N‐Phosphine Oxide‐Substituted Imidazolylidenes (PoxIms): Multifunctional Multipurpose Carbenes

Abstract

This article discusses the concept of N‐heterocyclic carbenes (NHCs) equipped with more than one functional moiety, which allows using these NHCs for multiple purposes. A pioneering example for such NHCs is N‐phosphine oxide‐substituted imidazolylidenes (PoxIms), and their synthesis and strategic use are highlighted. The utility of PoxIms by far exceeds the conventional use as multidentate ligands for metal complexes on account of the synergetic functions of the carbene and the N‐phosphine oxide group(s).

Main-Group-Catalyzed Reductive Alkylation of Multiply Substituted Amines with Aldehydes Using H2

“…To achieve our goal, we focused on the use of FLP-based catalysts, since the chemistry of the FLP-catalyzed hydrogenation of imines and carbonyls has been extensively developed. , For the optimization of the reaction conditions, we initially employed benzaldehyde ( 1a ), aniline ( 2a ), H 2 (80 atm), and 4 Å molecular sieves (MS) in THF in the presence of 5 mol % B­(C 6 F 5 ) 3 (entry 1, Table a). However, the targeted N -phenylbenzylamine ( 3aa ) was obtained in only 2% yield after heating to 100 °C for 2 h. Although the formation of N -benzylideneaniline ( A ) was confirmed in 91% yield, the subsequent hydrogenation of A did not proceed under these reaction conditions.…”

N‐Phosphine Imide‐Substituted Imidazolylidenes

“…These molecules contain two distinct Lewis bases, i. e. a carbene moiety and a phosphine oxide group, and an electrophilic phosphorus center (Figure 1). [1l,m] Given the multifunctionality of the N ‐phosphinoyl group, PoxIms have been used in frustration revival strategies, [2] for the facile transformation of carbon dioxide into unsymmetrical carbonyl compounds, [3] and the synthesis of mono‐ [4] or hetero‐bimetallic [5] complexes through different coordination modes (e. g. κ ‐ C , κ ‐ O , or κ ‐ C,O ). For example, the complexation of (S)PoxIms with E(C 6 F 5 ) 3 (E=B, Al) furnishes ( κ ‐ O ‐(S)PoxIm)E(C 6 F 5 ) 3 complexes that contain the carbene carbon atoms (Figure 1a).…”

“…The introduction of an unexplored functional group onto wellestablished and widely employed molecules can diversify both the structure and the reactivity of these compounds. This may eventually lead to the discovery of groundbreaking applications as exemplified by the advent of multifunctional N-heterocyclic carbenes (NHCs) [1] that contain at least one functional group, such as Lewis-acidic or-basic substituents, in addition to the carbene moiety.…”

Cyclometalated Ruthenium(II) NHC Complexes with Imidazo[1,5‐a]pyridine‐Based (C^C*) Ligands – Synthesis and Characterization

“…Moreover, in recent years NHC concepts beyond “simple” heterocycles, e.g. functionalized NHCs, or even acyclic carbenes have been studied intensively.…”