ELECTRON TRANSFER ACTIVATION OF THE DIELS-ALDER REACTION QUANTITATIVE RELATIONSHIP TO CHARGE TRANSFER EXCITED STATES

Article abstract of DOI:10.1016/0040-4020(82)80121-X

The Diels-Alder cycloaddition of anthracene to tetracyanoethylene (TCNE) is quantitatively compared to alkylmetal insertion under the same reaction conditions.In both systems, the observation of transient charge transfer (CT) absorption bands is related to the presence of 1:1 electron donor-acceptor complexes of anthracene (Ar) and alkylmetal (RM) donors with the TCNE acceptor.The activation free energies ΔG(excit.) for anthracene cycloaddition and alkylmetal insertion are found to be equal to the energetics of ion-pair formation, i.e. and ,which are evaluated from the CT transition energies hν_CT.Indeed,the differences in the rates of alkylmetal insertion and anthracene cycloaddition by a factor of more than 10E9, are shown quantitatively to arise from the differences in ion-pair solvation ΔG^s.The same differences in ΔG^s also apply quantitatively to the free ions, and , independently derived from the electrochemical and iron(III) oxidations of alkylmetals and aromatic compounds, respectively, by outer-sphere electron transfer.The charge transfer formulation of the activation process thus provides a unifying basis for comparing such diverse processes as Diels-Alder cycloadditions and organometal cleavages, when a common electron-deficient acceptor is employed.The relationship to the concerted mechanisms of the Diels-Alder reaction is discussed.