Search for: All records

We synthesized some of the longest unimolecular oligo(p-phenylene ethynylenes) (OPEs), which are fully substituted with electron-withdrawing ester groups. An iterative convergent/divergent (a.k.a. iterative exponential growth – IEG) strategy based on Sonogashira couplings was utilized to access these sequence-defined macromolecules with up to 16 repeating units and 32 ester substituents. The carbonyl groups of the ester substituents interact with the triple bonds of the OPEs, leading to (i) unusual, angled triple bonds with increased rotational barrier, (ii) enhanced conformational disorder, and (iii) associated broadening of the UV/Vis absorption spectrum. Our results demonstrate that fully air-stable, unimolecular OPEs with ester groups can readily be accessed with IEG chemistry, providing new macromolecular backbones with unique geometrical, conformational, and photophysical properties. 

Abstract Electrophilic aromatic substitution reactions are of profound importance for the synthesis of biologically active compounds and other advanced materials. They represent an important means to activate specific aromatic C–H bonds without requiring transition-metal catalysts. Surprisingly, few stereoselective variants are known for electrophilic aromatic substitutions, which limits the utility of these classical reactions for stereoselective synthesis. While many electrophilic aromatic substitutions lead to achiral products (due to the planar nature of aromatic rings), there are important examples where chiral products are produced, including desymmetrization reactions of aromatic cyclophanes and of prochiral substrates with multiple aromatic rings. This Synpacts article now illustrates how chiral arms, when placed precisely above and underneath delocalized carbocations, can act as chiral auxiliaries to convert classical electrophilic aromatic substitution reactions into powerful diastereo- and enantioselective transformations. 

This work presents the first transition metal-free synthesis of oxygen-linked aromatic polymers by integrating iterative exponential polymer growth (IEG) with nucleophilic aromatic substitution (S N Ar) reactions. Our approach applies methyl sulfones as the leaving groups, which eliminate the need for a transition metal catalyst, while also providing flexibility in functionality and configuration of the building blocks used. As indicated by 1) 1 H- 1 H NOESY NMR spectroscopy, 2) single-crystal X-ray crystallography, and 3) density functional theory (DFT) calculations, the unimolecular polymers obtained are folded by nonclassical hydrogen bonds formed between the oxygens of the electron-rich aromatic rings and the positively polarized C–H bonds of the electron-poor pyrimidine functions. Our results not only introduce a transition metal-free synthetic methodology to access precision polymers but also demonstrate how interactions between relatively small, neutral aromatic units in the polymers can be utilized as new supramolecular interaction pairs to control the folding of precision macromolecules. 

We report a general synthetic route toward helical ladder polymers with varying spring constants, built with chirality‐assisted synthesis (CAS). Under tension and compression, these shape‐persistent structures do not unfold, but rather stretch and compress akin classical Hookean springs. Our synthesis is adaptable to helices with different pitch and diameter, which allowed us to investigate how molecular flexibility in solution depends on the exact geometry of the ladder polymers. Specifically, we showed with molecular dynamic simulations and by measuring the longitudinal¹H NMR relaxation times (T₁) for our polymers at different Larmor frequencies, that increasing the helix diameter leads to increased flexibility. Our results present initial design rules for tuning the mechanical properties of intrinsically helical ladder polymers in solution, which will help inspire a new class of robust, spring‐like molecular materials with varying mechanical properties.

 

We report a general synthetic route toward helical ladder polymers with varying spring constants, built with chirality‐assisted synthesis (CAS). Under tension and compression, these shape‐persistent structures do not unfold, but rather stretch and compress akin classical Hookean springs. Our synthesis is adaptable to helices with different pitch and diameter, which allowed us to investigate how molecular flexibility in solution depends on the exact geometry of the ladder polymers. Specifically, we showed with molecular dynamic simulations and by measuring the longitudinal¹H NMR relaxation times (T₁) for our polymers at different Larmor frequencies, that increasing the helix diameter leads to increased flexibility. Our results present initial design rules for tuning the mechanical properties of intrinsically helical ladder polymers in solution, which will help inspire a new class of robust, spring‐like molecular materials with varying mechanical properties.

 

Enantioselective electrophilic aromatic nitration methodology is needed to advance chirality‐assisted synthesis (CAS). Reported here is an enantioselective aromatic nitration strategy operating with chiral diester auxiliaries, and it provides an enantioselective synthesis of aC_3v‐symmetric tribenzotriquinacene (TBTQ). These axially‐chiral structures are much sought‐after building blocks for CAS, but they were not accessible prior to this work in enantioenriched form without resolution of enantiomers. This nitration strategy controls the stereochemistry of threefold nitration reactions from above the aromatic rings with chiral diester arms. Dicarbonyl‐to‐arenium chelation rigidifies the reaction systems, so that remote stereocenters position the ester‐directing groups selectively over specific atoms of the TBTQ framework. Closely guided by computational design, a more selective through‐space directing arm was first predicted with density functional theory (DFT), and then confirmed in the laboratory, to outperform the initial structural design. This enantio‐ and regioselective TBTQ synthesis opens a new pathway to access building blocks for CAS.

 

Enantioselective electrophilic aromatic nitration methodology is needed to advance chirality‐assisted synthesis (CAS). Reported here is an enantioselective aromatic nitration strategy operating with chiral diester auxiliaries, and it provides an enantioselective synthesis of aC_3v‐symmetric tribenzotriquinacene (TBTQ). These axially‐chiral structures are much sought‐after building blocks for CAS, but they were not accessible prior to this work in enantioenriched form without resolution of enantiomers. This nitration strategy controls the stereochemistry of threefold nitration reactions from above the aromatic rings with chiral diester arms. Dicarbonyl‐to‐arenium chelation rigidifies the reaction systems, so that remote stereocenters position the ester‐directing groups selectively over specific atoms of the TBTQ framework. Closely guided by computational design, a more selective through‐space directing arm was first predicted with density functional theory (DFT), and then confirmed in the laboratory, to outperform the initial structural design. This enantio‐ and regioselective TBTQ synthesis opens a new pathway to access building blocks for CAS.