Probing the Diastereoselectivity of Staudinger Reactions Catalyzed by N‐Heterocyclic Carbenes

Abstract

The reaction of ethylphenylketene with 1,3-dimesitylimidazol-2-ylidene (IMes) or 1,3-dimesitylimidazolin-2-ylidene (SIMes) afforded the corresponding azolium enolates in high yields. The two zwitterions were fully characterized by various analytical techniques. Their thermal stabilities were monitored by thermogravimetric analysis and the molecular structure of SIMes⋅EtPhCCO was determined by means of X-ray crystallography. A mechanism was proposed to account for the trans-diastereoselectivity observed in the [2+2] cycloaddition of ketenes and N-protected imines catalyzed by N-heterocyclic carbenes and an extensive catalytic screening was performed to test its validity. The steric bulk of the NHC catalyst markedly affected the cis/trans ratio of the model β-lactam product. The nature of the solvent used to carry out the Staudinger reaction also significantly influenced its diastereoselectivity. Conversely, the nature of the substituent on the N-sulfonated imine reagent and the reaction temperature were less critical parameters.

Efficient synthetic protocols for the preparation of common N-heterocyclic carbene precursors

“…Other counterions were found to alleviate this tendency. In particular, the replacement of HCl with aqueous HBF 4 in the original procedure allowed us [ 48 ] and others [ 49 ] to isolate ICy·HBF 4 as a well-behaved, non-hygroscopic solid that could be easily purified by recrystallization from isopropanol. Typical yields were in the 70–80% range ( Scheme 3 ).…”

Accessible ring opening metathesis and atom transfer radical polymerization catalysts based on dimethyl sulfoxide ruthenium(II) complexes bearing N-heterocyclic carbene ligands

“…These NMR data matched those reported in the literature. [29,30] 2.3.3. 1,3-dicycloheptylimidazolium tetrafluoroborate 1c Paraformaldehyde (0.4 g, 10.0 mmol), cycloheptylamine (2.6 mL, 20.0 mmol), glyoxal (1.1 mL, 10 mmol), and tetrafluoroboric acid (1.1 g, 12.5 mmol) afforded 0.91 g (72%) of the title compound as white needles.…”

“…The one-pot procedure described for 1,3-dicyclohexylimidazolium tetrafluoroborate could be seamlessly translated to the preparation of 1,3-dicycloalkylimidazolium tetrafluoroborate (1a, 1c, and 1d) from glyoxal, cycloalkylamine (cyclopentylamine, cycloheptylamine, and cyclooctylamine), paraformaldehyde, and tetrafluoroboric acid [29,30]. This synthesis protocol of NHC ligands 1a-1d is an optimized route from the procedure reported by Nolan and co-workers, while searching for bulky NHC ligands to coordinate in transition metal catalysts [35][36][37][38][39][40].…”

N-Heterocyclic Carbene-Catalyzed [4 + 2] Cyclization of Saturated Carboxylic Acid witho-Quinone Methides through in Situ Activation: Enantioselective Synthesis of Dihydrocoumarins

“…Recently, Ye’s group and Wang et al also disclosed the NHC-catalyzed formation of 3,4-dihydrocoumarins . As an important intermediate in NHC catalysis, NHC-bounded enolates A could be generated from α-chloro aldehydes, aliphatic aldehydes, enals, ketenes, esters, and saturated carboxylic acids (Scheme ). Then, intermediate A could be trapped by other reactive species to assemble the heterocyclic skeletons readily.…”