Aromaticity and Pericyclic Reactions

Abstract

Pericyclic reactions formally resemble the conversion of one Kekult structure into another; the transition state may be "aromatic", "nonaromatic", or "antiaromatic". Thermally induced pericyclic reactions proceed preferentially via aromatic transition states whereas their photochemical counterparts lead to products that are formed via antiaromatic transition states.

Graph Theory of Aromatic Stabilization

“…We can now justify the occurrence of not only thermal but also photochemical pericyclic reactions explicitly in terms of aromatic transition states. 83 As expected by Zimmerman 90 and Dewar, 91 a photochemical pericyclic reaction can likewise be predicted to take place via an aromatic transition state, but in this case the transition state is iso-π-electronic with aromatic Hückel or Möbius annulene in the excited state. 83 Stereochemistry of the reaction product necessarily reflects that of the aromatic transition state.…”

“…Zimmerman 90 and Dewar 91 attempted to rationalize the allowed pericyclic reactions in terms of transition-state aromaticity. A thermally allowed pericyclic reaction really proceeds via an aromatic transition state that is iso-π-electronic with aromatic Hückel or Möbius annulene in the ground state.…”

Unambiguous assessments of reaction paths for selected pericyclic reactions

“…In accord with experiment, the π 4 a + π 2 s pathway is found, unambiguously, to be forbidden using (i) either the original Zimmerman-Dewar approach or my modification of it 6 or (ii) Fukui's frontier orbital approach 3,9 .…”

“…Three major approaches have been developed to predict outcomes for concerted reactions controlled by orbital symmetry -analysis using (i) correlation diagrams 1 , (ii) frontier orbital interactions in transition states 2 and (iii) incipient aromatic/ antiaromatic character in transition states [3][4][5] . The latter approach (Zimmerman-Dewar or Hückel-Möbius) is particularly simple to teach to undergraduate students.…”

Facets of the Tetsuo Nozoe Legacy Immortalized in an Enduring Series of International Symposia on Novel Aromatic Compounds (ISNA)

“…In time, Dewar realized that the concepts of aromaticity and anti-aromaticity could be extended even to transition states of pericyclic reactions and used these concepts to explain the rules of conservation of orbital symmetry. [17] Fortunately for historians of chemistry and devotees of aromaticity, most of the invited lectures presented at ISNAs, beginning with ISNA-1, have been published in the IUPAC journal Pure and Applied Chemistry which are available open access online. [18] Sadly, no lectures were published from ISNA-3 or ISNA-10.…”