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) catal ysts (4 and 8) in CH2Cl2 or nitroalkane solvents. The use of chiral ca talyst [S]-8 led to significant asymmetric induction in the adducts de rived from dienophiles 3 and cyclopentadiene, but only in nitroalkane solution at low temperatures and at lower catalyst loadings. The bindi ng 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. Th ese studies reveal that two isomeric, five-coordinate, monotriflate co mplexes (9a and 9b) are formed from 3a and 8 in both solvents. The min or isomer (9a) has the carbamate C=O coordinated to the metal at a cen tral site, while in the major isomer (9b) it is coordinated to a later al 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. T hese studies, along with the sense of asymmetric induction observed, s uggest that it is the minor isomer 9a that reacts most rapidly and sel ectively 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 rev ealed by VT NMR studies at different concentrations of 3a and 8 as wel l as 2D-EXSY spectra), while free triflate ion (formed via complexatio n of 3a to 8) undergoes rapid, associative exchange with bound triflat e in residual complex 8 (as independently revealed by VT F-19 NMR stud ies involving 8 and [(Bu4N)-Bu-n][OTf]). The rates of both of these pr ocesses were studied by VT F-19 NMR spectroscopy. The activation param eters 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 ni troalkane or dichloromethane solvent are very rapid, even at low catal yst loadings and low temperatures, and rates of reaction cannot be con veniently monitored by H-1 NMR spectroscopy. However, the enantioselec tivity was shown to vary with conversion, indicating that non-Curtin-H ammett conditions apply.