Synthesis, chemistry, and direct observation of (trifluoromethyl)carbenes synthetic approaches towards a fluorinated polyacetylene

Five different (trifluoromethyl)carbenes (1-5) have been observed directly in argon matrices at 12 Kelvin using infrared (IR) and ultraviolet (UV) spectroscopy. Photolysis of 3-halo-3-(trifluoromethyl)diazirines (X = F (13), X = Cl (11), and X = Br (12)) generates the respective singlet halo(trifluoromethyl)carbenes 1, 2, and 3. Photolysis of 3-methoxy-3-(trifluoromethyl)diazirine (14) generates singlet methoxy(trifluoromethyl)carbene (4), and photolysis of 2,2,2-trifluorodiazoethane (6) generates triplet 2,2,2-trifluoroethylidene (5). Each carbene was characterized by chemical trapping with hydrogen chloride (carbenes 1-4) or carbene dimerization (carbenes 1 and 5) in annealed matrices. Extended broad band irradiation at short wavelengths converts the carbenes to the respective alkenes (carbenes 1-3, 5) and 1,1,1-trifluoroacetone (carbene 4). A nitrogen matrix gave similar results for 1, 2, 4, and 5, but carbene 3 was found to photochemically add nitrogen and form diazo compound 7. Ab initio molecular orbital calculations predict that carbenes 1-4 have singlet ground states and that carbene 5 has a triplet ground state and are consistent with experimental results. The calculated IR (carbenes 1-5) and UV (carbenes 1-4) spectra for the ground state of each carbene were in good agreement with experimental values. Using calculated energies and IR frequencies as a guide, carbene 4 was found to exist only in the anti conformation. The selectivity of methoxy(trifluoromethyl)carbene (4) in cyclopropanation reactions was investigated. Using standard competition reaction analysis, 4 displayed low chemoselectivity towards electron rich and poor alkenes. In contrast, carbene 4 displayed exceptionally high diastereoselectivity. In an investigation unrelated to carbene studies, an approach towards a fluorinated derivative of polyacetylene (PA) was developed. Fluorinated monomer 1,2,3,4,5,6-hexafluoro-anti-tricyclo (4.2.0.0$\sp{2,5}$) octa-3,7-diene (50) was synthesized in two steps from hexafluorobenzene. Monomer 50 failed to polymerize under ring opening metathesis conditions using molybdenum alkylidene catalyst 58 or a tungsten hexachloride/tin tetramethyl catalyst. The failure to polymerize 50 under these conditions precluded investigation of the two steps anticipated to furnish the fluorinated PA