Boric acid as a precatalyst for BH3-catalyzed hydroboration

We report that boric acid, BO3H3, is a good precatalyst for the BH3-catalyzed hydroboration of esters using pinacolborane as a borylation agent. Using microwave irradiation as an energy source, we demonstrated that a dozen esters were converted into the corresponding boronate ethers in good yields. It was also possible to use boric acid as a precatalyst to reduce carbonates and alkynes. Considering the hazardous and pyrophoric nature of BH3 solutions, boric acid proves to be a safe and green precatalyst for the metal-free reduction of unsaturated species.

Coupling constants are reported in Hz. Deuterated-chloroform (CDCl3) was dried by distillation over P2O5. Mass spectrometry analyses were carried out on an Agilent 6210 LC Time of Flight Mass Spectrometer, using an electrospray ionization (ESI) method. Esters, carbonates, alkynes and all other chemicals bought from Sigma-Aldrich and were used without further purification. All solvents and boron reagents were used directly from the bottle, without additional purification steps. Microwave reactions were performed with a Monowave 400 from Anton Paar.

Procedure for the catalytic hydroboration of esters
The ester (0.60 mmol, 1 equiv) and 435 µL (3.00 mmol, 5 equiv) of HBpin were dissolved in 2methyltetrahydrofuran to give a total volume of 2.00 mL, then a catalytic amount of boric acid (3.71 mg, 0.06 mmol, 0.1 equiv) was introduced. All catalytic reactions were carried out using sealable microwave vials. The reaction mixture was subsequently heated at 200 o C for 1 h.
Afterward, all volatiles were evaporated in a rotatory evaporator at 40 o C. The products and the internal standard were dissolved in CDCl3 and added to a cap-sealed NMR tube for characterization. To measure the NMR yield, 10 μl (0.072 mmol) of mesitylene was weighted and incorporated to the NMR solution.

NMR yield calculation
This is an example on the determination of the NMR yield from a 1 H NMR ( Figure S1) corresponding to entry 12 of Table 1. The hydroboration reaction was performed at 150 °C with 1a.
Figure S1 NMR analysis of entry 12 of Table 1 after the hydroboration catalysis at 150 °C.

Characterization of ester hydroboration products
Characterization data for the products of ester reduction are given below. Reported products were characterized by 1 H, 11 B{ 1 H}, and 13 C{ 1 H} NMR and correspond to the values previously reported for these species. 3

Procedure for the catalytic hydroboration of carbonates
Carbonate substrates (0.60 mmol, 1 equiv) and 609 µL (4.2 mmol, 7 equiv) of HBpin were dissolved in 2-methyltetrahydrofuran to give a total volume of 2.00 mL, then a catalytic amount of boric acid (3.71 mg, 0.06 mmol, 0.1 equiv) was introduced. All catalytic reactions were carried out using sealable microwave vials. The reaction mixture was subsequently heated at 200 o C for 4 h. Afterward, all volatiles were evaporated in a rotatory evaporator at 40 o C. The products and the internal standard (mesitylene) were dissolved in CDCl3 and added to a cap-sealed NMR tube, wrapped with parafilm for NMR characterization.

Characterization of carbonates hydroboration products
Characterization data for the products of carbonates reduction are given below. Reported products were characterized by 1

Monitoring of the catalytic transformation
A time study of the catalytic transformation was carried out for benzyl benzoate 1a using boric acid as catalyst by collecting 6 data points early in the reaction.  c) The reaction between 1 equiv of HBpin and 1 equiv of BH3•SMe2 produces a little amount of multiple small singlets in 11 B{ 1 H} NMR. In 11 B NMR it is possible to see a multiplet at -13.2 ppm corresponding to the speculated adduct ( Figure S3).

Computational details
All calculations were performed on the full structures of the reported compounds using the Gaussian 16 suite of programs. 7 The ωB97XD functional 8 was used in combination with the 6-31+G** basis set for all atoms. All geometry optimizations were carried out without any symmetry constraints. The transition states were located and confirmed by frequency calculations (single imaginary frequency). Similarly, the stationary points were characterized as minima (no imaginary frequency). The energies were then refined by single point calculations to include solvent effect using the SMD solvation model 9 with the experimental solvent (tetrahydrofuran) at the same level of theory. 10 All structures with their associated free enthalpy and Gibbs free energies as well as their Cartesian coordinates are fully detailed in the following section.