Intercepting 1,2‐Diamantene: A Doubly Bridgehead <i>Anti</i> ‐Bredt Alkene

We prepared the first doubly bridgehead anti ‐Bredt alkene 1,2‐diamantene from 1,2‐diiododiamantane and trapped it as its Diels–Alder adduct with 11,12‐dimethylene‐9,10‐dihydro‐9,10‐ethanoanthracene; the structure of the adduct was confirmed through single crystal X‐ray diffraction. The structure of 1,2‐diamantene resembles a hypothetical trans ‐cyclohexene moiety at the central belt that we analyzed computationally and compared it with isomeric 2,3‐ and 3,4‐diamantenes as well as parent 1,2‐adamantene and several known bicyclic anti ‐Bredt olefins. The olefin strain energies (OSE) were computed through homodesmotic equations at the ab initio MP2/cc‐pVTZ level of theory. The choice of the computational method was based on results of CCSD(T)/cc‐pVTZ optimizations of bicyclo[2.2.1]hept‐1‐ene and norbornane as the hydrogenated reference hydrocarbon. Although isomeric diamantenes have different degrees of twisting and bending and altered substitution patterns, their OSEs are similar. Comparative orbital analyses using cis ‐cyclohexene, trans ‐cyclohexene, and 1,2‐adamantene reveal the critical role of CC hyperconjugation in stabilizing twisted alkenes. High‐yielding formation of 1,2‐diamantene is possible due to steric hindrance preventing dimerization in contrast to parent 1,2‐adamantene that readily forms its C 20 ‐dimer.