Reactions and Reverse Cycloadditions

Cheletropk Reactions and Reverse Cycloadditions.—The cheletropic addition of unsaturated carbenes, which were generated by Bu OK treatment of the appropriate vinyl triflate (e.g. Me2C=CHOTf) at low temperature, to dialkylacetylenes affords 

Bieniek, and F. Korte, Angew. Chem. Internal. Edn., 1975,14,181. 

Toyoda, A. Iwama, Y. Sakata, and S. Misumi, Tetrahedron Letters, 1975, 3203. 

Pyrolysis of 5,6-dimethylenebicyclo[2,2,l]hept-2-ene (251) yields indane, whereas the similar treatment of the analogous compounds (252)—(254) resulted in Diels-Alder retrogression to afford 1,2,3-butatriene. When a solution of the endo-epoxide (255) was injected directly into the inlet of a gas-liquid chromatograph, it was found that biacetyl and hexamethylcyclohexa-2,4-dienone could be detected as components of the eluted mixture. The presence of the dienone can be explained by assuming Diels-Alder retrogression, and the biacetyl could arise from intermediate dimethyl-oxiren (256) formed concomitantly. Since, however, neither dimethylketene nor its 

The cheletropic reactions of dihalogenocarbenes with norbomadienes have been the subject of further investigations. The main reactions involved are 1,2-addition, which can occur from the endo and exo directions, and homo-1,4-addition. For reaction with norbomadiene itself it has been found that the exo endo selectivity ratio increases som hat in the series difluoro-, fluorochloro-, dichloro-, and dibromo-carbene (respectively 1.2,1.8,4.0, and 2.0), whereas the selectivity ratio for the 1,2 1,4 

The application of thermolytic reactions (e.g. Diels-Alder and reverse Diels-Alder processes) for the synthesis of fluoro-organic compounds has been reviewed. The cycloaddition of the meso-ionic dithiolone or thiazolone compounds (209 X = S or NPh) with the Diels-Alder adduct of cyclo-octatetraene and dimethyl acetylenedicarboxylate occurs at the cyclobutene ring to give the exo-adducts (210). Pyrolysis of compounds (210) yields dimethyl phthalate and 7,9-diheteratricyclo[4,2, 1,0 ]non-3-en-8-ones (211 X = S or NPh) by retro-[4-t- 2] addition. The similar additions of (209) and related compounds to dimethyl 7-oxabicyclo[2,2,l]-hepta-2,5-diene-2,3-dicarboxylate afforded [4 -1-2] adducts which on pyrolysis suffered double thermal fragmentation e.g. loss of XCO and 3,4-dimethoxycarbonyl-furan) to afford five-membered aromatic heterocyclics.  

The activation entropy values are found to be zero or positive for the extrusion of nitrogen from the azo-compound (212), to give cycloheptatriene, in various solvents. The rate of nitrogen loss decreases with increasing solvent polarity and internal pressure of the solvent an inverse solvent dependence, of the same order of magnitude, is evident from previous studies on the bimolecular reverse (Diels-Alder) reaction. These and other considerations lend support to the continuity of mechanisms in these cases, and no distinction between concerted or radical pathways can be made from the sign of Another remarkable example has appeared that further 

Kinetic parameters for the gas-phase cyclo-reversion ( 2 +J1 + 2) processes of cis-tris-a-homobenzene (221), diademane (222), and 1,6-homodiademane (223) 

Stable o-xylylene derivatives have been prepared by the electrocyclic ring-opening of polycyclic benzocyclobutenes under photochemical control. Irradiation of 

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