The standard procedure for protecting a carbonyl group as an acetal uses dry acid, such as $\ce{TsOH}$, but when forming a thioacetal, the Lewis acid $\ce{BF3}$ seems to be the catalyst of choice. For example, see Corey and Seebach's original demonstration of dithiane acidity: Org. Synth. 1970, 50, 72.

Is there any particular theoretical or practical reason why this is the case? I am guessing that the boron trifluoride coordinates exclusively to the harder oxygen atom, making it a better leaving group and hence favouring the formation of products (since the reaction is under thermodynamic control). However, I don't see any reason why acid wouldn't be able to do the job equally well.


The pKa/leaving group ability between alcohols and thiols may have something to do with it.

Alcohol pKa around 17, thiol around 13.

As you pointed out the whole reaction is in equilibrium. But in the case of the thioacetal, after the initial addition theres a strong driving force to kick back out the sulfur to reform the carbonyl rather than committing to the thioacetyl formation since the sulfur is a relatively stable leaving group.

Protic acids are almost always wet, and as such the water would prevent the reaction, whereas Lewis acids are generally either neat, or in ethereal solutions.


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