Potassium tert-butoxide/18-crown

Various chiral crown ethers based on [1,1 -binaphthalene]-2.2 -diol, lactose or other chiral 1,2-di-ols were tested as catalyst in the addition of methyl benzcneacetate to methyl 2-propenoate using sodium amide or potassium tert-butoxide as the base. Some pertinent examples are given261 262 396. 

S. Aoki, S. Sasaki, K. Koga, Simple Chiral Crown Ethers Complexed with Potassium tert>Butoxide as Efficient Catalysts for Asymmetric Michael Additions , Tetrahedron Lett. 1989, 30, 7229-7230. 

Potassium acetate complex with dicyclohexyl-18-crown-6-polyether, 52, 71 Potassium amide, 52, 75 Potassium azide, 50, 10 Potassium tert-butoxide, 52,... 

Intramolecular cyclization of 6-(mesyloxymethyl)bicyclo[4.4.0]dcc-l-cn-3-one using lithium diiso-propylamide produced almost exclusively the y-alkylation product tricyclo[5.3.1.01,6]undec-5-en-4-one (17), together with a trace amount of the 2-alkylation product tricyclo[5.3.1 016]undec-5-en-8-one (18).17 Surprisingly, the a-alkylation product 18 was the major product when the cyclization was carried out using potassium tert-butoxide.17 The preference for y-alkylation over a-alkylation can be rationalized by the Hammond postulate which favors y-alkylation due to the less reactant-like transition state when lithium diisopropylamide is used. Alternatively, when potassium /ert-butoxide and 18-crown-6 in hexamethylphosphoric triamide is used, the reactivity of the enolate anion is significantly enhanced. As a result, the transition state becomes reactant-like so that a-alkylation is the predominant process.17... 

Akiyama s group employed naturally occurring L-quebrachitol 6 to prepare the C2-symmetrical 18-membered chiral crown ether 7 [14]. Compound 7 was found to be an active catalyst for the enantioselective Michael additions of glycine enolates. Thus, deprotonation of ester 8 using potassium tert-butoxide in dichloromethane (DCM) in the presence of crown ether 7 (20 mol %), followed by addition of a Michael acceptor, gave amino-acid derivatives 9 with up to 96% ee, as shown in Scheme 8.4. 

Crown ether 18-crown-6 plays a role m the generation of phenylfluoro-carbene by 1,1-dehydrobromination of a-bromo-a-fluorotoluene with potassium tert-butoxide [J] (equation 3)... 

Trans-l,2-bis[tris(trimethylsilyl)silyl]ethylene 5 was obtained from the reaction of tris(tri-methylsilyl)silyllithium with formic acid methyl ester [7]. The corresponding dianion 6 was obtained as the only reaction product in a fast reaction of 5 with two equivalents of potassium tert-butoxide in the presence of two equivalents of crown ether as the first example of a vinylidene-bridged oligosilyl a,co-dianion (Eq. 3), which is in good analogy with our previously synthesized alkynylidene- and alkylidene-bridged compounds [3,4, 8]. 

By far the most frequently used method is the deprotonation with potassium tert-butoxide, which gives the potassium salts in nearly quantitative yields. The method seems to be usable for any bis(chalcogenophosphinyl and -phosphoryl)imide and has been employed for a broad diversity of derivatives, regardless of the nature of the chalcogen.2,26,30,33,36-38,49,89,91,99 If the salts are needed for further use in reactions with metal halides to form complexes, the potassium salt can be used in situ, without isolation, e.g., with zinc(II) chloride or palladium and platinum chloro complexes.41,43 Potassium metal in THF also forms the salt K[SPh2PNPPh2S] in 82% yield, 38 but the method is not practical for preparative purposes. Potassium-crown ether complexes, [K(18-crown-6)][Q1Ph2PNPPh2Q1] with Q1 = O,92 Q1 = S,93 and Q1 = Se,98 have been prepared by direct complexation of the potassium salt with the macrocyclic ligand. 

Reaction of bromofluorophenylmethane (for a preparation see ref 118) and potassium tert-butoxide with an alkene afforded l-fluoro-l-phenylcyclopropanes (Houben-Weyl, Vol.4/3, p233 Vol. E19b, p980). To identify the reacting species, e.g. carbene or carbenoid, the reaction was carried out without and with an equimolar amount of 18-crown-6. ° Alternatively, chlorofluorophenylmethane, in place of the bromo derivative, can be used. Reactions with bromofluorophenylmethane were performed at 25 °C (sealed tube), whilst those with chlorofluorophenylmethane at 60-80 °C. Addition of fluoro(phenyl)carbene to alkenes is at least 98% stereospecific e.g. formation of . ... 

Chlorobicyclo[4.1.0]hept-2-ene (12) is obtained as a minor product from the reaction of 7,7-dichlorobicyclo[4.1.0]heptane (9) with potassium tert-butoxide and sodium methoxide in benzene or tetrahydrofuran containing catalytic amounts of 1 S-crown-O. The highest conversion (73%) was obtained in benzene, where the chloroalkene accounted for 26% of the product mixture. 

Guida WC, Mathre D1 (1980) Phase-transfer alkylaHon of heterocycles in the presence of 18-crown-6 and potassium tert-butoxide. 1 Org Chem 45 3172-3176... 

The present procedure evolved from our previous work on the conversion of allenals, allenones, and allenylcarbinols to furans and 2,5-dihydrofurans with catalytic silver nitrate (AgN03) in acetone.5-10 It has also been shown that allenylcarbinols can be converted to 2,5-dihydrofuran under these conditions.11 0- and y-Alkynyl allylic alcohols can also be isomerized to furans under strongly basic conditions with potassium tert-butoxide in tetrahydrofuran-tert-butyl alcohol-18-crown-6 or hexamethylphosphoramide (KO-t-Bu in THF-t-BuOH-18-crown-6 or HMPA).12 The AgNOa/silica gel method is milder, faster, and more efficient than the previously reported procedures.13 Moreover, it offers the potential advantage of catalyst recovery and possible applicability to a flow system in which a packed column, protected from light, could serve as the reactor.4... 

The E2 reaction can also be applied to the synthesis of alkynes. When a vinyl halide is treated with a strong base, loss of HX by what is essentially an E2 process leads to formation of a triple bond. In a simple example, 12-methyltridec-l-ene was treated with bromine in dichloromethane to give dibromide 193. When this was treated with potassium tert-butoxide in petroleum ether, in the presence of 18-crown-6, initial elimination gave vinyl bromide 194 but this reacted again with the base, and an E2 reaction gave alkyne 195 in 72% yield for both chemical steps. Alkyne 195 was used in Mori s synthesis of the sphingosine derivative sulfobacin... 

The asymmetric Michael reaction can be catalysed by enantiomerically pure crown ethers in the presence of base. For instance the addition of cyclic donor (11.16) with acceptor (11.22) occurs with up to 99% ee using an enantiomerically pure crown ether in the presence of potassium tert-butoxide. In fact, enantiomerically pure crown ethers have been used to catalyse other Michael reactions, including the use of crown ether (11.45) in the conjugate addition reaction between the ester (11.46) and Michael acceptor (11.47). The reaction is remarkably rapid (one minute at —78°C). 

Guido, W, and Mathre, D. J., Phase-transfer alkylation of heterocycles in the presence of 18-crown-6 and potassium tert-butoxide, J. Org. Chem., 45, 3172, 1980. Mariani, G., Modena, G., Pizzo, G. E, and Scorrano, G., The effect of crown-ethers on the reactivity of alkoxides. Part 2. The reaction of potassium isopropoxide and 2,4-dinitrohalogenobutanes in propan-2-ol-benzene, J. Chem. Soc., Perkin Trans. 2, 1187, 1979. 

As imidazole can also undergo C-alkylation, polyalkylation can be problematic in preparing NMI. Using a phase-transfer catalyst such as 18-crown-6 in presence of potassium tert-butoxide and iodomethane, NMI can be regioselectively formed in good yield (eq 9).i"... 

At room temperature, l,l,4,4-tetrakis(trimethylsilyl)octamethyl-cyclohexasilane (1.00 g, 1.72 mmol), potassium tert-butoxide (387 mg, 3.45 mmol), and 18-crown-6 (910 mg, 3.45 mmol) were reacted in toluene (5 mL) to give a yellow solution from which 1,4-bis(trimethylsilyl) octamethylcyclohexasilanyl-1,4-dipotassium (18-crown-6)2 (1.65 g, 92%) precipitates as yellow crystals. 

Some of the polymers slowly change their helicity in solution. A chiral crown ether-potassium ferf-butoxide combined system was reported to cause polymerization of methyl, tert-butyl, and benzyl methacrylate to form isotactic polymers that had high rotation values (164). Detailed scrutiny, however, raised questions about the result (135, 165). At first, in the presence of the initiator, the oligomers exhibit considerable activity, but after removal of the catalyst, the optical activity decreases. This decrease may be attributed to unwinding of the helixes in the chain the helicity could be caused by the anchored catalyst. 

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