Synthesis of Single‐Handed Helical Spiro‐Conjugated Ladder Polymers through Quantitative and Chemoselective Cyclizations**

Abstract

We report the unprecedented synthesis of onehanded helical spiro-conjugated ladder polymers with welldefined primary and secondary structures, in which the spirolinked dibenzo [a,h]anthracene fluorophores are arranged in a one-handed twisting direction, through quantitative and chemoselective acid-promoted intramolecular cyclizations of random-coil precursor polymers composed of chiral 1,1'spirobiindane and achiral bis[2-(4-alkoxyphenyl)ethynyl]phenylene units. Intense circular dichroism (CD) and circularly polarized luminescence (CPL) were observed, whereas the precursor polymers exhibited negligible CD and CPL activities. The introduction of 2,6-dimethyl substituents on the 4alkoxyphenylethynyl pendants is of key importance for this simple, quantitative, and chemoselective cyclization. This strategy is applicable to the defect-free precise synthesis of other varieties of fully p-conjugated molecules and coplanar ladder polymers that have not been achieved before.

Circularly Polarized Light Probes Excited‐State Delocalization in Rectangular Ladder‐type Pentaphenyl Helices

“…Such a helix-related amplification of the CD response with increasing chain length has previously been found for polypeptides, [14] and concluded from the influence of the molecular weight on g abs for so-called artificial chiral ladders. [7,8] There is no significant influence of temperature and concentration on the effects described (see Figure S23 in the Supporting Information). The perpendicular arrangement of constituent chromophores enforced by the spiro linkages suppresses aggregation of polymer chains.…”

Consecutively fused single‐, double‐, and triple‐expanded helicenes

“…With the above chiral chromatographic results in hand, we then performed trifluoroacetic acid‐promoted intramolecular alkyne benzannulations 54 of the precursors 1 – 3 existing as a few or a number of interconvertible meso ‐ and racemo ‐diastereomers to obtain ExH‐ C , ExH‐ S , and ExH‐ M , respectively (Figure 2a). The four‐, six‐, and eight‐fold intramolecular cyclizations of 1 – 3 completely proceeded, thus quantitatively producing the expanded helicenes without any byproducts, as supported by their simple 1 H NMR spectra and IR analyses (see below) (Figures 2bii, biv, bvi, and S5).…”

“…2c(i), (iii), (v), and S1). 54,71 The resulting chemoselectivity may arise from the steric effect of the 2,6-dimethyl substituents on the 4alkoxyphenylethynyl pendants. 54 The incorporated expanded helicene subunits in the DFT-calculated structures of (M,M)-ExH-S and (M,M,M)-ExH-M display similar helically twisted conformations with the angles between the two planes of the rings A and K (θ AK = 28.5-32.1 • ) and/or those of G and Q (θ GQ = 26.4 • ) (Figure 4) that are also comparable to the angle θ AK ( = 25.0 • ) of (M)-ExH-C in the single crystal (Figure 3a(ii)).…”

“…As a result, we, for the first time, succeeded in the perfect diastereospecific synthesis of the homochiral (all‐ P‐ or all‐ M ) consecutively fused double (ExH‐ S ) and triple (ExH‐ M ) expanded helicenes, which were exclusively produced among a number of diastereomers through quantitative and chemoselective acid‐catalyzed diastereospecific multistep cascade cyclizations of the precursors 2 and 3 even composed of a large number of complicated dynamic stereoisomers, respectively (for a multiple helicene synthesis via the gold(I)‐catalyzed cascade cyclizations with moderate to excellent diastereoselectivities, see refs 63, 69, and 70), without any side reactions, including rearrangements of aryl pendants (Figures 2c(i), (iii), (v), and S1). 54,71 The resulting chemoselectivity may arise from the steric effect of the 2,6‐dimethyl substituents on the 4‐alkoxyphenylethynyl pendants 54 . The incorporated expanded helicene subunits in the DFT‐calculated structures of ( M , M )‐ExH‐ S and ( M , M , M )‐ExH‐ M display similar helically twisted conformations with the angles between the two planes of the rings A and K ( θ AK = 28.5–32.1°) and/or those of G and Q ( θ GQ = 26.4°) (Figure 4) that are also comparable to the angle θ AK ( = 25.0 °) of ( M )‐ExH‐ C in the single crystal (Figure 3a(ii)).…”

“…52 We recently reported the first defect-free synthesis of one-handed helical spiro-conjugated ladder polymers 53,54 as well as coplanar fully conjugated ladder polymers 54 by the quantitative and chemoselective acid-promoted alkyne benzannulations of random-coil precursor polymers, [53][54][55][56] assisted by introducing the 2,6-dimethyl substituents on the phenyl pendant groups. 54 We now report the unprecedented acid-catalyzed quantitative, chemoselective, and diastereospecific formation of a series of defect-free single (ExH-C), double (ExH-S), and triple (ExH-M) expanded helicenes with C-, S-, and M-shaped fully conjugated helical ladder structures, respectively, from anthracene-based cyclization precursors (1-3) (Figures 1b and 2a) (for the S-and Mshaped homochiral double carbohelicenes, see refs. [57][58][59][60][61][62][63][64][65][66][67].…”