Bond cleavage boron-hydrogen

The iridium(I) salt containing 185 has also been employed as catalyst in hydrogenation of imines. In the hydrogenation of 2-substitufed quinolines to provide chiral tetrahydro derivatives iridium complexes of SYNPHOS and DIFLUORPHOS prove to he effective catalysts. Oximes undergo enantioselective reduction (and N—O bond cleavage) on treatment with borane and spirocychc boronate 186. ... 

Scheme 6.25. A representation of the oxidative hydroboration of 1-methylcyclopentene (as a typical alkene). The addition of boron and hydride (H ) is suprafacial with the double bond attacking the boron so as to build up charge on that carbon of the double bond that would be the most stable carbocation it is to this carbon of the double bond that the hydride (H") adds. In the oxidation step, the anion of hydrogen peroxide (H02 ) attacks the boron. Subsequent rearrangement of carbon to oxygen with oxygen-oxygen bond cleavage produces the alcohol. Only one ligand to boron is shown to emphasize the oxidation process.

Analogous cleavage reactions have been observed with B4H10 and B5H11. Such basic cleavage reactions are only observed to occur in boron hydrides in which a boron atom is bonded to the rest of the molecule by two boron-hydrogen-boron, three-center bonds. 

Reactions of this type are somewhat less satisfactory for preparation of mixed diboron derivatives containing boron-carbon bonds. Thus, while tetrakis(dimethylamino)diborane(4) is readily hydrolyzed to tetrahydroxy-diborane(4) in aqueous acid, l,2-bis(dimethylamino)-l,2-diethylborane(4) reacts with formation of hydrogen and cleavage of the boron-boron bond 14). Reaction of the dibutyl analog with a twofold excess of water gave the dimethylamine adduct of the boroxole... 

The chemical reactivity of several heterocyclic derivatives is closely associated with a reactivity one might expect for open-chain unsaturated derivatives. The boron heterocycles are sensitive to oxygen or hydrogen peroxide 111, 169) and so apparently is 1,1,2,5-tetraphenylsilacyclo-pentadiene 17, 246). These reactions probably lead to ring cleavage but the products have not been characterized. Vinyl-metal bonds are frequently cleaved by electrophilic reagents and the heterocycles are no exception. 

Boric Acid.9 Hydrolysis of boron halides, hydrides, etc., affords the acid, B(OH)3, or its salts. The acid forms white, needle-like crystals in which B(OH)3 units are linked together by hydrogen bonds to form infinite layers of nearly hexagonal symmetry the layers are 3.18 A apart, which accounts for the pronounced basal cleavage. 

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