Pyrazoles extrusion reactions

In this case, the 3H-pyrazole is stable to heat, and the reaction only proceeds on irradiation with light. Light may be used to initiate an extrusion reaction in which the carbonyl group is lost from a chain. Suggest the route for the conversion of R-CO-R to R-R and C=0. 

A novel one-step synthetic route for the construction of fully substituted pyrazol-4-ols by the condensation-fragmentation-cyclization-extrusion reactions of thietanone 62 with... 

Tandem sequences have also yielded some interesting pyrazole structures. Four-component coupling of terminal alkynes 37, hydrazines 38, carbon monoxide and aryl iodides furnished pyrazoles 39 in the presence of palladium catalyst <05OL4487>. Fully substituted 1/f-pyrazoles 42 were prepared from the condensation/fragmentation/cyclization/extrusion reactions of thietanone 40 with 1,2,4,5-tetrazines 41 <05JOC8468>. Reactions of isocyanides 43 and dialkyl acetylenedicarboxylates 44 in the presence of 1,2-diacylhydrazines 45 led to highly-functionalized pyrazolines 46 <05TL6545>. 

A simple, yet non-obvious method for the construction of pyrazol-4-ols 52 by a consecutive series of condensation-fragmentation-cyclization extrusion reactions of thietanone 50 with 1,2,4,5-tetrazines 51 has been described <05JOC8468>. 

Calculations have indicated that direct thermal extrusion of nitrogen from 3//-pyrazole to give cyclopropene is an unfavorable process, though more favorable in terms of heat of reaction than cycloreversion to diazomethane... 

The extrusion of SO from some 1,2,6-thiadiazine 1-oxides, such as 198, to give the corresponding pyrazole can be achieved by heating in toluene [79CC891 81JCS(P1)1891 83JCS(P 1)2273]. The reaction of bromine/ dichloromethane with 3-oxo l,2,5-thiadiazine212(R = Me, R2 = Me) in... 

One approach to the introduction of the trifluoromethylsulfinyl group in fipronil involves the reaction of 5-amino-3-cyano-1-(2,6-dichloro-4-trifluoromethylphe-nyl)pyrazole and trifluoromethylsulfinyl chloride in toluene (Fig. 8) [43], Exposure of fipronil to sunlight results in the extrusion of sulfur oxide, to give a 4-trifluoromethylpyrazole photoproduct derivative [44],... 

The generatiphotolysis resulted in nitrogen elimination. The formation of 4-(3-butenyl)-l,2-pyrazole (214) occurred via rDA reaction of adduct (213) (equation 93). Apparently the rigidity of the tricyclic azoalkane (213) prevented nitrogen extrusion under the usual thermolysis OHiditions. Again, photolysis resulted in nitrogen elimination rather than cycloreversion. 

The nitrogen extrusion from 1-pyrazolines and 3H-pyrazoles giving cyclopropanes and cyclopropenes, respectively, has been extensively reviewed The cyclopropane synthesis from 1-pyrazolines can be executed thermally as well as photochemically, but the latter method generally gives substantially better results than the former. The major side reaction observed in the thermal process is the production of olefins, which arise in the migration of a substituent from the C(4) to C(3) position. A retro-1,3-dipolar addition producing a diazoalkane and an olefin has been observed in certain cases. The decomposition of 3-acyl- or 3-alkoxycarbonyl-1-pyrazolines is catalyzed by acids, such as perchloric acid and boron trifluoride and by Ce The stereochemical course... 

Pyrazolines can also be prepared by the oxidation of pyrazolidines. 2-Pyrazolines, which are readily obtained in the reaction of a,j -unsaturated ketones with hydrazine, also undergo nitrogen extrusion at elevated temperature usually in the presence of a basic catalyst. The reaction is believed to proceed via 1-pyrazolines. Treatment of 3,3,5-trialkyl-2-pyrazolines with lead tetraacetate followed by thermolysis affords cyclopropyl acetates ". Oxidation of certain 2-pyrazolines with manganese dioxide gives 3H-pyrazoles, which in turn produce cyclopropenes in the photolysis (equation 7). ... 

The cycloaddition of a diazo compound to an alkyne followed by extrusion of nitrogen under various conditions from an isolated pyrazole represents one of the standard routes to cyclopropenes (Section 3.2.1.1.). In certain cases the reaction is carried out under conditions which might involve direct addition of a carbene derived from the diazo species, e.g. formation of 7, 8, and9. For8(R = cyclopropyl), the product rearranged under the reaction conditions to 1-cyclopropyl-1-trimethylsilylallene. ... 

Pyrazoles (87) are also formed, by extrusion of SO2, on submitting 2-alkyl-3,5-dimethyl-l,2,6-thiadiazine 1,1-dioxides (86 R = Me or PhCHz) to flash vacuum pyrolysis at 700 °C (Equation (2)) <93CJC4io>. With 2-unsubstituted thiadiazine 1,1-dioxides (86 R = H) no pyrazoles are formed, whereas with the 2-cyclohexyl derivative (86 R = cyclohexyl) the reaction is more complex and yields, in addition to starting material, pyrazole (87 R = cyclohexyl 50%), the decycloalkylated thiadiazine 1,1-dioxide (86 R = H 35%), and cyclohexene. The last two products most probably arise by a retro-ene reaction. 

Many enone systems readily undergo 1,3-dipolar addition reactions. This applies to steroids, as illustrated by the formation of the fused pyrazole 16-4 from reaction of the enone 16-2 with diazomethane. Pyrolysis of that pyrazole results in extrusion of nitrogen to leave behind a methyl group at position 1 (16-5). 

The reaction system (6-37) includes the thermal azo-extrusion of a cyclic azo compound to a cyclopropane derivative and the direct formation of cyclopropanes, catalyzed by metal complexes. Synthetic routes to cyclopropane derivatives became an important subject in the last two decades, and one frequently used method is the 1,3-dipolar cycloaddition of a diazoalkane to an alkene followed by thermal or photolytic azo-extrusion of the 4,5-dihydro-3//-pyrazole formed to the cyclopropane derivative (6-37 A). This route can be followed in many cases without isolation, or even without direct observation, of the 4,5-dihydro-3//-pyrazole. Therefore, it is formally very similar to cyclopropane formation from alkenes with diazoalkanes, in which a carbene is first formed by azo-extrusion of the diazoalkane (see Sect. 8.3). As shown in pathway (6-37 B), this step can be catalyzed by copper, palladium, or rhodium complexes (see Sects. 8.2, 8.7, and 8.8). There are cases where it is not clearly known whether route A or B is followed. Scheme 6-37 also includes... 

The nature of the cyclopropane-forming reaction is still controversial in the sense of a differentiation between a two-step cleavage of the two CN bonds and a concerted formation of a biradical in the azo-extrusion part of the reaction (for a review of the older literature, see Mackenzie, 1975, p. 354, and Engel, 1980, p. 118). This reaction does not strictly belong to the scope of this book. Therefore, we will not discuss it further except to refer to an investigation of Reedich and Sheridan (1988) who reported that, in contrast to earlier results (see, e. g., Cichra et al., 1980, and other references there), both thermal and photochemical processes take place by stepwise cleavage in a pair of isomeric dihydro-pyrazoles. 

Very active interest in a new addition reaction of aliphatic diazo compounds started in 1991 when WudPs group reported that diphenyldiazomethane forms diphenylmethanofullerene with buckminsterfullerene (C o Suzuki et al., 1991). Although this investigation showed that the reaction proceeds via the formation of a dihydro-pyrazole, i.e., in the mode of a 1,3-dipolar cycloaddition followed by an azo-extrusion, we shall discuss the syntheses of methanofullerenes in its entirety in the chapter on carbenes (Sect. 8.4) because Diederich s recent work (see review of Diederich et al., 1994b) shows that the methano bridge can also be obtained from a carbene. The question whether the dihydro-pyrazoles are intermediates or side-equilibrium products (see earlier in this section) is also open for the reaction of with diazoalkanes. 

The desulfuration of 6-unsubstituted-3-methyl-2,3-dihydro-677-l,3,4-thiadiazines 61 (R = Me, Pr ) in boiling glacial acetic acid affords pyrazoles 62 <2003SL2392>. In contrast to the rapid sulfur extrusion of 6-phenyl- or 6-ethoxycarbonyl-6/7-l,3,4-thiadiazines 59 (R = Ph, C02Et) to pyrazoles, a much longer reaction time (40h) was required to achieve a complete desulfuration of 61 (Equation 5). The treatment of 4/7-1,3,4-thiadiazine 63 with concentrated hydrochloric acid or hydrobromic acid (48%) afforded the 5-imino-l,2-dimethylpyrazoles 64 in 40% and 30% yield, respectively (Equation 6). The yield was increased to 53% by the use of glacial acetic acid. The sulfur extrusion proceeded very rapidly and precipitation of considerable amounts of sulfur was observed even after stirring for only 5 min. 

Benzofuran 332 reacts with 1-phenylthiosemicarbazide to give the benzofuran derivative 333, which undergoes ring cleavage to the intermediate 334 and subsequent cyclization to the 1,3,4-thiadiazine 335 <2005H(65)1569>. The reactions of benzofuran 332 with 4,4-dialkylthiosemicarbazides proceed with extrusion of a sulfur atom from the 1,3,4-thiadiazine intermediate and the formation of pyrazole derivatives (Scheme 46) <2005H(65)1569>. 

Palladium-catalyzed cyclization reactions with aryl bromides have been used to synthesize pyrazole derivatives. lV-Aryl-Ar -(o-bromobenzyl)hydrazines 20 or [A-Aryl-N -(o-bromobenzyl)hydrazinato-N ]-triphenylphosphonium bromides 21 participated in palladium-catalyzed intramolecular amination reactions to give l-aryl-l//-indazoles 22 <01TL2937>. Thermal extrusion of sulfur dioxide or carbon dioxide from their respective heterocyclic precursors 23 or 24 generated 1,2-diaza-l, 3-butadicnes 25, which underwent palladium(0)-catalyzed carbonylation to yield 2,3-pyrazol-l(5//)-ones 26 <01OL3651> or [4 + 2) Diels-Alder reactions with Af-phenyldiazamaleimide to afford cycloadducts 27 <01OL3647>. 

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