• Boron is electrophilic because of its empty p orbital

• Boron forms strong B–O bonds and weak B–C bonds

• Migration of alkyl groups from boron to O, N, or C is

stereospecific

Special features of organoboron chemistry

Reactivity of Organoboranes

Organoboranes R3B are very strong Lewis acids and readily react with

amines, etc to give crystalline Lewis acid-base adducts.

In contrast to the dimeric B2H6 (B-H-B banana bonds) and the usually dimeric

organo-alanes R3Al, organoboranes R3B are always monomeric (R2BH

dimerize like B2H6).

The monomeric nature of organoboranes makes their chemistry less complex

than the chemistry of hydridoboranes and organoaluminum compounds.

Organoboranes are extremely reactive towards oxygen and moisture.

Organoboranes are covalent compounds. They are soluble in hydrocarbons

and can usually be distilled / sublimed.

Compared to organic compounds R3C-R and amines R3N:, the very high

reactivity of organo boranes R3B may seem somewhat surprising.

The high reactivity of boranes is easier to understand if one considers that

boranes R3B are iso-lobal to carbeniumions:

The term iso-lobal was introduced to describe the fact that certain

compounds have very similar orbitals irrespective of the different elements

that are involved.

A more precise definition states that two compounds are isolobal if they

have the same energy, symmetry and spatial extension of HOMO and

LUMO.

Organoboranes From Haloboranes

• Organoboranes can be obtained from boron halides and Grignard reagents:

• BF3•Et2O, a fuming liquid that can be distilled without decomposition (does

not attack glass) is the least expensive boron halide

• BCl3 is a low boiling liquid and more difficult to handle. More important is

the fact that BCl3, BBr3, and BI3 decompose ethers very rapidly (BCl3 <

BBr3 < BI3 ). This is problematic because Grignard reagents require ethers

for their formation

• Of much greater importance is the hydroboration reaction.

The difference in the electronegativity of the CB bond is small, meaning that it is

an almost perfect covalent bond.

Second, the boron atom has an open p-electron structure, meaning that it might

be susceptible to nucleophilic reagents.

This suggested that the compounds might undergo reactions as shown in

Equation

Carbon–carbon bonds can also be made with alkyl boranes. The requirement for a carbon nucleophile that bears a suitable leaving group is met by a-halo carbonyl compounds.

The halogen makes enolization of the carbonyl compound easier and then departs in the rearrangement step.

Theproduct is a boron enolate with the boron bound to carbon. Under the basic conditions of the reaction,hydrolysis to the corresponding carbonyl compound is rapid.

In this example it is important which group migrates from boron to carbon as that is the group that forms the new C–C bond in the product.

The reaction of organic boron compounds with a,b-unsaturated

ketones.

Stereoselectivity and Regioselectivity of the Hydroboration

The hydroboration is an anti-Markovnikov cis-addition: