Communication
Cooperative Asymmetric Catalysis with Dendrimeric [Co(salen)] Complexes
Rolf Breinbauer Dr.,
Eric N. Jacobsen Prof.,
Rolf Breinbauer Dr.
Department of Chemistry and Chemical Biology Harvard University Cambridge, MA 02138 (USA) Fax: (+1) 617-496-1880
Search for more papers by this authorEric N. Jacobsen Prof.
Department of Chemistry and Chemical Biology Harvard University Cambridge, MA 02138 (USA) Fax: (+1) 617-496-1880
Search for more papers by this authorRolf Breinbauer Dr.,
Eric N. Jacobsen Prof.,
Rolf Breinbauer Dr.
Department of Chemistry and Chemical Biology Harvard University Cambridge, MA 02138 (USA) Fax: (+1) 617-496-1880
Search for more papers by this authorEric N. Jacobsen Prof.
Department of Chemistry and Chemical Biology Harvard University Cambridge, MA 02138 (USA) Fax: (+1) 617-496-1880
Search for more papers by this authorFirst published: 13 October 2000
Citations: 308
Prof. M. T. Reetz (Max Planck-Institut für Kohlenforschung, Mülheim/Ruhr) and Mr. J. Hong (Harvard University) are gratefully acknowledged for stimulating discussions. This work was supported by the NIH (GM 43214). R.B. thanks Fonds zur Förderung der Wissenschaftlichen Forschung, Vienna (Erwin-Schrödinger-Fellowship) and Land Oberösterreich (Sonderförderung außerhalb des KIP-Programms) for postdoctoral fellowship support. H2salen=bis(salicylidene)ethylenediamine.
Abstract
Dendrimeric catalysts can present catalytic units in close proximity, thereby inducing useful cooperative effects (see picture). Commercial dendrimers were derivatized with chiral [Co(salen)] complexes and found to display enhanced reactivity relative to monomeric or even dimeric analogues in the hydrolytic kinetic resolution of epoxides.
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- 10 Although oxidation to the [(salen)CoIII(acetate)] complexes using HOAc/O2 is the method of choice for the preparation of the monomeric catalysts (see ref. [7d]), this proved impractical with the PAMAM-Co complexes because of the basic amine sites in the dendrimer. The [(salen)CoIII(iodide)] complexes obtained by oxidation with I2 have been found to display similar reactivity to the corresponding acetate complexes.[8c]
- 11
4-Co-PAMAM: IR (KBr):
max=3339, 2948, 2864, 1663 (br.), 1609, 1526, 1435, 1253, 1170, 1031, 783; 1H NMR (400 MHz, [D6]DMSO): δ=8.1 (br., 8 H), 7.83 (s, 4 H), 7.79 (s, 4 H), 7.47 (s, 4 H), 7.43 (s, 4 H), 7.33 (s, 4 H), 7.29 (s, 4 H), 3.62 (br. s, 16 H), 3.38 (br. s, 12 H), 3.10 (br. s, 16 H), 2.6–2.2 (br., 16 H), 1.98 (br., 4 H), 1.89 (br., 4 H), 1.76 (s, 36 H), 1.72 (s, 36 H), 1.60 (br. m, 8 H), 1.28 (s, 36 H); 13C NMR (100 MHz, [D6]DMSO): δ=170.7, 164.4, 163.7, 162.5, 161.6, 141.9, 141.4, 135.6, 133.0, 132.2, 129.0, 128.5, 121.2, 118.4, 118.2, 69.2, 69.1, 41.2, 38.2, 35.7, 35.5, 33.5, 31.4, 30.3, 30.2, 29.5, 29.4, 24.2.
- 12
1 b: IR (KBr):
max=3365, 2950, 2864, 1630, 1611, 1526, 1436, 1254, 1170, 783; 1H NMR (400 MHz, [D6]DMSO): δ=8.00 (t, J=5.6 Hz, 1 H), 7.85 (s, 1 H), 7.80 (s, 1 H), 7.50 (d, J=2.4 Hz, 1 H), 7.43 (d, J=2.4 Hz, 1 H), 7.33 (s, 1 H), 7.30 (s, 1 H), 3.62 (br. s, 2 H), 3.35 (s, 2 H), 3.10 (br. s, 2 H), 3.05 (m, J=6.8 Hz, 2 H), 2.00 (br., 2 H), 1.91 (br., 2 H), 1.76 (s, 9 H), 1.60 (br. m, 2 H), 1.40 (m, J=7.2 Hz, 2 H), 1.32 (s, 9 H), 1.28 (m, J=7.2 Hz, 2 H), 0.85 (t, J=7.2 Hz, 3 H); 13C NMR (100 MHz, [D6]DMSO): δ=170.7, 164.6, 164.0, 162.8, 162.0, 142.2, 141.8, 135.9, 133.2, 132.3, 129.2, 128.7, 122.0, 118.8, 118.6, 69.3, 69.2, 41.4, 38.1, 35.7, 35.5, 33.5, 31.5, 31.2, 30.4, 30.3, 29.6, 29.4, 24.2, 19.5, 13.7. 2: IR (KBr): 
max=3307, 2949, 2862, 1658, 1620, 1526, 1435, 1253, 1170, 1030, 783; 1H NMR (400 MHz, [D6]DMSO): δ=8.1 (br., 4 H), 7.82 (s, 2 H), 7.78 (s, 2 H), 7.47 (s, 2 H), 7.43 (s, 2 H), 7.31 (s, 2 H), 7.27 (s, 2 H), 3.62 (br. s, 8 H), 3.35 (br. s, 7 H), 3.10 (br. s, 8 H), 2.6–2.2 (br., 8 H), 1.98 (br., 4 H), 1.89 (br., 4 H), 1.76 (s, 18 H), 1.72 (s, 18 H), 1.60 (br. m, 4 H), 1.28 (s, 18 H); 13C NMR (100 MHz, [D6]DMSO): δ=171.7, 165.3, 164.7, 163.5, 162.6, 142.8, 142.4, 136.6, 133.9, 129.9, 129.5, 122.3, 119.4, 119.2, 70.1, 70.0, 42.0, 39.2, 36.6, 36.4, 34.3, 32.3, 31.2, 31.1, 30.4, 30.3, 25.1; MS (FAB) of the corresponding CoII complex: 1456 [M+Na]+ (calcd exact mass: 1433.76).
- 13 Experimental procedure for the kinetic experiments: 10 mL vials were charged with a stir bar and 12.5 μmol (referring to Co) catalyst 1 b, 2, 4-Co-PAMAM, 8-Co-PAMAM, or 16-Co-PAMAM. The catalysts were dissolved in THF (4.00 mL), then (rac)-1,2-epoxyhexane (3.00 mL, 25 mmol) and bromobenzene (200 μL; as an internal standard) were added to the dark brown solution. After the mixture had been stirred for 1 h at 4 °C, water (250 μL) was added. Reaction conversion was monitored by GC analysis (HP-5 column) of 20 μL aliquots withdrawn periodically from the reaction mixture.
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