1,n dicarbonyl chemistry Flashcards
1,4 1,2 and 1,6 relationships
Why can’t 1,4 species be formed through a propanone and enone mixture
In theory, the propanone forms the enolate, and then attacks the enone however, in mixture, both the enone and the ketone are electrophilic, and so both could be attacked by the other
Examples of reagents used to form 1,4 species
Alpha-halocarbonyls, Epoxides and Nitro compounds
Outline the use and issues with uses of alpha-halocarbonyls
Used to make the alpha carbon of the ketone electrophilic which can then be attacked by a stabilised anion. There is a high potential for unwanted reactions due to their reactivity, often inducing electrophilicity of the ketone carbonyl
Outline the mechanism of reaction between a stabilised anion and an alpha-halocarbonyl
Check notes
Uses of alpha-bromo aldehydes
They are too reactive to be useful. More useful to form the ester and then decarboxylate
Why don’t lithium esters work well with alpha-halocarbonyls?
The basic conditions used to form lithium enolates makes them particularly basic themselves. The alpha proton of the halocarbonyl is too acidic for these basic conditions
Product of the reaction between epoxides and a specific lithium enolate
1,4 hydroxy carbonyl
Outline the mechanism for the formation of a 1,4 carbonyl species using an epoxide
Check notes
Benefits of using nitrocompounds and a con
They are very good at stabilising charge and are very effective as acidifying the alpha proton. The nitro group has to be converted to a carbonyl afterwards
Outline the mechanism between a nitro group an enone to form a 1,4 species
Check notes. Uses a mild base (EtOH). Acidic workup
Reactions for conversion from nitro to carbonyl groups
Nef and McMurray
Outline the Nef reaction and mechanism
2 stages. First strong base, then aqueous acid. Check notes for mechanism
Why was the Nef reaction replaced by McMurray/TiCl3 reaction
Low yield issues
Outline the mechanism for the McMurray reaction
Check notes
Outline the product and mechanism and conditions of benzoin condensation
Forms benzoin (1,2 phenyl hydroxy carbonyl species). Reagent is CN- and ketone/aldehyde. Involves initial nucleophilic attack and then self condensation reaction
What are tautomers
Species with the same molecular formula but different connectivity that can rapidly interconvert in equilibrium with each other
Key stage in the benzoin condensation
The interconversion between tautomer’s of the initial tetrahedral intermediate
Draw the other nucleophile that can be used during benzoin condensation
Thonzonium salts
Why does the enamine react as supposed to the enone in the intermediate of the alternate benzoin mechanism
The lone pair on nitrogen is more readily available and so the enamine collapses first to react with the electrophile
What does the acyloin condensation reaction produce
1,2 hydroxy carbonyl species with any substituents R from any ester
Reagents of the acyloin reaction
Na, MeSiCl3, xylene. Acid workup
What is special about xylene as a solvent?
The melting point is very high - higher than the melting point of sodium
Possible side reactions of acyloin condensation
The reaction releases methoxide which is a good base for forming enolates. If the R group can enolise, side reactions will occur.
How are side reactions minimised during acyloin condensation
MeSiCl3 captures the methoxide given off
Outline dihydroxylation
Formation of a dihydroxyl from an alkene using osmium tetroxide (OsO4)
What is the stereochemistry of a dihydroxylation product
The two OH groups are always both in the same direction (either both forwards or both backwards). Product is a racemic mixture
Outline the pinacol reaction
Uses Mg (instead of Na in acyloin) as a electron source to produce radical intermediates. The product is a 1,2 dicarbonyl unless worked up in acid (then dihydroxy)
Outline the reagents of Darzens reaction
Alpha-chloro ester, cyclonone and butoxide
Outline the mechanism for the Darzens reaction
Check notes
