Toward the Synthesis of a Functionalized [6.3.1]Helicene

Polymeric helicenes are examples of structurally predictable, nanoscale spring-like molecules belonging to a larger class of molecules known as polycyclic aromatic hydrocarbons. Though past attempts to synthesize polymeric helicenes (including photocyclizations and DielsAlder cycloadditions) have failed, it is thought that Suzuki-Miyaura polymerization of pentasubstituted benzene monomer units followed by ring-closing olefin metathesis may afford a polymeric [6.3.1]helicene. Final reduction of peripheral nitro-groups may afford a fully functionalized helicene capable of linear response to environmental fluctuations in pH. Introduction: Polymeric helicenes, or nanoscale spring-like molecules, are flexible polycyclic aromatic hydrocarbons (PAHs). As a class of molecules, PAHs have unique, well-defined molecular geometries, and recent calculations have shown that polymeric helicenes also exhibit structural predictability (Rempala and King 2006). To date, there are no examples of polymeric helicenes, helicenes which contain greater than thirty benzene units. However, the interest in synthesizing molecules with a circular arrangement of benzene rings extends back nearly three quarters of a century. In 1932, a planar, helicene analogue known as coronene was synthesized (Scholl and Meyer 1932). This molecule consists of six benzene rings angularly-fused to form a ring (Fig. 1). Fig. 1: Examples of circular and helical polycyclic aromatic compounds (PAHs). Expanding upon this molecule, an open form of coronene, hexahelicene (Fig. 1), was synthesized by Newman and Lednicer (1956). Likewise, this aromatic molecule is composed of six benzene units; however, it is no longer a planar molecule, but takes on the beginnings of a coil-like asymmetric structure due to the steric strain within the molecule. Similar helical molecules, including biphenylene-based heliphenes (Fig. 1) (Han, et. al. 2002) and azahelicenes (Fig. 1) (helicenes that incorporate nitrogen atoms into the carbon backbone of the helicene) have also been reported (Staab, et. al. 1989; Staab, et. al. 1994). At present, the largest known benzenoid helical molecule is [14]helicene (Martin and Baes 1975), a multi-layered spring-like molecule containing fourteen benzene units (Fig. 2). Yet, there are still no known examples of polymeric helicenes. Fig. 2: Molecular model of [14]helicene. One of the reasons that polymeric helicenes have not been prepared is due to shortcomings with previous synthetic routes devised to form such molecules. The most common synthetic approaches have been photocyclizations (Martin and Baes 1975; Staab, et. al. 1994; Mallory, et al. 2001) or repetitive Diels-Alder cycloaddition reactions (Nikolaos et. al. 1992; Chan, et. al. 2002; LeHoullier and Gribble 1983). In a photocyclization, light provides the activation energy for the reaction to occur. A Diels-Alder cycloaddition follows the mechanism shown in Fig. 3, by which two molecules, a diene and a dienophile (together containing 3 double bonds), react in one step to produce a final cyclic compound consisting of two new carboncarbon bonds, only one double bond, and a one more ring than was present in the reactants