Rearrangement of 1,1'-Spirobiindene and Thermochemical Evidence for Its Spiroconjugative Destabilization

Article abstract of DOI:10.1021/jo01313a028

Thermal rearrangement of 1,1'-spirobiindene (1) in a flow system at temperatures above 300 deg C leads exclusively to 3,4-benzofluorene, the product of vinyl migration.The same product is formed by direct irradiation at 254 nm or by treatment with potassium in refluxing THF.Determination of the heats of hydrogenation of 1 (-61.0 kcal/mol) and indene (-23.63 kcal/mol) reveals a destabilization in 1 of 13.7 kcal/mol relative to indene, in agreement with literature calculations that spiroconjugated systems like 1 possess high strain energies attributable to unfavorable ?-orbital interactions.