I have always been interested in how intermediates with supposedly well-understood behaviour take an occasional detour. At the moment, I am collecting papers of this kind with the goal of writing a review article at some point. There is a lot to talk about here, particularly because the “roads less travelled” are often influenced by subtle structural changes in reaction components. The trouble is that it is not easy to find these cases. Let me illustrate what I mean by using a process you all probably know – the Staudinger reduction of azides. The example below is not new, but is nonetheless instructive because it shows a rare departure from the accepted reaction course. Depending on the group on the phosphorus centre, the reduction of the indole-derived azide proceeds to the well expected iminophosphorane outcome or to the less common triazine heterocycle, whose ring system features three contiguous nitrogen atoms. The mechanism is fascinating, especially if you are programmed to see nitrogen extrusion whenever phosphorus meets azide. I thought this reduction was largely predictable, but I was wrong. Let me know if you are aware of other interesting cases.
http://www.sciencedirect.com/science/article/pii/S0040402000003227
amphoteros 
I have seen phosphazides in a medchem project about 10 years ago: I needed to prepare N-trityl ketimine from alpha- substituded cyclic ketone. A combination of Ph3C-N3 and Me3P instantly produces an adduct that is stable and isolable; it liberated N2 only after the addition of ketone. I think steric and electronic effects keep Trityl-N(-)-N=N-PMe3(+) quite happy at room temperature, and N2 extrusion happened through 6-membered ring TS with ketone.
This is very interesting, I bet the size of trityl is making its presence felt here…
A very interesting and very challenging review! How do you even conduct a search about the things that “didn’t exactly go according to textbooks”?
I’m interested in doing a similar review in comp.chem. but I’m not sure how to expand it beyond the single cases I know from direct experience.
Thanks!
It’s true… Not easy.
Not that I could have predicted but counter intuitively the aryl group on “P” is more stable compared to more reactive “t-butyl version” to a point that it can react with less reactive ketone. I wonder what will the outcome if you had a formyl group for the triphenyl phosphine? Just curious.
I am not sure, this is something that is difficult to predict without running the experiment…
I have seen several examples where N2 extrusion is not observed, but I think you are looking for more examples of triazines. Right? Not things like these below I guess.
http://pubs.acs.org/doi/abs/10.1021/ic048560c
This is an interesting example. Thanks! I will have to think about it.
Or may be this is a better example. Interesting nevertheless.
http://pubs.acs.org/doi/pdf/10.1021/om900135e
Thank you. The good old uranium chemistry…