Dihydroimidazolium salts

Imidazole nitrones 127 reacted with dimethyl acetylenedicarboxylate to yield imidazo[l,5-fc]isoxazoles 128, which in the presence of base afforded imidazoles 129 <00TL5407>. Chiral imidazoline nitrone 130 participated in a [3+2] cycloaddition reaction with various dienophiles to furnish imidazoisoxazoles 131 <00SL967>. A convenient synthesis of AyvyV -trisubstituted ethylenediamine derivatives from 2-methyl-2-imidazoline has been reported <00SC3307>. Dehydrogenation of 1,3-di- and 1,2,3-trisubstituted imidazolidines afforded l//-4,5-dihydroimidazolium salts <00SC3369>... 

More recently, two types of Ru complexes were obtained by the reaction of mesityl-substituted electron-rich olefins with [RuCl2(p-cymene)]2 [27]. Cleavage of the chlorine bridges occurs first to yield the expected (NHC)(p-cymene)Ru(II) complex 9. Under harsher reaction conditions (140 °C in p-xylene) further arene displacement takes place to yield the chelated ( 6-mesityl-NHC - Ru complex 10 (Scheme 6). The olefin 8 was easily obtained by deprotonation of the corresponding dihydroimidazolium salt. 

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],... 

Deprotonation of amidinium salts gives 1,1-enediamines112 and the method is well suited for the preparation of simple cyclic 1,1-enediamines18, 41. 4,5-Dihydroimidazolium salts 99, prepared easily by alkylation of imidazolines or cyclization of ethylenediamine with orthoesters, react with sodium hydride to afford 2-alkylideneimidazolidines 100 in good to excellent yields (equation 34)18. 

In 2006, Matsumoto and Tomioka developed a chiral symmetric N-heterocyclic earbene generated from a dihydroimidazolium salt and strong base (potassium hydride or /r-butyllithium) and applied these catalysts to asymmetric intramolecular Stetter reactions with up 80% ee. It is remarkable to note that high temperature (reflux in toluene) is crucial for the formation of cyclic products with relatively high enantioseleetivity. Racemization of the product was almost sufficiently suppressed by adjusting the ratio of base to dihydroimidazolium salt to 1 2. 

Dihydroimidazolium carbene precursors or "saturated N-heterocyclic carbene" precursors are generated by a related sequence that includes reaction of a diamine, often generated by reduction of the glyoxaldimine, with triethylorthoformate, in the presence of acid. This reaction generates the dihydroimidazolium salts, wliich can be deprotonated to form the free carbene or used directly in the synthesis of carbene complexes. Finally, the acid-catalyzed reaction of the diamine with formaldehyde, followed by iodine and base, generates the free carbene. The silver complexes are typically generated by reaction of the protonated carbenes with Ag O. ... 

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]. 

Technically significant is the finding that 4 can be generated in situ from 1 and l,3-dimesityl-4,5-dihydroimidazolium tetrafluoroborate (treatment of the salt with t-BuOK in THF at room temperature for < 1 min). The RCM is performed in the presence of ethereal HCl. ... 

The resulting amine hydrochloride salts were obtained with sufficient purity (>95 %) and were directly used for cychzation. The general reaction conditions required 1 mol% of Pd2(dba)3, 2 mol% of l,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium tetrafluoroborate as a ligand and four equivalents of t-BuOK. The mixture was heated in 1,4-dioxane at 160 °C for 10 min in a microwave reactor (Scheme 2.58). The crude product was typically purified by simply passing the crude reaction mixture through a short pad of silica gel. 

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