STEREOELECTRONIC EFFECTS IN IONIZATION REACTIONS OF CYCLIC ORTHO THIOESTERS

The objective of this work was to determine whether stereoselectivity observed in certain condensed-phase ionic reactions of ortho thioesters was also evident in related gas-phase reactions. Ionization of the C-2 epimers of cis-4,6-dimethyl-2-(methylthio)-1,3-dithianes 6 and 7 under FT-ICR conditions produced gaseous ions of m/z 147 corresponding to the cis-4,6-dimethyl-1,3-dithian-2-yl cation. Kinetics of ionization were followed by using mixtures of each epimer with 2-methylpropane or with 2-(ethylthio)ethanol. Kinetic parameters were calculated from the rates of decay of the precursor ions (m/z 43 from 2-methylpropane and m/z 75 from 2-(ethylthio)ethanol) at different pressures of reactants. Within experimental error, the specific rates of reaction of each epimer with m/z 43 and with m/z 75 were equal. Ionization is not therefore sensitive to stereochemical configuration at the C-2 reaction site in the gas phase. Solution-phase methylthiolation and methylation of the equatorial epimer (4c,6c-dimethyl-2r-(methylthio)-1,3-dithiane, 7) led to reversible cleavage at C-2 to produce the cis-4,6-dimethyl-1,3-dithian-2-yl cation. Addition of sodium methanethiolate quenched the equilibrium and led irreversibly to a mixture of epimers 6 and 7 in which the axial epimer 6 was in modest excess. The control of reaction stereoselectivity in the formation and trapping of 1,3-dithian-2-yl cations from ortho thioesters is discussed.