Pivaloylmetals (tBu‐COM: M=Li, MgX, K) as Equilibrium Components

Abstract

Short-lived pivaloylmetals, (H(3)C)(3)C-COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=CtBu(2) from the overcrowded metal alkoxides tBuC(=O)-C(-OM)tBu(2) (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(CD(3))(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=CtBu(2) (2 equiv). A common-component rate depression by surplus O=CtBu(2) proves the existence of some free tBu-COM (separated from O=CtBu(2)); but companion intermediates with the traits of an undissociated complex such as tBu-COM & O=CtBu(2) 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=CtBu(2) by H(3)CLi or PhCH(2)K, 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(SiMe(3))(3), but only tBu-COK is basic enough for being protonated by the precursor acyloin tBuC(=O)-C(-OH)tBu(2) .

Accessing Stable Magnesium Acyl Compounds: Reductive Cleavage of Esters by Magnesium(I) Dimers

“…One recent demonstration of the synthetic power of samarium diiodide has been its use in the in situ formation of samarium acyl radicals through reductive cleavage of unactivated esters . In light of this, we wondered if magnesium(I) dimers might reductively cleave esters to give stable magnesium acyl complexes, all prior examples of which exist only transiently at room temperature . Here we show that this is the case, and while we have not yet had success using these complexes as nucleophilic acyl anion sources in organic synthesis, this possibility remains.…”

Kinetics of α-(2,6-Dimethylphenl)vinyllithium: How To Control Errors Caused by Inefficient Mixing with Pairs of Rapidly Competing Ketones

“…As a sluggish chain carrier, the Li alkoxide 10 needed 10 days to consume 9a with creation of 4d and remnant 10 in a 95:5 ratio. However, the corresponding but much faster decay of the potassium alkoxide 11 required less than 150 min for the complete consumption of 9a via 11 and its fragment 12 with generation of t -Bu 2 CO and alkene 4d …”

Nucleofugal behavior of a β-shielded α-cyanovinyl carbanion

“…(iii) The alternative trapping [3] of the carbonyl component by means of a nucleophile was not tried here but might serve to accumulate the organometallic equilibrium component for the purpose of spectroscopic characterization or X-ray diffraction analyses.…”

“…Apart from deprotonation reactions or formation of a separated ion pair and other trivial examples, the (at least formally) heterolytic cleavage of C–C single bonds can provide cases of interest if it generates organometallic compounds under unusual conditions. The well-known cases of alkoxide fission [1–3] (top line of Scheme 1) may be viewed as a reversed formation of an alkoxide A 1 M 1 from an organometallic C–M 1 and a carbonyl compound (R 1 ) 2 C=O. Such a C–C fission (retro-addition reaction) can occur already near room temperature (rt) if the nucleofugal carbanion proves to be an electronically stabilized N≡C–CH 2 – [1] or allylic [2] species or a short-lived equilibrium component [3].…”

“…For this purpose, A 1 M 1 should be demetalated through work-up to give, for instance, the alcohol A 1 H ; the subsequent deprotonation of A 1 H by an M 2 -containing base BM 2 must be complete before the generated alkoxide A 1 M 2 begins to release C–M 2 that would otherwise be protonated by any unconsumed portion of A 1 H . We had used this technique [3] to generate the short-lived, NMR-invisible pivaloylmetals (H 3 C) 3 C–COM from the overburdened alkoxides of tri- tert -butylacyloin that were readily cleaved with relief of internal strain as a driving force. We now report here on the above-mentioned options (a) and (c) with the focus on some nucleofugal traits of the carbanion unit of the stable [4], β-shielded α-metalated acrylonitriles.…”