Free Radical-Mediated Aryl Amination and Its Use in a Convergent [3 + 2] Strategy for Enantioselective Indoline α-Amino Acid Synthesis

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Abstract
The scope of aryl radical additions to the nitrogen of azomethines is described. Aryl, trifluoromethyl alkyl, and α,β-unsaturated ketimines engage in regioselective aryl−nitrogen bond formation via 5-exo cyclizations of an aryl radical to azomethine nitrogen. Selectivity for carbon−nitrogen over carbon−carbon bond formation is generally high (>95:5) and competes only with direct aryl radical reduction by stannane (0−10%). α-Ketoimines are a promising new class of carbon radical acceptors for which no competitive aryl radical reduction is observed. The reaction conditions are pH-neutral and are therefore among the mildest methods available for amination of an aromatic ring. The ketimines examined did not suffer from competitive reduction by stannane, offering an advantage over the use of diazo and azide functional groups as nitrogen sources for carbon radicals. The free radical-mediated aryl amination was sequenced with the O'Donnell phase transfer-catalyzed enantioselective alkylation strategy of glycinyl imine to provide either enantiomer of indoline α-amino acids with high ee. These new constrained phenyl alanine derivatives are now readily available for evaluation across a variety of applications.

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