Corannulene 1 is an interesting aromatic compound because it is nonplanar, having a bowl shape. [6]helicene is an interesting aromatic compound because it is nonplanar, having the shape of a helix. Kato, Segawa, Scott and Itami have joined these together to synthesize the interesting quintuple helicene compound 3.¹

The optimized structure of 3 is shown in Figure 1. They utilized computations to corroborate two experimental findings. First, the NMR spectra of 3 shows a small number of signals indicating that the bowl inversion should be rapid. The molecule has C₅ symmetry due to the bowl shape of the corannulene core. Rapid inversion makes the molecule effectively D₅. (The inversion transition state is of D₅ symmetry, and would be a nice quiz question for those looking for molecules of unusual point groups.) The B3LYP/6-31G(d) computed bowl inversion barrier is only 1.9 kcal mol^-1, significantly less that the bowl inversion barrier of 1: 10.4 kcal mol^-1. This reduction is partly due to the shallower bowl depth of 3 (0.572 Å in the x-ray structure, 0.325 Å in the computed structure) than in 1 (0.87 Å).

Figure 1. Optimized structure of 3.

Second, they took the enhanced MMMMM-isomer and heated it to obtain the thermodynamic properties for the inversion to the PPPPP-isomer. (The PPPPP-isomer is shown in the top scheme.) The experimental values are ΔH^‡ = 36.8 kcal mol^-1, ΔS^‡ = 8.7 cal mol^-1 K^-1, and ΔG^‡ = 34.2 kcal mol^-1 at 298 K. They computed all of the stereoisomers of 3 along with the transition states connecting them. The largest barrier is found in going from MMMMM–3 to MMMMP–3 with a computed barrier of 34.5 kcal mol^-1, in nice agreement with experiment.

References

1. Kato, K.; Segawa, Y.; Scott, L. T.; Itami, K., "A Quintuple [6]Helicene with a Corannulene Core as a C₅-Symmetric Propeller-Shaped π-System." Angew. Chem. Int. Ed. 2018, 57, 1337-1341, DOI: 10.1002/anie.201711985.

InChIs

1: InChI=1S/C20H10/c1-2-12-5-6-14-9-10-15-8-7-13-4-3-11(1)16-17(12)19(14)20(15)18(13)16/h1-10H
InChIKey=VXRUJZQPKRBJKH-UHFFFAOYSA-N

2: InChI=1S/C26H16/c1-3-7-22-17(5-1)9-11-19-13-15-21-16-14-20-12-10-18-6-2-4-8-23(18)25(20)26(21)24(19)22/h1-16H
InChIKey=UOYPNWSDSPYOSN-UHFFFAOYSA-N

3: InChI=1S/C80H40/c1-11-31-51-41(21-1)42-22-2-12-32-52(42)62-61(51)71-63-53-33-13-3-23-43(53)44-24-4-14-34-54(44)64(63)73-67-57-37-17-7-27-47(57)48-28-8-18-38-58(48)68(67)75-70-60-40-20-10-30-50(60)49-29-9-19-39-59(49)69(70)74-66-56-36-16-6-26-46(56)45-25-5-15-35-55(45)65(66)72(62)77-76(71)78(73)80(75)79(74)77/h1-40H
InChIKey=XYUIBQJVZTYREY-UHFFFAOYSA-N