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I have been asked to identify suitable reagents for the following transformation: enter image description here

(Apologies it's back to front - the question is a map of reactions and I took a snapshot).

I really have no idea about this one. Don't even know where to begin, sorry. For the purposes of the question I only need reagents but I would appreciate a mechanism if you know it. Thanks.

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    $\begingroup$ The answers below are very good. If you have access to a book on protecting groups in organic synthesis (Greene and Wuts is a popular one), you'll find an extensive list of methods that have been used for demethylating a phenol. $\endgroup$
    – jerepierre
    Apr 22, 2015 at 20:53

2 Answers 2

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The reaction is demethylation of methoxybenzene. You can do it by using an acid which has a nucleophilic counter ion.

enter image description here

It is not actually obvious why the iodine nucleophile attacks the methyl group and not inside benzene ring. The other option would be:
enter image description here

This reaction doesn't happen because otherwise the iodine ion would appear inside the benzene ring and resulting product would be in a very strange conformation (high energy). More information:
Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. p. 434, 589.

As it was correctly stated in the other answer there are also other reagents which can be used to cleave the methyl ether. $\ce{BBr3}$ is a Lewis acid which also can be used in this reaction. A relatively new paper describes a computationally calculated bimolecular mechanism for this reaction.

enter image description here

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    $\begingroup$ I guess there are two reasons, why the iodine is not attacking from inside the ring. The first should be, because it is much too big to even fit in there. SN2 is just not possible, because that requires the donation in the antibonding orbital of the bond to be cleaved. The antibonding orbital corresponding to the phenyl oxygen bond is way higher in energy than the LUMO of the aromatic system and even higher than the CH bonds (this is an educated guess though). And also don't forget there is partial double bond character between the phenyl and the oxygen. Btw.: Excellent answer! $\endgroup$
    – Martin - マーチン ♦
    Apr 26, 2015 at 7:15
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Boron tribromide is a standard reagent for cleaving aryl methyl ethers to the corresponding phenol. It is a general method and can be used in aromatic methyl ethers with sensitive substituents where acid or base cannot be tolerated.

The above Wiki link also shows the following mechanism

1) complex or adduct formation

$$\ce{ArOCH3 + BBr3 -> ArO+(^{−}BBr3)CH3}$$

2) bimolecular reaction of 2 adducts $$\ce{2 ArO^{+}(^{−}BBr3)CH3 -> 2 ArO(BBr2) + 2CH3Br }$$

3) hydrolysis

$$\ce{ArOBBr2 + 3H2O → ArOH + B(OH)3 + 2HBr}$$

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