Short-lived pivaloylmetals, (H₃C)₃C-COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=CtBu₂ from the overcrowded metal alkoxides tBuC(=O)-C(-OM)tBu₂ (M=MgX, Li, K). In all three cases, this fission step is counteracted by a faster return process, as shown through the trapping of tBu-COM by O=C(tBu)-C(CD₃)₃ with formation of the deuterated starting alkoxides. If generated in the absence of trapping agents, all three tBu-COM species “dimerize” to give the enediolates MO-C(tBu)=C(tBu)-OM along with O=CtBu₂ (2 equiv). A common-component rate depression by surplus O=CtBu₂ proves the existence of some free tBu-COM (separated from O=CtBu₂); but companion intermediates with the traits of an undissociated complex such as tBu-COM & O=CtBu₂ had to be postulated. The slow fission step generating tBu-COMgX in THF levels the overall rates of dimerization, ketone addition, and deuterium incorporation. Formed by much faster fission steps, both tBu-COLi and tBu-COK add very rapidly to ketones and dimerize somewhat slower (but still fairly fast, as shown through trapping of the emerging O=CtBu₂ by H₃CLi or PhCH₂K, respectively). At first sight surprisingly, the rapid fission, return, and dimerization steps combine to very slow overall decay rates of the precursor Li and K alkoxides in the absence of trapping agents: A detailed study revealed that the fast fission step, generating tBu-COLi in THF, is followed by a kinetic partitioning that is heavily biased toward return and against the product-forming dimerization. Both tBu-COLi and tBu-COK form tBu-CH=O with HN(SiMe₃)₃, but only tBu-COK is basic enough for being protonated by the precursor acyloin tBuC(=O)-C(-OH)tBu₂.

Partial labeling by deuterium may be quantified through simple integrations of those ¹H (200 or 400 MHz) and ¹³C (100.6 MHz) NMR resonances that are split into pairs by chemical shifts ⁿΔ=δ(deuterated)−δ(nondeuterated) as induced by deuterium across n>2 chemical bonds. The relative intensities of the two components of a pair are shown to be influenced to practically equal degrees by relaxation effects, so that a deuterium fraction may be determined from ¹H and ¹³C integral pairs at more remote molecular positions under the routine conditions of fast accumulative spectral acquisition.

Die quantitative Bestimmung einer partiellen Deuterierung gelingt problemlos durch Integrieren derjenigen ¹H (200 oder 400 MHz) und ¹³C (100.6 MHz) NMR Resonanzen, welche durch Isotopieverschiebungen ⁿΔ=δ(deuteriert)−δ(undeuteriert) in Paare aufgespalten sind, verursacht durch die Deuteriummarkierung in Abständen von n>2 chemischen Bindungen vom Markierungsort. Die Relativintensitäten der beiden Komponenten eines Paares erweisen sich als praktisch gleich stark beeinflußt durch Relaxationseffekte, so dass sich der Deuteriumgehalt aus den ¹H- und ¹³C-Integralpaaren an entfernteren molekularen Positionen unter Routinebedingungen der raschen Spektrenakkumulation ermitteln läßt.

Dimethyl sulfoxide (DMSO) and tBu₂C=C=O in diglyme require heating to about 150 °C to furnish the Pummerer-type product tBu₂CHCO₂CH₂SCH₃ 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 tBu₂CHCO₂^– 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) 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 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 ClCH₂SCH₃ through a more common mechanistic variant involving neither the ketene nor the acylium cation tBu₂CHC≡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 ¹H and ¹³C NMR integrations of deuterium-shifted signals.

The ketene tBu₂C=C=O is prepared from tBu₂C=O in three steps (performable as a two-stage operation) through elimination of HCl from the intermediate product tBu₂CCl–CH=O. The acid tBu₂CH–CO₂H, obtainable in two, three, or four preparative stages from tBu₂C=O, adds slowly to the ketene to produce the anhydride (tBu₂CH–CO)₂O. Elemental lithium together with ClSiMe₃ converts tBu₂CCl–CH=O into tBu₂C=CH–OSiMe₃, which is a durable precursor of tBu₂CH–CH=O, making this aldehyde easily and cheaply available from tBu₂C=O. By exclusion of alternative mechanistic possibilities, the reduction of tBu₂CCl–CH=O by tBuMgCl is shown to involve at least one single-electron transfer, leading to the enolate tBu₂C=CH–OMgCl, which can be converted into tBu₂CH–CH=O (three steps from tBu₂C=O) or into tBu₂C=CH–OSiMe₃. Hydride transfer from NaBH₄ to tBu₂CCl–CH=O affords tBu₂CCl–CH₂OH, the transformations of which provide an entertaining set of S_N1-type reactions. Several other examples of carbenium-type behavior are encountered in this gem-tBu₂ system; they are attributed to steric congestion, which also impedes bond rotations in the anhydride and in two esters. A convenient route to tBu₂CH–C≡N (five steps from tBu₂C=O) uses the conversion of tBu₂C=CH–OSiMe₃ into tBu₂CH–CH=NOH. The slow thermal (Z)/(E) equilibration of tBu₂CH–NH–CH=O reveals the ranking of ecliptic repulsions as H₃C < tBu < tBu₂CH.