Structures and stereoinversions of β-shielded α-Li (or K or Cs)-acrylonitriles: A mechanism with neighborly assistance

Nucleofugal behavior of a β-shielded α-cyanovinyl carbanion

“…For instance (bottom line of Scheme 3), the heterogeneous, slow deprotonation of 13 in THF by the insoluble base potassium hydride (KH) afforded 11 and 1 as the only products via cleavage of the potassium alkoxide 14a and protonation of the emerging 2K by residual 13 , so that the final introduction of pivalaldehyde ( 4 ) produced no adduct 7 . However, the same result was also found when 13 was added to a homogeneous solution of PhCH 2 K in THF; this suggested that 13 had transferred its proton to the emerging, thermally stable [4] cleavage product 2K faster than to the residual PhCH 2 K (perhaps a problem of slow mixing). On the other hand, deprotonation of 13 by the Grignard reagent MeMgBr in THF was complete (quantitative CH 4 evolution) within 20 min at 0 °C; the subsequent cleavage reaction of the generated magnesium alkoxide 14b was very slow at rt, creating 2MgBr (Scheme 3) in the presence of t -BuCH=O ( 4 ) and therefrom 7 together with only a small amount of 1 .…”

“…The α-cyanoalkenyllithium 2Li (top line of Scheme 2) may be prepared [4] either by LDA-mediated deprotonation (LDA = iPr 2 NLi) of the β-shielded acrylonitrile derivative 1 or by an Sn/Li transmetalation reaction of n -butyllithium (“ n -BuLi”) with the α-stannyl derivative 3 . Due to intermolecular LiN coordination, 2Li was deduced [4] to form a clustered ground state that showed no obvious rate anomaly with previously [4] studied electrophiles; this should also hold true here for pivalaldehyde t -BuCH=O ( 4 , “ t -Bu” = tert -butyl). The chiral adduct 7 of 4 exhibited four different NMR signals for the four methyl groups at the 1- and 3-positions.…”

“…In view of the apparently modest spatial demand of the α-cyano substituent in the adducts of 2Li , it seemed surprising that the following ketones generated no (or no kinetically stable) C=O adducts at rt: t -Bu 2 C=O, pivalophenone ( t -Bu–C(O)–Ph), benzophenone (Ph 2 C=O) [7], bis(1-methylcyclopropyl)ketone [8], and dicyclopropyl ketone ( 19 ). Instead of an adduct of 19 to 2Li , the carboxyalcohol 21 (a known [9] product of 19 with LDA) in Scheme 5 and the acrylonitrile derivative 1 were obtained from 19 ; this run was conducted in the absence of LDA with a sample of 2Li that had been prepared in Et 2 O through an Sn/Li interchange reaction [4] of 3 with n -BuLi. Therefore, 2Li should have deprotonated 19 with formation of 20 that was trapped by 19 to give 21 .…”

“…This “normal” C=O addition reaction was obviously faster than the deprotonation of 19 and also apparently irreversible under the reaction conditions, whereas the unobserved C=O addition of 19 to 2Li might be possible yet quickly reversible with a terminating proton transfer from 19 to 2Li as shown in Scheme 5. Alternatively, carbonyl additions to 2Li might be generally retarded (relative to protonations) on account of the clustered [4] ground state structure of 2Li in solution.…”

“…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.…”