Diamantane. I. Preparation of Diamantane. Physical and Spectral Properties

The preparation of diamantane (I) by Lewis acid catalyzed rearrangements of various pentacyclic tetradecanes has been examined. The best yield (84%) was obtained from trans-tetrahydro-Binor-S (XXXV). However, the most convenient synthetic procedure involves rearrangement of hydrogenated Binor-S (XXVII/XXVIII), which gives I in ∼70% yield. Other more highly strained precursors give I in lower yield (1-47%) owing to disproportionation. The diamond lattice structure of diamantane, confirmed by X-ray analysis, is consistent with high thermodynamic stability. However, I, like adamantane, is not strain free. Molecular mechanics calculations show that this is due to an excess of repulsive over attractive nonbonded interactions in comparison with noncage hydrocarbons. The spectral properties of diamantane are characterized by a single-line proton nmr spectrum, resistance toward mass spectral fragmentation, and a simplified ir spectrum due to high symmetry.