Stereospecific Nickel-Catalyzed Borylation of Secondary Benzyl Pivalates

Published as part of the Cluster C–O Activation

Abstract

A stereoselective nickel-catalyzed direct borylation of enantioenriched secondary benzyl pivalates is described. This methodology is characterized by an intriguing cooperativity of simple nickel and copper salts to promote the targeted C–B bond formation under mild reaction conditions. Unlike classical S_N2-type processes, this protocol occurs with a neat retention of configuration, resulting in synthetically versatile benzyl boronic esters with excellent stereochemical fidelity.

Key words

borylation - nickel - cooperativity - stereoselective - C–O bonds

• References and Notes

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• 24 (R)-1-(6-fluoronaphthalen-2-yl)ethanol (2d) – Typical ProcedureA 5 mL oven-dried screw-capped test tube containing a stirring bar was charged with the benzyl pivalate 1d (54.8 mg, 0.2 mmol). The test tube was introduced in an argon-filled glovebox where B₂nep₂ (67.8 mg, 0.3 mmol, 1.5 equiv), CuF₂ (6 mg, 30 mol%), CsF (9.1 mg, 30 mol%), Ni(COD)₂ (304 μL, 7.5 mol%, 0.05 M in toluene), PCy₃ (152 μL, 7.5 mol%, 0.1 M), and toluene (1 mL) were then added sequentially. The tube with the mixture was taken out of the glovebox and stirred at 50 °C for 15 h. The mixture was then allowed to warm to room temperature, diluted with EtOAc (5 mL), and filtered through a Celite^® plug, eluting with additional EtOAc (5mL). The filtrate was concentrated removing the volatiles. Then the reaction was cooled to 0 °C (water/ice bath) and BHT (ca. 1 mg) was added followed by anhydrous THF (1 mL). An ice-cold degassed mixture of 3 M NaOH (1.2 mL) and 30% aq H₂O₂ (0.75 mL) was added all at once. The reaction mixture was stirred at room temperature for 2 h. Then, the reaction mixture was diluted with water (4 mL) and extracted with Et₂O (3 × 10 mL). The combined organic layers were washed with brine and dried over MgSO₄. The filtrate was concentrated and purified by silica gel chromatography to give the title product 2d (30.5 mg, 80%) as a white solid (mp 84–86 °C). The enantiomeric excess was determined to be 65% ee (88% ee_s) by chiral HPLC analysis (CHIRALPAK IB, 1 mL/min, 2% EtOH/hexane, λ = 220 nm): t _R (minor) = 12.4 min, t _R (major) = 13.9 min. [α]_D ²⁴ = 43.2 (c 0.06, CHCl₃).¹H NMR (500 MHz, CDCl₃): δ = 7.83–7.76 (m, 3 H), 7.53 (dd, J = 2.0 Hz, 1 H), 7.44 (dd, J = 2.6 Hz, 1 H), 7.26 (td, J = 2.6 Hz, 1 H), 5.06 (q, J = 6.4 Hz, 1 H), 1.97 (s, 1 H, OH), 1.58 (d, J = 6.4 Hz, 3 H) ppm. ¹³C NMR (126 MHz, CDCl₃): δ = 161.8, 159.4, 142.5, 133.6, 130.3, 127.7, 124.9, 123.8, 116.6, 110.7, 70.3, 25.2 ppm. ¹⁹F NMR (376 MHz, CDCl₃): δ = –115.1 ppm.