2,2-DIMETHYLISOINDENE AND 5,5-DIMETHYLBENZOBICYCLO[2.1.O]PENT-2-ENE

2, 2-Dimethylisoindene (1) is quite stable in solution at room temperature in the absence of oxygen. It can be prepared from the azoxy precursor 3 by irradiation in a glassy matrix or by room temperature deoxygenation with Si2C16, and from the dibromo precursor 4 by reaction with metals. The photochemical interconversion 1 < = > 2 is described; the activation energy for the facile thermal “forbidden” ring opening 2 → 1 is 18.9 kcal/mol. The NMR, UV, fluorescence, polarized excitation and emission, and MCD spectra of 1 are reported. Comparison with semiempirical calculations using singly and doubly excited configurations shows good agreement and suggests the presence of a low-energy “doubly excited” state, believed to be responsible for the photochemical reactivity of 1. © 1979, American Chemical Society. All rights reserved.