An early attempt to access 4,7-dihydroisoindole — the closest relative of thermodynamically unstable isoindole was performed 1985.

[1] It was based on the classical Paal-Knorr synthesis under conditions which probably harmed the electron-rich pyrrole ring.

Observed instability of 4,7-dihydroisoindole led researchers to conclude that it was not a useful intermediate in the porphyrin chemistry.

A likely reason for such stability is that the pyrrolic residue is more acidic (as NH-acid) as well as more nucleophilic than the respective reaction centers involved in the anticipated double bond migration; thus, the pyrrolic ring may serve to protect the double bond from the initiation of both carbocationic and carbanionic shifts.

4,7-Dihydroisoindole is universally a synthon of extended porphyrins, since its isolated double bond in the annelated cyclohexene ring can allow for modification by addition or cycloaddition reactions.