Alkenes

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Chapter: Organic Chemistry : Functional Group Synthesis

Alkenes are relatively low oxidation level hydrocarbons. The most common way to prepare alkenes is to carry out the elimination of a small molecule from between vicinal carbon atoms.


ALKENES

Traditional preparations of alkenes include the following:


Alkenes are relatively low oxidation level hydrocarbons. The most common way to prepare alkenes is to carry out the elimination of a small molecule from between vicinal carbon atoms. However, this is only a viable strategy if the regiochemistry of elimination can be controlled. That is, traditional dehydrohalo-genations or dehydrations often are regioselective but not regiospecific, so that mixtures of structurally isomeric olefins are formed. For example,


The formation of regioisomers is due to the presence of several sets of nonequiva-lent vicinal hydrogens of similar but not identical reactivity. The resulting mixture of similar products must be separated if only one of the regioisomers is desired. Since the alkene isomers are very similar in physical properties, such separations can be very difficult and certainly are not practical.

Several strategies to control the elimination regiochemistry have been devel-oped. These include placement of the leaving group, steric bulk of the base, and/or establishment of thermodynamic control. By placing the leaving group at the end of a chain, only terminal olefins can be produced by elimination because there is only one set of vicinal hydrogens that can be removed by the base. Diaz-abicycloundecane (DBU) is the base used in the example below. It is very useful for promoting olefin-forming eliminations since it is a strong nitrogen base which is also relatively nonnucleophilic.


By using very bulky alkoxide bases (t -butoxide or amyloxide), attack of the base occurs at the least hindered position — usually at the end of chains if possible. In this way the regioselectivity of elimination is controlled by steric factors so that one isomer is produced nearly exclusively.


Finally, when eliminations which give conjugated systems are possible, they are favored significantly by the greater stability of the conjugated π system.


Dehydrations produce olefins from alcohols by the acid-catalyzed elimination of a water molecule from between two carbons. Acid-catalyzed dehydrations often give mixtures of products because the intermediate carbocation is prone to cationic rearrangements to more stable carbocations prior to formation of the olefin product. Moreover, even when the intermediate carbocation is not subject to skeletal rear-rangement, as in the case of tertiary alcohols, mixtures of regioisomers are often produced during the loss of a proton from the carbocation. As a consequence, the acid-catalyzed dehydration of alcohols is generally not a viable synthetic method.


There are many other methods for carrying out 1,2 eliminations to give olefins. Several are particularly useful and widely used. Selenoxide eliminations are fre-quently used to install the double bond of α, β -unsaturated carbonyl compounds. They occur by concerted, cyclic, syn processes


Silyloxide eliminations (Petersen olefination) also proceed readily and regiospecifically to give olefins. When base is used to produce the oxyanion, the elimination occurs with syn stereochemistry. If an acid is used to promote the elimination, it occurs in an anti fashion, leading to the opposite olefin stereochemistry. This is a very useful way to generate either a Z or E olefin from the same starting material.


Similarly phosphine oxide eliminations (Wittig reaction) occur very readily to give olefins.


Both of the latter two methods of elimination are part of a longer sequence of reactions that produce olefins. Initial formation of a single bond to a carbonyl carbon is followed by elimination to an alkene. Thus the alkene is a condensation product of two smaller units. Schematically,


where X is an element (Si or P) which can remove oxygen to the alkene. It should also be noted that both anionic versions of these eliminations proceed with syn stereochemistry between the oxyanion and the heteroatom; thus the stereochemistry of the intermediate dictates the geometry of the olefin product.

Alkenes can also be produced effectively by the reduction of alkynes. The reduction can be carried out stereospecifically to give either cis or trans olefins as desired. This is a very useful method because of the stereocontrol. The P-2 nickel catalyst for the cis hydrogenation is produced in situ by the reduction of Ni[II] acetate with sodium borohydride and the reaction is carried out at atmospheric pressure making this a very simple method for the preparation of cis olefins. The lithium in liquid ammonia reduction of alkynes to the trans olefin is also very straightforward experimentally.



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