Addition to Carbon–Carbon Multiple Bonds

Introduction

There are four types of reactions in organic chemistry----------1. Addition reaction 

                                                                                                    2. Elimination reaction

                                                                                                    3. Substitution reaction

                                                                                                    4. Rearrangement reaction

Let us know more about the Addition reactions, particularly addition to carbon-carbon multiple bonds.

Basically addition to a double or triple bond can takes place in four different ways. 

                            1.Two-step processes, with initial attack by a nucleophile,

                            2. Two-step processes, with initial attack upon an electrophile

                            3.Two-step processes, with initial attack upon a free radical.

The second step of the reaction will consists of combination of thus formed intermediate with, a positive species, a negative species, or a neutral entity respectively.

                            4. One step process, which involves attack at the two carbon atoms of the double or                                   triple bond is simultaneously (concerted mechanism). 

which type of the mechanism involved in the given reaction, depends upon the nature of the substrate, the reagent, and the reaction conditions. Some reactions may take place by all four ways.

Mechanism and stereochemical aspects of the addition reactions involving electrophiles

In this type of mechanism, a positive species (electrophile) approaches the double or triple bond (as it is electron rich) and in the first step forms a bond by donation of the pair of pi electrons to the electrophilic species to form a sigma pair as follows

and in the second step intermediate formed in the first step combines with the species (W) carrying an electron pair, generally, it is negatively charged. 

Not all electrophilic additions follow the simple mechanism given above. In bromination the intermidiate formed in first step, very rapidly cyclizes to a bromonium ion :

In both the cases, the mechanism is called AdE2 (electrophilic addition, bimolecular).

Mechanism and stereochemical aspects of the addition reactions involving nucleophiles

In the first step of nucleophilic addition, a nucleophile donates its pair of electrons to one of the doubly/ triply bonded carbon atom, which generates a carbanion. The second step is a combination of this carbanion with a positive species as follows

This mechanism is the similar as the electrophilic addition, except that the charges are reversed. When the alkene contains a good leaving group, substitution is a side reaction.

Mechanism and stereochemical aspects of the addition reactions involving free radicals,

The mechanism of free-radical addition follows the pattern as shown below

then the reaction propagates as shown below

then termination of the chain reaction takes place by following manner

regioselectivity of addition reactionchemoselectivity of addition reactionreactivity and orientation of addition reaction.

reactivity

As in electrophilic aromatic substitution electron-donating groups increase the reactivity of a double bond toward electrophilic addition and electron-withdrawing groups decrease it. Similarly  the reactivity toward electrophilic addition of a group of alkenes increased in the order CCl3CH=CH2 < Cl2CHCH=CH2<ClCH2CH=CH2< CH3CH2=CH2. For nucleophilic addition the situation is reversed. These reactions are best carried out on substrates containing three or four electron-withdrawing groups, two of the most common being F2C=CF2 and (NC)2C=C(CN)2. The effect of substituents is so great that it is possible to make the statement that simple alkenes do not react by the nucleophilic mechanism, and polyhalo or polycyano alkenes do not generally react by the electrophilic mechanism.

orientation

When an unsymmetrical reagent is added to an unsymmetrical substrate, the question arises: Which side of the reagent goes to which side of the double or triple bond? The terms side and face are arbitrary, and a simple guide is shown to help understand the arguments used here. For electrophilic attack, the answer is given by

Markovnikov’s rule: The positive part of the reagent goes to the side of the double or triple bond that has more hydrogens. A number of explanations have been suggested for this regioselectivity, but the most probable is that Y+ adds to that side which can give the more stable carbocation.

In free-radical addition the main effect seems to be steric. All substrates CH2=CHX preferentially react at the CH2, regardless of the identity of X or of the radical. With a reagent such as HBr, this means that the addition is anti-Markovnikov:

Thus the observed orientation in both kinds of HBr addition (Markovnikov electrophilic and anti-Markovnikov free radical) is caused by formation of the secondary intermediate. In the electrophilic case it forms because it is more stable than the primary; in the free-radical case because it is sterically preferred.

Stereochemical Orientation: Some additions are syn, with both groups, approaching from the same side, and that others are anti, with the groups approaching from opposite sides of the double or triple bond. For cyclic compounds steric orientation must be considered. In syn addition to an unsymmetrical cyclic alkene, the two groups can come in from the more- or from the less-hindered face of the double bond. The rule is that syn addition is usually, although not always, from the less-hindered face. For example, epoxidation of 4-methylcyclopentene gave 76% addition from the less-hindered and 24% from the more-hindered face

In anti addition to a cyclic substrate, the initial attack on the electrophile is also from the less-hindered face. However, many (although not all) electrophilic additions to norbornene and similar strained bicycloalkenes are syn additions.In these cases reaction is always from the exo side, as in following reaction.

Hydrogenation of double bond, Dihydro-addition

Most carbon–carbon double bonds, whether substituted by electron-donating or electron-withdrawing substituents, can be catalytically hydrogenated, usually in quantitative or near-quantitative yields. Almost all known alkenes added hydrogen at temperatures between 0 and 275 C. The catalysts used can be divided into two broad classes, both of which mainly consist of transition metals and their compounds:

(1) catalysts insoluble in the reaction medium (heterogeneous catalysts). Among the most effective are Raney nickel, palladium-on-charcoal, NaBH4-reduced nickel (also called nickel boride), platinum metal or its oxide, rhodium, ruthenium, and zinc oxide.

(2) Catalysts soluble in the reaction medium (homogeneous catalysts). An important example is chlorotris (triphenylphosphine) rhodium, RhCl(Ph3P)3, (Wilkinson’s catalyst), which catalyzes the hydrogenation of many alkenyl compounds without disturbing such groups as COOR, NO2, CN, or COR present in the same molecule. Even unsaturated aldehydes can be reduced to saturated aldehydes, although in this case decarbonylation may be a side reaction. In general, for catalytic hydrogenation, many functional groups may be present in the molecule, for example, OH, COOH, NR2 including NH2, N(R)COR', including carbamates, CHO, COR, COOR, or CN. Vinyl esters can be hydrogenated using homogeneous rhodium catalyst. Enamides are hydrogenated, with excellent

enantioselectivity, using chiral rhodium catalysts.

 Hydrogenation of triple bonds

Triple bonds can be reduced, either by catalytic hydrogenation or by the other methods. The comparative reactivity of triple and double bonds depends on the catalyst. With most catalysts (e.g.,  Pd), triple bonds are hydrogenated more easily, and therefore it is possible to add just 1 equivalent of

hydrogen and reduce a triple bond to a double bond (usually a stereoselective syn addition) or to reduce a triple bond without affecting a double bond present in the same molecule.

Hydrogenation of aromatic rings

Aromatic rings can be reduced by catalytic hydrogenation, but higher temperatures (100–200 C) are required than for ordinary double bonds. although the reaction is usually carried out with heterogeneous catalysts, homogeneous catalysts have also been used; conditions are much milder with these. Mild conditions are also successful in hydrogenations with phase transfer catalysts. Hydrogenation in ionic liquids is known, and also hydrogenation in supercritical ethane containing water. Many functional groups, such as OH, COOH, COOR, NH2, do not interfere with the reaction, but some groups may be preferentially reduced. Among these are CH2OH groups, which undergo hydrogenolysis to CH3. Phenols may be reduced to cyclohexanones,  through the enol. Heterocyclic compounds are often reduced. Thus furan gives THF.

Mechanism of Michael reaction.

The Michael reaction or Michael addition is the nucleophilic addition of a carbanion or another nucleophile to an α,β-unsaturated carbonyl compound.

Mechanism:

 Mechanism of Sharpless asymmetric epoxidation

The Sharpless assymmetric epoxidation is an organic reaction used to stereoselectively convert an allylic alcohol to an epoxy alcohol using a titanium isopropoxide catalyst, t-butyl hydroperoxide (TBHP), and a chiral diethyl tartrate (DET).

 The mechanism begins with the displacement of the isopropoxide ligands on the titanium by DET, TBHP, and finally by the allylic alcohol reagent. This titanium complex is believed to exist as a dimer, but for simplicity is shown as a monomer in the mechanism. Oxidation of the olefin with TBHP then occurs where the chiral DET dictates the face of attack and leads to a steroselective epoxy alcohol

Example: