22847-18-3Relevant articles and documents
Imidazolium Based Fluorous N-Heterocyclic Carbenes as Effective and Recyclable Organocatalysts for Redox Esterification
?ervenková ??astná, Lucie,Bílková, Veronika,Cézová, Tereza,Cu?ínová, Petra,Karban, Jind?ich,?ermák, Jan,Krupková, Alena,Stra?ák, Tomá?
, p. 3591 - 3598 (2020/06/17)
A series of new highly fluorophilic ionic liquids (f > 110) was synthetized from 3-iodopropyltris(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane and N-alkyl imidazoles, followed by anion exchange. N-heterocyclic carbenes generated in situ from obtained imidazolium salts were employed to catalyze redox esterification (umpolung) of cinnamaldehyde with alcohols. The most effective N-methyl derivative with iodide as a counter anion was studied in detail with respect to the optimization of reaction conditions, substrate scope and recyclability. Recovery of the precatalyst was achieved using either fluorous extraction or performing the reaction in suitable fluorous biphase system with direct recycling of the fluorinated precatalyst phase. For both tested options, the catalytic activity did not significantly decrease within 5 subsequent cycles. The redox esterification was shown to proceed also in supercritical carbon dioxide (scCO2) as an alternative solvent where the activity of the fluorinated catalyst was also superior to the nonfluorinated model, while retaining the benefit of easy recycling.
Enantio- and Diastereoselective Hydrofluorination of Enals by N-Heterocyclic Carbene Catalysis
Wang, Leming,Jiang, Xinhang,Chen, Jiean,Huang, Yong
supporting information, p. 7410 - 7414 (2019/05/10)
In contrast to well-established asymmetric hydrogenation reactions, enantioselective protonation is an orthogonal approach for creating highly valuable methine chiral centers under redox-neutral conditions. Reported here is the highly enantio- and diastereoselective hydrofluorination of enals by an asymmetric β-protonation/α-fluorination cascade catalyzed by N-heterocyclic carbenes (NHCs). The two nucleophilic sites of a homoenolate intermediate, generated from enals and an NHC, are sequentially protonated and fluorinated. The results show that controlling the relative rates of protonation, fluorination, and esterification is crucial for this transformation, and can be accomplished using a dual shuttling strategy. Structurally diverse carboxylic acid derivatives with two contiguous chiral centers are prepared in a single step with excellent d.r. and ee values.
Catalytic, Enantioselective β-Protonation through a Cooperative Activation Strategy
Wang, Michael H.,Barsoum, David,Schwamb, C. Benjamin,Cohen, Daniel T.,Goess, Brian C.,Riedrich, Matthias,Chan, Audrey,Maki, Brooks E.,Mishra, Rama K.,Scheidt, Karl A.
, p. 4689 - 4702 (2017/05/12)
The NHC-catalyzed transformation of unsaturated aldehydes into saturated esters through an organocatalytic homoenolate process has been thoroughly studied. Leveraging a unique “Umpolung”-mediated β-protonation, this process has evolved from a test bed for homoenolate reactivity to a broader platform for asymmetric catalysis. Inspired by our success in using the β-protonation process to generate enals from ynals with good E/Z selectivity, our early studies found that an asymmetric variation of this reaction was not only feasible, but also adaptable to a kinetic resolution of secondary alcohols through NHC-catalyzed acylation. In-depth analysis of this process determined that careful catalyst and solvent pairing is critical for optimal yield and selectivity; proper choice of nonpolar solvent provided improved yield through suppression of an oxidative side reaction, while employment of a cooperative catalytic approach through inclusion of a hydrogen bond donor cocatalyst significantly improved enantioselectivity.
