Mayr, Herbert; Heigl, Ulrich W.; Baran, Janusz
A Stepwise [4 + 3] Cycloaddition Reaction of the 1,3-Diphenyl-2-azaallyl Anion.
In: Chemische Berichte, Vol. 126, No. 8: pp. 1913-1916
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.