I came across a question about reduction of a conjugated ketone with $\ce{LiAlH4}$. enter image description here Although it seemed quite easy at first sight, everything became cloudy as I dug deeper. My doubts regarding the question are:

  1. I have heard that conjugated double bonds are reduced by $\ce{LiAlH4}$. Is there any chance that the Oxygen from the ether group takes part in conjugation?
  2. If not, why?
  3. Is there a chance that the ether is cleaved during the acidic workup?
  4. What is the true mechanism of this reaction, and does it involve the following intermediate?enter image description here Please help me with this question. Thanks in advance.
  • $\begingroup$ The reagents should read 1) LiAlH4, 2) H3O+. After step 1), "d" is the product. After hydrolysis, "a" is formed. $\endgroup$
    – user55119
    Aug 12 '21 at 18:54
  • 1
    $\begingroup$ You might be interested in this: chemistry.stackexchange.com/questions/87138/… $\endgroup$
    – user55119
    Aug 12 '21 at 19:01
  • $\begingroup$ Note, chemical information may be advantageously formatted using on ChemSE with mhchem. Take moment to familiarize with this. You are encouraged to use it in the body of questions, answers, and comments. Because it is something special not all web browsers understand well, do not use it in the title of questions or answers. $\endgroup$
    – Buttonwood
    Aug 12 '21 at 19:07

I'll take the opportunity to expand on the Comments and links I provided previously. The intermediate you proposed does not occur during the LiAlH4 reduction of the enol ethers of dihydroresorcinols such as enone 1. The LiAlH4 reduction is a straightforward 1,2-reduction of the carbonyl group. If this reduction is worked up with water, no aqueous acid, allylic alcohol 2 would be expected. This species is no ordinary allylic alcohol because loss of water from protonated 2 -- namely, structure 3 -- leads to a highly stabilized allylic carbocation 4 with participation of the oxygen electron pair. Addition of water and loss of ethanol from 4 via intermediates 5 and 6 leads to the transposed enone 7.

This methodology is exploited in one aspect of the Stork-Danheiser reaction wherein the enol ether of dihydroresorcinol 8 (R1=isobutyl, R2=alkyl) is converted into a 4-alkyl cyclohexenone.


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