1,n dicarbonyl chemistry

Question 1

Why can’t 1,4 species be formed through a propanone and enone mixture

Answer 1

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

Question 2

Examples of reagents used to form 1,4 species

Answer 2

Alpha-halocarbonyls, Epoxides and Nitro compounds

Question 3

Outline the use and issues with uses of alpha-halocarbonyls

Answer 3

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

Question 4

Outline the mechanism of reaction between a stabilised anion and an alpha-halocarbonyl

Answer 4

Check notes

Question 5

Uses of alpha-bromo aldehydes

Answer 5

They are too reactive to be useful. More useful to form the ester and then decarboxylate

Question 6

Why don’t lithium esters work well with alpha-halocarbonyls?

Answer 6

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

Question 7

Product of the reaction between epoxides and a specific lithium enolate

Answer 7

1,4 hydroxy carbonyl

Question 8

Outline the mechanism for the formation of a 1,4 carbonyl species using an epoxide

Answer 8

Check notes

Question 9

Benefits of using nitrocompounds and a con

Answer 9

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

Question 10

Outline the mechanism between a nitro group an enone to form a 1,4 species

Answer 10

Check notes. Uses a mild base (EtOH). Acidic workup

Question 11

Reactions for conversion from nitro to carbonyl groups

Answer 11

Nef and McMurray

Question 12

Outline the Nef reaction and mechanism

Answer 12

2 stages. First strong base, then aqueous acid. Check notes for mechanism

Question 13

Why was the Nef reaction replaced by McMurray/TiCl3 reaction

Answer 13

Low yield issues

Question 14

Outline the mechanism for the McMurray reaction

Answer 14

Check notes

Question 15

Outline the product and mechanism and conditions of benzoin condensation

Answer 15

Forms benzoin (1,2 phenyl hydroxy carbonyl species). Reagent is CN- and ketone/aldehyde. Involves initial nucleophilic attack and then self condensation reaction

Question 16

What are tautomers

Answer 16

Species with the same molecular formula but different connectivity that can rapidly interconvert in equilibrium with each other

Question 17

Key stage in the benzoin condensation

Answer 17

The interconversion between tautomer’s of the initial tetrahedral intermediate

Question 18

Draw the other nucleophile that can be used during benzoin condensation

Answer 18

Thonzonium salts

Question 19

Why does the enamine react as supposed to the enone in the intermediate of the alternate benzoin mechanism

Answer 19

The lone pair on nitrogen is more readily available and so the enamine collapses first to react with the electrophile

Question 20

What does the acyloin condensation reaction produce

Answer 20

1,2 hydroxy carbonyl species with any substituents R from any ester

Question 21

Reagents of the acyloin reaction

Answer 21

Na, MeSiCl3, xylene. Acid workup

Question 22

What is special about xylene as a solvent?

Answer 22

The melting point is very high - higher than the melting point of sodium

Question 23

Possible side reactions of acyloin condensation

Answer 23

The reaction releases methoxide which is a good base for forming enolates. If the R group can enolise, side reactions will occur.

Question 24

How are side reactions minimised during acyloin condensation

Answer 24

MeSiCl3 captures the methoxide given off

Question 25

Outline dihydroxylation

Answer 25

Formation of a dihydroxyl from an alkene using osmium tetroxide (OsO4)

Question 26

What is the stereochemistry of a dihydroxylation product

Answer 26

The two OH groups are always both in the same direction (either both forwards or both backwards). Product is a racemic mixture

Question 27

Outline the pinacol reaction

Answer 27

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)

Question 28

Outline the reagents of Darzens reaction

Answer 28

Alpha-chloro ester, cyclonone and butoxide

Question 29

Outline the mechanism for the Darzens reaction

Answer 29

Check notes