GEAR EFFECTS IN POLYCYCLOHEXYL SYSTEMS - TETRACYCLOHEXYLETHENE

Tetracyclohexylethene (4) was synthesized by McMurry coupling of dicyclohexyl ketone. The H-1 and C-13 NMR spectra of 4 were analyzed by means of 2D NMR techniques. The compound exists both in solution and in the 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 cyclohexyl ring. Molecular mechanics calculations reproduce satisfactorily the structural parameters found in the crystal structure of 4. The barrier for rotation of the cyclohexyl rings was measured by dynamic NMR from the exchange of syn and anti methine protons. This barrier is 18.7 kcal mol(-1), i.e., higher than that of tetraisopropylethene (ca. 16 kcal mol(-1)). Molecular mechanics calculations of the rotational pathway suggest that the larger barrier observed for 4 is due to conformational restrictions imposed by the cyclohexyl rings.