Introduction to Organoboron Chemistry
The highly important thing to note about boron is that in its common trivalent state it has only 6 electrons and therefore is quite electron deficient.
It can be used to perform many useful synthetic operations, which often follow unusual mechanisms.
The Hydroboration Reaction
The most common of Boron’s reactions in organic chemistry (the exact details of what ‘hydroboration’ is will be explained below!).
The B-H unit is electrophilic (being highly electron deficient) and can thus undergo electrophilic additions with alkenes. The usual boron hydride used is BH3, which will be present as some kind of adduct e.g. B2H6 (diborane), H3B.SMe2 etc. where the electron deficient boron is stabilised by donation of electrons from some other source.
The addition of the B-H unit occurs in a concerted manner and it is seen to be ‘anti-Markownikov i.e. the boron adds preferentially to the least hindered end of the alkene, placing the H on the most hindered end (so the opposite of what would be expected from Markownikov). This is predominantly caused by sterics – the larger BR2 (R includes Hydrogen) adds in the least sterically hindered position.
An example of hydroboration;
Points to note
i. If the alkene is not too hindered it can react with all three B-H bonds to form a trialkylborane.
ii. Addition is cis due to the concerted process.
From the (tri)alkylborane, further reactions can be carried out which will be covered later in this chapter. Some of these reactions will only use one of the alkyl substituents (of the trialkylborane) leaving the other two alone – this is inefficient so often alkylboranes will be used which already have alkyl substituents (usually bulky ones) which do not participate in the reaction.
Some examples of commonly used reagents;