College Chemistry Carbocation Cinch

Now here's a paper that reiterates one simple freshman organic chemistry principle; tertiary carbocations are more stable than secondary carbocations. Of course, that need not be the case with enzymes at all, but in this case Tantillo's theoretical results indicate that that may be the case (DFT studies using the popular mpW1PW91 functional). He finds that the biasobolyl cation which traditionally goes through a sec carbocation intermediate to the terpene trichodiene, actually follows a lower energy pathway if it involves all tert carbocations. But he ends up proposing a novel proton transfer and a "temporary methyl shift", that is a colour different from all those hydride transfers that we learn in terpene biosynthesis. It would be interesting to investigate experimentally what happens in the active site; enzymes can stabilize prim and sec cations through cation-pi interactions for example.

Org. Lett.; (Letter); 2006; ASAP Article; DOI: 10.1021/ol061884f



Incidentally, the muddle of terpene pathways in the paper brings back fond memories of a master's level course on the topic which a really great, senior professor had taught us. It was actually fun deciphering painfully, the various ways involving methyl, hydride, and proton shifts and cyclizations which could get you from an intermediate to the product in those terpene biosyntheses.