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Hydroheteroarylation of Unactivated Alkenes Using N‑Methoxy­hetero­arenium Salts

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posted on 30.03.2017, 00:00 by Xiaoshen Ma, Hester Dang, John A. Rose, Paul Rablen, Seth B. Herzon
We report the first reductive coupling of unactivated alkenes with N-methoxy pyridazinium, imidazolium, quinolinium, and isoquinolinium salts under hydrogen atom transfer (HAT) conditions, and an expanded scope for the coupling of alkenes with N-methoxy pyridinium salts. N-Methoxy pyridazinium, imidazolium, quinolinium, and isoquinolinium salts are accessible in 1–2 steps from the commercial arenes or arene N-oxides (25–99%). N-Methoxy imidazolium salts are accessible in three steps from commercial amines (50–85%). In total 36 discrete methoxy­heteroarenium salts bearing electron-donating, electron-withdrawing, alkyl, aryl, halogen, and haloalkyl substituents were prepared (several in multigram quantities) and coupled with 38 different alkenes. The transformations proceed under neutral conditions at ambient temperature, provide monoalkylation products exclusively, and form a single alkene addition regioisomer. Preparatively useful and complementary site selectivities in the addition of secondary and tertiary radicals to pyidinium salts are documented: harder secondary radicals favor C-2 addition (2–>10:1), while softer tertiary radicals favor bond formation to C-4 (4.7–>29:1). A diene possessing a 1,2-disubstituted and 2,2-disubstituted alkene undergoes hydropyridylation at the latter exclusively (61%) suggesting useful site selectivities can be obtained in polyene substrates. The methoxy­pyridinium salts can also be employed in dehydrogenative arylation, borono-Minisci, and tandem arylation processes. Mechanistic studies support the involvement of a radical process.