The addition of carbon chains to molecules is important when synthesizing compounds in organic chemistry. One such synthetic reaction is the Wittig reaction.
In the Wittig reaction, a carbonyl group (ketone) or a formyl group (aldehyde) can be converted to alkenes. This means that C=O can be converted to C=C. A frequently used synthetic reaction to create carbon chains is the Wittig reaction.
So how can we perform the synthesis of a carbonyl compound to a double bond? What happens to EZ selectivity when making alkenes?
Without understanding the reaction conditions and the reaction mechanism, we cannot obtain the desired compound. So, we will explain how to use the Wittig reaction.
Table of Contents
- 1 Wittig Reaction, the Most Important Reaction in Alkene Synthesis
- 2 What IS the Ylide of the Wittig Reagent? Stable and Unstable Ylides
- 3 Applications of the Horner-Wadsworth-Emmons Reaction.
- 4 Wittig Reaction, the Most Important Reaction in Alkene Synthesis
Wittig Reaction, the Most Important Reaction in Alkene Synthesis
The famous synthetic reaction to create a double bond is the E2 reaction (elimination reaction). However, the conditions for the E2 reaction are extreme, such as reacting under strong base and high temperature conditions. In addition, the EZ isomer (cis-trans) is a problem.
In addition, the E2 reaction may cause a nucleophilic substitution, resulting in the formation of by-products.
In contrast, the Wittig reaction is selective for C=O and can synthesize a double bond. In addition, the reaction conditions are moderate and the EZ isomer can be controlled. Therefore, the Wittig reaction is the most important synthetic reaction in alkene synthesis.
If you want to synthesize a double-bonded carbon chain, the Wittig reaction is the first reaction to consider.
Alkenes Can Be Synthesized from Ketones and Aldehydes
There are different types of carbonyl compounds. Among them, the Wittig reaction can be performed with molecules containing carbonyl (ketone) and formyl (aldehyde) groups.
The Wittig reaction utilizes organophosphorus compounds. The organophosphorus compounds used in the Wittig reaction are called phosphonium ylides (Wittig reagents).
The Wittig reagent can be synthesized by reacting triphenylphosphine with alkyl halides and then processing the reaction under basic conditions. The reaction mechanism for the synthesis of the Wittig reagent is as follows
With the use of phosphonium ylide (Wittig reagent), ketones or aldehydes can be reacted to yield alkenes as follows.
The use of a special reagent, phosphonium ylide, makes it possible to synthesize alkenes.
The Wittig reaction is a synthetic method discovered by Georg Wittig, who won the Nobel Prize in Chemistry in 1979 for his work on organic synthesis using organophosphorus.
The Reaction Mechanism is Nucleophilic Addition of Phosphorus Ylides
So how does the Wittig reaction work? The phosphorus ylides attack the carbonyl carbon by nucleophilic addition.
The reaction mechanism is as follows.
After phosphorus ylides nucleophilic attack, the negatively charged oxygen atom attacks the phosphorus atom. This results in the formation of a four-membered ring intermediate. This intermediate is called oxaphosphetane.
Oxaphosphetane intermediates are highly unstable compounds. Therefore, after the oxaphosphetane intermediate is synthesized, an alkene can be obtained through a elimination reaction.
An important point is the fact that the P=O bond is very strong. After the oxaphosphetane intermediate is formed, the alkene is synthesized because the elimination reaction proceeds in a way that creates a P=O bond.
What IS the Ylide of the Wittig Reagent? Stable and Unstable Ylides
The Wittig reaction uses a special reagent called Wittig reagent. Naturally, depending on the type of phosphonium ylide (Wittig reagent) used, the compounds obtained are different.
What are ylides? Ylides are molecules in which a negatively charged carbon atom and a positively charged heteroatom (an atom other than carbon or hydrogen) exist next to each other. The most frequently used of the ylides are phosphorus ylides.
Phosphonium ylides (Wittig reagents) are type of phosphorus ylides.
-Stable and Unstable Ylides
When making Wittig reagents, it is important to understand stable and unstable ylides. There are two types of phosphorus ylides, stable and unstable ylides. In stable ylides, as the name implies, they are in a stable state. In unstable ylides, on the other hand, they exist in an unstable state.
When do they become stable ylides? This is when an electron withdrawing group is present next to a carbon atom that has a negative charge. The following functional groups are known to be electron withdrawing groups, for example.
- Nitro group (-NO2)
- Cyano group (-CN)
- Carbonyl group (-CO)
- Sulfur group (-SO3H)
Why does the presence of these functional groups make it a stable ylide? This is because we can write the following resonance structure.
Stable ylides can be isolated. The presence of electron withdrawing groups makes it possible to produce the Wittig reagent as a compound in a highly stable state.
Unstable ylides, on the other hand, do not have electron withdrawing groups like stable ylides have. In this case, it is impossible to isolate them as they decompose on contact with water and air.
Stable ylides can be isolated and have poor reactivity as ylides. On the other hand, unstable ylides cannot be isolated, but they have excellent reactivity as ylides. In any case, it is important to understand that there are stable and unstable ylides.
Possible to Control the Stereochemistry of Cis-Trans (EZ compounds)
So why is it important to understand stable and unstable ylides? It’s because we can control stereochemistry.
When you synthesize alkenes, there is always a problem with geometric isomers. The problem of cis-trans arises. So we have to synthesize EZ isomers so that we can selectively obtain either E or Z forms.
In the Wittig reaction, depending on whether the phosphonium ylides used are stable or unstable ylides, the compound to be synthesized will be either E or Z-form. Specifically, there are the following differences between the Wittig reagents.
- Stable ylides: the product is E-form (trans)
- Unstable ylides: the product is Z-form (cis)
In general alkene synthesis, it is difficult to synthesize EZ isomers in a distinct way. On the other hand, Wittig reaction allows us to synthesize these isomers separately.
Why the EZ Isomer Can Be Synthesized Differently
Why is it possible to separate the synthesis of EZ isomers depending on the stability of the ylides? The reason for this is explained by differences in steric hindrances and reversible reactions.
In stable ylides, the reaction in which the phosphorus atom attacks the carbonyl carbon to form oxaphosphetane is a reversible reaction. Therefore, when considering the stability of intermediates, steric hindrance occurs in the cis (Z-form). In the trans (E-form), the synthetic reaction proceeds in an E-selective manner in the stable ylides because a larger substituent exists on the opposite side.
On the other hand, when an unstable ylide attacks a carbonyl carbon, the oxaphosphetane produced is an irreversible reaction. Therefore, after the formation of oxaphosphetane, it is not converted to an arrangement with less steric hindrance (trans) by the reversible reaction.
The synthetic reaction proceeds as the first oxaphosphetane formed. In this case, in order to react with the lowest steric hindrance, it is necessary to pass through the intermediate of the four-membered ring as shown in the figure below.
When oxaphosphetane is synthesized in this way to reduce steric hindrance, it yields a Z isomer (cis) intermediate. It is not converted to an E isomer (trans) after forming the intermediate, as is the case with stable ylides.
The difference in whether an oxaphosphetane intermediate becomes a syn-form or an anti-form is explained by whether it is a reversible or irreversible reaction.
Applications of the Horner-Wadsworth-Emmons Reaction.
Double bond formation by the Wittig reaction is very useful and has many applications. Among them, the Horner-Wadsworth-Emmons reaction is known as a synthetic reaction that is an advanced version of the Wittig reaction.
The Horner-Wadsworth-Emmons reaction is a synthetic reaction that uses phosphate esters as reagents as organophosphorus compounds.
In addition to esters, it can also be used with other functional groups such as amines, ethers and allies. This makes it a highly useful synthetic reaction.
In the Horner-Wadsworth-Emmons reaction, the synthetic reaction proceeds in an E-selective manner. However, with special reagents, the reaction is known to be Z-selective. It is important to note that the cis-trans stereochemistry in the Horner-Wadsworth-Emmons reaction can be controlled in the same way as in the Wittig reaction.
Since there are other applications of the Wittig reaction with organophosphorus compounds, learning these synthetic reactions will enable us to synthesize a wide range of compounds.
Wittig Reaction, the Most Important Reaction in Alkene Synthesis
When considering the synthesis of compounds with double bonds, the first reaction to be aware of is the Wittig reaction. As an organic chemistry reaction to create carbon chains, the Wittig reaction is beneficial.
In alkene synthesis, geometric isomers are always a problem. In the Wittig reaction, however, selective synthesis of EZ isomers is possible by changing the phosphonium ylides used. The stable ylides will give E-form (trans) compounds and the unstable ylides will give Z-form (cis) compounds.
In addition, the reaction is selective for carbonyl groups and produces fewer by-products than other synthetic reactions. Many applications of the Wittig reaction have been reported and it is possible to synthesize a wide variety of compounds.
One of the synthetic reactions that many organic chemists use in the name reaction is the Wittig reaction. When you study organic chemistry in college, almost everyone learns the Wittig reaction, so make sure you understand what kind of synthetic reaction it is.