Acid-catalyzed rearrangements in arenes: interconversions in the quaterphenyl series.

Arenes undergo rearrangement of phenyl, alkyl, halogen and other groups through the intermediacy of ipso arenium ions in which a proton is attached at the same carbon as the migrating substituent. Interconversions among the six quaterphenyl isomers have been studied here as a model for rearrangements of linear polyphenyls. All reactions were carried out in 1 M CF3SO3H (TfOH) in dichloroethane at 150 °C in a microwave reactor for 30-60 min, with product formation assessed by high field NMR analysis. Under these reaction conditions, m,p'-quaterphenyl is the equilibrium product. This isomer is unchanged by the reaction conditions and all other quaterphenyl isomers rearrange to m,p' as the dominant or sole product. DFT computations with inclusion of implicit solvation support a complex network of phenyl and biphenyl shifts, with barriers to rearrangement in the range of 10-21 kcal/mol. Consistent with experiments, the lowest energy arenium ion located on this surface is due to protonation of m,p'-quaterphenyl. This supports thermodynamic control based on carbocation energies.

A Short and Efficient Synthesis of the [3]Triangulene Ring System.

Triangulenes are of current interest for potential applications in molecular electronics. We describe here a three step synthesis of the 4,8,12-trihydro[3]triangulenium cation by cascade cyclization of a tetra-benzyl alcohol precursor in triflic acid solution. This stable carbocation is easily observed by NMR and optical spectroscopy and is highly fluorescent. Quenching of the cation into basic solutions or by hydride transfer from triethylsilane provides access to stable dihydro and tetrahydro[3]triangulenes. These neutral species interconvert with cations in a complex series of proton and hydride transfers. This route provides several important [3]triangulene precursors. Preliminary experiments designed to generate [3]triangulene in the solution phase provide evidence for its formation and rapid oligomerization.

Experimental and Theoretical Investigations of the Bromination of Phenols with ß and γ Aliphatic Substituents, including Rings.

Bromination reactions of substituted and ring fused phenols were studied by both experiment (t-BuNH-Br) and computation (density functional theory). The outcomes support each other, indicating a clear and predictable regioselective preference among 3,4-bis-alkylated and 3,4-ring-fused phenols.