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Hydrogen‐Bond‐Assisted Helical Folding of Propeller‐Shaped Molecules: Effects of Extended π‐Conjugation on Chiral Selection, Conformational Stability, and Exciton Coupling
Author(s) -
Opsitnick Elizabeth A.,
Jiang Xuan,
Hollenbeck Andrew N.,
Lee Dongwhan
Publication year - 2012
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201101351
Subject(s) - chemistry , chromophore , circular dichroism , steric effects , folding (dsp implementation) , hydrogen bond , molecule , crystallography , aryl , exciton , stereochemistry , alkyl , photochemistry , electrical engineering , engineering , organic chemistry , physics , quantum mechanics
Cooperative interaction between multiple chiral centers dictates the absolute handedness of structural folding. We have designed and prepared a series of chiral C 3 ‐symmetric tris( N ‐salicylidenamine) derivatives that adopt three‐blade propeller‐like conformations. Synthetic access to an expanded family of such constructs was aided by enzymatic resolution and C–C cross‐coupling reactions of aryl‐substituted chiral propargylic alcohol derivatives. These key structural components were integrated into molecular propellers of predetermined helical screw sense. Through comparative studies on a homologous set of molecules, we found that installation of phenylene‐ethynylene‐derived π‐conjugation profoundly affected the stabilities of the helically folded structures, as evidenced by UV/Vis and circular dichroism (CD) studies. Increasing the number of hydrogen bonds through additional substitution also enhanced the populations of the folded conformations in solution. In addition to introducing steric bias to control structural folding, linearly π‐conjugated groups function as spatially well‐defined chromophores that give rise to characteristic exciton‐coupled circular dichroism. Absolute configurations of chiral centers could thus be further confirmed by comparing the torsional relationships between pairs of chromophores on adjacent subunits, which are fully consistent with the computationally predicted structural models.