The Direct Alkylation of π-Rich, Acid-Sensitive Heterocyclic Compounds via Essentially Free Carbocations1

Abstract

Alkylation of pi-rich heteroaromatics such as pyrroles and furans by the standard Friedel-Crafts approach is impractical because the acid catalysts employed (Brønsted or Lewis) induce polyalkylation, ring opening, and polymerization. The present study describes the facile benzylation of pi-excessive heteroaromatics using the nitrosoamide approach, which generates nitrogen-separated carbocation-counterion ion-pairs as the alkylating agent with no catalyst being required. N-Nitrosoamides are favorable sources of carbocations because of the following variables: mildness of the conditions required to generate cations, high reactivity of the unsolvated carbocations formed, solubility of the precursors in a wide range of solvents, homogeneity of the reactions, wide range of decomposition temperatures possible, straightforward chemistry, and excellent product balance. The majority of the cations that are generated in pyrrole (80%) are intercepted by the solvent, and only 20% are intercepted by the counterion; this result provides support for the intermediacy of nitrogen-separated ion-pairs in deamination. A nucleophilicity scale is presented for reactions of selected nucleophiles with essentially free carbocations.