ON THE HOMOCONJUGATION BETWEEN DONOR AND ACCEPTOR FRAGMENTS - SYNTHESIS AND PROPERTIES OF BISHOMOSQUARIC ACID-DERIVATIVES

The tetraketone 3,3,6,6-tetramethylcyclohexane-1,2,4,5-tetrone (11) an d its congener bicyclo[3.2.2]nonane-6,7,8,9-tetrone (17) were synthesi zed by conventional procedures. The photoreaction of 11 and 17 with te tramethylethylene yields thyl-2,5-dioxabicyclo[4.4.0]dec-1(6)-ene-8,9- dione (22) and atricyclo[6.3.2.0(2,7)]tridec-2(7)-ene-12,13-dione (24) , respectively. Both are formally diesters of a bishomosquaric acid. T he interactions between the two nonconjugated diketone moities in 11 a nd 17 as well as between the donor and acceptor fragments in 22 and 24 were studied. X-ray investigations of 11 and 17 show no deviation fro m the expected bond lengths and angles. In the case of 24 the distance between the carbon atoms of the donor and acceptor units is 2.39 Angs trom, this is 0.11 Angstrom shorter than the corresponding distance be tween the two acceptor units in 17. The PE-spectroscopic investigation s of 11 and 17 show a considerable interaction between the two accepto r units. This is concluded from the splitting of the PE bands (ca. 0.5 eV) of the four lone-pair combinations. For 22 and 24 no interaction between the donor and acceptor part was detected by means of PE spectr oscopy. The C-13-NMR chemical data show a high-field shift for the car bonyl carbon atoms in 11 and 17. These findings were substantiated by the investigation of the corresponding phenazine derivatives obtained from 11 and 17 by the reaction with o-phenylenediamine. In the case of 22 and 24 a high-field shift of the signals of the olefinic double bo nd was observed. The electronic absorption spectrum of 22 shows for th e higher excited states a charge-transfer contribution, whereas in the case of 24 a charge-transfer contribution was already detected in the first excited state.