On the Homoconjugation Between Donor and Acceptor Fragments. Synthesis and Properties of Bishomosquaric Acid Derivatives

Abstract

The tetraketone 3,3,6,6-tetramethylcyclohexane-1,2,4,5-tetrone (11) and its congener bicyclo[3.2.2]nonane-6,7,8,9-tetrone (17) were synthesized by conventional procedures. The photoreaction of 11 and 17 with tetramethylethylene yields 3,3,4,4,7,7,10,10-octamethyl-2,5-dioxabicyclo [ 4.4.01 dec-1 (6) -ene-8,g-dione (22) and 4,4,5,5-tetramethyl-3,6-dioxatricyclo-[6.3.2.02,7]tridec-2(7)-ene-12, 13-dione (24), respectively. Both are formally diesters of a bishomosquaric acid. The interactions between the two nonconjugated diketone moities in 11 and 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 from 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 A, this is 0.11 A shorter than the corresponding distance between the two acceptor units in 17. The PE-spectroscopic investigations of 11 and 17show a considerable interaction between the two acceptor 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 spectroscopy. The I3C-NMR chemical data show a high-field shift for the carbonyl 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 bond was observed. The electronic absorption spectrum of 22 shows for the 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.The fact that donor and acceptor groups strongly influence the structure and reactivity of 7c systems is familiar to all chemists. Although this concept has been known for more than 100 years, its consequences are still being investigated. Nowadays it is very helpful in devising new materials with special properties such as high conductivity or absorption in the infrared region or materials with nonlinear optical properties [']. One example, out of many, in which a donor and acceptor fragment are directly coupled, is the dianion of squaric acid (2). The low pK, values of squaric acid (1) (PIC,] = 0.5, pKaz = 3.5)L2] have been explained by an optimal interaction (resonance) between the donor and acceptor moiety in 2L3].In this interpretation the question arises whether an acceptor group and an electron-rich z system might still interact even if both units are not directly attached to each other, i.e. if spiro-or homoconjugation between both groups is detectable. Our interest in this question stems from earlier studies of possible homoconjugative interactions in bicyclo- as well as recent studies of transannular interactions in cyclic t e t r a k e t~n e d~~] and spiro compounds with donor acceptor fragments [4c]. In this paper we report ...

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