N-Heterocyclic Carbene Catalyzed Carba-, Sulfa-, and Phospha-Michael Additions with NHC·CO₂ Adducts as Precatalysts

Abstract

N-heterocyclic carbene catalyzed Michael additions have been revisited with 1,3-dialkyl- or 1,3-diarylimidazol(in)ium-2-carboxylates, that is, NHC·CO2 adducts, as the source of the free NHC catalysts in solution. Using these precatalysts, a number of efficient carba-, sulfa-, and phospha-Michael additions were achieved very conveniently, without the need for an external strong base to generate the NHC by deprotonation of an azolium salt. To further expand the scope of the procedure, some NHC-catalyzed sulfa-Michael/aldol organocascades were also investigated.

NHC‐Catalyzed Metathesis and Phosphorylation Reactions of Disulfides: Development and Mechanistic Insights

“…Current methods to produces these compounds normally involve toxic and moisture‐sensitive reagents or catalysts . NHC organocatalysts were previously known to also activate phosphites for a number of (1,ω)‐hydrophosphonylation to carbonyl compounds . Hence, it seems likely that NHCs should also act as effective catalysts for the P(O)−S coupling reactions between disulfides and P(O)H substrates.…”

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes

“…Recently, NHC CO 2 Adducts adducts were also shown as competent precatalysts for the Michael additions of thiols, diphenylphosphine oxide, diethylphosphonic acid, and 1,3-dicarbonyl compounds. 277 …”

1,4‐Dithiane‐2,5‐diol: An Attractive Platform for the Synthesis of Sulfur‐Containing Functionalized Heterocycles

“…[18] Knoevenagel condensation between various aromatic and heteroaromatic aldehydes 34 and 3,5-dimethyl-4-nitroisoxazole (35) followed by domino sulfa-1,6-Michael/intramolecular vinylogous Henry reaction sequences between the derived nitrostyrene derivatives 36 and 1 was elegantly designed by Kashinath et al [19] for rapid synthesis of highly functionalized tetrahydrothiophene (thiolane) derivatives 37 containing the biologically useful isoxazole nucleus. However, although the reaction between 1 and methyl vinyl ketone in the presence of ICy·CO 2 32 (2.5 mol-%) in THF produced the corresponding tetrahydrothiophene 33 in excellent yield, the diastereoselectivity remained modest.…”