First of all, let me apologize for the font size issue…I have no idea what’s going on, but I am trying to figure it out. Secondly, this is my fiftieth post! MGW has come a long way since post #1…maybe I’ll reminisce or something in the near future. “We’ll see what happens,” as I always say when I’m probably never going to make something happen. 🙂
The real point of this post is to report on this intriguing paper. At stake is the proper mechanism for the alkenyl shift/aromatic substitution shown, which was discovered by Oshima et al. in 2004. The authors postulate three potential mechanisms. Either the shift and aryl attack are:
(2) Stepwise in which the alkenyl shift is the rate-determining step
(3) Stepwise in which the aryl attack is the rate-determining step
Which is it?! In fact, using Gaussian calculations the authors found that, for every X they explored, the reaction was stepwise and proceeded through an intermediate that looks at first glance like a typical tertiary carbocation. However, on closer inspection one can see that the intermediate isn’t quite typical: its positive charge is adjacent to the alkenyl’s giant pi cloud of electrons! It’s got no problem waiting around for the moderately nucleophilic aryl functionality to attack. Not surprisingly, electron-donating X’s increase the rate of the reaction, while electron-withdrawing X’s have the opposite effect.
Some really neat physical chemistry here…the paper has more.