Alkene hydroboration with pinacolborane catalysed by lithium diisobutyl-tert-butoxyaluminum hydride

Here we developed a highly efficient alkene hydroboration protocol, showing that various alkyl boronates can be smoothly obtained in good yields by reacting alkenes with pinacolborane (HBpin) in the presence of 5 mol% lithium diisobutyl-tert-butoxyaluminum hydride. The coordination of aluminate ions with lithium cations allowed for effective hydride transfer during hydroboration, and the obtained boronate ester was further used for C–C coupling, trifluoroboronate salt formation, and oxidation to alcohol.


Experimental section:
General Information: All glassware used was dried thoroughly in an oven, assembled hot, and cooled under a stream of dry nitrogen prior to use. All chemicals were commercial products of the highest purity. HBpin and alkenes were purchased from Aldrich Chemical Company. 1 H NMR spectra were measured at 400 MHz with CDCl3 as a solvent at ambient temperature unless otherwise indicated and the chemical shifts were recorded in parts per million downfield from tetramethylsilane (δ = 0 ppm) or based on residual CDCl3 (δ = 7.26 ppm) as the internal standard. 13 C NMR spectra were recorded at 100 MHz with CDCl3 as a solvent and referenced to the central line of the solvent (δ = 77.0 ppm). The coupling constants (J) are reported in hertz. Analytical thin-layer chromatography (TLC) was performed on glass precoated with silica gel (Merck, silica gel 60 F254). Column chromatography was carried out using 70-230 mesh silica gel (Merck) at normal pressure.
GC analyses were performed on a Younglin Acme 6100M and 6500 GC FID chromatography, using an HP-5 capillary column (30 m). All GC yields were determined with the use of naphthalene as the internal standard and the authentic sample.
General procedures for the LDBBA catalyzed hydroboration of alkenes (2a-l): A 20 mL test tube was charged with styrene (1.0 mmol, 0.11 mL), pinacolborane (1.2 mmol, 0.18 mL) at room temperature. To this LDBBA (5 mol%, 0.39 M, 0.13 mL) was added under nitrogen atmosphere at the same temperature, reaction mixture was brought to 110 °C and stirred for 2 h. After this time, reaction mixture was cooled to room temperature, unreacted substrates were quenched by the addition of 1 mL of water. The crude mixture was extracted with ethyl acetate and combined organic layers were dried over MgSO4. (Conversions were determined by Gas chromatography). Solvents (volatiles) were evaporated under reduced pressure, residue mixture was subjected to column chromatography using silica gel. Isolated compounds were analyzed by spectroscopic data, compared with literature.

S2
Procedure for the synthesis of potassium phenethyltrifluoroborate (3b) S1,S2 The substrate 4,4,5,5-Tetramethyl-2-phenethyl-1,3,2-dioxaborolane (2a, 1 mmol, 232 mg) was taken in 25 mL round bottom flask and dissolved in 5 mL methanol. To this, KHF2 (1 mL, 4.5 M) was added drop wise at room temperature, reaction mixture was stirred for 30 min. After this time solvent was evaporated under reduced pressure. To this crude material, hot acetone was added (3x10 mL), decanted. Filtrate (decanted liquid) was evaporated to leave a white solid. To this solid, ether was and decanted to remove pinacol and dried under vaccume to obtain pure potassium phenethyltrifluoroborate as colorless solid in 93% yield.
After this time, solvent was evaporated and extracted with ethyl acetate. The combined organic layers were dried over MgSO4, solvent was evaporated under reduced pressure. The crude material was purified under column chromatography using silica gel with ethyl acetate and hexane to afford 1-nitro-4-phenethylbenzene (3c) as a pale yellow solid in 56 % yield.