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

Abstract

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 (MgH(2)) and magnesium bromide (MgBr(2)) 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 MgH(2) and MgBr(2) 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.

Synthesis of Catechol‐, Pinacol‐, and Neopentylglycolborane through the Heterogeneous Catalytic BB Hydrogenolysis of Diboranes(4)

“…[12] Similarly, bis(neopentylglycolato)diborane(4) (B 2 -A C H T U N G T R E N N U N G neop 2 ) [14a] was examined for cleavage of its B À B bond by hydrogenolysis to form neopentylglycolborane (HBneop). [18] Reaction of B 2 neop 2 in toluene for 1-5 h under a hydrogen atmosphere of 1 bar at 80 8C in the presence of Pt/C or Pd/C resulted in a doublet signal at d = 25.6 ppm (J = 172 Hz) in the 11 B NMR spectra. GCMS measurements also confirmed the formation of HBneop (m/z: 114 [M] + ).…”

Magnesium‐Catalyzed Tandem Dehydrogenation‐Dehydrocoupling: An Atom Economical Access to Alkynylboranes

“…[29] Alternatively, it can intermediately go through HMgBr loss which was proven to occur after addition of PhMgBr to HBpin for example. [30] To evaluatei ts potentiali mplication in the catalytic process, HMgBr was separately prepared by reaction between magnesium hydride and magnesium bromide. [31] When Scheme2.Structuralscope and limitationofa lkyne borylation.…”

Copper‐Catalyzed Amidation of Primary and Secondary Alkyl Boronic Esters

“…Numerous heteroaromatic compounds are compatible, including basic and non-basic indoles (5,6), pyridines (3, 8, 23, 26), aminopyrimidines (7), benzimidazoles (9), benzothiazoles (10), and thiophenes (4,6,11). Other tolerated groups include protic amine derivatives (17), aryl fluorides (e.g., 3-5), aryl and heteroaryl chlorides (6,8,9,15), alkenes (25), aryl ethers (19)(20)(21)(22), andc arbamates (5,17,18). Nitriles can be used (8,26), but aliphatic nitrilesl ead to some attenuation of the yield (16).…”