{"id":7885,"date":"2012-10-08T13:22:47","date_gmt":"2012-10-08T12:22:47","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=7885"},"modified":"2012-10-09T14:30:08","modified_gmt":"2012-10-09T13:30:08","slug":"alkyne-metathesis-a-comparison-with-alkene-metathesis","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7885","title":{"rendered":"Alkyne metathesis: a comparison with alkene metathesis."},"content":{"rendered":"
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Metathesis reactions are a series of catalysed transformations which transpose the atoms in alkenes or alkynes. Alkyne metathesis<\/a> is closely related to the same reaction for alkenes, and one catalyst that is specific to alkynes was introduced by Schrock (who with Grubbs won the Nobel prize for these discoveries) and is based on tungsten (M=W(OR)3<\/sub>).\"\"<\/p>\n

In the previous post, I expressed surprise<\/a> at the nature of the transition state for the alkene reaction, since the C-C or M-C bond (M=Ru) had hardly started to form by the time the transition state was reached. So what of the nature of the alkyne analogue? Firstly, I should mention that since the intention is to study the intrinsic reaction coordinate, which can be a lengthy calculation, it is necessary to prune the catalyst itself down to bare essentials. Unfortunately, it is now increasingly recognised that those sterically bulky groups that often adorn modern catalysts can in fact dramatically affect the nature of transition state. For one example where e.g. addition of bulky groups can transform a transition state to a minimum<\/a>, see here[1]<\/a><\/span> (and there may well be other examples of the reverse transformation of changing a minimum into a transition state). So with that caveat in place, take a look at the computed transition state<\/a> (\u03c9B97XD\/Def2-SVPD\/SCRF=dichloromethane) for a model where the t-Butoxy ligands of the real catalyst are replaced by simple OH groups.<\/p>\n

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Transition state for addition of alkyne to Schrock catalyst. Click for 3D<\/p><\/div>\n

The C-W and C-C forming bonds are respectively 2.45 and 2.59 \u00c5 long; both are significantly shorter than those for the alkene transition state. The latter however had four ligands other than the incoming alkene to reorganise, whereas this one has one fewer. With less reorganisation needed, it can start forming bonds earlier.<\/p>\n

The transition state leads to a fascinating product, being a metallacyclobutadiene. The carbocycle itself is of course famously transient and unstable (normally ascribed to its being anti-aromatic). In contrast, changing one carbon to tungsten makes the ring stable enough to be isolated as a crystal, one example of which is shown below (with an imine replacing one of the alkoxy groups).\u00a0<\/p>\n

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Crystal structure of a metallacyclobutadiene. Click for 3D.<\/p><\/div>\n

This is an example where replacing an atom carrying a p\u03c0<\/sub> orbital with one carrying a d\u03c0<\/sub> orbital inverts the aromaticity rules. However, the alternating bond pattern characteristic of cyclobutadiene (and anti-aromaticity) remains visible in this structure. Thus the two C-C lengths in the X-ray structure below are 1.39 and 1.53\u00c5, which perhaps corresponds to something like the following resonance form rather than implicating anti-aromaticity. More analysis is clearly needed here at some future stage. At any rate, the barrier for converting one bond-localized form into the other \u00a0(via<\/em> TS2) \u00a0looks likely to be very small, and that the rate-determining-step is going to \u00a0be TS1\/TS1′.<\/p>\n

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The intrinsic reaction coordinate appears thus, with the C-C forming only shortly after the W-C bond is formed.<\/p>\n

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\"Schrock \"Schrock<\/p>\n

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A postscript to the above<\/strong>. The IRC paths for these reactions are particularly difficult to compute; the secret lies in discovering the correct combination of parameters and step size to use. As a result, I have been able to chart a larger proportion of the IRC<\/a> than initially reported.<\/p>\n