Triazolin-5-ylidenes

Based on these assumptions many different heteroatom-substituted carbenes have been synthesized. They are not limited to unsaturated cyclic di-aminocarbenes (imidazolin-2-ylidenes Scheme 3, A) [17-22] with stericbulk to avoid dimerization like 1 l,2,4-triazolin-5-ylidenes (Scheme 3, B), saturated... 

Imidazolidinium salts can also be transformed into the corresponding diamino ortho-esters by alkaline alkoxylate, and upon alcohol elimination at elevated temperature the imidazolidin-2-ylidenes can be trapped. The reaction of tria-zolium salts with sodium methanolate in methanol yields 5-methoxy-4,5-dihydro-IH-triazole which also eliminates methanol upon heating in vacuo. The resulting triazolin-5-ylidenes can either be isolated or trapped by an appropriate metal precursor [Eq. (19)]. Benzimidazolin-2-ylidenes are similarly accessible by this route. 

As mentioned earlier, triazolium salts can be converted into 5-methoxy-4,5-dihydro-lH-triazoles by reacting them with sodium methanolate in methanol. The heterocycles eliminate methanol upon heating in vacuo [Eq. (21)] and the formed triazolin-5-ylidenes can then be isolated. " The same method works with imidazolium and benzimidazolium salts." ... 

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]. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

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]. 

An enantioselective version was recently reported by the same group (Scheme 9.23) [66]. 2,2-Dichloroaldehydes 76 react with phenols 77 in the presence of the chiral triazolin-5-ylidene carbene 35 to form a-chloroesters 78 in good yield and enantioselectivity. The key step in the proposed catalytic cycle is an enantioselec-... 

Ruthenium-arene catalysts bearing triazolin-5-ylidene ligands... 

This section focuses solely on the use of a commercially available triazolin-5-ylidene carbene, viz. l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazol-5-ylidene. When the stoichiometric reaction between this ligand and [RuCl2(/i-cymene)]2 was carried out in THF, a mixture of product and starting dimer was obtained. NMR analysis showed that coordination of the incoming carbene to the Ru center was accompanied by an orf/to-metaUation of the phenyl substituent in position 1, thereby releasing one equivalent of HCl in solution. Addition of a base (EtN/-Pr2 in excess) drove the reaction to completion and prevented side reactions between the protic acid and the highly reactive free carbene species. Thus, a single adduct could be obtained selectively and quantitatively (Eq. 

Huynh HV, Lee C-S. Pincer-type di(l,2,4-triazolin-5-ylidene)Pd(II) complexes and their catalytic activities towards Cu- and amine-free Sonogashira reaction. Dalton Trans. 2013 42 6803-6809. 

An efficient stereoselective synthesis of the (pyrrolidin-2-ylidene)glycinate intermediate 325 was reported in a total synthesis of carzinophilin (326), employing an intramolecular cycloaddition of an azide with an alkene (63) (Scheme 9.63). The arabinose derivative 319 was converted into the required azide 321 via the triflate 320. Thermolysis of the azide 321 at 50 °C in THF produced the unstable triazoline 322, which on rearrangement gave the (pyrrolidin-2-ylidene)glycinate 325 in 60-72% overall yield from the triflate 320. 

By contrast, 1,2,4-triazole carbene 354 displays electrophilic character. Thus, it reacts with alcohols and amines producing triazoline derivatives 355 in quantitative yields. Oxygen or sulfur gives triazolinone and triazolinethione derivatives 356 (similar reaction with tellurium is known for imidazol-2-ylidenes). Reactions of 354 with dimethyl maleate or dimethyl fumarate lead to compounds 357, probably via ring opening of a cyclopropane intermediate with subsequent 1,2-hydrogen shift. 

Diisopropyl- and l,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene ligands and complexes of Pd(ii) have been synthesized. The complexes were obtained via an Ag-carbene transfer reaction with PdCl2(NGMe)2 and X-ray structures determined. The complexes were found to be extremely effective in Heck coupling reactions with aryl bromides but much less so with aryl chlorides. Palladium complexes of the triazole-based carbenes, 1,4-dimethyl-l,2,4-triazolin-2-ylidene, and chelating l,T-methylenebis(4-alkyl-l,2,4-triazolin-2-ylidene) have been synthesized by... 

Certain imidazolinium salts, particularly derivatives with bulky N,N -sah-stituents and some triazolium salts could not be deprotonated with strong bases like NaH, but a thermally induced a-elimination reaction proved successful in these cases. This approach was attempted as early as 1960 by Wanzlick and Schikora who heated C2-trichloromethyl substituted imidazolidine but could only isolate the carbene dimer (see Scheme 1.2). Enders et al. successfully heated triazoline 67 under elimination of methanol from the C5 carbon atom and formation of triazol-5-ylidene 68 (Scheme l.lOa). ... 

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