O-Allylphenols

Section 24 13 On being heated allyl aryl ethers undergo a Claisen rearrangement to form o allylphenols A cyclohexadienone formed by a concerted six tt electron reorganization is an intermediate... 

The Claisen rearrangemenC is a thermal rearrangement of allyl aryl ethers and allyl vinyl ethers respectively. It may be regarded as the oxa-version of the closely related Cope rearrangement. Claisen has discovered this reaction first on allyl vinyl ethers 1, and then extended to the rearrangement of allyl aryl ethers 2 to yield o-allylphenols 3 ... 

If both ortho positions bear substituents other than hydrogen, the allyl group will further migrate to the para position. This reaction is called the para-Claisen rearrangement. The formation of the para-substituted phenol can be explained by an initial Claisen rearrangement to an ortho-2l y intermediate which cannot tautomerize to an aromatic o-allylphenol, followed by a Cope rearrangement to the p-allyl intermediate which can tautomerize to the p-allylphenol e.g. 6 ... 

Unlike the acid-catalyzed ether cleavage reaction discussed in the previous section, which is general to all ethers, the Claisen rearrangement is specific to allyl aryl ethers, Ar—O—CH2CH = CH2. Treatment of a phenoxide ion with 3-bromopropene (allyl bromide) results in a Williamson ether synthesis and formation of an allyl aryl ether. Heating the allyl aryl ether to 200 to 250 °C then effects Claisen rearrangement, leading to an o-allylphenol. The net result is alkylation of the phenol in an ortho position. 

Allyl phenyf Transition state Intermediate o-Allylphenol... 

Evidence for this mechanism comes from the observation that the rearrangement takes place with an inversion of the allyl group. That is, allyl phenyl ether containing a 14C label on the allyl ether carbon atom yields o-allylphenol in which the label is on the terminal vinylic carbon (green in Figure 18.1). It would be very difficult to explain this result by any mechanism other than a pericyclic one. We ll look at the reaction in more detail in Section 30.8. 

Claisen rearrangement reaction (Sections 18.4, 30.8) The pericyclic conversion of an allyl phenyl ether to an o-allylphenol by heating. 

In 1997, Uozumi and Hayashi found high enantioselective Wacker-type cycUza-tion of o-allylphenols or o-homoaUylphenols by using Pd(II) catalysts coordinated with chiral bis(oxazoline) ligands based on the 1,1 -binaphthyl backbone (Eq. 6.36)... 

The basic pattern of the Claisen rearrangement is the conversion of a vinyl allyl ether to a y,8-enone. The reaction is also observed for allyl phenyl ethers, in which case the products are o-allylphenols. 

The regiospecificity of intramolecular palladium-induced cyclization of o-allylphenols is determined by the precise nature of the catalyst. A mixture... 

Aresta (54) has investigated the platinum complexes formed with o-allylphenol and o-allylthiophenol. The phenolic ligand reacts with the PtCl4 ion (in a suitable acetate buffer) to form the chelate complex shown in Fig. 40. The coordinated double bonds of this compound are successively replaced by two equivalents of pyridine. 

Problem 19.17 Alkylation of PhO with an active alkyl halide such as CH2=CHCHjCI gives, in addition to phenyl allyl ether, some o-allylphenol. Explain. M... 

PhO " is an ambident anion with negative charge on both O and the ortho C atoms of the ring. Attack by O gives the ether attack by the ortho carbanion gives o-allylphenol. 

Allylic aryl ethers, when heated, rearrange to o-allylphenols in a reaction called the Claisen rearrangementIf both ortho positions are filled, the allylic group migrates to the para... 

Dithiocoumarins (50) are generally prepared by treatment of thiocoumarins with phosphorus pentasulfide, isolation being best effected by formation of a mercury complex by addition of mercurous chloride, followed by decomposition of this complex with sodium sulfide. An alternative ring-closure approach involves heating o-allylphenols (49) with sulfur in... 

Pathway G Synthesis of 2-(l-hydroxyalkyl)benzofuran derivatives from o-alkenylphenols. o-Allylphenols and their benzyl ethers (168), treated with a peroxy acid, give epoxides which, according to conditions, lead... 

Iodine, ring closure of o-allylphenols. Treatment with iodine chloride (in ethanol or CCI4) of o-allylphenols gives 2-iodomethyl-2,3-dihydro-benzofurans (196).455-457 The latter compounds can lead either to the... 

Iodomethyl-2,3-dihydrobenzofurans (196) and the corresponding 2-iodomercurimethyl derivatives can be obtained from o-allylphenols and mercuric acetate, via the 2-acetoxymercuri-2,3-dihydrobenzo-furans.420... 

AHC(18)337>. The 3-alkylbenzo[6 ]furans result from cyclodehydration of aryloxyacetones the most common dehydrating agents are sulfuric acid, phosphorus oxychloride and poly-phosphoric acid. The allyl ethers of phenols can be converted to 2-alkyl-2,3-dihydro-benzo[6]furan by heating with polyphosphoric acid, pyridine hydrochloride or magnesium chloride at 180 °C the intermediate o-allylphenol is not isolated. 

The allyl ethers of phenols may be converted via a Claisen rearrangement to o-allyl derivatives and thence to the benzo[6]furans. The products of addition of bromine to the o-allylphenols give good yields of the heterocycle on prolonged heating with alkali, but the photochemical rearrangement of O-allylphenol is not so effective. 

It is also possible to oxidize o-allylphenols to chromenes with potassium dichromate. The oxidant may be supported on an anionic exchange resin, but it is preferable to dissolve the dichromate in benzene using Adogen 464, a mixture of methyltrialkylammonium chlorides (77TL4167). The oxidation is assumed to proceed through the chromate ester (107) which yields the quinoneallide (79CC836). 

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