Unsubstituted quinoidal pyrrole and its reaction with oxygen, charge transfer and palladium(ii) complexes via DDQ oxidation

Abstract

The dehydrogenation reaction of 2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrole with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) followed by treatment with sodium bicarbonate afforded two new products containing oxygen atoms, 2,5-bis(3,5-dimethylpyrazolylcarbonyl)pyrrole (2) and 5-{bis-(3,5-dimethylpyrazolyl)methyl}pyrrole-2-carbaldehyde (3). Interestingly, the products are different, when the reaction mixture is not treated with base; they are 2,2-tri(3,5-dimethylpyrazolylmethyl)-5-(3,5-dimethylpyrazolylcarbonyl)pyrrole (4) and the adduct 5, the 2,3-dichloro-5,6-dicyanohydroquinone (DDQH₂) adduct of 2,5-bis{di(3,5-dimethylpyrazolyl)methene}-2,5-dihydropyrrole (6) isolated in 22% yield, in which 6 acts as an electron acceptor and DDQH₂ as an electron donor. The treatment of 5 with NaBH₄ or basic alumina afforded the free base 6 in good yield, representing a discrete unsubstituted quinoidal structure of pyrrole. 6 is fluorescent, reactive and could have a biradical resonance structure, which is demonstrated by its reaction with dioxygen only under the irradiation of sunlight, not reacting in darkness, giving the oxygen containing products: 2, bis-(3,5-dimethylpyrazolyl)methanone (7) and 2-(3,5-dimethylpyrazolylcarbonyl)-5-(3,5-dimethylpyrazolyl)pyrrole (8). Moreover, the quinoidal molecule loses its fluorescence property on coordinating with palladium metal atom and gives a binuclear complex, [Pd₂Cl₄{μ-C₄H₃N-2,5-(C(Me₂pz)₂)₂-N,N,N,N}], in good yield. The structures of most of the compounds, including this palladium complex, were determined by single crystal X-ray diffraction studies.