Spectral and Kinetic Properties of Radical Cations Derived from Oxoisoaporphines: Relevance to Electron-Transfer Processes Involving Phytoalexins

Artículo de publicación ISIThe thermally induced intermolecular electron transfer reaction in acetonitrile between the tetracyanoethylene (TCNE), a [pi]-electron acceptor with a large electron affinity, and six oxoisoaporphines (2,3-dihydro-7H-dibenzo[de,h]quinolin-7-one, 5-methoxy-2,3-dihydro-7H-dibenzo[de,h]quinolin-7-one, 1-azabenzo[de]anthracen-7-one, 5-methoxy-1-azabenzo[de]anthracen-7-one, 7H-benzo[e]perimidin-7-one, and 2-methyl- 7h-benzo[e]perimidin-7-one) is reported. Spectral and kinetic characteristics are presented for radical cations derived from these six oxoisoaporphines either generated by a thermal reaction or generated radiolytically in argon-saturated 1,2-dichloroethane, oxygen-saturated acetone, and acetonitrile. The radical cations of oxoisoaporphines are insensitive to oxygen and are mostly characterized by absorption maxima of their most intense bands located at [lamda]max = 400-410 nm, except of the radical cations derived from 2,3-dihydrooxoisoaporphines. For the latter compounds, the absorption maxima of the most intense absorption bands are located at [lamda]max = 290-295 nm. Their locations are independent of the presence of functional groups and the solvents used. They are formed in bimolecular processes with pseudo-first-order rate constants ranging from 2.1 x 10 5 to 1.5 x 10 6 s-1 (in solutions containing 10 -4 M of the substrate), depending on the derivative and the solvent used. They are stable either when formed via the electron-transfer reaction with TCNE or when generated in isolation in pulse radiolysis of Ar-saturated 1,2-dichloroethane. In acetone and acetonitrile they decay predominantly by first-order kinetics with the first-order rate constants ranging from 2.3 x 10 4 to 5.1 x 10 4 s-1. Formation of dimeric radical cations for all of the oxoisoaporphines studied was observed in acetonitrile solutions, and for azaoxoisoaporphines also in acetone solutions. The experimental spectra show a reasonably good agreement with the ZINDO/S semiempirical quantum mechanical calculations of radical cation absorptions.We thank Dr. Eduardo Sobarzo-Sanchez (USC) for the samples of oxoisoaporphines and the FONDECYT Grants Nos. 1100121, 1070623, and 7080096 for financial support that made possible short exchange visits of J.R.F. and K.B. in the INCT (Warsaw, Poland) and the Universidad de Chile (Santiago, Chile), respectively. C.A. thanks CONICYT for the Ph.D. fellowship, Project No. 24091001, and the final Fellowship to write the Doctoral Thesis and Universidad de Chile for the Travel Grant that made possible his 3 month visit in the INCT (Warsaw, Poland) to perform pulse radiolysis experiments