Majority‐Rules‐Type Helical Poly(quinoxaline‐2,3‐diyl)s as Highly Efficient Chirality‐Amplification Systems for Asymmetric Catalysis

Abstract

Ahighly efficient majority-rules effect of poly(quinoxaline-2,3-diyl)s (PQXs) bearing 2-butoxymethyl chiral side chains at the 6-and 7-positions was established and attributed to large DG h values (0.22-0.41 kJ mol À1 ), which are defined as the energy difference between P-and M-helical conformations per chiral unit. APQX copolymer prepared from amonomer derived from (R)-2-octanol (23 %ee) and amonomer bearing aP Ph 2 group adopted as ingle-handed helical structure (> 99 %) and could be used as ah ighly enantioselective chiral ligand in palladium-catalyzed asymmetric reactions (products formed with up to 94 %ee), in which the enantioselectivity could be switched by solvent-dependent inversion of the helical PQX backbone.Helical majority-rules-type polymers, [1,2] which are derived from chiral monomers with low ee values and adopt nonlinearly enhanced single-handed helical conformations,a re among the most promising platforms for the development of new asymmetric-amplification systems.[3] Thec oncept of the enantioselective synthesis of chiral compounds through asymmetric amplification [4] has attracted considerable interest, as it may afford useful model systems for the elucidation of the origin of natural homochirality [5] as well as practical methods for the generation of optically active molecules. [6] Although an umber of helical polymers [7] with efficient asymmetric amplification in terms of helix-sense induction have been developed, the highly enantioselective production of chiral compounds on the basis of asymmetric amplification by majority-rules-type helical polymers has not been reported. According to the theory developed by Green and co-workers for polymers containing helix-sense-reversal conformations (Green theory,c ase 3), [8] them aximum value of the screw-sense excess (se)ofahelical polymer,whose degree of polymerization (DP) is sufficiently large,islimited by DG h (the energy difference between P-a nd M-helical conformations) and DG r (the energy difference of the helix-sensereversal conformation relative to the helical conformation). Formost helical polymers,the DG h and/or DG r values are too small to induce ap urely single handed screw sense.Acistransoidal poly(aryl acetylene) bearing (R)-(1-phenylethyl)-carbamate side chains (DG h = 0.0048 kJ mol À1, DG r = 15.5 kJ mol À1 ), for example,exhibits even at very high degrees of polymerization merely 49.5 % se at room temperature. [9] Forthe enantioselective generation of chiral compounds from am ajority-rules-type helical polymer,t he use of ap olymer backbone exhibiting large DG h and DG r values is therefore of critical importance.Recently,w er eported sergeants-and-soldiers-type [10] poly(quinoxaline-2,3-diyl)s (PQXs) containing (R)-2-butoxymethyl and propoxymethyl groups in arandom manner.[11] A DG h value of 0.59 kJ mol À1 was estimated for the chiral unit of this polymer system, thus indicating that 22 chiral units are required for as crew-sense induction of 99.0 % se.W eh ave also reported that single-handed helically chiral PQ...

Fifty Years of Hydrosilylation in Polymer Science: A Review of Current Trends of Low-Cost Transition-Metal and Metal-Free Catalysts, Non-Thermally Triggered Hydrosilylation Reactions, and Industrial Applications

“…Using very low catalyst loadings, silylated products were obtained in very high yields and enantioselectivities ( Figure 36); the catalysts could be used in eight consecutive catalytic cycles without any loss of activity or selectivity. The same group reported another copolymer based on poly(quinoxaline-2,3-diyls) and employed it in a switchable asymmetric hydrosilylation reaction [104]. However, unlike the previous copolymer which was synthesized from a chiral monomer, the novel copolymer was prepared from a monomer derived from (R)-octan-2-ol (23% ee) and a monomer bearing a PPh 2 group that adopted a single-handed helical structure.…”

Asymmetric Catalysis in Chiral Solvents: Chirality Transfer with Amplification of Homochirality through a Helical Macromolecular Scaffold

“…This plot clearly indicated that (R)-limonene with 65% ee is enough to induce an exclusively homochiral right-handed helical structure in 5(1000). 23,25 Asymmetric Reaction in Chiral Solvents. We then prepared the random co-1000mers L1 and L2, which contain on average 10 and 50 coordination units, respectively, with 2-(diphenylphosphino)phenyl pendants, by living copolymerization for their use in asymmetric Suzuki-Miyaura coupling (SMC) reactions.…”

Double‐Gyroid Nanostructure Formation by Aggregation‐Induced Atropisomerization and Co‐Assembly of Ionic Liquid‐Crystalline Amphiphiles

“…This concept constitutes a new moleculardesign principle, not only for developing bicontinuous cubic liquid crystals, but also to obtain functional small molecules and polymers that self-organize into helical supramolecular assemblies. [24,25] For instance, the architecture of salts 1 n -X can be used to design new chiral ionic liquids, which have attracted attention as media for asymmetric organic synthesis and chiral chromatography. [26]…”