Aerobic oxidation of tetrahydrofuran by a series of iron (III) containing POSS compounds

A series of iron (III) containing POSS compounds, [Bu₄N][(R)₇Si₇O₁₂FeCl] (R = isobutyl- (1), ethyl- (2), phenyl- (3), and cyclopentyl- (4)) were investigated as potential catalysts for aerobic oxidation of tetrahydrofuran (THF). The oxidation products for this reaction were characterized by IR, GC and GC-MS. The major oxidation product was γ-butyrolactone (C₄H₆O₂, GBL); while the two minor products were 2-hydroxy-THF and its tautomer, 4-hydroxybutanal. A third minor oxidative product is believed to be an isomer of dihydroxy-THF. THF free syntheses of 2-4 have been developed and all three compounds were characterized by elemental analysis, IR, and X-ray diffraction studies. The crystallographic data for 1-4 demonstrate a similar iron coordination sphere between the four compounds, but the different substitutents on the various POSS. The corresponding turnover numbers for the aerobic oxidation of THF suggest that smaller substitutents on the POSS ligands improve the stability of the catalytic species. When compared to other published catalytic iron systems, these turnover numbers appear to be the largest for the aerobic conversion of THF to GBL.