Polar organometallic strategies for regioselective C–H metallation of N-heterocyclic carbenes

Abstract

N-Heterocyclic carbenes (NHCs) have become indispensable ligands across a broad swathe of the synthetic and catalytic landscape, not in small part due to their ease of electronic and steric tunability. One of the latest additions to this important family of ligands are anionic NHCs, which have become valuable precursors to access abnormal NHC complexes as well as shown great potential for further NHC functionalisation. Deprotonative metallation has emerged as one of the most versatile methodologies to access anionic NHCs, where judicious choice of reaction conditions and metallating agents can finely tune the regioselectivity of the reaction. This Feature Article focuses on the recent emergence of s-block metal-mediated NHC metallations and the new opportunities this methodology offers.

Molecular Manipulations of a Utility Nitrogen–Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

“…[14,15,22,23] While redistributionp rocesses on magnesiate chemistry have been previously noted, [24] this is as far as we can ascertain the first one induced by an N-heterocyclic carbene. This prompted us to try astepwise approach, by reactings equentially the single components of NaMgR 3 .T his strategy relies on the ability of NaR to metallatet he NHC to form a sodium anionic NHC which in turn can undergo transmetallation to al ower polaritym etal fragment [25] and it has been recently successfully applied by us for the synthesis of novel sodium ferrate [21] and sodium gallate [23] complexes containing anionic NHCs. [16] Here, by introducing IPr,a na lternative redistribution process on NaMgR 3 is activated, where ap utative [IPrNaMgR 3 ] complex evolves into 1 and MgR 2 .T he lack of metallation of IPr,a nd its preference to act as an eutral donor instead, contrast with our previous studies using heteroleptic sodium magnesiates [14] which combine TMP and Bu groups and seem to be more alignedw ith Hill's formation of coordination adducts when using [NaMg(HMDS) 3 ].…”

“…This prompted us to try a stepwise approach, by reacting sequentially the single components of NaMgR 3 . This strategy relies on the ability of NaR to metallate the NHC to form a sodium anionic NHC which in turn can undergo transmetallation to a lower polarity metal fragment and it has been recently successfully applied by us for the synthesis of novel sodium ferrate and sodium gallate complexes containing anionic NHCs. Thus, by treating IPr with a molar equivalent of NaR followed by the addition of MgR 2 and THF, heteroleptic [(THF) 3 Na(μ‐ IPr − )MgR 2 (THF)}] ( 2 ) [ IPr − =:C{[N(2,6‐ i Pr 2 C 6 H 3 )] 2 CHC] was obtained in a 58 % isolated yield (Scheme b)].…”

Towards a Paradigm Shift in Polar Organometallic Chemistry

“…In this approach the choice of organometallic trap should fulfil two key requirements: firstly, the metal should form strong enough M-C bonds to stabilize the generated anions, in the case of 4 the sensitive fluoroaryl; but secondly, in order to facilitate further functionalization of the aromatic fragment, the resulting M-C bond should also be labile enough to promote onward reactivity with electrophiles. [23] Reaching wider applications, mixed-metal combinations operating in a concerted-cooperative or stepwise-cooperative manner have also been exploited successfully for functionalising N-heterocyclic carbenes (NHCs), [24][25][26][27][28][29] establishing new ways to trap dianionic NHC fragments in reactions of unprecedented chemoselectivity [28] as well as enabling lateral metallation of saturated NHCs via stepwise (or trans-metal-trapping) approaches. [29] A second generation of cooperative bimetallics for deprotonative metallation has also been developed by replacing the low polarity s-block metal by divalent earth-abundant transition metals such as Mn(ii) and Fe(ii).…”

Aluminates with Fluorinated Schiff Bases: Influence of the Alkali Metal–Fluorine Interactions in Structure Stabilization

“…These species not only show unusual structures, they also display a wide-ranging reactivity which, on many occasions, is diverse from the one shown by the homometallic analogous [12,13,14,15]. As such, these compounds have become very popular reagents for reactions such as the activation of unreactive C-H bonds [16,17,18], direct orthometalation processes [19,20,21] or the formation of C-C and C-heteroatom bonds [22,23,24,25]. They are also active catalysts for the polymerization of polar alkenes [26,27,28,29].…”