Coaxial helices in chiral supramolecular aggregates from highly hindered chiral allenes

Abstract

Chiral allenes self-assembly following a cooperative mechanism into a supramolecular chiral aggregate consisting of two coaxial helices: the internal helix described by the allene stack and the external helix which consist in a 4-helix described by the four allene substituents. More precisely, this supramolecular aggregate possesses six axially chiral elements within its structure—the allene, the allene stack (internal helix) and the stacks of the four allene substituents (external 4-helix)—. Interestingly, slight variations in the magnitude of the tilting degree while keeping its P- or M- orientation (internal helix) can vary the orientation of the 4-axial motifs at the external helix. Thus, while (P)-1 produces a supramolecular helix with a Θ ca. 15° (Pint) and a M1/P2/M1’/P2’ orientation of the four axial motifs at the periphery, (P)-2 produces a supramolecular helix with a Θ ca. 23° (Pint) and a P1/P2/P1’/P2’ orientation of the four axial motifs at the external helix. As a result, the ECD spectra and the AFM images of the (P)-1 and (P)-2 supramolecular aggregates dominated by the 1 and 1’ substituents of the chiral allene indicate opposite handedness although the chirality of the building block and the orientation of the allene stack are the same