Imidazolium salts acidic 2-proton

The epoxide method can be used with epoxides of acyclic [ 165-168] and cyclic [169-172] alkenes with a visible bias for cyclohexene oxide as the epoxide of choice. The epoxide is usually reacted with unsubstituted imidazole creating a neutral molecule. If the epoxide is reacted with an N-substituted imidazole, a zwitterionic molecule is created as the hydroxide functional group in the sidearm lacks the imidazole NH hydrogen atom to be proto-nated. In this case, addition of one equivalent of acid provides protonation to the alcohol and the counteranion for the formation of the imidazolium salt. 

Carboxylic acid amide functionalised imidazolium salts can be reacted with group 10 halides directly in the presence of an inorganic base (e.g. K COj) to form homoleptic complexes (see Figure 4.41) [122]. Interestingly, the carbene units are cis to each other and not trans as would be expected (see Figures 4.30 and 4.38). The cis complexation is not aided by hydrogen bonding as the carboxylic acid amide functionality loses its proton upon complexation to nickel. 

Activation of the carboxylic acid amide proton occurs only after addition of potassium carbonate, similar to the reactivities observed with the monofunctionalised imidazolium salt in Figure 4.42. 

The synthesis of a cyclopentadienyl-annulated imidazolium salt 282 was accomplished through a Nazarov-type cyclization as a key transformation. This annulation step was affected by toluenesulfonic acid via protonation-dehydration of the tertiary allylic alcohol 278 to form a three-centered carbocation, which was then annulated, in an electrophilic fashion, onto the C-4 position of the imidazole to form 279. The formation of the alcohol 278 was achieved via lithiation of imidazole 276 and then quenching with ketone 277 to give the 1,2-addition product (Scheme 70) <2005TL6847>. 

Lin et al. have studied variations in the chemical shifts for a variety of methyl-imidazolium salts with different alkyl chain substituents and Br, BF , and BF" anions [8]. They observed a high sensitivity in chemical shift (depending, among other factors, predominantly on the alkyl chain length) for the 2-proton in the imidazolium ring. The effect was most pronounced for the bromide salts. Additionally, they observed unexpected H/D exchange for the 2-proton. Today, these results can be interpreted as H-bonding effects, and of course the acidity of H2 comes to no surprise ([3,9] and references therein). 

Acylation of imidazole produces A-acylimidazoles via loss of proton from the initially-formed A-3-acyl-imidazolium salt. A-Acyl-imidazoles are even more easily hydrolysed than A-acyl-pyrroles moist air is sufficient. The ready susceptibility to nucleophilic attack at carbonyl carbon has been capitalised upon commercially available l,l -carbonyldiimidazole (CDI), prepared from imidazole and phosgene, can be used as a safe, phosgene synthon, and also in the activation of acids for formation of amides and esters via the A-acyl-imidazole. ... 

Several other methodologies have been employed for the synthesis of hydroxy-lated TSILs. 2-Hydroxypropyl-fiinctionalized imidazolium salts have been prepared in excellent yields by the reaction of protonated 1-methylimidazole with propylene oxide, the acid providing the anionic component of the resultant ionic liquid (Scheme 5.5-4) [22]. 

Although substitution at the 2-position of the imidazolium cation was considered to prevent the side reaction in the MBH reaction, Handy and Okello have found that even the 2-methyl substituted imidazolium cation was not completely inert. They found that the 2-methyl group underwent slow proton exchange even in the presence of a weak base such as triethylamine (Scheme 1.113). The acidic nature of this methyl group was further verified by analyzing the products obtained from attempted methylation of the imidazolium salt 302. When 302 was treated with excess NaH and CH3I, none of the expected product 304 was detected, instead product 303 was obtained (Scheme 1.114). ° ... 

Several methodologies for the preparation of metal-carbene complexes have been developed (Scheme 2.153). In the best case, imidazolium salts are submitted directly to a solution of a metal complex. The active catalyst is formed under catalytic conditions. Mandatory removal of the acidic proton and subsequent formation of the carbene is carried out in situ and can be promoted by a basic ligand already present in the metal salt. This task can be fulfilled, for example, by acetate in the palladium precursor, which is basic enough to remove the proton and to form the carbene-metal complex. Also, counter-ions of imidazolium salts can participate in the coordination reaction of the newly formed carbene complex by the displacement of less chelating ligands from the metal center [2]. 

Garbene-transfer reactions are particularly valuable in the formation of Pd-carbene complexes in which the free carbene is not readily accessible, or where a functionalized imidazolium salt, which contains acidic protons other that the G2 proton, is used. Therefore, an important development in the synthesis of carbene complexes was the use of Ag-carbene compounds to transfer the carbene to other metal centers. The complex PdCl2(A,jV -diethylbenzimidazol-2-ylidene) was prepared by this route.The Ag-carbenes are prepared by the reaction of imidazolium salts with the weakly basic Ag20. Thermally stable liquid crystalline Pd(ii)-carbene complexes 67 and 68 were also prepared by this method.In this case, the Ag-carbene compounds were generated and used in situ to form the palladium complexes. The unsymmetrical complex PdCl2(l-ethyl-3-methylimidazol-2-ylidene)2 complex, with different A-alkyl substituents, has also been synthesized via the Ag transfer route and the complex has been structurally characterized. 

Mixed imidazolium triazolium salt 24 was readily metalated at the imidazolium heterocycle with either [Pd(OAc)2] or [Rh(OMe)(COD)]2 (COD = 1,5-cyclooctadiene), due to the higher acidity of the imidazolium C2-bound proton compared to the triazolium proton. Extended reaction times for palladation or the use of the acetate analogue of the rhodium precursor, [Rh(OAc)(COD)]2, induced cyclometalation and produced ehelate complexes... 

Besides, imidazolium salts can also be employed as carbine precursor for organometallic catalysis, where hydrogen/ deuterium exchange reaction of the C(2)-proton occurs. Formation of the inclusion complexes (ICs) of imidazolium salts with the native P-CD and the heptakis-(2,6-di-0-methyl)-P-CD (DM-P-CD) is a simple and efficient method to modify the acidity of the imidazolium H(2) and its environment (Leclercq Schmitzer, 2009b). Encapsulation of 1, 3-disubstituted imidazolium chloride by P-CDs results in the inhibition of the H(2)/D exchange in the complex. 

It is also relatively easy to functionalise imidazolium cations at the 2-position.[88] For example, the phosphine derivatised salts shown in Figure 2.7 illustrate such a substitution pattern and they are easily prepare by virtue of the acidity of the 2-proton.[74] Substitution of the 2-proton tends to yield relatively high melting salts instead of ionic liquids. Despite this limitation the imidazolium-phosphine compounds are good ligands for catalysis improving the immobilisation potential of complexes dissolved in ionic liquids. 

Nevertheless, in the category of acidic ILs can be also included ILs based on protic ammonium, pyrrolidinium, pyridinium and imidazolium cations. These salts are formed through the transfer of a proton from a Bronsted acid to a Bronsted base ... 

Imidazole, thiazole and alkyl-oxazoles, though not oxazole itself, form stable crystalline salts with strong acids, by protonation of the imine nitrogen, N-3, known as imidazolium, thiazoUum and oxazolium... 

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