Pyrazoline, from pyrazole

The important synthesis of pyrazoles and pyrazolines from aldazines and ketazines belongs to this subsection. Formic acid has often been used to carry out the cyclization (66AHQ6)347) and N-formyl-A -pyrazolines are obtained. The proposed mechanism (70BSF4119) involves the electrocyclic ring closure of the intermediate (587) to the pyrazoline (588 R = H) which subsequently partially isomerizes to the more stable trans isomer (589 R = H) (Section 4.04.2.2.2(vi)). Both isomers are formylated in the final step (R = CHO). 

Hydrazoyl halides are useful reagents for the synthesis of pyrazolines and pyrazoles (80JHC833). The elimination of HX, usually with triethylamine, is now the preferred method for the generation of the nitrilimine (621) in situ. Although in some cases it is not clear if the mechanism involves a nitrilimine (621) (as for example in the Fusco method in which sodium salts of /3-diketones are used), in other reactions it is the most reasonable possibility. For example, the synthesis of pyrazolobenzoxazine (633) from the hydrazoyl halide (631) probably occurs via the nitrilimine (632). Trifluoromethylpyrazoles (634) have been prepared by the reaction of a hydrazoyl halide and an alkynic compound in the presence of triethylamine (82H(19)179). 

Pyrazoline compounds are partially unsaturated pyrazoles. Jeong et al. [95, 96] and Moustafa and Ahmad [92] described the formation of these compounds from chalcones (e.g., 196) using hydrazine hydrate to form the pyra-zolines (e.g., 197, Scheme 53 [95]). Chimenti et al. also described the synthesis of the pyrazolines from reaction of hydrazine with chalcones but included acetic acid in the reaction mixture [97]. 

Oxidation of chalcone phenylhydrazone 13 leads to a pyrazole and the expelled proton catalyses formation of a pyrazoline from the chalcone phenyl-hydrazone [43]. The latter undergoes further anodic oxidation (p. 308). In the presence of pyridine as a proton acceptor, the pyrazole becomes the major product. A further example of oxidative cyclization is the conversion of a-oximino phenylhydrazones to 1,2,3-triazole-l-oxides 14 [44]. 

Nitrile imine cycloadditions provide access to pyrazolines and pyrazoles. Intramolecular cyclizations to alkynes were first reported in 1974.88 Perhaps the most useful method for generation of nitrile imines involves 1,3-elimination of hydrogen chloride from an a-chlorohydrazone. Tetrazoles and sydnones are also precursors to nitrile imines. 

Section 2.1 is devoted to the aromatic pyrazole 1-oxides. Further structures derived from pyrazole and embedding an N-O bond are the nonaromatic 1,2-dioxides also called diazapentalene 4,4 -dioxides 75-78, the pyrazoline 1-oxides also called diazapentalene 4-oxides 79-84, and the 2-substituted pyrazoline 1-oxides represented in Scheme 22 by the parent structure 85. Structures 77, 78, 79, 81, and 82 are known and are discussed in Section 2.2 but since the present review deals with aromatic /V-oxides the nonaromatic N-oxides will only be discussed when their chemistry relates them to the aromatic pyrazole N-oxides. 

Deactivation processes competing with fluorescence are mainly nonradiative deactivation to the S0 state (IC) and nonradiative transition to a triplet state (intersystem crossing, ISC). Photochemical products are often formed from this triplet state. Important photochemical reactions are the E—yZ isomerization of ethylene, the oxidation of pyrazoline to pyrazole, and the dimerization of cou-marins. 

In a new approach 3-aminopyridazines are formed from acetylenic hydroxynitriles that add hydrazine and subsequently cyclize [Eq. (19)]. The reaction may also form pyrazolines or pyrazoles, depending on the starting nitrile. 

In the historical introduction to this volume (Sect. 1.1), it was mentioned that Buchner studied reactions of ethyl diazoacetate with ethenedicarboxylic acid in 1888. In the following year, he discovered pyrazole, obtained from methyl diazoacetate and dimethyl acetylenedicarboxylate followed by thermolysis. Finally, in 1893 Buchner et al. synthesized 4,5-dihydro-l//-pyrazole (2-pyrazoline) from methyl diazoacetate and methyl acrylate. Von Pechmann, the discoverer of diazomethane, performed analogous reactions of fumarates and maleates (von Pechmann, 1894, von Pechmann and Burkard, 1890). ... 

Results of a theoretical study of the mechanism of Br2 oxidation of IH-pyrazolines to pyrazoles suggest that a bromo substituted 2- or 1-pyrazoline is involved in the reaction. Br2 and SbCls oxidize bis(4-t-butylphenyl)nitroxyl (A) to the corresponding oxoammonium salts. The tribromide salt on heating is converted into a mixture of bromodiphenylamines which is also formed from A as a result of three-electron reductive bromination. ... 

Dipolar cycloaddition with ring opening Pyrazoles and Zl -pyrazolines from sydnones... 

Further evidence showed this mechanism to be incorrect, especially the fact that it was methyl cinnamate and not (347) which was isolated from the reaction (73CPB2026). Also 1-phenylpyrazoles did not react with DMAD under the reaction conditions (74BSF2547). The origin of (346) remains obscure, but in no circumstances does it imply a Diels-Alder reaction of a pyrazole. For Ogura et al., it has its origin in an intermediate A -pyrazoline (73CPB2026). 

---