Mechanochemical borylation of aryldiazonium salts; merging light and ball milling

Merging of photo- and mechanochemical activation permitted studying the role of eosin Y in the borylation of aryldiazonium salts in a ball mill. Simultaneous neat grinding/irradiation of the reactants and the photocatalyst led to the formation of boronates in a molten state. On the other hand, the catalyst-free liquid-assisted grinding/irradiation reaction also led to product formation, featuring a direct photolysis pathway facilitated by substrate–solvent charge-transfer complex formation.


General information
NMR spectra were recorded on a VNMRS 400 or a VNMRS 600 spectrometer. Proton chemical shifts are reported in parts per million on the δ scale and are calibrated using the residual non-deuterated solvent signal as an internal reference (MeCN-d 3 : δ = 1.94 ppm). Spectral data is provided as follows: chemical shift in ppm (from downfield to upfield), multiplicity (s = singlet, d = doublet, m = multiplet), integration and coupling constant J. GC-MS analysis was performed on an Agilent 7890A GC with an Agilent 5975C inert XL EI/CI MSD with triple axis detector and an Agilent DB-5ms column (30 m × 0.25 mm × 0.25 µm) using helium as the carrier gas. detector. Differential scanning calorimetry (DSC) analysis was conducted on a Mettler Toledo DSC System. All samples were heated at a rate of 5 °C/min from 25 to 200 °C. Thermograms were analyzed using Mettler Toledo software. Mechanochemical reactions were carried out in a RETSCH MM400 Mixer mill using milling jars made of Plexiglas ® [(Poly(methyl methacrylate) (PMMA)], or Teflon ® with ZrO 2 milling balls of 5 or 1.5 mm in diameter. Blue and green irradiation was provided by LEDs strips of 70 cm and 90 cm in length respectively, coiled around the milling jar. Substrates 1c, 1b, 1d were prepared according to reported methods. [1] Aryldiazonium salts 1a and 1e were purchased from commercial suppliers.

Neat grinding experiments:
A mixture of 1 (0.369 mmol), 2 (0.369 or 0.554 mmol) and eosin Y (11.96 mg; 5 mol %) was mixed in a 25 mL PMMA milling jar with 15 ZrO 2 balls of 5 mm in diameter at 25 Hz. Irradiation of the reaction mixture was achieved by wrapping the milling jar with a green-LED strip (90 cm; see picture in section 2). After the milling was stopped, the reaction mixture was recovered from the milling jar and the product was purified by column chromatography (SiO 2 , eluent 100:1 n-pentane/ethyl acetate).

LAG experiments:
A mixture of 1a (50 mg; 0.185 mmol) and 2 (70 mg; 0.277 mmol) was mixed in the presence of the solvent (30 L, = 0.25), in a 25 mL PMMA milling jar (higher values of LAG can cause chemical damage to the plastic milling jar). Irradiation of the reaction mixture was achieved by wrapping the PMMA milling jar with a blue-LED strip (70 cm). After the milling was stopped, the reaction mixture was recovered from the milling jar and the product was purified by column chromatography (SiO 2 , eluent 100:1 n-pentane/ethyl acetate).

Thermal decomposition of iodophenyldiazonium tetrafluoroborate (1d)
100 mg of 1d were weighted in a threaded screw cap glass test tube. The sample was heated in an oil bath for 10 min at 130 °C. Then the sample was cooled down and analyzed by 1 H NMR spectroscopy (see figure below). For a thermal decomposition of aryldiazonium salts observation by ageing, see ref. [5].

Borylation of 1a in the presence of 1,1-diphenylethene (4)
A mixture of 1a (100 mg; 0.369 mmol), 2 (93.7 mg; 0.369 mmol) and 4 (66.4 mg; 0.369 mmol), eosin Y (11.96 mg; 5 mol%) was mixed for 1 h in a 25 mL PMMA milling jar with 15 ZrO 2 balls of 5 mm in diameter at 25 Hz. Irradiation of the reaction mixture was achieved by wrapping the milling jar with a blue-LED strip. After the milling was stopped, the reaction mixture was analyzed by gas chromatography-mass spectrometry.