Imidazolium salts amino

Chichibabin reaction, 5, 409-410 UV spectra, 5, 356 Naphthimidazoles, 2-amino-tautomerism, 5, 368 Naphth[2,3-h]imidazoles oxidation, 5, 405 Naphth[l,2-d]imidazolium salts nucleophilic substitution, 5, 412 Naphth[l, 2-h]isoquinolines... 

Also the use of moisture stable ionic liquids as solvents in the Diels-Alder reaction has been carried out, and in all examples an enhanced reaction rate was observed [182,183]. The application of pyridinium-based ionic liquids allowed the utilization of isoprene as diene [184]. The chiral ionic liquid [bmim][L-lactate] was used as a solvent and accelerated the reaction of cyclopentadiene and ethyl acrylate, however, no enantiomeric excess was observed [183]. In addition several amino acid based ionic liquids have been recently tested in the Diels-Alder reaction. Similar exo. endo ratios were found but the product was obtained as racemate. The ionic liquids were prepared by the addition of equimolar amounts of HNO3 to the amino acids [185]. Furthermore, an enantiopure imidazolium salt incorporating a camphor motive was tested in the Diels-Alder reaction. No enantiomeric excess was found [186]. 

The amino functionalised imidazolium salt was then used to prepare a palladium NHC complex that was used in the Suzuki-Miyaura coupling between phenylboronic acid and phenyl bromide, albeit with rather moderate success. 

The obtained amino functionalised imidazolium salts could be used to generate the corresponding palladium(ll) carbene complexes using the silver(l) complexes as carbene transfer agents. Application of these palladium(ll) complexes (predominantly in situ) in asymmetric allylic alkylation reactions between ( )-l,3-diphenylprop-3-enyl acetate and dimethyl malonate (a standard reaction for this catalytic process [145]) gave up to 80% ee,... 

Figure 3.50 Synthesis of a chiral amino functionalised NHC ligand by reduction of a chiral imino functionalised imidazolium salt using NaBH. ...

A second source of amino functionalised imidazolium salts is the quest for functionalised ionic liquids [146,147]. Wan et al. used a piperidine functionaUsed imidazolium salt as a base in a palladium catalysed Heck reaction in an ionic liquid medium. The system was designed such that the phosphane Ugand, the base and the solvent were all imidazolium based ionic Uquids [148] (see Figure 3.51). Given the fact that imidazolium based ionic liquids can act as carbene precursors for coordination to transition metals [149] that coordinate better than phosphane Ugands (with cationic substituents in the present case), it is far from certain which is acmally the ligand in that carefully designed system. 

From the three components, the 2-phosphino substituted imidazolium salt cannot serve as a normal carbene ligand, but both the amino functionalised imidazoUum salt (given the role as the base) and the nonfunctionalised imidazolium salt can act as carbene ligands and would enhance the catalytic activity of the system, if they did [149]. Furthermore, the amino functionalised imidazolium salt would stabilise the Pd(0) species due to its hemila-bile behaviour. One should use these ionic liquid systans with caution. 

A truly hemilabile amino functionalised NHC ligand was introduced by Jimdnez et al. who synthesised a series of rhodium(I) compounds using anunonium functionalised imidazolium salts as starting materials [150] (see Figure 3.52). Interestingly, initially an ionic rhodium(I) compound was obtained that did not contain a carbene-rhodium bond. The rhodium carbene complex could be obtained after further deprotonation and coordination of the amine sidearm to the metal occurred only after chloride abstraction with AgBF. ... 

Note Protection of the amino group during synthesis of the amino functionalised imidazolium salt can become necessary as the amino group can itself react with the functionalised aminoalkyl halide (see Figure 3.55). 

Belokon et al. have published a somewhat intricate protocol for the N -methylation of a histidine isomer, itself an imidazolium derivatised a-amino acid [158] (see Chapter 6). The amino group can be N-boc protected and the resulting imidazolium salt reacted with silver(I) oxide to yield the corresponding silver(l) carbene complex. The functional groups in the wingtip group... 

Figure 3.55 Possible side reactions in the synthesis of amino functionalised imidazolium salts.

The coordination of amido functionalised carbene ligands, where the amido function is introduced by the acetylation of an amino functionalised imidazolium salt, to transition metals has already been discussed in Section 3.1. Here it suffices to mention that the synthesis is similar to other carbonyl functionalised NHC ligands and their transition metal complexes. A suitable example is given in Rivera and Crabtree [226]. 

A similar protocol was developed by Wang et al. starting from benzaldehyde, N,N-dimethylaminomethyl benzene was formed, Uthiated in the ortho position and reacted with ClPPhj to introduce the phosphino group. Then the amino group is substituted with chloride and the molecule reacted with the respective imidazole to generate the mono- or bisphosphino imidazolium salt (see Figure 3.81). 

The simpler architecture is the 1,1 -biphenyl scaffold, likewise introduced by Hoveyda and coworkers [19]. The synthesis of the imidazolium salt starts with a chiral diamine and a substituted, achiral biphenyl [82-84], Subsequent introduction of a Mes substituent on the remaining primary amino end and ring closure reaction yields the chiral saturated imidazolium salt after hydrolysation of the methoxy group to liberate the phenolic hydroxy group (see Figure 4.22). Reaction with silver(I) oxide and carbene transfer to a Grubbs (Hoveyda) catalyst sets up the ruthenium catalyst complex. 

The hydroxy-binaphthyl functionalised saturated imidazolium salt is readily available from 1-amino-I -hydroxy-binaphthyl in a reaction with a ( oc-protected mesitylamine aldehyde [86] (see Figure 4.24). The resulting Schiff base is reduced to the diamine by Na(OAc)3BH. Subsequent deprotection and ring closure reaction with triethyl orthoformate yields the corresponding hydroxy-binaphthyl functionalised saturated imidazolium salt. Reaction with silver(I) carbonate and subsequent carbene transfer to the ruthenium(II) precursor yields the asymmetric olefin metathesis precatalyst. 

Synthesis of amide functionalised NHC ligands is facile and follows a standard protocol of unsymmetrically substituted imidazolium salts (see Chapter 1). In the present case, Arnold et al. reacted iV-fcrt-butyl-imidazole with iV-tcrt-butyl-aminoethyl bromide hydro-bromide [34]. Subsequent reaction with potassium hydride yields the amino functionalised carbene, probably in the form of a hydrogen bond stabilised zwitterion (see Figure 4.28). Stepwise reactiou of the amino functionalised imidazolium salt hydrobromide first strips off the hydrobromide and then deprotonates the imidazolium ring (not the secondary amino group) to form a lithium chelate complex. 

Figure 4.28 Synthesis of an amino functionalised imidazolium salt and its lithium carbene...

The reaction of the amino imidazolium salt hydrobromide with silver(I) oxide yields the corresponding silver(I) carbene complex (see Figure 4.29) that has the same central structural unit as the lithium complex [35]. 

Figure 6.20 Synthesis of a chiral imidazolium salt from the amino acid histidine.

Note Imidazolium salts can be generated directly from amino acids. 

Ionic liquids (imidazolium salts) were utilized as green solvents in the Friedlander synthesis. Reactions of o-amino-substituted aromatic carbonyls with a variety of ketones (cyclic, acyclic, aromatic) result in the desired quinolines in uniformly high yield. Conditions are mild and no hazardous acids or bases are used <03JOC9371>. 

Benzamidinc combines with 2-amino-3-phenacyl-l,3,4-oxadiazolium bromides to give l-acylamino-2-benzimidoylamino-4-arylimidazolcs. Yields arc only moderate (14-43%), but the reaction works for a variety of 4-arylimidazoles f22. Reactions of /V-methyl-JV-(JV -phenylbenzimidoyl)amino-acetonitrile (13) under acidic conditions lead to imidazolium salts which have amino (14) or amido (15) groups in the 4-position (Scheme 2.2.6). The 4-amino salt (14) undergoes Dimroth rcaaangement to the 4-phenylaminoimidazole (16) direct conversion of (13) into (16) also occurs in warm alkali [8]. A Claisen rearrangement of the adduct (17), which forms from interaction of an arylamidoxime and a propiolate ester, provides a method... 

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