Imidazolium saturated

Without special drying procedures and completely inert handling, water is omnipresent in ionic liquids. Even the apparently hydrophobic ionic liquid [BMIM][(CF3S02)2N] saturates with about 1.4 mass% of water [15], a significant molar amount. For more hydrophilic ionic liquids, water uptake from air can be much greater. Imidazolium halide salts in particular are laiown to be extremely hygroscopic, one of the reasons why it is so difficult to make completely proton-free chloroaluminate ionic liquids. 

Homoenolate Protonation The p-protonation of homoenolates has been observed by Scheidt and co-workers, resulting in a redox transformation of enals to afford saturated esters 48. This process is catalysed by the NHC derived from imidazolium salt 46 and utilises phenol as a proton source [14]. A range of primary and secondary alcohols, and phenol itself, are competent nucleophiles with which to trap the acylazolium intermediate 47 generated by protonation (Scheme 12.8). 

Although these two methods have found widespread application for the synthesis of free carbenes, they failed for selected saturated imidazolidin-2-ylidenes and especially in the preparation of triazolin-5-ylidenes. In these cases the free carbene species 7 can be obtained from 2-alkoxyimidazolidines 6 [44] or 5-aUcoxytriazoles [36] by thermally induced ot-elimination of an alcohol (Fig. 5). In addition to 2-aUcoxyimidazolidmes, 2-(pentafluorophenyl)imidazolidines [45, 46] have also been used for the generation of NHCs by cx-elimination. The adduct 8 eliminates acetonitrile upon heating [47] to yield the benzimidazolin-2-ylidene 9. In a more exotic procedure, imidazolium salts have been reduced electrochemically to give the free imidazolin-2-ylidenes [48]. 

The ionic liquid [bmim][BF ] is known to catalyze the aza-Diels-Alder reaction in the synthesis of pyrano- and furanoquinolines [190]. This reaction was also catalyzed by the enantiopure bis-imidazolinium salt 67 in 67% yield with an endo. exo ratio of 60 40 (Scheme 69) [191]. The product was obtained as a race-mate. In addition the aza-Diels-Alder reaction with imines and Danishefsky s diene was catalyzed by the salt 67 giving racemic product. The salt and its analogues could be easily prepared via the oxidation of the corresponding aminals [192]. Investigation of the influence of the counter anion in achiral C2-substituted imidazolinium salts, which can be also described as 4,5-dihydroimidazolium or saturated imidazolium salts, in the aza-Diels-Alder reaction showed, that the catalytic activity increased, the more lipophilic the counter anion and therefore the more hydrophobic the salt was [193]. 

The oxidative potency of dichromates and chlorochromates decreases under less acidic conditions. This is so, for example, when a less acidic ammonium salt is included as counter-ion of a dichromate or chlorochromate anion. Thus, a number of ammonium dichromates and chlorochromates possessing a milder oxidative potency has been described with the specific purpose of allowing very selective oxidations of unsaturated alcohols in the presence of saturated ones. These selective dichromates and chlorochromates include bis(benzyltriethylammonium)dichromate,135 tetramethylethy-lenediammonium dichromate (TMEDADC),136 imidazolium dichromate (IDC),137 N, A -dimeth y I a m i n o py r id i n i u m chlorochromate (DMAPCC),138 l-(benzoylamino)-3-methylimidazolium chlorochromate (BAMICC)139 and butyltriphenylphosphonium chlorochromate (BTPPCC).140... 

Amyes et al. point out that the difference in pAas for C- and N-protonation of these carbenes provides the difference in stability of the carbene and parent heterocyle as shown by the cycle of Scheme 27 for the imidazolyl carbene 50. Interestingly, Yates and coworkers have calculated similar pAas for the imidazolium ion 46 and its saturated imidazolinium counterpart 49.208 This... 

The most common way to prepare N-heterocyclic carbenes is the deprotonation of the corresponding azolium salts, like imidazolium, triazolium, tetrazolium, pyrazolium, benzimidazolium, oxazolium or thiazolium salts or their partly saturated pendants, with the help of suitable bases. The pJCa value of imidazolium and benzimidazolium salts was determined to be between 21 and 24, which puts them right in between the neutral carbonyl carbon acids acetone and ethyl acetate [41,42], Arguably, imidazolium-based carbenes have proven to be especially versatile and useful and their synthesis should be discussed in more detail. The synthesis of imidazolium salts has been developed over many decades and numerous powerful methods exist [43]. 

The ortho-arylation of aromatic aldehydes in the presence of a combination of Pd(II)/saturated imidazolium salt has also been reported [174]. Remarkably, the formation of the mono- or bi-orf/zo-substituted product could be easily controlled depending on the nature of the aromatic halide employed (Scheme 23). Both electron-donating and electron-withdrawing substituents were well tolerated by the catalytic system and heteroaromatic aldehydes could also be coupled. 

Figure 4.15 Synthesis of a saturated phenoxy functionalised imidazolium salt.

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 protocol leading to the unsaturated 1,1 -hydroxy-binaphthyl saturated NHC was a chance discovery, enabled by an aqueous workup procedure. The yield is only moderate (40%) and has to be separated from the main product, the targeted bis-imidazolium salt (51%). A better synthesis for this ligand might be desirable, although the saturated NHC equivalent is readily available [18,86,88,89]. 

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. 

The second wingtip group on the saturated imidazolium ring can be chosen almost at will and examples include Mes, DIPP and adamantyl [88]. 

Imidazolium dichromate is a selective oxidant for allylic and benzylic hydroxy groups. (Allylic alcohols are oxidized faster than benzylic alcohols.) The selectivity over saturate alcohols is similar to that of 4-(dimethylamino)pyridinium chlorochromate. DMF is recommended as the solvent for oxidations, since it appears that the choice of solvent is critical to obtaining high yields. This reagent has also been observed to cause some ( )/(Z)-isomerization during the oxidation of allylic alcohols. 

A striking feature of the imidazol-2-ylidenes is the small N-C-N bond angles (around 100-102°). These angles are smaller than in the imidazolium ion precursors and this trend, which is reproduced by good quality calculations, continues with the saturated imidazolidin-2-ylidenes right through to the comparison of acyclic diaminocarbene 6 (121°) [57] and its precursor 22 (133°). [58]... 

By applying a new mode of cooperative catalysis involving the combination of a chiral Bronsted acid and a -symmetric biaryl saturated imidazolium precatalyst, Lee and Scheldt disclosed a highly enantioselective NHC-cata-lyzed [3 + 2] annulation reaction between a,p-alkynals and a-keto esters to generate the desired y-crotonolactones in high yields and excellent levels of enantioselectivity (up to 92% yield, 92% ee). The authors proposed that NHC-bound allenolate underwent addition to the a-keto ester activated by the chiral Bronsted acid derived co-catalyst (Scheme 7.43). 

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