Reactions of Diisopropylaminoborane and Metal Dialkylaminoborohydrides
Diisopropylaminoborane (H2B-N(iPr)2) is prepared as a monomer from the reaction of lithium diisopropylaminoborohydride with trimethylsilyl chloride at ambient conditions. Monomeric aminoboranes, such as H2B-N(iPr)2, can reduce nitriles and esters in the presence of catalytic amounts of LiBH4. Diisopropylaminoborane serves as an inexpensive boron source in the palladium-catalyzed borylation of aryl bromides, iodides, and triflates, affording the corresponding boronic acid upon work-up. From aryl bromides and iodides, the borylation reaction affords the boronic acids in moderate to good yields. Borylation of aryl triflates affords the corresponding boronic acids in high yields.
Boronic acids can be synthesized upon reaction of H2B-N(iPr)2 with Grignard reagents at ambient conditions. Aliphatic, aromatic, and heteroaromatic (diisopropylamino)boranes are readily synthesized at ambient temperature by the reaction of Grignard reagents with H2B-N(iPr)2. This borylation reaction can be carried out under Barbier conditions, where H2B-N(iPr)2 traps the in situ formed Grignard reagent from the corresponding organic halide and magnesium metal. Simple acid hydrolysis of the product organo(diisopropylamino)borane leads to the corresponding boronic acid in good to excellent yield.
Reaction of H2B-N(iPr)2 with two equivalents of Grignard reagents results in the formation of B,B-dialkyl(diisopropylamino)borane within one hour at room temperature. This has allowed for the preparation of a series of symmetrical and unsymmetrical aminoboranes. Unsymmetrical B,B-dialkyl(diisopropylamino)boranes are prepared by reaction of H2B-N(iPr)2 with a sterically demanding Grignard reagent, followed by a less hindered one, or by reaction with dialkylzinc followed by subsequent reaction with a Grignard or organolithium reagent. Furthermore, H2B-N(iPr)2 reacts only once with the mild dialkylzinc reagents, affording the boronic acid upon aqueous work-up.
Magnesium aminoborohydride, a by-product of the reaction of Grignard reagents with H2B-N(iPr)2, is an unknown class of borohydride. Chloromagnesium dimethylaminoborohydride (ClMg+ [H3B-NMe2]-, MgAB) is an analogue of the versatile lithium dialkylaminoborohydrides (LAB reagents), prepared by the reaction of dimethylamine-borane with methylmagnesium chloride. MgAB can reduce amides to alcohols or aldehydes under ambient conditions and is complementary to the commonly utilized lithium aluminum hydride (LiAlH4) and diisobutylaluminum hydride (DIBAL) reagents, though it exhibits enhanced chemoselectivity. MgAB can reduce amides in the presence of esters, nitriles, and units of unsaturation. MgAB has also been shown to reduce lactams to the corresponding amines.