# Tag Info

33

tl;dr: I don't think there is any mechanism that is 100% correct, and, in cases like this especially, I think it would completely depend upon what set of carbonyl/ylid/base/solvent etc. was used. But, of course, we like being able to generalise, and to my knowledge theres a lot more evidence to support a concerted type mechanism. General Background The ...

11

Overview of mechanisms There have been recent excellent summaries of the reaction profiles of the Wittig1 and the Corey–Chaykovsky2 reactions. For the purposes of answering this question, there's no need to go into exquisite detail about each individual step (a lot of the research has been poured into understanding stereoselectivities which isn't relevant ...

10

TL;DR: With stabilised ylids, the trans oxaphosphetane is formed, under kinetic control. This is due to minimisation of dipole-dipole repulsions in the transition state for oxaphosphetane formation: There is no equilibrium established between cis and trans oxaphosphetanes. The formation of the trans oxaphosphetane is irreversible, and its collapse leads to ...

9

First, note that with the mechanism you have drawn you should expect to get a mixture of cis and trans isomers. In the intermediate you drew, rotation about the C–C bond will be fast and will lead to loss of any stereochemical information that was previously there. The only way to get exclusively trans-2-butene is for the reaction to occur in a single step ...

8

Because the target structure contains the motif of a cyclohexane-1,2-diol, I suggest an approach including a Diels-Alder reaction (DA in the illustration below) of 1,3-butadiene and the commercially available diethyl acetal of propargyl aldehyde: The hydrolysis of the acetal then sets the stage to install the methyl group with the cuprate and 1,4-...

7

I am fairly sure that for both, there are practicality issues at hand. This is almost certainly so for the choice of phosphine. Even though PPh3 is absolutely terrible from an atom economy point of view, and even though PPh3O can be a real pain to remove after the reaction is done, at least it's a nice white solid which doesn't smell. On the other hand, the ...

6

Think about how the PPh3 first reacts with the epoxide - it does a nucleophilic attack on one of the carbons from below the epoxide ring to give the intermediate shown below left (I have drawn the P substituents as Me for clarity). This then rotates to form the oxaphosphetane (4-membered ring with P-O bond shown right) putting the Me groups cis to each other....

5

The Wittig reaction has been known for decades, but research into its mechanism has continued to throw up surprises even until now. The literature is unfortunately very difficult to read, as numerous hypotheses have been proposed, each having subtle but important differences. Historically, betaines were postulated as the main intermediates in the Wittig ...

3

This works because the acidic starting material is going to be deprotonated as there is excess bicarbonate present. Because of the adjacent carboxylate group, the pKa of the phosphonium species is going to be around 6-7 pKa Table here so within the range where bicarbonate can deprotonate it. The reaction is between the anion of the starting material and the ...

3

Wittig reactions can be performed with all sorts of phosphanes. The resulting phosphonium salts can be deprotonated by all sorts of bases, as long as the base is basic enough. Triphenylphosphane has the advantage that its phosphonium salt can only be deprotonated on one carbon atom: the three ipso-phenyl carbon atoms are quarternary and thus cannot be ...

2

I was wrong, this is not a Wittig reaction. Just posting the answer to let anybody who comes in hunt for this in the near future. On googling for about an hour, I discovered that this is indeed a preparation of phosphonium ylide which is the witting reagent. On adding $\ce{Ph_3P}$ to 1,2-dichloroethane , $\ce{PhLi}$ acts as an catalyst and eliminates \$\...

1

It might not be the only factor, but I think that the steric effect of the huge phenyl groups in Wittig ylides might contribute in making them less nucleophile. If you give a fast look at a "space filling" model of the two molecules, you see that the negatively charged carbon in a Wittig Ylide is much more masked by the phenyl groups Note that in order for ...

1

It seems that the method to circumvent this problem is indeed to use a protecting group to protect one end of the alkyl chain, which is the end to be converted to a bromide after the Wittig reaction. As Waylander aptly mentioned, another problem which comes with using an alkyl dibromide is that there may be an intramolecular substitution reaction, in which ...

1

A good system for this is using DBU as the base in refluxing chloroform. Bull. Chem. Soc. Japan (2003) 76 1675

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