Hydride as a Leaving Group in the Reaction of Pinacolborane with Halides under Ambient Grignard and Barbier Conditions. One-Pot Synthesis of Alkyl, Aryl, Heteroaryl, Vinyl, and Allyl Pinacolboronic Esters

Grignard reagents (aliphatic, aromatic, heteroaromatic,
vinyl,
or allylic) react with 1 equiv of 4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(pinacolborane, PinBH) at ambient temperature in tetrahydrofuran (THF)
to afford the corresponding pinacolboronates. The initially formed
dialkoxy alkylborohydride intermediate quickly eliminates hydridomagnesium
bromide (HMgBr) and affords the product boronic ester in very good
yield. Hydridomagnesium bromide (HMgBr) in turn disproportionates
to a 1:1 mixture of magnesium hydride (MgH2) and magnesium
bromide (MgBr2) on addition of pentane to the reaction
mixture. DFT calculations (Gaussian09) at the B3LYP/6-31G­(d) level
of theory show that disproportionation of HMgBr to MgH2 and MgBr2 is viable in the coordinating ethereal solvents.
This reaction also can be carried out under Barbier conditions, where
the neat PinBH is added to the flask prior to the in situ formation
of Grignard reagent from the corresponding organic halide and magnesium
metal. Pinacolboronic ester synthesis under Barbier conditions does
not give Wurtz coupling side products from reactive halides, such
as benzylic and allylic halides. The reaction between PinBH and various
Grignard reagents is an efficient, mild, and general method for the
synthesis of pinacolboronates.