Mechanistic Studies and Expansion of the Substrate Scope of Direct Enantioselective Alkynylation of α-Ketiminoesters Catalyzed by Adaptable (Phebox)Rhodium(III) Complexes

Abstract

Mechanistic studies and expansion of the substrate scope of direct enantioselective alkynylation of α-ketiminoesters catalyzed by adaptable (phebox)rhodium(III) complexes are described. The mechanistic studies revealed that less acidic alkyne rather than more acidic acetic acid acted as a proton source in the catalytic cycle, and the generation of more active (acetato-κ(2)O,O')(alkynyl)(phebox)rhodium(III) complexes from the starting (diacetato)rhodium(III) complexes limited the overall reactivity of the reaction. These findings, as well as facile exchange of the alkynyl ligand on the (alkynyl)rhodium(III) complexes led us to use (acetato-κ(2)O,O')(trimethylsilylethynyl)(phebox)rhodium(III) complexes as a general precatalyst for various (alkynyl)rhodium(III) complexes. Use of the (trimethylsilylethynyl)rhodium(III) complexes as precatalysts enhanced the catalytic performance of the reactions with an α-ketiminoester derived from ethyl trifluoropyruvate at a catalyst loading as low as 0.5 mol % and expanded the substrate scope to unprecedented α-ketiminophosphonate and cyclic N-sulfonyl α-ketiminoesters.

Recent Advances in the Carbon–Carbon Bond-Forming Reactions of N-Acylketimines

“…Based on this result, the authors developed a second-generation catalyst (method B: alkynyl complex 10). 46 As expected, the second-generation catalyst 10 showed higher catalytic activity than the first-generation catalyst 8 and could be applied over a wide substrate scope including related activated imines such as α-ketimino phosphonates and cyclic N-sulfonyl-α-ketimino esters.…”

Regioselective decarboxylative addition of malonic acid and its mono(thio)esters to 4-trifluoromethylpyrimidin-2(1H)-ones

“…This complementary method takes advantage of specific reagents featuring original fluorinated motives and/or functional groups which affords more complex derivatives via conventional functionalization or (hetero)cyclization [ 15 – 17 ]. Among these reagents, trifluoromethylketimines have drawn much research interest in recent years as key starting materials for the synthesis of trifluoromethyl-substituted amines [ 18 – 19 ], α-amino acids [ 20 – 23 ] as well as nitrogen-containing heterocyclic compounds [ 24 – 29 ]. It should be noted that the presence of a strong electron-withdrawing trifluoromethyl group is responsible for the sufficient reactivity of the electrophilic ketimine function with various carbon nucleophiles in these reactions.…”

Catalytic Asymmetric Synthesis of α‐Trifluoromethylated Carbinols: A Case Study of Tertiary Propargylic Alcohols

“…Therefore, the authors proposed that am onoalkynyl speciesw as responsible fort he catalysis. [29] Gong, Song, and co-workers reported as imilar study based on the use of ar hodiumc omplext hat contained an imidazoline moietyi nstead of an oxazoline. The additional nitrogen atom allowed the catalytic activity to be finely tuned.…”