Regioselective C–H Borylation of Heteroaromatic Aldimines with Iridium Complexes

Abstract

An iridium-catalyzed regioselective C–H borylation of pentafluoroaniline-derived heteroaromatic aldimines has been developed. Various heteroaromatic aldimines underwent borylation by bis(pinacolato)diboron to afford the corresponding borylated products in good yields in the presence of an iridium complex formed in situ from bis(cyclooctadienyl)(methoxy)iridium {[Ir(OMe)(cod)]₂} and 1,10-phenanthroline.

Key words

iridium - catalysis - borylation - heteroarenes - regioselectivity - aldimines

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• 13 Borylation of Heteroaromatic Aldimine 1a; General Procedure An oven-dried two-necked flask was charged with [Ir(OMe)(cod)]₂ (5.0 mg, 7.5 μmol), borane 2 (152.4 mg, 0.60 mmol), and 1,10-phen (2.7 mg, 15 μmol). The flask was connected to a vacuum/N₂ manifold through a rubber tube, evacuated, and backfilled with N₂. This cycle was repeated three times. Mesitylene (3.0 mL) was then added in the flask through a rubber septum by using a syringe and the mixture was stirred at r.t. for 15 min. Next, 1 (0.50 mmol) was added from a syringe and the mixture was stirred at r.t. When the reaction was complete, the mixture was initially purified by Kugelrohr distillation. The resulting mixture was crystallized from hexane. See the Supporting Information for characterization data of all compounds 3. Data for 3a are given here as an example. The NMR yield of 3a in the crude mixture was 79%. The reaction mixture was first purified by Kugelrohr distillation (34 Pa, 180 °C). Then, the resulting mixture was recrystallized from hexane to obtain 3a (76.2 mg, 0.19 mmol, 38%) as a white solid from 1a (137.6 mg, 0.50 mmol); mp 149 °C. ¹H NMR (396 MHz, CDCl₃): δ = 1.32 (s, 12 H), 2.45 (d, J = 1.2 Hz, 3 H), 6.40 (s, 1 H), 8.69 (s, 1 H). ¹³C NMR (99 MHz, CDCl₃): δ = 13.7 (CH₃), 24.7 (CH₃), 84.2 (C), 113.5 (CH), 127.0 (dt, J = 4.2, 12.4 Hz, C), 136.4–136.8 (m, CF), 139.0–139.3 (m, CF), 141.5–141.6 (m, CF), 155.7 (C), 156.2 (CH), 158.1 (C). The carbon directly attached to the boron atom was not detected, likely due to quadrupolar relaxation. ¹⁹F NMR (373 MHz, CDCl₃): δ = –153.6 (d, J = 14.2 Hz, 2 F), –161.5 (t, J = 21.6 Hz, 1 F), –163.9 (t, J = 17.9 Hz, 2 F). HRMS-EI (m/z): [M]⁺ calcd for C₁₈H₁₇ ¹⁰BF₅NO₃, 400.12580; found, 400.12638.
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