Chiroptical Signature and Absolute Configuration of Tröger's Base-Based Triangular Macrocycles

Chiral macrocycles are an attractive subject in supramolecular chemistry because of their proven relevance in appealing applications such as asymmetric catalysis, selective absorption, or chiral recognition. Taking into account the strong topological nature of these topics, the determination of the exact 3D structure of a chiral macrocycle becomes critical to developing all of its potential usages. In this paper, we have addressed the absolute configuration determination of a Tröger's base-based triangular macrocycle, 3MC, by means of vibrational and electronic chiroptical spectroscopy with the support of DFT quantum chemistry calculations. The molecular orbital topologies, together with a dipole moment vector analysis of the electronic transitions, showed the connection between the ground and the first electronic states by means of the vibrational modes that describe the quinoidization of the side biphenyl groups. DFT ab initio methods were employed to calculate the ROA spectra of the enantiomeric pair (+)-(S,R,R)-3MC and (-)-(R,S,S)-3MC. The agreement with the recorded spectra, together with the good fitting between the experimental and theoretical Circular Intensity Difference spectra, led us to unambiguously assess the AC of this macrocycle in solution.