Thermolysis 1-pyrazoline

Finally, A -pyrazolines lose N2 (73KGS64) but it is possible that this takes place thermally before ionization, since thermolysis of A -pyrazolines usually gives cyclopropanes (Section 4.04.2.2.2(iv)). 

Thermolysis of the pyrazoline 1037 in chlorobenzene gave (76TL2513) triazinotetrazole 1038, which on further heating decomposed to 1040 and 1039. The reaction occurred by sequential ring expansion and contraction (Scheme 195). 

Alkenyl-substituted diarylnitrilimines 325, generated by photolysis or thermolysis of corresponding tetrazoles 324, undergo a regioselective INIC reaction to yield fused 2-pyrazolines (Scheme 34) [93]. Similarly, with alkynyl derivatives... 

Thermolysis of 6-substituted l,5-diazabicyclo[3.1.0]hexanes 326, easily available from 325, leads to a diaziridine ring opening and to the intermediate formation of labile azomethine imines 327. These compounds can be stabilized by a proton shift to form 1-substituted 2-pyrazolines 328. However, when the thermolysis is carried out in the presence of a 1,3-dipolarophile, the corresponding products of dipolar cycloaddition can be obtained. For example, iV-arylmaleimides provide mixtures of the major trans- and minor air-products 329 and 330, respectively (Scheme 47) C1999RJO110, 2001RJ0841, 2003RJ01338, 2004RJ067>. 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R = Ac, Bz no reaction with R = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRs was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-aIkenylidene-l,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRa = SiPhaf-Bu, a formal intramolecular [3 + 2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

The photolysis or thermolysis of certain 1,3-bis(diazomethyl)trisilanes furnished products that are derived from bicyclic pyrazolines formed by intramolecular 1,3-dipolar cycloaddition at the Si= bond of a diazosilene intermediate (343,344). 

From the correspondingly substituted A -pyrazolines (18), 1-alkoxy- and l-acetoxy-l,2,2,3,3-pentasubstituted cyclopropanes were obtained in good yields by thermolysis in cyclohexane solution under nitrogen using a high-pressure vessel (91JHC1773). 

Diazomethane and its simple analogs undergo cycloaddition to unsaturated compounds both directly and after conversion to carbenes. The direct cycloadditions are 1,3-dipolar for the most part and provide access to pyrazolines and pyrazoles. Intramolecular cyclizations were recognized as early as 1965 95 The two main methods used in generation of diazo compounds for subsequent intramolecular cycloaddition include thermolysis of tosylhydrazone salts and thermolysis of iminoaziridines. Decomposition of nitros-amines has also been employed. 

Oxa-l -silabicyclo[ . 1,0 alkanes (n = 3 111 n = 4 113) were the only products isolated from the photochemical, thermal or transition-metal catalyzed decomposition of (alkenyloxysilyl)diazoacetates 110 and 112, respectively (equation 28)62. The results indicate that intramolecular cyclopropanation is possible via both a carbene and a carbenoid pathway. The efficiency of this transformation depends on the particular system and on the mode of decomposition, but the copper triflate catalyzed reaction is always more efficient than the photochemical route. For the thermally induced cyclopropanation 112 —> 113, a two-step noncarbene pathway at the high reaction temperature appears as an alternative, namely intramolecular cycloaddition of the diazo dipole to the olefinic bond followed by extrusion of N2 from the pyrazoline intermediate. A direct hint to this reaction mode is the formation of 3-methoxycarbonyl-4-methyl-l-oxa-2-sila-3-cyclopentenes instead of cyclopropanes 111 in the thermolysis of 110. 

Access to alkyl substituted derivatives of the homotropylidene complexes is provided via the / -tropone iron complex by reaction with diazoalkanes, followed by mild thermolysis of the 3+2 pyrazoline adduct to give the corresponding homotropone complexes (equation 149)217,218 jYiQ 8,8-dimethyl derivative was used as starting material for the preparation of the fluxional ( -2,8,8-trimethylbicyclo[5.1.0]octa-2,4-dienylium)Fe(CO)3 cation com-plex More recently (homotropone)Fe(CO)3 was used for the synthesis of unique chiral 1,2-homoheptafulvene iron complexes . ... 

Thermolysis of enantiopure sulfonyl pyrazolines 241 afforded sulfonyl cyclopropanes 242 in a completely stereoselective manner (Scheme 17) <2004OL4945>. Both cyclopropanes 243 and alkylidenecyclopropanes 244, containing one or two chiral carbon atoms, one of them being quaternary, were obtained by hydrogenolysis of the G-S bond and under Julia conditions, respectively. Thermal dediazotization of 3-(l-fluorovinyl)-3-methylpyrazoline 245 at 250°G afforded l-(l-fluorovinyl)-l-methylcyclopropane 246, which was capable of isomerizing further, at 600°G, to... 

Subsequent to these developments Mishra and Crawford , in a further attempt to elucidate the decomposition mechanism and nature of intermediates, have investigated the stereometric distribution of products in the thermolysis of (3R 5R)-(+)-rranj-3,5-dimethyl-l-pyrazoline. If the reaction proceeds as postulated by Crawford et al., the tra/w-l,2-dimethylcyclopropane product would result from the disrotatory closure of the planar biradical and should be racemic, while the above two alternatives, step-wise elimination of nitrogen or the involvement of pyramidal diradicals by inversion at both asymetric centres would be expected to yield optically active tranj-1,2-dimethylcyclopropane. 

The 1,3-diaryl 1-pyrazolines are much more labile than the alkyl derivatives. They undergo thermolysis with activation energies ranging from 12 to 18 kcal.mole" (c/. Table 8). The trans isomers yield predominantly trans-cyc o-propanes while the m-3,5-di-p-methoxyphenyl-l-pyrazoline gives only slightly higher yield of the trans- than the c/s-cyclopropane. 

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