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I want to form methylenecyclohexane from cyclohexylmethanol

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I'm thinking about an $\mathrm{E1cB}$ mechanism since the β-hydrogens will be quite acidic (similar electronegativity to fluorine). So I can use a strong and hindered base (to reduce $\mathrm{S_N2}$) to form methylenecyclohexane.

Another possibility is a weak hindered base for an $\mathrm{E1}$ Hofmann elimination (since the carbocation will rearrange). But not only will this lead to a some amount of Zaitsev product, $\mathrm{S_N1}$ will dominate because of better thermodynamics (net 1 σ formed vs net 1 π formed that is not very stabilized either).

I feel like $\mathrm{E1cB}$ would be a better idea.

Are there any better methods?

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    $\begingroup$ Tosylate and E2? $\endgroup$ – orthocresol Aug 10 '17 at 7:41
  • $\begingroup$ I can't pick any compound. I gotta do it through the one I've attached in the picture. OH- is a poor leaving group. It won't undergo E2. $\endgroup$ – xasthor Aug 10 '17 at 7:50
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    $\begingroup$ Tosylate the alcohol, and E2 $\endgroup$ – orthocresol Aug 10 '17 at 7:58
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    $\begingroup$ As @orthocresol has suggested I would form the mesylate or tosylate and treat with DBU $\endgroup$ – Waylander Aug 10 '17 at 8:27
  • $\begingroup$ Off the top of my head no direct methods will work as $\ce{OH-}$ won’t leave no matter what you throw at it and strong bases will deprotonate the alcohol before any $\ce{C-H}$ bond of your compound. So you have to modify and use a two-step (or two-stage) procedure. $\endgroup$ – Jan Aug 10 '17 at 10:23
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Whichever method of elimination you end up preferring, there is no way to directly eliminate the hydroxy group without derivatising it first in some way. The hydroxide ion is a very bad leaving group (but a rather good nucleophile) and transforming a hydroxy group into a better leaving group by protonation ($\ce{R-OH2+}$) requires acidic conditions that do not really allow the (basic) abstraction of a β hydrogen. For each of the simple options:

  • A hydroxide ion will never leave as part of an $\mathrm{S_N2}$ or $\mathrm{E2}$ reaction; it is too weak a leaving group.
  • An $\mathrm{E1}$ reaction would require strongly acidic conditions to protonate the hydroxy group; under these conditions the β hydrogen cannot be abstracted. $\mathrm{S_N1}$ would dominate.
  • An $\mathrm{E1cb}$ mechanism is absolutely impossible. The $\mathrm pK_\mathrm{a}$ value of the β hydrogen is around $50$ while that of the hydroxy group is around $15$. The only conjugate base being formed will be the alcoholate anion.

This means we have to resort to two-step procedures. A number of them have been published in literature in various different contexts. Some bear a name, others don’t. Some follow one of the paradigms mentioned above, others follow a completely different mechanism. Since there is only one elimination product possible, you need not worry about selectivity. I shall present a few possibilities grouped by the mechanistic type where possible but note that this list is not exhaustive and does not even begin to scratch the surface.

$\mathrm{E2}$ type mechanisms

The simplest is the one Orthocresol mentioned in the first comment: transform the alcohol into the corresponding tosylate, e.g. by adding $\ce{TsCl}$ and pyridine, then eliminate with additional potentially stronger base.

A variant of this would be to use mesyl chloride/DBU instead. Depending on how activated your proton is, a simple addition of more DBU to the reaction mixture may suffice.

Considering that your starting material is a primary alcohol, $\mathrm{E1}$-based reactions are very unlikely. Considering again the acidity of the β hydrogen, so are $\mathrm{E1cB}$; no matter which the reaction protocol.

syn-selective eliminations based on cyclic transition states

One example is the ester pyrolysis: use an acid chloride (e.g. acetyl chloride $\ce{AcCl}$) to synthesise an ester, then heat up to initiate thermal rearrangement to the corresponding acid and alkene. The transition state is cyclic, so this reaction is syn-selective.

A similar reaction is the Chugaev elimination. React the alcohol with a base and $\ce{CS2}$, then add an alkyl halide (typically iodomethane) to generate a xantogenate. This also fragmentates thermally to give an alkene and a dithiocarbonate while syn-selectively removing the β hydrogen.

And finally my favourite one: the selenoxide pyrolysis. First synthesise a selenoether with $\ce{NCSe-C6H4-NO2}$ (ortho-nitrophenyl selenocyanate) and $\ce{PMe3}$ in a Mitsunobu reaction, then oxidise the selenoether e.g. by using $\ce{H2O2}$ at room temperature to smoothly give the corresponding alkene — again via a cyclic transition state practically identical to the former two.

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