Pyrazole tetramers

We have published some papers concerning some less common heterocycles such as 1,2,4-diazaphospholes 257 and 258 [203], Using X-ray crystallography, CPMAS NMR, and GIAO-type calculations we have found that 257 is a cyclic dimer with localized N-H protons (similar to pyrazole dimers) while 258 is probably a tetramer (similar to pyrazole tetramers) showing ISSPT (intramolelecular solid state proton transfer). This prediction was only partly true because 258 crystallizes in two cyclic dimers, both presenting proton disorder [204] (Fig. 8). 

By contrast, the tautomerization of soUd pyrazole tetramers 20 (Figure 14.9) proceeds via two consecutive concerted double proton transfers via a zwitteri-onic intermediate [44]. This was to be expected as a concerted hydrogen bond compression of a large number of hydrogen bonds is not feasible. On the other... 

Figure 14.9 Solid state tautomerization of a substituted pyrazole tetramer 20 [44] (a) and of pyrazole-4-carboxylic acid 21 (b) [45, 46].

Lifnbach et al. [92JA9657 97BBPG889] made an exhaustive study of proton transfer in solid pyrazoles. For instance, the activation barriers, isotope and tunneling effects of the dimer 67, the trimer 68, and the tetramer 69 were determined. Catemers, like pyrazole itself, do not show dynamic behavior. 

After these findings, evidence was presented that pyrazoles which crystalline as cyclic dimers 9a/9b and tetramers 11a are subject to double and quadruple proton transfers respectively (92JA9657 95JOC1965). An im-... 

With hydrazines in aqueous ethanolic solution 3-(pyrazol-3-yl)-substi-tuted tetramic acids (45) are formed. Hydroxylamine gives the corresponding compound 46 (87TH2). (See Fig. 21.)... 

We studied, at the B3LYP/6-31+G theoretical level, four monomers and 12 NH-pyrazole cyclamers, C-unsubstituted or bearing fluoro, chloro and bromo substituents at positions 3 and 5 [100], Two mechanisms of proton transfer, stepwise and synchronous, were calculated for dimers, trimers, and tetramers. The set of values of energies and geometries thus obtained provide useful insights about the dynamics of NH-pyrazoles in the solid state. It has been shown that pyrazole cyclamers exist not only in condensed phases but in the gas phase as well [101], thus our gas-phase calculations will provide information about the solid state. 

Fig. 4 Representation of the discrimination of NH-pyrazoles using sum-F (2dimer, 4 tetramer, C catemer). Sum-F corresponds to the accessible surface of the N atom [119]...

Condensation of acetyl-tetronic or -tetramic acids with methyl- or phenyl-hydrazine leads to 3-(l-hydrazinoethyl)-tetronic and -tetramic add, respectively, which upon dehydration form 1-substituted 4-oxo-l,4-dihydro-6//-furo[3,4-c]pyrazoles (X=0) and -pyrrolo[3,4-c]pyrazoles (X=NH) (equation 20) (82SC431). 

The geometries of hydrogen bonded trimers and tetramers in solid 3,5-sub-stituted pyrazoles (Fig. 6) have been studied from consideration of both 15N chemical shift tensors and dipolar interactions involving 15N [122]. 

Copper(I) Carboxylates, Triflate, Alkoxides, and Dialky lamides. Thecarbox-ylates have varied structures. The acetate that is obtained as white air-sensitive crystals by reduction of Cu11 acetate by Cu in pyridine or MeCN has a planar chain structure (17-H-I). By contrast the trifluoroacetate [Cu02CCF3]4-2C,H6, and benzoate [Cu02CPh]4 complexes are tetramers with bridging carboxylates as in (17-H-II). This is only one type of Cut polynuclear structure (see later). There are also bridged pyrazole and pyrazolylborate compounds. 

H-Pyrazol 4-Oxo-3,3,5,5-tetrame-thyl-4,5dihydro E17b, 1111 (1,3-Br2 — 2-on/N2H4 Oxid.) Pyrimidin... 

Self-assembly of monopyrazolylporphyrins by hydrogen bonding has been examined by Ikeda and coworkers [144]. IR spectral measurements of the pyrazole NH stretches indicate that self-aggregates hydrogen bond at concentrations above 10 M. The identities of cyclic dimer 61 and tetramer 62 are confirmed by electrospray ionization mass spectrometry (ESI). H NMR titrations in CDCI3 give association constants of 39 M and 9.3 x 10 M for 61 and 62, respectively. No electron transfer studies on these systems have yet been undertaken. 

A combination of N CPMAS NMR, N quadrupole double resonance, and x-ray studies of solid 3,5-dimethylpyrazole (19) between 270 K and 350 K has been necessary to discuss the intermolecular proton transfer in a structure like (55) <89JA7304> the experimental evidence (species HHH and DDD) is consistent with a correlated triple hydrogen jump with an activation energy of 11 kcal mol". The cases of 3,5-diphenyl-4-bromopyrazole (a dimer like (54)) and 3,5-diphenyl-pyrazole (a tetramer like (56)) were subsequently studied <92JA9657> here also proton jumps were found with activation energies similar to those found for (19). The dynamic behavior of pyrazole... 

Two related compounds were prepared afterwards <93ccii39> and one of them, a 1 1 complex of the host and pyrazole (7) crystallizes in the form of a tetramer which presents proton transfer in the solid state involving nitrogen and oxygen atoms. The system has been studied by a combination of x-ray crystallography and dynamic high resolution solid state NMR spectroscopy. 

Monopyrazolylporphyrins self-assemble to form dimers and tetramers (1) by hydrogen bonding between the pyrazole units (Fig. 3). The association constant of both processes in chloroform, iCa = 39 and = 9.3 X 10 M , respectively, is, however, low [19]. 

Acylation and metallation studies on monomers, dimers, trimers, and tetramers containing linked thiophen, pyridine, pyrimidine, furan, benzofuran, benzothiophen, and indole moieties have been published/ Deuteriation, halogenation, and diazo-coupling reactions of 2-oxo- and 2-thioxo-l,2-dihy-dropyrimidinium salts have been studied and compared with results for 2,2-dialkyl-1,2-dihydropyrimidinium and 2,3-dihydro-1,4-diazepinium salts in order to demonstrate the effect of an adjacent 0x0- or thioxo-group on the properties of a 1,5-diazopentadienium system/ Vilsmeier formylation of, and tautomerism in, 2-hydroxypyrazolo[5,l-h]quinazolone and l-phenylpyrazolo[5,l-A]-quinazoline-2,9-dione have been studied/ The pyrazolo[3,4-c]pyrazole (71) has been methylated and acetylated, the major products being (72)/ ... 

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