Alkynes are capable of undergoing carbometalation, e.g. with organocopper reagents:

Alkyne carbocupration

Why don't the corresponding alkenes undergo analogous carbometalation reactions? Is β-elimination the issue?

  • 1
    $\begingroup$ In carbometalation, an alkyne will become an alkene before the hydrolysis occurs (in the TS with the metal). Using the same logic, alkene will become an alkane. Pi bonds are higher in energy and more easily accessible spatially compared to sigma bonds, pi backbonding is also is present in alkene metal complexes. For these reasons, the latter reaction suffers from a high activation energy barrier compared to the carbometalation of alkynes. $\endgroup$ – AS_1000 Jul 15 '17 at 13:59
  • $\begingroup$ Out of learning curiosity: is this anywhere related to chelating? $\endgroup$ – bonCodigo Jul 27 '17 at 9:10
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    $\begingroup$ What about steric hindrance? Hydrolysis is a separate step, so you have this cuprate floating around and you're trying to add a second alkyl cuprate. It would be interesting to think about what other ligands exist on the copper but aren't represented... $\endgroup$ – Zhe Aug 8 '17 at 17:20

Alkenes are very capable of a similar carbometallation reaction as you can see in the scheme below (ref: R. M. Sultanov et al. / Tetrahedron Letters 54 (2013) 6619–6623):

enter image description here

I think the reasons why such reaction with alkenes is not so common is:

  1. It is useless (correct me if i am wrong). Hence the reason why the work in the scheme above was published in a low impact journal. Why would anyone want to make alkanes in such a complicated way instead of just getting them from crude oil for example?

  2. With alkenes, the reaction prefers to keep incorporating alkene to form really useful dimers or polymers. If one takes a look in the step wise mechanism of olefin polymerization (e.g. Ziegler-Natta) and dimerization the transmetallation/insertion step is equivalent to the one in the question posted:

enter image description here

  • $\begingroup$ I don't think it's useless. The organomettalic intermediate could react with all kind of electrophiles not just with a proton to give alkanes. $\endgroup$ – RBW Mar 6 '18 at 23:44
  • $\begingroup$ @Marko true, but surely you could just make a Grignard in that case? I don't know. Maybe this saves a step somewhere along the way. But I can see why this isn't a particularly... groundbreaking transformation. $\endgroup$ – orthocresol Mar 7 '18 at 0:27
  • $\begingroup$ @Marko Yes, i am referring to the formation of short chain common alkanes only which is a bit of a waste. The organometallic intermediate is really useful. But if we are discussing only about the organometallic intermediate then carbometallation with alkenes is as common (or maybe more) as with alkynes. $\endgroup$ – AMM Mar 7 '18 at 0:36
  • $\begingroup$ @orthocresol Don't forget that by carbometallation you add an alkyl chain to the substrate, which rapidly increases complexity. $\endgroup$ – RBW Mar 7 '18 at 8:42

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