Borane monoborane

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

In monoborane (BH3), monoalkylboranes RBH2, or dialkylboranes R2BII there is only an electron sextet at the boron atom. In comparison to the more stable electron octet, the boron atom thus lacks two valence electrons. It obtains them by bonding with a suitable electron pair donor. When no better donor is available, the bonding electron pair of the B—H bond of a second borane molecule acts as the donor so that a two-electron, three-center bond is produced. Under these conditions, boranes are consequently present as dimers BH3, for example, as B2H6. Still, small fractions of the monomers appear as minor components in the dissociation equilibrium of the dimer B2H6, for example, thus contains some BH3. 

In the Cp2TiMe2-catalyzed hydroboration of alkenes, a titanocene bis(borane) complex is responsible for the catalysis. This bis(borane) complex initially dissociates to give a monoborane intermediate. Coordination of the alkene gives rise to the alkene-borane complex, which is likely to be a resonance hybrid between an alkene borane complex and a 3-boroalkyl hydride. An intramolecular reaction extrudes the trialkylborane product, and coordination of a new HBR2 regenerates the monoborane intermediate. 

There are two important differences between these monoborane reagents that affect the outcome of the overall reaction and provide some control of product formation. Hrst, the presence of a large excess of borane reduces the possibility of metal hydride products. Second, the reaction of borane with the first formed metallaborane can be facile thereby leading to addition of one or more BH fragments and cluster expansion. Indeed, we have now explicitly demonstrated examples of cluster expansion for M2B2. y M2B3.j.y y = 0,... 

Nucleophilic attack at other atoms. The chemistry of phosphine-borane adducts has continued to generate interest. Simple borane adducts of primary vinyl-, ethynyl- and allenyl-phosphines have been prepared and studied by a range of spectroscopic and theoretical techniques. The same group has also shown that attachment of the BH3 unit to a variety of primary phosphines results in a substantial increase in the intrinsic acidity of the system in the gas-phase. Group III halide adducts of the type Bu 2PH EX3 (E = B, Al, Ga or In X = Cl or Br) are accessible from the reactions of the secondary phosphine with the trihalides at room temperature. The solid state structure and reactivity of these adducts was also reported. Treatment of l,8-bis(diphenylphosphino)naphthalene with the borane-dimethylsulfide complex in ether solvents affords a simple monoborane adduct of the diphosphine irrespective of the molar ratio of the... 

---