Aryne Chemistry Flashcards
What are Arynes?
- Arynes are neutral intermediates derived from aromatic rings by the removal of two substituents, leaving behind two electrons to be distributed between two orbtials
- There are several possibilities shown before for benzyne
What is the main characteristic of this Benzyne
partial triple bond character
The Ortho-benzyne can be considered as a highly strained alkyne - weak
Theoretic calculations support this structure, which bond has the longest and shortest bond length?
Shortness of the short one gives the indication of triple bond character
The generation of Arynes can be formed from the elimination of aryl halides
What is the mechanism and product?
- Arynes are made BUT then rapidly react in subsequent reactions, forming biphenyl
- In the second intermediate you have an anion associated with a lithium cation
- Phenyl lithium works as a base and a nucleophile
The generation of Arynes can be formed from the elimination of aryl halides
What is the mechansim and product?
The generation of Arynes can also be done from Benzedediazonium 2-carboxylate
What is the mechanism and product?
- Thermodynamic driver for this reaction with the potential to lose nitrogen as a gas AND a molecule of carbon dioxide gas
- Remember this is a reactive intermediate so will go on to react with other things OR itself (timerise)
Arynes can also be generated from 2-(trimethylsilyl)phenyl triflate
What is the mechanism and product
- Preferred process due to safe and milder conditions
Arynes can also be generated from oxidative degradation of aminobenzotriazoles
- Quadruple nitrogen bond moiety - see the formation of two dinitrogen gas by-product
- Lead acetate acts as an oxidant
The strained, highly reactive triple bond of arynes is readily attacked by nucleophiles
What is the mechanism?
- Results in the formaton of an anion which attacks an electrophile (similar to lithium example)
The regiochemistry of nucleophilic attack on unsymmetrical arynes depends on the nature of the substituent(s)
Here is an example of an inductive effect through σ-bonds
Where does the carbonanion form?
- Allows for the build up of charge on the carboanion closer to the electron withdrawing group
The regiochemistry of nucleophilic attack on unsymmetrical arynes depends on the nature of the substituent(s)
Here is an example of an inductive effect through σ-bonds
Where does the carbonanion form?
Here is a ‘real’ example with a benzyne containing electron-withdrawing group - nucleophilic addition
What is the mechanism and product
- The nitrogen attacks the position will allows for the build up of negative charge next to the electron-withdrawing group
Here is a ‘real’ example with a benzyne containing electron-donating group - nucleophilic addition
What is the mechanism and product
(note can avoid this product if we replace the methyl with something big - sterics)
Intramolecular nucleophilic additions to arynes are useful for forming rings
What is the mechanism and product?
- Last step involves quenching with water to give cyclobutane
- We have an alkyl group where the anion formed should be further away from this group - EDG
- Forms a 4-MR due to the closeness of that carbon
Arynes readily undergo a vareity of cycloaddition reactions
For example Diels-Alder reactions
What is the mechanism and product?
Arynes readily undergo a vareity of cycloaddition reactions
For example Diels-Alder reactions
What is the mechanism and product?
Arynes readily undergo a vareity of cycloaddition reactions
For example Diels-Alder reactions
What is the mechanism and product?
- Within this structure there is an azodiene which will allow a hetero-Diels-Alder reaction occurs
- Protonation occurs within the workup
- This reaction is quite favourable due to the aromatic nature of the final product
Arynes also react with alkenes to give products of formal [2+2] cycloadditions
These reactions are unlikely to be concerted, as thermal [2+2] cycloadditions are disallowed
What is the mechanism and product?
- The carbanion formed will then trap the oxonium cation formed
- Last step will involve hydrolysing the the dimethyl acetal with acid to form a ketone
Arynes readily undergo 1,3-dipolar cycloadditions
What is the mechanism and the product
Arynes readily undergo 1,3-dipolar cycloadditions
What is the mechanism and the product
