Zum Mechanismus der Cyclopropan‐„Walk”︁‐Umlagerung: Synthese und Eigenschaften von 2‐Diazopropan‐Aren‐Addukten; eine regiospezifische N 2 ‐Eliminierung

Mechanism of the Cyclopropane “Walk” Rearrangement: Syntheses and Properties of 2‐Diazopropane ‐ Arene Adducts; a Regiospecific N 2 Elimination The synthesis and the thermal and photochemical behavior of 2‐diazopropane adducts of toluene 1a , m ‐xylene 1b,c , and Dewar benzene 2a,b are reported. At 60°C the thermal N 2 elimination of the specifically deuterated compound 1c proceeds regiospecifically leading to 1,7,7‐trimethyl‐5‐(trideuteriomethyl)‐1,3,5‐cycloheptatriene ( 12c ), whereas the photolysis of 1c sensitized by benzophenone yields both regioisomeric cycloheptatrienes 12c and 13c in a 51:49 ratio. This finding excludes the symmetrical diradical 3c as an intermediate in the thermal N 2 elimination of 1c and suggests a novel pericyclic process for it. The formation of both regioisomeric cycloheptatrienes 25b and 25c in a 74:26 ratio in the thermolysis of the diazopropane‐tetradeuterio Dewar benzene adduct 2b indicates that in this case a multi‐step N 2 elimination passing through the diradical intermediate 3e competes with the concerted process. According to force‐field calculations the course of the N 2 elimination in 1a ‐ e might be controlled by the conformational equilibrium A ⇆ B