2- 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... 

CLAISEN - IRELAND Rearrangment Rearrangement ol allyl phenyl ethers to o (or p-)allylphenols or of allyl vinyl ethers to y.S-unsaturated aldehydes or ketones (Claisen) Rearrangement ol allyl esters as enolale anions to y.S-unsaturated acids (Ireland)... 

The oxazoloquinolinequinone derivative 610 was prepared from the allylphenol 605 which was transformed to the alcohol 606 and then to the aldehyde 607. Subsequent debenzylation gave 608 that was oxidized with Ce(NH4)2(N02)6 to the quinone 609 that upon cyclization with ethanolamine gave the oxazoloquinolinequinone 610 which showed... 

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 ... 

Mechanistically this reaction is described as a concerted pericyclic [3,3] sigma-tropic rearrangement. A carbon-oxygen bond is cleaved and a carbon-carbon bond is formed. In a subsequent step the initial product 4 tautomerizes to the stable aromatic allylphenol 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 ... 

An analogous sequence on the allyl ether of catechol (63) leads to oxyprenolol (64) in the same vein, ortbo-allylphenol (65) affords alprenolol (66). ... 

It forms long thin prisms, which when melted tend to remain in the liquid condition, but when the liquid is sown with a crystal of the phenol it at once solidifies. It forms a benzoyl compound, melting at 72° to 73°. It is apparently dihydro-para-allylphenol of the constitution—... 

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. 

The results were compared with the bromination with bromine. It was apparent that bromine gave /jora-bromides exclusively except 2-allylphenol. As the reaction of 2-allylphenol with bromine gave the mixture of many products (bromine adduct as main product, some ring bromides as by-products, etc.), 2-allylphenol was treated with benzyltrimethylammonium tribromide (BTMA Br3) which was already developed as mild and easy bromine (ref. 10). 

Consequently the bromine adduct was obtained in high yield (83 %). Using NBS and a catalytic amount of the amine, the ratio of the ort/io-brominated phenols was remarkably raised. 2-Allylphenol and o-cresol were considerably orr/io-brominated by NBS even without the amine. In NBS-amine system dibromides as by-products were obtained slightly and any p jra-bromide and bromine adduct in the case of 2-allylphenol were not detected. 

Fig. 2 Selectivity of orf/io-bromide (left) and dibromide (right) depending on reaction temperature in the bromination of 2-substituted phenols with NBS (solid powder) and (/-Pr)2NH. 2-allylphenol ( ), o-cresol (A), 2-bromophenol (A), and 2-chlorophenol (O).

Useful methods for the preparation of various bromosubstituted 2-allylphenols were proposed in Scheme 4. 

Hydrogen bonding between Br2 and 2-substituted phenols having electron-donating group is strong enough to orf/io-brominate these phenols. Therefore, o/t/io-bromination of phenol, o-cresol and 2-allylphenol was promoted by only NBS without amines. 

Faster ort/zo-bromination than bromine addition is the reason why bromine adduct was not obtained in the bromination of 2-allylphenol with NBS (ref. 15). 

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... 

Allyl phenyl ether was heated with water in the MBR for 1 h at different temperatures [46]. It underwent Claisen rearrangement to 2-allylphenol (56% yield) at 200 °C, 2-(2-hydroxyprop-l-yl)phenol (37% yield) at 230 °C and 2-methyl-2,3-dihydro-furan (72% yield) at 250 °C (Scheme 2.12). Support for the reaction sequence was obtained through experiments with authentic intermediates. 

Scheme 4.5 Rearrangement of phenyl vinyl ether to 2-allylphenol.

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