Cyclic (Alkyl)(amino)carbenes: Synthesis of Iminium Precursors and Structural Properties

Abstract

Using readily available preallylated aldehydes, we report a simple and divergent synthesis of cyclic (alkyl)­(amino)­carbene (CAAC) iminium precursors. Using a combination of crystallographic data and steric maps, we further elaborate on the specific steric properties of CAAC ligands with respect to state-of-the-art phosphine and carbene ligands.

Ambivalent Role of Rotamers in Cyclic(alkyl)(amino)carbene Ruthenium Complexes for Enantioselective Ring-Opening Cross-Metathesis

“…3 Since then, CAAC ligands have been shown to afford robust ruthenium precatalysts with remarkable catalytic performances in a number of metathesis transformations. 4,5 Their growing popularity has been attributed to the highly modular steric environment of the CAAC framework 6 and their propensity to generate stable, yet very reactive, catalysts. Using a new class of CAACs, namely, chiral cyclic(alkyl)(amino)carbenes (Chi-CAACs), 7 we recently extended their application to asymmetric olefin metathesis.…”

“…In 2007, Bertrand, Grubbs, and co-workers demonstrated that this symphony could greatly benefit from the use of cyclic­(alkyl)­(amino)­carbenes (CAACs) as ligands for ruthenium-catalyzed olefin metathesis (Figure ). Since then, CAAC ligands have been shown to afford robust ruthenium precatalysts with remarkable catalytic performances in a number of metathesis transformations. , Their growing popularity has been attributed to the highly modular steric environment of the CAAC framework and their propensity to generate stable, yet very reactive, catalysts. Using a new class of CAACs, namely, chiral cyclic­(alkyl)­(amino)­carbenes (ChiCAACs), we recently extended their application to asymmetric olefin metathesis. , To overcome the synthetic drawback plaguing chiral ligands and catalyst preparation (complex/low-yielding procedures), meanwhile facilitating the screening of CAAC-Ru catalysts, we capitalized on preparative high-performance liquid chromatographic resolution ( Prep HPLC), a technology very popular in the industrial setting .…”

Highly Robust and Efficient Blechert-Type Cyclic(alkyl)(amino)carbene Ruthenium Complexes for Olefin Metathesis

“…Olefin metathesis represents a highly versatile synthetic tool to build a plethora of valuable building blocks and organic compounds. , Thanks to the development of bench-stable and well-defined ruthenium-arylidene complexes, this catalytic reaction is intensively used in polymer chemistry , and fine chemistry, and it has also found applications in the valorization of biomasses and the depolymerization of polyethylene . Despite these remarkable achievements, the quest for more efficient Ru-complexes remains a very active research topic across academia and industry. , A significant breakthrough was accomplished in 2007 when it was reported that the use of cyclic­(alkyl)­(amino)­carbene (CAAC) ligands − could provide very active catalysts for the ethenolysis of vegetable oils (0.0001 mol % Ru-1 ; TON up to 390.000). , This peculiar behavior, surpassing state-of-the art N-heterocyclic carbene (NHC) analogues, was shown to result from the improved stability of the corresponding Ru-methylidene intermediate, imparting resilience toward bimolecular decomposition. − Obviously, structural modifications of the CAAC ligand framework were thoroughly investigated, which led to several new CAAC Ru-complexes ( Ru-2 – Ru-7 , Figure A), − including optically pure congeners Ru*-8 recently developed by our groups. − Based on a benchmark reaction (i.e., RCM of diethyldiallylmalonate 1a (DEDAM; see Table )), a map of structure–activity relationship (SAR) allowed one to highlight two key features: (i) bulky ortho- N -aryl substituents such as 2,6-diisopropylphenyl (DIPP) have a detrimental effect on the catalyst activity; and (ii) the introduction of NO 2 on the labile styrenyl-ether ligand reduces reaction times.…”

Chiral Cyclic Alkyl Amino Carbene (CAAC) Transition-Metal Complexes: Synthesis, Structural Analysis, and Evaluation in Asymmetric Catalysis

“…Very recently in 2022, the synthesis benefited from an improvement shortening the previously established routes 1 and 2. During the preparation of this manuscript, Bertrand and Jazzar independently reported the strategy of conveniently accessed key methallylated aldehydes as synthetic cornerstones facilitating scaffold diversification focusing on the preparation of a range of achiral or racemic carbene precursors with various anilines (Scheme C) . Our main focus lies on the exploitation of chiral amines and the preparation of sets of the corresponding enantiopure metal complexes.…”