Reversible Frustrated Lewis Pair Addition of N-Heterocycles to Unsaturated C–C Bonds

Abstract

N-Heterocycle-tethered olefins and alkynes are cyclized by addition of the N-heterocycle and B(C₆F₅)₃ across the unsaturation. This activation is demonstrated to be reversible at high temperatures.

Key words

frustrated Lewis pairs - olefin activation - borane - N-heterocycles

• References and Notes

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• 23 Representative Procedure for Cyclizations – Synthesis of 2c from 1b In a nitrogen-filled glovebox, alkene 1b (16 mg, 0.1 mmol) was dissolved in toluene (1 mL) and transferred to a vial containing B(C₆F₅)₃ (51 mg, 0.1 mmol). After stirring for 13 h the reaction was concentrated in vacuo, and the resulting precipitate was washed with hexanes (3 mL) to yield the desired cylic product as a white precipitate (61 mg, 90%). ¹H NMR (400 MHz, CD₂Cl₂): δ = 7.99 (t, J = 7.8 Hz, 1 H), 7.52 (d, J = 7.8 Hz, 1 H), 7.43 (d, J = 7.5 Hz, 1 H), 4.66–4.63 (m, 1 H), 3.60–3.52 (m, 1 H), 3.21–3.14 (m, 1 H), 2.63–2.56 (m, 1 H), 2.40–2.32 (m, 1 H), 2.34 (s, 3 H), 2.09–2.01 (m, 1 H), 1.78–1.64 (m, 2 H), 1.34–1.26 (m, 1 H). ¹³C(¹H) NMR (101 MHz, CD₂Cl₂): δ = 157.82 (s), 154.03 (s), 143.01 (s), 127.88 (s), 127.37 (s), 65.46 (s), 29.08 (s), 27.5 (br s), 25.63 (s), 20.45 (s), 15.53 (s). ¹⁹F NMR (377 MHz, CD₂Cl₂): δ = –131.4 (d, J = 22.1 Hz, o-C₆F₅), –162.4 (t, J = 20.3 Hz, p-C₆F₅), –166.1 (q, J = 4.7 Hz, m-C₆F₅). ¹¹B NMR (128 MHz, CD₂Cl₂): δ = –14.2 (s).
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