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‐dienylidene (3) to 4,4‐dimethylbicyclo‐[3.1.0]hexa‐1(6),2‐diene (5) was investigated by means of the matrix isolation technique. Carbene 3 was generated in argon matrices at 10 K by photolysis of the corresponding diazo compound γ (>550 nm) and was characterized by IR and UV/Vis spectroscopy and its characteristic thermal reaction with triplet oxygen. Long‐wavelength irradiation (γ > 515 nm) induced an irreversible rearrangement of carbene 3 to the highly strained cyclopropene 5. This is in contrast to the previously investigated 1 H‐bicyclo[3.1.0]hexa‐3,5‐dien‐2‐one (1), which is thermally labile even at 10 K and rearranges back to 4‐oxo‐2,5‐cyclohexa‐dienylidene. The experimental findings were confirmed by ab initio calculations at the MP 2/6‐31 G(d) level. MP 2 and experimental IR frequencies are in good agreement. The strain energies of 5 and 1 are 75 and 78 kcalmol‐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 π delocalization in the diene unit of the bicyclic system.