4-Allylphenol

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). 

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.

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

Si-H bonds add across olefins over platinum catalysts. This reaction (hydrosi-lation) is used in silicone polymer manufacture, when the silane is a hydrogen bearing oligo(alkylsiloxane). With some types of olefin there have been reports of runaways to explosion because of unexpectedly fast reaction. Dangerous substrates recorded are 2-allylphenols and ethenylsiloxanes. Very low levels of catalyst (ppm) and good cooling are recommended. 

Bis[2-(2,3-dihydrobenzofuranyl)methyl]tellurium dichloride (typical procedure) Te02 (2.4 g, 15.0 mmol), 2-allylphenol (4.0 g, 30.0 mmol) and LiCl (3.0 g, 70.8 mmol) are refluxed in HOAc (50 mL) for 1.5 h. The clear solution is then filtered hot from a small amount of elemental tellurium and, upon cooling (freezing), the product crystallizes as a white solid [5.4 g (78%)]. The product is recrystallized from EtOH/CH2Cl2 (1 1) (m.p. 180-184°C). 

Bis[2-(2,3-dihydrobenzofuranyl)methyl] ditelluride (typical procedure) Te02 (2.0 g, 12.5 mmol) is dissolved in concentrated HCI (10 ml) and the yellow solution diluted with MeOH (40 mL). 2-Allylphenol (1.98 g, 14.8 mmol) is added and the mixture is heated... 

In addition to the development of new catalysts and reaction conditions for aerobic oxidative heterocycUzation, considerable effort has been directed toward asymmetric transformations. Hosokawa and Murahashi reported the first example of asymmetric Pd-catalyzed oxidative heterocycUzation reactions of this type [157,158]. They employed catalytic [(+)-(Ti -pinene)Pd (OAc)]2 together with cocatalytic Cu(OAc)2 for the cycUzation of 2-allylphenol substrates however, the selectivity was relatively poor (< 26% ee). 

Analogous reactions have been achieved recently with molecular oxygen as the sole stoichiometric oxidant by employing (-)-sparteine (sp) as the chiral ligand [153,163]. Stoltz and coworkers demonstrated asymmetric oxidative cychzation of a 2-allylphenol substrate (Eq. 34). A stoichiometric quantity of the sp hgand was necessary, perhaps because it also serves as a base in the reaction. Enantioselective oxidative tandem cychzation of 2-allyl anilides was achieved by Yang and coworkers (Eq. 35). The reactions proceed exclusively to the five-membered exocyclization products. 

Phenolic oxygen participates in facile oxypalladation. The intramolecular reaction of 2-hydroxychalcone (105) produces the flavone 106(127]. The ben-zofuran 107 is formed from 2-allylphenol by exo cyclization with Pd(OAc)2, but benzopyran 108 is obtained by endo cyclization with PdCl2[128]. Normal cyclization takes place to form the furan 109 from 2-(l-phenylethenyl)phe-nol[129]. Benzofuran formation by this method has been utilized in the synthesis of aklavinione (110)[130]. 

A reaction flask was charged with 2-allylphenol (0.536 mol) and ethanedithol (0.536 mol) and then heated under reflux for 4 hours. GPC indicated the product was 65% active and the material used without purification. 

A mixture consisting of 2-allylphenol (35 g), 1,10-dibromodecane (39 g), and potassium carbonate (36 g) dissolved in 150 ml of methyl ethyl ketone was heated to 80°C for 24 hours and then treated with 200 ml of water and 200 ml of hexane. The resulting mixture separated into an organic phase and an aqueous phase. The organic phase was washed with water, dried over Na2S04, and then concentrated. After distillation at 210-215°C 0.1 mmHg, 26 g of product was isolated as a liquid. 

An interesting variant on this theme involves the cyclization of substituted 2-allylphenols by A-iodosuccinimide. The 3-iodochromans, which presumably arise via a cyclic iodonium species, are readily dehydrohalogenated by base. The overall yields are generally impressive, although ring iodination can compete (79TL2545). 

The palladium(n)-catalyzed reaction of 2-allylphenols 27 with carbon monoxide and hydrogen (Equation 7) leads to mixtures of five-, six-, and seven-membered lactones, the content of the latter 28 in the mixture being up to 90-100% <1996JA4264>. 

Dihydrobenzofuranes.2 The reaction of 1 with 2-allylphenol at —17° in CH2C12 results in the 2,3-dihydrobenzofurane 2 in 48% yield. At higher temperatures, thiomethylation is observed at the position para to the oxygen. 

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