Another foray into one of the more famous anecdotal chemistry “models”, the analysis of which led directly to the formulation of the WoodWard-Hoffmann (stereochemical) rules for pericyclic reactions.

The quote of the post title comes from R. B. Woodward explaining the genesis of the discovery of what are now known as the Woodward-Hoffmann rules for pericyclic reactions.[cite]10.1021/ja01080a054[/cite] I first wrote about this in 2012, noting that “*for (that) blog, I do not want to investigate the transition states”.* Here I take a closer look at this aspect. I will start by explaining my then reluctance to discuss transition states.

In an earlier post, I pondered on how the “arrow pushing” for the thermal pericyclic reactions of some annulenes (cyclic conjugated hydrocarbons) could be represented in terms of either two separate electrocyclic reactions or of one cycloaddition reaction.

I occasionally spot an old blog that emerges, if only briefly, as “trending”. In this instance, only the second blog I ever wrote here, way back in 2009 as a follow up to this article.[cite]10.1021/ed084p1535[/cite] With something of that age, its always worth revisiting to see if any aspect needs updating or expanding, given the uptick in interest.

I noted in my WATOC conference report a presentation describing the use of calculated reaction barriers (and derived rate constants) as mechanistic reality checks. Computations, it was claimed, have now reached a level of accuracy whereby a barrier calculated as being 6 kcal/mol too high can start ringing mechanistic alarm bells.

In the previous posts, I explored reactions which can be flipped between two potential (stereochemical) outcomes. This triggered a memory from Alex, who pointed out this article from 1999[cite]10.1070/MC1999v009n02ABEH000995[/cite] in which the nitrosonium cation as an electrophile can have two outcomes A or B when interacting with the electron-rich 2,3-dimethyl-2-butene.

This post, the fifth in the series, comes full circle. I started off by speculating how to invert the stereochemical outcome of an electrocyclic reaction by inverting a bond polarity. This led to finding transition states for BOTH outcomes with suitable substitution, and then seeking other examples.

One thing leads to another. Thus in the previous post, I described a thermal pericyclic reaction that appears to exhibit two transition states resulting in two different stereochemical outcomes.

In my first post on the topic, I discussed how inverting the polarity of the C-X bond from X=O to X=Be (scheme below) could flip the stereochemical course of the electrocyclic pericyclic reaction of a divinyl system.

In my earlier post on the topic, I discussed how inverting the polarity of the C-X bond from X=O to X=Be could flip the stereochemical course of the electrocyclic pericyclic reaction of a divinyl system. An obvious question would be: what happens at the half way stage, ie X=CH ₂ ? Well, here is the answer.