Multiple Functionalization of Benzophenones
A R T I C L E S
Scheme 1. Synthesis of the Branching Unit 3,4-Bis-[4-(2-{4-
inner dendrimer generations: (i) the use of a branching unit
carrying the desired function (a priori) or (ii) a postsynthetic
approach (a posteriori). The latter offers a larger flexibility
because once the dendrimer has been synthesized, the func-
tionalization can easily be varied. Recently, Majoral and
Caminade et al.8 introduced phosphorus dendrimers, in which
a layer-specific grafting of different reagents has been achieved.
Schlu¨ter et al.9 presented a concept based on internal aryl bromo
functions that could be reacted using Suzuki cross-coupling.
[(triisopropylsilylethynyl)-ethynyl]-phenyl}-[1,3]dithiolan-2-yl)-
phenyl]-2,5-diphenylcyclopentadienone (7)a
Polyphenylene dendrimers have attracted great attention, as
they possess stiff and shape-persistent dendrons,10 which allowed
the efficient shielding of core moieties11 as well as the
topologically defined placement of functions on the surface.12
Up to now, however, defined postsynthetic functionalizations
in the scaffold of polyphenylene dendrimers are still elusive.
In this paper, we present the synthesis of a benzophenone-
substituted tetraphenylcyclopentadienone branching unit that was
employed in a Diels-Alder cycloaddition to build up polyphen-
ylene dendrimers with a defined number of incorporated keto
groups. Because keto groups are known to be important
substrates for several reactions, for example, Aldol- and Claisen-
condensation or Grignard-reactions,13 they exhibit both prereq-
uisites for a postsynthetic reaction, high reactivity as well as
high selectivity. A postsynthetic functionalization was therefore
applied by reacting the dendrimers with organolithium reagents.
In that way, suitable precursors for the synthesis of trityl radicals
and trityl cations were obtained. Moreover, because the direct
reduction of benzophenone with potassium is known to form
the ketyl radical anion,14 this was also tested for the benzophen-
one bearing dendrimers. Entrapping radicals or charged species
in polyphenylene dendrons was expected to lead to a delocal-
ization of their charge/spin into the all-phenyl backbone.
Moreover, the intramolecular recombination or dimerization of
radical centers should be avoided due to the hindered motion
of the shape-persistent dendritic arms. EPR and NMR techniques
were used to investigate the role of the polyphenylene shell upon
the accessibility and the chemical properties of the charge/spin
carrying centers.
a (i) Oxalyl chloride, DMSO, triethylamine, CH2Cl2, -78 °C, 93%; (ii)
ethane-1,2-dithiole, BF3‚OAc2, CH2Cl2, 95%; (iii) 2/4, 1.1 equiv of
triisopropylsilylethyne, [Pd(PPh3)2]Cl2, PPh3, CuI, toluene/triethylamine, 97/
96%; (iv) 3 equiv of 5, Pd(PPh3)4, K2CO3, toluene/ethanol, 80 °C, 78%.
Results and Discussion
Synthesis of the Scaffold-Functionalized Polyphenylene
Dendrimers. A key reagent for the construction of polyphen-
ylene dendrimers with embedded keto groups is a tetraphenyl-
cyclopentadienone branching unit, carrying benzophenones, and,
in addition, ethynyl groups allowing further growth in a Diels-
Alder cycloaddition. Recently, we have shown that the boronic
acid-functionalized tetraphenylcyclopentadienone 6 is a versatile
building block, as bromo- or iodo-substituted aromatic com-
pounds with the desired functionality can easily be introduced.15
Therefore, the asymmetric benzophenone derivative 3 was
synthesized in which the bromo function was designed for the
coupling with 6 (Scheme 1).
After mono-lithiation of 1,4-diiodo-benzene, the reaction was
quenched with 4-bromo-benzaldehyde to afford (4-bromo-
phenyl)-(4-iodo-phenyl)-methanol (1) in 80% yield.16 Subse-
quent Swern oxidation17 generated 2 in almost quantitative yield.
The desymmetrization of the benzophenone made sure that the
following Hagihara-Sonogashira cross-coupling18 with triiso-
propylsilylethyne could be carried out selectively on the iodo
function to obtain a single product in high yield. Unfortunately,
the Suzuki cross-coupling19 of 3 with 6 gave the desired
benzophenone-functionalized tetraphenylcyclopentadienone (struc-
ture not shown) in only 20% yield, even when different catalyst/
base systems were employed. However, after the introduction
of a dithiolane protecting group,20 the reaction of 5 with 6
furnished the tetraphenylcyclopentadienone 7 in 78% yield
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