Diels-Alder reactions of oxazolidinone dienophiles catalyzed by zirconocene bis(triflate) catalysts: Mechanism for asymmetric induction

The Diels-Alder cycloaddition reaction between enoyl-oxazolidinones 3 and dienes is efficiently catalyzed by zirconocene bis(triflate) catalysts (4 and 8) in CH2Cl2 or nitroalkane solvents. The use of chiral catalyst [S]-8 led to significant asymmetric induction in the adducts derived from dienophiles 3 and cyclopentadiene, but only in nitroalkane solution at low temperatures and at lower catalyst loadings. The binding of acryloyloxazolidinone 3a to 8 was studied in detail by H-1, C-13, and F-19 NMR spectroscopy in both CD2Cl2 and nitroalkane solvent. These studies reveal that two isomeric, five-coordinate, monotriflate complexes (9a and 9b) are formed from 3a and 8 in both solvents. The minor isomer (9a) has the carbamate C=O coordinated to the metal at a central site, while in the major isomer (9b) it is coordinated to a lateral site. In nitromethane solvent, the ratio of 9a:9b approximate to 1:2.6 at -30 degrees C, while in CD2Cl2 the ratio is similar to 1:6.7. These studies, along with the sense of asymmetric induction observed, suggest that it is the minor isomer 9a that reacts most rapidly and selectively with dienes under catalytic conditions. The binding of 3a to 8 is strongly favored in nitromethane solvent with K-eq = 33.3 +/- 1.5 at -30 degrees C. Thermodynamic parameters for substrate binding were derived from VT H-1 NMR spectroscopic studies (Delta H-o = -9.8 +/- 1.0 kcal mol(-1); Delta S-o = -33 +/- 3 cal mol(-1) K-1). Complexes 9a and 9b interconvert predominantly by an intramolecular process (as revealed by VT NMR studies at different concentrations of 3a and 8 as well as 2D-EXSY spectra), while free triflate ion (formed via complexation of 3a to 8) undergoes rapid, associative exchange with bound triflate in residual complex 8 (as independently revealed by VT F-19 NMR studies involving 8 and [(Bu4N)-Bu-n][OTf]). The rates of both of these processes were studied by VT F-19 NMR spectroscopy. The activation parameters for triflate exchange involving 8 are Delta H-double dagger = 2.9 +/- 0.3 kcal mol(-1) and Delta S-double dagger -26 +/- 3 cal mol(-1) K-1 while the barrier to interconversion between 9a and 9b is 13.8 +/- 0.7 kcal mol(-1) at -30 degrees C with Delta H-double dagger = 17.2 +/- 0.9 kcal mol(-1) and Delta S-double dagger = 13.9 +/- 0.7 cal mol(-1) K-1. Diels-Alder reactions of 3a with, e.g., cyclopentadiene in nitroalkane or dichloromethane solvent are very rapid, even at low catalyst loadings and low temperatures, and rates of reaction cannot be conveniently monitored by H-1 NMR spectroscopy. However, the enantioselectivity was shown to vary with conversion, indicating that non-Curtin-Hammett conditions apply.