Molecular gymnastics in 2+2 cycloadditions.

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In this earlier post, I described how the stereochemistry of π22 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. Here I illustrate how the stereochemistry of a thermal π22 cycloaddition can indeed avoid an antarafacial component by performing appropriate gymnastic contortions instead of a mechanistic change (a WH violation certainly in the letter of their law, if not their spirit).

The IRC for the reaction (R=CH=S) shows the two molecules moving towards each other in a trapezoidal motion, with the axes of the C=C bonds parallel and the faces clearly suprafacial. At IRC ~-4, the nature of motion changes into a slide of one C=C axis along the other, converting the trapezoidal geometry into a square. By doing so, a full frontal “violation” is changed into a more subtle sideways insertion.

Animation of a 2+2 thermal cycloaddition with no antarafacial components. Click for 3D.

This trapezoidal motif for a thermal reaction does require the two R groups to have electron withdrawing nature, but it is noteworthy that the same motif was found for the photochemically induced π22 cycloaddition. That both the thermal AND the photochemical pathways have essentially the same geometry is indeed a violation of WH!

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4 Responses to “
Molecular gymnastics in 2+2 cycloadditions.

  1. […] Henry Rzepa Chemistry with a twist « Molecular gymnastics in 2+2 cycloadditions. […]

  2. […] But there is a (rarely cited or observed) alternative, as was illustrated for the π2+π2 cycloaddition of ethene to itself. There we saw the gymnastics of a limbo dancer, with one ethene sliding up to the other rather than […]

  3. […] asymmetric example is the  2+2 closed shell cycloaddition of two ethenes, which adopt a different form of […]

  4. […] bond, and one can normally rely on that bond length being in the range 2.0 – 2.3Å. Thus the thermal 2+2 cycloaddition of two ethenes can have a C-C length of 2.0Å, albeit accompanied by a fascinating trapezoidal geometry. My […]

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