The addition of an electrophile to the alkylidenecycloproparenes (2a-d) is dominated by capture at the exocyclic centre with formation of the corresponding cycloproparenyl cation, e.g. (15). Subsequent reaction with the counter ion is usually accompanied by cleavage of the three-membered ring. Thus compounds (2) give the ethanones (4) with aqueous acids whilst anhydrous acetic acid yields the vinyl acetates (5). Silver(1)-catalysed methanolysis of (2) leads to vinyl ethers (6); the alkyne (7) is formed only from (2d) which carries a vinylic proton. Brominations and bromine water additions lead to products of ring expansion (8)-(10) or ring cleavage (11)-(14) depending upon the conditions employed. These latter reactions demonstrate a delicate balance between cycloproparenylcarbinyl cation formation and cleavage of the three-membered ring.
Oxygen transfer from dimethyldioxiran to 1-diphenylmethylidene-1 H-cyclopropabenzene (8b) proceeds to give the corresponding epoxy derivative (7b), the simplest and most stable oxaspiropentene yet recorded. The compound has been characterized spectroscopically prior to ring expansion and rearrangement to the cyclobutenone (9b) above 20°C; this is the sole product isolated when the reaction is performed at or above room temperature. If the oxygenation reaction is performed under conditions other than strictly anhydrous, the rearrangement (7b) → (9b) competes with protonation of the oxygen atom and sequential opening of the three-membered ring to provide the hydroxyethanone (11b) via the diol (10b).