Anion addition, phase-transfer-catalyze

Phase-Transfer-Catalyzed Addition of Anion Supplied as Metal Salt... 

Acid-catalyzed addition of aliphatic, aromatic or heteroaromatic cyanohydrins to ethyl vinyl ether, n-butyl vinyl ether or dihydro-4//-pyran provides base stable, protected cyanohydrin derivatives. Phase transfer catalyzed alkylation of aliphatic cyanohydrins with allylic bromides gave a-substituted a-allyl-oxyacetonitrile. Carbonyl compounds react wiA cyanide under phase transfer catalysis to give cyanohydrin anions, which are trapped by an acyl chloride or ethyl chloroformate to give acyl- or alkoxycarbonyl-protected cyanohydrins respectively. The reduction of the carbonyl group of an acyl cyanide by NaBH4 under phase transfer conditions followed by esterification serves as an alternative route to aldehyde-derived cyanohydrin esters. ... 

Enantioselective aldol reactions also can be used to create arrays of stereogenic centers. Two elegant ot-amino anion approaches have recently been published. Fujie Tanaka and Carlos F. Barbas III of the Scripps Institute, La Jolla, have shown (Org. Lett. 2004,6,3541) that L-proline catalyzes the addition of the aldehyde 6 to other aldehydes with high enantio- and diastereocontroJ. Keiji Maruoka of Kyoto University has developed (J. Am. Chem. Soc. 2004,126,9685) a chiral phase transfer catalyst that mediates the addition of the ester 9 to aldehydes, again with high enantio- and diastcrcocontrol. 

Activation of the carboxylic acid as the acyl chloride permits direct reaction with azide anion to form the acyl azide substrates for Curtius rearrangement. Sodium azide is commonly used, and the reaction has been used on the process chemistry scale for the synthesis of benzyl-A-vinyl carbamate. Acryloyl chloride was combined with sodium azide in a biphasic system with phase-transfer catalysis (PTC), providing acyl azide 25. Upon heating, Curtius rearrangement provided vinyl isocyanate, which was distilled directly into benzyl alcohol containing phenothiazine (27) to inhibit polymerization of 26 and triethylamine to catalyze addition of the alcohol to the isocyanate. The vinyl carbamate product 28 was isolated by crystallization. As the autiior clearly pointed out, preparation and reaction of acyl azides, particularly on large scales, require appropriate safety precautions. 

However, it is one of the most valuable and frequently used anions in PTC reactions. It was shown that the quantity of OH ion extracted into the organic phase decreased as its concentration in the aqueous phase increased, and the observed overall activity of OH ion actually increased due to the desiccating effect of the concentrated aqueous solution of OH ion [110]. Addition of a small amount of alcohol to a hydroxide-promoted PTC system usually causes a dramatic increase in reaction rate. One reason is that the alkoxide anions produced are more easily transferred into the organic phase than the highly hydrated OH ion and are at least as basic as OH ions. The other reason is that the solvation of the OH ion with alcohol rather than with water increases its organophilicity [111]. In the PEG-catalyzed dehydrohalogenation of 2-bromo-octane in toluene/KOH(aq) medium, a maximum 126-fold increase in the reaction rate was observed in the presence of methanol [112]. It was observed that the decomposition of various quaternary ammonium cations was retarded by the addition of methanol [113]. 

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