Functionalized [3 + 3]cycloalkynes: Substituent effect on self-aggregation by nonplanar π-π interactions

Optically active (M)-2,11-dihydroxy-1,12-dimethylbenzo[c]phenanthrene-5,8- dicarbonitrile was synthesized from (M)-1,12-dimethyl-2,11-dinitrobenzo[c] phenanthrene-5,8-dicarbonitrile by the reduction and hydroxylation of nitro groups. The compound was converted to several oxygen-functionalized [3 + 3]cycloalkynes with -OH, -OSiMe₂-t-Bu, -OAc, -OTf, or -ONf groups, which are chiral arylene ethynylene macrocycles containing three helicenes. The aggregation behaviors of these [3 + 3]cycloalkynes were examined in CHCI ₃, THF, and acetone using ¹H NMR, CD, and vapor pressure osmometry (VPO) studies and were compared with that of the parent [3 + 3]cycloalkyne. An increasing strength of aggregation in CHCI₃ was observed in the following order of the substituted derivatives: -H > -ONf > -OTf > -OAc > -OSiMe₂-t-Bu. In THF the following strength of aggregation was observed: -OTf > -ONf > -OAc > -H > -OSiMe ₂-t-Bu > -OH. The aggregation of the functionalized [3 + 3]cycloalkynes is stronger for the compounds with electron-withdrawing substituents than for those with electron-donating substituents. (M)-1,12-dimethylbenzo[c]phenanthrene-2,5,8,11-tetraol was also synthesized from the same intermediate. This electron-rich helicene was readily oxidized to 5,6-quinone in air, and the quinone was suggested to form a self-charge-transfer complex in solid state.