E. M.
Schneider
a,
M.
Zeltner
a,
N.
Kränzlin
b,
R. N.
Grass
a and
W. J.
Stark
*a
aInstitute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland. E-mail: wendelin.stark@chem.ethz.ch
bLaboratory for Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland
First published on 21st May 2015
For the first time Knoevenagel condensation has been catalyzed by elemental copper with unexpected activity and excellent isolated yields. Inexpensive, widely available copper powder was used to catalyze the condensation of cyanoacetate and benzaldehyde under mild conditions. To ensure general applicability, a wide variety of different substrates was successfully reacted.
Entry | Catalyst | Time (h) | Yieldb (%) |
---|---|---|---|
a Reaction conditions: catalyst (4 mg), benzaldehyde (0.12 mmol) and ethyl cyanoacetate (0.1 mmol) in 1 mL of EtOH for 2 h. b Only the E-isomer was detected, yield determined via HPLC-UV (MS) using commercial product 3 as the reference standard. | |||
1 | None | 2 | 3 |
2 | C/Co | 2 | 8 |
3 | Co powder | 2 | 9 |
4 | C/Fe | 4 | 4 |
5 | FeCl3 | 4 | 4 |
6 | Fe powder | 4 | 6 |
7 | C/Cu | 4 | 12 |
8 | Cu powder | 2 | 32 |
9 | Brass alloy 260 | 2 | 5 |
10 | Ag powder | 4 | 7 |
11 | Au powder | 4 | 6 |
12 | ZnCl2 | 6 | 4 |
Entry | Solvent | Loading (mg) | t (h) | Cub (ppm) | Yieldc (%) |
---|---|---|---|---|---|
a Reaction conditions: copper (4 mg), benzaldehyde (0.12 mmol) and ethylcyanoacetate (0.1 mmol). b Cu leaching measured by ICP-OES. c Determined via HPLC-UV using commercial product 3 as the reference standard. d Isolated yield. | |||||
1 | EtOH | 0.4 | 2 | 3 | 6 |
2 | EtOH | 2 | 2 | 38 | 14 |
3 | EtOH | 4 | 2 | 44 | 32 |
4 | EtOH | 10 | 2 | 216 | 96 (95)d |
5 | EtOH | 40 | 2 | 114 | 99 (85)d |
6 | EtOH | 4 | 6 | 44 | 99 (95)d |
7 | Acetone | 4 | 20 | 18 | 1 |
8 | Cyclohexane | 4 | 20 | 32 | 3 |
9 | Toluene | 4 | 20 | 30 | 3 |
10 | DMF | 4 | 6 | 17 | 99 (98)d |
11 | DMSO | 4 | 2 | 241 | 99 |
12 | MeCN | 4 | 2 | 230 | 99 (98)d |
Entry | Reactant | Reactant | Product | Yieldb (%) |
---|---|---|---|---|
a Reaction conditions: Cu (20 mg), aldehyde (0.21 mmol) and active methylene (0.2 mmol) in EtOH for 6 h at 56 °C.
b Yield determined via HPLC-UV or GC-FID.
c Stirred for 16 h.
d Isolated yield, only the E-isomer was detected.
e 6 h in DMF.
f Stirred for 1 h at RT.
g Stirred for 15 h in DMSO at 70 °C, E/Z = 65%![]() ![]() |
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1 | 2 |
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99c (98d) |
2 | 2 |
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99 (87d) |
3 | 2 |
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90c (86d) |
4 | 2 |
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R 1 85 (73d) R2 90 (68d) R3 98 (93d) |
5 | 1 |
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R a 98e (97d) Rb 93e (89d) |
6 | 1 |
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57e |
7 | 2 |
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87e |
8 | 1 |
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99f (99d) |
9 | 1 |
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23g |
10 | 1 |
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87c (85d) |
A possible first hypothesis was that copper leaching, i.e. soluble Cu(I) or Cu(II) species, could be the reason for catalytic activity. Several Cu(I/II) compounds have been tested (entries 1–5, Table 4) to investigate the activity of copper ions in solution. None of these copper species showed substantial activity, instead of a high measured solvated Cu amount (entries 2 and 3). Also, there are reports of metal-alkoxide catalysed Knoevenagel condensations, and activity of in situ generated CuOMe may also be possible.11 However, if the solid catalyst is filtered (Fig. 1), conversion stops immediately. This leads to the conclusions that either there is a very unstable homogeneous reactive species formed on the surface of Cu or leaching is not the reason for catalytic activity. Hence, a second hypothesis can be formulated around Cu surface based catalysis, where the yield is expected to correlate with the specific surface area (SSA) of the catalyst. Cu(D) has dendritic structures between 0.5–1 μm (see Fig. S4, ESI†) and thus a relatively low BET surface area of 0.2 m2 g−1. While a tenfold larger amount of Cu(D) resulted in a tenfold larger surface area, leaching was only increased by factor 2.5 (entries 6 and 7), yet the yield substantially changed from 32% to 99%. This is another argument against a mechanism based on solvated Cu species, since the relatively small amount of additional leached species, for example in the case of Copper hollow-spheres (Cu(Hol), entry 8, Table 4), can hardly be responsible for the large increase in yield. Thus, a Cu surface based mechanism can be assumed. However, the nature of the surface state required for catalytic action is still unknown. Thus, surface activation experiments were done: the copper catalyst was pre-treated under different reducing conditions, in order to reduce copper oxide species. This resulted in higher yields, most obviously for toluene, where the pre-treated copper yielded 53% product after 20 h, while the untreated copper did not catalyze the reaction at all (entries 4 and 5, Table 5). This finding leads to the conclusion that a non-oxidized Cu(0) surface reacts with the substrates. Kinetic measurements support the surface catalyzed mechanism too, as a zero-order kinetic model fits well in the beginning of the reaction (see Fig. S1 and S2 in the ESI†). An initial TOF of 0.88 s−1 was calculated using optimal conditions (Fig. 1, 41% yield after 30 min) and the conservative assumption that every Cu(0) atom on the surface is an active site. Further mechanistic studies are a subject of ongoing research. Moreover, similar copper(0) species with different BET surface areas (see Table S1 in the ESI†), such as copper hollow spheres Cu(hol),12 copper nanoparticles A Cu(NPA) and copper nanoparticles B Cu(NPA) were tested (entries 8–10). A correlation between the surface area and conversion could be confirmed. Also, an experiment was conducted to prove the reusability of the catalyst. The standard setup in DMSO afforded constant high yields after 1 h for 5 consecutive cycles (Fig. S1, ESI†).
Entry | Catalyst | Time (h) | SSA (m2 10−4) | Cu (ppm) | Yieldb (%) |
---|---|---|---|---|---|
a Reaction conditions: catalyst (4 mg), benzaldehyde (0.12 mmol) and ethyl cyanoacetate (0.1 mmol) in EtOH at 56 °C. b Yield determined via HPLC-UV (MS) using commercial product 3 as the reference standard. c 40 mg of Cu(D) used. | |||||
1 | Cu(II)2(OH)2CO3 | 2 | — | 6 | 2 |
2 | Cu(II)Cl2 | 16 | — | 740 | 1 |
3 | Cu(I)Cl | 16 | — | 460 | 3 |
4 | Cu(II)O | 20 | — | 6 | 4 |
5 | Cu(I)2O | 20 | — | 12 | 14 |
6 | Cu(D) | 2 | 8 | 44 | 32 |
7c | Cu(D) | 2 | 80 | 114 | 99 |
8 | Cu(Hol) | 2 | 84 | 62 | 65 |
9 | Cu(NPA) | 2 | 168 | 176 | 96 |
10 | Cu(NPB) | 2 | 636 | 197 | 99 |
Entry | Pretreatment | t P (h) | T P (°C) | t cat (h) | Solvent | Yieldb (%) |
---|---|---|---|---|---|---|
a Reaction conditions: pre-treated catalyst (4 mg), benzaldehyde (0.21 mmol) and ethyl cyanoacetate (0.2 mmol) at 56 °C. b Yield determined via HPLC-UV (MS) using commercial product 3 as the reference standard. c In 2% formic acid. | ||||||
1 | — | — | — | 2 | EtOH | 32 |
2 | Reduction w/FAc | 1 | RT | 2 | EtOH | 49 |
3 | Reduction w/H2 | 2 | 60 | 2 | EtOH | 52 |
4 | — | — | — | 20 | Toluene | 3 |
5 | Reduction w/H2 | 2 | 110 | 20 | Toluene | 53 |
To illustrate the practical synthetic utility of our method, a 100 gram scale experiment was performed (Scheme 1). Benzaldehyde 1 (81.2 mL, 0.8 mol) was reacted with ethyl cyanoacetate 2 (84.8 mL, 0.8 mol) and 16.00 ± 0.001 g of Cu(D) in EtOH at 56 °C for 16 h. After filtration through aluminum oxide and recrystallization, 91% (146 g) pure isolated yield was obtained and almost all of the solid catalyst (15.94 ± 0.001 g) could be recovered. This satisfying result highlights the simplicity of both catalyst and work-up and hence is of interest for industrial scale Knoevenagel reactions. It should be noted that this kind of upscale experiment is difficult to perform with soluble base catalysts as large amounts of solvent and neutralization agent have to be used.
In summary, we have developed a simple and mild method for the Knoevenagel condensation using commercially available semi-noble copper. In this catalysis, tedious separation procedures are not necessary and no traces of base remain. Further experiments revealed that a Cu(0) surface is necessary to catalyze the condensation and, surprisingly, other noble metals such as gold and silver were not active. Moreover, a substantial scale up (>100 g) experiment demonstrated simple applicability.
Financal support was provided by ETH Zurich and the Swiss Science Foundation (No. 200021-150179).
Footnote |
† Electronic supplementary information (ESI) available: Experimental procedures, supplementary kinetic figures, SEM micrographs, X-ray crystallographic analysis and 1H and 13C NMR spectra. See DOI: 10.1039/c5cc02541a |
This journal is © The Royal Society of Chemistry 2015 |