Determination of absolute configuration using vibrational circular dichroism spectroscopy: The chiral sulfoxide 1-(2-methylnaphthyl) methyl sulfoxide

We report the determination of the absolute configuration (AC) of the chira l sulfoxide, 1-(2-methylnaphthyl) methyl sulfoxide, 1, using vibrational ci rcular dichroism (VCD) spectroscopy. The VCD of 1 has been measured in the mid-IR spectral region in CCl4 solution. Analysis employs the ab initio DFT /GIAO methodology. DFT calculations predict two stable conformations of 1, E and Z, Z being lower in energy than E by <1 kcal/mol. In both conformatio ns the S-O bond is rotated from coplanarity with the naphthyl moiety by 30- 40 degrees. The predicted unpolarized absorption ("IR") spectrum of the equ ilibrium mixture of the two conformations permits assignment of the experim ental IR spectrum in the mid-IR spectral region. The presence of both E and Z conformations is clearly evident. The VCD spectrum predicted for S-l is in excellent agreement with the experimental spectrum of (-)-1, unambiguous ly defining the AC of 1 as R(+)/S(-).