Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King
Caught on Tape: Catalyst Recovery
by syringe. The solid catalyst residue was washed, recharged with reactants, warmed to allow reaction three more times, each cycle giving a yield of 98%. In an effort to lower the amount of catalyst used, the scientists then completed the process, but used only 0.15 mol% of catalyst and added Teflon tape. The familiar white tape became lightly colored during reaction and orange–red when the sample was cooled. The catalyst rest-state separates onto the tape rather than in a second solid phase. GC analysis indicated an excellent percent yield for three cycles, but a substantial decrease in the fourth cycle, most likely due to catalyst deactivation. Researchers applied the Teflon tape method to other ketones as well, including 2-octanone, acetophenone, and benzophenone with similar results. The catalyst can also be precoated onto the Teflon tape. This work now offers the advantages of “catalyst-on-a-tape”, including measuring catalyst with a ruler instead of a balance.
Lots of work has gone into the development of recovery methods for homogeneous catalysts. Beginning in the late 1990s, scientists used fluorous “ponytails”, (CH2)m (CF2)n-1 CF3((CH2)mRfn) moieties, to allow the catalyst to be recovered by fluorous solvents. To avoid the high cost of these solvents, scientists then turned to temperature manipulation, using catalysts designed to be practically insoluble at low temperatures while soluble when the temperature is elevated. Scientists can then use filtration to separate the solid catalyst from reaction products at low temperature. Often insoluble fluorous support, such as Teflon shavings, is used to aid the recovery of small amounts of catalyst. Now researchers from the Institut für Organische Chemie at Friedrich-Alexander-Universität ErlangenNürnberg in Germany have determined that common Teflon tape not only aids catalyst recovery, but also delivery! The research team, led by John A. Gladysz, employed a ketone hydrosilation as a model reaction. These reactions are catalyzed by red–orange fluorous rhodium complexes that are soluble in organic solvents only if the temperature is increased. A solution of cyclohexanone in dibutyl ether was treated with 1 mol% of catalyst complex and warmed to 65 ⬚C for 8 hr. GC analysis showed the silyl ether product was present in 98% yield, so the reaction mixture was cooled, eventually in the freezer, and the product-containing supernatant removed
1. Dinh, Long V.; Gladysz, J. A. “Catalyst-on-a-Tape”-Teflon: A New Delivery and Recovery Method for Homogeneous Fluorous Catalysts. Angew. Chem., Int. Ed. 2005, 44, 4095–4097. 2. J. A. Gladysz’s research Web page is available at http:// www.chemie.uni-erlangen.de/gladysz/ (accessed Oct 2005). 3. This Journal has reported on Teflon tape being used to take IR spectra. See Oberg, K. A.; Palleros, D. R. Teflon Tape as a Sample Support for IR Spectroscopy. J. Chem. Educ. 1995, 72, 857.
Figure 1. Recycling of a thermomorphic fluorous rhodium hydrosilation catalyst by liquid–solid phase separation. Images provided by L. Dinh.
Figure 2. Hydrosilylation of cyclohexanone with pre-coated tape. Images provided by L. Dinh.
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Reports from Other Journals Secondary Structure Switch Biochemistry students are familiar with secondary structures such as ␣-helices and -sheets. What they may not realize is that many neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease involve a transformation between these two peptide and protein structures. While the formation of amyloid fibrils from proteins associated with the diseases is the pathogenic parameter, preliminarily formed -sheets are thought to be a precursor. Compared to the rest of the human body, the Cu2⫹ and Zn2⫹ concentration in grey matter is exceptionally high. ␣-Helix to -sheet transformations mediated by metal ions are thought to play a role in assembling deposits in the brains of people with Alzheimer’s disease. Understanding such metallochemical reactions is crucial to identifying the mechanism of disease. Now a team of scientists at the Freie Universität Berlin have crafted a peptide model to study the role that metal ions play in converting ␣-helix to -sheet. Led by Beate Koksch, the chemists based their model on an antiparallel doublestranded ␣-helical coiled coil motif that displays stability due to side-by-side packing of hydrophobic residues in some regions and interhelical salt bridges formed by charged residues in other areas. The resulting model peptide CC (for coiled coil) was compared to peptide CCM, which had four histidine residues per helix introduced to ligate metal ions. Researchers then used CD spectrometry to study the effect of the environment (concentration of peptide, pH, and metal ion concentration) on the structures of both peptides. At 0.1 mM concentration in 40% trifluoroethanol (TFE), both peptides prefer an ␣-helical structure to a sheet, and thus these conditions were used as the starting point for observation of metal ion effects on structure. Peptide CC, lacking any histidine residues, showed no change in conformation upon addition of one equivalent of Cu2+ or Zn2+. On the contrary, peptide CCM displayed a vigorous change from an ␣-helical structure to a -sheet upon addition of either metal ion. Upon addition of metal scavenging EDTA, CCM switched back to its original helical structure. CD spectroscopy was used to follow the titration of peptide CCM with solutions of Cu2+ or Zn2+. As metal ions were added, the helical content of samples decreased while at the same time the -sheet increased. This clearly indicated that complexation of the metal ions triggered the change in conformation. The peptide backbone can also participate in metal ion binding in addition to the histidine residues. However, the complete absence of change in conformation of CC (the model peptide lacking any histidine residues) upon addition of metal ions reveals the importance of metal complexation by histidine as a trigger mechanism.
More Information 1. Pagel, K.; Vagt, T.; Kohajda, T.; Koksch, B. From ␣-Helix to -Sheet—A Reversible Metal Ion Induced peptide Secondary Structure Switch. Org. Biomol. Chem. 2005, 3, 2500–2502. 2. This Journal has published an undergraduate lab labeling histidine residues. See Bonser, A. M.; Moe, O. A. Labeling His-
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Figure 3. A new method for controlling peptide and protein folding. From Chem. World 2005, 2 (8), 16. Reproduced by permission of The Royal Society of Chemistry.
tidines in Cytochrome c: An Integrated Laboratory Project. J. Chem. Educ. 1996, 73, 794. 3. Koksch’s research group Web page can be found at http:// userpage.chemie.fu-berlin.de/~akkoksch/ (accessed Oct 2005).
DNA-Based Chiral Catalysts Catalytic RNAs and enzymes are used by synthetic chemists to transfer their chirality to chemical reactions, but “DNA-zymes” have not seen as much success because they form double helices and lack a 2⬘⫺OH functional group. Now a team of researchers from the University of Groningen have demonstrated DNA’s potential by transferring chirality from a double helix of DNA to a copper(II)-catalyzed Diels– Alder reaction. The catalyst, formed in situ, is a complex of the metal ion and a ligand (1) with three important features. First, the ligand must have the ability to intercalate DNA. It also needs a spacer group that ties the intercalating moiety to a metalbinding group, the third required feature. Without DNA, these ligands bind copper to yield chiral complexes as racemic mixtures. Such racemic mixtures cannot be the source of enantiomeric excess in the catalyzed reaction. UV–vis spectroscopy was employed to demonstrate that the copper is bound by the ligand and not sequestered by the DNA. As a model reaction, the research team led by Gerard Roelfes and Ben Feringa studied the Diels–Alder reaction between cyclopentadiene (2) and an aza chalcone (3) catalyzed by complexes of DNA intercalating ligand (1) and copper(II) ions in the presence of commercially available salmon testes or calf thymus DNA. The reaction product was obtained in high yield as a mixture of endo and exo isomers. The enantiomeric excess seemed to greatly depend on the length of the spacer (n) and the substituent on the ligand. These features of the ligand structure also determined which enantiomer of the product was formed in excess. Neither the ratio of catalyst to substrate nor the source of DNA affected the outcome.
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Chemical Education Today
Figure 4. CD spectra of peptide CCM in 40% TFE with (a) 1 equiv of CuCl2/EDTA and (b) 1 equiv of Zn(OAc)2/EDTA. From Pagel, K.; Vagt, T.; Kohajda, T.; Koksch, B. Org. Biomol. Chem. 2005, 3, 2500–2502; http://xlink.rsc.org/UDOI=b505979h (accessed Oct 2005). Reproduced by permission of The Royal Society of Chemistry.
Figure 5. Coiled coil-based peptide (CC) and coiled coil-based peptide with histidine for metal complexation (CCM). (a) Helical wheel model of CC and CCM (in parentheses); (b) schematic -sheet layer; sequences shown correspond to the complete primary structure. From Pagel, K.; Vagt, T.; Kohajda, T.; Koksch, B. Org. Biomol. Chem. 2005, 3, 2500–2502; http://xlink.rsc.org/ ?UDOI=b505979h (accessed Oct 2005). Reproduced by permission of The Royal Society of Chemistry.
Table 1. Results of the Catalytic Diels–Alder Reaction with 1-Naphthylmethyl and 3,5-Dimethoxy Benzyl Substituted Ligand 1. Ligand 1 Entry Ligand # 11[a] 12[a] 13[b] 14[a] 15[a] 16[a] 17[a] 18[a]
1a 1a 1a 1a 1a 1b 1c 1d
19[a] 10[a] 11[a] 12[b] 13[c] 14[a] 15[a]
1c 1f 1f 1f 1f 1f 1f
Diels–Alder product (4)
⫺R
OMe
OMe
n
Dienophile
Endo:Exo
E.E. Endo (%)
E.E. Exo (%)
3 3 3 3 3 4 5 2
3a 3a 3a 3b 3c 3a 3a 3a
98:20 97:30 98:20 96:40 98:20 98:20 97:30 96:40
⫺49 ⫺49 ⫺47 ⫺37 ⫺48 ⫺33 ⫺
