Abstract
t-BuOK reacts with cationic 18-electron d 6 Fe II complexes of aryl ethers ArOR as their PF6 - salts to cleave the C-O bond of the aryl ether, yielding Fe II –hexahapto-phenolate complexes and the ethers t-BuOR, only in the presence of a salt, the best one being [n-Bu4N][PF6]. Salt effects in THF result from either enhancement of nucleophilic reactivity of t-BuO - with a large cation or polarization of the aryl ether C-O bond with a THF-insoluble salt such as KBr or NaCl. This nucleophilic reaction of t-BuO - is of special interest for the 8-step synthesis of a triallyl phenol dendron obtained when the cleavage reaction of the complexes of aryl ethers is carried out in the presence of allyl bromide. During this reaction that is carried out without addition of a salt, it is the in situ formation of KBr during the three deprotonation-allylation sequences that further facilitates the crucial aryl ether cleavage at low temperature.
Highlights
There has been considerable interest for the potential of anionic reagents to serve as bases and nucleophiles,[1] and the role of ion-pairing,[2] salts[3] and solvents[4] has been shown to be essential
We report studies showing that the cleavage of this aryl ether complex is a nucleophilic substitution reaction of potassium tert-butoxide, and we examine the influence of the medium on this reaction, in particular that of salts
The key role of ion pairing in the nucleophilic property of the anion t-BuOWhereas potassium tert-butoxide is an excellent base for the facile deprotonation of such iron complexes of polymethylarenes, the present study shows that it cannot cleave the coordinated aryl ether per se, but the presence of [n-Bu4N][PF6] is required
Summary
There has been considerable interest for the potential of anionic reagents to serve as bases and nucleophiles,[1] and the role of ion-pairing,[2] salts[3] and solvents[4] has been shown to be essential. Potassium tert-butoxide has been found in the early 1960’s to be an efficient base in DMSO,[5] and the cation effect was recognized in these early days.[6] Potassium tert-butoxide subsequently became a popular reagent that is used by everyone as a convenient base whose properties are seemingly well known.[7] Its reactivity is that of a base, but it is multifold. Its nucleophilic character can sometimes be expressed, as for instance in the presence of 18-crown-6 ether that segregates the potassium cation, providing an enhanced reactivity of the anion.[8] potassium tert-butoxide is little known as a nucleophile despite these seminal studies
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