Pinacolborane is used as a precursor of boronic esters by hydroboration or coupling reactions. It acts as a monofunctional hydroborating agent. It is used in the synthesis of unsymmetrical biaryls through aromatic C-H borylation-cross-coupling sequence. It is involved in the preparation of 4,4,5,5-tetramethyl-2-thiophen-2-yl-[1,3,2]dioxaborolane by reacting with 2-iodo-thiophene. Further, it is used to prepare vinylboronates by palladium-catalyzed coupling with alkenyl triflates and iodides.
Pinacolborane is used for synthesis of unsymmetrical biaryls via aromatic C-H borylation-cross-coupling sequence.
Pinacol (177 g, 1.5 mol) and CH2Cl2 (1.6 L) were added to a 5 L round bottom flask. Borane dimethyl sulfide (10 M, 159 mL, 0.16 mol) was added to the flask via cannula and the mixture was stirred at 0C for 4 hours. The reaction mixture was then heated to room temperature overnight with stirring.17 After 17 h, the reaction mixture gave off some gas (hydrogen); however, the reaction was heated and slowly depressurized (760 mm to 200 mm). This process allowed CH2Cl2 (1.3 L) to move away from the reaction. The remaining liquid was transferred to a 500 mL flask through a cannula and the pressure was slowly reduced from atmospheric to 50 mm over 2.5 h. Some initial foaming was observed in the viscous liquid due to continued outgassing. The method provided pinacol borane suitable for subsequent reactions. The yield was 157 g, 81.7%.
