Organometallics 2002, 21, 1001-1003
1001
Su ccessive Activa tion a n d P er fu n ction a liza tion of
+
+
Ar en es by π-Com p lexa tion to Cp F e a n d Cp *Ru for th e
Con str u ction of Or ga n om eta llic Sta r s a n d Den d r im er s
†
‡
,†
Beatriz Alonso, J ean-Claude Blais, and Didier Astruc*
Laboratoire de Chimie Organique et Organom e´ tallique, UMR CNRS 5802,
Universit e´ Bordeaux I, 33405 Talence Cedex, France, and Laboratoire de Chimie Structurale
Organique et Biologique, EP CNRS No. 103, Universit e´ Paris VI, 4 Place J ussieu,
7
5252 Paris Cedex, France
Received October 3, 2001
Summary: Temporary complexation and activation of
C6H4Br)6}][PF6], 2 (Scheme 1), a yellow solid character-
+
5
hexamethylbenzene by CpFe (Cp ) η -C5H5) allow mild
hexa-p-bromobenzylation. Then regioselective π-complex-
ation of the six peripheral arene rings of the resulting
ized inter alia by elemental analysis and its MALDI
+
TOF mass spectrum (m/z 1297.76 [M - PF ] , 100). This
6
opens the route to divergent classic Pd-catalyzed func-
+
5
star molecule by Cp*Ru (Cp* ) η -C5Me5) activates
nucleophilic substitution of the six bromides by phenol
dendrons to give an 18-allyl hexaruthenium dendrimer.
6
tionalization. To investigate the use of π-arene activa-
tion, however, we have envisaged the less classic
complexation and activation of the peripheral bro-
+
mobenzene rings by the CpM group (M ) Fe or Ru).
The central iron group was first removed by photolysis
with visible light in the presence of 0.95 equiv of PPh3
in MeCN (1 day, Xenon lamp), giving an 84% yield of
C6(CH2CH2p-C6H4Br)6, 3, an off-white solid (step ii)
characterized inter alia by elemental analysis and its
In tr od u ction
Whereas organometallic activation of hydrocarbons
and catalysis have provided a considerable advance in
organic synthesis including aromatic transformations,
tandem-type reactions whereby two different metals are
combined in successive reactions to reach sophisticated
molecules have gained recent interest. We would like
to illustrate this principle here in metallodendrimer
synthesis starting from hexamethylbenzene, with two
new versions of arene activation using π-complexation.
1
+
MALDI TOF mass spectrum (m/z 1283.43, [M + Ag] ,
+
1
00). Complexation of bromobenzene by CpFe leads to
4
2
some loss of bromide and, thus, is inadequate. On the
+
other hand, multiple complexation by Cp*Ru (Cp* )
5
3
η -C5Me5) proceeds regioselectively in the periphery in
7
7
2
{
2% yield by reaction with [RuCp*(CH3CN)3][OTf] in
h at reflux in CH2Cl2 to give the complex [(RuCp*)6-
C6(CH2CH2η -p-C6H4Br)6}][OTf]6, 4, a tan solid (step
6
Resu lts a n d Discu ssion
iii of Scheme 1 and Figure 1). The hexaruthenium
complex 4 was characterized by elemental analysis and
by the prominent molecular peak at m/z 3339.89 cor-
The first sequence involves the complexation of hex-
+
6
amethylbenzene to CpFe giving [FeCp(η -C6Me6)][PF6],
4
+
1
, followed by hexa-p-bromobenzylation (Scheme 1, step
responding to [M - CF3SO3] (100) observed in the
i).
MALDI TOF mass spectrum. Complexation of the
central hexasubstituted benzene ring is thus precluded
by the large steric bulk of the star branches.
+
The CpFe -induced hexabenzylation of hexamethyl-
5
benzene has been known for some time, but extension
to functional benzylbromides has been demonstrated so
far only with alkoxy substituents.5b We now find that
the hexabromobenzylation of 1 using p-Br-C6H5CH2Br
and KOH in DME proceeds in large scale at 40 °C in 6
Activation of the nucleophilic substitution of halogens
in the cationic complexes of the type [MCp(C H X)][PF ]
6
5
6
4
b
(M ) Fe or Ru) is well known. This nucleophilic
activation has not yet been demonstrated for the less
activating Cp*Ru fragment, however, although an
6
+
days with 80% yield, giving [FeCp{η -C6(CH2CH2p-
example of activation of the substitution of fluoride by
†
Universit e´ Bordeaux I.
Universit e´ Paris VI.
+ 8a
amine has been reported with Cp*Fe , whose activat-
‡
+
8b
(
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use of this strategy in metallodendritic synthesis, we
(
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0.1021/om010871x CCC: $22.00 © 2002 American Chemical Society
Publication on Web 02/06/2002