We all know about allylic strain. Today I want to talk about Inomata’s syn-effect (see the graphic below), which is on display in certain base-promoted isomerization reactions. I have seen Professor Inomata’s thought-provoking work for many years and I think it stands as a very useful example of stereoelectronic balance in acyclic stereocontrol. The peculiar aspect here is that the Z-olefin is favoured. I don’t think cases like this belong to the usual repertoire of topics we discuss while teaching the foundations of acyclic stereocontrol. Indeed, the syn-effect seems to run counter to some of our intuition regarding the build-up of unfavourable steric interactions in transition states. The effect is neat, yet I am kind of happy that there is not too many instances of it out there. I, for one, would not want to mention it when I teach allylic strain. Why confuse all those bright young minds?
In syn-effect, the electronic contribution made when CH sigma orbitals interact with the pi-system, overrides the unfavourable A(1,3) interaction that develops when the R group is forced to be coplanar with the vinyl C-H bond. At least this is what the authors propose, although I have not seen theoretical papers dealing with this phenomenon. It is exceptional cases like this that keep chemistry rolling. There might be other outliers out there and we need to find them.