A Spectroscopic Investigation of Donor-Acceptor-Substituted Heptalenes

It is shown that the heptalene-4,5-dicarboxylates 5 react with their Me group at C(1) with N,N-dimethylformamide dimethyl acetal or other acetals of this type in N,N-dimethylformamide (DMF) to give the corresponding 1-[(E)-2-(N,N-dialkylamino)ethenyl]-substituted heptalene-4,5-dicarboxylates 8a – 8e as well as 8k and 8i in good yields (Table 1). In a similar manner, the 1-[(E)-2-pyrrolidinoethenyl]-substituted heptalene-5-carboxylates 8f – h were synthesized from the corresponding heptalene-carboxylates 10 – 12, carrying a CHO, CN, or (E)-2-(methoxycarbonyl)ethenyl group at C(4) (Table 1). All new heptalenes with the π-donor and π-acceptor groups at C(1) and C(4), respectively, exhibit a strongly enhanced heptalene band I in the spectral region of 450 – 500 nm in MeCN (Table 7 and Figs. 4 – 7), whereby the specific position is dependent on the π-donor quality of the N,N-dialkylamino substituent at C(2′) and the π-acceptor property of the group at C(4). The position of heptalene band I is also strongly solvent-dependent as is demonstrated in the case of heptalene 8i (Table 9). A good linear correlation with the CT band of 1-(diethylamino)-4-nitrobenzene or (E)-4-(dimethylamino)-β-nitrostyrene (Figs. 11 and 12) characterizes the heptalene band I also as an electronic CT transition. Irradiation into this band of 8i leads, as observed in other cases (cf. [1]), to a double-bond shift in the heptalene moiety (→8′i; Figs. 8 – 10). On warming in solution, 8′i is converted quantitatively to 8i.