ABSTRACT
A new strategy towards tubular hydrogen-bonded polymers based on the self-assembly of isocytosine tautomers in orthogonal directions is proposed and experimentally verified, including by 1 H fast magic-angle spinning (MAS) solid-state NMR. The molecular tubes obtained possess large internal diameter and tailor-made outer functionalities rendering them potential candidates for a number of applications.
ABSTRACT
The assembly of a discrete hydrogen-bonded molecular tube from eight small identical monomers is reported. Tube assembly was accomplished by means of selective heterodimerization between isocytosine and ureidopyrimidinone hydrogen-bonding motifs embedded in an enantiopure bicyclic building block, leading to the selective formation of an octameric supramolecular tube. Upon introduction of a fullerene guest molecule, the octameric tube rearranges into a tetrameric inclusion complex and the hydrogen-bonding mode is switched. The dynamic behavior of the system is further explored in solvent- and guest-responsive self-sorting experiments.
ABSTRACT
The formation of an unprecedented decameric capsule in carbon disulfide, held together by the combination of double and triple hydrogen bonds between isocytosine units embedded in an enantiomerically pure bicyclic framework is reported. The aggregation occurs via symmetry breaking of the enantiopure intrinsically C2-symmetric monomer brought about by solvent, induced tautomerization of the hydrogen-bonding unit. We show that the topology of the aggregate is responsive to the solvent in which the assembly takes place. In this study we demonstrate that in carbon disulfide the chiral decameric cavity aggregate consisting of three forms of the same monomer, differing in their hydrogen bonding to each other is selectively formed, representing a tube-like structure capped with two C2-symmetric monomers. The large cylindrical cavity produced selectively accommodates one partially solvated C60 molecule, and molecular dynamic simulations revealed the special role of the solvent in the inclusion mechanism. The strategy described herein represents the first step toward the creation of a new class of hydrogen-bonded tubular objects from only one small symmetric building block by solvent-responsive aggregation.
ABSTRACT
The synthesis of the enantiomerically pure N-Boc 9-azabicyclo[3.3.1]nonane-2,6-dione (4b), a potentially useful chiral building block, from N-Bn and N-Boc 9-azabicyclo[3.3.1]nonane-2,6-diols 2a and 2b was accomplished. The enantiomer resolution of diols 2a and 2b was achieved by crystallization of their diastereomeric esters or by kinetic resolution of the racemic diol 2a using lipase from Candida rugosa (CRL). Both enantiomers of N-Boc protected diol 2b were converted into the corresponding enantiomerically pure diones 4b, the absolute configuration of which was determined by comparison of the experimental and simulated circular dichroism (CD) spectra, obtained by ab initio time-dependent density functional theory (TDDFT) calculations. The (-)-(1R,5R)/(+)-(1S,5S) absolute configuration of 4b inferred from the TDDFT calculations was confirmed via analysis of the CD spectrum of endo,endo-dibenzoate (+)-7 derived from diol (+)-2b and application of the benzoate exciton chirality method. The assigned absolute configuration was further supported by the results of kinetic resolution of diol 2a using Candida rugosa lipase, which exhibited kinetic preference toward the (1R,2R,5R,6R)-enantiomer in agreement with the Kazlauskas' rule.
