Quaternary ammonium with active hydrogen

Another mechanism of copolymerization was reported by Feltzin et al.73). These authors also considered activation of tertiary amine by a compound with active hydrogen but with the formation of a quaternary ammonium base (Eq. (45)), being regarded as the real initiator of copolymerization. Initiation, propagation and termination reactions according to Feltzin are illustrated by schemes (46)-(49). 

As regards phosgene reactions with active hydrogen substrates, mechanistic studies were indicative of substrate activation as a result of the nucleophilidty of chloride anion in the case of Q" C1 type catalysts (e.g. quaternary ammonium chloride). The mechanism of nucleophilic assistance by these catalysts can be ra-... 

The asymmetric synthesis achieved when the base is an optically active one is proof that the base is present in a transition state with the carbonyl and not just an agent for removal of protons from hydrogen cyanide. It has further been shown that asymmetric synthesis is still achieved even if the only optically active molecules present are quaternary ammonium compounds, i.e., positive ions without any protons to donate. This probably means that the important thing is to have some positive ion near the carbonyl oxygen, an actual covalent... 

There are relatively few reports of phase-transfer catalysed syntheses of phenols from activated haloarenes using quaternary ammonium salts, presumably because of the instability of the ammonium salts under the reaction conditions. A patented procedure for the conversion of, for example, 2,6-dichloropyridine into 6-chloropyrid-2-one (98%) using aqueous sodium hydroxide in the presence of benzyl-triethylammonium chloride at 120-150°C has been filed [32], A possible route to the phenols, however, comes from the observed reaction of phenols with potassium carbonateipotassium hydrogen carbonate to yield the aryl carbonates (80-85%) using the procedure described for the preparation of dialkyl carbonates (3.3.13) [50]. 

When L=H, (ki/kj) = RkR/TkR, and when L = D, (h/kj) = DkR/TkR, the subscript represents the transferred atom, the superscript represents the atom remaining behind. The (k /ki) values have been determined by comparing the activity of the product olefin at the beginning of the reaction, Rs°, with the activity of the quaternary ammonium salt, Ro. Equation 208 has been used for calculating the IE in the reaction of 373 and equation 209 in the case of compound 374, which has three reactive hydrogens. 

Characteristic of this process is the low concentration of chiral cation necessary to induce optical activity (10% of the substrate concentration is sufficient) and that the chiral salt is recovered mostly unchanged (except for no. 3). Quaternary ammonium ions work as well as substituted ammonium ions, thereby making less probable the mechanism first suggested (Gourley et al., 1967, 1970), namely, hydrogen atom transfer from an ammonium radical R3NH to the substrate. Today it seems reasonably well established that adsorption of the chiral cation, possibly as a complex with an intermediate is connected with the asymmetric induction observed (Horner et al., 1972 Kariv et al., 1973a). 

Electrondeficient olefins can be epoxidized by hydrogen peroxide in the presence of bases. This method, known as the Weitz-Scheffer reaction, is used especially for the epoxidation of o,p-unsaturated ketones. Wynberg et al. [4] epoxidized a series of such ketones with hydrogen peroxide in the presence of quaternary ammonium salts of optically active alkaloids. Especially when phase-transfer conditions were used they yielded an e.e. of about 50% (Scheme 3). 

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