Alkali metal phenoxides reactions

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. 

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.)... 

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). ... 

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]. 

Triaiyl phosphates are prepared by phosphorylation of alkyl phenols without phosphorylation catalyst. This simplifies purification of the plasticizer by eliminating the need to withdraw the pirrified product as a distillate. Mixed alkyl diaiyl esters are produced in the presence of a catalytic amoimt of an alkali metal phenoxide. The plasticizer needs to be separated by distillation. Figure 2.39 shows the distillation process of pmifi-cation of triaryl phosphates. A crude triaryl phosphate ester reaction mixtrrre is passed... 

The aryl ether linkage is important in many natural products and pharmaceuticals, including the antibiotic vancomycin [196]. Diaryl ethers have been efficiently prepared by the coupling of fluo-robenzonitriles with a series of phenols in DMSO using potassium carbonate as a base catalyst and with microwave irradiation [197]. A DPT study of the reaction of alkali metal phenoxides with fluo-robenzenes indicated that the role of the metal cation is to aid the binding of the aryl halide and to facilitate fluoride displacement [198]. 

Trifluorovinyl aromatic ethers can be prepared by several methods. In 1966, phenyl trifluorovinyl ether was first obtained by the reaction of an alkali metal phenoxide (PhONa or PhOK) with tetrafluoroethylene in Parr bombs [25]. The yield of the reaction was mediocre because of side reactions, where reactive fluorocarbanions are trapped by the available proton donors in the reaction system to give saturated 1,1,2,2-tetrafluoroethyl phenyl ethers (Scheme 14.1). 

A number of compounds of the types RBiY2 or R2BiY, where Y is an anionic group other than halogen, have been prepared by the reaction of a dihalo- or halobismuthine with a lithium, sodium, potassium, ammonium, silver, or lead alkoxide (120,121), amide (122,123), a2ide (124,125), carboxylate (121,126), cyanide (125,127), dithiocarbamate (128,129), mercaptide (130,131), nitrate (108), phenoxide (120), selenocyanate (125), silanolate (132), thiocyanate (125,127), or xanthate (133). Dialkyl- and diaryUialobismuthines can also be readily converted to secondary bismuthides by treatment with an alkali metal (50,105,134) ... 

By reactions with alkali metal alkoxides and phenoxides 338... 

The carboxylation of phenols is a well established process for synthesis of salicylic acid according to the Kolbe-Schmitt method (Table 4, entry 39). The exothermic reaction is carried out at slightly elevated temperatures around 150 °C and pressures of approximately 5 bar. Batch processes are still mainly used. The main task is to exclude water from the reaction mixture, because this would release the alkali metal hydroxide from the phenoxide salt. 

---