Issue Date: July 23, 2007
Isomerization At a Distance
A new "zipper" catalyst (shown) can isomerize a variety of functionalized alkene derivatives, including diallyl ether, geraniol, and tert-butylpent-4-enyloxydimethylsilane, to form high-purity (E)-isomer products. Depending on the alkene selected, the catalyst can shift the double bond as many as 30 positions along an alkyl chain, according to Douglas B. Grotjahn and colleagues at San Diego State University (J. Am. Chem. Soc., DOI: 10.1021/ja073457i). This feat tops the previous record of 20 positions accomplished in a reaction that requires stoichiometric amounts of a dicyclopentadienylzirconium reagent. Using the new catalyst, Grotjahn's team found that full isomerization of substrates with polar groups such as hydroxyls and amides require heating to 70 ° C. Nonpolar substrates only require 25 ??C. The researchers believe the catalyst begins the isomerization by releasing acetonitrile and forming a complex with the alkene. The basic nitrogen in the imidazole ring then transfers an allylic proton to the alkene, shifting the location of the double bond.
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