Pyrazoline dimerization

Cl6H2 NftOg, 3-Carbomethoxy-5-methyl-5-formyl-A2-pyrazoline dimer, 41B, 348... 

Pyrazolones show a great variety of reactions with carbonyl compounds (B-76MI40402). For instance, antipyrine is 4-hydroxymethylated by formaldehyde and it also undergoes the Mannich reaction. Tautomerizable 2-pyrazolin-5-ones react with aldehydes to yield compound (324) and with acetone to form 4-isopropylidene derivatives or dimers (Scheme 8 Section 4.02.1.4.10). 

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). 

Cycloadditions are in general an effective way of constructing cyclobutane rings. A wide variety of heterocyclic systems dimerize in this way. 1,3-Diacetylindole, for example, affords the head-to-tail dimer 242 on irradiation in ethanol.185 Ethyl 2-ethoxy-l,2-dihydroquinoline-l-carboxy-late is similarly converted in diethyl ether into the trans head-to-head dimer.186 Notable among many analogous photodimerizations are those reported in 1,4-dihydropyridines,187 in furo[3,2-b]pyridin-2(4//)-ones,188 in 8-methyl-s-triazolo[4,3-a]pyridine,189 and in 2H-2-benzazepine-1,3-diones.190 The [ 2 + 2] dimerization of amidopyrine is the first reported example of a photocycloaddition in a 4-pyrazolin-3-one.191... 

Photoelimination of nitrogen from 1-pyrazolines has also been employed in the synthesis of tricyclo[3.2.1.02,4]oct-6-ene,338 prismane,339 quadri-cyclane,340 snoutene ,341 and marasmic acid.342 The trimethylenemethanes 414 have been prepared by photolysis of azoalkanes 415 and characterized spectroscopically.343 Dimerization and cycloaddition to alkenes of these biradicals have been reported.344... 

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. 

Isoxazolin-5-ones and pyrazolin-5 ones have been shown to give the anhydro dimers (140a and b, respectively), when treated with a base such as piperidine.191,259... 

Trimethylenemethane biradicals have been proposed as intermediates in the photodecomposition of fluorine-substituted 4-methylenepyrazolines both in the gas phase and in solution.Evidence for the intermediacy of two trimethylenemethane biradicals on direct irradiation of the bi-(l-pyrazolin-4-ylidene) (29) has also been reported. Benzophenone-sensitized irradiation of the same pyrazoline, however, takes a different course and affords the allene dimer (30), presumably via the tetramethylene-ethane biradical (31). 

Dimers of 3-alkoxycarbonyl-5-formyl-5-methyl-A -pyrazoline 9-Oxo-2a,3,5,6,7,8,8a,9-octahydroindolizino[ 1, 2 5,4]pyrazolo- 75AX(B)548, 76AX(B)2216... 

Acyl-3-pyrazolin-5-ones dimerize by an aldol condensation.901... 

This presumably goes by dimerization of the malononitrile to give a substituted /3-amino-crotononitrile. This dimer also reacts with hydrazines to give 5-imino-2-pyrazolin-4-carbonitriles. These can be hydrolyzed to amides. 

Diazo compound 2 is obtained by thermal ring opening of the bicyclic pyrazoline 1." Catalytic decomposition of 2 with copper(I) iodide in refluxing benzene yields the bicyclo[1.1.0]butane 3. The same result is achieved with bis(acetonitrile)palladium(II) chloride as catalyst, whereas rhodium(II) acetate dimer and copper(I) cyanide are unreactive. 

The copper(I) trifluorosulfonate catalyzed photoreaction of cycloheptene (38a) leads to the dimer 95 (114c). Formation of the latter was taken as support for the generation of frans-cycloheptene (39a) as an intermediate. However, isomerization of free (ran,s-cycloheptene (39a) seems to be faster than its cyclodimerization. 7 ran.s-cycloheptene (39a), obtained by photoreaction with methyl benzoate as sensitizer, isomerizes to ds-cycloheptene (38a) at 0°C (114b) but reacts at low temperature with acidified methanol to form 96 or with diazomethane to form 97 (114a). The stereochemistry of the pyrazoline 97 is trans, in accord with the stereospecific addition to trans-cycloheptene (39a). 

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