Comments on: Cyclopentadiene: a hydrocarbon at the crossroads of …

By: The conformation of cyclohexane « Henry Rzepa

The conformation of cyclohexane « Henry Rzepa — Fri, 12 Oct 2012 14:29:02 +0000

[...] of path known as a valley-ridge inflection path. An example of such a surface can be seen for the dimerisation of cyclopentadiene (DOI: 10.1021/ja016622h) and effectively it connects one transition state to a second transition [...]

By: Henry Rzepa

Henry Rzepa — Wed, 01 Aug 2012 07:13:08 +0000

I am adding here another IRC animation. This time it starts from the green dot, itself of course a transition state, and tracks the [3,3] sigmatropic shift that occurs. The endo nature of both start and end dimers can be seen.

By: Henry Rzepa

Henry Rzepa — Tue, 31 Jul 2012 09:04:25 +0000

Thanks Paul for the insights. The rapid [1,5] shifts were first discovered, I think, when its 1H NMR spectrum was first measured, probably in the mid to late 1950s. I know that Geoff Wilkinson (who was interested in it in terms of the ferrocenes) had asked that an early low temperature probe be constructed on one of the first commercial NMR systems (40 MHz?) round about that period in order to show that the spectrum became more complex as you cooled it. This is what I alluded to in my reference to fluxional NMR behaviour. Whether he was the first to do this remains unknown. I have however speculated that if he HAD observed the behaviour, and rationalised it as a rapid [1,5] shift, and then attempted to explain it theoretically, he would have anticipated Woodward by 5-6 years! But he did not! Regarding the endo stereochemistry on the rhs, I actually did not show any stereochemistry at the bridgehead (the implicit hydrogens need a wedge). I was trying to depict how a [3,3] sigmatropic shift converts the dimer into its mirror image; this of course preserves the endo relationship of the rings throughout. It has been argued that this particular system is the one that best illustrates the secondary orbital interactions, via the ambiguity of which monomer acts as a diene and which as the alkene.

By: Paul Schleyer

Paul Schleyer — Tue, 31 Jul 2012 08:50:42 +0000

CPD itself also brought us rapid 1,5-H shifts as well as hyperconjugative aromaticity. Mulliken realized long ago that its CH2 group could function as a 2 π electron donor. The readily available CPD dimer, dicyclopentadiene (DCPD), gives adamantane in two steps.
The CPD dimerization led to the Alder-Stein rule of endo-addition (unfotunately, Henry depicts DCPD with the incorrect exo stereochemistry in the two right side representations). The role of secondary orbital interactions in determining the correct endo stereochemistry is not brought out explicitly, but should be apparent from this post. Endo (but not exo) DCPD undergoes the degenerate Cope rearrangement.