Zum Mechanismus der Cyclopropan‐“Walk”‐Umlagerung: Synthese und Eigenschaften eines optisch aktiven Diazoalkan‐Addukts von Toluol; eine hochstereoselektive N2‐Eliminierung

Mechanism of the Cyclopropane “Walk” Rearrangement: Synthesis and Properties of an Optically Active Diazoalkane‐Toluene Adduct; a Highly Stereoselective N2 Elimination The synthesis and the thermal and photochemical behavior of the optically active 2‐diazo‐1‐methoxypropane‐toluene adduct 3 are reported. The N2 elimination of 3 ocurs with one and the same stereochemical course upon thermolysis as well as direct photolysis: 7‐(Methoxymethyl)‐1,7‐dimethyl‐1,3,5‐cycloheptatriene (1) is formed with retention of configuration at C‐7 and 7‐(methoxymethyl)‐2,7‐dimethyl‐1,3,5‐cycloheptatriene (2) with inversion of configuration at C‐7. These stereochemical findings are explained by an intermediary singlet diradical 1[4] in which the ring closure between C‐1 and C‐7 or C‐5 and C‐7 is faster than the rotation around the single bond between C‐6 and C‐7. In the case of the thermally induced formation of 2 a concerted N2 elimination has to be considered as a competitive pathway. The results presented here are well in accord with the stereochemical analysis of the cyclopropane “walk” rearrangement 1⇋2 reported previously and are therefore a good evidence that the diradical intermediate 4 is also involved in the “walk” rearrangement. In the photochemically induced N2 elimination of 3, sensitized by benzophenone and leading again to 1 and 2, a surprisingly high degree of stereoselectivity is observed. Thus, the intermediary triplet diradical 3[4] must have a much shorter life time than ordinary 1,3‐diradicals in the triplet state.