Triazoles carbenes from

Cycloaddition of aryl imines 155 with diazomethane followed by oxidation of the intermediate triazoline with potassium permanganate afforded l-(4-methylsulfonylphenyl)-5-ary 1-1,2,3-triazoles 156 <05JHC33>. l,2,3-Triazolo-3 -deoxycarbanucleosides and their analogs were prepared efficiently by palladium(0)-catalyzed reactions <05T11744>. Base-induced generation of aryl(l,2,3-triazol-lyl)carbenes from l-[(7V-phenylsulfonyl)benzohydrazonoyl)-... 

Scheme 1.4 Formation of metal-bound imino carbene from iV-sulfonyl-l,2,3-triazole.

Scheme 14.9 Proposed mechanism for ROP of lactide by triazole carbene. (Adapted from Ref [28].)...

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

The intermediate produced by loss of nitrogen from a l.ff-triazole can be written as an imino-carbene, as a zwitterion, or as a diradical. Subsequent reactions of the intermediate which have been observed (Scheme 57) include (a) photochemical WoUf rearrangement (suggesting that the intermediate may have a singlet carbene structure) ... 

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). 

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]. 

On pyrolysis, 1-arylimidazoles rearrange to 2-arylimidazoles. In other systems, pyrolysis causes more deep-seated changes. 1-Arylbenzotriazoles on pyrolysis or photolysis give carbazoles via intermediate nitrenes (see Section 3.4.1.2.1). 1-Phenyl-1,2,4-triazole 789 is converted by pyrolysis into isoindole 790 via a carbene intermediate and another example of the participation of A-phenyl groups is found in the formation of benzimidazoles from tetrazoles (see Section 3.4.1.2.1). In the oxadiazolinone series, a nitrene intermediate 791 is also probably formed, which then ring closes. 

With respect to the application of asymmetric carbene catalysis as a tool for enantioselective synthesis, the last decade s major success is based on substantial improvements in catalyst development. Early reports dealt with implementing chirality in thiazolium scaffolds (Sheehan and Hunneman 1966 Sheehan and Hara 1974 Dvorak and Rawal 1998), but their catalytic performance suffered from either low yields or low ee-values. In this regard, the investigation of triazole heterocycles as an alternative core structure (Enders et al. 1995) has played a crucial role to provide heterazolium precatalysts improving both asymmetric benzoin and Stetter reactions. An intramolecular Stetter reaction yielding chromanones upon cyclization of salicylaldehyde-derived substrates is commonly used as a benchmark reaction to compare catalyst efficiency (Scheme 1 Ciganek 1995 Enders et al. 1996 Kerr et al. 2002 Kerr and Rovis 2004). 

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. 

Thermolysis of triazole 520 in acetonitrile affords imidazole 521 in 75% yield. Its formation was rationalized in terms of a 1,3-dipolar cycloaddition of the intermediate a-cyaniminocarbene 522 to the C=N bond of the solvent [81AG(E)113], although the mechanism of ylide formation with concomitant cyclization must not be ruled out. The similar nitrile with carbene 523, generated from diazo tetrazole 524, leads to imidazotetrazolic systems 525 (R = Me, Ph) in 42 and 51% yield, respectively (85T4621). 

The route shown succeeded in making the desired diazene 23 but the yield in the intramolecular carbene addition step was poor and initial attempts to obtain prismane by deazetization of compound 23 failed.A much more successful synthesis made use of the reaction of A-phenyl-l,2,4-triazole-3,5-dione with benzvalene. This gave an adduct which was simply converted to the same diazene 23 and it was subsequently shown that a low yield (ca. 6%) of prismane (24) could be obtained from the photolysis. 

Ring enlargement of aryl(l,2,3-triazol-l-yl) carbenes, generated from 1-[A -(phenylsulfonyl)benzohydrazonoyl-1,2,3-triazoles 8, into 3-aryl-l,2,4-triazines 9 has been described <05H279>. 

Phenylnitrene and 2-pyridylcarbene interconvert in the gas phase, as evidenced by the formation of identical products from phenyl azides and 2-(diazomethyl)pyridines (Scheme 30).1 99,200 Although the carbene precursor 157 exists as a triazole in the solid state, the diazomethane valence tautomer... 

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