Alkyl allyl ethers, isomerization

Allylic double bonds can be isomerized by some transition metal complexes. Isomerization of alkyl allyl ethers 480 to vinyl ethers 481 is catalysed by Pd on carbon [205] and the Wilkinson complex [206], and the vinyl ethers are hydrolysed to aldehydes. Isomerization of the allylic amines to enamines is catalysed by Rh complexes [207]. The asymmetric isomerization of A jV-diethylgeranylamine (483), catalysed by Rh-(5)-BINAP (XXXI) complex to produce the (f )-enaminc 484 with high optical purity, has been achieved with a 300 000 turnover of the Rh catalyst, and citronellal (485) with nearly 100% ee is obtained by the hydrolysis of the enamine 484 [208]. Now optically pure /-menthol (486) is commerically produced in five steps from myrcene (482) via citronellal (485) by Takasago International Corporation. This is the largest industrial process of asymmetric synthesis in the world [209]. The following stereochemical corelation between the stereochemistries of the chiral Rh catalysts, diethylgeranylamine (483), diethylnerylamine (487) and the (R)- and (5)-enamines 484... 

EtOH, RhCli, reflux, 86% yield. Cleavage proceeds by isomerization and enol ether hydrolysis. See the section on alkyl allyl ether cleavage for other methods to perform the isomerization. 

Metalated alkyl allyl ethers 365 can concurrently undergo [1,2]- and [1,4]-isomerizations leading through l-alken-3-olates and enolates to 1-alken-3-ols and aldehydes, respectively. This possibility was first recognized by Hugh Felkin et a/., " who obtained, after hydrolysis, a mixture of l-hepten-3-ol (23%) and heptanal (29%) after having treated allyl butyl ether with propyllithium in THF (Scheme 1-283). Only 1-hexene (73%) was formed when the same reaction was conducted in pentanes. 

Scheme 1-283. [1,2]- and [l,4]-Isomerizations of metalated alkyl allyl ethers 365. 

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 . 

Compounds of type LXXIII cannot be made by the rearrangement of the 7-substituted allyl ethers, because these compounds yield LXXIV by inversion. a,7-Dimethylallyl bromide,16 7,7-dimethylallyl bromide,29 cinnamyl chloride,84 and phytyl bromide 86 (a vitamin K synthesis) have been used in C-alkylation procedures. The silver salt of 2-hydroxy-l,4-naphthoquinone is converted to a mixture of C-alkylation product and two isomeric ethers by treatment with allylic halides and benzyl halides. ... 

The reaction is highly stereospecific and is always accompanied by inversion of the structure of the allyl group. This inversion is experimentally detectable when the a carbon or y carbon bears an alkyl substituent and is detectable in the case of allyl vinyl ether itself if the a or y carbons of the allyl group are labeled with (ref. 257). That the a carbon of the vinyl group of the ether becomes the carbonyl carbon of the product is demonstrated by the isomerization of isopropenyl allyl ether to hex-5-ene-2-one, and by the isomerization of CH2=CDOO H to CH2=CHCH2CH2CD02S7. 

If the allyl group of the aryl allyl ether featured a tri- or tetra-substituted alkene, then the thermodynamic isomerization to the aromatic benzofuran could not occur, and dihydroben-zofurans were consequently produced in good to excellent yields (Figure 9.8). As in the benzofuran systems, a host of aryl and alkyl substitution patterns were tolerated in this oxidative cyclization. A number of polycyclic and highly functionalized dihydrobenzofurans were obtained via this palladium(II)-catalysed oxidation. 

Allylic and 2-methylallylic ethers are cleanly isomerized to the corresponding vinyl ethers on heating in benzene or toluene with a catalytic quantity of palladium on carbon (e.g. Scheme 41). The method is applicable to both alkyl and aryl allylic ethers, and, importantly, is compatible with functional groups such as aldehyde, ketone, epoxide, and alcohol. 

Included among the many types of vinyl monomers that have been subjected to photoinitiated cationic polymerization are styrene," substituted styrenes, a-methylstyrenes, N-vinylcarbazole, alkyl vinyl ethers, prop-l-en-l-yl ethers, ketene acetals, and alkoxyallenes. Most useful in the crosslinking photopolymerizations employed for UV curing applications are multifunctional vinyl ethers and multifunctional prop-l-en-l-yl ethers. A number of multifunctional vinyl ether monomers are available from commercial sources, while multifunctional prop-l-en-l-yl ethers can be readily prepared by catalytic isomerization from their corresponding allyl ether precursors. The photoinitiated cationic... 

The air-stable complex [Ir(cyclo-octadiene)(PMePh2)2]PF6, after activation with hydrogen, isomerizes allyl ethers to the corresponding frans-propenyl ethers at room temperature with very high stereoselectivity ( 97%) and in high yield (3=95%) [equation (6)]. This appears to be the first stereoselective conversion of alkyl ethers into rrans-propenyl ethers, but the reaction is limited to primary allyl ethers, secondary allyl ethers being unaffected even at 65 °C. 

Dienes can be readily accessed by treating allylic ethers with LIDA in the presence of catalytic or stoichiometric amounts of potassium tert-butoxide. If there is a choice between methyl and alkyl at the 5-center, the former is deprotonated. Thus, both geranyl and neryl methyl ether afford myrcene (7-methyl-3-methylene-l,6-octadiene). (Z)-Alk-2-enyl ethers (Z-254) give mainly or solely ( )-dienes as the ( )-isomeric substrates ( -254) give (Z)-dienes (Scheme 1-201). ... 

Some allyl phenyl ethers with an alkyl substituent on the end carbon of the allyl group rearrange to give the normal ortho-Claisen product together with another isomeric O-allyl phenol. The latter, formed by the rearrangement of the normal product, has been established. This is called abnormal Claisen rearrangement, is illustrated by the following example. 

Ceric ammonium nitrate promoted oxidative addition of silyl enol ethers to 1,3-butadiene affords 1 1 mixtures of 4-(/J-oxoalkyl)-substituted 3-nitroxy-l-butene and l-nitroxy-2-butene27. Palladium(0)-catalyzed alkylation of the nitroxy isomeric mixture takes place through a common ij3 palladium complex which undergoes nucleophilic attack almost exclusively at the less substituted allylic carbon. Thus, oxidative addition of the silyl enol ether of 1-indanone to 1,3-butadiene followed by palladium-catalyzed substitution with sodium dimethyl malonate afforded 42% of a 19 1 mixture of methyl ( )-2-(methoxycarbonyl)-6-(l-oxo-2-indanyl)-4-hexenoate (5) and methyl 2-(methoxycarbonyl)-4-(l-oxo-2-indanyl)-3-vinylbutanoate (6), respectively (equation 12). 

The overwhelming majority of literature devoted to isomerizations of allyl aryl ethers is connected with the aromatic Claisen rearrangement and is summarized in detail in many reviews . Although the [3,3]-sigmatropic isomerization of phenol ethers to the corresponding C-alkylated derivatives has enjoyed widespread application in organic synthesis for over seventy years, it continues to be a very important reaction for the construction of a carbon-carbon bond. This section presents only recent reports. 

---