Phenols complexes with Lewis acids

A broad range of compounds can be O-alkylated with carbene complexes, including primary, secondary, and tertiary alcohols, phenols, enols, hemiaminals, hydroxylamines, carboxylic acids, dialkyl phosphates, etc. When either strongly acidic substrates [1214] and/or sensitive carbene precursors are used (e.g. aliphatic diazoalkanes [1215] or diazoketones) etherification can occur spontaneously without the need for any catalyst, or upon catalysis by Lewis acids [1216]. 

Lewis acids catalyze electrophilic reactions of carbonyl compounds with phenols. Reaction of ethyl pyruvate and 1-naphthol in the presence of a dibomacyclopentadienyl Zr(IV) complex affords the ortho-hy-dtoxyalkylated product in up to 87% ee (Scheme 121). Intervention of a Zr naphthoxide intermediate has been proposed, and addition of a small amount of water increases the enantioselectivity (293). 

The direct formation of racemic a-tocopherol from trimethylhydroquinone and isophytol occurs at low temperature in the presence of boron trifluoride or aluminum chloride (71JOC2910). It is important that the solvent should not be able to complex with the Lewis acid rather, it is the phenol-catalyst complex which is alkylated. 

The Gatterman-Koch formylation is unsuitable for the preparation of aldehydes from phenols and phenolic ethers owing to the formation of complexes with the Lewis acid. 

Without protonation of the carbonyl group, weak nucleophiles (TV,/V-dime-thylaniline and phenol) would only react slowly or not at all with the carbonyl groups. Similarly, complexation with Lewis acids can enhance the electrophilic reactivities of carbonyl compounds. This occurs by decreasing participation (using Winstein s concept) of the neighboring oxygen... 

Phenol complexes of [Os] display pronounced reactivity toward Michael acceptors under very mild conditions. The reactivity is due, in part, to the acidity of the hydroxyl proton, which can be easily removed to generate an extended enolate. Reactions of [Os]-phenol complexes are therefore typically catalyzed using amine bases rather than Lewis acids. The regio-chemistry of addition to C4-substituted phenol complexes is dependent upon the reaction conditions. Reactions that proceed under kinetic control typically lead to addition of the electrophile at C4. In reactions that are under thermodynamic control, the electrophile is added at C2. These C2-selective reactions have, in some cases, allowed the isolation of o-quinone methide complexes. As with other [Os] systems, electrophilic additions to phenol complexes occur anti to the face involved in metal coordination. 

This reaction, known as the Gatterman-Koch reaction, does not work with phenolic or amino aromatic species due to complex formation with the Lewis acid. It does work well with aromatic hydrocarbons and is used industrially to prepare benzaldehyde and, as here, p-tolualdehyde. 

Photocationic initiators. Epoxy resins can be cross-linked by compounds containing active hydrogen, e.g., carboxylic acids, anhydrides, amines, phenols etc., or by the ionic polymerization process. Lewis acids such as BF3 and usually a crystalline complex of BF3 with amines, e.g., BF3 NH2C2H5, can be used for curing reaction at 80-100° C [124]. 

PCDEs can be prepared by direct chlorination of diphenyl ether and they were intended to be used for similar purposes as commercial PCB mixtures, chlorination mixtures of biphenyl. Direct chlorination of diphenyl ether at room temperature in acetic acid solution without catalyst yields mainly a monochlorinat-ed diphenyl ether (4-monoCDE) [9], whereas direct halogenation of diphenyl ether with Lewis acid catalysts such as A1C13 results a complex mixture of PCDEs [10]. The synthesis of most individual PCDE congeners can be performed via coupling of biaryliodonium salts and phenols [10]. 

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