ELECTRONIC AND STRUCTURAL-PROPERTIES OF THE CYCLOBUTENODEHYDROANNULENES

Two series of cyclobutene-annelated dehydro[n]annulenes (n= 18, 24, 30) 10-12 and 13-15 were obtained by oxidative coupling of 1,2-diethyny 1-1-cyclobutenes 8 and 9. The properties of these macrocycles are analyzed in an experimental and computational study and compared to those of the parent 1,3,7,9,13,15-hexadehydro[18]annulene (18) and 1,3,7,9,13,15,19,21-octadehydro[24]annulene (19). The analysis of the electronic absorption and 1H NMR spectra shows that the trimers 10 and 13 both possess planar diatropic [18]annulene perimeters, while the tetramers 11 and 14 both possess planar paratropic [24]annulene perimeters. The chromophores in the pentamers 12 and 15 are conformationally more flexible, and the diatropic character of these [30]annulenes is strongly reduced. Cyclobutene annelation stabilizes all macrocycles. In particular, the fusion of cyclobutene rings stabilizes dramatically the planar [24]annulene perimeter in the tetramers 11 and 14 and is responsible for their preferred formation in the oxidative coupling reactions. AMI and MM2 computational studies reproduce well the experimentally observed conformational preferences of the parent dehydroannulenes 18 and 19 and the cyclobutene-fused 10-11 and 13-14. The calculations show that the peculiar stereochemistry of the 1,2-diethynyl-1-cyclobutene unit defines the unique properties of the cyclobutenoannulenes. With its large C(l')-C(l)-C(2)-angle of 136°, the 1,2-diethynyl-1-cyclobutene unit is accommodated in a nearly strain-free way into the planar tetramers 11 and 14, whereas its incorporation into the planar trimers 10 and 13 generates considerable angle strain. The enhanced kinetic stability of the cyclobutene-fused dehydroannulene perimeters is explained by energetically more difficult bending and out-of-plane distortions, normally required to reach reaction transition states. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.