In this earlier post, I described how the stereochemistry of π ₂ +π ₂ cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating a change in mechanism requiring stepwise pathways with intermediates along the route.

There are many treasures in Woodward and Hoffmann’s (WH) classic monograph.

My very first post on this blog, in 2008, was to describe how Jmol could be used to illustrate chemical themes by adding 3D models to posts. Many of my subsequent efforts have indeed invoked Jmol.

Thus famously wrote Woodward and Hoffmann (WH) in their classic monograph about the conservation of orbital symmetry in pericyclic reactions.

Previously, I had noted that Corey reported in 1963/65 the total synthesis of the sesquiterpene dihydrocostunolide. Compound 16, known as Eudesma-1,3-dien-6,13-olide was represented as shown below in black; the hydrogen shown in red was implicit in Corey’s representation, as was its stereochemistry. As of this instant, this compound is just one of 64,688,893 molecules recorded by Chemical Abstracts.

My previous three posts set out my take on three principle categories of pericyclic reaction. Here I tell a prequel to the understanding of these reactions.

Another common type of pericyclic reaction is the migration of hydrogen or carbon along a conjugated chain, as in the [1,3] migration of a carbon as shown below. As before, I explore the stereochemistry of the thermal and photochemical reactions.

The π ₂ + π ₂ cyclodimerisation of cis -butene is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions. The photochemical reaction is known to give a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with the all- cis isomer apparently predominating.

Woodward and Hoffmann published their milestone article  “Stereochemistry of Electrocyclic Reactions” in 1965. This brought maturity to the electronic theory of organic chemistry, arguably started by the proto-theory of Armstrong some 75 years earlier. Here, I take a modern look at the archetypal carrier of this insight, the ring opening of dimethylcyclobutene.

In the previous post, I wrote about the processes that might be involved in a molecular wheel rotating. A nano car has four wheels, and surely the most amazing thing is how the wheels manage to move in synchrony.

The world’s smallest nano car was recently driven a distance of 6nm along a copper track. When I saw this, I thought it might be interesting to go under the hood and try to explain what makes its engine tick and its fuel work.