|Two research groups have almost simultaneously reported the synthesis of 5-cycloparaphenylene (5CPP), the chemical compound with 5 phenylene groups joined together in a loop via 1,4-connnections and of some relevance because it also makes up the equatorial belt of fullerene. Team Evans, Darzi & Jasti (DOI) beat team Kayahara, Patel & Yamago (DOI) by just a month. Both teams are veterans in the field with a track record of research on similar but larger nanohoops (earlier report here). |
Team Jasti considered itself lucky to find traces of a 5CPP precursor amidst a 10CPP synthesis based on a dimerisation. This precursor (already strained by 32 kcal/mole) was then reduced by sodium naphthalenide to a dianion and reaction with methanol produced a neutral methanol-subtstituted 5CPP. Removing those groups with lithium diisopropylamide gave 5CPP itself as a "dark-red solid with a deep ruby colour" solution in toluene and stable at RT. Based on the crystal structure, Team Jasti reports a benzenoid structure with elongated phenyl-phenyl bonds and identical bond lengths for the phenyl hexagons. The displacement of each tertiary carbon atom out of the hexagon plane is 15.6°. The strain energy has increased to 119 kcal/mole.
In the meanwhile the Yamago team in their publication presented their strategy towards 5CPP as a hybrid of earlier Jasti and Yamago schemes and coincidently arrived at an almost identical 5CPP precursor with 4 methanol groups instead of just two. After reduction with tin chloride 5CPP was isolated as a "dark-purple solid".
Is 5CPP relevant? Here is a quote from Graham Bodwell in his Nature Chemistry News & Views item on the topic: "Just as the most skilled golfer makes the most difficult of shots look easy, the Jasti and Yamago groups have made the synthesis of 5CPP look straightforward. Ironically, this can make it very difficult to convey the significance of the accomplishment to the scientific community."