Efficient Borylation of Reactive Aryl Halides with MPBH (4,4,6-Trimethyl-1,3,2-dioxaborinane)

Abstract

The combination of 4,4,6-trimethyl-1,3,2-dioxaborinane, a particularly stable and inexpensive borylation reagent, and Buchwald’s palladium catalyst provides a simple, fast, cost-effective borylation of electron-rich, reactive iodides, bromides, and triflates to produce stable, easily purified boronic esters.

Key words

boron - palladium - catalysis - arylboronic - Suzuki

References and Notes

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• Mechanism:
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• Arene C-H borylation has been recently reviewed:
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7

Preparation of MPBH
Under a well-ventilated fume hood (H₂ evolution), a 100 mL flask equipped with a Claisen distillation head was charged with 10% Pd/C (320 mg) and anhyd dioxane (10 mL). The flask was flushed with nitrogen and freshly distilled 2-methyl-2,4-pentanediol (50 mmol, 6.4 g) in dioxane (10 mL) was added. A solution of N,N-diethylaniline-borane (60 mmol, 9.78 g) in dioxane (10 mL) was added at 20 ˚C over 30 min. The reaction mixture was stirred for an additional 30 min at 20 ˚C. Low pressure distillation gave neat MPBH (4.8 g, 75%, bp 45 ˚C/0.05 bar).

8

Our attempts to use N,N-diethylaniline as the base in subsequent borylations failed.

10

Typical Procedure for the Borylation
An oven-dried Schlenk vessel (or a 10 mL microwaves vial) was charged with Pd₂ (dba)₃ (2.3 mg, 2.5 µmol, described as 0.5% in Table  [¹] ) and CyJohnPhos (3.5 mg, 10 µmol, always 2 equiv/Pd) and placed under an atmosphere of Argon. Anhydrous dioxane (0.6 mL), the aryl halide (0.5 mmol), Et₃N (152 mg, 1.5 mmol) and MPBH (96 mg, 0.75 mmol) were introduced (solid aryl halides were added along with the other solid reagents). The reaction mixture was then heated at the indicated temperature until the aryl halide has been completely consumed as determined by gas chroma-tography. The reaction was allowed to cool to r.t., and filtered through a short pad of Celite (eluent Et₂O). The eluate was concentrated and the crude material purified by flash chromatography on silica gel. Products 2a-j ^²,³, 3g ^¹b and 3h ^¹i have been previously described.
4,4,6-Trimethyl-2-naphthalen-2-yl[1,3,2]dioxaborinane (2g) ^¹H NMR (400 MHz, CDCl₃): δ = 8.34 (s, 1 H), 7.89-7.85 (m, 2 H), 7.82-7.76 (m, 2 H), 7.48-7.40 (m, 2 H), 4.38 (dqd, J = 11.8, 6.2, 2.9 Hz, 1 H), 1.87 (dd, J = 13.7, 2.9 Hz, 1 H), 1.62 (dd, J = 13.7, 11.8 Hz, 1 H), 1.41 (s, 3 H), 1.39 (s, 3 H), 1.38 (d, J = 6.2 Hz, 3 H) ppm. ^¹³C NMR (75.5 MHz, CDCl₃): δ = 134.8 (2×), 132.99, 130.14, 128.68, 127.67, 129.5 (v br, CB), 126.62, 126.39, 125.44, 71.16, 65.14, 46.15, 31.38, 28.27, 23.30 ppm. ^¹¹B NMR (128 MHz, CDCl₃): δ = 27.10. IR (neat): 3053, 2972, 2932, 2911, 1630, 1598, 1503, 861, 823, 765, 746, 685 cm^-¹. MS (EI, 70 eV): m/z (%) = 255 (26), 254 (100), 253 (28).

16

See Supporting Information for a detailed table of these experiments.