Question

I was reading some enolate chemistry from Carruthers textbook and came across the selective alkylation of unsymmetrical ketones. The reaction involves blocking one alpha-position of the ketone using an enol thioether, as shown below.

I cannot understand the mechanism of the final step and while searching, I ended up with this from Sciencedirect (Ref.1)

Removal of the thioenol ether protecting group gives the aldehyde function, and under the acidic conditions the β-hydroxy group eliminates to give the mainly (E)-unsaturated enal. This reaction also works if the ketone is reduced to a secondary alcohol with LiAlH4 (69% yield).

My questions are:

1. Are enol thioethers the same as thioenol ethers? If not, please draw an example.
2. If they are the same, does the deprotection take place through an aldehyde intermediate as stated above?
3. Also, please explain the mechanism of this deprotection.

Ref. 1: Warren J. Ebenezer, Paul Wight, in Comprehensive Organic Functional Group Transformations, 1995

Ref. 2: Modern Methods of Organic Synthesis 4th edition, page no. 10

Answer 1

Enol thioethers and thioenol ethers are the same class of compounds that function as protected forms of enols. The deprotection of enol thioethers involves the removal of the thioenol ether protecting group, leading to the formation of an aldehyde or ketone. The deprotection mechanism involves attack of the acidic proton by a hydroxide ion, followed by elimination of sulfur and oxygen to generate the aldehyde.

Step-by-step explanation:

Enol thioethers and thioenol ethers refer to the same class of compounds, which are protected forms of enols. An enol thioether is a compound where the oxygen atom of the enol is replaced by a sulfur atom, while a thioenol ether is a compound where the oxygen atom of the enol is attached to a sulfur atom. An example of an enol thioether is acetone enol thioether:

The deprotection of an enol thioether involves the removal of the thioenol ether protecting group and the formation of an aldehyde or a ketone. In the presence of acidic conditions, such as strong acid or acid-catalyzed conditions, the thioenol ether is cleaved, leading to the formation of an aldehyde through an aldehyde intermediate. This process is known as deprotection and typically occurs due to the positive charge on the thioenol ether group attracting the negatively charged hydroxide ion.

The mechanism of deprotection involves the attack of the acidic proton of the thioenol ether by the hydroxide ion, leading to the formation of a sulfide intermediate. This intermediate then undergoes elimination of sulfur and oxygen, resulting in the formation of an aldehyde. The reaction is typically carried out under mild acidic conditions, and the choice of acid can influence the selectivity and yield of the reaction.