A Stepwise [4 + 3] Cycloaddition Reaction of the 1,3‐Diphenyl‐2‐azaallyl Anion

The 1,3‐diphenyl‐2‐azaallyl anion ( 1 ) undergoes [3 + 2] cycloaddition reactions with the s‐cis ‐fixed 1,3‐dienes 8–11 . In contrast, 1,1,2,2,3,3‐hexamethyl‐4,5‐bis(methylene)cyclopentane ( 7 ) reacts with 1 to give the [4 + 3] cycloadduct 13 and the linear 1,4‐addition product 14 . This reaction is four orders of magnitude slower than the corresponding reaction of 1 with 1,2‐bis(methylene)cyclopentane ( 8 ), which exclusively yields [3 + 2] cycloadducts. A change of mechanism – concerted cycloaddition of 8 and stepwise cycloaddition of 7 – is suggested, but not unequivocally proven. It is concluded that reactions of 1 with ordinary dienes cannot profit from concertedness by more than 5 kcal · mol –1 .