Gas-phase reactivity of sulfate esters and analogs: Why is the sulfur center unreactive?

作者：Correra Thiago C; Riveros Jose M^*

来源：International Journal of Mass Spectrometry, 2013, 354: 326-332.

DOI：10.1016/j.ijms.2013.07.008

• Gas-phase sulfate esters reactivity
• Nucleophilic substitution at S vs C
• DFT calculations
• Energy profiles of reactions
• Dynamic hindrance of reaction channels

Sulfate esters and analogs are important species in synthetic chemistry, in processes catalyzed by sulfatases and in atmospheric chemistry because of their role as precursors in the formation of aerosols. In order to understand the intrinsic reactivity of these systems toward simple nucleophiles and the lack of reactivity in the sulfur center, calculations were carried out for (MeO)(2)SO2, MeS(OMe)O-2 and (MeO)(2)SO reacting with OH-, NH2-, SH-, F-, MeO- and EtO- at the B3LYP/6-311+G(3df,2p) level of theory. Our results are in agreement with previous experimental gas-phase studies that showed substitution at the C atom (S(N)2@C) and elimination promoted by abstraction of a proton followed by release of formaldehyde (E(CO)2) to be the main reaction channels for dimethyl sulfate and sulfite, and proton transfer (PT) for methyl methanesulfonate. Our calculations show that the barriers for substitution at the S atom (S(N)2@S) are comparable, and sometimes even lower in energy, than the barriers calculated for the S(N)2@C pathway for strong nucleophiles such as OH- and NH2-. Yet, S(N)2@S reactions are only observed in a few cases in dimethyl sulfite in reactions promoted by delocalized carbanions. Calculations that map out the energy profile of the nucleophile approaching the C and S center reveal a much more favorable energy pathway for reaction at the C center due to the electrostatic repulsion of the oxygens surrounding the S center. Therefore, reactivity at the S center in these systems is dictated-by a delicate balance between the reaction dynamics and the thermochemistry of these systems and attack of strong nucleophiles at the S center, although thermochemically favorable, is dynamically hindered and thus promote crossing over to the C route pathway.

• 出版日期2013-11-15