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/ $\endgroup$ – Aditya suresh May 9 at 8:43
  • 4
    $\begingroup$ 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. $\endgroup$ – Mathew Mahindaratne May 9 at 8:44
  • $\begingroup$ This may assist en.wikipedia.org/wiki/Reactions_of_organocopper_reagents $\endgroup$ – Waylander May 9 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. $\endgroup$ – user93057 May 10 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 May 19 at 22:13

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.
| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.