Triazol-5-ylidene carbene

Synthesis of Poly([R,S]-P-Butyrolactone), (x-Methoxy, coCarboxylic Acid from Commercially Available 5-Methoxy-1,3,4-Triphenyl-4,5-Dihydro-lH-l,2-Triazol-5-Ylidene Carbene [78] 

2 Danheiser, R.L. and Nowick, J.S. (1991) The Journal of Organic chemistry, 56, 1176-85 and references within. 

5 Arnold, LD., Drover, ).C.G. and Vederas, ).C. (1987) Journal of the American Chemical Society, 109, 4649 and references dted therein. 

8 Testa, E., Fontanella, L, Cristiani, G.F. and Mariani, L (1961) Liebigs Annalen Der Chemie, 639,166. 

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The selectivity of the commercially available l,3,4-triphenyl-4,5-dihydro-lH-l,2-triazol-5-ylidene carbene 1 (Figure 9.2) for the ROP of lactide prompted Coulem-bier and coworkers to investigate its reactivity in the ROP of BL [76]. The group showed that, when the polymerization of BL was initiated from primary alcohol and 1 at 80 °C in toluene, the expected polymer chains were contaminated with crotonate byproducts. 

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. 

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... 

Dimethyl-1,2,4-triazolium iodide with nickel(II) acetate gives the carbene complex l2Ni( 1,4-dimethyl-l,2,4-triazol-5-ylidene)2 (97OM2209). 

Dimethyl-1,2,4-triazolium iodide with palladium acetate yields the carbene adduct 182 (97JOM(530)259). Under water it undergoes cis-trans isomerization to 183. Some other derivatives were reported in 1981 (81BCSJ800). 1,1 -Methylenebis(4-alkyl-l,2,4-triazolium)diiodides (alkyl = /-Pr, n-Bu, octyl) with palladium(II) acetate give the mononuclear complexes [L Pdl ] (99EJIC1965), where L2= l,l -methylenebis(4-R-l,2,4-triazol-2-ylidene) (R = /-Pr, n-Bu, octyl). Thermolysis of the products in THF gives the rran -dinuclear complexes 184... 

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... 

Scheme 32. Reaction of bicyclopropylidene (1) with the stable carbene l,3,4-triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene (143) [126]...

N-Heterocyclic carbenes are an example of a family of nucleophilic catalysts [84-87]. For instance, the polymerization of p-butyrolactone was catalyzed by l,3,4-triphenyl-4,5-dihydro-l//,l,2-triazol-5-ylidene in the presence of methanol as an initiator [86]. This reaction was carried out in toluene at 80 °C. Nevertheless, an undesired elimination (Fig. 4) reaction was observed and control of the polymerization was lost. This issue was overcome by using ferf-butanol as a co-solvent, which reacts reversibly with the free carbene to form a new adduct. Owing to the decrease in the concentration of the free carbene, the elimination is disfavored and the polymerization is then under control provided that a degree of polymerization below 200 is targeted. As a rule, the reactivity of N-heterocyclic carbenes depends on their substituents. Hindered N-heterocyclic carbenes turned out to be not nucleophilic enough for the ROP of sCL. Recently, it was shown that unencumbered N-heterocyclic carbenes were more efficient catalysts [87]. 

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). 

Different synthetic routes have been used to prepare these carbenes (Scheme 8.6). The most common procedure is the deprotonation of the conjugate acid. In early experiments, sodium or potassium hydride, in the presence of catalytic amounts of either f-BuOK or the DMSO anion were used. ° Then, Herrmann et al. showed that the deprotonation occurs much more quickly in liquid ammonia as solvent (homogeneous phase), and many carbenes of type IV have been prepared following this procedure. In 1993, Kuhn and Kratz" developed a new and versatile approach to the alkyl-substituted N-heterocyclic carbenes IV. This original synthetic strategy relied on the reduction of imidazol-2(3//)-thiones with potassium in boiling tetrahydrofuran (THF). Lastly, Enders et al." reported in 1995 that the 1,2,4-triazol-5-ylidene (Vila) could be obtained in quantitative yield from the corresponding 5-methoxy-l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazole by thermal elimination (80 °C) of methanol in vacuo (0.1 mbar). 

Note that the other type of aromatic carbene isolated by Enders et al.," namely, the l,2,4-triazol-5-ylidene (Vila) is stable enough to be prepared by thermal elimination at 80 °C, and it became the first carbene to be commercially available. 

The nitrogen heteroatoms in imidazole and some closely related heterocycles can stabilize a carbene center at the 2-position (97AG(E)2162). Thus, 1,3-disubstituted imidazole-2-ylidenes (163)-(170), l,3-dimesitylimidazoline-2-ylidene (171), 1,3,4-triphenyl-1H-1,2,4-triazole-5-ylidene (172), and their silylene (173) and germylene (174) analogues are stable (in the absence of oxygen and moisture) solids with definite melting points, which can be recrystallized from appropriate hydrocarbon solvents. The exception is carbene (163) which is an unstable liquid however, it is stable in solution. 

In cooperation with Teles and colleagues, our research group has studied the triazole heterocycle as an alternative core structure of nucleophilic carbenes. First, the triazol-5-ylidene 12 (Fig. 9.3 see also Scheme 9.2) was synthesized and shown to be stable at temperatures up to 150 °C in the absence of air and moisture [22]. Compound 12 exhibited the typical behavior of a nucleophilic N-heterocyclic car-bene, and was found to be sufficiently stable to become the first commercially available carbene [23]. As shown in Scheme 9.2, the crystalline carbene was obtained from the corresponding triazolium salt precursor 13 by the addition of methanolate and subsequent thermal decomposition of the adduct 14 in vacuo via a-elimination of methanol [24]. 

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. 

On the other hand, generation of free carbenes can be thermally achieved from C-protected NHC . In 1995, Enders reported the thermal elimination of methanol from 5-methoxy-l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazole (12) affording the corresponding carbene l,2,4-triazol-5-ylidene (13) in quantitative yield (Scheme 2). Methanol adduct (12) is easily synthesized from reaction of triazolium perchlorate (11) and NaOCH3 in methanol. 

Most active is an in situ generated Ni complex with the Enders carbene, 1,3,4-triphenyl-4,5-dihydro-lH-l,2,4-triazol-5-ylidene. The catalyst performance reads as follows 2.5 H2 equiv in 4h at 60°C for a dilute solution of AB in diglyme (0.14M, catalyst amount 10 mol%) 2.5 H2 equiv in 2.5 h at 60 °C for a highly concentrated solution of AB in diglyme (25 wt.%, catalyst amount 1 mol%). The obtained product mixture consists mainly of soluble polyborazylene. Besides the high yield of more than 2 equiv H2, this catalytic system is also beneficial in respect to a very low borazine... 

There have been two published reports on the syntheses of stable 1,2,4-triazolyl carbenes. Thermal decomposition in vacuo of 5-methoxytriazoline 208 provided in quantitative yield l,2,4-triazol-5-ylidene 209, a stable carbene in the absence of oxygen and moisture <0381292>. This nucleophilic carbene 209 could react with a variety of alcohols, thiols, amines, oxygen, sulfur, selenium, isocyanantes, and metal carbonyls to form a myriad of addition products. Reactions of 1,2,4-triazolyl perchlorate salts 210 with base afforded stable nucleophilic 1,2,4-triazol-5-ylidenes 211, which could react with acetonitrile and elemental sulfur and selenium to yield addition products <03JOC5762>. 

New pyrazolyl ligands containing an aminomethyl group, in position 4, have been coordinated to rhodium(I) and show a good solubility in water, presumably because the ammonium group does not interact with the metal center. The resulting complexes have been characterized and, in our opinion, they are good candidates for biphasic catalysis [82], such as l,2,4-triazol-2-ium-5-ylidene complexes of Rh(I), Ir(I), Ni(0), Ni(II), Pd(II), and Hg(II) [83]. Due to their ionic character, these carbene complexes are extremely soluble in water. 

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