Gas‐Phase Thermolysis of 4‐Substituted 3,5‐Dihydro‐3,3,5,5‐tetramethyl‐4H‐ pyrazoles Studied by Photoelectron Spectroscopy and Flash Vacuum Pyrolysis

Gas‐phase thermolyses of the 3,5‐dihydro‐4 H‐pyrazoles 1 – 4 have been studied by photoelectron spectroscopy and flash vacuum pyrolysis. In the PE‐controlled thermolysis, 1–4 extrude molecular nitrogen, and the fragments cyclize immediately to different three‐membered rings. In the case of 2 and 3, the former 4‐substituent becomes part of the three‐membered ring, whereas 1 and 4 afford short living 0x0‐ and iminocyclopropanes. At higher temperatures and in flash vacuumpyrolysis, acyclic compounds and smaller molecules are formed in a second step. Based on MNDO, AM1, and PM3 calculations, the ionization potentials of 3,5‐dihydro‐3,3,5,5‐tetramethyl‐4‐methylene‐4H‐pyrazole (3) have been assigned to molecular orbitals. To aid the identification of the pyrolysis products, PE spectra of the methylenethiirane 9 and the methylenecyclopropanes 11 and 13 have been measured and interpreted with the aid of SCF calculations.