Formation of thioacetals from aldehydes and ketones

by James

Description: In analogy to the formation of acetals from alcohols and acid, treatment of aldehydes or ketones with thiols in the presence of an acid produces a thioacetal.
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Notes: A common Lewis acid for this process is boron trifluoride (BF3).

Examples:

Notes: BF3 (or H+) promotes this reaction. The byproduct here is water.

Mechanism: The reaction is similar in essentially all respects to acetal formation with alcohols.

The reaction begins by addition of acid to the carbonyl oxygen (Step 1, arrows A and B) which makes the carbonyl carbon a better electrophile. Then, one equivalent of the thiol attacks the carbonyl carbon in a 1,2-addition reaction (Step 2, arrows C and D) followed by a migration of a proton to the OH group (Step 3, arrows E and f). This event creates OH2(+), a superior leaving group to OH(–), setting up 1,2-elimination of water (Step 4, arrows G and H) followed by another 1,2-addition of the second thiol (Step 5, arrows I and J). All that remains is to deprotonate the resulting thioaetal (Step 6, arrow K).

Notes: There are other reasonable mechanisms for proton transfer in step 3. Furthermore there are several other species that could act as bases in step 6. TsO(–) is chosen for simplicity. Although each step is reversible, single arrows are shown here for simplicity.

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