Acylation Mechanisms of DMSO/[D6]DMSO with Di-tert-butylketene and Its Congeners

Dimethyl sulfoxide (DMSO) and tBu(2)C=C=O in diglyme require heating to about 150 degrees C to furnish the Pummerer-type product tBu(2)CHCO(2)CH(2)SCH(3) through a novel mechanistic variant. The "ester enolate" tBu(2)C=C(O-)-O-S+(CH3)(2) arising through the reversible addition of DMSO (step 1) to C-1 of tBu(2)C=C=O must be trapped through protonation (step 2) at C-2 by a carboxylic acid catalyst to form tBu(2)CH-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 tBu(2)CHCO(2)(-) and H3CS-CH2+, whose combination (step 4) generates the final product. With a mixture of DMSO and [D-6]DMSO competing for tBu(2)C=C=O in diglyme, the small value of the kinetic H/ D isotope effect (KIE) k(H)/k(D) = 1.26 at 150 degrees 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 tBu(2)C=C=O generating the acylium cation tBu(2)CHC O+ can be excluded. Preparatory studies were undertaken to compare the mechanistic behavior of tBu(2)C=C=O with that of two related acylating agents: (i) The anhydride (tBu(2)CHCO)(2)O affords the same Pummerer-type product more slowly, again with an unexpectedly small KIE of 1.24 at 150 degrees 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 tBu(2)CHCOCl affords ClCH2SCH3 through a more common mechanistic variant involving neither the ketene nor the acylium cation tBu(2)CHC O+: The modestly enhanced k(H)/k(D) value of 2.4 at 55 degrees 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 H-1 and C-13 NMR integrations of deuterium- shifted signals.