Fluorine-decoupled carbon spectroscopy for the determination of configuration at fully substituted, trifluoromethyl- and perfluoroalkyl-bearing carbons: comparison with 19F-1H heteronuclear Overhauser effect spectroscopy.

Abstract

The synthesis of a series of α-trifluoromethylcyclohexanols and analogous trimethylsilyl ethers by addition of the Ruppert–Prakash reagent to substituted cyclohexanones is presented. A method for the assignment of configuration of such compounds, of related α-trifluoromethylcyclohexylamines and of quaternary trifluoromethyl-substituted carbons is described based on the determination of the 3JCH coupling constant between the fluorine-decoupled 13CF3 resonance and the vicinal hydrogens. This method is dubbed fluorine-decoupled carbon spectroscopy and abbreviated FDCS. The method is also applied to the configurational assignment of substances bearing mono-, di-, and perfluoroalkyl rather than trifluoromethyl groups. The configuration of all substances was verified by either 19F−1H heteronuclear Overhauser spectroscopy (HOESY) or X-ray crystallography. The relative merits of FDCS and HOESY are compared and contrasted. 2JCH, 3JCH, and 4JCH coupling constants to 19F decoupled CF3 groups in alkenes and arenes have also been determined and should prove to be useful in the structural assignment of trifluoromethylated alkenes and arenes.