Intermolecular reactions phenol arylation

At the beginning of the new millennium, Hashmi et al. presented a broad research study on both intramolecular and intermolecular nucleophilic addition to alkynes and olefins [18]. One of the areas covered by these authors was the isomerization of co-alkynylfuran to phenols [19]. After that, Echavarren and coworkers identified the involvement of gold-carbene species in this type of process, thus opening a new branch in gold chemistry [20]. And subsequently, Yang and He demonstrated the initial activation of aryl —H bonds in the intermolecular reaction of electron-rich arenes with O-nucleophiles [21, 22]. 

Intermolecular arylation of carbocyclic arenes are rare. Exceptionally, phenols are easily arylated. As treated in Chapter 3.7.4, the aryl ether 48 is formed by reaction of aryl halides with hydroquinone monomethyl ether when bulky and electron-rich phosphines are used, which accelerate reductive elimination. Miura and co-workers found new completely different reactions of phenol with bromobenzene by using Pd(OAc)2, PPhs and CS2CO3 to give the unexpected pentaphenylated product 49 in 58% yield [lb, 15]. 

In 2006, Li and coworkers reported a Au-catalyzed aryl C(sp )-H functionalization of phenols and naphthols 3 with dienes toward the synthesis of benzofuran 4 (Scheme 12.2) [6]. This reaction proceeds via a Au(III)-catalyzed intermolecular addition of aryl C-H bond to dienes followed by the intramolecular addition of the phenol O-H bond to afford benzofuran 4. 

The Pd-catalyzed intermolecular C—O bond formation has also been achieved [105-108]. Novel electron-rich bulky phosphine ligands utilized by Buchwald et al. greatly facilitated the Pd-catalyzed diaryl ether formation [109], When 2-(di-tert-butylphosphino)biphenyl (95) was used as the ligand, the reaction of triflate 93 and phenol 94 elaborated diaryl ether 96 in the presence of Pd(OAc)2 and K3PO4. The methodology also worked for electron-poor, neutral and electron-rich aryl halides. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

Intermolecular Arylation Reactions of Phenols and Aromatic Carbonyl Compounds... 

The same catalyst was used by Li and co-workers for an efficient annulation of electron-rich phenols or naphthols with cyelic dienesJ Tricyclic or tetracyclic benzofuran derivatives were obtained as mixtures of syn and anti isomers with the former predominating. The reaetion probably proceeds by intermolecular C-C bond formation via addition of the phenol to the activated diene, followed by protodeauration and intramolecular formation of the C-O bond. In a related transformation, naphthalenes were obtained by treatment of aryl-substituted propargyl esters with eationic gold(I) eatalysts. The reaction may proceed via consecutive 1,3- and 1,2-rearrangements to afford conjugated dienes that then undergo an intramolecular hydroarylation. 

A review describes the asymmetric epoxidation of allylic alcohols,369 another the role of metal oporphyrins in oxidation reactions.370 jhe TiiOPrMi, catalysed self-epoxidation of allylic peroxides proceeds via an intermolecular mechanism.371 Racemic allyl alcohols can be resolved by asymmetric epoxidation (eq.35).372 a Pd(II)/Mn02/benzoquinone system catalyses the oxidative ring-closure of 1,5-hexadienes (eq.36).373 propenyl phenols are oxidatively degraded to aryl aldehydes and MeCHO in the presence of Co Schiff-base catalysts.374 An Oppenauer-type oxidation with Cp2ZrH2/cyclohexanone converts primary alcohols selectively into aldehydes.375 co macrocycles catalyse the oxidation of aryl liydrazones to diazo compounds in high yields.376 similar Co complexes under CO oxidise primary amines to azo compounds.377 Arene Os complexes in the presence of base convert aldehydes and water slowly into carboxylic acids and H2.378... 

Regioselective synthesis of polysubstituted benzofurans from phenols and alkynes using a Cu(OTf)2 catalyst and ZnClj as Lewis acid in O2 atmosphere in a one-pot procedure has been reported by Jiang et al. (Scheme 8.79). The transformation consists of a sequential nucleophihc addition of phenols to alkynes and oxidative cyclization. A wide variety of phenols and alkynes can be used in this reaction. The reaction might proceed via a Cu-catalyzed intermolecular nucleophilic addition and intramolecular Cu-catalyzed aryl C(sp )-H functionahzation [149]. 

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