Syntheses and Properties of Enantiomerically Pure Higher (n ≥ 7) [n−2]Triangulanedimethanols and σ-[n]Helicenes

• Published in: Chemistry : a European journal 2006-07, Vol.12 (22), p.5697-5721
• Main Authors: de Meijere, Armin, Khlebnikov, Alexander F., Kozhushkov, Sergei I., Yufit, Dmitrii S., Chetina, Olga V., Howard, Judith A. K., Kurahashi, Takuya, Miyazawa, Kazutoshi, Frank, Daniel, Schreiner, Peter R., Rinderspacher, B. Christopher, Fujisawa, Mari, Yamamoto, Chiyo, Okamoto, Yoshio
• Format: Article
• Language: English
• Subjects: chirality; helical structures; optical rotations; self-assembly; small ring systems
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200600111

(P)‐(+)‐Hexaspiro[2.0.0.0. 0.0.2.1.1.1.1.1]pentadecane [(P)‐17] as well as (M)‐(−)‐ and (P)‐(+)‐octaspiro[2.0.0.0.0.0.0.0.2.1.1.1.1.1.1.1]nonadecanes [(M)‐ and (P)‐25]—enantiomerically pure unbranched [7]‐ and [9]triangulanes—have been prepared starting from racemic THP‐protected (methylenecyclopropyl)methanol 6. The relative configurations of all important intermediates as well as the absolute configurations of the key intermediates were established by X‐ray crystal structure analyses. This new convergent approach to enantiomerically pure linear [n]triangulanes for n=7, 9 was also tested in two variants towards [15]triangulane. Some of the most prominent and unexpected features of the newly prepared compounds are the remarkable modes of self‐assembly of the diols (P)‐14, (E)‐(3S,3′S,4S,4′S,5R,5′R)‐21, (P)‐(+)‐22, and (E)‐31 in the solid state through frameworks of intermolecular hydrogen bonds leading to, depending on the respective structure, nanotube‐ [(P)‐14, (P)‐(+)‐22, and (E)‐31], honeycomb‐like structures [(E)‐(3S,3′S,4S,4′S,5R,5′R)‐21] or a supramolecular double helix [(P)‐(+)‐ and (M)‐(−)‐22]. Liquid crystalline properties of the esters and ethers of the diols (P)‐14, (P)‐, and (M)‐22 have also been tested. Although all of these [n]triangulanes have no chromophore which would lead to significant absorptions above 200 nm, they exhibit surprisingly high specific rotations even at 589 nm with [α]${{{20\hfill \atop {\rm D}\hfill}}}$=+672.9 (c=0.814 in CHCl3) for (P)‐(+)‐17, +909.9 (c=0.96 in CHCl3) for (P)‐(+)‐25, −890.5 (c=1.01 in CHCl3) for (M)‐(−)‐25, and −1302.5 (c=0.36 in CHCl3) for (M)‐(−)‐39, and the specific rotations increase drastically on going to shorter wavelengths. This outstanding rotatory power is in line with their rather rigid helical arrangement of σ bonds, and accordingly these helically shaped unbranched [n]triangulanes may be termed “σ‐[n]helicenes”, as they represent the σ‐bond analogues of the aromatic π‐[n]helicenes. Density functional theory (DFT) computations at the B3 LYP/6‐31+G(d,p) level of theory for the geometry optimization and time‐dependent DFT for determining optical rotations with a triplet‐ζ basis set (B3 LYP/TZVP) reproduce the optical rotatory dispersions (ORD) very well for the lower members (n=4, 5) of the σ‐[n]helicenes. For the higher ones (n=7, 9, 15) the computed specific rotations turn out increasingly larger than the experimental values. The remarkable increase of the specific rotation with an increasin