Triazol-5-ylidenes

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). 

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. 

Very recently, new ruthenium catalysts, for example RuCl2(triazol-5-ylidene) (p-cymene) [11] and the catalytic system generated in situ from [RuCl2(p-cym-ene)]2, tris(p-chlorophenyl)phosphine, and 4-dimethylaminopyridine [12], have provided efficient catalysts for synthesis of the same type of enol ester. The regio-selective cyclization of acetylenic acids containing a terminal triple bond to give unsaturated lactones was performed in the presence of catalytic amounts of Ru(tris(pyrazolyl)borate)(PhC=C(Ph)C CPh)(PMe2iPr2) [13],... 

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]. 

The triazol-5-ylidene 12 was found to be a powerful catalyst for the conversion of formaldehyde to glycolaldehyde in a formoin reaction [25.] The concept of triazolium salt catalysis appeared to show promise, and consequently our research group undertook the synthesis of a variety of chiral triazolium salts for the asymmetric benzoin reaction [26]. However, the ce-values and catalytic activities shifted widely with slight structural changes in the substitution pattern of the triazolium system. The most active catalyst 15 (Fig. 9.4) afforded benzoin (6, Ar = Ph) in its (R -configuration with 75% ee and a satisfactory yield of 66%. 

The introduction of two chloride atoms on the NHC backbone has little effect on the reactivity of the resulting complex 41 (Fig. 8) in the RCM of dienes [86]. Catalyst 42, exhibiting a cyclohexene group as part of the NHC heterocycle, displays a decrease in productivity in the RCM reaction of N,N-diallyltoluene-4-sulfonamide compared to the parent catalyst 15 [96] (Eq. 29). The triazol-5-ylidene catalyst 43 allows for the cyclization of disubstituted di-... 

Very recently,new catalysts precursors derived from [RuCl2(p-cymene)]2 such as RuCl2(triazol-5-ylidene)(p-cymene) (D) and RuCl(p-cymene)(o-Ph-(triazol-5-ylidene) (E) [24], or the in situ generated catalytic system based on [RuCl2(p-... 

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>. 

Toluenesulfonic acid effects the formation of diethyl ( )-2-(6-alkylthiazolo[3,2-6][l,2,4]triazol-5-yl)ethene-l-phosphonate (149) by the 1,4-elimination of ethanol from diethyl 2-(6-alkyl-thiazolo[3,2-/ ][l,2,4]-triazol-5-yl)-2-ethoxyethanephosphonate (147) as well as the 1,2-elimination of ethanol from diethyl 2-(6-alkyl-6-ethoxy-5,6-dihydrothiazolo[3,2-Z ][l,2,4]triazol-5-yliden)-ethanephosphonate (148) (Scheme 10) <88CB977>. 

The chemistry of stable carbenes has not been fully explored. However, Enders and coworkers have performed a range of organic reactions involving a triazol-5-ylidene. These reactions are outUned in Scheme 5.74 and may be considered as a model for other carbenes. 

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