1H-BICYCLO[3.1.0]HEXA-3,5-DIEN-2-ONE - A STRAINED 1,3-BRIDGED CYCLOPROPENE

Triplet 4-oxocyclohexa-2,5-dienylidene (5) gives 1H-bicyclo[3.1.0]hexa-3,5-dien-2-one (4) on irradiation into its long-wavelength triplet-triplet absorption band (lambda = 508-566 nm). Bicyclus 4 was characterized by IR spectroscopy in partially oriented matrices, by deuterium and oxygen-18 isotopic labeling and by comparison of experimental data with ab initio calculations. 4 is highly labile and readily rearranges back to carbene 5 thermally or on visible light (lambda = 470 nm) or infrared irradiation. The rates of the thermal 4 --> 5 rearrangement have been measured in argon, krypton, xenon, and nitrogen matrices, and deuterium kinetic isotope effects have been determined. The data show that 4 is directly transformed into 5, with intersystem crossing being rate determining. At low temperatures (< 20 K), the rates are independent of temperature, which indicates that the rearrangement occurs via quantum mechanical tunneling. MP2/6-31G(d) calculations show that the cyclopropene ring in 4 is tilted by 129.6-degrees with regard to the cyclopentene ring. The torsional angle between the two carbon 2p-pi-orbitals in the cyclopropene ring is 9-degrees, and the pyramidalization angles at C5 are 19.2-degrees. The extra strain energy caused by distortion of the cyclopropene double bond is compensated by the pi-stabilization energy of the dienone system. Thus, the total strain energy is only 54 +/- 1 kcal/mol-comparable to the strain energy of cyclopropene.