Naoya Kanbayashi

Enantioselective Synthesis of Allylic Esters via Asymmetric Allylic Substitution with Metal Carboxylates Using Planar-Chiral Cyclopentadienyl Ruthenium Catalysts

An asymmetric allylic substitution with sodium carboxylate using a planar-chiral cyclopentadienyl ruthenium complex has been developed. Optically active allylic esters were prepared in good yields with high regio- and enantioselectivities.

Ruthenium‐Catalyzed Regio‐ and Enantioselective Allylic Substitution with Water: Direct Synthesis of Chiral Allylic Alcohols

Enantioselective allylic substitution catalyzed by transitionmetal complexes is an important process in organic synthesis. For many years, mainly palladium complexes that contain chiral ligands have been employed as efficient catalysts in these reactions. Recent studies have demonstrated that chiral catalysts based on other transition metals show different regioselectivity in the synthesis of branched allylic products via monosubstituted p-allyl intermediates. Although a variety of carbon and nitrogen nucleophiles can be used in those reactions, applicable oxygen nucleophiles are still limited to phenols and alcohols, which produce allylic ethers. Thus, enantioenriched branched allylic alcohols, which serve as useful chiral building blocks, are often synthesized by other processes, such as the hydrogenation of a,b-unsaturated ketones, the nucleophilic addition of vinylmetal reagents to aldehydes and ketones, and the kinetic resolution of racemic allylic alcohols. Recently, new ways to access these compounds have been developed, and they involve allylic substitution by a two-step conversion involving allylic esters and silyl ethers (Scheme 1; OPG= ester or silyl

Asymmetric Auto‐Tandem Catalysis with a Planar‐Chiral Ruthenium Complex: Sequential Allylic Amidation and Atom‐Transfer Radical Cyclization

The efficient synthesis of complex molecules with multiple stereogenic centers is a challenging task in synthetic organic chemistry. One-pot reactions have received considerable attention for the improvement of reaction efficiency, because they can avoid time-consuming workups and the often formidable isolation of intermediary products. A representative example is domino catalysis, in which two or more mechanistically similar reactions proceed in only one operation. 2] Another method, auto-tandem catalysis is also an ideal and eco-friendly synthetic process, which involves two or more mechanistically distinct reactions promoted by only a single catalyst. Despite numerous examples of domino catalysis, there are limited numbers of reports on autotandem catalysis, this is probably due to the difficulty of optimizing the reaction conditions. We have shown that planar-chiral cyclopentadienylruthenium (Cp’Ru) complex 1 is a proficient catalyst for asymmetric allylic substitutions. 6] Recently, we succeeded in the development of regioand enantioselective reactions of monosubstituted allylic halides with oxygen nucleophiles, which produced enantiomerically enriched branched allylic ethers, esters, and alcohols in good yields. These products possess a reactive terminal olefin, which can be potentially applied in a further transformation. 8] As the catalytic activity of 1 is preserved, even at the end of the allylic substitution, and ruthenium complexes show various desirable oxidation states for catalytic behavior, we conceived an extension of our system to auto-tandem asymmetric catalysis. As a candidate for the transformation of a terminal olefin on a branched allylic compound, we focused on the atomtransfer radical cyclization (ATRC) because half-sandwiched Ru complexes similar to 1 are known to promote this reaction. ATRC is an atom-economical method for the formation of cyclic compounds, which proceeds under mild conditions and exhibits broad functional group tolerance. Hence, it was hypothesized that complex 1 could realize an asymmetric auto-tandem catalysis consisting of allylic substitution and ATRC. To test this theory, we envisioned the facile synthesis of optically active g-lactams, an important structural motif found in a variety of biologically active molecules. Nagashima and co-workers reported that ATRC reaction of branched allylic amides using a Ru catalyst proceeded diastereoselectively, 13] with the configuration at the new stereogenic carbon controlled by the stereochemistry of the substrate. The preparation of optically active allylic amides by 1-catalyzed enantioselective allylic amidation would therefore provide g-lactams with multiple stereogenic centers in a pure form through 1-catalyzed ATRC (Scheme 1). Herein, we report

Post-polymerization modification of the side chain in optically active polymers by thiol–ene reaction

In this study, we examined novel synthetic methods for optically active polymers bearing various side chains from post-polymerization modification of a single optically active polymer. The side chain in the optically active polymer (poly-1) which was synthesized by asymmetric polymerization is modified using the thiol–ene reaction without racemization of the main chain. This transformation enables quantitative introduction of a wide range of functional groups, and the solubility of each resulting optically active polymer (poly-2) can be changed depending on the introduced substituent. The CD spectra of the modified polymer revealed the formation of a dynamic secondary structure through a new Cotton effect around 270 nm. In the experiments, the introduction of a dodecylthio group into the side chain restricted the flexibility of the main chain and suppressed the conformational fluctuations in the polymer to some extent. Moreover, when a hydroxyl group was introduced into the side chain, the Cotton effect was decreased, with the degree of intensity dependent on the solvent. Thus, the introduced substituents and polarity of the solvents influence the conformational fluctuations of the polymer.

Enantio- and diastereoselective polymerization: asymmetric allylic alkylation catalyzed by a planar-chiral Cp′Ru complex

In this study, we examined the regio-, diastereo-, and enantioselective polymerization using asymmetric allylic alkylation catalyzed by a planar-chiral cyclopentadienyl-ruthenium (Cp′Ru) complex ((S)-1). Achiral AB type monomers bearing 1,3-diketone and allylic chloride moieties were polymerized by (S)-1 with quantitative monomer conversion. The resulting polymer (3) was characterized by NMR analyses; the polymerization reaction catalyzed by (S)-1 proceeded with high regioselectivity, and the vicinal stereocenters of the monomer unit can be controlled. The polymer showed a Cotton effect at around 270 nm in its CD spectrum, which was ascribed to the bisignate coupling between the π–π* transition moment of each benzene chromophore. The results suggested that the polymerization proceeded in a stereoselective manner to give optically active polymers.

Polymerization based on alternating insertion of an isocyanide and alkyne into palladium–carbon bonds

The first polymerization system based on the alternating insertion of an isocyanide and alkyne into palladium–carbon bonds has been developed. An aryl isocyanide containing isocyanide and alkyne moieties in a single molecule was polymerized using chloro(methyl)bis(triphenylphosphine)palladium as a catalyst. The resulting polymer was characterized by IR and NMR spectroscopic analyses, which showed that the polymerization reaction proceeded through the intramolecular alternating insertion of an isocyanide and alkyne to produce indole units in the main chain.