Org Prep Daily

April 1, 2013

It curdles if you don’t stir it

Filed under: mechanisms — milkshake @ 5:07 pm


Trityl group on sulfur is unstable to LiAlH4 reduction. It falls off as triphenylmethyl anion – that’s where the gorgeous blood-red color is coming from. (Unlike trityl cation, which is canary yellow). I did not know about this S-trityl instability – my Greene book (3rd edition) for example mentions only the electrochemical reduction at highly negative potentials – and so I presume it is not as widely known.

In my hands, sulfur de-tritylation with LAH happens both with primary and secondary thiols protected as trityl thioethers. The rate of trityl loss seems structure-dependent: metal coordinating groups (such as OH, amino) in the vicinity of sulfur accelerate the LAH-promoted de-tritylation to a point that it cannot be avoided even under mild reaction conditions. In such cases all that remains to be done is completing the de-tritylation by overnight reflux and isolating the free-thiol product from the Al basic salt cake after the usual Fieser workup. The thiol actually ends up stuck within the salt cake as a thiolate; the filtrates contain only triphenylmethane.

Trityl-S group seems to be reasonably stable to borane-THF at room temperature.

Update: Trityl-O reductive cleavage to trityl anion is also facile, it takes place at room temperature with potassium naphthalenide solution in THF


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