Novel Aromatic and Antiaromatic Systems

Abstract

Note from the Editor: August Kekulé's dream and his theory of the structure of benzene (1865). Julius Thomsen's logic that benzene contains equivalent electrons between its carbon atoms (ca. 1900). Richard Willstätter's synthesis and observation that 1,3,5,7‐cyclooctatetraene is not aromatic (early 1900s). Sir Robert Robinson's proposal of the ‘aromatic sextet’ (1925). Erich Hückel's molecular orbital treatment of benzene and other unsaturated compounds that separated sigma and pi electrons (early 1930s). Tetsuo Nozoe and Michael Dewar's independent proposal of a new aromatic structure for a cycloheptatrieneone (mid‐1940s). Ronald Breslow's demonstration that cyclopropenyl cation is aromatic (1958). Breslow's proposal of ‘antiaromaticity’ (1965). I offer several comments. First, in the field of aromaticity, Breslow is in mighty exceptional company. Second, his research has extended well beyond aromaticity. Yes, Breslow is applying the concepts of antiaromaticity toward the development of highly conductive organic materials. But his research has encompassed enzyme mimics, remote functionalization reactions, unnatural DNA analogues and cancer chemotherapy. Indeed, Breslow is the co‐discoverer of a highly successful drug, Vorinostat, which is FDA‐approved for the treatment of cutaneous T‐cell lymphoma. We thank Professor Breslow for joining our project honoring his and our friend, Tetsuo Nozoe. Tetsuo Nozoe would have would have been deeply touched. —Jeffrey I. Seeman Guest Editor University of Richmond Richmond, Virginia 23173, USA E‐mail: jseeman@richmond.edu Ronald Breslow with Koji Nakanishi (left) and Tetsuo Nozoe (center), Sendai, 1970. Abstract We contributed to the field of non‐benzenoid aromatic compounds by creating the cyclopropenyl cation and various of its derivatives, including cyclopropenone; it was the first aromatic system with other than six pi electrons in a single ring, and the simplest aromatic system. The pioneering work of Tetsuo Nozoe in tropolone chemistry was celebrated with the founding of ISNA, the International Symposium on Non‐Benzenoid Aromatic Compounds, where I described our work in the field. It fit the prediction that aromaticity would be found in systems with 4n + 2 pi electrons, where n is an integer. I was also concerned with the properties of monocyclic systems with 4n cyclically conjugated pi electrons. They were expected not to be aromatic, but the interesting question was whether they were actually antiaromatic, especially destabilized by the cyclic conjugation in such 4n species as the cyclopropenyl anion, cyclobutadiene, and cyclopentadienyl cation. The evidence supports antiaromaticity in these cases. We also examined compounds where 4n cyclic pi systems were fused with aromatic systems, and most interestingly systems in which two 4n pi systems were fused. In these cases the periphery of the molecules had 4n + 2 pi electrons, for aromaticity, but the components were antiaromatic. Recently we have studied electrical conductivities in aromatic molecules such as thiophe...

Anti‐Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro‐diazatrioxa[9]circulene with a Proton Placed Directly above the Central Ring

“…Aromaticity and anti‐aromaticity are phenomena that have received significant attention both from experimental and from theoretical chemists over the years . The famous Hückel 4 n +2 rule describes aromatic compounds such as benzene ( 1 ) while the 4 n rule describes anti‐aromatic systems such as planarized cyclooctatetraene ( 2 ; Figure ) .…”

“…In recent years, several examples of chemically stable anti‐aromatic molecules have been described, including a range of planar cyclooctatetraenes imbedded in [8]circulene structures ( 3 ), nor‐corrole structures, and indeno[1,2‐ b ]fluorene structures . A prevalent strategy to stabilize anti‐aromatic rings, within a molecule, is to anneal the anti‐aromatic segment with aromatic rings . This is the case for diazadioxa[8]circulene 3 , where the eight aromatic rings form a planar cyclooctatetraene motif (Figure ) .…”

“…Aromaticity and anti-aromaticity are phenomena that have received significant attention both from experimental and from theoretical chemists over the years. [1][2][3] Thefamous Hückel 4n + 2r ule describes aromatic compounds such as benzene (1)while the 4n rule describes anti-aromatic systems such as planarized cyclooctatetraene (2;F igure 1). [4] Generally speaking, aromatic compounds are chemically stable, while anti-aromatic compounds are unstable.S tudies of planar, thus anti-aromatic,c yclooctatraene (2)a re only possible in the realm of computational chemistry.…”

Antiaromatic character of cycloheptatriene-bis-annelated indenofluorene framework mainly originated from heptafulvene segment

“…While the majority of these systems are based on fused [4 n  + 2] π-electron aromatic molecules such as pentacene, the utilization of a [4 n ] π-electron antiaromatic character has emerged as a useful strategy to create new molecules with unusual electronic structures 8–11 . Since it is well known that the representative 8π- and 12π-electron frameworks pentalene and indacene (ID), respectively, are unstable due to the high antiaromatic character, the incorporation of methylenecyclopentadiene (fulvene) segments into an acene system has become a rational molecular design to construct [4 n ] π molecules.…”

The Nozoe Autograph Books Project: An Assessment

“…[1,[11][12][13][14][15][16] A number of essays were highly technical. [17][18][19][20][21] One editorial was entirely instructive: how to use the project's website. [2] This article is the second editorial.…”