Azolium salts, reaction with

Azolines 13C NMR, 5, 20 H NMR, 5, 17 NMR, 5, 14 Azolinethiones reactions, 5, 102-103 Azolinones acidity, 5, 72 13C NMR, 5, 16, 19 H NMR, 5, 14, 15 halogenation, 5, 58 hydrolysis, 5, 64 IR spectra, 5, 24, 27 reactions, 5, 81 with aldehydes, 5, 60 with ketones, 5, 60 Azolin-2-ones electrophilic attack, 5, 99 Azolium cations nucleophilic attack at carbon, 5, 61 reactivity, 5, 42 Azolium ions reactions with alkali, 5, 63 with amines, 5, 65 Azolium salts hydrogen exchange, 5, 70 nucleophilic attack at hydrogen, 5, 69-72 reactions... 

Condensation Reaction of Cyclopentadienides with Azolium Salts. 130... 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

In 2007, Fernandez et al. demonstrated that transition-metal complexes with heterobidentate S/C ligands based on imidazopyridin-3-ylidene and thioether functionalities could be readily prepared from the corresponding azolium salts by reaction with Ag20 and transmetalation of the resulting silver carbenes with appropriate metal sources. The cationic Pd(allyl)(carbene-S) complexes have proven to be active catalysts in the test reaction, reaching enantioselectivities of... 

In many cases the synthesis of NHC complexes starts from iV,A/ -disubstituted azolium salts. Imidazolium salts as precursors for imidazolin-2-ylidenes are generally accessible by two ways complementing each other (i) nucleophilic substitution at the imidazole heterocycle or (ii) a multicomponent reaction building up the heterocycle with the appropriate substituents in a one-pot reaction. 

The two most common methods for the synthesis of complexes with NHC ligands are the reaction of a free carbene (a) or its enetetramine dimer (b) with a suitable metal precursor or the in situ deprotonation of an azolium salt (c) depicted in Fig. 8 using diaminocarbenes with five-membered heterocycles as examples. 

The carbene transfer reaction from silver NHC complexes has developed into a standard procedure for the synthesis of NHC complexes. This versatile procedure was introduced by Lin et al. in 1998 [102]. It is based on the preparation of silver NHC complexes which are obtained in good yield by the in situ deprotonation of azolium salts with silver oxide (Fig. 9). Depending on the counter ions present in the azolium salt and the steric demand of the N,N -substituents, complexes 25a-25c... 

A large number of NHC complexes tolerate moisture, air, and elevated temperatures [1, 2]. The stability of these compounds is, however, still limited. One of the most important decomposition route for these complexes is the reductive elimination of 2-aIkyl or 2-aryl substituted azolium salts from NHC complexes with alkyl or aryl groups in cA-position to the NHC ligand. This route can be considered as the reverse reaction of the previously discussed oxidative addition of C2-X bonds to transition metals and has been reviewed [94, 123]. 

Relatively little is known concerning the oxidation of azolium salts. Most of the publications deal with thiazolium salts due to the significant biochemical role of thiamin as a coenzyme in a variety of enzyme-catalyzed decarboxylations and aldol-type condensations. The chemistry of thiamin has been extensively reviewed (83MI1). Depending on the reaction conditions, thio-chrome (197) and the disulfide 198 are formed by oxidation of thiamin (57JA4386). 

Dihydro allyl adducts like (254) are obtained by reaction of thiazoles with allyltributyl tin in the presence of alkyl chloroformates acting as activators of the thiazole ring (Scheme 28) (94JOC1319). This reaction most likely takes place via the intermediate azolium salt. Under these conditions even organolithium compounds can add to thiazoles (84TL3633). Similarly, direct ethynylation of thiazole and benzothiazole can be achieved by reaction with bis(tributylstannyl)acetylene (Scheme 29) (94SL557). 

There are three groups of substrates for which SNH reactions are especially characteristic (i) neutral azines and azoles (ii) azinium and azolium salts and (iii) nitro-arenes. Their electron-deficiency and, thus, their ability to react with nucleophiles strongly differ from each other. Azinium salts are the more electrophilic and are able to add even neutral nucleophiles very easily. Triazines,. v-tetrazines and polynitro-arenes also possess high electrophilicity. At the same time substrates such as pyridine (82JHC1285, 72JA682) and cinnoline (03CHE87), at a low temperature (e.g. in liquid ammonia as a solvent), do not react even with sodium amide. It should also be... 

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 in situ deprotonation of an azolium salt to produce the desired NHC has the advantage that the carbene does not have to be isolated, thus simplifying the reaction workups when the aim is preparation of the metal complex. This avoids the handling of the free NHCs, which most of the times are air-and moisture-sensitive. Two types of azolium in situ deprotonation reactions can be found in the literature, depending on the deprotonation process employed (i) addition of an external base and (ii) use of metal complexes with basic ligands. 

Methods (1) and (2) are classical routes for the preparation of pyridinium quaternary salts [74HC( 1)309], which together with route (3) are almost general approaches, and each has its own area of application. Alternatively, condensation reactions (4) offer a rather specific methodology, and as will be seen, they may be useful for the synthesis of vinylogues 36-38. Other more or less unusual methods (5) may be applied for the preparation of specific azolylpyridinium(azolium) salts. 

In a similar manner the l-aza-2-azoniaallene salts derived from coumarin and camphor react with diisopropylcarbodiimide to give the [2-1-3] cycloadducts. °i Also, l,3-diaza-2-azoniaallene salts 304 undergo the [2-1-3] cycloaddition reaction with diisopropylcarbodiimide or DCC to give 1,3,4,5-tetrasubstituted 4,5-dihydroten azolium salts. 305.11 ... 

An interesting variant is the in situ preparation of transition metal alkoxides from the corresponding halogenides and subsequent reaction with an azolium salt to form the NHC transition metal complex [69]. This works particularly well with rhodium, iridium and ruthenium where [(ii -cod)MCl]j (M = Rh, Ir) and [Cp RuCl]2 are readily available [57,58,71]. 

One of the most generally used methods to prepare a NHC transition metal complex is the reaction of an azolium salt with silver oxide to form the silver carbene complex [24,25,72], It is so general that it has its own name, the silver(I) oxide (Ag O) method... 

A similar mechanism might operate in the activation of an azolium salt by a transition metal compound forming the metal carbene complex. However, since a basic substituent on the metal (acetate, alkoxide, hydride) usually reacts with the H -proton, the proton is removed from the reaction as the conjugate acid and reductive elimination does not occur. 

Steric protection can be provided by adamantyl wingtip groups that are introduced by the reaction of the l,r-diaminobiphenyl to the Schiff base (with 2-adamantone) followed by reduction with LiAlH [79,80] (see Figure 5.24). Ring closure with triethyl orthoformate to the corresponding azolium salt proceeds as usual and subsequent reaction with [Pd(allyl)Cl]j in the presence of a suitable auxiliary base (potassium tcrt-butylate) yields the respective palladium allyl complex. The paUadium(II) allyl complex can then be reacted with hydrochloric acid to form the [Pd(NHC)Cy dimer. Subsequent reaction with a silver(I) salt predictably breaks up the dimer under chloride abstraction and anion transfer from silver to palladium. 

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