Issue in Honor of Prof. Armand Lattes ARKIVOC 2006 (iv) 173-188
anion.8 Nevertheless, potassium tert-butoxide is little known as a nucleophile despite these seminal studies.
We have extensively used potassium tert-butoxide as a base together with an organic halide for the CpFe+-induced perfunctionnalization of polymethylbenzene. This set of reactions opened the route to stars and dendritic cores. For instance, in this way, hexamethylbenzene leads, with R = allyl, to either hexabutenylbenzene under ambient conditions (eq. 1)9 or a dodecallyl arene in one pot,10 whereas mesitylene leads to nona-allylation also under ambient conditions in a one-pot reaction (eq.2).11
t-BuOK, excess
+
R
+RX (X = Br or I)
R
h. (vis.)R
Fe-FeII
excess, RT
II
PF6
R
R PF6
MeCN R R
THF
R = alkyl, alkenyl
RR
R
ferrocenylalkyl, allyl, benzyl, RR p-alkoxybenzyl p-bromobenzyl (eq. 1)
Fe PF6 -Fe PF6 - MeCN CH2=CHCH2Br t-BuOK, THF, RT 85% h. (vis.) 95% + + (eq. 2) This latter reaction is the most commonly used one in our laboratory, as it is so easy and convenient to carry out on a large scale either using potassium tert-butoxide or potassium hydroxide. The nona-allyl aromatic obtained can be hydroborated9 or hydrosilylated9,11 for further dendrimer construction using a phenoltriallyl brick (dendron A in Scheme 1)12 that allows multiplying the number of branches by three at each dendritic generation. The largest dendrimers known have been built in this way and characterized until the 9th generation bearing a theoretical number of 311 branches,11 for the first time far beyond the de Gennes “dense-packing” limit13 (Scheme 1).
Thus, the production of the phenoltriallyl brick that is standard is crucial. It is carried out in a one-pot reaction from the cationic 18-electron d6 FeII complex of an aryl ether such as paraethoxytoluene and involves eight steps using potassium tert-butoxide and allyl bromide in THF, including the cleavage of the arylether complex (Scheme 2). Note the multiple roles of potassium tert-butoxide in this complex reaction: first as a base to deprotonate the iron complex in benzylic position, then for the cleavage of the aryl ether in the complex, then finally in the decomplexation reaction providing the desired free phenoltriallyl brick A.
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