Aromatic chemistry and the impossibility of a spiral stair

David Bradley recently blogged about a molecule that resembles the Penrose Steps, the post caught the eye of Dan Lednicer who reveals here a tale of aromatic chemistry from the pioneering days of 1950s organic synthesis and alludes to the impossibility of a spiral staircase.

There was no question in my mind in the fall of 1952 that Melvin S. Newman would be my preceptor Ohio State University. The choice was strongly influenced by his elegant demonstration of carbonium ions – known as carbocations today – using the limited techniques then available.

At the time, Newman had an ongoing program aimed at synthesizing aromatic hydrocarbons that would exist as a pair of chiral isomers as a result of overcrowding. The molecule as a whole would be asymmetric in these cases unlike the run-of-the-mill chiral compounds that are asymmetric by reason of a carbon atom that carries four different substituents. Progress in the area had by then proceeded to the molecule that comprised five benzene rings.

Newman proposed that I undertake the synthesis of the six-ring analog. He expected that the synthetic route for the compounds with fewer rings would be applicable. Models showed that the last two rings in the crescent would actually overlap. At the time I found the name given to the polynuclear aromatic built from a circle of six rings, coronene for crown, quite intriguing. Cogitating about my goal, and consulting a dictionary led to the conclusion that my molecule should be named helicene as it would form the start of a helix.

Proud of this first small step in my program I walked into Newman’s office and proposed that my target be named helicene. The reply “Make it first, then we’ll talk about a name”. Back at the bench, it became evident that the later steps in the synthesis no longer worked and had to be modified as the chemistry reflected the buildup of steric hindrance, chemical crowding, in other words. The modified ten-step scheme finally gave me just over one gram of the material!

With the thesis already half-written, at least in my imagination, I again walked into Newman’s office only to be greeted with the injunction “Good, now resolve it”. Not the easiest thing to accomplish with a molecule without any functional chemical groups at all were it not for the fact that another student, Wilson Lutz had just finished preparing a reagent to do just that by forming complexes. Mixing a solution of my helicene with Lutz’s reagent instantly gave the deep-red color that signaled that, as per plan, a complex had just formed. Not according to plan was the fact that the complex was perfectly happy to just sit there and stay in solution. In desperation, as a last resort, I added more ethanol, in which helicene was not very soluble.

After the helicene that precipitated was cleaned up it indisputably rotated polarized light indicating that the sample represented a partly resolved helicene isomer. [The rotation of a beam of plane polarized light will occur if the molecules through which the beam is passing are chiral; this is the basic test for the presence of handed molecules] A fully resolved sample should give the same reading after a second recrystallization. The rotation just kept going up after each recrystallization until no more helicene was left.

There was nothing for it but to go back and go through the ten steps to prepare more helicene. This time I had just over two grams on hand. This was put through the complex formation twice more and then recrystallized and recrystallized again, but the rotation just kept going up. Optical rotation it should be noted is a calculated value. In any event, I finally got to the point where two successive recrystallizations gave the same calculated optical rotation. The people in the analytical chemistry lab at that kindly weighed the three plus milligrams of the purest sample on one of their sensitive balances. This gave an actual observed reading of 17 degrees on the polarimeter…this calculates to the astounding rotation of 3600 degrees…

This time I did start writing my thesis. As a final note, the thesis referred to the synthesis and resolution of phenathro[3,4-c]phenanthrene. The name hexahelicene waited for the full paper that appeared in 1956; Newman had added the hexa rightly anticipating the synthesis of higher benzologs. When Newman won the SOCMA Award he had tie tacks custom made for all his students who had worked in the area.

The take-home lesson from all this is that spiral staircases are mathematically impossible as a spiral is a planar figure. They should more properly be referred to as helical staircases

– by Dan Lednicer