zoic acid, eqn. (2). Water is the sole solvent in this case. Table 2
displays the results of these heterogeneous experiments
Investigaciones Químicas (CSIC-La Cartuja, Sevilla) is also
thanked for the use of their NMR facilities.
Notes and references
† Homogeneous catalytic experiment: 0.05 mmol of the catalyst (1 or 2) was
dissolved in 10 mL of acetonitrile and styrene (2 mmol) was added. An
aqueous solution of Oxone (0.5 mmol, 1 mmol of KHSO5, 10 mL H2O) with
three equiv. of sodium bicarbonate (1.5 mmol) was prepared and added to
the catalyst-containing solution. The mixture turned greenish, and was
stirred for 18 h. The products were quantified by GC, using acetophenone
as an internal standard (added at the end of the reaction, immediately before
quantification). The products were identified by NMR from experiments
carried out in deuterated solvents, and by comparison with pure samples.
The results are shown in Table 1.
Heterogeneous catalytic experiment: the copper complexes were sup-
ported as reported in ref. 12. One gram of silica-gel containing BpCu (0.02
mmol) or alternatively Tp*Cu (0.04 mmol) was suspended in 20 mL of H2O
along with 2 mmol of styrene. Neutralised Oxone (prepared as above) was
added, and the mixture was stirred for 8 h. The solid was filtered off, and
reused twice in an identical manner. Table 2 displays the average results of
the three cycles.
(2)
(average of three cycles with the same reused catalyst). The
yields, based on oxidant, are similar to those observed in the
homogeneous case, although the reaction is faster in the former.
The unique difference seems to be the appearance of the
epoxide-opening product, the diol. This must be a consequence
of the acidic nature of the solid support, the silica gel, since this
transformation is usually acid-catalysed.18 The epoxide and the
diol account for 70–85% of the products (Table 2), very close to
the 90% value obtained with the soluble catalysts. It is
important to point out that the immobilisation of the catalyst
does not seem to significantly alter the catalytic capabilities of
this system, i.e., it provides an excellent route to accomplish one
of the permanent goals in catalysis: to heterogenise an
homogeneous system without loss of the activity and/or
selectivity of the latter, in order to achieve the well-known
advantages of the heterogeneous system.
1 M. P. Doyle, M. A. McKervey and T. Ye, Modern Catalytic Methods for
Organic Synthesis with Diazo Compounds, John Wiley & Sons, New
York, 1998.
2 G. Das, R. Shukla, S. Mandal, R. Singh and P. K. Bharadwaj, Inorg.
Chem., 1997, 36, 323.
3 W. Nam, H. J. Kim, S. H. Kim, R. Y. N. Ho and J. S. Valentine, Inorg.
Chem., 1996, 35, 1045.
In conclusion, we have discovered that copper( ) complexes
I
containing polypyrazolylborate ligands present catalytic activ-
ity towards the epoxidation reaction, as they did with the olefin
cyclopropanation and aziridination reactions. These complexes
can operate under both homogeneous and heterogeneous
conditions, with no significant differences when moving from
the former to the latter, thus achieving the well-known
advantages of the recycling and reuse of the catalyst in the
heterogeneous system. Moreover, the parallels between the
epoxidation and the carbene and nitrene transfer reactions are
reinforced since the same metal complexes assure those three
catalytic conversions. Mechanistic, comparative studies for the
three reactions are currently underway in this laboratory.
We thank the Universidad de Huelva (Plan Propio de
Investigación) and the Ministerio de Educación y Ciencia
(PB98-0958) for financial support. M. M. D.-R. also thanks the
Junta de Andalucía for a research studentship. The Instituto de
4 G. Rousselet, C. Chassagnard, P. Capdevielle and M. Mauny,
Tetrahedron Lett., 1996, 47, 8497.
5 S.-I. Murahashi, Y. Oda, T. Naota and N. Komiya, J. Chem. Soc., Chem.
Commun., 1993, 139.
6 C. C. Franklin, R. B. VanAtta, A. F. Tai and J. S. Valentine, J. Am.
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7 M. B. Andrus and B. W. Poehlein, Tetrahedron Lett., 2000, 41, 1013.
8 E. N. Jacobsen, Catalytic Asymmetric Synthesis, ed. I. Ojima, VCH
Publishers, New York, 1993; J. P. Collman, Z. Wang, A. Straumanis and
M. Quelquejeu, J. Am. Chem. Soc., 1999, 121, 460.
9 S. Trofimenko, Scorpionates, The Coordination Chemistry of Poly-
pyrazolylborate Ligands, Imperial College Press, 1999.
10 P. J. Pérez, M. Brookhart and J. L. Templeton, Organometallics, 1993,
12, 261.
11 M. M. Díaz-Requejo, M. C. Nicasio and P. J. Pérez, Organometallics,
1998, 17, 3051.
12 M. M. Díaz-Requejo, T. R Belderrain, M. C. Nicasio and P. J. Pérez,
Organometallics, 2000, 19, 285.
13 M. I. Bruce and J. D. Walsh, Aust. J. Chem., 1979, 32, 2753.
14 C. Mealli, C. S. Arcus, J. L. Wilkinson, T. J. Marks and J. A. Ibers,
J. Am. Chem. Soc., 1975, 98, 711.
Table 2 Epoxidation of styrene catalysed by complexes 1 and 2 supported
on silica gel
15 Y. Golberg and H. Alper, Applied Homogeneous Catalysis with
Organometallic Compounds, ed. B. Cornils and W. A. Herrman, VCH
Publishers, Weinheim, 2000.
Styrene
oxidea
1-phenyl-
Catalyst
ethanediola PhCHOa PhCOOHa Yieldb
16 J. T. Groves, Z. Groos and M. K. Stern, Inorg. Chem., 1994, 33,
5065.
17 A. Sanjuan, M. Alvaro, A. Corma and H. García, Chem. Commun.,
1999, 1641.
18 J. March, Advanced Organic Chemistry, Wiley Interscience, 4th edn.,
New York, 1992.
BpCu
Tp*Cu
28
17
38
70
22
3
11
10
57
68
a Quantified after 6 h by GC, mol% of the products. Average of three cycles.
b Based on oxidant.
1854
Chem. Commun., 2000, 1853–1854