Alkyl vinyl ether elimination reactions

Alkyl vinyl ethers decompose to olefins and aldehydes at measurable reaction rates in the 600 °K region, viz. 

Kinetics of five such reactions have been reported and the data are given in Table 17. With the single exception of /-butyl vinyl ether, all the observed -factors are 

Static system no surface effect was observed in seasoned vessels. First order to only 45 % reaction. 

Substituent effects in the alkyl vinyl ether decompositions are in qualitative agreement with those found in the ester elimination reactiors. This can be seen from comparisons of relative reaction rates per j3-H at 600 °K for various a-carbon and jS-carbon substituents as given in Table 18. Similar mechanisms for these two types of reactions are suggested by the data. Charge distribution in the vinyl ether transition state may be represented as 

The substituent effect data indicates that the charge polarity in the vinyl ether transition states is less than in the corresponding ester elimination transition states. The activation energies (corrected) naturally parallel the rate coefficient variations and show a systematic decrease of 2.3 to 2.4 kcal.mole for successive methyl substitutions for hydrogen at the a-carbon. Corrected for gauche destabilizations of the ether ground states, this indicates a transition state charge stabilization by methyl of about 1.6 kcal per CH3. Ester stabilizations have previously been estimated to be about 3.3 kcal per CH3. 

---

The first palladium-catalyzed formation of aryl alkyl ethers in an intermolecular fashion occurred between activated aryl halides and alkoxides (Equation (28)), and the first formation of vinyl ethers occurred between activated vinyl halides and tin alkoxides (Equation (29)). Reactions of activated chloro- and bromoarenes with NaO-Z-Bu to form /-butyl aryl ethers occurred in the presence of palladium and DPPF as catalyst,107 while reactions of activated aryl halides with alcohols that could undergo /3-hydrogen elimination occurred in the presence of palladium and BINAP as catalyst.110 Reactions of NaO-/-Bu with unactivated aryl halides gave only modest yields of ether when catalyzed by aromatic bisphosphines.110 Similar chemistry occurred in the presence of nickel catalysts. In fact, nickel catalysts produced higher yields of silyl aryl ethers than palladium catalysts.108 The formation of diaryl ethers from activated aryl halides in the presence of palladium catalysts bearing DPPF or a CF3-subsituted DPPF was also reported 109... 

Partially fluorinated vinyl ethers of fluoroolefins are quite susceptible to the action of Lewis acids. Reaction usually proceeds with ionization of the allylic C-F bond and results in formation of C=0 group and elimination of alkyl halide. Indeed, 3-chloro-2-methoxyhexafluoro-2-butene 82 reacts with A1C13 with formation of trichlorovinyl ketone 83, and cyclic alkoxyfluoroalkenes demonstrate similar behavior in reaction with aluminum or tin(IV) halides [170] ... 

Unequivocal 5-exo cyclization can be achieved by treating the allyl acetate-vinyl ether system with methylaluminum bis(trifluoroacetate)3. After elimination of the acetate, the resulting allylic cation is attacked by the silyl enol ether at the least alkylated terminus and the five-mem-bered ring is generated exclusively to form 4 with no 1-endo reaction being observed,... 

Chain Transfer. Chain transfer is the most important chain-breaking reaction in carbocationic polymerization. It can be considered as a subcase of termination, when this latter is accompanied by reinitiation. In chain-transfer reactions the active center is transferred to a new site to imreacted monomer or initiator, the preformed polymer, or the solvent. This can happen by /3-proton elimination from the growing chain, or Friedel-Crafts alkylation of aromatic rings. /8-Hydrogen atoms of the propagating carbenium ions are quite acidic owing to the delocalization of the positive charge. In isobutylene, vinyl ether, and styrene... 

Jin and Fuchs reported that vinyl sulfones, using basic phase-transfer catalyst conditions, were regiospecifically alkylated at the a-position.78 No P-elimination products were observed in systems capable of undergoing anion-promoted P-elimi-nation. He also reported that y-methoxy vinyl sulfones 116 can be converted to the corresponding substituted enones 119 using this protocol (Scheme 32).79 On reaction with tert-butyllithium, 116 is converted to the y-methoxy allylsulfonyl anion 117, which was regiospecifically trapped by a variety of electrophiles to provide the enol ether 118. On hydrolysis the -substituted enone 119 was obtained. 

Treatment of 1,3-dicarbonyl compounds with DBP in a methoxide/methanol system affords 2-alkyl-4-[(phenylsulfonyl)methyl]furans, where reaction proceeds by Initial addition-elimination on the vinyl sulfone moiety. In contrast, silyl enol ethers in the presence of silver tetrafluoroborate resulted in products derived from Sn2 displacement at the allylic site.11 Anions derived from 1,3-dicarbonyls substituted at the C-2 position are found to induce a complete reversal in the mode of ring closure.12 The major products obtained are 3-[(phenylsulfonyl)methyl]-substituted cyclopentenones. The internal displacement reaction leading to the furan ring apparently encounters an unfavorable Ai -interaction in the transition state when a substituent group is present at the 2-position ol the dicarbonyl compound. This steric Interaction is not present in the transition state leading to the cyclopentenone ring. 

An intramolecnlar palladium-mediated allylic alkylation via a ketone enolate of piperidinone 54 was reported by Williams and coworkers for the synthesis of R)-l-hydroxyquinine 57 (eqnations 17 and 18) °. The key step involves a palladium-mediated Sjv2 -type cyclization reaction of enol ether 55 in the presence of BnsSnF, giving rise to a quinuclidine ketone, which was immediately rednced to 56 to avoid equilibration and /3-elimination. Interestingly, none of the undesired C3-vinyl stereoisomer was observed. 

Dehydrohalogenation by oxygen bases from vinyl halides proceeds readily by trans elimination via an 2-type of mechanism. c/s-Elimination is sluggish or does not occur at all. Thus trans elimination from bromovinyl ethers 2 (R = alkyl) furnishes the acetylenes 3 in a fast reaction in about 90% yield, whereas elimination from 4 (R = alkyl) is very sluggish and yields a mixture of the acetylene 3 and allene 5 (equations 17 and 18). Similar observations were shown in Schemes 1 and... 

---