Deoxygenation of Tertiary Amine Oxides with Carbon Bisulfide

Reduction of various tertiary amine oxides with carbon disulfide was examined and kinetic experiments were carried out. Trialkylamine oxides and N,N-dialkylarylamine N-oxides were readily reduced by CS2 to give the corresponding tertiary amines in good yields, while heteroaromatic amine N-oxides such as picoline N-oxide were not affected. The oxygen atom in the N-oxide was found to be transferred to CO2 upon mass spectral analysis of the gas evolved. The kinetic experiments were carried out following the UV spectra of N,N-dimethylaniline N-oxide in CH3CN containing much excess of CS2 and the rate was found to be of 2nd order in the N-oxide and CS2. Activation parameters (ΔH=55.7 kJ mol−1, ΔS=−78.2 J K−1 mol−1 at 20 °C are characteristic of a normal bimolecular reaction. The logarithms of the rate constants for para-substituted N,N-dimethylaniline N-oxides are nicely correlated with Hammett σ values and a small negative ρ value (ρ=−0.2) was obtained. The rate of reaction was faster in polar aprotic solvents than in nonpolar or protic solvents. These observations seem to suggest that the reaction proceeds via an initial nucleophilic attack of the N-oxide oxygen at carbon disulfide followed by the rate-determining N–O bond fission to give the tertiary amine.