Imidazolidinium

Asymmetric versions of this transformation were also developed by using chiral imidazolium pro-ligands as NHC precursors, or silver transmetallation methodology with chiral NHC ligands (Fig. 2.23) [106]. Imidazolium salts with chiral A-substituents (132) or imidazolidinium salts with chirality at the backbone of the heterocycle (133) gave quantitative conversions at -78°C with good ee (58% and 70% respectively). 

Improvement in the catalyst activities and enantioselectivities was realised by the development of the chiral, bidentate alkoxy-functionalised imidazolium and imidazolidinium pro-ligands (134 and 136). 134, after deprotonation, was used to prepare the well-defined complex 135. Both 136 in the presence of BuLi and Cu(OTf)2 or 135 without any additional co-reagents were efficient catalysts in the asymmetric 1,4 addition of dialky Izincs and Grignards to cyclohexen-2-one giving higher ee (83% at rt and 51% at -30°C, respectively) [107, 108]. 

A variety of imidazolidinium and pyrrolidinium salts (Fig. 2.38) have been found to catalyze the reaction between nitrones and cyclic a, p-unsaturated aldehydes, affording bicyclic adducts with high diastereoselectivity and enantioselec-tivity (Scheme 2.256) (759). 

The reaction of an orf/io-ester, e.g., HC(OEt)3, with a secondary bisamine in the presence of an anunonium salt yields imidazolidinium salts (Scheme The necessary secondary diamines can be generated by a classical condensation-reduction sequence or by applying the palladium-catalyzed Buchwald-Hartwig amination." The latter reaction offers convenient access to imidazolidinium salts with chiral backbones starting from chiral diamines, a number of which are commercially available. ... 

Imidazolidinium salts can also be transformed into the corresponding diamino ortho-esters by alkaline alkoxylate, and upon alcohol elimination at elevated temperature the imidazolidin-2-ylidenes can be trapped. The reaction of tria-zolium salts with sodium methanolate in methanol yields 5-methoxy-4,5-dihydro-IH-triazole which also eliminates methanol upon heating in vacuo. The resulting triazolin-5-ylidenes can either be isolated or trapped by an appropriate metal precursor [Eq. (19)]. Benzimidazolin-2-ylidenes are similarly accessible by this route. 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

Asymmetric homogeneous catalysis generally requires chiral ligands. Approaches to chiral NHCs have focused on the generation of chiral centers either in the 4- and 5-position of imidazolidinium salts 71 or in the a-position of the nitrogen substituents for imidazolium salts 72. 

The saturated imidazolidinium salts can be obtained by alkylation of dihydroimidazole or by selected cyclization reactions [22]. A multicomponent reaction leading to unsymmetric derivatives of type 2 has also been reported (Fig. 3c) [30]. 

Fig. 3 Syntheses of imidazolium salts 1 (a, b), unsymmetrically substituted imidazolidinium salts 2 (c) and of cyclic 2-thiones 3 and 4 (d, e)...

Additional factors which lead to an increased stability of the carbene complexes towards reductive ehmination are the type of NHC ligand and the NA -substitution pattern. The stability of NHC complexes depends strongly on the electronic situation at the carbene center. The oxidative addition of p-tolyl chloride to linear Pd° complexes bearing two unsaturated imidazolin-2-ylidenes (type 5, Fig. 6) or two saturated imidazolidin-2-ylidenes (t3q>e 7, Fig. 6) proceeds readily. The Pd complex with the imidazolin-2-yhdene ligands is stable, while the one with the imida-zolidin-2-ylidene ligands reductively ehminates the C2-arylated imidazolidinium salt [134]. 

MacMillan s catalysts 56a and 61 allowed also the combination of the domino 1,4-hydride addition followed by intramolecular Michael addition [44]. The reaction is chemoselective, as the hydride addition takes place first on the iminium-activated enal. The enamine-product of the reaction is trapped in a rapid intramolecular reaction by the enone, as depicted in Scheme 2.54. The intramolecular trapping is efficient, as no formation of the saturated aldehyde can be observed. The best results were obtained with MacMillan s imidazolidinium salt 61 and Hantzsch ester 62 as hydride source. As was the case in the cyclization reaction, the reaction affords the thermodynamic trans product in high selectivity. This transformation sequence is particularly important in demonstrating that the same catalyst may trigger different reactions via different mechanistic pathways, in the same reaction mixture. 

The reversible formation of a N,N-dibenzyl iminium intermediate, which is reduced by hydride capture from the Hantzsch ester 1 was proposed. Subsequent hydrolysis regenerates catalyst 2 and releases the saturated aldehyde. The transition state A has been suggested for the hydride transfer. An example of the asymmetric version of this reaction was also realized, by using a chiral imidazolidi-none catalyst (the McMillan imidazolidinium salt 3 [13]) (see Scheme 11.4). 

Only recently an additional, elegant access to unsymmetrically substituted imidazolium salts has been disclosed (Fiirstner et al. 2006). Unsymmetric bisaryl-substituted IV-heterocyclic carbenes (imidazolidinium derived) can be prepared from ethyl chlorooxoacetate (Waltman and Grubbs 2004). 

Fig. 40 (a) Crystal structure of the tris-imidazolidinium cage 233+ [80]. Only C-H fragments of the imidazolidinium subunits are shown, (b) Top view of the trication indicates that such C-H fragments do not point towards the cavity... 

NHCs have become popular ligands in coordination chemistry owing to the facile access to this type of ligands and to metal-NHC complexes. Most NHC ligands are prepared from azohum compounds such as imidazolium, triazolium, benzimidazo-lium, imidazolidinium, or thi azolium salts [1]. Alternatively, the reductive desulfurization of imidazolin-, benzimidazolin-, and imidazolidin-2-thiones to yield a variety of NHCs has been described. The preparation of suitable azolium salts and imidazolin-2-thiones is presented in Sect. 2.1. This is followed by the description of methods to liberate the free NHCs from these compounds. Today, stable singlet... 

The reaction usually requires palladium catalysis. In the case of aromatic tosylates [23] or arylchlorides, Ni-catalysts [20, 21] or Pd-imidazole-2-ylidene complexes had to be employed ]33]. The latter catalyst is generated in situ from 1,3-bisaryl or 1,3-bisalkyl imidazolidinium chloride ]34, 35] and a base, such as CS2CO3 or KOtBu ]36]. 

Reacting nitrogen-stabilized a-carbenium dithioates 110 and 111 (inner salts) with electrophiles yields the a-carbenium dithioesters 112 and 113, respectively, in high yields, as shown in Scheme 23 [64] and Scheme 24 [65]. Hydrolysis of 112 and 113 gives a-oxo dithioates 114 and 115, respectively. In the case of imidazolidinium dithioate 117, prepared from the inner salt 116, the hydrolysis is followed by intramolecular cyclization to give a 28% yield of 3-thioxo piperazin-2-one 118 (Scheme 25) [64]. The alkylation of p-carbenium dithioate 119 proceeds similarly (Scheme 26) [66]. 

Scheme 3.49 Preparation of cyclic imidazolidinium salts from acyclic amidines...

Addition of 21.5 g (0.13 mole) of the imidazolidinium chloride dissolved in 100 ml of chloroform to excess ammonia in 100 ml of chloroform at room temperature affords a mixture of ammonium chloride and 2 imino-l,3-dimethylimidazolidine hydrochloride, m.p. 225-228°C. The latter can be separated from the ammonium chloride by extraction with hot chloroform. The free base XXVII, b.p. 50-51°C/0.1 mm, 1.5010, can be generated with sodium hydroxide. 

A variety of cationic surfactants are derived from pyridine and imidazole, such as the alkylpyridinium salts and the alkyl imidazolidinium salts. These salts are very stable in aqueous solutions, but absorb UV light which makes them difficult to be used in MLC. Also, surfactants associating a nonionic polyoxyethylene chain with a cationic terminal group have been designed [5],... 

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