Metal phenoxides

Phenol-, phenol, phenol-, phenolic (in com-bination with the name of a metal) phenoxide (or phenolate) of, as Phenolaluminium, aluminum phenoxide, aluminum phenolate, phenolartig, a. of the nature of or like a phenol, phenoloid. 

This is the most common route, the reagent being a metal compound/solvent combination. Typical conditions call for the metal salt (e.g., acetate) in a buffer system (e.g., NaOAc/AcOH) and a co-solvent such as chloroform. Generally the reaction mixture is refluxed until the metal complex spectrum (see Section 9.22.5.6 and Table 4) is fully developed. Metal acetylacetonates and metal phenoxides have also been employed. The topic has been reviewed in detail by Buchler,51 who has also summarized the history and classification of metal complexes of this series, and the mechanisms of metalation.52... 

Williams225 has recently reported the synthesis of etherimides by treating halogeno- or nitroamides (152) with alkali metal phenoxides (153) in a nonpolar organic solvent in the presence of a phase transfer catalyst. 

The combination of a metal phenoxide and a Lewis acid facilitates alkylation of the phenolic moiety by 1,1,3,3-tetramethoxypropane and promotes specific ortho attack. Spontaneous cyclization leads to a mixture of two stereoisomers of 2,4-diethoxychroman (81JHC1325). 

The use of lithium in toluene for the preparation of alkali metal phenoxides appears to be the most convenient and least expensive procedure. The procedure also has the merit of giving the salt as a finely divided powder. 

These studies, which employed density functional theory (DFT) methods (B3LYP/LANL2DZ/Gaussian 98) proposed that the reactions of all alkali metal phenoxides with C02 followed a similar ground mechanism that comprised three intermediates and three transition states. In step 1, C02 must first be activated by an alkali metal phenoxide. In the case of the sodium phenoxide [24a], C02 can only attack at the polarized O-Na bond to form a Ph0Na/C02 complex as the first intermediate (structure 4). The calculation definitely rules out a direct C-C bond formation at the aromatic ring. 

G. Sartori, G. Casiraghi, L. Bolzoni, and G. Casnati, General synthesis of 2//-bcnzo fc]pyrans (chrom-3-enes) from metal phenoxides and ,p-unsaturated carbonyl compounds,/. Org. Chem., 44, 803-805 (1979). 

Structures of [alkali metal phenoxide C02] complex In the Kolbe-Schmitt reaction, phenyl carbonate (I) was originally proposed as the intermediate[2], but infrared absorption (i.r.) spectra of the intermediate showed a band at 1684 cm which disagrees with I, because an absorption band at 1754 cm of methyl phenyl carbonate is not much different from 1748 cm for dimethyl carbonate.[3] Therefore, the carbonyl of the complex might be in the structures such as (II) - (IV). 

The mechanism of the Kolbe-Schmitt reaction was investigated since the late 1800s, but the mechanism of the carboxylation could not be elucidated for more than 100 years. For a long time, the accepted mechanism was that the carbon dioxide initially forms an alkali metal phenoxide-C02 complex, which is then converted to the aromatic carboxylate at elevated temperature. The detailed mechanistic study conducted by Y. Kosugi et al. revealed that this complex is actually not an intermediate in the reaction, since the carefully prepared phenoxide-C02 complex started to decompose to afford phenoxide above 90 °C. They also demonstrated that the carboxylated products were thermally stable even at around 200 °C. The CO2 electrophile attacks the ring directly to afford the corresponding ortho- or para-substituted products. (When the counterion is large (e.g., cesium) the attack of CO2 at the ortho-position is hindered therefore, the para-substituted product is the major product.)... 

Kosugi, Y., Takahashi, K., Imaoka, Y. Solvent-assisted carboxylation of alkali metal phenoxide with carbon dioxide. J. Chem. Res., Synop. 1999, 114-115. 

Kosugi, Y., Imaoka, Y., Gotoh, F., Rahim Mohammad, A., Matsui, Y., Sakanishi, K. Carboxylations of alkali metal phenoxides with carbon dioxide. Org. Biomol. Chem. 2003, 1,817-821. 

Thus, for example, chloral is usually a better acceptor than acetaldehyde, and a metal alkoxide is a better addendum than a metal phenoxide. 

Addition of phenols to activated C—C multiple bonds is another method for O-aUcylation. Conjugated carbonyl compounds with -leaving groups react with metal phenoxides, giving the substituted products via addition-elimination, and the resulted /3-aryloxylated carbonyls are versatile intermediates for synthesis of heterocyclic compounds . Addition... 

Metal phenoxides are structurally related to metal enolates, and undergo aldol reaction to give C-hydroxyalkylated phenols. Reaction of formaldehyde and phenol to give phenol resins is of industrial importance, and occurs under either basic or... 

The classical Kolbe-Schmidt reaction treats alkali metal phenoxides and carbon dioxide at higher than atmospheric pressure, giving salicylic acid. Hirao and Kato developed several modifications for industrial production ". Recently, phenol phosphate was enzymatically carboxylated, giving p-hydroxybenzoic acid ". As for related reactions, Sartori and coworkers conducted o-carbamoylation of aluminum or boron phenoxides with alkyl isocyanate ", and Adachi and Sugasawa o-cyanated phenols using methyl thioisocyanate in the presence of BCI3 (equation 54). ... 

Metal phenoxides are utilized extensively in organic synthesis as reagents, since they can readily be prepared from phenols and appropriate metal reagents, and the phenol moiety can easily be modified either sterically or electronically. Particularly, 2,2 -dihydroxy-l,l -binaphthyl (BINOL), salicylideneamine and Af,Af -ethylenebis(salicylideneamine) (salen) proved to be excellent phenol ligands for asymmetric synthesis. Since some of their reactions have recently been reviewed , it may not be appropriate to reproduce all of them. Instead, this section concentrates on the effect of the phenol moiety on the chemical reactivity and selectivity, and tries to provide structure-activity relationships for the metal phenoxide reagents. Metalated derivatives of monophenols, biphenols and salicylaldehyde imines are discussed separately. 

As indicated by Miyano, alkali metal phenoxides derived from (R)-l,l -bi-2-naphthol (BINOL) possessing an oligoether moiety promoted the asymmetric addition of ketoesters to 7 [39]. Simple BINOL or its monomethyl derivative gave the racemic product, and the appropriate length of the polyoxyethylene chain... 

Polymerization. Typically, the dihydric phenol (1 mole) and aqueous alkali metal hydroxide (2 moles) are mixed under an inert atmosphere in sulfolane and benzene. The water from the aqueous solution plus metal phenoxide formation is removed by distillation of a benzene-water azeotrope between 110° and 140°C. After water removal has been completed, the excess benzene is distilled off, the anhydrous salt in sulfolane cooled to 70°-80°C, and bis(4-chlorophenyl)-sulfone (I) added. The temperature is increased gradually to 200°C and held for four to five hours. Methyl chloride is bubbled in at the end of the polymerization to convert any terminal phenoxide groups to methyl ethers (10). 

Aromatic hydroxycarboxylic acids, especially salicylic acid, have a wide range of applications, for example, as valuable raw materials and intermediates in the production of pharmaceutical chemicals. Originally, salicylic acid was synthesized by the Kolbe-Schmitt reaction [57], which consists of two steps (1) the synthesis and purification of alkali metal phenoxides and (2) carboxylation (Scheme 4.4). Another possible synthetic method is via the attack of a trichloromethyl cation (generated by a copper catalyst from carbon tetrachloride) on the phenoxide anion, followed by hydrolysis of the C—Cl bonds with concentrated sodium hydroxide, because it is fairly difficult to replace an aromatic hydrogen with carboxyl functionality [58]. 

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