Pyrimidines from furfural. A convenient synthesis of 5-bromo- and 5-chloropyrimidine

A new method for the preparation of 5-bromo- and 5-chloropyrimidine from furfural has been described.     

Theoretical Investigation of Macrodipoles in Supramolecular Columnar Stackings

Supramolecular columnar assemblies are known to form intrinsic macrodipoles, which play an important role in intercolumnar interactions and govern the self‐assembly on the mesoscale. A prominent class that provides this feature are trisamide derivatives, namely, 1,3,5‐benzenetrisamides and 1,3,5‐cyclohexanetrisamides. The understanding of how subtle changes in the chemical structure influence the columnar order and consequently the macrodipole formation is of fundamental interest. Here we report on the theoretical investigation of trisamide derivatives and how the formed macrodipole is related to the properties of the columnar aggregates. Calculations were carried out on a semiempirical level using the PM6 approximation, which is able to treat weak interactions like hydrogen bonding and dispersion forces with a sufficient accuracy. We have compared the influence of a benzene core with a cyclohexane core on the macrodipole formation. It was revealed that columnar aggregates based on 1,3,5‐cyclohexanetrisamides have much higher dipole moments than those formed with aromatic cores. A cooperative effect was found during aggregation, as longer aggregates show stronger hydrogen bonding, thereby facilitating the addition of the next molecule. We have also investigated the influence of the amide connection on the strength of the formed macrodipole. The trends observed for the macrodipole strength correlate with the calculated heat of formation. If the amide groups are inverted, the strength of the macrodipole is reduced and the negative heat of formation is increased. HOMO–LUMO gaps were correlated with the inverse of the dipole moment per monomer unit, thus indicating that the macrodipole might act as a perturbation to the supramolecular assemblies.     

On the denseness of Herglotz wave functions and electromagnetic Herglotz pairs in Sobolev spaces

Let D??³ be a bounded domain with connected boundary δD of class C². It is shown that Herglotz wave functions are dense in the space of solutions to the Helmholtz equation with respect to the norm in H¹(D) and that the electric fields of electromagnetic Herglotz pairs are dense in the space of solutions to curl curl E=k²E with respect to the norm in H_curl(D). Two proofs are given in each case, one based on the denseness of the traces of Herglotz wave functions on δD and the other on variational methods. Copyright © 2001 John Wiley & Sons, Ltd.     

Polymerization of Norbornene with CoCl2 and Pyridine Bisimine Cobalt(II) Complexes Activated with MAO

Addition polymerization of norbornene was performed with several pyridine bis(imine) cobalt dichloride complexes activated with methylaluminoxane (MAO), first described for ethylene polymerization. For the first time, norbornene was also polymerized with CoCl₂ associated to MAO. The influence of several reaction parameters has been investigated. Quite different behavior was observed compared with ethylene polymerization. Moreover, the copolymerization of ethylene and norbornene with these complexes was not possible but led to a mixture of both homopolymers.

The pyridine bis(imine) cobalt dichloride complexes used in this study.     

Polymerization of norbornene with Co(II) complexes

The norbornene polymerization was studied in the presence of 6 pyridine bis(imine) cobalt(II) complexes activated with methylaluminoxane (MAO). Norbornene was also polymerized with CoCl₂ associated to MAO. All these catalytic systems generate an addition polymerization of norbornene, yielding fully saturated polymers. It was shown that the polymerization yield and the molar masses are highly dependant on several reaction parameters (monomer concentration, [Al]/[Co] ratio, polymerization temperature and time) and the frame of the ligand.