Carbenes, triazoles

Reaction of the keto ylide (588) with cyanazide (589) provided a convenient route to the 1,2,3-triazole-l-carbonitrile (590). On heating, (590) lost N2 readily to form the a-cyanoimino carbene (591) (8lAG(E)ll3). 

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

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). 

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... 

The dicationic ditriflate salt of 1,2,4-trimethyltriazolium with silver acetate gives the bis-carbene complex 185 (00JOM(600)l 12). In excess silver acetate, the one-dimensional polymeric species with alternating silver ions and 1,2,4-triazol-3,5-diylidene carbenes result, where both carbon atoms of each heteroring are engaged in coordination. 

For 1,2,3-triazole, the Ti (N ), i-ti (N, N ), and Ti fC) carbene coordination modes prevail in organometallic compounds. Benzotriazole has the same general pattern but often operates as a composite unit of benzotriazole and benzotriazolate linked by the hydrogen bond. In some organoosmium compounds, the i-ti (N, ) mode is realized, where the C -center refers to the annulated benzene ring. In some variations, together with the i-ti (N, N ) mode, the Ti fN" ) monodentate coordination occurs. The I-ti (N, N ) mode can also be traced. 

Triazole reveals the T) (C) carbene function, J.-ti (N, N ) bridges with subsequent coordination via the site, J.-ti (N, N ) bridges with subsequent coordination to the N" site. 

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... 

The catalyst is also effective for the reduction of styrenes, ketones, and aldehydes. Cyclohexenone 16 was reduced to cyclohexanone 11 by transfer hydrogenation, and using a higher catalyst loading, styrene 17 was reduced to ethylbenzene 18. The elaboration of [Ir(cod)Cl]2 into the triazole-derived iridium carbene complex 19 provided a catalyst, which was used to reduce aUcene 20 by transfer hydrogenation [25]. 

Attempted formation of the 4-silyl-substituted nucleophilic carbene (111) by deprotonation of the corresponding triazolium salt with KH led to the triazole (112), the product of apparent [1,2]-Si migration.A crossover experiment indicated that silyl transfer is intermolecular. 

This degenerate rearrangement has not been observed with 5-dia-zomethyl-l,4-diphenyl-l,2,3-triazole. The diazo function decomposes and the carbene formed reacts, when benzene is used as solvent, into a cyclo-heptatriene. The requirement to have the ester function present at C-4 apparently has to do with stabilization of the diazo function. 

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) ... 

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]...

DiazotriazoIe 28 (R = Ph) reacted with /-butyl alcohol and 2-propanol to give compounds 148 and 149 (Scheme 40) in comparable yields by carbenic C—H insertion and nucleophilic substitution, respectively [81DIS(B)(42)1892]. In the case of 2-propanol, an oxidation-reduction process, to give the parent triazole and acetone, was also observed to a smaller extent. Also, it was previously reported that 3-diazotriazole 28 (R = COOH) oxidizes primary and secondary alcohols to the corresponding aldehydes and ketones (1898LA33). 

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]. 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

Diazomethyl)-1,4-diphenyl-1,2,3-triazole (354) reacts with 1,4-disubstituted benzenes or naphthalenes to give a mixture of carbene derived norcaradiene/cycloheptatriene products (355) and (356) (Scheme 68). Attack is favored adjacent to the more highly branched substituent. When R = = Pr, a mixture of triazole derivatives (357) and (358), formed by addition to benzene and... 

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. 

The most important chemistry of azidoazoles is the fragmentation of derived nitrenes of which the prototypes are (616 — 617) and (618 — 619). Thus, 5-azido-l,4-diphenyltriazole (620) evolves nitrogen at 50°C (70JOC2215). 4-Azido-pyrazoles and-1,2,3-triazoles (621) undergo fragmentation with formation of unsaturated nitriles (8lAHC(28)23i) cf corresponding carbene reactions (Section 3.4.3.4.6). 

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