The Search for Homoaromatic Semibullvalenes. 5.¹1,5-Dimethyl-2,4,6,8-semibullvalenetetracarboxylic Dianhydride

Abstract

The synthesis and study of a novel bisannelated semibullvalene that is calculated to be a neutral homoaromatic ground state molecule is presented. Through variable-temperature X-ray, 13 C solution-phase NMR, and 13 C solid-state CP-MAS NMR studies and by a modified Saunders' isotopic perturbation method, 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride is shown to be a highly fluxional molecule that is not homoaromatic in the solution or solid states. The barrier to the Cope rearrangement of this dianhydride, in solution, is estimated (from 125 MHz 13 C NMR data) to be less than 14 kJ mol -1 .

Beyond static structures: Putting forth REMD as a tool to solve problems in computational organic chemistry

“…This second type of case has been explored less often, as most REMD simulations to date have been performed together with a fixed‐bonds, nonreactive force field. The CR of SBV represents a prototypical example for which a great deal of mechanistic detail has already been amassed, including information regarding: the (a)synchronous nature and the role of tunneling in the reaction mechanism, the presence and magnitude of homoaromaticity in the transition state, and the link between molecular properties and structure . In its simplest form, the CR transitions between two equivalent structures each characterized by three‐membered ring on one side of the bridging ethyl subunit (Fig.…”

The Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes—Direct Observation of Transition States of Degenerate Cope Rearrangements

“…[14] We report here the quantitative analysis of this phenomenon. Enthalpy differences of as little as 1 kJ mol À1 are found to govern the equilibrium between the two degenerate ground states A, A' ' and the species A* which absorb at long wavelengths.…”

Homoaromaticity in aza‐ and phosphasemibullvalenes. A computational study

“…Over the last few decades, researchers have discovered and explored a wealth of structures based on 1 ,3, 4 the intriguing fundamental goal being the experimental identification of a semibullvalene having a delocalized bishomoaromatic ground state. Strategies towards this goal have mainly been twofold: (a) introduction of substituents5–15 or coordinating entities16, 17 aiming at electronic stabilization of 1b relative to 1a and (b) annelation of small rings7, 18–24 which by strain destabilizes 1a relative to 1b . Both strategies have been pursued by experimental and computational means, the latter proving to be a valuable tool in the search for homoaromatic structures 5–8, 11–17, 19–21, 23.…”