ipso-Arylative Ring-Opening Polymerization as a Route to Electron-Deficient Conjugated Polymers

Pd-Catalyzed Ring-Opening Polymerization of Cyclobutanols through C(sp3)-C(sp3) Bond Cleavage

A new approach to ring-opening polymerization (ROP) based on C(sp3)-C(sp3) bond cleavage is reported. This process is based on the ability of Pd to promote both the β-carbon elimination of a bifunctional cyclobutanol precursor and the C-C coupling process with the resulting Pd-alkyl intermediate. Consequently, novel polyketone materials are obtained. Owing to the modular synthesis of the used cyclobutanol monomers, the present ROP reaction allows the introduction of substitution patterns in the polymeric chain that are not accessible by current polyketone synthesis methodologies. We have explored in detail the initiation, propagation, and termination steps of this new polymerization process.

Transition Metal-Catalyzed Biaryl Atropisomer Synthesis via a Torsional Strain Promoted Ring-Opening Reaction

ConspectusArising from the restricted rotation of a single bond caused by steric or electronic effects, atropisomerism is one of the few fundamental categories for molecules to manifest their three-dimensional characters into which axially chiral biaryl compounds fall. Despite the widespread occurrence of axially chiral skeletons in natural products, bioactive molecules, and chiral ligands/organocatalysts, catalytic asymmetric methods for the synthesis of these structures still lag behind demand. Major challenges for the preparation of these chiral biaryls include accessing highly sterically hindered variants while controlling the stereoselectivity. A couple of useful strategies have emerged for the direct asymmetric synthesis of these molecules in the last two decades.Recently, we have engaged in catalytic asymmetric synthesis of biaryl atropisomers via transition metal catalysis, including asymmetric ring-openings of dibenzo cyclic compounds. During these studies, we serendipitously discovered that the two substituents adjacent to the axis cause these dibenzo cyclic molecules to be distorted to minimize steric repulsion. The distorted compounds display higher reactivity in the ring-opening reactions than the non-distorted molecules. In other words, torsional strain can promote a ring-opening reaction. On the basis of this concept, we have successfully realized the catalytic asymmetric ring-opening reaction of cyclic diaryliodoniums, dibenzo silanes, and 9H-fluoren-9-ols, which delivered several differently substituted ortho tetra-substituted biaryl atropisomers in high enantioselectivity. The torsional strain not only activates the substrates toward ring-opening under mild conditions but also changes the chemoselectivity of bond-breaking events. In the palladium-catalyzed carboxylation of S-aryl dibenzothiophenium, the torsional strain inversed the bond selectivity from exocyclic C-S bond cleavage to the ring-opening reaction.In this Account, we summarize our studies on copper-, rhodium-, or palladium-catalyzed asymmetric ring-opening reactions of dibenzo cyclic compounds as a useful collection of methods for the straightforward preparation of ortho tetra-substituted biaryl atropisomers with high enantiopurity on the basis of the above-mentioned torsional strain-promoted ring-opening coupling strategy. In the last part, the torsional strain energies are also discussed with the aid of density functional theory (DFT) calculations.

Atroposelective Kinetic Resolution of 8H-Indeno[1,2-c]thiophen-8-ols via Pd-Catalyzed C-C Bond Cleavage Reaction

The present work demonstrates a palladium-catalyzed kinetic resolution/ring-opening reaction of 8H-indeno[1,2-c]thiophen-8-ols. The reaction proceeds in a highly regioselective manner, and both optically active thiophene-phenyl atropisomers and stereogenic 8H-indeno[1,2-c]thiophen-8-ols were obtained with high enantiomeric excesses. The synthetic applications of the obtained thiophenyl atropisomers were briefly investigated.