Question

Must Friedel-Crafts acylations be protected from atmospheric moisture? I didn't think so either, but apparently these researchers [http://pubs.acs.org/doi/pdf/10.1021/ol007056i] did what appears to be a typical acylation under argon. Is this to prevent hydration of the lewis acid or is there some unforeseen reaction with atmospheric contaminants that is being prevented here? It would seem like the use of inert gases in academic papers is simply a precaution, but I would like to verify if this is the case.

Answer

While searching for information specifically on whether or not Friedel-Crafts acylations are affected by exposure to the atmosphere I could find nothing stating that they specifically were sensitive to such things.  In academia as you probably know, protection from the atmosphere is somewhat rigorous to help exclude atmospheric effects from having an impact on scientific research.  Additionally, in industry the same practices are in place for the additional reason of inerting the atmosphere to prevent static discharges and the like from preventing mixtures from catching fire.

Atmospheric moisture can react with your Lewis acid and hydrolyze it, it could also react with your carbocation intermediate creating a carboxylic acid and your hydrogen halide.  However, unless you are constantly drawing air over your mixture you are only going to react with the moisture already in there and unless it is a really humid day then it should be minimal, at least not enough to be extremely detrimental to your yield.  Also, in this reaction your Lewis acid is present in roughly equivalent quantities, since it is not catalytic entirely then you are not killing your catalyst.  If this were a radical reaction then the reaction could terminate on your atmospheric oxygen or water and it would be enough to kill it potentially.  But since this is not a reaction that worry is unfounded.

Initially I wondered about the carbocation reacting directly with atmospheric oxygen, these types of reaction are usually radical initiated (like the reaction between atmospheric O₂ and Grignards) however it looks like it is not something you should be worried about.

If you wanted to be prudent about it you could add additional Lewis acid to mop up the water that you might worry about.  In this case the inerting seems to be more of a safety/control mechanism than something that is essential to preserving the reaction.