-bond of BCB 1a affording the cyclobutyl radical Int-2 and Int-2a was found to be highly exergonic. atom, affording the observed product 3a (Supplementary Fig. and) was highly exergonic (G = -27.1 kcal/mol). The alternative cyclization pathway via the base-mediated nucleophilic attack from the amidyl nitrogen atom, leading to 2-aza-BCP 3a' was kinetically as well as thermodynamically disfavored due to the high strain energy associated with its formation (G ‡ = 14.2 kcal/mol for TS-2' and G = 1.0 kcal/mol for 3a').