Reversible <i>Cis-Cisoid</i> to <i>Cis-Transoid</i> Helical Structure Transition in Poly(3,5-disubstituted phenylacetylene)s

A series of novel 3,5-disubstituted phenylacetylenes, <b>rM-1</b>, <b>sM-1</b> to <b>sM-5</b>, bearing an achiral methoxy­carbonyl pendant group and various chiral <i>N</i>-alkylamide pendant groups, were synthesized. They were converted to the corresponding polymers, <b>rP-1</b>, <b>sP-1</b> to <b>sP-5</b>, with high <i>cis</i>-structure contents under the catalysis of [Rh­(nbd)­Cl]₂, aiming to understand how the environmental variation and the structure of pendant group influence the chiroptical properties of polymers. <b>sP-1/rP-1</b> were soluble in CHCl₃ and THF at the molecular level and exhibited much larger optical rotations with opposite signs to those of <b>sM-1/rM-1</b> and displayed the intense Cotton effects centered at 360 nm in the circular dichroism (CD) spectra, ascribed to the one-handed, contracted <i>cis-cisoid</i> helical polyene backbone. The reversible conformation transition between the contracted <i>cis-cisoid</i> helix and the frustrated, extended <i>cis-transoid</i> helix was achieved by alternately adding trifluoro­acetic acid (TFA) and triethyl­amine into the hydrogen bond donating solvent (i.e., CHCl₃), as evidenced by UV–vis absorption and CD spectroscopy, dynamic and static laser light scattering, DSC, and WAXD results. However, the addition of TFA into the <b>sP-1</b> solution in the hydrogen bond accepting solvent (i.e., THF) caused no discernible halochromism. The competing interaction of THF with TFA was considered to account for the observed difference in acid-induced chromism. The small modification in the chiral alkylamide pendant group was found to remarkably affect the solubility and helical conformation of the polymer. <b>sP-2</b> was insoluble in all the solvents tested, <b>sP-3</b> and <b>sP-4</b> dissolved in polar DMF, while <b>sP-5</b> dissolved in both polar and apolar solvents. Depending on the nature of solvents and additives, <b>sP-3</b> and <b>sP-4</b> took either contracted or frustrated helical conformation, whereas <b>sP-5</b> took only a stretched helical conformation due to the highly branched alkyl group

Trivalent Zirconium and Hafnium Metal–Organic Frameworks for Catalytic 1,4-Dearomative Additions of Pyridines and Quinolines

We report the quantitative conversion of [M^IV₆(μ₃-O)₄(μ₃-OH)₄Cl₁₂]^6– nodes in the MCl₂-BTC metal–organic framework into the [M^III₆(μ₃-O)₄(μ₃-ONa)₄H₆]^6– nodes in M^IIIH-BTC (M = Zr, Hf; BTC is 1,3,5-benzenetricarboxylate) via bimetallic reductive elimination of H₂ from putative [M^IV₆(μ₃-O)₄(μ₃-OH)₄H₁₂]^6– nodes. The coordinatively unsaturated M^IIIH centers in M^IIIH-BTC are highly active and selective for 1,4-dearomative hydroboration and hydrosilylation of pyridines and quinolines. This work demonstrated the potential of secondary building unit transformation in generating electronically unique and homogeneously inaccessible single-site solid catalysts for organic synthesis

Helical Conformations of Poly(3,5-disubstituted phenylacetylene)s Tuned by Pendant Structure and Solvent

A series of novel <i>cis</i> poly­(phenylacetylene)­s (PPAs) substituted at the <i>meta</i>-position(s) by both achiral alkoxycarbonyl and chiral alkylamide groups, i.e., <b>rP-I</b>, <b>sP-I</b> to <b>sP-V</b>, or by just a chiral alkylamide group, i.e., <b>rP-VI</b>, were synthesized under catalysis of [Rh­(nbd)­Cl]₂. The dependence of the elongation, screw sense, and stimuli response of helical polyene backbone on the structure and number of substituent was systematically investigated in both solution and solid states. Stretched <i>cis–transoid</i> helices with opposite signs coexisted in the DMF solution of either <b>sP-I</b> or <b>rP-I</b>, but a single handed, contracted <i>cis–cisoid</i> one formed in the mixture of DMF/THF (10/90, v/v). Increasing the substituent size made the polymers <b>sP-III</b>, <b>sP-IV</b>, and <b>sP-V</b> to take only single handed stretched <i>cis–transoid</i> helical conformations regardless of the solvent polarity. The <i>N</i>-methylation of the amide group in <b>sP-II</b> caused a similar effect. With the removal of achiral methoxycarbonyl substituent, <b>rP-VI</b> took just a stretched <i>cis–transoid</i> helical conformation in polar DMF, whereas it existed as a mixture in equilibrium of stretched <i>cis–transoid</i> and contracted <i>cis–cisoid</i> helices with identical screw sense in less polar solvents such as dioxane, THF, and chloroform. The twisting directions of substituent array and polyene backbone were found to be coincident in a dynamic, contracted helix, but the opposite in a less dynamic, stretched helix. These results suggested that the 3,5-disubstitution, strong intramolecular hydrogen bonding, and small substituent favored the formation of contracted <i>cis</i>–<i>cisoid</i> helices for PPAs

Single-Site Cobalt Catalysts at New Zr₁₂(μ₃‑O)₈(μ₃‑OH)₈(μ₂‑OH)₆ Metal–Organic Framework Nodes for Highly Active Hydrogenation of Nitroarenes, Nitriles, and Isocyanides

We report here the synthesis of a robust and porous metal–organic framework (MOF), Zr₁₂-TPDC, constructed from triphenyl­dicarboxylic acid (H₂TPDC) and an unprecedented Zr₁₂ secondary building unit (SBU): Zr₁₂(μ₃-O)₈­(μ₃-OH)₈­(μ₂-OH)₆. The Zr₁₂-SBU can be viewed as an inorganic node dimerized from two commonly observed Zr₆ clusters via six μ₂-OH groups. The metalation of Zr₁₂-TPDC SBUs with CoCl₂ followed by treatment with NaBEt₃H afforded a highly active and reusable solid Zr₁₂-TPDC-Co catalyst for the hydrogenation of nitroarenes, nitriles, and isocyanides to corresponding amines with excellent activity and selectivity. This work highlights the opportunity in designing novel MOF-supported single-site solid catalysts by tuning the electronic and steric properties of the SBUs