The Synthesis and Reactivity of 5H-Cycloprop[f]isobenzofuran and Related Compounds. A Kinetic Probe for the Mills-Nixon Effect.

5H-Cycloprop[f]isobenzofuran (6) and the sulfur analogue 5H-cyclopropa[f][2]benzothiophen (18) have been prepared by a sequence of reactions involving trapping of 1,2-dibromocyclopropene with 3,4-dimethylidenetetrahydrofuran and 3,4-dimethylidenetetrahydrothiophen followed by sequential dehydrogenation and di-dehydrobromination. Both cyclopropa-fused heterocycles, like their parents, have limited stability. Several other 5,6-methylene-bridged and 5,6-disubstituted isobenzofurans (32) have been generated and characterized as their adducts with dimethyl fumarate. Second-order rate constants for the reaction of dimethyl fumarate with isobenzofuran, 5H-cycloprop[f]isobenzofuran as well as the series of substituted derivatives have been measured. The reactivity span is only one order of magnitude suggesting that pi-bond fixation (the Mills-Nixon effect) does not play a significant role in determining the reactivity of (6).