Group 13: Boron

Organoboranes of type R₃B can be prepared by reaction 1.1, or by the hydroboration reaction mentioned above.

                               (1.1)

Trialkylboranes are monomeric and inert towards water, but are pyrophoric; the triaryl compounds are less reactive. Both sets of compounds contain planar 3-coordinate B and act as Lewis acids towards amines and carbanions. Reaction 1.2 shows an important example; sodium tetraphenylborate is water-soluble but salts of larger monopositive cations (e.g. K‏) are insoluble. This makes Na[BPh₄] useful in the precipitation of large metal ions.

                                                             (1.2)

   Compounds of the types R₂BCl and RBCl₂ can be prepared by transmetallation reactions (e.g. equation 1.3) and are synthetically useful (e.g. reaction 1.4).

            (1.3)

                                       (1.4)

    The bonding in R₂B(µ-H)₂BR₂ can be described in a similar manner to that in B₂H₆. An important member of this family is 1.1, commonly known as 9-BBN, which is used for the regioselective reduction of ketones, aldehydes, alkynes and nitriles.

                                          (1.1)

By using bulky organic substituents (e.g. mesityl = 2,4,6-Me₃C₆H₂), it is possible to stabilize compounds of type R₂B^_BR₂. These should be contrasted with X₂B^_BX₂ where X = halogen or NR₂ in which there is X → B π- overlap. Two-electron reduction of R₂B^_BR₂ gives [R₂B=BR₂]^2-, an isoelectronic analogue of an alkene. The planar B₂C₄ framework has been confirmed by X-ray diffraction for Li₂[B₂(2,4,6-Me₃C₆H₂)₃Ph], although there is significant interaction between the B=B unit and two Li‏ centres. The shortening of the B^_B bond on going from B₂(2,4,6-Me₃C₆H₂)₃Ph (171pm) to [B₂(2,4,6-Me₃C₆H₂)₃Ph]^2- (163 pm) is less than might be expected and this observation is attributed to the large Coulombic repulsion between the two B^- centres.