Experimental Section
Enantiomeric excesses were determined by gas chromatography, with a 25 m, CHIRAL-DEX-bonded silica column. Electrospray M S was performed by Dr. H. E. K. Matimba and Dr. R. T. Aplin. All NMR spectra were recorded on a BrukerAMX800 spectrometer ('H: 500 MHz) with an inverse gradient probe at low temperatures; the heteronuclear experiments ("P: 202 MHz, 13C: 125 MHz) werecarried out with a broad-band probe. TOCSY was performed with the MLEV-17 spin lock (8 3 kHz) with a mixing time of 70 ms (bracketed by 2.8 ms "trim pulses"). Signals for a modified I-D gradient ROESY (GROESY) were selected by 25 ms Gaussian 180' pulses with application of a continuous-wave spin lock (2.6 kHzJ, a relaxation delay, and mixing times of 4 s and 300 ms. The gradients used were 14. -6: -20% of the maximum gradient strength (about 45 Gcm-I).
Typical procedure for preparing samples for in situ NMR experiments: Silver triflate (0.006 g, 0.02 mmol) was added to a vigorously stirred solution of [(S)-(BINAP)Pd(Ph)(l)] [17] (0.020 g, 0.02 mmol) in [DJTHF (0.5 mL) at -78 "C. Stirring was continued for 10 min, durlng which time a white precipitate formed. The suspension was centrifuged cold, and the pale colored liquid rapidly decanted over a cannula into a 8 mm NMR tubecontaining 2,3-dihydrofuran (10 pL, 0.13 mmol, excess) at -78 "C. The tube was transferred to the pre-cooled probe, and spectra measured as described.