Electron-transfer-induced tautomerization in methylindanones: Electronic control of the tunneling rate for enolization

The radical cations generated from 4-methyl- and 4,7-dimethylindanone, as w ell as their deuterated isotopomers, isolated in Argon matrices, were found to undergo enolization to the corresponding enol radical cations at rates that differ by orders of magnitude. It is shown by quantum chemical calcula tions that the effect of the remote methyl group in the 4-position is of pu rely electronic nature in that it stabilizes the unreactive jr-radical rela tive to the reactive sigma -radical state of the 7-methylindanone radical c ation. The observed kinetic behavior of the two compounds can be reproduced satisfactorily on the basis of calculated heigth and width of the thermal barrier for enolization, using the Belt model for quantum mechanical tunnel ing. High-level calculations on the methylacrolein radical cation show that barriers for enolization in radical cations are overestimated by B3LYP/6-3 1G*