Synthesis and properties of stable heteroazulene analogues of a triphenylmethyl cation

A general synthetic route to a novel type of fairly stable heteroazulene analogues of triphenylmethyl cations, tris(2-oxo-2H-cyclohepta[b]furan-3-yl)-, tris(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl)-, and tris(1,2-dihydro-N-methyl-2-oxocyclohepta[b]pyrrol-3-yl)methyl cations, 4a-c, is reported. The synthetic method is based on a simple, TFA-catalyzed electrophilic aromatic substitution on the heteroazulenes 2H-cyclohepta[b]furan-2-one and 1,2-dihydro-N-phenyl-2H-, and 1,2-dihydro-N-methyl-2H-cyclohepta[b]pyrrol-2-one with trimethyl orthoformate, followed by oxidative hydrogen abstraction with DDQ and subsequent exchange of the counter-anion by using aq. HPF6 solution. The stability of the cations 4a-c is expressed as the pK(R+)-value, which is determined spectrophotometrically as 9.7, 12.2, and 13.1, respectively. The electrochemical reduction of 4a-c exhibits reversible waves and low reduction potentials at -0.31, -0.58, and -0.62 (V vs. Ag/Ag+) upon cyclic voltammetry (CV), which values are lower than that of tri(azulen-1-yl)methyl cations.