Potassium halide-18-crown-6 systems

Potassium halide-18-crown-6 systems were shown effective to exchange halogens of epihalohydrins. Recently, we have also shown that tetrabutylammonium fluoride is a very effective reagent to replace mesylate of glycerol 1,2-acetonide. ... 

Three PTC methods are worthy of note. In the first the alkyl halide is treated with sodium cyanide in decane solution in the presence of catalytic amounts of hexadecyltributylphosphonium bromide.1673 In the second tetraethylammo-nium cyanide in molar quantities is used in dichloromethane solution with the alkyl halide.1676 In the third method, which is reported to be most satisfactory in the case of allylic and benzylic halides, a solid/liquid system of potassium cyanide and 18-crown-6 ether is employed.168... 

Naked cyanide ion. Alkyl halides are converted into nitriles by potassium cyanide in the presence of a catalytic amount of 18-crown-6. Acetonitrile (or benzene) is used as solvent, and the two-phase system is stirred vigorously at 25-83°. Little or no reaction occurs in the absence of the crown ether, an indication that the ether is functioning also as a phase-transfer catalyst. Primary halides are also converted quantitatively into nitriles chlorides react much faster than bromides. A few percent of elimination products are formed in the reaction of secondary halides. Cyclohexyl halides give only cyclohexene by elimination. o-Dichlorobenzene fails to react. Methacrylonitrile undergoes hydrocyanation to 1,2-dicyanopropane (92% yield). 

Reactions between dialkyl hydrogenphosphonates and haloalkanes have been performed under phase-transfer conditions some initial experiments used diethyl and diisopropyl hydrogenphosphonates with either non-functionalized (e.g. alkyl, allyl or benzyl) halides, or functionalized halides, and employed K2CO3 as base in the presence of tetra-butylammonium salts or 18-crown-6. Other workers have also reported successes in their use of the same or similar systems, but more recent work has raised doubts about the value of the potassium salt, and has illustrated the evident superiority of CS2CO3 as base ... 

Under phase transfer conditions (boiling benzene, 50% aquous sodium hydroxide solution and tetia-n-butylammonium hydrogenphosphate) benzamides are monoalkylated. In the solid-liquid PTC system potassium hydride, tetrahydrofuran and 18-crown-6, acidic trifluoroacetamides are rapidly deprotonated and alkylated with lower kyl, allyl and benzyl halogenides in high yield. Methylation of trifluoroacetamides is carried out under less basic conditions in KOH/acetone. A similar reaction in KOH/DMSO generally allows the efficient reaction of benzamide and acetanilide with primary alkyl halides with secondary halides only moderate yields are accomplished, whereas tertiary halogenides are not alkylated due to dehydrohalogenation. ... 

N-Substituted amides can be N-alkylated by alkyl halides using a two-phase system of solid NaOH-KaCOs in refluxing benzene with tetra-n-butylammonium hydrogen sulphate as phase-transfer catalyst." This method works well for the limited range of compounds studied. Potassium t-butoxide in ether containing small amounts of a crown ether has similarly been used to N-alkylate some iV-arylbenzanilides." N-Aryl-jS-keto-amides can be N-alkylated under standard conditions (NaH-DMF-RX) after first blocking the sensitive jS-keto-amide function by formation of a difluoro-oxyborane complex with BFs-EtjO. ... 

Catalysis by 18-crown-6 of the reaction of solid potassium cyanide with halides in acetonitrile has been studied using a variety of chlorides and bromides. With primary bromides yields were high and reaction times were 15-30 hr at reflux (83 C). Interestingly, in this system chlorides are more reactive than bromides and have reaction time of only about 2 hr. Secondary halides react considerably more slowly and yields drop to 50-60%, because of competing elimination. Tertiary halides do not react successfully since elimination processes dominate. A good example of the utility of these nonaqueous conditions is the preparation of trimethyl-silyl cyanide from the hydrolytically sensitive trimethylsilyl chloride. ... 

---