Pyrazolyl groups

From UV studies of 4-phenyl-, 4-nitro- and 4-nitroso-pyrazoles, Habraken et al. (67RTC1249,72JHC939) conclude that the 4-pyrazolyl group acts as an electron-donating group. UV spectra of pairs of 1-aryl- and 2-aryl-indazoles and their utility in the determination of isomeric structures are discussed in (67BSF2619) many other UV data on indazole derivatives can be found in (71PMH(3)67). 

We have conducted the comparative study of gold (III), platinum (IV) and palladium (II) acidocomplexes solution on macroporous granular sorbents on the basis of polystyrene with functional groups of methyleneamine, 3-methylpyrasolyl, N,N-dimethylaminomethylene, dimethylmethylene-P-oxyethylamine and with functional 6-(3-methylpyridine) groups on polyvinylpyridine basis as well as fibrous polystyrene sorbent with pyrazolyl groups. 

M[pzTp](H20) (M = Na, K) have been determined by x-ray diffraction (30). In contrast to the discrete monomeric structures of the tris (pyrazolyl)hydroborato complexes Tl[TpRR ], K[TpBut2], and Cs[TpBut2] described earlier, the hydrated tetrakis(pyrazolyl)borato complexes M[PzTp](H20) (M = Na, K) exhibit an interesting polymeric-type structure, a portion of which is illustrated in Fig. 14. In each case, the cations occupy two different crystallographic sites, and the cation in one of the sites is coordinated to two pyrazolyl groups in a rr-type fashion. 

Another illustration of the structural changes that may result as a consequence of alkyl substitution at the 3-position of the pyrazolyl group is provided by the structures of the dimeric copper(I) complexes [Tp]Cu 2 (36), [TpMe2]Cu 2 (36), [TpPh2]Cu 2 (37), and [TpBut]Cu 2 (37), which differ in the manner in which the tris(pyrazo-lyDhydroborato ligand bridges the two copper centers (Fig. 16). 

TpBut,Me]Cu 2 is also observed at temperatures up to 90°C It is possible that marked difference in fluxionality of [TpBut]Cu 2 and [TpBut,Me]Cu 2 is a consequence of increased steric interactions between the 5-methyl substituents that would result in a structure for a transition state (or intermediate) in which all three pyrazolyl groups are coordinated to the dicopper moiety in a similar fashion to that of [TpMe2]Cu 2 (Fig. 16). 

The structure of [pzTp]2GaMe, determined by x-ray diffraction, demonstrates that the [pzTp] ligands adopt both bidentate and tridentate coordination. However, in solution, the complexes [Tp]2GaMe and [pzTp]2GaMe are fluxional on the NMR time scale at room temperature, exhibiting a single set of resonances for the pyrazolyl groups. 

The course of the reaction of [TpRR ]ZnOH with C02 is strongly influenced by the presence of different substituents on the pyrazolyl groups, and the reaction between [Tp lZnOH and C02 presents some interesting differences from that of the [TpBut Me] system. 

Pentadentate ligands with N4S donors based on methyl 2-aminocyclopent-l-ene-l-dithiocar-boxylate with pendent pyrazolyl groups (113) form complexes with zinc including a structurally characterized trigonal-bipyramidal zinc compound.891... 

Due to the trans influence of the pyrazole acceptor ligands a coordination of the phosphine donor trans to the carboxylate (isomer C) is very unlikely. The remaining isomers A and B are both racemic mixtures of two enantiomers with two different sets of NMR signals for the two pyrazolyl groups. 

In compound 191 one of the pyrazolyl groups is lacking and thus only two nitrogen atoms can coordinate to zinc [Zn—N 2.018(5) and 2.021(5) A]. This leads to a trigonal planar geometry at zinc (the sum of the bond angles around zinc is 360° within experimental error). It has been suggested that an additional weak interaction exists between one of the hydrides and zinc. 

The bis(mercaptoimidazolyl)(pyrazolyl)hydroborato unit present in 193 (Figure 93) coordinates via both sulfur atoms [Zn—S 2.404(1) and 2.410(1) A] of the two mercap-toimidazolyl moieties and the nitrogen atom [Zn—N 2.043(4) A] of the pyrazolyl group to zinc. Because this ligand system is more flexible than the tris(pyrazolyl)hydroborate, the bond angles around zinc are closer to the tetrahedral value than in 190. 

Although variations in the substituent groups on Tp are expected to result in differing reactivities, synthetic problems arise when attempts to vary the substituents are made. With less sterically demanding ligands than tert-butyl, the pyrazolyl group can bind to the boron center in two different fashions, yielding a difficult-to-separate mixture of isomers. For example (45), the reaction of 3-iso-propyl-5-methylpyra-... 

The ligands consist of two to four pyrazolyl groups attached to a boron atom, i.e. H2B(pz), HB(pz)7 and B(pz)+, where pz = 1-pyrazolyl. Thus, excellent bidentate (1) and tridentate (2)... 

Unsurprisingly, dynamic exchange of pyrazolyl groups on the NMR timescale was universally observed, equivalence of all pyrazolyl groups being noted for 192,223, and 224 at ambient temperature, with the process slowed at 40 °C.49,52 In contrast, the platinum analogue 225 exhibits... 

When l-(2,4-dinitrophenyl)pyrazole is heated with aqueous alkali, the C—N link is cleaved by a nucleophilic displacement with the formation of pyrazole and the sodium salt of 2,4-dinitrophenol.718 The carbon-nitrogen link is also cleaved readily when more than one pyrazolyl group is joined via nitrogen to the same carbon atom. Thus tri-l-pyrazolylmethane (78) on heating with aqueous acetic acid decomposes to pyrazole and 1-formylpyrazole.719... 

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