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Table 3 Cycloisomerisation of the g-alkynoic acid 4c catalysed by the
iminophosphorane–Au(I) complex 3 in 1ChCl/2Urea: catalyst recycling
Green Chemistry, Marcel Dekker, New York, 2001; (c) M. Poliakoff,
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a
(d) M. Lancaster, Green Chemistry: An Introductory Text, RSC Publishing,
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R. A. Sheldon, I. W. C. E. Arends and U. Henefeld, Green Chemistry
and Catalysis, Wiley-VCH, Weinheim, 2007.
A recent editorial in Organic Process Research and Development
discourages chemists to use solvents that are either known to be
toxic, dangerous for large scale preparations or expensive to dispose
as waste. T. Laird, Org. Process Res. Dev., 2012, 16, 1.
2
3
b
c
Cycle
Time [h]
Yield [%]
TON
1
2
3
4
1
1
1.75
2
99
90
95
90
99
189
284
374
4 (a) A. P. Abbott, R. C. Harris, K. Ryder, C. d’Agostino, L. Gladden and
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4
1, 4996; (d) Q. Zhang, K. de Oliveira Vigier, S. Royer and
a
General conditions: reactions performed under air, at r.t. using
F. J ´e r ˆo me, Chem. Soc. Rev., 2012, 41, 7108; (e) M. Francisco,
A. van den Bruinhorst and M. C. Kroon, Angew. Chem., Int. Ed.,
1
mmol of the alkynoic acid 4c and 1 mol% of catalyst 3 in 1 gr of
b
c
the eutectic mixture 1ChCl/2Urea. Determined by GC. Cumulative
TON values (turnover number = (mol product/mol Au)).
013, 52, 3074; ( f ) G. Yu and F. J ´e r ˆo me, Chem. Soc. Rev., 2013,
2, 9550; (g) F. Del Monte, D. Carriazo, M. C. Serrano, M. C.
2
4
Gutierrez and M. L. Ferrer, ChemSusChem, 2014, 7, 999.
5
6
J. Garc ´ı a- A´ lvarez, Deep Eutectic Solvents and Their Applications as
reaction or the addition of the components of DESs. Finally, this
catalytic system was also active with internal alkynes (entry 7).
However, in this case, a small amount of the corresponding
New Green and Biorenewable Reaction Media, in Handbook of Solvents,
vol. 2, 2nd edn: Use, Health, and Environment, ed. G. Wypych, ChemTec
Publishing, Toronto, 2014.
(a) G. Imperato, S. H o¨ ger, D. Leinor and B. K o¨ nig, Green Chem.,
15
6-membered ring enol-lactone was detected (in a ratio of 10 : 1).
2
006, 8, 1051; (b) G. Imperato, R. Vasold and B. K o¨ nig, Adv. Synth.
The lifetime of a catalytic system and its level of reusability are
Catal., 2006, 348, 2243; (c) F. Illgen and B. K o¨ nig, Green Chem., 2009,
11, 848; (d) F. J ´e r ˆo me, M. Ferreira, H. Bricout, S. Menuel, E. Monflier
and S. Tilloy, Green Chem., 2014, 16, 3876.
(a) B. M. Trost, Science, 1991, 254, 1471; (b) B. M. Trost, M. U. Frederiksen
and M. T. Rudd, Angew. Chem., Int. Ed., 2005, 44, 6630; (c) R. A. Sheldon,
Green Chem., 2007, 9, 1273.
16
very important factors. Thus, under the previously optimised
reaction conditions (1 mol% of 3, 1ChCl/2Urea, r.t. and air) and
using the cycloisomerisation of alkynoic acid 4c as a model
reaction (see Table 3), we found that the catalytic system remains
active (90–99%) after recycling up to four consecutive runs, with a
gradual decrease of the activity after each cycle. Thus, for the first
two cycles 1 hour was needed to achieve high conversions, while
7
8
9
N. T. Patil, R. D. Kavthe and V. S. Shinde, Tetrahedron, 2012, 68, 8079.
(a) J. Garc ´ı a-Alvarez, J. D ´ı ez and J. Gimeno, Green Chem., 2010,
´
12, 2127; (b) J. Garc ´ı a-A
Seifried, Chem. Commun., 2011, 47, 6470; (c) J. Garc ´ı a- A´ lvarez,
´
lvarez, J. D ´ı ez, J. Gimeno and C. M.
J. D ´ı ez, J. Gimeno, F. J. Su ´a rez and C. Vicent, Eur. J. Inorg. Chem.,
2
hours were required in the fourth cycle, probably due to both
leaching during the work-up and decomposition of the catalyst.
012, 5854; (d) J. Garc ´ı a-Alvarez, J. D ´ı ez and C. Vidal, Green Chem.,
´
2
2012, 14, 3190; (e) J. Garc ´ı a-Alvarez, J. Dıez, C. Vidal and C. Vicent,
´
´
Inorg. Chem., 2013, 52, 6533; ( f ) J. Garc ´ı a- A´ lvarez, J. D ´ı ez, J. Gimeno,
In summary, we have designed a new air-stable catalyst, i.e. the
C. M. Seifried and C. Vidal, Inorg. Chem., 2013, 52, 5428.
1
Au(I) complex [AuCl{k -S-(PTA )Q NP (Q S)(OPh)
2
}] (3), for the cyclo-
´
1
0 (a) C. Vidal, F. J. S u´ arez and J. Garc ´ı a-Alvarez, Catal. Commun., 2014,
44, 76; (b) C. Vidal, J. Garc ´ı a-A
´
lvarez, A. Hern ´a n-G ´o mez, A. R. Kennedy
isomerisation of g-alkynoic acids in the eutectic mixture 1ChCl/2Urea.
The reaction proceeds under remarkably mild and aerobic conditions,
displaying a broad substrate scope and functional compatibility. The
following catalytic features of the catalytic system merit highlighting:
and E. Hevia, Angew. Chem., Int. Ed., 2014, 53, 5969.
1 (a) A. Corma, A. Leyva-P ´e rez and M. J. Sabater, Chem. Rev., 2011, 111,
1
1
657; (b) Gold Catalysis, an Homogenous Approach, Catalytic Science
Serie, ed. F. D. Toste and V. Michelet, Imperial College Press, London,
014, vol. 13.
2 It has been previously described that the presence of an external base
typically a tertiary amine or K CO ) is mandatory, in some cases, to
2
(
i) complex 3 is the first active catalyst reported to date for the cyclo-
1
isomerisation of g-alkynoic acids in DESs, (ii) the catalytic reaction
takes place under standard bench experimental conditions (at room
temperature, under air and in the absence of co-catalysts), providing a
pivotal contribution to green chemistry, (iii) its high selectivity precludes
either the addition of the components of DESs to the CRC bond or
the concomitant hydrolysis of enol-lactones 5a–g, and (iv) the catalytic
system can be efficiently recycled (up to four consecutive runs). Thus,
this methodology represents an important contribution to the almost
unexplored field of metal-catalysed organic reactions in DESs. Further
efforts devoted to the development of new catalytic systems active and
recoverable in DESs are currently underway.
(
2
3
promote the metal-catalysed cycloisomerisation of a variety of alkynoic
acids, by catching temporarily the acid proton of the substrate. See for
example: (a) G. Chaudhuri and N. G. Kundu, J. Chem. Soc., Perkin Trans.
1, 2000, 775; (b) H. Harkat, J.-M. Weibel and P. Pale, Tetrahedron Lett.,
2006, 47, 6273; (c) H. Harkat, A. Y ´e nim ´e gu ´e Dembel ´e , J.-M. Weibel,
A. Blanc and P. Pale, Tetrahedron, 2009, 65, 1871.
1
3 It is well-established that a more polar medium would facilitate the
dissociation of the Au–Cl bond making the catalyst more active. For a
recent article which studies the polarity of several choline chloride-based
DESs through solvatochromic optical spectroscopic responses of several
UV-vis absorbance and molecular fluorescence probes, see: A. Pandey,
R. Rai, M. Pal and S. Pandey, Phys. Chem. Chem. Phys., 2014, 16, 1559.
14 Related Au(I)- and Au(III)-NHC catalysts have been successfully employed
in the cycloisomerisation of g-alkynoic acids in water: (a) E. Tom ´a s-
Mendivil, P. Y. Toullec, J. D ´ı ez, S. Conejero, V. Michelet and V. Cadierno,
Org. Lett., 2012, 14, 2520; (b) E. Tom ´a s-Mendivil, P. Y. Toullec, J. Borge,
S. Conejero, V. Michelet and V. Cadierno, ACS Catal., 2013, 3, 3086.
We are indebted to the MICINN of Spain (CTQ2010-14796/BQU)
and COST action SIPs-CM1302 for financial support. J.G.-A. thanks
the MICINN and the European Social Fund for the award of
1
5 Similar formation of 6-membered ring enol-lactone in the gold-
catalysed cycloisomerisation of internal g-alkynoic acids in water
has been recently reported. See ref. 14.
‘
‘Ram o´ n y Cajal’’ contract.
Notes and references
16 (a) D. Cole-Hamilton and R. Tooze, Catalyst Separation, Recovery
and Recycling. Chemistry and Process Design, Springer, Dordrecht,
The Netherlands, 2006; (b) M. Benaglia, Recoverable and Recyclable
Catalyst, John Wiley & Sons, Chichester, UK, 2009.
1
(a) P. T. Anastas and J. C. Warner, Green Chemistry Theory and Practice,
Oxford University Press, Oxford, 1998; (b) A. S. Matlack, Introduction to
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.