Amido group deprotonation

In order to investigate further the relationship between in-plane interaction and ease of oxidation of the central metal ion, a study of the oxidation of Ni(ii) and Cu(n) complexes of the cyclam derivative (287) was investigated (Fabbrizzi, 1985). This dioxocyclam ligand (287) coordinates to a divalent metal ion with deprotonation of the amido groups. 

Figure 3-13. Polarisation of a chelated amino acid amide leading to deprotonation of the amido group. Notice the change in bonding mode from N,0 to N,EF associated with this process.

The first group 1 carbene complex with an N-bound anionic functional group was reported in 2004.12 An alkylamino carbene is readily deprotonated using //-butyl lithium to afford 4 (Fig. 3). The solid state structure comprises a discrete dimer via bridging amido groups. Although there is severe distortion of the lithium-NCN bond (147.9° compared to the closer to linear 161.8° in 3), the lithium-NHC bond distance of 2.124(4) A is still short, suggesting that the interaction is predominantly ionic. 

According to the above equilibrium, the dioxotetramine moiety of 1 chelates the metal centre and effective complexation takes place with the simultaneous deprotonation of the two amido groups. Cu promoted deprotonation of the amido (or peptido) group in polyaza ligands is a well known phenomenon in coordination chemistry, leading in any case to the formation of pink-violet species. ... 

Reaction of CO with the tautomeric mixture of the two aforementioned rhodium complexes (several / flra-substituted imidoaryl groups were tested) afforded a unique bridging isocyanate complex Rh2(CO)2(ii -N,T] -C, x-ArNCO)(p-DPPM)2. The CO insertion is irreversible. Since the two initial tautomers are in equilibrium in solution, insertion of CO may in principle proceed by either of the two (Scheme 20)(next page). However, evidence was given in favour of the amido-path (path b in the Scheme), based on the fact that the cationic complex [Rh2(p-NHPh)(CO)2(DPPM)2] rapidly reacted with CO. No complex could be isolated from this last reaction, but the formation of PhNCO was detected. Two features of this mechanism are worth of note. The first is the contrast between the conclusion reached for this system (amido complex more reactive than imido one in the insertion reaction of CO) and the one reached by Bhaduri et al. [161] for the trinuclear complex Ru3(p-H)(p-NHPh)(CO)io, which, upon deprotonation of the amido group by OH, affords the inserted product [Ru3(p-H)(T] -N,ii -C,p3-PhNCO)(CO)9]. The difference is likely due to the fact that, in this latter case, the complex is trinuclear, so that the inserted CO is already coordinated to the third ruthenium atom and, especially, the formation of the new C-N bond does not require the breaking of any of the pre-existing Ru-N bonds. 

As in the reaction of the sulfide complex just described, the product of C—C coupling was found to be too tmstable for isolation, but could be characterized by NMR at low temperature. Stable 2,6- Pr-BIAN analogs were isolated and characterized, including by X-ray dif action, for P(CH3)3, P(CH3)2Ph, and P(CH3)Ph2 complexes (see Fig. 5 for the P(CH3)3 derivative). The deprotonation has been found to take place exclusively at the methyl—not at the phenyl—substituents. The amido groups in the C—C... 

The majority of examples of metal-assisted hydrolysis of peptides which have been reported recently involve the use of cobalt(II) centers. However, use of copper(II) for the specific hydrolysis of the C-terminal residue of polypeptides has been reported. The polypeptides coordinate to the copper with concomitant deprotonation of the amido group of the C-terminal residue. Treatment with persulfate results in an oxidative decarboxylation to yield an iV-acylimine, which undergoes subsequent hydrolysis to generate a carbonyl compound and carboxamide. This results in an overall process, Eq. (3). In contrast, treatment with [IrCl ] results in the alternative reaction (4), although this process is dependent upon the redox potential of the copp r(II)/copper(III) couple. 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

Among the polydentate carbene ligands, particular interest has recently been placed on cyclic polycarbenes. ImidazoUum precursors like 23 [89] or 24 [90, 91], which upon C2 deprotonation would lead to tetradentate or even hexadentate double-pincer NHC ligands, have been prepared. Their interesting coordination chemistry will be discussed in Sect. 4. Finally, Arnold et al. developed and reviewed NHC ligands which are functionalized with additional anionic (alkoxide or amido) donor groups [92]. 

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