1-Oxoindan-2-yl and 1,3-dioxoindan-2-yl esters as photoremovable protecting groups

1-Oxoindan-2-yl and 1,3-dioxoindan-2-yl carboxylic acid esters react with an excess of hydrogen atom or electron donors to release the corresponding acids, in addition to indan-1-one and indan-1,3-dione, respectively, as by-products. The maximum degradation quantum yields of 1-oxoindan-2-yl esters in H-donating propan-2-ol were found to approach 10, indicating that a chain reaction process, involving hydrogen transfer from the ketyl radical intermediates formed from an excited ester by hydrogen abstraction from an alcohol, participates. Such a cleavage mechanism parallels that observed earlier in photolysis of phenacyl esters. The corresponding 4,7-dimethyl substituted derivatives showed no contribution of the photoenolization mechanism apparently because of electronic and geometric reasons. Both 1-oxoindan-2-yl and 1,3-dioxoindan-2-yl chromophores are proposed to be utilized as photoremovable protecting groups in applications when higher concentrations of the hydrogen/electron donors are experimentally feasible.