GEAR EFFECTS IN POLYCYCLOHEXYL SYSTEMS - TETRACYCLOHEXYLETHENE

Tetracyclohexylethene (4) was synthesized by McMurry coupling of dicyc lohexyl ketone. The H-1 and C-13 NMR spectra of 4 were analyzed by mea ns of 2D NMR techniques. The compound exists both in solution and in t he crystal in a conformation with a ''tongue and groove'' arrangement, i.e., in an arrangement in which the methine hydrogens are located in the notch provided by the methylene protons of a neighboring cyclohex yl ring. Molecular mechanics calculations reproduce satisfactorily the structural parameters found in the crystal structure of 4. The barrie r for rotation of the cyclohexyl rings was measured by dynamic NMR fro m the exchange of syn and anti methine protons. This barrier is 18.7 k cal mol(-1), i.e., higher than that of tetraisopropylethene (ca. 16 kc al mol(-1)). Molecular mechanics calculations of the rotational pathwa y suggest that the larger barrier observed for 4 is due to conformatio nal restrictions imposed by the cyclohexyl rings.