Enantiopure Turbo Chirality Targets in Tri-Propeller Blades: Design, Asymmetric Synthesis, and Computational Analysis

Abstract

Enantiopure turbo chirality in small organic molecules, without other chiral elements, is a fascinating topic that has garnered significant interest within the chemical and materials science community. However, further research into and application of this concept have been severely limited by the lack of effective asymmetric tools. To date, only a few enantiomers of turbo chiral targets have been isolated, and these were obtained through physical separation using chiral HPLC, typically on milligram scales. In this work, we report the first asymmetric approach to enantiopure turbo chirality in the absence of other chiral elements such as central and axial chirality. This is demonstrated by assembling aromatic phosphine oxides, where three propeller-like groups are anchored to a P(O) center via three axes. Asymmetric induction was successfully carried out using a chiral sulfonimine auxiliary, with absolute configurations and conformations unambiguously determined by X-ray diffraction analysis. The resulting turbo frameworks exhibit three propellers arranged in either a clockwise (P,P,P) or counterclockwise (M,M,M) configuration. In these arrangements, the bulkier sides of the aromatic rings are oriented toward the oxygen atom of the P=O bond rather than in the opposite direction. Additionally, the orientational configuration is controlled by the sulfonimine auxiliary as well, showing that one of the Naph rings is pushed away from the auxiliary group (-CH₂-NHSO₂-tBu) of the phenyl ring. Computational studies were conducted on relative energies for the rotational barriers of a turbo target along the P=O axis and the transition pathway between two enantiomers, meeting our expectations. This work is expected to have a significant impact on the fields of chemistry, biomedicine, and materials science in the future.

Keywords: asymmetric synthesis; enantiopure turbo chirality; phosphine oxide; propeller blades; propeller chirality.

Figure 1

Turbo chirality in multilayer frameworks.

Figure 2

Enantiopure turbo chiral targets.

Scheme 1

Synthesis of the building block 1a.

Scheme 2

Synthesis of the building block 2a.

Scheme 3

Assembly of the final turbo target.

Figure 3

Turbo chirality represented by phosphine oxide target (4a).

Figure 4

Synthetic results of P(O)–turbo chiral targets I.

Figure 5

Synthetic results of P(O)–turbo chiral targets II.

Figure 6

Structures of the stationary points along the pathway (minima and transition states) connecting the two enantiomers. (A) The (a)-P,P,P enantiomer. (B) The (a)-M,M,M enantiomer. (C) The transition state. The C, N, O, S, and P atoms are colored in cyan, blue, red, yellow, and brown, respectively.