Electronic Tuning of a Carbene Center via Remote Chemical Induction, and Relevant Effects in Catalysis

Abstract

The present report develops the idea that an N-heterocyclic carbene incorporating a remote anionic functionality--here, a malonate group--as a backbone component of its heterocyclic framework, can be "post-functionalized" directly from its transition-metal complexes, upon simple addition of a variety of electrophiles interacting directly with the malonate group in the outer coordination sphere. From a palette of selected electrophilic reagents, it was thus possible to modulate the electronic donor properties of the carbene center over a rather broad range. Both the zwitterionic complex [Rh{malo-NHC}(cod)] and the cationic derivatives [Rh{malo-NHC(E)}(cod)](+) (where "malo-NHC(E)" represents the ligand modified by a selected electrophile "E") were used as pre-catalysts in two types of catalytic reactions, namely, the polymerization of phenylacetylene and the hydroboration of styrene. The results indicate that, in both cases, the zwitterionic species is by far the best catalyst, whereas a decrease in the ligand donicity induced by the added electrophile results in a concomitant reduction of catalytic activity. Apparent deviations to such a trend in the case of the hydroboration of styrene were rationalized in terms of an interaction between the reactive catecholborane substrate and the remote functionality of the N-heterocyclic carbene leading to an in situ modification of the nature of the active species. These observations serve as a useful basis to define the scope and limitations of the present conceptual approach in catalysis.

The Ambivalent Chemistry of a Free Anionic N‐Heterocyclic Carbene Decorated with a Malonate Backbone: The Plus of a Negative Charge

“…Model system and DFT calculations on the electronic structure: The lithium salt [1]·Li of the anionic carbene [1] À was chosen as the preferred model carbene system for the present study, since we had previously shown that [1]·Li is cleanly and quantitatively generated from its mesoionic precursor [1]·H with a slight excess of n-BuLi and that it is stable in solution for days at room temperature (Scheme 2). [22] When the carbene [1] À is coordinated to a metallic center in zwitterionic or anionic complexes, [22,23] its internal ligand geometry reflects the quasi-absence of electronic communication between the proximal diaminocarbene moiety NCN, a 4p e À system, and the distal malonate unit, a 6p e À system. The electronic structure of the coordinated carbene [1] À is thus best described by the average resonance form [1 d ] À , analogous with that of its precursor [1]·H, a heterocyclic member of the family of cross conjugated mesoionic betaines (CCMB).…”

“…We have already demonstrated that the stable carbene B is capable of coordinating transition-metal centers to yield zwitterionic complexes active in catalysis. [23] Nonetheless, its structural similarities with the diamidocarbene A prompted us to explore its potential reactivity as a "free" anionic entity. Indeed, very little is known about the reactivity of anionic NHCs, [24] with the notable exception of the anionic compound C, whose electronic properties and reactivity were shown to match better with those of a phenylide-type carbanion than of a real NHC.…”

From betaines to anionic N-heterocyclic carbenes. Borane, gold, rhodium, and nickel complexes starting from an imidazoliumphenolate and its carbene tautomer

“…1). The carbenes 3 [31] and 4 [32–33] originate from a conjugated ylide and a cross-conjugated mesomeric betaine, respectively. A review elucidates the interconversions of mesomeric betaines to different types of N-heterocyclic carbenes (NHC, aNHC, rNHC, MICs) and vice versa [34].…”

Postmodification of the Electronic Properties by Addition of π-Stacking Additives in N-Heterocyclic Carbene Complexes with Extended Polyaromatic Systems

“…6a The effects of modifying the electronic prop--erties of NHCs ligands are known to afford important implications in the context of homogeneous catalysis and bioorganometallic chemistry, 9 and some elegant examples of fine electronic tuning of NHC ligands via remote chem--ical induction have shown to produce relevant effects in catalysis. 10 Benzo--fused nitrogen heterocycles continue to com--mand a great deal of attention because a large number of natural products and pharmaceutical compounds incor--porate these heterocyclic systems.…”