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.
1b) through hydrophobic aromatic–aliphatic and/or aromatic–olefinic interactions in 1:1 water/methanol solutions. 2a) with two embedded anthracene panels to design molecular tube 1. 26) were reacted in 1-propanol at 85 °C over 7 days, molecular tube 1 formed and was isolated as a yellow solid (9% yield).
The host–guest structure revealed that the bound alkane with branched methyl groups fully occupies the cavity of the tube with multiple CH–π interactions (Fig. Linear alkanes such as 4b, n-dodecane (4c) and n-hexadecane (4d) were also bound by tube 1 under similar conditions to give host–guest complexes 1·4b, 1·4c and 1·4d (92, 46 and 12% yields, respectively). (e) Molecular structure of 1·4a: the framework of tube 1 is the crystal structure (substituents are omitted for clarity) and the structure of bound 4a is optimized by semiempirical calculation (PM3 level).
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.
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.
A competitive binding experiment of 1 with a mixture of cis-5c and trans-5c displayed the formation of 1·cis-5c and 1·trans-5c complexes in a 2:3 ratio as thermodynamically stable products (Fig. Finally, we determined the ability of tube 1 to bind long hydrocarbons containing both branches and unsaturated moieties and showed the selective binding of squalene and coenzyme Q4, bearing multiple methyl groups and unsaturated carbon–carbon double bonds, by 1 in aqueous solutions, due to the synergistic effect of hydrophobic aromatic–aliphatic and aromatic–olefinic interactions.
When tube 1, squalene (6a) and squalane (6b), which is the fully hydrogenated squalene, were combined in a 1:1 D99%) at room temperature to afford a 1·6a complex (24% yield), as confirmed by NMR analysis (Fig. The size and shape of 6b are comparable to that of 6a, but surprisingly 6b was not bound at all by 1 even in the absence of 6a. and −0.22 p.p.m., respectively, with huge upfield shifts (Δδ=2.2 p.p.m.), indicating tight contacts between the central part of 6a and the anthracene frameworks of 1.