Imidazole Imidazolidines

Imidazole-5-thione, 4,4-diphenyl-tautomerism, 5, 368 3 H-Imidazole-2-thione, 1,3-dimethyl-structure, 5, 367 Imidazole-2-thiones acidity, 5, 367 betaines, 5, 372 synthesis, 5, 481 tautomerism, 5, 367 3H-Imidazole-2-thiones synthesis, 5, 473, 6, 992 Imidazolides deacylation, 5, 453 mass spectra, 5, 360 phosphoric acid reactions, 5, 454 reactions, 5, 451-453 Imidazolidine, l-alkyl-3-phenyl-N-oxidation, 5, 427 Imidazolidine, 1,3-benzyl-2-phenyl-oxidation, S, 427... 

Dioxo-3-alkyl-imidazolidine werden mit Lithiumalanat in der Hitze zu 3-A1-kyl-imidazolidinen reduziert z.B. 2,4-Dioxo-3-methyl-5-phenyl-imidazolidin zu 7-Methyl-4-phenyl-imidazolidin (43% d.Th.) bei 23° und inverser Zugabe wird dagegen (vgl. S. 252) 2-Hydroxy-l-methyl-4-phenyl-imidazol (66% d. Th.) erhalten, das sich unter energischen Bedingungen zum 7-Methyl-4-phenyl-imidazol (25 % d. Th.) reduzieren laftt3 ... 

Imidazole and its derivatives continued to play an important role in asymmetric processes. Optically active pyrroloimidazoles 26 were prepared by the cycloaddition of homochiral imidazolium ylides with activated alkenes <96TL1707>. This reaction was used in the enantioselective preparation of pyrrolidines <96TL1711>. A review of the use of chiral imidazolidines in asymmetric synthesis was published <96PAC531> and the preparation and use of a new camphor-derived imidazolidinone-type auxiliary 27 was reported < 6TL4565> <96TL6931>. 

These results suggest that imidazolidin- and imidazol-based skeletons transfer similar amounts of electron density to the metal. The conclusion that changes in the bridge of the NHC skeleton have such a small effect on the electronic properties of the NHC is quite surprising, considering that SIMes- and IMes-based catalysts often show remarkably different catalytic behaviour. It is still unclear if these small changes in the electronic properties of the NHC ligand confer such different catalytic behaviours, or other effects (steric, flexibility, etc.) should be invoked. 

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

In the course of studying a large nnmber of examples where the side chains of the imidazol- and imidazolidin-2-ylidene were altered, several research groups found that NHCs bearing exclnsively alkyl side chains did not provide catalysts with better characteristics when compared to SIMes- and DVIes-derived systems 14 and 15. While Herrmann and co-workers showed that an unsaturated NHC bearing cyclohexyl wing tips conld be incorporated into a second-generation catalyst that was active in metathesis [20-23], more recent studies showed that similar complexes were either very difficult to prepare or were unstable and showed dramatically decreased catalytic properties [24-26] (complexes 17-19, Fig. 3.4). 

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>... 

Since this discovery, a few types of other stable singlet carbenes have been described and reviewed5 imidazol-2-ylidenes,6 l,2,4-triazol-3-yli-denes,7 imidazolidin-2-ylidenes,8 acyclic diaminocarbenes,9 thiazol-2-yli-denes,10 and acyclic aminooxy- and aminothiocarbenes.11... 

Hydrazine-induced rearrangements have been observed with imidazoles, which contain an A -arylcarboxamide function as part of the ring system. 3-Aryl-5-arylidene-2-methylthioimidazolin-4-one (79) reacts with hydrazine to produce 5-arylidene-3-amino-2-arylimino-imidazolidin-4-one... 

Ruthenium(n) systems containing imidazol-2-ylidene or imidazolidin-2-ylidene have been used to catalyze the synthesis of 2,3-dimethylfuran starting at (Z)-3-methylpent-2-en-4-yn-l-ol [Eq. (54)]. The activity of the catalyst strongly depends on the nature of the NHC ligand. Benzimidazolin-2-ylidenes give the best results for this transformation. Similar systems have also been used for olefin metathesis reactions. ... 

A comparative study of the reduction of 4ff-imidazoles and their iV-oxides is shown in Scheme 33. ° Reduction stops at the imidazoline stage even with a large excess (2 1 mol ratio) of borohydride unless a 1-oxide group is present in this case it may proceed further to give an imidazolidine. 

The reaction of 2-aminobenzyl alcohol 376 with 2-chloro-4,5-dihydroimidazole afforded [2-(4,5-dihydro-177-imidazol-2-ylideneamino)phenyl]methanol hydrochloride 377, which upon treatment with carbon disulfide gave l-(477-3,l-benzoxazin-2-yl)imidazolidine-2-thione 378 (Scheme 71). The assumed reaction mechanism involved the initial formation of the dithiocarbamate 379, which underwent intramolecular nucleophilic addition to furnish the unstable thiazetidine 380. By nucleophilic attack of the hydroxy group on the carbon atom of the thiazetidine ring, thiocarbamate derivative 381 was formed, which gave the final 3,1-benzoxazine 378 by an intramolecular cyclocondensation with the evolution of H2S <2006H(68)687>. 

Within the last decade many variations of the basic imidazole-2-ylidene structure (Scheme 2,A) have been synthesized [14-19]. They are not limited to sterically hindered unsaturated cyclic diaminocarbenes like 1, also 1,2,4-triazolin-5-ylidenes (Scheme 2,B), saturated imidazolidin-2-ylidenes [6,7,20] (Scheme 2,C), tetrahydropyrimid-2-yhdenes [21,22] (Scheme 2,D), acychc structures [23,24] (Scheme 2,E), systems with larger ring sizes [25,26] (Scheme 2,F) or constrained geometry [27,28] (Scheme 2,G). Reviews on the different possible synthetic routes from various precursors can be found in the literature [29-31]. 

Werden Aldehyde mit Amin-Hydrochloriden, lsonitrilen und Kaliumcyanat in Methanol/Was-ser-Gemischen umgesetzt, so werden 4-Imino-2-oxo-imidazolidine (4-Amino-2-hydr-oxy-imidazole) erhalten42d ... 

Unsubstituierte Imidazole konnen durch Umsetzungmit Kaliumcarbonat und Kohlendioxid unter Druck im Autoklaven bei 150° zu 4(5)-Carboxy-imidazolen carboxyliert werden701. Die Umsetzung von 2,5-Dioxo-l-phenyl-imidazolidin mit Methoxy -methoxycarbonyloxy-magnesium fuhrt zu 4-Carboxy-2,5-dioxo-l-phenyl-imidazolidin (s. Bd.E5/l, S.328) ... 

Apart from imidazol-2-ylidenes (IV), eight other types of carbenes are included in this category imidazolidin-2-yhdenes (III), tetrahydropyrimid-2-yhdene (V)," ° benzimidazol-2-ylidene (VI)," l,2,4-triazol-5-ylidene (VII)," l,3-thiazol-2-yli-denes (VIII), as well as acyclic diamino- aminooxy- and aminothio-carbenes (XI) (Fig. 8.5). 

---