Yvonne Chiang, Martin Gaplovsky, A. Jerry Kresge,* King Hung Leung, Christian Ley, Marek Mac, Gabriele Persy, David L. Phillips, Vladimir V. Popik, Christoph Rödig, Jakob Wirz* and Yu Zhu, "Photoreactions of 3-Diazo-3H-benzofuran-2-one; Dimerization and Hydrolysis of its Primary Photoproduct, a Quinonoid Cumulenone. A Study by Time-Resolved Optical and Infrared Spectroscopy",J. Am. Chem. Soc. 2003, 125, 12872-12880.
Abstract: Light-induced deazotization of 3-diazo-3H-benzofuran-2-one (1) in solution is accompanied by facile (CO)-O bond cleavage yielding 6-(oxoethenylidene)-2,4-cyclohexadien-1-one (3), which appears with a risetime of 28 ps. The expected Wolff-rearrangement product, 7-oxabicyclo[4.2.0]octa-1,3,5-trien-8-ylidenemethanone (4), is not formed. The efficient light-induced formation of the quinonoid cumulenone 3 opens the way to determine the reactivity of a cumulenone in solution. The reaction kinetics of 3 was monitored by nanosecond flash photolysis with optical (lambda_max 460 nm) as well as Raman (1526 cm-1) and IR detection (2050 cm-1). Remarkably, the reactivity of 3 is that expected from its valence isomer, the cyclic carbene 3H-benzofuran-2-one-3-ylidene, 2. In aqueous solution, acid-catalyzed addition of water forms the lactone 3-hydroxy-3H-benzofuran-2-one (5). The reaction is initiated by protonation of the cumulenone on its beta-carbon atom. In hexane, cumulenone 3 dimerizes to isoxindigo ((E)-[3,3']bibenzofuranylidene-2,2'-dione, 7), coumestan (6H-benzofuro[3,2-c]benzopyran-6-one, 8), and a small amount of dibenzonaphthyrone (benzopyrano[4,3-c]benzopyran-5,11-dione, 9) at a nearly diffusion-controlled rate. Ab initio calculations (G3) are consistent with the observed data. Carbene 2 is predicted to have a singlet ground state, which undergoes very facile, strongly exothermic (irreversible) ring opening to the cumulenone 3. The calculated barrier to formation of 4 (Wolff-rearrangement) is prohibitive. DFT calculations indicate that protonation of 3 on the beta-carbon is accompanied by cyclization to the protonated carbene 2H+, and that dimerization of 3 to 7 and 9 takes place in a single step with negligible activation energy.