4,4-DIMETHYLBICYCLO[3,1,0]HEXA-1(6),2-DIENE - A HIGHLY STRAINED 1,3-BRIDGED CYCLOPROPENE

The photochemical rearrangement of 4,4-dimethylcyclohexa-2,5-dienylide ne (3) to 4,4-dimethylbicyclo[3.1.0]hexa-1(6),2-diene (5) was investig ated by means of the matrix isolation technique. Carbene 3 was generat ed in argon matrices at 10 K by photolysis of the corresponding diazo compound (lambda > 550 nm) and was characterized by IR and UV/Vis spec troscopy and its characteristic thermal reaction with triplet oxygen. Long-wavelength irradiation (lambda > 515 nm) induced an irreversible rearrangement of carbene 3 to the highly strained cyclopropene 5. This is in contrast to the previously investigated 1H-bicyclo[3.1.0]hexa-3 ,5-dien-2-one (1). which is thermally labile even at 10 K and rearrang es back to 4-oxo-2,5-cyclohexadienylidene. The experimental findings w ere confirmed by ab initio calculations at the MP 2/6-31 G(d) level. M P2 and experimental IR frequencies are in good agreement. The strain e nergies of 5 and 1 are 75 and 78 kcal mol(-1), more than 20 kcal mol(- 1) larger than that of cyclopropene. The somewhat greater stability of 5 than 1 is a consequence of better pi delocalization in the diene un it of the bicyclic system.