Solvent Effect as the Result of Frontier Molecular Orbital Interaction. VII. The Retro-Diels-Alder Reaction

The solvent effect on the retro-Diels-Alder (R.D.A.) reaction of 1,4,4a-9a-tetrahydro-4a-methyl-1(1α,4α,4aα,9aα)-1,4-methanoanthrac ene-9,10-dione to 2-methyl-1,4-naphthoquinone and cyclopentadiene was investigated kinetically in 16 solvents. The hyperbolic relationship between the kinetic data and the Acceptor Number of the solvents is strong evidence that the solvent acts as an electrophile which lowers the activation energy of the reaction. Furthermore when these rats constants are plotted vs those of the previously investigated Diels-Alder (D.A.) reaction of 1,4-napthhoquinone and 2,3-dimethylbutadiene, a linear relationship is obtained. The linearity of the graph is a good indication that the nature of the solvent effects is the same in both D.A. and R.D.A. reactions. The above relationships and the thermodynamic parameters strongly suggest that the R.D.A. reaction is a 'late transition state' pericyclic reaction whose solvent effect derives from a specific interaction between the solvent and the product.