1351850-18-4Relevant academic research and scientific papers
Acylation mechanisms of DMSO/[D6]DMSO with Di-tert-butylketene and its congeners
Knorr, Rudolf
, p. 6335 - 6342 (2011)
Dimethyl sulfoxide (DMSO) and tBu2C=C=O in diglyme require heating to about 150 °C to furnish the Pummerer-type product tBu 2CHCO2CH2SCH3 through a novel mechanistic variant. The "ester enolate" tBu2C=C(O -)-O-S+(CH3)2 arising through the reversible addition of DMSO (step 1) to C-1 of tBu2C=C=O must be trapped through protonation (step 2) at C-2 by a carboxylic acid catalyst to form tBu2CH-C(=O)-O-S+(CH3)2 so that the reaction can proceed. The ensuing cleavage (step 3) of the O-S bond and one of the C-H bonds in the-S(CH3)2 group (E2 elimination, no ylide intermediate) results in the formation of tBu2CHCO 2- and H3CS-CH2+, whose combination (step 4) generates the final product. With a mixture of DMSO and [D6]DMSO competing for tBu2C=C=O in diglyme, the small value of the kinetic H/D isotope effect (KIE) kH/kD = 1.26 at 150 °C indicates that the cleavage of the C-H/C-D bonds (step 3) does not occur in the transition state with the highest free enthalpy. Therefore, the practically isotope-independent steps 1 and 2 determine the overall rate. The alternative slow initial protonation at C-2 of tBu2C=C=O generating the acylium cation tBu2CHC≡O+ can be excluded. Preparatory studies were undertaken to compare the mechanistic behavior of tBu2C=C=O with that of two related acylating agents: (i) The anhydride (tBu2CHCO)2O affords the same Pummerer-type product more slowly, again with an unexpectedly small KIE of 1.24 at 150 °C, which indicates that the overall rate is limited here by the almost isotope-independent initial O-acylation of DMSO in the addition/elimination (AE) mechanism. (ii) The acyl chloride tBu2CHCOCl affords ClCH 2SCH3 through a more common mechanistic variant involving neither the ketene nor the acylium cation tBu2CHC≡O +: The modestly enhanced kH/kD value of 2.4 at 55 °C shows that the C-H/C-D bond fissions contribute to the overall rate in cooperation with the retarded initial O-acylation. Deuterium labeling was quantified through 1H and 13C NMR integrations of deuterium-shifted signals.
