Synthesis and reactivity of difluorocyclopropenone: Synthetic approaches toward difluoroketene

Difluorocyclopropenone (1) was prepared by gas-phase photolysis of difluoromaleic anhydride and was characterized spectroscopically. It is unstable at room temperature but may be stored for long periods of time at $-78\sp\circ$C. The structure and dipole moment of 1 were determined by microwave spectroscopy. Ab initio calculations were also used to calculate the structure and dipole moment as well as bond orders and thermodynamics of 1. These results as well as the fluorine NMR data all suggest that 1 possesses aromatic character similar to that of the parent cyclopropenone. Numerous reactions of difluorocyclopropenone have been investigated including nucleophilic, electrophilic and cycloaddition reactions. The reaction of isobenzofuran and resulted in the formation of a 2:1 adduct as opposed to the expected Diels-Alder product. Difluorocyclopropenone was extremely sensitive to nucleophilic addition and reacted instantly with water. However, did not react as readily with electrophiles when compared to cyclopropenone. A second topic, approaches toward the synthesis of difluoroketene, is also discussed. Difluoroketene is predicted to be an unusually unstable compound and decomposes to carbon monoxide and difluorocarbene at 35$\sp\circ$C. The structure and vibrational spectra of difluoroketene were calculated using ab initio methods at HF/6-31G$\sp\*,$ CISD/6-31G$\sp\*,$ MP2/6-31G$\sp\*,$ and QCISD/6-31G$\sp\*$ levels of theory. At the MP2/6-31G$\sp\*$ and QCISD/6-31G$\sp\*$ level of theory difluoroketene is predicted to have a bent structure. The initial approach to the synthesis of difluoroketene was to photolyze 4,4-difluoropyrazoline-3,5-dione in an argon matrix at 11 K. However, efforts to synthesize the difluoropyrazoline precursor failed. This failure was perceived to be a result of the instability of this compound at room temperature. In order to increase the stability of this compound the diazo functionality was protected as a 2,5-thiadiazolidinedione to give 3-thia-7,7-difluorodiazobicyclo (3.3.0) octane-2,4,6,8-tetraone (6). This compound was photolyzed ($\gamma=265$ nm) in an argon matrix at 11 K. Comparison of the experiment IR spectrum to calculated frequencies revealed that difluoroketene is not formed or is not observable under these conditions. Difluorocarbene is found in the matrix and may be a secondary photoproduct of difluoroketene