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arylation of 1,2,3-triazole
144
in non-toxic polyethylene glycol (PEG)
115
under a
carboxylic acid-assisted pathway (Scheme 12.87).
Furthermore, non-toxic diethylcarbonate (DEC) has also been used as the
reaction medium.
145-147
In 2005, Ackermann highlighted the use of water
along with NMP.
115
Dixneuf and co-workers showed that the Ru-catalyzed
arylation is feasible even in a completely aqueous medium in the presence of
KOPiv.
148
Interestingly, under these conditions the reactivity order of the aryl
halides is PhCl 4PhBr 4PhI, owing to their relative solubilities in water.
12.4.5 Mechanistic Illustration
The Ru-catalyzed arylation reaction can occur via two possible pathways,
each of which involves two steps and differs in the order of these two steps.
These steps are (a) oxidative addition of the aryl halide to form an aryl-Ru
species and (b) ruthenation of the aromatic ring assisted by the directing
group (Figure 12.20).
68
Ackermann and Mulzer showed that complex 4 did not react with aryl
chloride even at elevated temperature, whereas cyclometallation occurred
readily (Figure 12.21). This phenomenon suggests facile cyclometallation
prior to oxidative addition (pathway B).
141
The reports of Davies et al. and Fernandez et al. on the beneficial effect
of NaOAc as additive
149,150
and the DFT calculations by Dixneuf and co-
R
R
Br
RuCl
3
(H
2
O)
n
](5mol%)
MesCO
2
H(30mol%)
PEG- 2000
K
2
CO
3
,120
°
C, 24 h
N
N
X
X
+
Scheme 12.87 Reaction in the presence of PEG-2000 as solvent.
K
2
CO
3
+
N
Ar-RuX
2
-Br
oxidative
addition
cyclometallation
KBr
Ar-Br
+
pathway A
KHCO
3
N
Ar
RuX
2
RuX
2
N
Ph
+
N
pathway B
K
2
CO
3
oxidative
addition
N
cyclometallation
Ru
Ar-Br
KBr
KHCO
3
+
X
Figure 12.20 Probable catalytic cycles.
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