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Within the confined nanospaces of synthetic capsule and barrel-shaped molecular hosts, long hydrocarbons can adopt unusual helical or holding conformations of the molecules.
However, the recognition of long hydrocarbons by synthetic hosts is a challenging task due to their conformational flexibility and the lack of distinguishing recognition sites.
Herein we report the synthesis and host properties of molecular tube 1 which presents a hydrophobic cavity encircled by polyaromatic anthracene panels (Fig. The tube is obtained by the transition-metal-free cross-coupling reaction of end-functionalized, bent compounds with two embedded anthracene rings.
The girded cylindrical cavity (~1 nm in diameter and length) of tube 1 selectively binds long hydrocarbons with branched methyl groups and/or unsaturated moieties, such as 2,2,4,4,6,8,8-heptamethylnonane (4a), nervonic acid methyl ester (5a), squalene (6a) and coenzyme Q4 (6c) (Fig.
The open cylindrical cavity of 1 is fully girded by the polyaromatic anthracene panels with inner diagonal distances of 0.9–1.1 nm.
The tubular structure provides a well-defined hydrophobic nanospace and hydrophilic pyridinium and methoxyethoxy groups so that we examined the host capability of the amphiphilic tube 1 for long hydrocarbons in aqueous solutions.) but shows selectivity for a multiply branched alkane over nonbranched alkanes.
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The selective recognition and effective binding of hydrocarbons within synthetic host compounds are problematic owing to their conformational flexibility and the lack of specific binding sites.For example, when branched 2,2,4,4,6,8,8-heptamethylnonane (4a; 2.2 μmol) and linear n-nonane (4b; 2.2 μmol) were suspended in a 1:1 D=−2.08 p.p.m., due to shielding effects from the encircling anthracene rings.The 1:1 host–guest ratio was supported by ESI-TOF MS measurement, where a prominent signal was observed at m/z=843.8 corresponding to a [1·4a−2Cl ion.In the crystal structure of 1·4a, the hydrocarbon chain of bound guest 4a is highly disordered in the cavity of 1 and cannot be located in the crystallographic analysis.
Thus the structure of 4a within 1 was optimized by semiempirical calculation (PM3 level). 4d), respectively, with significant upfield shifts. However, competitive binding experiments revealed the binding preference of tube 1 for the branched and nonbranched alkanes to be 4a OD=1:1, r.t.) of (c) 1·4b and (d) 1·4d.
Electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) analysis confirmed the formation of 1 with a molecular weight of 1,532.67 Da: a prominent signal was observed at m/z=730.9, assigned to the [1−2Cl species.