1,2-Shifts of hydrogen atoms in aryl radicals and synthetic routes toward cyclo[12]phenacene: A molecular belt.

摘要

Hydrogen atom 1,2-shifts in aryl radicals have not been previously reported, to the best of our knowledge. We have found evidence for such a rearrangement in the radical derived from 2-bromobenzo[c]phenanthrene ( 9) by flash vacuum pyrolysis (FVP) at temperatures ranging from 950–1100°C.; The aryl hydrogens in the bay region of 9 are sterically congested. After homolytic cleavage of the aryl bromide bond at the high temperatures of FVP, this steric congestion is relieved by a 1,2-shift of hydrogen out of the cove region to the adjacent radical. The new aryl radical subsequently undergoes an irreversible cyclization reaction to form benzo[ghi]fluoranthene (4). Deuterium labeling studies provide further evidence that the major reaction pathway involves migration of an adjacent hydrogen atom to the site where the original radical was generated. Calculations performed at the BP/DN** level of theory (ZPE corrected) also support this mechanism.; Nonplanar benzenoid hydrocarbons are anticipated to have unusual properties due to their curved molecular surface and aromaticity. A fully conjugated 'molecular belt' in which the π-electron system is circularly deformed can be viewed as an extreme example of a nonplanar benzenoid hydrocarbon. Our objective was to synthesize a molecular belt consisting of twelve benzenoid rings in a zig-zag arrangement, cyclo[12]phenacene (1), utilizing flash vacuum pyrolysis (FVP) in the final step. The high temperatures in FVP would provide the energy necessary to induce the curvature and inherent strain in the target molecule; this strategy has proven highly successful for the synthesis of numerous bowl-shaped fullerene subunits.; Synthesis of a macrocyclic pyrolysis precursor was pursued using a range of strategies. The two main synthetic pathways utilized either naphthalene or phenanthrene building blocks to form a macrocylic tetramer or trimer, respectively. Described herein are the syntheses of numerous naphthalene and phenanthrene building blocks along with the synthetic routes toward cyclo[12]phenacene (1) that were explored.