Electrophilic substitutions allylic ethers

As an alternative to electrophilic substitution as a means for introducing functional groups into the calixarenes, a reaction sequence has been developed that involves the conversion of calix[4]arene (59) to the tetraallyl ether 63. When 63 is heated in diethylaniline it undergoes a four-fold p-Claisen rearrangement to afford p-allyl-calix[4]arene (62) in excellent yield 126). From the tetra-tosyl ester of 62 (i.e. compound 66 a) a variety of functionalized calixarenes have been obtained, including the aldehyde 66b, alcohol <56 c, bromide 66 d, azide 66 e, amine 66f, and nitrile 66g. Removal of the tosyl group occurs under mildly basic conditions to yield, for example, p-(2-hydroxyethyl)calix[4]arene (66 h). 

These organocopper intermediates were successfully trapped with electrophiles such as allyl bromide, acyl chlorides, and diphenylchlorophosphine, but not with benzaldehyde and methyl iodide. The resulting products were subjected to in situ oxidation with elemental sulfur to form stable alkenylphosphine sulfides 353 [103]. Owing to its synthetic potential, the regioselective carbometallation of substituted ynol ethers has recentiy witnessed a renaissance [91aj. Thus, the alkynyl ethers 354 were treated under different carbocupration conditions (Scheme 10.121). 

Nucleophilic substitutions of allylic functional groups (allyl esters, ethers, alcohols, halides, fluorosilanes, etc.) are efficiently catalyzed by palladium derivatives a very electrophilic jr-allyl palladium being formed as an intermediate. 

Nickel catalysts have been shown to promote allylic substitutions with hard organometallic nucleophiles and allylic ethers or acetals as the electrophiles [38, 105, 106]. In a series of investigations, Hoveyda has documented the use of chiral nickel catalysts for asymmetric allylic substitution reactions [38]. A brilliant illustration is the conversion of acetal 120 into cyclohexanone 123 (90 %, 92 % ee) by employment of EtMgBr in the presence of a chiral Ni catalyst prepared in situ from diphosphine 121 (Equation 11) [106]. 

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