Synthesis of 1,4-diethynyl- and 1,1,4,4-tetraethynylbutatrienes

In this paper, we report the synthesis and opto-electronic properties of differentially substituted 1,4-diethynyl- and 1,1,4,4-tetraethynylbuta-1,2,3- trienes. These novel chromophores greatly extend the series of building modules for oxidative coupling, which includes 1,2-diethynyl- and 1,1,2,2- tetraethynylethenes and 1,3-diethynylallenes (Fig. 1). A general synthesis of 1,1,4,4-tetraethynylbutatrienes, which tolerates a significant number of peripheral substituents, starts from pentadiynols that are oxidized to the corresponding dialkynyl ketones, followed by Corey-Fuchs dibromo-olefination, and transition metal mediated dimerization (Schemes 2 and 3). A similar protocol, including oxidation of propargyl aldehydes, dibromo-olefination, and dimerization yields the less stable 1,4-diethynylbutatrienes (Scheme 4). Attempts to prepare 1,1,4,4-tetraethynylbutatrienes with four terminal electron-donor-substituted aryl groups failed so far, mainly due to difficulties in the dibromoolefination step (Scheme 6). cis-trans-Isomerization of differentially substituted 1,1,4,4-tetraethynylbutatrienes is remarkably facile, with barriers to rotation in the range of those for peptide bond isomerization (ΔG‡ ≈20 kcal mol^-1). Barriers to rotation of 1,4-diethynylbutatrienes are higher (ΔG‡ ≈25 kcal mol ^-1), allowing in some cases the isolation of pure isomers. Both UV/VIS spectroscopy (Figs. 2 and 3) and electrochemical studies (Table) demonstrate that the all-C-cores in diethynyl- and tetraethynylbutatrienes have strong electron-acceptor properties that are greatly enhanced with respect to those of diethynyl- and tetraethynylethenes with two C(sp)-atoms less. Substitution with peripheral electron donor groups leads to efficient intramolecular charge-transfer interactions, as evidenced by intense, bathochromically shifted longest-wavelength bands in the UV/VIS spectra.