Question

What happens when acetone is treated with alpha-bromomethylacetate in presence of zinc. Give the reaction and mechanism.

Answer 1

Answer:

The Reformatsky reaction usually commences with the oxidative addition or insertion of the zinc into the carbon-halogen bond of α-haloester. The main purpose of using zinc is that it allows the generation of an enolate even without using Bronsted base which normally condenses with the ketone or aldehyde itself.

Answer 2

The Reformatsky reaction (sometimes spelled Reformatskii reaction) is an organic reaction which condenses aldehydes or ketones, with α-halo esters, using a metallic zinc to form β-hydroxy-esters:[1][2]

Reformatsky reaction

Named after

Sergey Reformatsky

Reaction type

Coupling reaction

Identifiers

Organic Chemistry Portal

reformatsky-reaction

RSC ontology ID

RXNO:0000036

The Reformatsky reaction

The organozinc reagent, also called a 'Reformatsky enolate', is prepared by treating an alpha-halo ester with zinc dust. Reformatsky enolates are less reactive than lithium enolates or Grignard reagents and hence nucleophilic addition to the ester group does not occur. The reaction was discovered by Sergey Nikolaevich Reformatsky.

Some reviews have been published.[3][4]

Contents

Structure of the reagent Edit

The crystal structures of the THF complexes of the Reformatsky reagents tert-butyl bromozincacetate[5] and ethyl bromozincacetate[6] have been determined. Both form cyclic eight-membered dimers in the solid state, but differ in stereochemistry: the eight-membered ring in the ethyl derivative adopts a tub-shaped conformation and has cis bromo groups and cis THF ligands, whereas in the tert-butyl derivative, the ring is in a chair form and the bromo groups and THF ligands are trans.

Ethyl-bromozincacetate-from-xtal-3D-sticks-C.png

Ethyl-bromozincacetate-THF-dimer-from-xtal-2D-skeletal-D.png

Tert-butyl-bromozincacetate-from-xtal-3D-sticks-C.png

Tert-butyl-bromozincacetate-THF-dimer-from-xtal-2D-skeletal-D.png

ethyl bromozincacetate dimer

tert-butyl bromozincacetate dimer

Reaction mechanism Edit

Zinc metal is inserted into the carbon-halogen bond of the α-haloester by oxidative addition 1. This compound dimerizes and rearranges to form two zinc enolates 2. The oxygen on an aldehyde or ketone coordinates to the zinc to form the six-member chair like transition state 3. A rearrangement occurs in which zinc switches to the aldehyde or ketone oxygen and a carbon-carbon bond is formed 4. Acid workup 5,6 removes zinc to yield zinc(II) salts and a β-hydroxy-ester 7.[7]