The Photochemical Generation of Isoindene and Its Dimerization to Diastereomeric 1-(Indian-1-yl)indenes

Abstract

Isoindene (1), generated by ultraviolet irradiation ( Vycor filter, acetone solution) of 1,4- dihydro-1,4-methanonaphthalene-2,3-dione (14), is shown to undergo homodimerization at -60°. This cycloaddition involves the intermolecular transfer of a hydrogen atom from one isoindene molecule to a second isoindene molecule in an [(8π+2σ)+ Sπ ] pericyclic process. The gross structure of the resulting diastereomeric dimers was confirmed by synthesis from 1-bromoindan (25) and indenyl anion (26). The stereochemistry of the dimers was determined by comparing their dihydro derivatives (33) and (34) with authentic samples. The authentic samples were themselves prepared by cis hydrogenation (Pd/C) of (E)-2,2′,3,3′-tetrahydro-1,1′-bi-1H-indenylidene (30) which yielded (34), and trans hydrogenation (Na/NH₃) of (30) which formed (33) together with (34) (ratio 7:3). The (E) stereochemistry of (30), which was a critical feature in the stereochemical assignments to (33) and (34), was confirmed by n.O.e. studies on it and its (D₄)derivative (32).

Complete analysis of the proton, proton coupling was conducted on (32). This identified a ⁵J coupling between protons C 3-H and C 7-H which contributed a 6% scalar coupling to the observed n.O.e . of 39% at C 7-H. This information allowed unambiguous stereochemical assignments to be made to (33).

The chirality of 1,1′-bi-1H-indenyls (43) and (44), prepared by the radical coupling of 1H-inden-1-ylmagnesium bromide, was also determined by a similar hydrogenation process.