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.
|Published (Last):||11 July 2008|
|PDF File Size:||10.57 Mb|
|ePub File Size:||16.88 Mb|
|Price:||Free* [*Free Regsitration Required]|
Recall from section Notice that the enzyme specifically takes the pro-R proton in this step. Alkene isomerization Recall from section It is adiicon accurate to use the more general principle that has already been stated above: This rule of thumb is known as Markovnikov’s ruleafter the Russian chemist Vladimir Markovnikov who proposed it in To continue the chain elongation process, another IPP molecule can then condense, in a very similar reaction, electrofilca C 1 of geranyl diphosphate to form a carbon product called farnesyl diphosphate FPP.
That reaction proceeded through a negatively-charged, enolate intermediate.
These results strongly suggest indicate the formation of a carbocation intermediate in an S N 1-like displacement. Conjugated alkenes can undergo 1,2 or 1,4 electrophilic addition:.
Use a Grignard reaction and a tetrahydropyran protecting group. 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. While it is useful in many cases, Markovikov’s rule does not apply to all possible electrophilic additions. The electrophile is an allylic tertiary carbocation that is generated by the departure of pyrophosphate, an excellent leaving group.
Synthetic parallel – electrophilic additions in the laboratory We have already seen one common laboratory electrophilic addition reaction: First of all, recall that the question of whether a substitution is dissociative S N 1-like or associative S N 2-like is not always clear-cut – it could be somewhere in between, like the protein prenyltransferase reaction section 9.
Additional evidence that the first step is a carbocation-intermediate adiciom addition was provided by an experiment in which the reaction was run in D 2 O with PEP and adicino deoxy analog of S3P Acc. However, attack by the pi electrons does not occur directly at the carbon that was bound to the diphosphate, but instead at a carbon two positions away which bears some of the positive charge due to resonance.
This process has intrigued and perplexed enzymologists for decades, and is thought to proceed via a short-lived tetrahedral intermediate that is the result of acicion electrophilic addition of S3P to PEP.
Show a mechanism for the deprotection step the last step in the reaction above. Because the pi electrons in this case are attacking an aldehyde carbonyl electofilica 1this can be thought of as an electrophilic version of a carbonyl addition chapter 11 reaction.
The product is alpha-terpineol, a component in sap from pine trees. In the elimination phase, elechrofilica abstraction leads to re-establishment of a double bond in the GPP product. The two electrons shared in this pi bond are, on average, further away from the carbon nuclei than the sigma-bonded electrons, and thus are held less tightly.
File:Electrophilic addition hydron mechanism 2nd step.png
The key to understanding electrophilic reactions is to consider aficion the geometry of alkene groups and the orbitals that overlap to form the pi bond. Now, imagine that an electrophile approaches an alkene.
In the acid-catalyzed electrophilic addition of electroilica to dihydropyran Electrofiliczthe 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 resonance electron-donating effects of the adjacent oxygen. So it makes perfect sense that the chain elongation reaction should more S N 1-like than S N 2-like. Rather, addition of the S3P nucleophile at C2 implies an electrophilic carbocation intermediate mechanism, with protonation occurring prior to nucleophilic attack by the S3P hydroxyl:.
The next reaction is adiicion good example of an intra molecular electrophilic addition: In the next step of isoprenoid biosynthesis, the two five-carbon elctrofilica condense to form a carbon isoprenoid product called geranyl diphosphate GPP. The first step is ionization of the electrophile – in other words, the leaving group departs and a carbocation intermediate is formed.
The addition is completed by attack of a water nucleophile step 2which then collapses into a carbonyl step 3driving off the phosphate. Electrophilic addition reactions – the general picture In electrophilic reactions, pi-bonded electrons act as bases and nucleophiles.
File:Electrophilic addition 3-center – Wikimedia Commons
The alkyne can now be abstracted with a strong base, then used to form a new carbon-carbon bond. Consider the following electgofilica 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 adixion by highly electron-withdrawing fluorines. If this happens, a new sigma bond is formed between one of the carbons and the electrophile and the carbon, which was sp 2 -hybridized when it was part of the alkene, is now sp 3 -hybridized.
In the condensation addition electeofilica, the C 3 -C 4 double bond in IPP attacks the positively-charged C 1 of DMAPP, resulting in a new carbon-carbon bond and a second carbocation intermediate, this time at a tertiary carbon. Predict the product of the following reaction: The following has nothing to do with electrophilic addition, but while we are on the subject of protecting groups for alcohols it is worthwhile to mention tert -butyldimethylsilyl TBDMS ethers.
Notice another point about the regiochemical course of the reaction: In the process of isoprenoid chain construction, isopentenyl diphosphate IPPwhich is the essential ‘building block’ for all isoprenoid moleculesis first isomerized to dimethylallyl diphosphate DMAPP by an enzyme called ‘IPP isomerase’.