Copper pyrazolates

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Analogously, pyrazolyl-aluminate and -indate ligands have been prepared <75JCS(D)749) and their chelating properties evaluated with cobalt, nickel, copper and zinc. Gallyl derivatives of pyrazoles and indazoles have been extensively studied by Storr and Trotter e.g. 75CJC2944) who determined several X-ray structures of these compounds. These derivatives exist in the solid state as dimers, such as (212) and (288). A NMR study in acetone solution showed the existence of a slow equilibrium between the dimer (212) and two identical tautomers (289) and (290) (Section 4.04.1.5.1) (81JOM(215)157). 

Another example of the analogy between pyrazole and chlorine is provided by the alkaline cleavage of l-(2,4-dinitrophenyl)pyrazoles. As occurs with l-chloro-2,4-dinitrobenzene, the phenyl substituent bond is broken with concomitant formation of 2,4-dinitrophenol and chlorine or pyrazole anions, respectively (66AHC(6)347). Heterocyclization of iV-arylpyrazoles involving a nitrene has already been discussed (Section 4.04.2.1.8(i)). Another example, related to the Pschorr reaction, is the photochemical cyclization of (515) to (516) (80CJC1880). An unusual transfer of chlorine to the side-chain of a pyrazole derivative was observed when the amine (517 X = H, Y = NH2) was diazotized in hydrochloric acid and subsequently treated with copper powder (72TL3637). The product (517 X = Cl, Y = H) was isolated. 

Finally, NHC complexes of copper, formed in situ from the hnidazolium salt and CuBr have been used in the monoarylation of aniline [167]. Trinuclear Cu(I) catalysts have been applied to the arylation of pyrazoles, triazoles, amides and phenols [168] (Scheme 6.50). 

Also N, N -bisprotected pyrazole-l-carboxamidines are applied for preparation of guanidines.[91,[9al A variation of this reaction permitted the copper salt of ornithine to be converted to the copper salt of arginine [10]... 

The pyrazole phosphazine ligand, hexakis(3,5-dimethylpyrazolyl)cyclotriphosphazene (16) forms a dizinc complex, that has been structurally characterized, and the analogous copper zinc complex.158 Two ZnCl2 units are coordinated, one distorted tetrahedral and one distorted trigonal-bipyramidal, separated by 7.51 A. Cu1 and ZnCl2 can also be coordinated giving a heteronuclear compound with a Zn Cu separation of 6.798 A. 

Bis(pyrazolyl)borate copper complex 47 has been employed as a catalyst in the homogeneous and heterogeneous styrene epoxidation reactions <00JCS(CC)1653>. Pyrazole palladacycles 48 have proven to be stable and efficient catalysts for Heck vinylations of aryl iodides <00JCS(CC)2053>. An asymmetric borane reduction of ketones catalyzed by N-hydroxyalkyl-/-menthopyrazoles has been reported <00JHC983>. 

An easy synthesis of the 2-oxo-2,3-dihydro-l/7-pyrrolo[l,2- ]pyrazole system can be performed by reaction of 1,2-diaza-1,3-butadienes 33 with dialkyl 1,3-acetonedicarboxylate 34 in the presence of potassium carbonate. At first, 1-aminopyrroles 36 was produced by dehydration in the presence of copper(n) trifluoromethanesulfonate. Treatment of these compounds with sodium hydride led to ATZ-substituted 2-oxo-2,3-dihydro-17/-pyrrolo[l,2-A]pyrazole 38. Under the same reaction conditions, and after acidic treatment, NH-BOC-protected 1-aminopyrrole was transformed to NH-unsubstituted 2-oxo-2,3-dihydro-l//-pyrrolo[l,2-A]pyrazole 37 (BOC =/-butylcarbonyl) (Scheme 1). 

Synthesis of some pyrazole derivatives from amidoximes was reviewed by Karbonits and Horvath. It has been shown that acrylophenone or methacrylophenone oximes (110) on treatment with BuONO in the presence of pyridine and copper(II) sulfate, and with subsequent interaction with dilute NaOH and acidification, gives 3(5)-phenyl-1-hydroxypyrazole 2-oxide or 4-methyl-3(5)-phenyl-l-hydroxypyrazole 2-oxide (111), respectively (equation 47) ... 

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