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HAT & DKR

(Taken from Dalton Trans., 2012, 41, 13423–13428. DOI:10.1039/c2dt31781h)

I have difficulties understanding the hydrogen transfer in the image above. I always thought that hydrogen transfers followed the following formula:

$$\ce{X^. + H-Y -> X-H + Y^.}$$

I can't really see the connection between the picture and the formula. Is it the whole ethanol group that is transferred?

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  • $\begingroup$ The hydrogen transfer is between the enantiomers. The dashed hydrogen is transferred in the picture. $\endgroup$
    – DHMO
    Commented Jan 22, 2017 at 14:47
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    $\begingroup$ Is there any more context provided as to how the racemisation occurs? Hydrogen atom transfer via a radical process seems plausible to me since the resultant hydroxybenzyl radical is quite stabilised, but you need something to take the hydrogen atom, since C-H bonds don't just fall apart and re-form spontaneously. $\endgroup$
    – orthocresol
    Commented Jan 22, 2017 at 15:01
  • $\begingroup$ I think, a deprotonation/reprotonation mechanism would be even more plausible than implying radical formation ($420~\mathrm{kJ/mol}$). (CC @orthocresol) $\endgroup$
    – Jan
    Commented Jan 22, 2017 at 15:10
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    $\begingroup$ Your image appears to be from this source: pubs.rsc.org/en/Content/ArticleHtml/2012/DT/c2dt31781h It's good practice to cite or link to such source material when you use it in a question (or answer). $\endgroup$
    – Curt F.
    Commented Jan 22, 2017 at 15:20
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    $\begingroup$ @Jan Imo a radical process would be more selective for the C-H bond (benzylic C-H ~370 kJ/mol) over the O-H bond (~440 kJ/mol) whereas an ionic deprotonation/reprotonation would happen at the O-H preferably. I don't know, though, that's just what I remember from lectures :) Of course, I'm not suggesting homolytic cleavage, I was thinking more of a reversible hydrogen atom abstraction by something else. $\endgroup$
    – orthocresol
    Commented Jan 22, 2017 at 15:24

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Did you continue reading the paper? :) The source is important and is what was needed to answer the question!

I'm just going to quote from it.

In DKR [dynamic kinetic resolution] a chemical catalyst is added in order to racemize the enantiomers, ensuring that all the substrate can be utilized. [...] Highly successful catalysts are often Ru(II) complexes [...] The metal complex catalyses racemization by a hydrogen transfer mechanism (Scheme 2).

enter image description here

So, the supposed hydrogen transfer occurs via coordination to a metal, followed by β-hydride elimination. Readdition of the metal hydride across the opposite face of the resultant ketone (acetophenone in this case) produces the other enantiomer.

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  • $\begingroup$ I feel like a complete idiot... sorry i wasted your time. I'll double check any further source based questions in the future. Thank you $\endgroup$
    – Oak Staff
    Commented Jan 22, 2017 at 15:36
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    $\begingroup$ No problem, it was an interesting question. $\endgroup$
    – orthocresol
    Commented Jan 22, 2017 at 15:36

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