Matthias Lobert

Screening the synthesis of 2-substituted-2-oxazolines.

2-Oxazolines are well-known organic compounds which are included in a variety of complex biologically active structures and play a role as catalyst ligands and intermediates for functional compounds. In addition, 2-oxazolines serve as monomers for the synthesis of substituted poly(imine)s by cationic ring-opening polymerization. For the latter application, the feasibility of preparing new 2-substituted-2-oxazolines was investigated using an automated synthesizer. The reaction of various nitriles with 2-aminoethanol under Lewis acid catalysis was utilized for this purpose. Twenty-nine different substituted nitriles were selected out of more than 2000 commercial available nitriles to form the corresponding 2-oxazolines. At first, the reaction conditions were optimized for seven nitriles with regard to solvent and catalyst, including reproducibility tests in an automated parallel robot system. In the next step, the synthesis of all 29 2-oxazolines was screened in an automated parallel manner, whereby the reactions were monitored by GC-MS measurements providing novel insights in the scope of this synthesis route. These insights resulting from the high-throughput screening were validated by performing representative larger-scale syntheses of selected 2-oxazolines.

Synthesis, Microwave‐Assisted Polymerization, and Polymer Properties of Fluorinated 2‐Phenyl‐2‐oxazolines: A Systematic Study

We present a detailed systematic study of the synthesis and ability of fluorinated 2-phenyl-2-oxazolines to undergo polymerization. The synthesis of these compounds is based on a two-step procedure that gives the desired 2-oxazolines in moderate-to-good yields. All the compounds were fully characterized by IR and NMR ((1)H, (13)C, and (19)F) spectroscopy, mass spectrometry, and elemental analysis. The 2-oxazolines were subsequently used as monomers for living cationic ring-opening polymerization (CROP) with microwave irradiation as the heat source (T=140 degrees C), nitromethane as the solvent, and methyl tosylate as the initiator. The linear first-order kinetic plots of the polymerizations accompanied by a linear increase of the molecular weight with conversion and low polydispersity index (PDI) values (generally below 1.30) indicate a living polymerization mechanism. The resulting polymerization rates reflect a strong sensitivity to the quantity of fluorine substituents in general and the presence or absence of ortho-fluoro substituents of the phenyl ring in particular. All the polymers were isolated and characterized by size-exclusion chromatography and MALDI-TOF mass spectrometry. Finally, a detailed investigation of selected polymer properties was performed by using differential scanning calorimetry, thermogravimetric analysis, and contact-angle measurements, thus resulting in structure-property relationships. Whereas the thermal properties of the polymers are mostly influenced by the presence of ortho-fluoro substituents, the surface properties are mainly determined by the presence of para- and/or meta-fluoro substituents.

Synthesis and Supramolecular Properties of Trimethylene‐Bridged Clips

The novel trimethylene-bridged clips 3 and 4 have been synthesized by using repetitive stereoselective Diels-Alder reactions of the benzo- and naphthobismethylenenorbornenes 8 and 19 as dienes and norbornadiene 9 as bisdienophile, and subsequent dehydrogenation of the primary cyclobisadducts 10 and 20 by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Clips 3 and 4 serve as receptors for a variety of electron-deficient neutral and cationic aromatic substrates, comparable to the molecular tweezers 1 and 2. The thermodynamic parameters of the complex formation, K(a) and DeltaG, were determined by (1)H NMR titration experiments and, in the case of the highly stable complex TCNB 32@4, by the use of isothermal titration microcalorimetry. The finding that clip 4 forms more stable complexes than 3 can be explained by the larger van der Waals contact surfaces of the naphthalene sidewalls in 4 compared to the corresponding benzene systems in 3. In the complexes with 4 as receptor, the plane of each aromatic substrate molecule is calculated to be oriented almost parallel to the naphthalene sidewalls. However, in the complexes of tweezers 2, the substrate is usually oriented parallel to the central naphthalene spacer unit. Due to the more open topology of 4, most complexes were calculated to consist of two or more equilibrating noncovalent conformers.

Luminescent host–guest complexes involving molecular clips and tweezers and tetracyanobenzene

The molecular tweezer 1 and the clips 2 and 3, containing naphthalene or anthracene in the sidewalls, form host–guest complexes in CHCl3 solution with the guest TCNB (1,2,4,5-tetracyanobenzene). The interaction leading to formation of the adduct is essentially of CT (charge-transfer) nature. A luminescence emission of CT origin is observed from the host–guest complexes both in fluid solution at 298 K and in rigid matrix at 77 K; to our knowledge, this is the first case of CT luminescence from a host–guest complex. At room temperature, the luminescence maxima are observed at 570, 614, and 668 nm, respectively, for the complexes based on the hosts 1, 2, and 3. Spectrophotometric and spectrofluorimetric titrations were performed to investigate the association processes. In all cases 1∶1 complexes are formed, with association constants 7.3 × 105, 5.4 × 106, and 1.24 × 104 L mol−1, respectively, for the receptors 1, 2, and 3. In the case of 2, a species with a 2∶1 host∶guest ratio is also formed. The system 1+TCNB was further investigated by cyclic and differential pulse voltammetry, giving the rate constants for adduct formation (1.9 × 108 L mol−1 s−1) and disassembling (2.0 × 102 s−1). The association/dissociation dynamics between the receptors 1 and 2 and the guest TCNB is discussed in relation to the receptor topology.