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What are the reagents for the following reactions:

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Typically, when thinking about ketone to alkene reactions, one thinks of the Wittig reaction. However, I don't see an appropriate form of that reaction here. This was posed as a question on my problem sheet and I suspect - after having given it some thought - that it's just a Hoffman/Saytzev question.

For reaction A, I think reaction with $\ce{MeMgBr}$ to make the tertiary alcohol. At this point, one has to think of a way to get the least thermodynamically stable product (the kinetic product). Perhaps sulfuric acid? It would protonate the hydroxyl so the transition state wouldn't have significant alkene character and the "base" is bulky. Correct?

For reaction B, I think it should be reacted with $\ce{NaBH4}$ to make the secondary alcohol then elimination can be performed using whatever. Maybe sulfuric acid again or just tosylate it then use $^t\ce{BuOK}$. Does this sound right?

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    $\begingroup$ Why can't you see a Wittig reaction working for reaction A? $\endgroup$
    – ron
    Commented May 19, 2015 at 16:04
  • $\begingroup$ How can it be done? $\endgroup$
    – RobChem
    Commented May 19, 2015 at 16:06
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    $\begingroup$ $\ce{Ph3P + MeI}$, then $\ce{NaH}$, then cyclohexanone. Unless you want to use $\ce{(Cp)2Ti=CH2}$ instead ;-) $\endgroup$
    – Klaus-Dieter Warzecha
    Commented May 19, 2015 at 16:16
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    $\begingroup$ B can be done either using the Shapiro reaction (tozylhydrazine, BuLi) or using the Bamford-Stevens rearrangement (tozylhydrazine, somewhat less strong base eg. NaOMe). You can do Wittig for 1, or a Schrock carbene as Klaus suggests, or a Peterson olefination (TMS + base). $\endgroup$
    – J. LS
    Commented May 19, 2015 at 19:25

1 Answer 1

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Part (a)

I agree with the previous comments that a Wittig reaction is the best way forwards especially in terms of regioselectivity of the double bond. PPh3, MeI and NaH would be an excellent combination. Perhaps draw the mechanism and you'll see how/why this work so well?

If you are interested, you could use (Cp)2Ti=CH2 also known as Tebbe's Reagent. Klaus Warzecha mentioned this already (above).

Part (b)

I like your idea of reducing, tosylating and using the non-nucleophilic base tBuOK

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