl)cyclopentadienone ( 5 ) and diphenylbutadiyne ( 3 ) gave only the single adduct, 1‐(phenylethynyl)‐2‐phenyl‐3,5,6‐tri‐ tert ‐butylbenzene ( 6 ), and even extreme conditions gave no second addition. When tetracyclone ( 4 ) was added to bis(trimethylsilyl)butadiyne ( 8 ), two additions were achieved, but one silyl group was lost either during, or immediately following, the second addition to give 2‐(trimethylsilyl)‐2′,3,3′,4,4′,5,5′,6‐octaphenylbiphenyl ( 11 ). However, when 3,4‐diphenyl‐2,5‐dimethylcyclopentadienone ( 12 ) was added to 8 , the fully substituted 2,2′‐bis(trimethylsilyl)‐4,4′,5,5′‐tetraphenyl‐3,3′,6,6′‐tetramethylbiphenyl ( 14 ) was formed. The X‐ray structures of compounds 11 and 14 show them to be quite crowded, but the central biphenyl rings do not exhibit the distortions previously observed in decaphenylbiphenyl. In an alternative approach, arynes were added to 5,5′‐bis(4‐chlorophenyl)‐3,3′,4,4′‐tetraphenyl‐2,2′‐bis(cyclopentadienone) ( 18 ). Simple benzyne added twice to give 4,4′‐bis(4‐chlorophenyl)‐2,2′,3,3′‐tetraphenyl‐1,1′‐binaphthyl ( 19 ) in low yield, but tetraphenylbenzyne, generated from tetraphenylanthranilic acid, added only once.