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V. Calo et al. / Journal of Organometallic Chemistry 692 (2007) 4397–4401
4400
ammonium chloride, insoluble in TBAB, afforded an
intractable medium inadequate for further recycling
purposes.
Acknowledgements
This work was in part financially supported by Minis-
`
Indeed, one of the most observed limitation in recycling
the Pd-catalyzed reactions in molecular solvents, as well as
in imidazolium or pyridinium ILs, is represented by the
gradual catalyst deactivation due to an increase of inor-
ganic salts or ammonium halides deriving from H–Pd–Cl
neutralization by inorganic or organic bases such as ter-
tiary amines. On the contrary, the reaction of TBAA with
H–Pd–Cl affords, besides the restoration of the Pd(0) cata-
lyst, acetic acid and tetrabutylammonium chloride miscible
with TBAB. When necessary, tetrabutylammonium chlo-
ride may be reconverted into TBAA by simple methatesis
with sodium acetate in acetone. The effectiveness of TBAA
in neutralizing the PdH is due to the higher basicity of the
acetate in TBAA than in water. In this IL, for steric rea-
sons, the acetate is distant from the bulky tetraalkylammo-
nium and therefore less solvated than in water [26].
Results reported above point out the fundamental role
of TBAB as an IL in governing the catalyst life and activity
by means of the stabilization of the Pd-nanoparticles.
However, despite the observed beneficial effects exerted
by quaternary ammonium salts on the Heck reaction [27],
the exact nature of this influence cannot be ascribed to a
single effect such as the high polarity or phase-transfer abil-
ity, but rather to a superposition of several factors. For
example, Reetz and Maase [19] found that Pd-nanoparti-
cles were stabilized by large ammonium cations. Further-
more, Amatore and Jutand [28], demonstrated that
Pd(0)(PPh3)2, the proposed catalyst in the C–C coupling
reactions, was unstable in the absence of halide or acetate
ions which transform this complex into a more stable and
catalytically active 16-electron anionic complex as
[Pd(PPh3)2X](À). Following these conclusions and to eluci-
date the ionic liquid effects, we propose some consider-
ations that would assist in explaining our results. In
TBAB the collapse of the Pd-nanoparticles into inactive
‘‘Pd-black’’ is inhibited by the capping of the catalyst due
to the large tetraalkylammonium cations [23]. In this situ-
ation the Coulombic repulsion amongst the nanoparticles
impedes their aggregation [19,20].
tero dell’Universita e della Ricerca Scientifica e Tecnolog-
ica, Rome, and the University of Bari (National Project:
‘‘Stereoselezione in Sintesi Organica: Metodologie ed
Applicazioni’’).
References
[1] (a) Reviews: F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102
(2002) 4009;
(b) M. Wilde, K. Anders, Chem. Tech. (Leipzig) 46 (1994) 316;
(c) S. Ordonez, H. Sastre, F.V. Diez, Recent Res. Dev. Chem. Eng. 4
˜
´
(2000) 327.
[2] M.J. Morra, V. Borek, J. Koolpe, J. Environ. Qual. 29 (2000) 706.
[3] G.H. Eduljee, R.E. Hester, R.M. Harrison (Eds.), Waste Incineration
and the Environment, Royal Society of Chemistry, Cambridge, 1994.
[4] (a) M.E. Logan, M.E. Oinen, Organometallics 25 (2006) 1052;
(b) Y. Zhu, C. Ching, K. Carpenter, R. Xu, S. Selvaratnam, N.S.
Hosmane, J.A. Maguire, Appl. Organometal. Chem. 17 (2003) 346,
and for ligandless homogeneous conditions see;
(c) R.E. Maleczka Jr., R.J. Rahaim Jr., Tetrahedron Lett. 43 (2002)
8823.
[5] (a) Selected recent examples: Y. Shindler, Y. Maratov-Meytal, M.
Sheintuch, Ind. Eng. Chem. Res. 40 (2001) 3301;
(b) H. Sajiki, A. Kume, K. Hattori, H. Nagase, K. Hirota,
Tetrahedron Lett. 43 (2002) 7251;
(c) A. Arcadi, G. Cerichelli, M. Chiarini, R. Vico, D. Zorzan, Eur. J.
Org. Chem. (2004) 3404;
(d) Y. Monguchi, A. Kume, K. Hattori, T. Maegawa, H. Sajiki,
Tetrahedron 62 (2006) 7926.
[6] P.P. Cellier, J.-F. Spindler, M. Taillefer, H.-J. Cristau, Tetrahedron
Lett. 44 (2003) 7191.
[7] (a) G. Evdokimova, S. Zinovyev, A. Perosa, P. Tundo, Appl. Catal.
A: Gen. 271 (2004) 129;
(b) T. Janiak, J. Błazejowski, Appl. Catal. A: Gen. 271 (2004) 103.
[8] P. Selvam, S.U. Sonavane, S.K. Mohapatra, R.V. Jayaram, Tetra-
hedron Lett. 45 (2004) 3071.
[9] H.M. Roy, C.M. Wai, T. Yuan, J.-K. Kim, W.D. Marshall, Appl.
Catal. A: Gen. 271 (2004) 137.
[10] (a) Y. Gao, S. Liao, F.D. Wang, X.G. Jian, Chin. Chem. Lett. 11
(2000) 743;
(b) R. Nakao, H. Rhee, Y. Uozumi, Org. Lett. 7 (2005) 163.
[11] P. Giannoccaro, M. Gargano, A. Fanizzi, C. Ferragina, A. Leoci,
M. Aresta, J. Mol. Cat. A: Chem. 227 (2005) 133.
[12] T. Hara, K. Mori, M. Oshiba, T. Mizugaki, K. Ebitani, K. Kaneda,
Green Chem. 6 (2004) 507, and references therein.
[13] F.D. Kopinke, K. Mackenzie, R. Koehler, A. Georgi, Appl. Catal.
A: Gen. 271 (2004) 119.
_
4. Conclusions
[14] O. Navarro, H. Kaur, P. Mahjoor, S.P. Nolan, J. Org. Chem. 69
(2004) 3173.
[15] C. Desmarets, S. Kuhl, R. Schneider, Y. Fort, Organometallics 21
(2002) 1554.
[16] (a) See for example: C.-B. Wang, W.-X. Zhang, Environ. Sci.
Technol. 31 (1997) 2154;
Palladium-nanoparticles, stabilized by tetrabutylammo-
nium bromide as the solvent and in the presence of
tetrabutylammonium acetate as the base, do allow the hyd-
rodehalogenation of aryl chlorides under hydrogen at
atmospheric pressure. In this solvent the nanoparticles
are so stable to permit an extensive recycling of both the
catalyst and the ionic liquid. In addition, as the catalyst
is air-stable, the experimental procedure does not require
special cares during both work-up and recycling opera-
tions, such as for example the use of the inert atmosphere,
thus increasing the value of this methodology.
(b) J.J. Davis, C.B. Bagshow, K.L. Busuttil, Y. Hanyu, K.S.
Coleman, J. Am. Chem. Soc. 128 (2006) 14135;
`
(c) For reviews see: M. Kralik, A. Biffis, J. Mol. Catal. A: Chem. 177
(2001) 113;
`
(d) B. Corain, M. Kralik, J. Mol. Catal. A: Chem. 173 (2001) 9.
[17] D.V. Davydov, I.P. Beletskaya, Organomet. Chem. USSR 3 (1993)
11.
[18] L.A. Fowley, D. Michos, X.-L. Luo, R.H. Crabtree, Tetrahedron
Lett. 34 (1993) 3075.