Transition state pyrazolines

We claim, however, that this reaction is likely to be more complex. The isolated intermediate salt may be the prototropic isomer 4.23 formed intermolecularly from 4.21, which is the primary steady-state intermediate. Compound 4.23 is energetically more favorable because in 4.23 — in contrast to 4.21 — conjugation (Ti-orbital overlap) between the arylcarbonyl part and the 4-toluenesulfonyl azide part is not interrupted by an sp C-atom. Intermediate 4.21 may, however, also react directly to give the diazoketone 4.22 via a cyclic transition state 4.24 that contains, however, a less favorable (Z)-azo group. The prototropy 4.21 4.23 was included at an early date for the mechanism of the diazo transfer from 4-toluenesulfonyl azide to the cyclopentadienyl anion by Roberts (see review Roberts, 1990, p. 217) and by Huisgen (1990). A transition state similar to 4.24 was mentioned by Balli et al. (1974) for the diazo transfer of azidinium salts to pyrazolin-5-one and 5-aminopyrazole compounds (see below). 

The problem of bending diazomethane for the cycloaddition transition state was basically answered by Leroy and Sana s investigations (1975, 1976a). Using ab initio SCF calculations at the STO-3G level, they evaluated some 150 points on the hypersurface of diazomethane, ethene, and the primary cyclization product 1-pyrazoline. The essential results are shown in Scheme 6-12. There is no second transition state. 

Nitrile oxides add in a 1,3-dipolar fashion to the double bond of cyclopropenes and diazomethane adds to give a pyrazoline (729) in good yield. Phenyl azide shows a very fast rate of reaction with 3,3-dimethylcyclopropene compared with additions to other olefins, evidently due to the influence of strain release in lowering the transition state of reaction, but the initial adduct is not stable and undergoes a reverse... 

The Curtius reaction of l-phenyl-3-azidocarbonyl-2-pyrazolin-5-one (320) in glacial acetic acid does not lead to the expected product (321), giving instead 2-acetyl-3-amino-l-phenyl-3-pyrazolin-5-one (322). A six-membered transition state involving the intermediate mixed anhydride (323) is proposed. 

Thermal extrusion of dinitrogen from the adduct of benzene with diazomethane, and from the pyrazoline adducts of fullerenes with diazoalkanes (to give 6-5-open methanofullerenes), are believed to feature a Mobius aromatic transition state with a highly anisotropic magnetic susceptibility. ... 

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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