Enhancer Phenoxide

Structure and Crystallinity. The mechanical—optical properties of polycarbonates are those common to amorphous polymers. The polymer may be crystallized to some degree by prolonged heating at elevated temperature (8 d at 180°C) (16), or by immersion ia acetone (qv). Powdered amorphous powder appears to dissolve partially ia acetone, initially becoming sticky, then hardening and becoming much less soluble as it crystallizes. Enhanced crystallization of polycarbonate can also be caused by the presence of sodium phenoxide end groups (17). 

Cram has published several studies indicating that complexation of ammonium salts was greatly enhanced by using cyclic polyethers over the corresponding acyclic ones. These molecules were analogs of the binaphthyl systems under study and were generally prepared by phenoxide substitution on the appropriate tosylate. The approach is illustrated below in Eq. (7.9). ... 

The dimerization of isoprene was carried out by a catalyst system of PdBr2/bis(diphenylphosphino)-1,2-ethane (39) sodium phenoxide at 150°C for I hour in benzene (85). Catalytic activity was enhanced by the addition... 

Displacement by cyanide works particularly well, and many other nucleophilic substitution reactions are enhanced by PTC. Most monovalent anions can be transferred, including alkoxides, phenoxides, thiocyanates, nitrates, nitrites, superoxides and all of the halides. Divalent anions are usually too hydrophilic to be transferred into the organic phase. 

Because of the differential partitioning of hydroxide and phenoxide anions into organic solvents by quaternary ammonium cations, the catalysts generally have little effect on the Reimer-Tiemann reaction of phenols with dihalocarbenes [15]. Cetyltrimethylammonium bromide has been used in the two-phase dichloromethyl-ation of polysubstituted phenols (Scheme 7.21, Table 7.10) under Makosza s conditions [16,17] ring expansion of the reaction products provides an effective route to tropones. The rate of the reaction is enhanced by ultrasonic radiation [16]. 

It has been found that the use, as ligands, of electron-poor phosphines, such as 1,1 -bis(diphenylphosphino)ferrocene, enhances yields ill the palladium-catalysed reaction of aiyl bromides with sodium phenoxides to yield diaryl ethers.87 Palladium-catalysed coupling of ethynylated derivatives of bipyridine or terpyridine with the corresponding bipyridine or terpyridine hahdes or triflates has been used in the synthesis of preorganized polytopic ligands.88... 

The formation of the phenoxide anion enhances the reactivity of the ortho and para positions of the aromatic ring towards electrophilic reagents. The reaction of the phenoxide anion with carbon dioxide at 130 °C leads to ortho carboxylation (the Kolbe reactior. Thus phenol gives salicylic acid (4.4), the acetate of which is aspirin. The reaction is reversible and ortho phenolic acids undergo decarboxylation on heating. 

Nucleophilicity of the guanidine must be. carefully controlled to avoid arylation of the catalyst itself. This could be easily accomplished through a proper choice of the substituents. Note also that delocalization of charge over the three nitrogens in the assumed intermediate guanidinium cation enhances the nucleophilicity of its counter anion, e.g. the p.nitro phenoxide anion. 

The second class of catalysts are zinc(II) mono- or dialkoxides obtained from polyhydric phenols and dialkylzinc with partly polymeric stmctures. This system, extensively studied by Kuran [84], is an optimization of the water/diethylzinc and polyphenol/diethylzinc systems developed by Inoue [85]. The use of soluble zinc phenoxides and their analogous cadmium complexes as catalyst for the copolymerization of CO2 and epoxide was studied extensively by the Darensbourg group [86]. This work focused on the use of mononuclear phenoxide derivatives with bulky substituents, e. g., phenyl- and fe/t-butyl groups, on the aromatic ring to a homogeneous catalytic system and thus enhance the activity of the Zn phenoxides. The catalysts developed are stabilized through ancillary neutral... 

Phenols are more acidic than alcohols because one of the non-bonding electron pairs on oxygen is drawn into the benzene ring by resonance. This stabilizes the phenoxide ion that is formed upon ionization and thus the acidity of phenol is enhanced by the phenomenon. This same withdrawal of electrons by the benzene ring stabilizes aniline and decreases the availability of the nonbonding electron pair on nitrogen. Both effects decrease the basicity of aniline relative to alkyl amines. 

Resonance stabilization of the anion of umbelliferone gives not only the same three forms as the phenoxide anion, but in addition, gives an extra resonance form with (-) charge on a carbon, and the most significant resonance contributor, another form with the (-) charge on the other carbonyl oxygen. This anion is much more stable than phenoxide which we interpret as enhanced acidity of the starting material, umbelliferone. In fact, the pKa of umbelliferone is 7.7 while phenol is about 10. 

Claisen had reported that the proportion of C- to O-alkylation could be greatly enhanced by the use of sodium phenoxides in benzene suspension and indeed this method has been beneficially employed for o-C-isoprenylation. In a general procedure (ref.9), an ethereal solution of the phenol (0.05 mole), was treated with metallic sodium (0.2 mole) and after 1.5 hours the prenyl chloride (RCI) was introduced. Upon refluxing for 10 hours, removal of excess sodium and acidic work-up, monohydric phenols afforded yields from 66-90%. For example, 2-methylphenolwith3-methylbuten-2-ylchloridegave2-methyl-6-(3-methylbuten-2-... 

Resonance effect can also be transmitted via the jt bonds of aromatic rings. A comparison of phenol (pATai 10.0), 4-nitrophenol (pA a. 7.2) and 2,6-dinitrophenol (pATa, 3.6) clearly shows that the presence of a nitro group at these positions enhances acidity, in part by delocalizing electron density in the conjugated phenoxide base. 4-Nitrophenol has one nitro group and its pKa is measured to be 7.2. In 2,6-dinitrophenoxide, there is more resonance stability due to the second nitro group and the pA a was measured to be 3.6.1 ... 

An alternative procedure for the permethylation of polysaccharides is the well-known one described by Hakomori, which employs the methyl-sulfinyl carbanion in methyl sulfoxide and methyl iodide. This newer approach was based on the observation that conversion of phenols into phenoxides is greatly enhanced by virtue of the high basicity of this car-... 

Nucleophilic attack of the phenoxide anion opens the phosphonium ring due to enhanced electrophilic reactivity of the mixed anhydride and acid structures. Salicylyl phenylphosphonite, however, in combination with 2-methyl-2-oxazoline behaves as a Me monomer. ... 

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