The construction of nanotubes with well-defined structures, although synthetically challenging, offers the prospect of studying novel chemical reactions and transportation within confined spaces, as well as fabricating molecular devices and nanoporous materials. Here we report a discrete molecular nanotube, namely the covalent organic pillar COP-1, synthesized through a [2 + 5] imine condensation reaction involving two penta-aldehyde macrocycles and five phenylenediamine linkers. A pair of enantiomeric nanotubes, obtained in a quantitative and diastereoselective manner, were characterized and resolved readily. NMR spectroscopy, isothermal titration calorimetric and X-ray crystallographic studies revealed that the 2-nm-long and 4.7-Å-wide one-dimensional channel inside COP-1 can accommodate α,ω-disubstituted n-alkyl chains with complementary lengths and electron density distributions. Furthermore, in a length-mismatched host–guest pair, we found that the nonamethylene dibromide thread not only displays a diminished binding constant in solution, but adapts an energetically unfavoured gauche conformation inside COP-1 in the solid state.
A "rim-differentiated" pillar[6]arene ) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C 60 adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.
Transition
metal catalyzed difunctionalization reactions of alkenes
using simple chemical feedstocks are powerful strategies for the synthesis
of valuable compounds and materials. Density functional theory (DFT)
calculations reported in the present paper reveal detailed mechanistic
insight and the origin of the substrate-dependent regioselectivity
in the titled reactions. Computational results demonstrate that these
reactions are generally composed of several steps including oxidative
addition, carbonickelation, H-shift/transmetalation, and reductive
elimination. Among these steps, the key one responsible for the regioselectivity
depends upon the organic halides utilized. Natural bond orbital (NBO)
analysis, energy decomposition analysis (EDA), and buried volume calculations
indicate that steric effect is a common contributor of the regioselectivity,
while other energy terms, such as electrostatic interaction, have
significant and even dominant effects on the specific regioselectivity.
Furthermore, the key reason for successfully suppressing Heck and/or
Suzuki products lies in that the formation of Heck and/or Suzuki products
is thermodynamically less favorable. Comparison of the total reaction
barriers of the rate-limiting step (combined transmetalation and reductive
elimination processes) demonstrates that the electron-induced effect
of various organic halides significantly causes the different bonding
ability between Ni atom and allyl moiety. As a result, transmetalation
and reductive elimination processes made distinct contributions to
reducing the total activation free energy of the rate-limiting step
of various difunctionalized reactions.
Palladium-catalyzed alkenylation of δ-C(sp3)–H bonds with alkynes was conducted by DFT calculations, showing that the dimeric Pd2(OAc)4 mechanism reproduces experimental observations well.
The mechanism, origin of stereoselectivity, and ligand-dependent reactivity of Pd(II)-catalyzed methylene C(sp 3 )−H alkenylation−aza−Wacker cyclization to form (E)-β-stereogenic γ-lactam have been comprehensively studied by density functional theory (DFT) calculations. The calculated results reveal that the methylene C−H activation assisted by K 2 CO 3 via the concerted metalation− deprotonation mechanism is found to be the most preferred pathway, where the enantioselectivity is distinguished by the orientation of the methyl group of a substrate relative to a chiral ligand. However, the stereochemistry of the olefin moiety in the generated product is mainly determined by the oxidative addition step, where the coulombic interaction and dispersion effect differentiate the energy difference of diastereomeric transition states. In terms of the agostic interaction nature of "three-center two-electron" transition states, the discrepancy of reactivities caused by different Pd catalysts is attributed to the electron induction effect of substituents on the chiral ligands. In other words, the use of an electron-withdrawing group (e.g., −CN) in place of an electron-donating group (e.g., −OMe) enhances the oxidation state of the Pd atom and lowers vacant d orbitals of the palladium atom of the catalyst and in turn facilitates a larger amount of σ-electronic-charge injection into an empty 3d shell of the palladium center. Thus, the higher catalytic activity of the Pd catalyst with ligands substituted by an electron-withdrawing group is anticipated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.