Taming Ambident Triazole Anions: Regioselective Ion Pairing Catalyzes Direct NAlkylation with Atypical Regioselectivity
Controlling the regioselectivity of ambident nucleophiles towards alkylating agents is a fundamental problem in heterocyclic chemistry. Unsubstituted triazoles are particularly challenging, often requiring inefficient stepwise protection-deprotection strategies and pre-functionalization protocols. Herein we report on the alkylation of archetypal ambident 1,2,4-triazole, 1,2,3-triazole and their anions, analyzed by in situ 1 H/ 19 F NMR, kinetic modelling, diffusion-ordered NMR spectroscopy,
... R spectroscopy, X-ray crystallography, highly correlated coupled-cluster computations [CCSD(T)-F12, DF-LCCSD(T)-F12, DLPNO-CCSD(T)] and Marcus theory. The resulting mechanistic insights allow design of an organocatalytic methodology for ambident control in the direct N-alkylation of unsubstituted triazole anions. Amidinium and guanidinium receptors are shown to act as strongly-coordinating phase-transfer organocatalysts, shuttling triazolate anions into solution. The intimate ion-pairs formed in solution retain the reactivity of liberated triazole anions but, by virtue of highly regioselective ion-pairing, exhibit alkylation selectivities that are completely inverted (1,2,4-triazole) or substantially enhanced (1,2,3-triazole) compared to the parent anions. The methodology allows direct access to 4-alkyl-1,2,4triazoles (rr up to 94:6) and 1-alkyl-1,2,3-triazoles (rr up to 99:1) in one step. Regioselective ion-pairing acts in effect as a noncovalent in situ protection mechanism, a concept that may have broader application in the control of ambident systems.