Highly Efficient and Site-Selective [3 + 2] Cycloaddition of Carbene-Derived Ambident Dipoles with Ketenes for a Straightforward Synthesis of Spiro-Pyrrolidones

The [3 + 2] cycloaddition reaction of 2-arylthiocarbamoyl
benzimidazolium, -imidazolinium, and -triazolium inner salts
(the ambident C−C−N and C−C−S 1,3-dipoles derived
from carbenes) with ketenes proceeded efficiently in a highly
site-selective manner to produce the C−C−N cycloaddition
products benzimidazoline-, imidazolidine-, or triazoline spiro-pyrrolidones in 58−93% yields. Theoretical calculation
suggests a stepwise mechanism for the reaction and indicates
that the C−C−N cycloaddition of the dipoles with ketenes
is both a dynamically and thermodynamically favored
reaction pathway. Their easy availability, high reactivity, and
reaction selectivity render the benzimidazolium, -imidazolinium, and -triazolium inner salts powerful and versatile 1,3-dipoles in the construction of novel spiro heterocyclic
systems, which are not easily accessible by other methods.