Direct Reactions with Imidazolium Salts

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

The carbene complexes can also be formed by direct oxidative addition of ze-rovalent metal to an ionic liquid. The oxidative addition of a C-H bond has been demonstrated by heating [MMIM]BF4 with Pt(PPh3)4 in THF, resulting in the formation of a stable cationic platinum carbene complex (Scheme 15) (189). An effective method to protect this carbene-metal-alkyl complex from reductive elimination is to perform the reaction with an imidazolium salt as a solvent. 

Direction of borate structures by specific cations is also illustrated by the formation of the unusual nonaborate anion in the presence of the imidazo-lium, [C3H7N2], and guanidinium, [C(NH2)3], cations [42]. The reaction of imidazole with three molar equivalents of boric acid in aqueous solution results in the spontaneous formation of the imidazolium salt of the [B90i2(OH)6] anion, shown in Fig. 11, associated with three [C3H7N2] ... 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

The reaction of triazolium and benzimidazolium salts with sodium methoxide yields the corresponding methoxy-triazoles and benzimidazoles [30,31], which can be also used as triazolilydene and benzimidazo-lilydene precursors. Notably, adduct formation does not occur for certain unsaturated imidazolium salts with a C = C backbone. For the latter, reaction with KOBu results in direct deprotonation to the free NHC (Scheme 8, also shows the reaction of a dihydroimidazolium salt with KOBu ) [32],... 

Oxidative addition of C2 - H bonds of imidazolium salts to low valent metals was first observed by Nolan and coworkers in 2001, who proposed a NHC - Pd - H intermediate in the catalytic cycle of the dehalogenation of aryl halides with Pd(dba)2 in the presence of imidazolium salts [154]. More direct evidence of this process was described by Crabtree and coworkers two years later [155]. The reaction between a pyridine-imidazolium salt and Pd2(dba)3 afforded the preparation of bis-NHC - Pd(II) complexes by C2 - H oxidative addition (Scheme 40). The presumed Pd - H intermediates were not detected. The authors proposed a mechanism via two successive C - H oxidative additions followed by reductive elimination of H2 [ 155]. 

Gade and co-workers reported the synthesis of an oxazolinyl-carbene which is obtained by direct linkage of the two heterocycles. The new ligand system was obtained by reacting the 2-bromooxazoline 65 [122] with an imidazolium precursor in THF (Scheme 46) [123]. N-heterocyclic carbene rhodium complexes could be obtained by reaction of the imidazolium salt 66 with [ Rh(/i.-OfBu)(nbd) 2] generated in situ [124]. 

A few years earlier, Herrmann et al. published a carboxylic ester functionalised imi-dazolium salt that was synthesised directly from imidazole and bromoacetic acid ethyl ester [216]. Owing to its method of synthesis the imidazolium salt is C -symmetric with two ester functional wingUp groups. Generation of the rhodium(I) and palladium(II) carbene complexes was realised by reaction of the imidazolium salt with a rhodium alkoxide precursor or with palladium(II) acetate in the presence of NaOEt and Nal (see Figure 3.76). The silver(I) oxide method had not been discussed in the literature at the time [11]. 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

Direct arylation of methyl imidazole with 2,7-dichloronaphthyridine leads to a potentially tetradentate and practically tridentate bis-carbene ligand on a naphthyridine scaffold [363]. Reaction of the bis-imidazolium salt with silver(l) oxide in the usual way yields a linear trinuclear silver carbene complex with this tris-bridging hgand (see Figure 3.115). 

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... 

The room-temperature chloroaluminate(lll) ionic liquids are the most important members of the first generation of ionic liquids, developed in the second half of the last century The room-temperature halogenoaluminate(III) ionic liquids are extremely sensitive to moisture and must be handled imder an inert atmosphere. Preparation of the halogcno-aluminate(III) ionic liquids is simple a quaternary ammonium (QUAT) halide, e.g. an imidazolium or pyridinium halide, is directly mixed with AICI3 in the ratio necessary to generate the composition required. Upon mixing, an exothermic reaction occurs and the two solids melt into a liquid. The first report on the formation of a room temperature liquid salt, based on the combination of 1-butylpyridinium with AICI3 in the relative molar proportions 1 2 (X =... 

Reactions of Imidazoles. The action of sodamide on the chloro(ethynyl)-imidazole (413 R = Cl, = H) results in the formation of the isomer (413 R = H, R = Cl) similar transhalogenations have been observed for chloro(ethynyl)-pyrazoles and -1,2,4-triazoles. The imidazolium salt (414) undergoes base-induced ring-expansion to the pyrazine derivative (415). 2,4,5-Tribromo-l-methylimidazole reacts with xenon difluoride to give the imidazolidinetrione (416). Low-temperature H n.m.r. studies of the sensitized photo-oxygenation of imidazoles (417 R, R, R = H or Me) give direct evidence for the formation of unstable enr/o-peroxides (418) 2-methyl-4,5-diphenylimidazole yields the peroxy-derivative (419) in this reaction.Irradiation of the N-oxide (420) produces the diamine (422) by way of the oxaziridine (421)."" ... 

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

In view of these previous developments, we directly coupled various N-substituted imidazoles, which display nucleophilic reactivity, with 2-bromo-oxazolines to give the imidazolium precursors (Scheme 15.15) [55]. This direct condensation of an oxazoline and an imidazole salt provided a straightforward and modular route to the development of a new family of stereodirecting ligands. NHC - rhodium complexes could be obtained by reaction of the imidazolium salt with [ Rh(p-OtBu)(nbd))2] (nbd, norbornadiene) generated iw sifn [56]. 

It has been observed that reductive elimination can also occur for aryl-Pd-carbene complexes. Such complexes were investigated in mechanistic studies on Heck coupling and catalyst decomposition routes. Reductive elimination products with direct imidazolium-aryl coupling were observed and in one case fully characterized. Such products provide direct evidence of the Heck coupling mechanism and of intermediates in the catalytic cycle. Important mechanistic studies on the oxidative addition of aryl chlorides to a 14-electron Pd(0)(carbene)2 complex have demonstrated that oxidative addition occurs via a dissociative process and this step is probably the rate-determining step in the amination of aryl chlorides. Aryl-carbene reductive coupling was observed in this study of the amination reaction, and directly coupled aryl-imidazolium compounds were isolated. A further study on an (aryl)Pd(carbene) complex has also demonstrated that such complexes undergo facile reductive elimination to form aryl-imidazolium salt. ... 

In order to separate structural effects from the electronic differences of these two catalyst classes. Bode synthesized chiral imidazolium salt 57 (Scheme 14.28). This allowed direct comparison of imidazolium versus triazolium precatalysts across a number of different reaction manifolds including those involving the catalytic generation of homoenolate equivalents, ester enolate equivalents, and acyl anions. These studies conclusively demonstrated that imidazolium-derived catalysts are superior for homoenolate reactions with less reactive electrophiles, while the triazolium-derived pre-catalysts are preferred for all other reactions. Interestingly, from the currently published body of the work, it does not appear to be any effects from the counterion of the azolium pre-catalysts in the presence of bases. 

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