1,5-Dimethyl-2,4,6,8-semibullvalenetetracarboxylic Dianhydride:  A Close Approach to a Neutral Homoaromatic Semibullvalene

Abstract

We report in this paper the syntheses and the results of an investigation of the structure (delocalized homoaromatic or localized Cope system) of 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride 1 and the 5-ethyl analog 2. Although many homoaromatic cationic species are known, there is to date no well-accepted example of an electrically neutral homoaromatic molecule. 1 The semibullvalenes have long been recognized as the system most closely approaching neutral homoaromaticity. 1,2 The Cope rearrangements of semibullvalenes (e.g. 1a a 1b) proceed through the homoaromatic transition state (e.g. 1c) often with very low activation barriers. 1,3 A number of attempts to design semibullvalenes having homoaromatic ground states have been considered. 1 One suggestion, by Paquette, is that the bisannelated semibullvalene (3) might be such a system since the "breathing motion" of the Cope process would be inhibited. 4 Semiempirical 5,6 and ab initio 7 quantum mechanical calculations on other bisannelated semibullvalenes support this hypothesis. Müllen et al. subsequently prepared the first example 8 (4) of a bisannelated semibullvalene and demonstrated that its Cope rearrangement is rapid on the NMR time scale even at -160°C. 10 Similarly, our theoretical studies predict the bisanhydride 1 to have a single minimum potential energy surface with a homoaromatic ground state. 6 The known 11 tetraester 5 was viewed as an ideal precursor to bisanhydride 1. Grohmann's route 11 to 5 (Scheme 1A) was modified to give this tetraester more easily and in improved yield (Scheme 1B). The tetraester 5 was converted to the desired anhydride 1 Via the tetraacid 7 (Scheme 1B).1 proved to be remarkably stable; it can be purified by sublimation (200°C at ambient pressure) and only undergoes slight hydrolysis of one of the anhydride moieties upon prolonged exposure to the atmosphere. This is in contrast with other semibullvalenes, which frequently yield cyclooctatetraene upon mild thermolysis 12 and react more or less readily with oxygen. 10,13 The anhydride 1 crystallizes in space group Cc or C2/c, which are crystallographically indistinguishable. In the latter, the molecule uses a crystallographic 2-fold axis, while in the former no symmetry is imposed on the molecule. Data were all taken on the same crystal with Mo KR radiation to 2θ max of 70°at 293, 243, 163, 148, and 123 K. For the three higher temperatures the refinement of coordinates was straightforward in C2/c (Figure 1a) but not in Cc where the ratio of shifts with respect to error remained high, even though the R-factor was somewhat lower in Cc than in C2/c (0.045 and 0.055, respectively). The difference in R-factors is not significant, because the number of parameters in Cc is twice that in C2/c. An analysis of the thermal ellipsoids in C2/c at room temperature ( Figure 1a) did not agree with rigid body motion. The weighted agreement between observed and calculated components of the thermal ellipsoids is 0.22, while for a rigid motion this is expected to be less...

Semibullvalenes and Related Molecules: Ever Closer Approaches to Neutral Homoaromaticity

“…At ambient temperature there is accidental degeneracy of the equilibrating and otherwise nondegenerate tautomers of 21 which results in the apparent C 2 symmetry at this temperature. [35] The known [36] semibullvalene tetraester 23 (Scheme 2) appeared to be an ideal starting material for the synthesis of the dianhydride 22. The attempted synthesis of bisannelated semibullvalene 20 enes.…”

A High-Precision Carbon-13 Shift Thermometer for the Temperature Range 100–300 K

“…Strongly temperature-dependent chemical shifts in carbon-13 spectra have been reported for certain substituted semibullvalenes that exist as a pair of different valence tautomers equilibrating rapidly on the NMR time scale by nondegenerate Cope rearrangements (13)(14)(15) e.g., 1a and 1b. Based on the following considerations, it should be possible to design semibullvalenes of this type that could serve as secondary standards at very low temperatures.…”

“…Although several nondegenerate semibullvalenes meet these requirements for large temperature dependence of carbon-13 shifts (13)(14)(15), most of them still possess barriers to exchange which are too high, resulting in line broadening and the emerging of the spectra of the nonrearranging valence tautomers at temperatures well above 100 K. The known accelerating effect of suitably placed cyano groups on the degenerate Cope rearrangement of semibullvalene (20) has led us to conceive the pair of valence tautomers 1 ‫-(‬ 1a and 1b), which is also useful in another context (21). It is a stable, crystalline material (m.p.…”

Gas‐Phase Electron‐Diffraction Investigation and Quantum‐Chemical Calculations of the Structure of 1,5‐Dimethylsemibullvalene‐2,4,6,8‐tetracarboxylic Dianhydride

“…A promising candidate for demonstration of a homoaromatic ground state is the molecule 1,5-dimethylsemibullvalene-2,4,6,8-tetracarboxylic dianhydride [10] [11] (hereafter SBDA) diagrammed in Fig. 1.…”

“…± The sample of SBDA was synthesized [10] at the University of Idaho and sent to Oregon State University, where it was used without further purification in the electron-diffraction (GED) experiments. 2 ) These distances are symbolized as r a , the distance between average nuclear positions; r g , the thermal average distance; and r a , the parameter figuring in the scattered intensity.…”