Polar vinyl ethers

Arylation of butyl vinyl ether with an arylpalladium chloride complex predominantly delivers the terminally arylated product (as expected if a neutral oxidative addition complex is involved). Internal arylation of the highly polarized vinyl ether dominates with the corresponding iodide precursor, suggesting that in this case the cationic intermediate is more plausible as the key intermediate. ... 

Figure 13.2 Polar vinyl ethers for ruthenium removal from metathesis reactions.

A long-standing goal in polyolefins is the synthesis of polymers bearing polar functional groups such as acrylate, esters, or vinyl ethers, etc [24,40]. These copolymers might endow polyolefins with useful properties such as adhesiveness, dyeability, paintability, and print-ibility. Advances have recently been made in polymerizing polar monomers with cationic metallocene catalysts... 

Thus, the preferred intramolecular stabilization of the cationic chain end by the formation of 5- and 3-membered cycles, which is possible for DME but impossible for vinyl ethers, can explain the characteristics of the cationic polymerization of DME in solvents of low polarity. 

The diion mechanism c has been reported for at least some of the reae-tions in categories 3 and as well as some ketene dimerizations. For example, the rate of the reaction between l,2-bis(trifluoromethyl)-l,2-dicyanoe-thene and ethyl vinyl ether was strongly influenced by changes in solvent polarity.Some of these reactions are nonstereospecific, but others are stereo-specific. As previously indicated, it is likely that in the latter cases the di-ionic... 

The corresponding methyl ester similarly underwent the facile rearrangement. A solvent polarity study on the rearrangement rate of the allyl vinyl ether was conducted in solvent systems ranging from pure methanol to water at 60°C.156 The first-order rate constant for the rearrangement of the allyl vinyl ether in water was 18 x 10-5 s 1, compared with 0.79 x 10-5 s 1 in pure methanol. 

P, N, O, S, or C based, which favor covalent bonding and stabilize low oxidation states) due to the metals higher electronegativity and lower oxidation states [24], In recent years, late transition metal catalysts [25-29] have attracted attention not only for the polymerization of a-olefins, but more importantly for the copolymerization of hydrocarbon monomers with readily available polar monomers such as acrylates, vinyl ethers, and vinyl acetate [27 and references therein]. 

The resulting complexes can be effectively employed as single component catalysts to homopolymerize ethylene or copolymerize ethylene with acrylates [50, 51] and a variety of other polar monomers including vinyl ethers, [51,52] vinyl fluoride [53], iV-vinyl-2-pyrrolidinone, and AMsopropylacrylamide [54], In fact, the resulting catalysts are so robust that they can be used as single component catalysts in aqueous emulsion homo-polymerization of ethylene and copolymerization of ethylene with norbomenes and acylates [55]. 

The addition of a cation to an olefin to produce a carbonium ion or ion pair need not end there but may go through many cycles of olefin addition before the chain is eventually terminated by neutralization of the end carbonium ion. Simple addition to the double bond is essentially the same reaction stopped at the end of the first cycle. The addition of mineral acids to produce alkyl halides or sulfates, for example, may be prolonged into a polymerization reaction. However, simple addition or dimerization is the usual result with olefins and hydrogen acids. The polymerization which occurs with a-methyl-styrene and sulfuric acid or styrene and hydrochloric acid at low temperatures in polar solvents is exceptional.291 Polymerization may also be initiated by a carbonium ion formed by the dissociation of an alkyl halide as in the reaction of octyl vinyl ether with trityl chloride in ionizing solvents.292... 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

The vinyl ethers, being polar, are much better solvating agents than alkenes, so that both cations and anions have a stabilizing solvation shell. (Hayashi et al. 1971). 

Complexation by monomer will be most prevalent with polar monomers, e.g., alkyl vinyl ethers, and also with less polar monomers in solvents of low polarity and polarisability, such as alkanes, because in these conditions the monomer is the most polar or polarisable part of the system and thus primarily involved with the solvation of the ions. 

Unless the monomer contains a hetero-atom, the polymer is likely to be much less polar and polarisable than the monomer and the solvent and therefore negligible as an ion solvator but for monomers such as p-MeO-styrene, the alkyl vinyl ethers, or N-vinylcarbazole, the polarity of their polymers is such that they can make a significant contribution to the solvation of Pn+, especially if the solvent is of low polarity. Therefore the species Pn+Pm and, presumably, Pn+PmA must be considered as parts of the ionic population in certain systems. 

Models (Hi) and (iv). Strictly, the only way of finding out definitely whether there is any complexation between the growing cation and the monomer or the polymer, or both, is to investigate whether (and if so, how) the apparent kp+ depends on monomer concentration [16, 17]. We have such evidence only for ACN and styrene and for these the value of kp does not depend on m. This is in accord with the prediction [15,17] that in a highly polar solvent the complexation of Pn+ by a Jt-donor monomer or its polymer is likely to be negligible. The likely behaviour of the w-donor vinyl ethers and their polymers is less clear, but a consideration of the dipole moments and concentrations involved makes it extremely unlikely that these monomers or their polymers could compete successfully for a place in the solvation shell of the growing cations. 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

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