For many years, helicenes were regarded as little more than an academic curiosity. More recently, their extraordinary optical, [1] electronic, [2] and chelating [3] properties have rekindled an interest in these nonplanar condensed aromatic compounds in fields as diverse as liquid crystals, [4] sensors, [5] dyes, [5] asymmetric synthesis, [3, 6,7] molecular recognition, [3, 6,7] polymer synthesis, [8] and materials science. [8] Helicenes have classically been prepared by the oxidative photocyclization of bis(stilbene)s. [9, 10] Though useful, complex product mixtures often arise as a result of poor regiocontrol in the photocyclization step or competitive side reactions, such as photoinduced dimerization or benzo[ghi]perylene formation. [10] The challenge of developing efficient routes to helicenes that are amenable to scale-up has inspired many imaginative approaches, [11, 12] including strategies based on Diels-Alder cycloadditions, [13] carbenoid insertions, [14] radical cyclizations, and metal-mediated cycloisomerizations. [15,16] Herein, we report a short and efficient entry to helicenes, azahelicenes, and phenanthrenes in which the use of halo and alkoxy substituents to control both the stereochemical course of Wittig reactions and the regiochemical course of homolytic aromatic substitution reactions is a key feature.
The strategy developed is exemplified by the synthesis of 5-aza[5]helicene (4) outlined in Scheme 1. [17] This synthesis relies on the use of cooperative ortho effects to control the stereochemical course of a Wittig reaction [18] and the counterintuitive use of a halide atom as a protecting group in a homolytic aromatic substitution reaction. Thus, phosphonium salt 2 and 2-chloroquinoline-3-carboxaldehyde were first condensed under standard Wittig conditions to give azastilbene 3 in near quantitative yield as a 16:1 mixture of Z and E isomers (as determined by 1 H NMR spectroscopic integration [18a] ). Isomer (Z)-3 was then exposed to 2.4 equivalents of tributyltin hydride and 0.4 equivalents of VAZO to induce selective homolysis of the carbon-iodine bond. [19] Homolytic substitution at C4 of the quinoline moiety followed to give 5aza[5]helicene 4 in 75 % yield. [20] Notably, no products arising from the addition of the aryl radical intermediate to C2 of the