Abstract
Here we report a new class of synthetic receptors, acyclic pillar[n]naphthalene (n = 2–4, Dimer, Trimer, and Tetramer) oligomers, which are made up of 2,3-diethoxynaphthalene units linked by methylene bridges at the 1- and 4-positions. They can be synthesized through a one-step condensation of 2,3-diethoxynaphthalene monomer and paraformaldehyde in the presence of BF3•(Et)2O catalyst. The crystal structure of Tetramer has an interesting pseudo-cycle shaped structure in the solid state. Their complexation behaviors toward several organic ammonium cations (1+-15+) and electron–deficient neutral guests (16–17), were examined by means of 1H NMR spectroscopy. Tetramer shows good host-guest properties toward the ammonium guests, giving association constants (Ka) in the magnitude of 102-104 M−1, which are comparable with those for some macrocyclic hosts.
Highlights
Since the discover of crown ethers, the development of hosts for recognizing various guest species has mainly focused on macrocyclic structures (Cram, 1988; Lehn, 1988; Pedersen, 1988; Gong et al, 2010; Chun et al, 2013; Jurícek et al, 2014; Liu et al, 2019)
No cyclic oligomers have been obtained after many attempts; a possible reason is that big SCHEME 2 | Synthesis of acyclic pillar[n]naphthalenes Dimer, Trimer, and Tetramer
Acyclic pillarnaphthalenes with 2,3diethoxynaphthalene units bridged by methylenes at 1,4-positions were synthesized through a one-pot reaction of 2,3-diethoxy naphthalene monomer and paraformaldehyde by using Lewis acid as the catalyst
Summary
Since the discover of crown ethers, the development of hosts for recognizing various guest species has mainly focused on macrocyclic structures (Cram, 1988; Lehn, 1988; Pedersen, 1988; Gong et al, 2010; Chun et al, 2013; Jurícek et al, 2014; Liu et al, 2019). Methylene–bridged macrocyclic arenes, for example calixarenes (Baldini et al, 2007; Guo and Liu, 2012), pillararenes (Ogoshi et al, 2008; Xue et al, 2012; Wang et al, 2016; Yang et al, 2016), coronarenes (Wang, 2018), helic[6]arene (Zhang et al, 2016), biphenarenes (Chen et al, 2015; Dai et al, 2017; Li et al, 2019; Wang et al, 2019b), and etc. Considering that pillararenes with pillar-shape topologic structures have shown nice host-guest properties, we wondered whether we can create acyclic pillarnaphthalenes (Scheme 1), which would have deep, pillar-shape, and π-rich cavities, and maybe better binding abilities than calixnaphthalenes. We did not get such macrocycles, but succeed in making acyclic pillarnaphthalene oligomers
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