Silver® pyrazolates

Sodium and silver pyrazole salts (Section 4.04.2.1.3(vl)) are often used Instead of neutral pyrazoles to facilitate electrophilic attack on the ring nitrogen atoms. For example, pyrazolyl-methanes (234)-(236) have been prepared from pyrazole anions (Section 4.04.2.1.3(vlll)). Unstable 2-acetyllndazoles are obtained from the reactive silver salts of indazoles (Section 4.04.2.1.3 (x)). Electrophilic attack on the ring carbon atoms also occurs more readily in... 

In the crystal, most trinuclear carbeniate complexes are known to be stacked with close aurophilic interactions, and organic 7r-acids can be intercalated into these stacks.252 Similar intercalation takes place with the cations of simple silver and thallium(i) salts. The cations become attached to form polymetallic core units through metallophilic bonding. With the trinuclear silver pyrazolates, both metal and ligand exchange have been observed to give new stacks of trinuclear units.254... 

Surprisingly, treatment of 1 with Ag20 does not decompose the diazo ester but rather leads to the silver pyrazolate 6. A related observation has been made recently [6], Similary, a thermally induced intramolecular 1,3-dipolar cycloaddition reaction generates pyrazole 7. This reaction mode is limited, however, to specific substitution patterns. In other cases, an intermolecular rather than the intramolecular [3+2] cycloaddition comes into play (see next section). 

With iodine in carbon tetrachloride, 4-methylpyrazole affords a deep-red oil for which the structure (266) has been proposed. Nitric acid, silver nitrate and iodine together convert pyrazole into 1,3,4-triiodopyrazole (267 = R" = I, = H). The fV-iodopyrazoles are... 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

Pyrazole does not react with iodine although pyrazolylsilver is converted into 4-iodopyrazole. 3-Iodoindazole can be obtained by the reaction of iodine with the silver salt of indazole. Kinetic studies on pyrazole iodination have been carried out by Vaughan et al. (71PMH(4)55, B-76MI40402). Coordination of pyrazole by nickel(II) in aqueous solution increases the rate of iodination by factors of two at pH 6 and eight at pH 7.2 (72JA2460). 

A -Pyrazolines such as (410) are oxidized by iodine, mercury(II) acetate and trityl chloride to pyrazolium salts (411), and compound (410) even reduces silver nitrate to Ag° (69JOU1480). Electrochemical oxidation of l,3,5-triaryl-2-pyrazolines has been studied in detail (74BSF768, 79CHE115). They Undergo oxidative dimerization and subsequent transformation into the pyrazole derivative (412). 

We have already noted (Section 4.04.2.1.4(xi)) that alkyl groups on pyrazoles are oxidized with permanganate to carboxylic acids. Silver nitrate and ammonium persulfate transform 4-ethyl-1-methylpyrazole (436) into the ketone (437) (72JHC1373). The best yield was obtained starting with the alcohol (438) and using an acid dichromate solution as oxidizing agent. 

Tetraazafulvalenes bearing two pyrazole subunits could be prepared by an original way. Tlius, treatment of benzylidene acetophenone with iso-pentylnitrite leads to an A, A -dihydroxy-bipyrazolyl-A, A -oxide, which in turn can be oxidized to TAF of type 100 (72CC961, 79JOC3211). Another type of oxidative dimerization was observed by the reaction of the electron-rich l-methyl-2,4-bis(dimethylamino)imidazole with silver salts (83TL3563). A bis-cation was isolated in 30% yield in the presence of sodium tetrafluo-roborate an unsymmetrical structure 101 was predicted from its NMR data (Scheme 40). 

Aqueous solutions of dimetiiylgold(III) nitrate easily react with pyrazole and 3,5-dimethylpyrazole to form the pyrazolate complexes 271 (R = H, Me) [85 JOM (295)401]. However, 3,5-diphenylpyrazole gives the complex [Me2Au(3,5-Ph2pzH)2], where the ligand is not deprotonated. To obtain the 3,5-diphenylpyra-zolate complex, dimethylgold(III) iodide must be reacted with silver diphenylpyra-zolate. 

Au-C bonds, mixed metal gold-silver dimers of planar, trinuclear complexes are readily formed by mixing gold(I) carbeniates and gold(I) benzylimidazolates with silver(I) pyrazolates in stoichiometric ratios. The complexes retain the ligands associated with the metal atoms of the starting materials. 

Barranco, E.M., Gimeno, M.C., Laguna, A. and Villacampa, M.D. (2005) Gold and silver complexes with the ferrocenyl-pyrazolate ligand FcCH2pz. Inorganica Chimica Acta, 358 (14), 4177 182. 

Other tri-coordinate silver(I) complexes are the bridging pyrazolate complexes such as [Ir(775-C5Me5)(pz)3Ag(PPh3)], [Ir(//5-C3Mc5)(pz)3 Ag(PPh3) 2]BF4, and [Ir(j 5-C5Me5)(PPh3)-... 

The cycloaddition of nitrilimines toward furo[3,4-c]thieno[2,3-r7]-pyrazoles has been investigated as a function of the electron-withdrawing or electron-donating character of the substituents attached to the aromatic rings <2000H(53)917>. For instance, nitrilimine 459, obtained in situ from hydrazonoyl chloride 458, by base treatment with silver carbonate in dioxane, when reacted with 460, gives a mixture of compounds 461 and 63 (Scheme 53) <2000H(53)917>. 

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