Ethers, alkyl vinyl solvent effects

AA sec acrylic acid abstraction sec hydrogen atom transfer abstraction v,v addition and micleophilicity 35 by aikoxy radicals 34-5, 124-5, 392 by alkoxycarbonyloxy radicals 103,127-8 by alkyl radicals 34 5, 113, 116 by f-amyloxy radicals 124 by arenethiyl radicals 132 by aryl radicals 35, 118 by benzovloxy radicals 35, 53, 120, 126 wilh MM a" 53, 120 by /-butovy radicals 35, 53, 55, 124 solvent effects 54, 55. 123 with alkenes 122 3 with ally I acrylates 122 wilh AMS 120, 123 wilh BMA 53, 123 with isopropenvl acetate 121 with MA 120 with MAN 121 with MMA 53, 55, 120.419 with VAc 121 with vinyl ethers 123... 

A third mechanistically distinct [3 -1- 2] cycloaddition between vinyl ethers and vinyl-carbenoids was discovered and reported in 2001 [26]. This reaction is remarkable because when Rh2(S-DOSP)4 is used as the catalyst, the cis-cyclopentenes 142 are formed in up to 99% enantiomeric excess. The reaction occurs between vinylcarbenoids unsubstituted or alkyl-substituted at the vinyl terminus and vinyl ethers substituted with an aryl or vinyl group. Some illustrative examples are shown in Tab. 14.12. The reaction is considered to be a concerted process, which would be consistent with the highly stereoselective nature of the reaction [26]. Contrary to the [3-1-2] cycloaddition derived by means of vinylogous carbenoid reactivity, this latest [3 -1- 2] cycloaddition is not influenced by solvent effects. Due to steric demands on the carbenoid, the [3-1-2] cycloaddi-tion only occurs with cis-vinyl ethers. 

For example, the polymerization of alkyl vinyl ethers using an HC1/ SnCU (or adduct S/SnCl4) initiating system in methylene chloride is very fast even at - 15° C to give polymers with broad and often bimodal MWDs (Figure 17D) [105], Similar effects of solvent polarity are found in the polymerizations of p-alkoxystyrenes [107], styrene [25], and iV-vinylcar-bazole [108],... 

More systematic studies with truns-p-methyl-p-methoxystyrene and with alkyl vinyl ethers confirmed and extended these remaikably large solvent effects on reaction rates the reactions of such alkenes with TCNE show large solvent effects and good correlations between solvent polarity, as represented by the t parameter, and reaction rates. These large effects were interpreted as support for the intervention of zwitterionic intermediates. " ... 

A number of years ago triphenylmethyl cation, Ph3C, formed in situ by dissociation of triphenylmethyl chloride, was shown [73] to initiate the polymerization of 2-ethylhexyl vinyl ether in m-cresol solvent. More recently certain stable carbonium ion salts, notably hexachloroantimonate (SbCls) salts of cycloheptatrienyl (tropylium, C7H7) and triphenylmethyl cations have been shown [74, 50] to be very efficient initiators of the cationic polymerization of many reactive monomers [27, 29, 75]. Since the discovery of the effectiveness of the SbClg salt, triphenylmethyl salts with different anions have also been used [76—78]. The most detailed kinetic studies using these initiators have been carried out on alkyl vinyl ethers [27, 30] and A-vinylcarbazole [39] in homogeneous solution in methylene chloride. 

Although the interpretation offered above for the isotope effect is consistent with the solvent and substituent effects, as well as with secondary isotope effects on other reactions, it will probably be resisted as counterintuitive unless its predictive power is established experimentally. A finding that the isotope effect at Cl is inverse in the cycloaddition of DPK to relatively unhindered alkyl vinyl ethers and reversed as the steric requirements of the substituents on Ci increase will go a long way towards its confirmation. Such an investigation was undertaken by the late Professor E.A. Koerner von Gustorf but discontinued on his untimely death in September 1975. ... 

Copolymerization of TFE with perfluoroalkyl vinyl ethers proved to be quite facile, either by dispersion techniques similar to that employed with FEP [7] or by a newly developed process ploying a fluorocarbon solvent. However, from the earliest studies it was evident that some complications had to be overcome. The most significant of these was a tendency for the alkyl vinyl ethers to rearrange when exposed to free radicals. In the extreme case a chain reaction could be initiated which would result in incomplete rearrangement to the isomeric acid fluoride. During polymerization at temperatures low enough to prevent excessive reaction by this route, the process, nevertheless, competes effectively with free radical coupling as a termination mechanism. 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

The tendency of vinyl ethers to polymerization decreases with the lowering of the alkyls inductive effect, i. e. ter/.-Bu > iso-Pr > Et -Bu, iso-Bu. Polymerization only takes place in strongly polar solvents such as acetonitrile or nitromethane. 

Anions from the Schiffs base (78) can be C- or A -alkylated with ethyl iodide or diethyl sulphate. The ratio of the products depends both on the solvent and on the presence of 18-crown-6. In non-polar solvents, the crown ether increases the solubility of the base, and C-alkylation is the major pathway. In dipolar aprotic solvents, the 18-crown-6 breaks up ion pairs by solvation of the Na" cation, and favours A -alkylation. A nerylsulphonamide, formed from (79), undergoes regiospecific reductive desulphonylation to give nerol (80), which implies that (79) is an effective synthon for cisoid iso-prenoids. Chiral complexes of crown ethers, e.g. (81), catalyse the Michael addition reaction of j3-keto-esters and methyl vinyl ketone to give adducts in high optical yields. ... 

Since 0i refers to a proton in flight , which is loosely bound, it will be considerably less than unity, leading to k - /kP- > 1, although the factor P (with / = 0.69) will produce a solvent isotope effect which is smaller than that commonly found for pure primary effects. This is in fact what is found for a number of reactions in which the rate-determining step is believed to involve proton-transfer from hydronium ion to carbon, typical values being 1.7 for the reaction of the enolate ion of 2-acetylcyclohexanone with hydronium ion, 1.7-3.0 for the hydrolysis of vinyl ethers by strong aqueous acids,1.7-3.2 for the acid-catalysed cleavage of alkyl-mercuric iodides, and 1.7-2.5 for the hydrolysis of a number of secondary diazo-ketones. " ... 

Addition of polar solvents like ethers and amines causes an increase in side chain vinyl content resulting from 1,2 or 3,4 polymerization. This effect is particularly marked in polymerizations with lithium alkyls, which are the only alkali metal alkyls that are soluble in bulk monomer or hydrocarbon solutions. Polar media also lend to increase the proportion of 1,4 units with trans configurations. 

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