Steric Communication of Chiral Information Observed in Dendronized Polyacetylenes

• Published in: Journal of the American Chemical Society Vol. 128; no. 50; pp. 16365 - 16372
• Main Authors: Percec, Virgil, Aqad, Emad, Peterca, Mihai, Rudick, Jonathan G, Lemon, Lance, Ronda, Juan C, De, Binod B, Heiney, Paul A, Meijer, E. W
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 20-12-2006
• Subjects: Alicyclic compounds; Alicyclic compounds, terpenoids, prostaglandins, steroids; Chemistry; Exact sciences and technology; General and physical chemistry; Organic chemistry; Preparations and properties; Terpenoids; Cyclohexane derivatives; Acetylenic compound; Distortion; Chiral compound; Branched compound; Conformational changes; Hexagonal crystals; Polyynic compound; Steric effect; Circular dichroism spectrometry; Helical structure; Chemical synthesis; Terpenoid; Polymerization; Polymer; Free energy; Circular dichroism; Acetylene derivatives; Acetylene polymer; Side chain; Macromolecule; Monoterpene; Comparative study; Dendritic structure; Conformation
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0665848

Structural and retrostructural analysis of helical dendronized polyacetylenes (i.e., self-organizable polyacetylenes containing first generation dendrons or minidendrons as side groups) synthesized by the polymerization of minidendritic acetylenes with [Rh(nbd)Cl]2 (nbd = 2,5-norbornadiene) reveals an ∼10% change in the average column stratum thickness (l) of the cylindrical macromolecules with a chiral periphery, through which a strong preference for a single-handed screw-sense is communicated. The cylindrical macromolecules reversibly interconvert between a three-dimensional (3D) centered rectangular lattice (Φr - c,k) exhibiting long-range intracolumnar helical order at lower temperatures and a two-dimensional (2D) hexagonal columnar lattice (Φh) with short-range helical order at higher temperatures. A polymer containing chiral, nonracemic peripheral alkyl tails is found to have a larger l as compared to the achiral polymers. In methyl cyclohexane solution, the same polymer exhibits an intense signal in circular dichroism (CD) spectra, whose intensity decreases upon heating. The observed change in l indicates that the chiral tails alter the polymer conformation from that of the corresponding polymer with achiral side chains. This change in conformation results in a relatively large free energy difference (ΔG h) favoring one helix-sense over the other (per monomer residue). The capacity to distort the polymer conformation and corresponding free energy is related to the population of branches in the chiral tails and their distance from the polymer backbone by comparison to recently reported first and second generation dendronized polyphenylacetylenes.