• Question: what is electrophilic boron reagents

    Asked by jordan to Ian on 10 Nov 2014.
    • Photo: Ian Cade

      Ian Cade answered on 10 Nov 2014:

      I suppose firstly I should say what I mean by electrophilic – Essentially this describes something that ‘wants to’ accept electrons (more technically, and from a chemistry point of view, an atom that has an empty low energy orbital into which electron density can be donated would be described as an electrophilic centre)

      Many boron compounds are quite electrophilic (in that they are relatively electron-poor and are able to accept electrons).

      An example you might be more familiar with is the electrophilic nature of molecules with C=O bonds, such as esters. The carbon atom of these compounds is electrophilic and is the site where nucleophiles (such as primary amines) will attack (donate electrons), for example:

      (1) Me-C(O)-OEt plus Pr-NH2 -> Me-C(O)-NH-Pr plus H2O

      This is similar to the case of compounds with a 3-coordinate boron atom such as BCl3. This is a flat molecule that looks a bit like a three-bladed propeller. Importantly, above and below the plane of atoms (where the cone and axle of the propeller would be) there is a relatively positively charged region of space (an empty p-orbital) that can accept up to two electrons. The following reaction (similar to (1)) is quite typical of three-coordinate boron compounds

      (2) BCl3 plus amine -> amine:BCl3

      The colon between amine and BCl3 indicates the pair of electrons that have been donated to boron.

      In my case I work with ‘especially’ electrophilic boron compounds. The already quite electrophilic BCl3 (above) can be made more electrophilic by removing electrons to make it even more eager to accept some. This is done by removing an anionic chloride using AlCl3, and replacing it with a neutral amine (though in practice the amine is added first), as below:

      (3) BCl3 plus amine -> amine:BCl3
      (4) amine:BCl3 plus AlCl3 -> amine:BCl2+ AlCl4-

      The “amine:BCl2+” is sufficiently keen to get more electrons it is able to react with molecules you would normally expect to be quite unreactive, such as benzene.

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