Isomerization allyl ethers

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

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.

Allyl ethers are isomerized by (Ph3P)3RhCl, and r-BuOK/DMSO in the following order ... 

Other catalysts that have been used to isomerize allylic ethers are Polystyrene-CH2NMe3-RhCl4 (EtOH, H20), RuCl2(PPh)3 (NaBH... 

Isomerization of the double bond in allylic alcohols may result in aldehydes or ketones (I07a). The reaction can have synthetic value (8bJ3c). If isomerization is desired, palladium is probably the preferred catalyst, operated best under hydrogen-poor conditions (/47fl). Allylic ethers can be converted to alcohols by isomerization with (Ph3P)3RhCl at pH 2 to the vinyl ether, which undergoes hydrolysis (36a). 

Allyllithium reagents have also been used in the synthesis of (Z)-y-alkoxyallylboronates 23 2 5. Stereoselectivity is excellent in these reactions since the (Z)-y-alkoxyallyl carbanions prepared by metalation of allyl ethers are stabilized by chelation. The (Z)-y-alkoxyallyl(diisopinocam-pheyl)boranes are prepared at low temperature by an analogous procedure and must be used at — 78 "C otherwise reaction diaslereoselectivity suffers owing to the facile isomerization to the -isomer26. 

Alkoxyallylstannanes can be generated in situ by stannylation of allyl ethers or by 1,3-isomerization of isomers, and trapped by boron trifluoride-diethyl ether complex induced addition to aldehydes to give syn-diol derivatives 13,120. 3-Alkylthioallylstannanes can similarly be generated and trapped84. 

The rearrangement of an O-allyl ether (LXXV) to form the isomeric ortho allyl phenol (LXXVI)... 

Allyl groups attached directly to amine or amide nitrogen can be removed by isomerization and hydrolysis.228 These reactions are analogous to those used to cleave allylic ethers (see p. 266). Catalysts that have been found to be effective include Wilkinson s catalyst,229 other rhodium catalysts,230 and iron pentacarbonyl.45 Treatment of /V-allyl amines with Pd(PPh3)4 and (V,(V -dimethylbarbi Lurie acid also cleaves the allyl group.231... 

Run(H20)6(tos)2 is an efficient catalyst for the isomerization of allylic alcohols and allylic ethers under mild conditions in aqueous media to yield the corresponding carbonyl compounds.39... 

The isomerization of allyl ethers and allyl acetals to vinyl ethers or vinyl acetals, respectively, has found many applications in organic synthesis (Equation (17)). Various transition metal catalysts have been reported in the literature for the isomerization of allyl ethers and allyl acetals. 

The efficient isomerization of allyl ethers and related compounds was accomplished by using NaHFe(CO)4 as catalyst. Isomerization of a homoallyl ether was also carried out in high yield (Scheme 32).56... 

Cis- and // / - -propenyI 4-methoxyphenyl ethers, which were used for Diels-Alder reaction, were synthesized through PdCl2-catalyzed isomerization of 4-methoxyphenyl allyl ether (Equation (18)).59... 

The ruthenium-catalyzed isomerization of aryl allyl ethers or amines followed by ring-closing metathesis with ruthenium catalyst 1 furnishes fused benzo-heterocycles in good yield (Scheme 40).66 67... 

A polymer-supported iridium catalyst 4 has been prepared and used in the isomerization of the double bonds in aryl allyl ethers and aryl allylic compounds with excellent trans-scIcctivity and without conventional workup procedures (Scheme 45).73... 

Alternatively, as shown in Scheme 8, we envisioned that styrenyl allylic ethers, in the presence of an appropriate catalyst, might undergo a net skeletal rearrangement to yield the desired isomeric heterocyclic products [14]. Rearrangement substrates would be synthesized in the non-racemic form by the Zr-catalyzed kinetic resolution [5c]. 

Claisen rearrangementThe allyl ether 1 when heated rearranges and cyclizes slowly and in low yield to the dihydrobenzofuran 2. However, the dianion (NaH) of 1 rearranges in refluxing DMF mainly to the isomeric dihydrobenzofuran 3, a precursor to the antibiotic ( )-atrovenetin (4). 

Similarly, the isomerization can be performed with an allylic ether [65, 76] and ketones such as 42 (with preservation of the double bond geometry ) [77] have also been reported to undergo these isomerizations (Scheme 1.17). 

An irreversible consecutive reaction as a driving force to shift an unfavorable Cope rearrangement equilibria in the needed direction can be illustrated by the Cope-Claisen tandem process used for the synthesis of chiral natural compounds243. It was found that thermolysis of fraws-isomeric allyl ethers 484 or 485 at 255 °C leads to an equilibrium mixture of the two isomers in a 55 45 ratio without conversion into any other products (equation 184). Under the same conditions the isomer 487 rearranges to give the Cope-Claisen aldehyde 491 (equation 185). Presumably, the interconversion 484 485 proceeds via intermediate 486 whose structure is not favorable for Claisen rearrangement. In contrast, one of the two cyclodiene intermediates of process 487 488 (viz. 490 rather than 489) has a conformation appropriate for irreversible Claisen rearrangement243. 

Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl-l-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of allyl ethers to enol ethers has been observed with complexes B [582]. 

Ru(H20)6], which is a precursor of ROM polymerization of cyclic dienes has also been found to possess good alkene isomerization activity [1], Among others it catalyzed the isomerization of allylphenyl ether to a vinylphenyl ether (Scheme 9.1) at room temperature. Allyl ethers are stable to acids and bases, while vinyl ethers are easily cleaved in acidic solutions. Therefore this isomerization gives a mild method for removal of protecting allyl groups under exceedingly mild conditions. 

Finally, addition of the carbanions derived from 83 to non-enolizable aldehydes is a facile process. Aryl and tertiary alkyl aldehydes gave trimethylsilyl allyl ethers 85 by a [1,4]-Brook isomerization (equation 30). The stereochemistry of the intermediate alkoxides 84 dramatically influences the reaction conditions required . 

As for allylic ethers, there are relatively few reports of highly enantioselective rhodium-catalyzed isomerizations of allylic alcohols (for example, >80% ee). Relative to the other processes described above, reactions of allylic alcohols are more atom economical [6] since they obviate the need for a separate hydrolysis step (Eq. 5). 

A series of pyrido[2,3-rf pyrimidine-2,4-diones bearing substituents at C-5 and/or C-6 were synthesized using palladium-catalyzed coupling of uracil derivative 417 with vinyl substrates or allyl ethers to give the regioisomeric mixtures of 418/419 and 420/421, respectively. The ratio of the isomeric structures was dependent on the substituent R. In the case of the reaction with -butyl vinyl ether, only the product 419 was obtained. However, the reactions with acrylonitrile, ethyl acrylate, 2-trifluoromethylstyrene, and 3-nitrostyrene afforded only 418. Also, reaction with allyl phenyl ether gave only 420. The key intermediate 417 was prepared by the reaction of 6-amino-l-methyluracil with DMF-DMA (DMA = dimethylacetamide), followed by N-benzylation with benzyl chloride and vinyl iodination with iV-iodosuccinimide (NIS) (Scheme 15) <2001BML611>. 

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