Miyaura Borylation Reaction
The Miyaura borylation reaction enables the synthesis of boronates by cross-coupling of bis(pinacolato)diboron (B2pin2) with aryl halides and vinyl halides.
Borylated products derived from B2pin2 allow normal work up including chromatographic purification and are stable towards air. Pinacol esters are difficult to hydrolyze, but they may serve as coupling partners in the Suzuki Coupling and similar reactions without prior hydrolysis.
Mechanism of the Miyaura Borylation
Crucial for the success of the borylation reaction is the choice of an appropriate base, as strong activation of the product enables the competing Suzuki Coupling. The use of KOAc (J. Org. Chem., 1995, 60, 7508) and KOPh (J. Am. Chem. Soc., 2002, 124, 8001. DOI) is actually the result of a screening of different reaction conditions by the Miyaura group.
The starting material bis(pinacolato)diboron is a poor Lewis acid and 11B-NMR of KOAc and B2bin2 in DMSO-d6 (J. Org. Chem., 1995, 60, 7508) shows no evidence of the coordination of the acetoxy anion to a boron atom leading to a tetrahedral activated species (compared to Suzuki coupling). However, the formation of an (acetato)palladium(II) complex after the oxidative addition of the halide influences the reaction rate of the transmetalation step. The Pd-O bond, which consists of a hard Lewis base with a soft Lewis acid, is more reactive than a Pd-X (X=Br, I) bond. In addition, the high oxophilicity of boron has to be considered as a driving force for the transmetalation step, which involves an acetato ligand.
The mild reaction conditions allow the preparation of boronates which are not accessible via lithium or Grignard intermediates followed by borylation. The use of HBPin instead of B2Pin2 allows similar reactions in large scale synthesis, and also tolerates various reducible functional groups, although side products may arise due to dehalogenation of the aryl halide.
Palladium-Catalyzed, Direct Boronic Acid Synthesis from Aryl Chlorides: A Simplified Route to Diverse Boronate Ester Derivatives
G. A. Molander, S. L. J. Trice, S. D. Dreher, J. Am. Chem. Soc., 2010, 132, 17701-17703.
Efficient Monophosphorus Ligands for Palladium-Catalyzed Miyaura Borylation
W. Tang, S. Keshipeddy, Y. Zhang, X. Wei, J. Savoie, N. D. Patel, N. K. Yee, C. H. Senanayake, Org. Lett., 2011, 13, 1366-1369.
Synthesis of β-Boryl-α,β-unsaturated Carbonyl Compounds via Palladium-Catalyzed Cross-Coupling Reaction of Bis(pinacolato)diboron with Vinyl Triflates
J. Takagi, A. Kamon, T. Ishiyama, N. Miyaura, Synlett., 2002, 1880-1882.
Palladium-Catalyzed Cross-Coupling Reaction of Bis(pinacolato)diboron with 1-Alkenyl Halides or Triflates: Convenient Synthesis of Unsymmetrical 1,3-Dienes via the Borylation-Coupling Sequence
J. Takagi, K. Takahashi, T. Ishiyama, N. Miyaura, J. Am. Chem. Soc., 2002, 124, 8001-8006.