solution (20 times) and cooling the system to 5 ◦C. The catalyst
was thus isolated by organic solvent evaporation, and to this,
water, CTABr and nitrile substrate were added for the recycling
experiments. The reaction led to 95% yield in benzamide in
the first cycle, 81% in the second followed by a decrease of
catalytic activity in the third cycle, where the yield was 54%.
This loss of activity is probably due to incomplete recovery of the
catalyst during work-up. GC-MS analysis on the first extracted
organic phase showed the presence of p-cymene, suggesting that
the active catalytic species lacks this aromatic ligand, which is
probably displaced by the incoming nitrile moiety.
Summarizing the results reported in this paper, complex 1af
(5 mol%) associated with TritonX114 provides good yields
for amide formation from a representative range of substrates
at 100 ◦C, demonstrating comparable catalytic activity to
intrinsically water-soluble RuII catalysts bearing hydrophilic
ligands.18
Nitriles and surfactants are all commercially available products
and were used as received. The identities of the amide products
were assessed by comparison of their 1H, 13C and GC-MS
spectra.
General procedure for the catalytic reactions
The RuII catalysts 1 (7.5 mM, 5 mol% of Ru), water (0.5 mL), the
appropriate amount of surfactant and the corresponding nitrile
(0.15 M, 75 mmol) were introduced to a vial equipped with a
screw-capped septum. The reaction mixture was stirred at 100 ◦C
for the time required. The course of the reaction was monitored
by regular sampling of aliquots of the solution, followed by dilu-
tion with MeOH and analysis by GC. Quenching of the reaction
was found to be unnecessary. Catalyst recycle experiments were
performed on 1 mmol amount of benzonitrile using CTABr
as surfactant following the experimental conditions reported in
Table 2.
In conclusion, we have demonstrated that micelles represent
suitable media for reactions involving water as a reagent, such as
nitrile hydration catalyzed by RuII species 1. Neutral surfactants
showed the best catalytic activity, probably because they do not
interfere (as charged species do) with ligand exchange on the
catalyst, enabling almost quantitative hydration on benzonitrile.
With respect to the use of intrinsically water-soluble catalysts,
micellar media represent an alternative approach to solubilize
hydrophobic complexes in water, and allow the facile screening
of different catalysts without the need for elaborate ligand
modification, but with a less straightforward separation and re-
cycling. Specifically, this reaction medium enabled the screening
of common monophosphine ligands, with the observation being
made that neither electron-rich nor electron-poor phosphines
ensured high catalytic activity, but rather an appropriate balance
of steric and electronic features are required.
Catalysis in water offers wide opportunities and deserves deep
investigation, since positive results are still largely unpredictable.
On the other hand, the use of water as the solvent is a
strong argument in ‘going green’. The micellar approach can,
in principle, be applied to exploit libraries of existing catalysts
for libraries of reactions without the need to synthesize a specific
water-soluble catalyst for each reaction.
Acknowledgements
The authors thank MIUR, the Universita´ Ca’ Foscari di Venezia
and “Consorzio Interuniversitario Nazionale per la Scienza e
Tecnologia dei Materiali” for financial support. Thanks are
expressed to Professors G. Albertin and S. Antoniutti for
providing RuII complexes and to Dr L. Sperni for GC-MS
analysis.
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1
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◦
◦
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This journal is
The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 790–794 | 793
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