Acylation Mechanisms of DMSO/[D 6 ]DMSO with Di‐ tert ‐butylketene and Its Congeners

Dimethyl sulfoxide (DMSO) and t Bu 2 C=C=O in diglyme require heating to about 150 °C to furnish the Pummerer‐type product t Bu 2 CHCO 2 CH 2 SCH 3 through a novel mechanistic variant. The “ester enolate” t Bu 2 C=C(O – )–O–S + (CH 3 ) 2 arising through the reversible addition of DMSO (step 1) to C‐1 of t Bu 2 C=C=O must be trapped through protonation (step 2) at C‐2 by a carboxylic acid catalyst to form t Bu 2 CH–C(=O)–O–S + (CH 3 ) 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(CH 3 ) 2 group (E2 elimination, no ylide intermediate) results in the formation of t Bu 2 CHCO 2 – and H 3 CS–CH 2 + , whose combination (step 4) generates the final product. With a mixture of DMSO and [D 6 ]DMSO competing for t Bu 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 °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 t Bu 2 C=C=O generating the acylium cation t Bu 2 CHC≡O + can be excluded. Preparatory studies were undertaken to compare the mechanistic behavior of t Bu 2 C=C=O with that of two related acylating agents: (i) The anhydride ( t Bu 2 CHCO) 2 O 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 t Bu 2 CHCOCl affords ClCH 2 SCH 3 through a more common mechanistic variant involving neither the ketene nor the acylium cation t Bu 2 CHC≡O + : The modestly enhanced k H / k D 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 1 H and 13 C NMR integrations of deuterium‐shifted signals.