Kiyotaka Onitsuka

Convergent Route to Organometallic Dendrimers Composed of Platinum–Acetylide Units

The third-generation dendrimer with 45 platinum atoms (shown schematically) was synthesized efficiently by a convergent methodology in which two different trialkylsilyl groups (symbolized by the nodal points) are used to protect the bridging triethynylbenzene derivatives. PT=[Pt(PEt3 )2 ].

Enantiomer-Selective and Helix-Sense-Selective Living Block Copolymerization of Isocyanide Enantiomers Initiated by Single-Handed Helical Poly(phenyl isocyanide)s

Rigid-rodlike right (P)- and left (M)-handed helical polyisocyanides (P-poly-L-1 and M-poly-L-1) prepared by the living polymerization of an enantiomerically pure phenyl isocyanide bearing an L-alanine pendant with a long n-decyl chain (L-1) with the mu-ethynediyl Pt-Pd catalyst were found to block copolymerize L-1 and D-1 in a highly enantiomer-selective manner while maintaining narrow molecular weight distributions. The M-poly-L-1 preferentially copolymerized L-1 over the antipode D-1 by a factor of 6.4-7.7, whereas the D-1 was preferentially copolymerized with P-poly-L-1 composed of the same L-1 units, but possessing the opposite helicity by a factor of 4.0. Circular dichroism and high-resolution atomic force microscopy revealed that the enantiomer-selective block copolymerizations proceed in an extremely high helix-sense-selective fashion, and the preformed helical handedness determines the overall helical sense of the polyisocyanides irrespective of the configuration of the monomer units of the initiators during the block copolymerizations. The block copolymers are rigid-rod helical polymers with a narrow molecular weight distribution and exhibit a lyotropic smectic liquid crystalline phase.

Regio‐ and Enantioselective O‐Allylation of Phenol and Alcohol Catalyzed by a Planar‐Chiral Cyclopentadienyl Ruthenium Complex

Optically active allylic aryl ethers have high potential for use as precursors of biologically active organic molecules. Whereas stereospecific allylation of phenol derivatives has been demonstrated with transition-metal catalysts, the enantioselective version catalyzed by chiral palladium complexes has been successfully applied to the synthesis of natural products. However, compared to the myriad reports on enantioselective allylic alkylation and amination, there has been little research on enantioselective allylic substitutions with alcohol and phenol. Recently, an efficient Ir-catalyzed system has been reported to give allyl aryl ethers as well as allyl silyl ethers with high regioand enantioselectivities. Although Ru complexes with chiral bisoxazoline ligands also show catalytic activity towards asymmetric allylic substitution with oxygen nucleophiles, the regioand enantioselectivities are not very high. We have been investigating the syntheses and stereoselective reactions of planar-chiral cyclopentadienyl (Cp’; see Eq. (1) for ligand) ruthenium complexes. Previously, we reported the first example of Ru-catalyzed asymmetric allylic amination and alkylation of symmetrically substituted allyl carbonates using planar-chiral Cp’Ru complexes 1. We describe herein the Ru-catalyzed reaction of unsymmetrically substituted allyl halides with phenol and alcohol to give ethers with high regioand enantioselectivities. To optimize the conditions, we chose the reaction of cinnamyl chloride (2a, LG=Cl) with o-cresol (3a) [Eq. (1)]. After careful examination, we found that the reaction of 2a (2.0 equiv) with 3a was effectively catalyzed by 3 mol% (S)1a in THFat 3 8C in the presence of K2CO3 (3.0 equiv) to give the branched ether 4a with R configuration in 92% yield and 95% ee, along with a trace amount of the linear ether 5a. The proper selection of cinnamyl derivatives and Ru catalyst was essential to achieve high selectivity (Table 1). Although methyland phenyl-substituted planar-chiral Cp’Ru complexes (S)-1b and (S)-1c also catalyzed the reaction with high regioselectivity, the enantioselectivities were lower than that obtained with (S)-1a (Table 1, entries 1–3). The substituent at the 4-position on the Cp’ ring has a similarly large effect on the enantioselectivity in the allylic aminations and alkylations. The reaction of cinnamyl bromide (2b) also produced 4a in good yield, albeit with slightly lower enantioselectivity than the reaction of 2a (Table 1, entry 4). When cinnamyl phosphate (2c) or cinnamyl carbonate (2d) was used as a substrate, a mixture of 4a with low enantioselectivities and 5a was obtained in good yield (Table 1, entries 5 and 6). Cinnamyl acetate (2e) was not suitable for the present reaction owing to its low reactivity (Table 1, entry 7). The scope of the present O-allylation catalyzed by (S)-1a under the optimized conditions was examined [Eq. (2)], and the results are summarized in Table 2. The reactions of 2a with various phenol derivatives 3 selectively produced the corresponding branched ethers 4 in good yields with more than 90% ee even when the phenol has a bulky substituent at Table 1: Reaction of cinnamyl derivatives 2 with o-cresol (3a).

PdII/PdIV Catalytic Enantioselective Synthesis of Bicyclo[3.1.0]hexanes via Oxidative Cyclization of Enynes

The first asymmetric Pd(II)/Pd(IV) catalysis has been achieved by employing the combination of a hypervalent iodine reagent and a chiral ligand, SPRIX. Enantioselective cyclization of enyne derivatives catalyzed by the Pd-i-Pr-SPRIX complex furnished lactones bearing a bicyclo[3.1.0]hexane skeleton with up to 95% ee.

A novel route to 2,3-disubstituted indoles via palladium-catalyzed three-component coupling of aryl iodide, o-alkenylphenyl isocyanide and amine

Three-component coupling reactions of aryl iodide, o-alkenylphenyl isocyanide and amine in the presence of palladium catalysts produced 2,3-substituted indoles in moderate yields.

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.

Screw-Sense-Selective Polymerization of Aryl Isocyanides Initiated by a Pd-Ptμ-Ethynediyl Dinuclear Complex: A Novel Method for the Synthesis of Single-Handed Helical Poly(isocyanide)s with the Block Copolymerization Technique

Living polymerization of chiral aryl isocyanides, such as m- and p-menthoxycarbonylphenyl isocyanides 2 and 5, initiated by the Pd-Pt mu-ethynediyl dinuclear complex 1, proceeds with a high screw-sense selectivity to give the poly(isocyanide)s 3 and 6, which exhibit a large specific rotation and an intense CD band at lambda = 364 nm as a consequence of a helical chirality. The molar optical rotation and molar circular dichroism of the resulting polymers 3 and 6 reach a constant value at a degree of polymerization (Pn) of more than 30. Screw-sense-selective polymerization of achiral aryl isocyanides that bear very bulky substituents, such as 3,5-di(propoxycarbonyl)phenyl isocyanide (11), 3,5-di(butoxycarbonyl)phenyl isocyanide (13), and 3,5-di(cyclohexyloxycarbonyl)phenyl isocyanide (15), is achieved by the use of chiral oligomer complexes 3(30) and 6(30), prepared from the reaction of 1 with 30 equivalents of 2 or 5, as an initiator to give predominantly single-handed helical polymers. In contrast, smaller aryl isocyanides are also polymerized by 3(30) and 6(30) with screw-sense selectivity in the initial stage of the reaction, but the single-handed helix is not preserved up to high molecular weight. Kinetic studies of the polymerization of (L)- and (D)-2, or (L)- and (D)-5 with chiral oligomer complexes (L)-3(50) or (L)-6(100) suggests that the screw sense of the polymer backbone is not controlled kinetically, but rather that the thermodynamically stable screw sense is produced.

Enantioselective intramolecular oxidative aminocarbonylation of alkenylureas catalyzed by palladium-spiro bis(isoxazoline) complexes.

An enantioselective synthesis of tetrahydropyrrolo[1,2-c]pyrimidine-1,3-diones via a palladium-catalyzed intramolecular oxidative aminocarbonylation is described. The carbon-carbon double bond of alkenylurea substrates has been shown to react intramolecularly with a nitrogen nucleophile in the presence of a palladium catalyst under a carbon monoxide atmosphere. The use of a chiral spiro bis(isoxazoline) ligand (SPRIX) is essential to obtain the desired products in optically active forms. In comparison with the coordination ability of other known ligands, this peculiar character of SPRIX originates from two structural characteristics: low sigma-donor ability of the isoxazoline coordination site and rigidity of the spiro skeleton.

Synthesis of organometallic dendrimers with a backbone composed of platinum-acetylide units

Abstract Novel organometallic dendrimers containing platinum-acetylide units in the main chain have been synthesized up to a henicosanuclear complex (12) using triethynylmesitylene (4) as a bridging ligand. The molecular structure of trinuclear complex (5b), which is a model core of the dendrimer, has been established by an X-ray analysis.

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