Ethers Isomerism

The advance of sulfur trioxide as sulfating agent largely depended on advances in sulfonation/sulfation reactor development and changes in raw material quality. Undiluted sulfur trioxide cannot be used as a sulfating agent except in special cases where suitable equipment is used because of its violent nature. Sulfur trioxide diluted in an inert gas, usually air, when used in batch processes can cause excessive dehydration and dark-colored products. However, batch processes were used years ago and inert liquid solvents were often suggested or used to moderate the reaction. Inadequate reaction conditions lead to a finished product that can contain dialkyl sulfate, dialkyl ether, isomeric alcohols, and olefins whereas inadequate neutralization conditions can increase the content of the parent alcohol due to hydrolysis of the unstable acid sulfate accompanied by an increase of mineral sulfate. 

Write the formulas of all ethers isomeric with CH3CH2CH2CH2OH. 

Molybdenum complexes A (Figure 3.46) react efficiently with terminal and internal alkenes in toluene (e.g. 500 eq. Z-2-pentene are metathesized in 2 min at 25 °C 20 eq. of styrene in 2 h at 25 °C). These catalysts also oligomerize 2,4-hexadiene [808] and 1,5-hexadiene [809] and promote RCM of enol ethers. Isomerization of alkenes by catalysts A is a potential catalytic side-reaction [810-812]. 

Allylalumination, with zirconium compounds, 10, 271 Allylamides, isomerization, 10, 75 Allyl aryl ethers, isomerization, 10, 87 Allylation reactions... 

Allylboron compounds, synthesis, 9, 170-171 5-Allylboron compounds, cross-coupling reactions, 9, 212 Allylboronic esters, synthesis, 9, 194—195 Allyl boryl ethers, isomerization, 10, 88 Allylchlorodimethylsilanes, for carbonyl allylations,... 

Allyl electrophiles, addition to zirconacycles, 10, 281 Allyl ethers, isomerization, 10, 85... 

Arylallenes, in rhodium complexes, 7, 221—222 Arylallyl ethers, isomerization, 10, 88 Arylamido complexes with Zr(IV), 4, 771... 

Dialkynyl ligands, in platinum(II) complexes, 8, 545 iV,iV-Diallylamine, via allyindium reagents, 9, 703-704 Diallyl clusters, in trirutheniums and triosmiums, 6, 773 gem-Diallyl esters, via indium-mediated allylation, 9, 677 Diallyl ethers, isomerization, 10, 90 Diamagnetic ylides, with gold(II), 2, 278 Diamides... 

Pentafluorophenyl propargyl ether isomerizes in the gas phase on silica gel at 370°C to give 2-monofluoromethyl-4,5,6,7-tetrafluorobenzo[6]-furan [81JCS(P1)1417]. Via the same route, naphtho[2,l-/ ]furans [82JCS(P1)107, 82JFC(20)173] and 4,5,6,7-tetrafluoro-2,3-dihydro-2-methyl-1-benzothiophene [81JCS(P1)1659] can be synthesized (Scheme 54). 

Engman reported that the acetoxyselenenylation of alkenes could be better carried out with PhSeBr in acetic acid in the presence of acetic anhydride and KNO3 [40]. In the case of terminal olefins the addition proceeds with poor regio-control. However, when the chloroform solution of the two products was treated with catalytic amounts of boron trifluoride etherate, isomerization takes place and the anti-Markovnikov adduct was transformed into the Markovnikov product. The acetoxyselenenylation of alkenes can be cleanly effected also by oxidation of diphenyl diselenide with iodobenzene diacetate in acetonitrile [22]. 

In addition to the formation of silyl enol ethers, isomerization of epoxides to allylic alcohols is another highly typical transformation performed by combination of a silicon Lewis acid with a tertiary amine. Reaction with la was examined, and its scope and limitation reported, by Noyori [61]. Epoxide 44 can be successfully converted into the corresponding allyl silyl ether 45 (Sch. 34). 

Alkyl aryl ethers are qnite stable on heating. Phenyl benzyl ether isomerizes slowly at 250 °C to afford 4-benzylphenol and its ortho-i ovasc as a minor prodnct" . The conditions of isomerizations of O-aUcylated and O-aralkylated phenols were reviewed . 

The preparative Claisen rearrangement was studied in aqueous media at temperatures up to 300 °C. The experiments were conducted in the recently created pressurized microwave batch reactor and in conventional heated autoclaves. It was found that allyl phenyl ether isomerizes in water during 10 min at 240 °C to give the ortho-Claisen rearrangement product in 84% conversion . 

On treatment with LDA, methyl 1-trimethylsilylallenyl ether isomerizes to methyl 3-lithio-l-trimethyl-silylpropargyl ether, which can be used efficiently for the preparation of l-methoxy-l-alken-3-ynes and 2-methoxy-2,5-dihydrofurans. The isomerization in the first step may proceed through a sequence of deprotonation and protonation processes with LDA and diisopropylamine, respectively, since it does not take place under the influence of butyllithium (Scheme 21). ... 

The sulfoxides 215 and 216 also show what is essentially alkene photochemistry [109], Photostationary states of E/Z isomeri2ation were obtained for the analogous sulfides and sulfones as well. Interestingly, if the sulfoxide is replaced by an ether, isomerization is followed by internal ketone hydrogen abstraction from R and five-membered ring formation. 

Allylic alcohols, esters, and ethers undergo acid-catalyzed isomerizations, and allylic alcohols and ethers isomerize at appreciable rates only in the presence of acids. The migrating groups in these reactions are uncharged molecules derived from the oxonium ion conjugate acids of the starting materials. For this reason, and because they are mechanistically related to anionotropic isomerizations, Braude named these reactions oxotropic rearrangements. 

The only kinetic study of acid-catalyzed isomerization of a simple allylic ether was published by Braude and Gore who found that methyl a-phenylallyl ether isomerizes to methyl cinnamyl ether in anhydrous dioxane containing either boron trifluoride or hydrogen chloride. At 25°C, the ether is a quarter to an eighth as reactive as the corresponding alcohol. 

The first-order kinetics, lack of dependence of rate upon pressure in the gas phase above 1 torr, absence of catalysis, and stereospecificity all show that vinyl allyl ether isomerizations are unimolecular reactions. The effects of substrate structure and solvents on reactivity indicate that the rate-limiting transition state does not resemble an ion pair. Methyl substituents on the a and y carbons of the allylic group increase reactivity by only about 10- and 2.5-fold respectively, which is very much less than substituent effects on ionic allylic reactions. While the isomerization of vinyl a-methylallyl ether is about ten times faster in organic solvents than in the vapor phase, the solvent effect is small and does not correlate with solvent polarity. 

Okamoto et al. [31] reported an enantioselective Rh-BINAP-catalyzed allyl ether isomerization-cycloisomerization domino sequence of phenol- or naphthol-linked 1,7-enynes 17 to give dihydrobenzofurans and dihydronaphtho-furans 18 (Scheme 12.9). 

Scheme 12.9 Enantioselective sequentially Rh(l)-BINAP-catalyzed allyl ether isomerization-cycloisomerization reaction.

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