2500 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
MaraVal et al.
that the surface is always very easily functionalized.18 Thus,
our aim was to prepare a dendron possessing one function
located at the core and several easily reactive functions located
on the surface. We report here the synthesis of a third generation
phosphorus-containing dendron possessing a vinyl group at the
level of the core and 16 chlorines on the surface, its subsequent
functionalization at both levels independently, and the use of
such functionalized dendrons as building blocks for the synthesis
of surface-block, layer-block, and segment-block dendrimers.
Figure 1. Schematized special dendritic architectures.
Results and Discussion
to the rapid synthesis of dendrons. In these cases, the surface
of one dendron reacts with the core of several dendrons.10
Association of these larger dendrons by their core is another
way to obtain layer-block dendrimers when layered dendrons
are used.11
The main difficulty in using the divergent strategy for the
synthesis of dendrons is to keep unreacted the function located
at the level of the core when growing the dendron and then to
activate it later. The function we choose to place at the core is
an activated vinyl group which should undergo Michael type
reactions with amines, but which should not react during the
synthesis of the dendron. The first step of this synthesis is a
Staudinger reaction between diphenylvinylphosphine and several
functionalized phosphorus azides 1a-d (Scheme 1). Azides
1a-c were obtained reacting one equivalent of P(S)Cl3, first
with two equivalents of functionalized phenols in basic condi-
tions at low temperature and then with NaN3. An X-ray structure
analysis of compound 1b (Figure 2) indicates that the bond
lengths and bond angles of the P-N-N-N linkage compare
well with those found for other phosphorus azides.19
The Staudinger reaction induces the quantitative formation
of compounds 2a-d-[G′0], possessing a PdN-PdS linkage,
characterized in 31P NMR by the appearance of two doublets.
The X-ray structure analysis of compound 2c-[G′0] reveals the
presence of two crystallographically independent molecules. The
ZORTEP drawing of one of them (2c-[G′0a]) is represented in
Figure 3. Bond lengths and angles of the PdN-PdS linkage
in both molecules are similar to those we already reported,20 as
well as to those concerning the P-CHdCH2 group.21
Interestingly, very special macromolecular architectures called
“segment-block” or “surface-block” dendrimers9 (Figure 1) have
been obtained mainly using dendrons. In the first case, several
different types of dendrons are associated, either two generations
or more of the same dendron,12 or the same generation with a
different skeleton to obtain segment-block dendrimers.13 Surface-
block dendrimers possess two different types of end groups
located in definite areas on the surface of the dendrimers; they
are obtained by grafting dendrons with different end groups
together or on a single core. The ratio of both surface groups
depends on the relative amount of each type of dendrons14 or
on the generation used for both dendrons, which may be
different.15
To the best of our knowledge, no example of synthesis of all
these special dendritic architectures (layer-block, segment-block,
and surface-block dendrimers), using only subtle modifications
of a unique dendron in the last step of its synthesis has been
described up to now. However, such an approach is particularly
interesting, because it would dramatically fasten the synthesis
of special dendritic architectures. Pursuing our researches
concerning tailored dendrimers,16 we reasoned that the divergent
method we used to synthesize phosphorus-containing dendri-
mers17 could be applied to the synthesis of dendrons. The
advantage of the divergent method over the convergent one is
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