[workingundergrad1896]

I was recently at an interview for a postgrad position in organic synth, and they asked for an explanation on the origin of the endo/exo selectivity in a diels alder reaction. I gave the explanation I was taught; endo product is kinetic due to lower energy transition state that results from secondary orbital overlap of electrons in substituents and the forming double bond, but is less stable than the exo product due to larger steric interactions.

The interviewers were almost instantly offended by my response, which threw me off a lot. They asked me what I thought of the explanation and I didn't really know how to answer. They told me it was a poor explanation as the orbitals are too far away for such overlap to occur, but didn't really give much other information.

I spoke to a student who had studied at the university I interviewed at, and he told me the lecturers there do teach them the explanation i gave above, but in an exam situation they're told to write ''this is what i was taught'' and that it's actually the wrong explanation, and that some DFT calculations had been reported in the literature that point to something else. I was never taught that this explanation was up for debate, and I guess I'm still in that mind set where you just assume that everything you're taught is correct (which is obviously not always the case...it's just not something I'd thought about)

Does anyone have any resources/literature references for me to read up on this topic, as I've done a bit of searching and can't find anything that seems immediately relevant. I don't want to be caught off guard by this again and I'm interested to what the new (if any exists yet) explanation could be.

[kriggy]

I am by no means expert on this topic but I recently read a chapter about DA reaction for my stereochemistry class and from the text it seems that the secondary orbital ovelap is specific for some substrates so it is not a general answer. The selectivity seems to be very substrate dependant