Phenoxides, coupling reactions with

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

Although the cross-coupling reaction with organic halides generally requires the assistance of bases, allylic phenoxide and cinnamyl acetate react with 1-alkenylborates under neutral conditions to yield 75% and 12%, respectively, of the corresponding 1,4-dienes (Scheme 2-21) [45, 54]. Thus, the jT -(allylphenoxo)- and r-... 

Coupling reactions with phenoxides. Influence of electronic character and effect of ion-pair dissociation. 

We will now give a more detailed analysis of the available data, using the following four cases 1. Coupling reactions with phenoxides influence of the electronic character and the effect of ion-pair dissociation. 2. Coupling reactions with alkoxides influence of solvation. 3. Nucleophilic substitution at silicon by carbon nucleophiles. 4. Reduction reactions by alanes, AlH Y3 n. 

The optimal pH-value for the coupling reaction depends on the reactant. Phenols are predominantly coupled in slightly alkaline solution, in order to first convert an otherwise unreactive phenol into the reactive phenoxide anion. The reaction mechanism can be formulated as electrophilic aromatic substitution taking place at the electron-rich aromatic substrate, with the arenediazonium ion being the electrophile ... 

Methylene bromide can function as the coupling reagent if it is used in an excess. This unusual coupling reaction succeeds, presumably, because the intermediate a-bromo ether, 3, reacts much more rapidly with the phenoxide endgroup of another polymer than methylene bromide does to produce the formal linked product, 4. 

Alkoxide or aryloxide anions are also reputed to be inactive in Sr I reactions. There is, however, one example of such a reaction at an sp carbon the nitro-derivative of 4-nitrocumyl reacts with phenoxide and 1-methyl-2-naphthoxide ions yielding the corresponding ethers (Kornblum et al., 1967). A similar reaction has been reported for halobenzenes in t-butyl alcohol upon stimulation by sodium amalgam (Rajan and Sridaran, 1977). This reaction could not, however, be reproduced (Rossi and Pierini, 1980) and other attempts to make phenoxide ions react at sp carbons have been equally unsuccessful (Ciminale et al, 1978 Rossi and Bunnett, 1973 Semmelhack and Bargar, 1980). It has been found, more recently, that phenoxide ions react with a series of aryl halides under electrochemical induction, but that the coupling occurs at the p- or o-phenolic carbon rather than at the phenolic oxygen (Alam et al, 1988 Amatore et al, 1988). This is... 

The regiochemistry of the reaction of phenoxide and naphthoxide ions with radicals was explained on the basis of the perturbation theory of the frontier MO involved in the coupling reaction, which are the HOMO of the nucleophile and the SOMO of the radical2626. The position of the coupling depends on the charge distribution in the HOMO of the nucleophile. Perturbation theory and thermodynamic stability of the formed radical anions follow the same tendency. 

It has been shown that reaction of Mn30(02CPh)6(py)2(H20) with [NEt3H]2(biphenoxide) results in the formation of [Mn(III)(biphen)2-(biphenH)]2- (biphen = 2,2 -biphenoxide) (Fig. 5) (143). The complex is a rare example of five-coordinate Mn(III) without softer ligands such as thiolate or chloride. The monodentate biphen-Mn bond represents the first stable example of the Mn phenoxide couple. The monomer displays several intense absorption bands (Table VI). In DMSO (where the compound may be six coordinate), a band at 430 nm, very similar to that of the enzyme, is assigned to a phenoxide - Mn(III) charge transfer band. 

Reaction with phenols and naphthols are usually carried out in the pH range 8-11, when the coupling species is the phenoxide ion. A cold, acidic solution of the diazonium salt is added to an alkaline solution of the phenol, when a fast electrophilic aromatic substitution occurs at the 4-position (Scheme 8.25). If this position is already occupied, attack occurs at the 2-position. 2-Naphthol couples at the 1-position. 

The coupling reactions of phenols with diazonium salts take place most readily using the phenoxide anions. Many of the products are dyestuffs (see below). 

Aromatic diazonium ions normally couple only with active substrates, such as amines and phenols.Many of the products of this reaction are used as dyes (azo dyes) Presumably because of the size of the attacking species, substitution is mostly para to the activating group, unless that position is aheady occupied, in which case ortho substitution takes place. The pH of the solution is important both for phenols and amines. For amines, the solutions may be mildly acidic or neutral. The fact that amines give ortho and para products shows that even in mildly acidic solution they react in their un-ionized form. If the acidity is too high, the reaction does not occur, because the concentration of free amine becomes too small. Phenols must be coupled in slightly alkaline solution where they are converted to the more reactive phenoxide ions, because phenols themselves are not active enough for the... 

Diazonium Coupling Reactions. The union of a diazonium ion with a phenol or an amine is known as a diazonium coupling reaction. The process appears to be an example of aromatic substitution in which the reactive species are the diazonium cation and phenoxide ion or the substituted aniline molecule 9... 

Like perfluoroalkylation, electrophilic a,a-dihydroperfluoroalkylation can be efficiently performed by the use of the appropriate (a,a-dihydroperfluoroalkyl)phenyliodonium salts. A wide range of nucleophiles were alkylated under mild conditions. Most of the reactions can occur by a ligand coupling taking place on an intermediate trisubstituted iodane(UI) compound. This is the case of the reactions with carbanions, amines, alkoxides, phenoxides, thiolates and carboxylic acid ammonium salts. Dialkylation of aniline was also obtained by using two molar equivalents of the iodonium reagent.219,220 (Table 5.11)... 

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