Pyrazolines, oxidative dimerization

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). 

Triphenyl-zl -pyrazoline electrolyzed in acetonitrile containing tetraethyl-ammonium perchlorate, pyridine added, and electrolysis continued until the current decreases to 1% of its initial strength 4,4 -biS (3,5 diphenylpyrazol-l-yl)biphenyl. Y 84%. F. e. s. F. Pragst, Z. Chem. 14, 236 (1974) oxidative dimerization also with FeClg (s. Synth. Meth. 3, 603) s. J. pr. 316, 195 (1974). 

Dimerization of pyrazolines-2 on the action of oxidants includes the formation and doubling of cation-radicals (Morkovnik and Okhlobystin 1979). The doubling process is usually characterized by the terms head and tail. The term head is applicable to the position bearing a polar fragment, whereas the term tail is adopted for an unsaturated molecular site. The reaction under consideration follows the head-to-head order. This means that the doubling cation-radical has only a single position with the maximal density of an unpaired electron (Scheme 3.11). 

The primarily formed radical-cation dimerizes at C-3 to bis(l-phenyl-A2-pyrazolin-3-yl) if C-3 is unsubstituted.281 By-products include biphenyl derivatives formed by an alternative dimerization of the parent radical-cation. The major product is further oxidized under the reaction conditions to stable cations. By blocking the para position of the phenyl ring in the 1-position, a persistent radical would be expected, and in the presence of a base, e.g., pyridine, the corresponding pyrazole was obtained.284,285 The anodic oxidation of 1,5-diphenyl-3-(4-hydroxycoumarinyl)-A2-pyrazoline in CH3CN-Et4NC104 solution, containing pyridine, resulted in the isolation of l,5-diphenyl-3-(4-hydroxycoumarinyl)pyrazole in 95% yield when the para position was not blocked.290... 

The most interesting direction of the oxidation of 7V-phenyl-substituted pyrazolines 105 is their dimerization to compounds like 107 under the action... 

The process is followed by the appearance of a bright blue, violet or green color of the reaction mixture and can be applied for qualitative analysis of pyrazolines. At the present time, it is proven that this dimerization passes via the preliminary formation of cation-radical particles 106, which are responsible for the deep color of pyrazolines under acidic conditions in the presence of oxidizing agents. 

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. 

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