Synthesis and supramolecular properties of trimethylene-bridged clips.
The novel trimethylene-bridged clips I and II have been synthesized by using repetitive stereoselective Diels-Alder reactions of the corresponding benzo- and naphthobismethylenenorbornenes as dienes and a norbornadiene as bisdienophile, and subsequent dehydrogenation of the primary cyclobisadducts by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Clips I and II serve as receptors for a variety of electron-deficient neutral and cationic arom. substrates, comparable to the corresponding mol. tweezers. The thermodn. parameters of the complex formation, Ka and DG, were detd. by 1H NMR titrn. expts. and, in the case of the highly stable complex TCNB 32@4, by the use of isothermal titrn. microcalorimetry. The finding that clip I forms more stable complexes than II can be explained by the larger van der Waals contact surfaces of the naphthalene sidewalls in I compared to the corresponding benzene systems in II. In the complexes with I as receptor, the plane of each arom. substrate mol. is calcd. to be oriented almost parallel to the naphthalene sidewalls. However, in complexes of tweezer mol. III, complexed substrates are usually oriented parallel to the central naphthalene spacer unit. Due to the more open topol. of I, most complexes were calcd. to consist of two or more equilibrating noncovalent conformers.
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