Protic NHCs

Apart from providing hydrogen bond donors, coordinated protic NHC ligands can be deprotonated at the ring-nitrogen atom followed by reaction with an electrophile. Thus, complexes of types B and C are easily mono- or dialkylated. Such alkylation reactions can be utilized for the template-controlled synthesis of linear or macrocyclic homodonor (E) or heterodonor (F) ligands (Scheme 9.1) [15d]. 

Scheme 9.2 Synthesis of complexes bearing protic NHCs from C2-lithiated azoles.

Complexes with protic NHC ligands have been prepared from neutral azole precursors. In spite of the problems often encountered during the deprotonation of neutral Af-alkyUmidazoles, some complexes bearing protic NHC ligands have been generated from C2-lithiated azoles followed by N-protonation as depicted in Scheme 9.2. Complex 1 is stable [20a], but complexes of type 2 have been shown to tautomerize to the complexes bearing N-bound azoles [17]. The C2-deprotonation of various thiazoles and the use of the resulting salts for the preparation of NHC complexes have been demonstrated by Raubenheimer [20b]. 

Tautomerization of N-bound azoles can be utilized for the generation of complexes bearing protic NHC ligands. For example, Taube and coworkers [21] reported as early as 1974 the acid-promoted tautomerization of the N-coordinated imidazole in 3 to yield complexes 4—6 bearing protic NHC ligands (Scheme 9.3). 

The C2-deprotonation of N-coordinated imidazoles followed by partial N->C metal transfer gave a mixture of isomeric complexes 7. Complex 8 bearing a protic NHC ligand 8 was obtained from 7 after addition of a proton source (Scheme 9.4a) [18a]. A similar reactivity was observed for the Re complex 9 bearing three N-coordinated imidazole ligands. On C2-deprotonation, one of these reacted under N-> C metal shift to give 10 that on protonation yielded the complex 11 bearing... 

Scheme 93 Acid-promoted tautomerization of N-coordinated azoles to yield complexes with protic NHC ligands.

The tautomerization reactions of N-coordinated azoles described earlier is limited to selected metal centers. A different reaction leading to complexes with protic NHC ligands starts the ubiquitous MA -dialkylated azolium salts. Such salts can be deprotonated to yield the free NHCs that on reaction with suitable metal precursors yield NHC complexes. Alternatively, azolium salts react directly with suitable metal precursors in the presence of internal or external bases to yield NHC complexes [15], If the azolium salt and subsequently the coordinated NHC are functionalized with removable N-substituents, protic NHC ligand can be generated at the metal template by the removal of an N-substituent from the coordinated NHC. 

Such a reaction was discovered in 2006 for complex 12, which was obtained by coordination of a free NHC to Ru with concurrent C—H activation at one of the methyl groups of an N-substituent (Scheme 9.5). Heating of 12 leads to cleavage of an exocyclic N—C bond and, among other reaction products, to the formation of the NHC complex 13 bearing a protic NHC and its tautomer 14 (Scheme 9.5) [22],... 

Scheme 9.5 Cleavage of an exocyclic N—C bond in 12 to give the complex with a protic NHC ligand.

The accidental removal of an Af-acetonyl substituent during silica gel chromatography has been observed for complex 15 giving complex 16 with a protic NHC ligand (Scheme 9.6a) [23a], This type of accidental removal of an N-wingtip substituent has also been described for complexes bearing Af-silyl-substituted NHCs [23b]. 

Scheme 9.6 (a-c) S5mtheses of complexes bearing protic NHC ligands by the removal of N-substituents from the coordinated NHC... 

While the protonation of benzimidazoles by NH4BF4 is not very likely (pathway b in Scheme 9.20), the formation of 69 does not provide any information on which pathway is operative. Such information can be obtained if the oxidative addition is performed in the absence of a proton source. The reaction of 2-chloro-A-methylbenzimidazole with [Pt°(PPh3)4] in toluene yields a mixture of complexes 70 and 71 (Scheme 9.21). Complex 71 is formed by dimerization of the reaction product of the initial oxidative addition clearly demonstrating that the oxidative addition precedes the protonation step (pathway a in Scheme 9.20). Related dinuclear complexes have been obtained by N-deprotonation and dimerization of complexes bearing protic NHC ligands [32, 75]. 

Complexes of type G are rather rare and have so far only been obtained by deprotonation of coordinated NR,NH-NHCs [67, 68]. The direct access to complexes featuring NR,N-NHC ligands by the oxidative addition of the C2—Cl bond of a 2-halogenato-Al-alkylbenzimidazole is much more facile and allows a rich follow-up chemistry like the N-protonation or Al-alkylation of the NR,N-NHC ligand. Particularly, complexes bearing protic NHC ligands can be obtained easily by this way. 

Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH-NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent smdies demonstrate that complexes with protic NHCs obtained from C2-H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N—H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent smdies indicate that even biomolecules such as caffeine can be C8-metallated. It... 

He F, Braunstein P, Wesolek M, Danopoulos AA. Imine-functionalised protic NHC complexes of Ir direct formation by C—H activation. Chem Commun. 2015 51 2814-2817. 

---