Elimination reactions tetrahydrofurans

As a result, trisylhydrazones 95 allow the use of just 2 equivalents of alkyllithinm, and thus only a single equivalent of electrophile (E) is needed to trap the resultant vinyllithinm 97 to form the functionalized aUtene 98. Moreover, trisylhydrazone dianions 96 nndergo the elimination reaction much faster than do those derived from tosylhydrazones. This rate enhancement allows the use of even more acidic solvents, such as tetrahydrofuran. 

Astonishingly enough, enantioenriched lithiated cyclooctene oxides 142, originating from (—)-sparteine-mediated lithiation of 124 by i-BuLi/(—)-sparteine (11), could be trapped by external electrophiles, resulting in substituted epoxides 143 (equation 31) ° . Again, the use of i-PrLi furnished better enantioselectivities (approx. 90 10). Lithiated epoxides, derived from tetrahydrofurans and A-Boc-pyrrolidines, undergo an interesting elimination reaction . ... 

Elimination reactions are facile processes as far as they have been studied in the gas phase. It is, however, often difficult to distinguish them from SN2 substitution reactions since both reactions mostly lead to the same product ions, but not to the same neutral products which in most experiments are not known (Smith et al., 1980 Jones et al., 1985). In that respect the reactions of cyclic compounds, such as cyclic ethers, are good probes for the study of elimination reactions because the leaving group remains with the anion. For example, the reaction of NH2 with tetrahydrofuran leads to (M — H) ions. Deuterium labelling has shown that the proton is abstracted exclusively from the P-position (DePuy and Bierbaum, 1981b DePuy et al., 1982b). 

The dehydrohalogenation products of poly(ethylene-a/z-chlorotrifluor-oethylene) were also studied using solid-state NMR spectroscopy and the cross-polarization magic angle spinning (CP MAS) technique [18]. The elimination reaction induced by potassium ZerZ-butoxide (z-BuOK) in tetrahydrofuran (THF) was found to proceed slowly only 50% of hydrogen... 

The 2,2 -bipyridine functional monomer M4 can be synthesized using an elimination reaction [13,14]. 5,5 -Dimethyl-2,2 -bipyridine is first lithiated with lithiumdiisopropyl amide (LDA) at one of the methyl groups. This lithium derivative is subsequently reacted with chloromethoxymethane to form the methoxy-ethyl derivative. Finally, the vinyl group is produced by elimination of the methoxy group in basic tetrahydrofuran (THF) at low temperature. 

Acetoxylation of poly(vinyl chloride) can be carried out under homogeneous conditions. Crown ethers, like 18-crown-6, solubilize potassium acetate in mixtures of benzene, tetrahydrofuran, and methyl alcohol to generate unsolvated, strongly nucleophilic naked acetate anions. These react readily with the polymer under mild conditions. Substitutions of the chlorine atoms on the polymeric backbones by anionic species take place by a Sn2 mechanism. The reactions can also proceed by a Sivl mechanism. That, however, requires formations of cationic centers on the backbones in the rate-determining step and substitutions are in competition with elimination reactions. It is conceivable that anionic species may (depending upon basicity) also facilitate... 

The properties and usefulness of the final polymer depend on its structure and can be directed by appropriate control of process variables during polymerization. Temperature control and the choice of catalyst are critical in minimizing the side reactions, such as formation of carboxyl end groups by elimination of tetrahydrofuran (THE) from 4-hydroxybutyl ester end groups. The proper choice of co-catalyst could promote direct polycondensation reaction with less extent of side reactions. For example, tetrapropyl zirconate is known as an efficient co-catalyst [23]. The temperature and pressure are also significantly important in controlling the rate of polycondensation reaction and degradation. 

The unimolecular gas-phase elimination kinetics of 2-methoxy-l-chloroethane, 3-methoxy-l-chloropropane, and 4-methoxy-l-chlorobutane has been studied using density functional theory (DFT) methods. Results calculated for 2-methoxy-l-chloroethane and 3-methoxy-l-chloropropane suggest that the corresponding olefin forms by dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state. In the case of 4-methoxy-l-chlorobutane, in addition to the 1,2-elimination mechanism, anchimeric assistance by the methoxy group, through a polar five-centered cyclic transition state, provides 4-methoxybutene, tetrahydrofuran, and chloromethane. Polarization of the C-Cl bond is rate limiting in these elimination reactions. 

Kinetic investigation of the reaction of cotarnine and a few aromatic aldehydes (iV-methylcotarnine, m-nitrobenzaldehyde) with hydrogen eyanide in anhydrous tetrahydrofuran showed such differences in the kinetic and thermodynamic parameters for cotarnine compared to those for the aldehydes, and also in the effect of catalysts, so that the possibility that cotarnine was reacting in the hypothetical amino-aldehyde form could be completely eliminated. Even if the amino-aldehyde form is present in concentrations under the limit of spectroscopic detection, then it still certainly plays no pfi,rt in the chemical reactions. This is also expected by Kabachnik s conclusions for the reactions of tautomeric systems where the equilibrium is very predominantly on one side. 

The elimination of sulfur dioxide from thiirane dioxides leading to the corresponding alkenes is not the only result of base-induced reactions other products are also formed. This fact raises the question of the mechanistic pathway of this reaction. In general, the thiirane dioxide is treated with a large excess of the base in an appropriate solvent for several hours at room temperature or below. Bases commonly used are 2n NaOH (in water), NaOCH3 (in methanol), t-BuO-K + (in f-BuOH) and BuLi (in tetrahydrofuran) or KOH-CCU (in t-BuOH)16-19"112 113. 

This procedure illustrates a general method for the stereoselective synthesis of ( P)-disubstitnted alkenyl alcohols. The reductive elimination of cyclic /3-halo-ethers with metals was first introduced by Paul3 and one example, the conversion of tetrahydrofurfuryl chloride [2-(chloromethyl)tetrahydrofuran] to 4-penten-l-ol, is described in an earlier volume of this series.4 In 1947 Paul and Riobe5 prepared 4-nonen-l-ol by this method, and the general method has subsequently been applied to obtain alkenyl alcohols with other substitution patterns.2,6-8 (I )-4-Hexen-l-ol has been prepared by this method9 and in lower yield by an analogous reaction with 3-bromo-2-methyltetra-hydropyran.10... 

Part C of the present procedure illustrates a mild method for effecting the elimination of thiophenol from thioacetals and thioketals under essentially neutral conditions. The reaction of simple thioacetals and thioketals with bis[copper(I) trifluoro-methanesulfonate] benzene complex in benzene-tetrahydrofuran at room temperature affords vinyl sulfides in high yield (Table I). The reaction presumably occurs by coordination of the thiophilic copper(I) reagent with sulfur, heterolysis to a phenylthio-stabilized... 

ISOC reaction was employed to synthesize substituted tetrahydrofurans 172 fused to isoxazolines (Scheme 21) [44b]. The silyl nitronates 170 resulted via the nitro ethers 169 from base-mediated Michael addition of allyl alcohols 168 to nitro olefins 167. Cycloaddition of 170 followed by elimination of silanol provided 172. Reactions were conducted in stepwise and one-pot tandem fashion (see Table 16). A terminal olefinic Me substituent increased the rate of cycloaddition (Entry 3), while an internal olefinic Me substituent decreased it (Entry 4). 

Unlike 1,3-dithiepin anion 144a, the evidence for the instability of 145a and for the lack of aromaticity associated with lOn-electron delocalization through the sulfur atom has been reported 91,92). The reaction of the disodium salt of c/s-dimercaptoethylene (155) with either l,2-dibromo-3-propanol or l,3-dibromo-2-propanol yielded 6,7-dihydro-5f/-l,4-dithiepin-6-ol (156). Treatment of the methoxy derivative 157 derived from 156 with two equivalents of lithium dicyclohexylamide resulted in an effective elimination of methanol to give 5//-l,4-dithiepin (145) as a colorless liquid. Lithiation of 145 with n-butyllithium in tetrahydrofuran at —70 °C... 

Present methods for solubilizing coal (including reductive alkylation in tetrahydrofuran (15) or liquid ammonia (8)) entail cleavage of oxygen ethers, scission of C-C bonds in certain polyaryl-substituted ethylenes and, in the case of reactions in tetrahydrofuran, extensive elimination of hetero-atoms (16). 

Tellurium sources, 22-24 Thermodynamics in cyclo-oligomerization, 185-186 butadiene insertion, 187-188 reductive elimination, 193, 194 selectivity control, 212 polysilane isomerisation, 158-160 see also Stability Thermolysis, 135, 136, 158 THF (tetrahydrofuran), 97, 150, 153 Thio-Wittig reaction, 37 Tin, 121... 

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