(Solved): 4. ( 30 points) The "helicenes" are a simple but structurally unusual class of polycyclic aromatic h ...

4. ( 30 points) The "helicenes" are a simple but structurally unusual class of polycyclic aromatic hydrocarbons. Formally, they can be thought of as coming from successively fusing additional rings to phenanthrene (in practice, the synthesis is more complicated). The first "official" helicene is [5]helicene, which has 5 total rings. At this point, steric hindrance forces the rings out of planarity, causing the molecule to twist into a helix, hence the name. The geometry of [6]helicene, from its crystal structure, is shown for reference. (a) One of the protons in [6]helicene, marked with an asterisk (*) in the structure, appears at 6.65 ppm in the \( { }^{1} \mathrm{H} \) NMR spectrum, unusually low for a proton on an aromatic ring. Why is this? (b) A tetramethylated derivative of [6]helicene is thought to undergo a [1,7]-sigmatropic hydrogen shift as the first step of longer series of rearrangements, as shown below. Explain, using molecular orbital arguments, why the helicene structure is well suited to this [1,7]-shift. In theory, the H atom could move to many other sites 7 atoms away from the initial point of attachment-why does it go to this one in particular? (c) Helicene subunits can be found within more complex structures. A synthesis of what is essentially two [6]helicenes fused together yields a mixture of three stereoisomers, which are shown below (H atoms omitted for clarity). List symmetry elements for [6]helicene itself and the three structures shown. Which are chiral? (a) (b) (c)