Indene-isoindene rearrangement

It has been suggested that walk rearrangements are responsible for the photoisomerization of benzanelated heterocycles and carbocycles (30). A remarkable example is the indene-isoindene rearrangement (31). Irradiation of 1,1-dimethyl-indene 28 produces the relatively stable 2,2-dimethylisoindene 29, an o-chino-dimethane derivative which was also synthesized by independent routes (32). 

Figure 3. Possible pathways for the indene - isoindene rearrangement.

These cycloadditions with o-quinodimethanes provide a broad variety of useful fullerene functionalizations, since o-quinodimethanes can be prepared using several routes and the resulting cycloadducts are thermally stable [42], There exist several alternatives to the iodide-induced bromine 1,4-elimination of 1,2-bis (bromomethyl)-benzenes [44-47]. o-Quinodimethanes have been prepared by thermolysis of 3-isochromanone (42) [43], benzocyclobutenes (43) [48-50], isobenzothiophene 2,2-dioxides (44) [42] and sultines [51,52] or by photolysis of o-alkylphenones such as 45 [53-55] and could be added to Cjq in good yields (Scheme 4.7). Indene, thermally rearranged to isoindene, also adds to Cjq in similar fashion to quinodimethanes [56]. 

Thermal rearrangements of indenes (8) to isoindenes by hydrogen shifts have been revealed by some early investigations9,22 Z4. 

Indene (benzcyclopentadiene) 99 also reacts with MA. In this case, a 1,4 addition requires rearrangement to isoindene 100 and hence loss of aromaticity during the reaction occurs as shown below ... 

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