Estabilidad de los alquenos. Electrofilo – Nucleofilo. Flechas curvas dirección. Adición electrofilica en alquenos. Procedimiento Adición Nucleofílica Enolizaciones Parte Experimental #1. Parte Experimental #2. Parte Experimental #3. Adición Electrofílica. juanvict. Guía de adición nucleofílica. qcaorg1. Ejemplos de reacciones de sustitución nucleofílica alifática. Rodolfo Alvarez Manzo. Aromaticos.
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In the acid-catalyzed electrophilic addition of methanol to dihydropyran DHPthe methoxy group adds to carbon 1 rather than carbon 2, because the intermediate with a positive charge on carbon 1 can be stabilized by the electrlfilica electron-donating effects of the adjacent oxygen.
Sección 15.4: Otro tipo de adición-eliminación electrofílica: Shikimato a corismato
The product is alpha-terpineol, a component in sap from pine trees. Consider the following hypothetical reaction, which is similar to the HBr addition shown above except that the six methyl hydrogens on the left side of the double bond have been replaced by highly electron-withdrawing fluorines. If the mechanism is S N 2-like, the fluorine substitutions should not have a noticeable effect, because a carbocation intermediate would not be formed.
Because of the symmetry in the immediate vicinity on either side of the double bond, there is not a large energy difference between the two possible carbocation intermediates that could form. This chemical step is part the pathway by which some bacteria -including those that cause tuberculosis and leprosy – form distinctive branched-chain fatty acids for incorporation into their cell walls J.
The electrophile is an allylic tertiary carbocation that is generated by the departure of pyrophosphate, an excellent leaving group.
Notice that the enzyme specifically takes the pro-R proton in this step. Notice another point about the regiochemical course of the reaction: Of course, the two reaction courses involve two different carbocation intermediates, which may have different energy levels. Enzymatic electrophilic additions Enzymatic electrophilic additions are, like virtually all enzymatic reactions, highly regiospecific, a result of the precise architecture of the enzyme active site. The unhybridized p orbitals on the two alkene carbons overlap, in a side-by-side fashion, to form the pi bond, which protrudes above and below the plane formed by the sigma bonds.
Predict the product of adocion following reaction:. In order to form only the desired product, the enzyme must stabilize one carbocation intermediate over the other, but exactly how this is accomplished is not yet clear. In most cases, biochemical pathways have evolved in such a way that electrophilic addition reactions to asymmetrical alkenes proceed through the more stable of the two possible carbocation intermediates, which of course makes the enzyme’s job easier it doesn’t have to ‘force’ the formation of the inherently less stable intermediate.
In the same sense, it is easy to see how an electron-rich enolate carbon is nucleophilic in the context of aldol and Claisen condensation reactions. Two different regiochemical outcomes are possible:. A fascinating example of the richness of electrophilic mechanisms in biochemical pathways is found in the conversion of skikimatephosphate S3P and phosphoenolpyruvate PEP to chorismate, a key phase of aromatic amino acid biosynthesis in plants and bacteria.
But the pi bonds of alkenes? The electrons in a pi elevtrofilica, in contrast, are only weakly nucleophilic, and thus need to be pulled in by a powerful electrophile – ie.
DMAPP ratio of about 6: Enzymatic electrophilic additions are, like virtually all enzymatic reactions, highly regiospecific, a result of the precise architecture of the enzyme active site. The carbocation that does form is clearly the more stable of the two, due mainly the electron-donating resonance effect of the adjacent phosphate oxygen.
File:Electrophilic addition hydron mechanism 2nd step.png
As a result, the predominant product is the secondary rather than the tertiary bromoalkane. When the reaction was stopped midway by denaturing the enzyme with the mild base triethylamine, the intermediate was actually stable enough to be isolated and directly characterized by NMR J.
The alkene substrate in this reaction PEP is asymmetrical electrifilica, meaning that two carbocation intermediates could potentially form. Glyphosate is a potent inhibitor of EPSP synthase, and thus plants exposed to it die because they are not able to synthesize aromatic amino acids.
These electrons are not free, they are already involved in a bond! An alternate regiochemical course could result in a seven-membered ring and a secondary carbocation, a much less energetically favorable intermediate in terms of both carbocation stability and ring size recall that six-membered rings are lower in energy then seven-membered rings. In this case, protonation of the substrate occurs first, followed by deprotonization the opposite order of the enolate mechanism.
ExampleEnd The following has nothing to do with electrophilic addition, but while we are on electrofiliac subject of protecting groups for alcohols it is worthwhile to mention tert -butyldimethylsilyl TBDMS ethers. First of all, given that the addition is taking place on a double bond that is conjugated to a carbonyl group, why is this not characterized as a Michael carbanion intermediate addition?
Show a mechanism for the deprotection step the last step in the reaction above. Also notice that this is an anti addition to the double bond. Conjugated alkenes can undergo 1,2 or 1,4 electrophilic addition:.
File:Electrophilic addition hydron mechanism 2nd – Wikimedia Commons
In the lab, this reaction is very useful because it serves as a method for ‘protecting’ an alcohol group while reactions are carried out elsewhere on the molecule under strongly basic conditions.
In the next step of isoprenoid biosynthesis, the two five-carbon isomers condense to form a carbon isoprenoid product called geranyl diphosphate GPP. If the reaction is S N 1-like, the electron-withdrawing fluorines should destabilize the allylic carbocation intermediate and thus slow the reaction down considerably.
In this reaction, the methyl group of SAM is the electrophile which attracts the pi electrons of a double bond in an unsaturated fatty acid. When an asymmetrical alkene undergoes electrophilic addition, the product that predominates is the one that results from the more stable of the electrofilia possible carbocation intermediates.
This is a good example of a non-enzymatic organic reaction that is highly regiospecific.