The π-Electron Delocalization in 2-Oxazolines Revisited: Quantification and Comparison with Its Analogue in Esters.
The single crystal X-ray analysis of the ester-functionalized 2-oxazoline, methyl 3-(4,5-dihydrooxazol-2-yl)propanoate, revealed π-electron delocalization along the N-C-O segment in the 2-oxazoline pentacycle to significant extent, which is comparable to its counterpart along the O-C-O segment in the ester. Quantum chemical calculations based on the experimental X-ray geometry of the molecule supported the conjecture that the N-C-O segment has a delocalized electronic structure similar to an ester group. The calculated bond orders were 1.97 and 1.10 for the N=C and C-O bonds, and the computed partial charges for the nitrogen and oxygen atoms of -0.43 and -0.44 were almost identical. In the ester group, the bond orders were 1.94 and 1.18 for the C-O bonds, while the partial charges of the oxygen atom are -0.49 and -0.41, which demonstrates the similar electronic structure of the N-C-O and O-C-O segments. In 2-oxazolines, despite the higher electronegativity of the oxygen atom (compared to the nitrogen atom), the charges of the hetero atoms oxygen and nitrogen are equalized due to the delocalization, and it also means that a cationic attack on the nitrogen is possible, enabling regioselectivity during the initiation of the cationic ring-opening polymerization of 2-oxazoline monomers, which is a prerequisite for the synthesis of materials with well-defined structures.
Polymer Competence Center Leoben, Roseggerstrasse 12, 8700 Leoben, Austria.|Institute for Chemistry and Technology of Materials, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.
Martin
M
Fimberger
Polymer Competence Center Leoben, Roseggerstrasse 12, 8700 Leoben, Austria.|Institute for Chemistry and Technology of Materials, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.
Klaus P
KP
Luef
Polymer Competence Center Leoben, Roseggerstrasse 12, 8700 Leoben, Austria.|Institute for Chemistry and Technology of Materials, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.
Claudia
C
Payerl
Institute of Inorganic Chemistry, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.
Roland C
RC
Fischer
Institute for Chemistry and Technology of Materials, Graz University of Technology, NAWI Graz, Stremayrgasse 9, 8010 Graz, Austria.
Franz
F
Stelzer
MTA-BME Lendület Quantum Chemistry Research Group, Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, P.O. Box 91, 1521 Budapest, Hungary.
Mihály
M
Kállay
Polymer Competence Center Leoben, Roseggerstrasse 12, 8700 Leoben, Austria.
Frank
F
Wiesbrock
2-oxazoline monomers; ester-functionalized 2-oxazoline; hydrolysis of 2-oxazoline; methyl 3-(4,5-dihydrooxazol-2-yl)propanoate; regioselectivity of the initiation of cationic ring-opening polymerizations; π-electron delocalization