Photoinduced formation of an azobenzene-based CD-active supramolecular cyclic dimer.

A series of new photo-responsive amino acid-derived azobenzenedicarboxylic acid derivatives (S)-1 a-e were synthesized. Compound (S)-1 a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)-1 a'] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)-1 a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis-azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)-4, an analogue of (S)-1 b, confirmed the formation of a dimer. A theoretical CD study revealed that (S)-1 a in the cis form should be present as a cyclic chiral dimer.

Synthesis and helical structures of poly(ω-alkynamide)s having chiral side chains: effect of solvent on their screw-sense inversion.

New ω-alkynamides, (S)-HC≡CCH2CONHCH2CH(CH3)CH2CH3 (1) and (S)-HC≡CCH2CH2CONHCH(CH3)CH2CH2CH2CH2CH3 (2) were synthesized and polymerized with a rhodium catalyst in CHCl3 to obtain cis-stereoregular poly(ω-alkynamide)s (poly(1) and poly(2)). Polarimetric, CD, and IR spectroscopic studies revealed that in solution the polymers adopted predominantly one-handed helical structures stabilized by intramolecular hydrogen bonds between the pendent amide groups. This behavior was similar to that of the corresponding poly(N-alkynylamide) counterparts (poly(3) and poly(4)) reported previously, whereas the helical senses were opposite to each other. The helical structures of the poly(ω-alkynamide)s were stable upon heating similar to those of the poly(N-alkynylamide)s, but the solvent response was completely different. An increase in MeOH content in CHCl3/MeOH resulted in inversion of the predominant screw-sense for poly(1) and poly(2). Conversely, poly(3) was transformed into a random coil, and poly(4) maintained the predominant screw-sense irrespective of MeOH content. The solvent dependence of predominant screw-sense for poly(1) and poly(2) was reasonably explained by molecular orbital studies using the conductor-like screening model (COSMO).

New Approach to the Polymerization of Disubstituted Acetylenes by Bulky Monophosphine-Ligated Palladium Catalysts.

Bulky monophosphine-ligated Pd complexes served as unprecedented admirable catalysts for the polymerization of a disubstituted acetylene. The moderately high polymer yields and cis content of the formed polyacetylene contrasted with those observed for traditional Mo catalyst-based polymer. These Pd catalysts are strong tools to promote the understanding of the structure-property relationships of disubstituted acetylenes.

Synthesis of end-functionalized polyacetylenes that contain polar groups by employing well-defined palladium catalysts.

A series of [(dppf)PdBr(R)]-type complexes (dppf=1,1'-bis(diphenylphosphino)ferrocene; R=p-cyanophenyl (1 a), o-hydroxymethylphenyl (1 b), and triphenylvinyl (1 c)), in combination with silver trifluoromethanesulfonate (AgOTf), were demonstrated to be active for the polymerization of monosubstituted polar acetylene monomers HC≡CCONHC(4)H(9) (2), HC≡CCO(2)C(8)H(17) (3), HC≡CCH(2)OCONHC(6)H(13) (4), HC≡CCH(2)OCO(2)C(6)H(13) (5), and HC≡CCH(CH(3))OH (6). The yields and molecular weights of the polymers depended on the combination of the Pd catalyst and monomer that was employed. Matrix-assisted laser-desorption/ionization-time of flight (MALDI-TOF) mass spectrometric analysis indicated the formation of polymers that contained the "R" and "H" groups at the chain ends. IR spectroscopic analysis supported the R-end-functionalization of the polymers. NMR spectroscopy and MS identified the presence of species that were formed by single, double, and triple insertion of the monomers into the Pd-C(6)H(4)-p-CN bond, thereby giving solid evidence for an insertion mechanism for the present system. Density functional theory (DFT) calculations suggested the preference for 1,2-insertion of the monomer compared to 2,1-insertion.

Alternating ring-opening metathesis copolymerization of amino acid derived norbornene monomers carrying nonprotected carboxy and amino groups based on acid-base interaction.

Amino acid derived norbornene monomers having carboxy (1) and amino groups (2) were synthesized and subjected to ring-opening metathesis copolymerization with various feed ratios using the Grubbs second generation ruthenium catalyst. The M(n)'s of the copolymers ranged from 5300 to 9400 (M(w)/M(n) = 1.40-1.69). The monomer conversion and M(n) of the copolymer were maximized when the monomer feed ratio was 1:1. The monomer unit ratios in the copolymers were almost 1:1 at 10% conversion, irrespective of the feed ratios. The monomer reactivity ratios r(1) and r(2) were estimated to be 0.08 and 0.02, which confirmed that alternating copolymerization occurred. It is considered that alternating copolymerization is brought about by the acid-base interaction between the monomers and/or between the propagating polymer end and the incoming monomer.

Synthesis and helical structure of poly(1-methylpropargyl ester)s with various side chains.

Optically active 1-methylpropargyl esters bearing various substituents were polymerized with [(nbd)Rh](+)[eta(6)-C(6)H(5)B(C(6)H(5))(3)](-) (nbd=norbornadiene) as a catalyst to afford the corresponding poly(1-methylpropargyl ester)s with moderate molecular weights in good yields. The polymers have a cis-stereoregular structure, which was determined by (1)H NMR spectroscopy. Large optical rotations and clear CD signals demonstrated that all these polymers take on a helical structure with a predominantly one-handed screw sense. The polymers exhibited large viscosity indices in the range 1.14-1.75. Chiral amplification was observed in R/S copolymerization. Conformational analysis revealed that the polymers form a tightly twisted helical structure with a dihedral angle of 70 degrees at the single bond of the main chain.

Mechanistic studies of platinum(II)-catalyzed ethylene dimerization: determination of barriers to migratory insertion in diimine Pt(II) hydrido ethylene and ethyl ethylene intermediates.

The diimine platinum(II) ethylene hydride complex [(N/\N)Pt(H)(ethylene)][BAr'4] (1, N/\N = [(2,6-Me2C6H3)N=C(An)-C(An)=N(2,6-Me2C6H3)], An = 1,8-naphthalenediyl, Ar' = 3,5-(CF3)2C6H3) was prepared by protonation of the diethyl complex (N/\N)PtEt2 with [H(OEt2)2][BAr'4]. The energy barrier to interchange of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N/\N)Pt(Et)(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed beta-agostic ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The 1-butene hydride complex, (N/\N)Pt(H)(1-butene)+ (3), is a key intermediate in the dimerization cycle and has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of free nitrile with bound nitrile in (N/\N)Pt(Et)(CH3CN)+ are reported.

Synthesis and extremely high gas permeability of polyacetylenes containing polymethylated indan/tetrahydronaphthalene moieties.

Polymethylated poly(diphenylacetylene) derivatives, a new category of substituted polyacetylenes, were successfully synthesized, and proved to show extremely high gas permeability.

Control of helix sense by composition of chiral-achiral copolymers of N-propargylbenzamides.

N-Propargylbenzamides 1-7 were polymerized with (nbd)Rh(+)[eta(6)-C(6)H(5)B(-)(C(6)H(5))(3)] to afford polymers with moderate molecular weights (M(n) = 26,000-51,000) in good yields. The (1)H NMR spectra demonstrated that the polymers have fairly stereoregular structures (81-88 % cis). The optically active polymers, poly(1) and poly(2), were proven by their intense CD signals and large optical rotations to adopt a stable helical conformation with an excess of one-handed screw sense when heated in CHCl(3) or toluene. The sign of Cotton effect could be controlled by varying the content in the copolymers of either chiral bulky 1 and achiral nonbulky 3, or chiral nonbulky 2 and achiral bulky 7. The smaller the pendant group in the copolymerization of achiral monomers with 1, the more easily did the preferential helical sense change with the copolymer composition. However, the copolymers of chiral nonbulky 2 and achiral nonbulky 3 did not change the helical sense, irrespective of the composition. The free energy differences between the plus and minus helical states, as well as the excess free energy of the helix reversal, of those chiral-achiral random copolymers were estimated by applying a modified Ising model.

Rapid ruthenium-catalyzed synthesis of pyranopyrandiones by reconstructive carbonylation of cyclopropenones involving C[bond]C bond cleavage.

Pyranopyrandiones were prepared by a novel ruthenium-catalyzed carbonylative dimerization of cyclopropenones via C-C bond cleavage. For example, treatment of dipropylcyclopropenone with a catalytic amount of Ru3(CO)12 and NEt3 in THF under 15 atm of carbon monoxide at 140 degrees C for 20 h gave a novel functional monomer, 3,4,7,8-tetrapropylpyrano[6,5-e]pyran-2,6-dione, in an isolated yield of 81%. Unsymmetrically substituted pyranopyrandiones were also obtained by ruthenium-catalyzed carbonylative coupling of cyclopropenones with alkynes under similar reaction conditions.