Racemic and Optically Pure Heptahelicene-2-carboxylic Acid: Its Synthesis and Self-Assembly into Nanowire-Like Aggregates

Heptahelicene‐2‐carboxylic acid was effectively synthesised from suitably functionalised naphthalene building blocks. Methoxy‐substituted 1,1′‐ethyne‐1,2‐diylbis(2‐but‐3‐yn‐1‐ylnaphthalene) was cyclised in the presence of CpCo(CO)2/PPh3 to 2‐methoxy‐7,8,11,12‐tetrahydroheptahelicene, which was converted into heptahelicen‐2‐yl trifluoromethanesulfonate. This reactive intermediate underwent Pd(OAc)2/dppp‐catalysed methoxycarbonylation reaction to provide, after hydrolysis, heptahelicene‐2‐carboxylic acid. The racemate was resolved into enantiomers by semipreparative HPLC on a chiral column. The helicity of (+)‐(P)‐heptahelicene‐2‐carboxylic acid was assigned by correlating its CD spectrum to that of the known (+)‐(P)‐heptahelicene. Racemic heptahelicene‐2‐carboxylic acid was deposited on calcite (10‐14) to undergo self‐assembly into nanowire‐like aggregates as demonstrated by noncontact atomic force microscopy (NC‐AFM). The preparation of racemic as well as nonracemic [7]helicene‐2‐carboxylic acid by [2+2+2] alkyne cycloisomerisation is described. When deposited on a calcite(10‐14) surface, the racemic acid undergoes self‐assembly to nanowire‐like aggregates as demonstrated by NC‐AFM.