Ambimodal Pericyclic Rearrangements of Dialkenyl-Bullvalenes Give Tetrahydro-1,8-ethenoheptalenes

Abstract

The fluxional structure of bullvalene is expanded by the discovery of a [5,5]-sigmatropic rearrangement of dialkenyl substituted derivatives. This gives rise to tetrahydro-1,8-ethenoheptalenes (THEH), representing the first examples of this tricyclic scaffold. Variation of the substitution pattern alters the product distribution, including one thermodynamically balanced between THEH and bullvalene isomers. DFT calculations are used to explore the thermodynamic landscape and reaction mechanism revealing a pretransition state bifurcation leading to a concerted ambimodal rearrangement pathway.

The Journey to Fluorescent Bullvalenes: Pitfalls and Prospects

“…Equipment NMR spectra were recorded on a Jeol JNM-ECZR ( 1 H: 500 MHz, 13 C: 125 MHz) or on a Varian V NMR-S 600 ( 1 H: 600 MHz, 13 C: 150 MHz). The chemical shifts (δ) are given in ppm and are referenced relative to tetramethylsilane (TMS, 1 H: δ = 0 ppm, 13 C: δ = 0 ppm) or the respective residual solvent signals (acetone-d 5 , 1 H: δ = 2.05 ppm, 13 C: δ = 29.8 ppm; CHCl 3 , 1 H: δ = 7.27 ppm, 13 C: δ = 77.2 ppm). For the structural assignment of the 1 H-NMR and 13 C-NMR data of new compounds they are classified as cPr (cyclopropyl position), -CH < (methanetriyl proton) CH Ar (aromatic carbon atom that is attached to a proton), C qAr (quaternary aromatic carbon atom) and C Ar (inconclusive to distinguish between CH Ar and C qAr ).…”

“…Equipment NMR spectra were recorded on a Jeol JNM-ECZR ( 1 H: 500 MHz, 13 C: 125 MHz) or on a Varian V NMR-S 600 ( 1 H: 600 MHz, 13 C: 150 MHz). The chemical shifts (δ) are given in ppm and are referenced relative to tetramethylsilane (TMS, 1 H: δ = 0 ppm, 13 C: δ = 0 ppm) or the respective residual solvent signals (acetone-d 5 , 1 H: δ = 2.05 ppm, 13 C: δ = 29.8 ppm; CHCl 3 , 1 H: δ = 7.27 ppm, 13 C: δ = 77.2 ppm).…”

“…Hence, it has been demonstrated that among the 15 possible isomers of disubstituted derivatives, some are more frequently observed (Scheme 4C). [6,8,9,[10][11][12][13] Specifically, disubstituted derivatives such as the bishydroxymethylbullvalene (3 b) [6] and the bispropargylbullvalene (9) [9] estab-lish an evenly distributed isomeric ratio with nine main interconverting isomers, whereas aryl-substituted bullvalenes predominantly form only four isomers at low temperatures (Scheme 4C, cf. SI).…”

“…[6] With this synthetic route, a large series of compounds has been synthesized and investigated, for example, with respect to their interconverting networks, their supramolecular association with cyclodextrins, metal ions, bacteria, and proteins. [6][7][8][9][10][11][12][13][14] Recently, bullvalene (3 a) has regained much interest because of its potential to serve as a suitable fluxional platform for the development of adaptive sensor arrays or dynamic supramolecular structures. [1,7,8,9,15,16] Specifically, Bode and coworkers have introduced several different approaches for the application of adaptive host-guest interactions of substituted bullvalones and bullvalenes for the detection of analytes, such as fullerenes, catechols, or carbohydrates by means of 13 Clabelling and NMR-spectroscopic analysis as well as by CD-and differential absorption spectroscopy (Figure 1).…”

“…[6][7][8][9][10][11][12][13][14] Recently, bullvalene (3 a) has regained much interest because of its potential to serve as a suitable fluxional platform for the development of adaptive sensor arrays or dynamic supramolecular structures. [1,7,8,9,15,16] Specifically, Bode and coworkers have introduced several different approaches for the application of adaptive host-guest interactions of substituted bullvalones and bullvalenes for the detection of analytes, such as fullerenes, catechols, or carbohydrates by means of 13 Clabelling and NMR-spectroscopic analysis as well as by CD-and differential absorption spectroscopy (Figure 1). [17][18][19][20][21][22] But although the bullvalene structure has been established successfully as dynamic, adaptive scaffold in these molecular chemosensors, none of the employed sensing approaches is based on fluorescence as an output signal, so far.…”