Stereoselektive Protonierung von Carbanionen, 2. Diastereoselektive Protonierung von Schöllkopf‐Bislactimether‐Anionen

Stereoselective Protonation of Carbanions, 2[1]. — Diastereoselective Protonation of Schöllkopf's Bislactim Ether Anions The anions of the bis‐lactim ethers 5–7 serve as models for a systematic investigation of diastereoselective protonation. From 5Li, 6Li, and 7Li in THF at −78°C 30 proton sources (OH, NH, SH, CH) of different acidity and bulkiness produce the following cis/trans ratios: 5 8:92 → 37:63; 6 15:85 → 48:52; 7 25:75 → 59:41 (equilibria ≈ 50:50). Substitution of R1 = iPr in 5 by Me (→ 6) decreases the stereoselectivity slightly, but substitution of OMe in 6 by OEt (7) much stronger (Table 1). Most of the added Lewis acids show week or even leveling effects, whereas with 5K a dramatic change is observed with cis/trans‐5 = 27:73 → 89:11 (Table 2). By variation of solvent and several additives only minor effects on the cis/trans ratio are observed (Table 3). Backed by 13CNMR spectra of the ion pairs and literature data protonation of 5Li‐7Li is supposed to occur on a dimer, which will mainly react to give trans‐5–7. The prevailing formation of cis‐5 from 5K is explained by the reaction of the monomeric ion aggregat.