Chiral Induction from Allenes into Twisted 1,1,4,4-Tetracyanobuta-1,3-dienes (TCBDs): Conformational Assignment by Circular Dichroism Spectroscopy

The chiral induction from allenes into twisted 1,1,4,4-Tetracyanobuta-1,3- dienes (TCBDs) using circular dichroism (CD) spectroscopy is examined. CD spectroscopy revealed a chiral induction from the chiral allene moiety into the sterically crowded TCBD chromophores. The PES was scanned for compound (M)-3, bearing two TCBD moieties, by systematic variation of the torsion angle in the TCBDs with all possible combinations every 60°, including non-symmetric conformers, but excluding those which were severely sterically hindered. A quantum-mechanical method was applied to confirm the AC of (M)-2, which involved determining the electronic CD curve for comparison with the experimental one. The results show that the exciton couplet in allene (M)-2 is in agreement with the absolute configuration (AC) determined previously by X-ray crystallographic analysis.