Addition of boron to C-C multiple bond

Electrophilic Addition to Carbon–Carbon Multiple Bonds

Introduction

Strong electrophiles shift the electron density of the double bond.

Since the electron density of a pi bond is above and below the internuclear axis, the electrons in pi bonds are not as tightly held as those in sigma bonds.

Thus the approach of a strong electrophile (or Lewis acid, bearing a partial or formal positive charge, to the pi bond distorts the electron cloud toward the electrophile, leaving one of the carbons of the double bond with a partial or formal positive charge (carbocation)

OR

Many reagents add to the alkene bond.

The reactions are called electrophilic additions and thus a carbocation is formed on a carbon atom.

The reaction is regioselective as controled by the formation of the more stable carbocation.

The reactions follow somewhat complex kinetic expressions showing that a simple mechanism is not being followed.

For example : below possibly two molecules of the reagent are reacting with the substrate in the rate determining step.

1) Additions of HCl, HBr :

a) Markovnikov Addition

The ionic addition of HX produces the more stable carbocation which then reacts with the bromide ion.

The result is that the bromide is attached to the more substituted part of the alkene.

This stereoselective reaction is called Markovnikov Addition.

Due to the stability of cabocation it will form secondary carbocation and having 6 hyperconjugative structures so it will go to more substituted site.

b) Anti-Markovnikov:

IMPORTANT NOTE: Free radical additions to double bonds: Anti-Markovnikov regiochemistry:

The addition of HBr in the presence of a radical iniatiator such as a peroxide follows a regioselective results which is opposite to the ionic addition.

The two reactions are referred to as Markovnikov and anti-Markovnikov additions but result because of the operation of two entirely different mechanisms.

The ionic mechanism (Markovnikov) produces the more stable carbocation which reacts with bromide.

The radical mechanism (anti-Markovnikov) adds the Br radical first to give the more stable radical which reacts with HBr to abstract a hydrogen atom.

2) Additions of Halogens

Halogenation of organic compounds occurs readily with halogens (X2), mixedhalogens (X-Y).

The reaction kinetics contain higher level terms just as addition of HX and thus may have more than one molecule of halogen in the rate controlling step.

The possible stereochemical result of halogenation is shown in the mechanism below.

Most electrophilic halogenations occur through the path involving a halonium ion to give anti addition, but syn addition is also observed as well as non stereoselective addition.

In the bromination of an adamantyl alkene a stable bromonium ion has been isolated.

Addition Mechanism:

The order of Nucleophility:

The order of nucleophility of the alkene increases with increasing electron donating groups on the alkene for the electrophilic halogen or for any electrophilic addition.

Examples of Halogen Addition

In the following example iodine forms Iodonium ion followed by attack of acid oxygen to form a lactone.

Stereochemistry of halogen addition

Bromine addition on Trans-2-butene gives an meso compound :

Other hand Bromine addition on cis-2-butene gives an DL (Recimic) compound :

IMPORTANT NOTE: It proves that the addition of X2 to the olefin is an Anti

Pseudo Halogens

Pseudo halogens are composed of compounds such as I-NCO and I-N3 .

These reagents add with the halogen positive followed by the negative functional part.

The function allows for further organic transformations.

Mixed Halogens

Mixed halogens can be added to alkenes.

While addition of F2 is usually a hazardous proposition, the use of BrF or IF proceed smoothly.

IF can be produced in situ from the reaction of xenon difluoride and iodine.

In the example shown the F anion adds at the site of the more stable cation.

NGP In Halogenation:

In the following example Protonation has occurred at C5, that the ether oxygen has acted as an internal nucleophile across the ring at C4, and that the chloride ion has attacked C7.

Halogenation of Alkynes:

Bromination of benzyl alkynes in acetic acid gave the products of addition of one molecule of bromine the 1,2-dibromoalkenes.

The reaction was successful with a variety of para-substituents and there seems at first to be no special interest in the structure of the products.

Allylic compounds can react efficiently with nucleophiles by either the SN1 or SN2 mechanisms as in these two examples.

In these experiments one of the methyl groups was changed for a CF3 group—exchanging a weakly electron-donating group for a strongly electronwithdrawing group.

If a cation is an intermediate, as in the SN1 reaction, the fluorinated compound will react much more slowly.

Here is the result in the first case.

The SN2 mechanism makes good sense with its concerted bond formation and bond breaking requiring no charge on the carbon skeleton.

3) Hydration of Alkenes

Acid-catalyzed hydration of alkenes to form alcohol.

The equilibrium is driven in the direction of the alcohol product in the presence of excess water.

Markovnikov Regiochemistry is observed; each step is an equilibrium.

The pi- electrons of the alkene attack the proton to form a new bond between the C-C of the alkene and the hydrogen atom.

The most stable carbocation is generated as in the previous examples.

Water behaves as Nucleophile and attacks the carbocation to form an oxonium ion.

Loss of a proton from the oxonium ion gives the alcohol product.

The rate determining step (rds) of the reaction is formation of the carbocation.

An additional reaction intermediate (oxonium ion) forms in this reaction.

The reaction energy diagram for this process has three transition states (TS) and two reaction intermediates (RI).

Acid-catalyzed hydration of alkenes is usually not a practical process, since presence of cationic intermediate indicates that rearrangements and polymerization may occur: