The Energy-Well of Diradicals, IV[ll. -2-Methylene-1,4-cyclohexadiyl*The energy surface of the title compound 2 is derived from the kinetics of its precursors 5, 6, 7 , and 8, the oxygen dependance of its trapping rate, and heat of hydrogenation measurements of 5, 6, 7, and 15. These data lead to a heat of formation for the diradical 2 of 69.8 kcal/mol and an energy well of 8.2 kcal mol-'. From the Curie plot of the ESR spectrum and oxygen trapping experiments a singlet-triplet splitting of 1.4 kcal mol-' is indicated, with the triplet being the ground state.

The recombination of free radicals is a collision-controlled process requiring very little activation enthalpy."] In the analogous intramolecular reactions the situation is far less clear, and older concepts of substantial barriersL2] have been ~ontradicted.'~] Systematic investigations of resonance stabilized diradicals have revealed a marked dependence of the recombination barrier on the structure (see Scheme I), which d c j " -9 Scheme 1. Energy wells of diradicals in kcal mol-' 121.has been explained in terms of the conformational stability of the diradical~.~~' Determination of the recombination threshold of the title compound 5 (Scheme 2) enables a critical test of this hypothesis.In the systems outlined in Scheme 1 interaction of the unpaired electrons requires a considerable rotation of the respective orbitals, which is possible only with more or less great sacrifice of resonance stabilization. In the case of 4-methylene-I ,3-cyclopentanediyl5 the situation is completely different. Here, recombination leads to a three-membered ring, whose Walsh orbitals still allow an interaction with the Land Nordrhein-Westfalen.[**I This work was supported by the Fonds der Chemischen Industrie and the exocyclic double bond even in the transition state of the reaction. The recombination enthalpy should therefore be distinctly smaller than in the homologous 2-methylene-I ,4-cyclohexanediyl (see Scheme 1). 5 is accessible thermally starting from 1,[5361 2['] and 4 or 6,I8] and, in the case of 1 also photochemically. According to ESR investigations by DowdL5' 5 has a triplet ground state. Our interest turned to the equilibrium 4 e 5 ~6 , which we have tried to detect by the mutual rearrangement 4 b e 6 b . The substrates were synthesized analogously to that of the parent compound 18a1 by addition of diazoethane to methylenecyclobutene and photolysis of the resulting pyrazolines (Scheme 3). The structure of the gas-chromatographically separated isomers is confirmed by the spectroscopic data. ''] The mutual rearrangement 4 b e 6 b was carried out in pentane at six temperatures between 20 and 60 "C. The resulting rate constants of the first order reactions (

Dieses Ergebnis belegt eindrucksvoll, daB der Ligand 4 b sowohl die P-H-als auch die P'-H-Positionen von 5 a in gleichem Mane fur eine Eliminierung aktiviert. Durch Kontrollversuche wurde sichergestellt, dab die isomeren Carbonsaureamide 7 und 8 als Folge unterschiedlicher P-H-Abstraktionen entstehen und nicht Produkte einer nachtraglichen Isomerisierung sind. So werden weder 7 noch 8 unter dem Einflun von (Lig)Nio oder 2 N H,SO, wechselseitig ineinander uberfuhrt.Die stochiometrische Umsetzung 1aBt sich unter bestimmten Bedingungen['I auch in eine katalytische umwandeln. Bei einer Gesamtcyclenzahl von vier bleibt beim Liganden 4b das Produktverhaltnis 7:8 rnit 1 : 1 konstant. Durch gezielte Veranderung der Ligandeigenschaften war es moglich, eine Steuerung der Produktbildung zu erreichen. So induziert das sterisch weniger anspruchsvolle Triisopropylphosphit 4c die P-H-Eliminierung am Metallacyclus, und es resultiert eine Isomerenverteilung 7:s von 95 : 5. Hingegen aktiviert der sterisch anspruchsvollere Ligand Tri-ortho-phenylphenylphosphit 4d die v-Position. Es stellt sich die umgekehrte Produktverteilung 7:s (14: 86) ein (Schema 2).

The Energy-Well of Diradicals.Part 4. 2-Methylene-1,4cyclohexadiyl -(thermochemistry of the title compound; determination of its energy-well by oxygen trapping reaction).-(ROTH, W. R.; WOLLWEBER, D.; OFFERHAUS, R.; REKOWSKI, V.; LENNARTZ, H.-W.; SUSTMANN, R.; MUELLER, W.; Chem.

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