cyclopent-2-en-1-one and gilman's reagent with acid workup


It seems to me that the carbon-oxygen bond would be more electrophilic due to the higher electronegativity of oxygen. Hence I believe $\ce{CH3-}$ should attack the carbonyl site. But this doesn't seem to be the case.

Is it because that the intermediate carbanion can undergo resonance with oxygen, had $\ce{CH3-}$ attacked carbon- carbon double bond?

What can be a valid reason?

  • $\begingroup$ Google for gilmans reagenthttps://www.masterorganicchemistry.com/2016/02/05/gilman-reagents-organocuprates-what-theyre-used-for/ $\endgroup$ Commented May 9, 2020 at 8:43
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    $\begingroup$ Your title doesn't match your question. You should have said "In a $\alpha,\beta$-unsaturated system.." An isolated $\ce{C=C}$ bond is not electrophilic. $\endgroup$ Commented May 9, 2020 at 8:44
  • $\begingroup$ This may assist en.wikipedia.org/wiki/Reactions_of_organocopper_reagents $\endgroup$
    – Waylander
    Commented May 9, 2020 at 9:14
  • $\begingroup$ of course, C=C bond is less electrophilic when compared to C=O bond, but the CH3 group seems to attack the C=C bond due to 1,4-addition. detailed explanation below. $\endgroup$
    – KaBe2003
    Commented May 10, 2020 at 8:00
  • $\begingroup$ Here is useful info on Gilman reagents and conjgate addition: www-oc.chemie.uni-regensburg.de/OCP/ch/chb/oc5/… $\endgroup$
    – user55119
    Commented May 19, 2020 at 22:13

1 Answer 1


step-1: cleavage of C=O bond:

  • the C=O bond undergoes heterolytic cleavage to give positive charge on carbon and negative charge on oxygen just like almost all other organometallic reagents react with a ketone, for example a grignard reagent (not exactly, but similar to it).

step-2:(the real step which caused deviation from your answer)-resonance of intermediate:

step 3:(as usual):-attack of CH3 group:

  • now on as usual, the CH3 group from our organo copper reagent ( called as gilmann reagent, used in Corey-house synthesis), just like a grignard reagent, attacks the new carbocation formed at carbon no.3 and the water molecule in the second step donates it's proton to the remnant oxygen of our old C=O group and it becomes an enol. tautomerisation takes place and we bet 3-methyl cyclohexanone as a result.

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