Fluorous catalysis under homogeneous conditions without fluorous solvents: a "greener" catalyst recycling protocol based upon temperature-dependent solubilities and liquid/solid phase separation.
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abstract
The thermomorphic fluorous phosphines P((CH(2))(m)()(CF(2))(7)CF(3))(3) (m = 2, 1a; m = 3, 1b) exhibit ca. 600-fold solubility increases in n-octane between -20 (1a = 0.104 mM) and 80 degrees C (63.4 mM) and 1500-fold solubility increases between -20 and 100 degrees C (151 mM). They catalyze conjugate additions of alcohols to methyl propiolate under homogeneous conditions in n-octane at 65 degrees C and can be recovered by simple cooling and precipitation and used again. This avoids the use of fluorous solvents during the reaction or workup, which are expensive and can leach in small amounts. Teflon shavings can be used to mechanically facilitate recycling, and (31)P NMR analyses indicate >97% phosphorus recovery (85.2% 1a, 12.2% other). (19)F NMR analyses show that 2.3% of the (CF(2))(7)CF(3) moieties of 1a leach, in some form, into the n-octane (value normalized to phosphorus). 1a similarly catalyzes additions in the absence of solvent. Yield data match or exceed those of reactions conducted under fluorous/organic liquid/liquid biphase conditions. The extra methylene groups render 1b more nucleophilic than 1a and, thus, a more active catalyst. The temperature dependence of the solubility of 1a is measured in additional solvents and compared to that of the nonfluorous phosphine PPh(3).
