Buchwald-Hartwig Amination of Aryl Chlorides Catalyzed by Easily Accessible Benzimidazolyl Phosphine-Pd Complexes

Abstract

This study describes the efficacy of benzimidazolyl phosphine ligands, which are easily synthesized from inexpensive and commercially available o-phenylenediamine, 2-bormobenzoic acid, and chlorophosphines, in the Buchwald-Hartwig amination of aryl chlorides. Primary and secondary aromatic/aliphatic amines are effective substrates in this catalytic system. Functional groups such as keto and esters are also compatible. The catalyst loading can be reduced to 0.1 mol% Pd.

palladium - cross-coupling - amination - aryl chlorides - phosphine

References and Notes

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16

According to the list price from Aldrich (21-2-2011), o-phenylenediamine costs 0.18 USD/G and 2-bromobenzoic acid costs 0.88 USD/G.

17

CCDC-865333 contains the supplementary crystallographic data for L2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

18

There was no detectable phosphine oxide signal of L2 in the ^³¹P NMR spectrum when the solid form of the ligand was allowed to stand under air for one month.

19

Palladium-Catalyzed Amination of Aryl Chlorides; General Procedure: A stock solution of [Pd₂ (dba)₃] (0.023 g, 0.10 mmol) and ligand L (Pd/L = 1:4) were loaded into a reaction tube equipped with a Teflon-coated magnetic stir bar. The tube was evacuated and flushed with nitrogen (3 cycles). A stock solution was prepared by adding freshly distilled toluene (5.0 mL). Bases (2.5 mmol) were loaded into an array of Schlenk tubes. The tubes were evacuated and flushed with nitrogen (3 cycles). Aryl chlorides (1.0 mmol), amines (1.5 mmol) and the stock solution (0.1 mol% Pd per 0.5 mL stock solution) were loaded into the tubes. Toluene was then added to give a total volume of 3.0 mL in each tube. The solutions were stirred at room temperature for several minutes and then placed into a preheated oil bath (135 ˚C) for the time period indicated in the Tables. After completion of reaction as judged by GC analysis, the reaction tube was allowed to cool to room temperature and the reaction was quenched with water and diluted with EtOAc. The organic layer was separated and the aqueous layer was washed with EtOAc. The combined organic layer was dried, filtered and concentrated under reduced pressure and the crude products were purified by flash column chromatography on silica gel (230-400 mesh) to afford the desired product. 4-Methyl- N -phenylaniline (Table 2, entry 1): ^¹H NMR (400 MHz, CDCl₃): δ = 2.50 (s, 3 H), 5.70 (s, 1 H), 7.05-7.09 (m, 1 H), 7.16-7.19 (m, 4 H), 7.26-7.30 (m, 2 H), 7.40-7.46 (m, 2 H); ^¹³C NMR (100 MHz, CDCl₃): δ = 20.6, 116.8, 118.8, 120.1, 129.2, 129.7, 130.7, 140.2, 143.9; MS (EI):
m/z (%) = 183.1 (100) [M]⁺, 167.1 (20), 91.0(14), 77.1 (10).