Synthesis of 1,4-Bis(diaryl)-1,3-Butadiynes Bearing Two Amino Moieties by Electrochemical Reaction Site Switching and Their Solvatochromic Fluorescence

Recently, π-conjugated compounds have been on focus because they are known to be as key building blocks for pharmaceuticals and potent candidates for electrochemical and optical materials. While the optical and electrochemical properties of π-conjugated compounds highly depends on the structure, it has not been easy to predict these properties. It is the reason why facile and diversity-oriented methodologies to construct π-conjugated structure have been strongly required. From such a point of view, we have investigated electrochemically assisted coupling reactions for constructing π-conjugated compounds, and recently reported sequential coupling reactions which include Pd/Cu-catalyzed electro-oxidative homo-coupling of p-bromophenylacetylene to give 1,4-bis(p-bromophenyl)-1,3-diyne and subsequent Suzuki–Miyaura coupling with arylboronic acids leading to 1,4-bis(diaryl)-1,3-butadiynes.¹ Two reaction sites of p-boromophenylacetylene, terminal alkyne and Br group, could be discriminated in a virtually complete manner by on/off application of electricity, and we called the strategy as “Electrochemical Reaction Site Switching (e-RSS)”. Several arylboronic acids could be applied to the reactions, and the reactions proceeded selectively to give 1,4-bis(diaryl)-1,3-butadiynes in good to excellent yields.

During the course of the investigation, we unexpectedly found that 1,4-bis(diaryl)-1,3-butadiyne bearing two dimethylamino groups at p-position exhibited solvatochromic fluorescence. To the best of our knowledge, while there have been many reports on the amines which exhibit solvatochromic fluorescence, there have been few reports on the solvatochromic fluorescence of diamines which have small dipole moment. Most of the amines exhibiting solvatochromic fluorescence so far have a strong acceptor moiety. This unexpected result prompted us to get deeper insight into this unusual phenomenon; however the poor solubility of diaminodiynes prevented us from further investigation. We therefore designed novel 1,4-bis(diaryl)-1,3-butadiynes bearing two dialkylamino or diphenylamino groups at p-position of the terminal benzene rings. The butadiynes could be easily synthesized by e-RSS with the corresponding p-aminoarylboronic acids. As expected, thus obtained diynes could be solved into several solvents and they exhibited solvatochromic fluorescence. We will discuss the detail of the synthesis of 1,4-bis(diaryl)-1,3-butadiynes bearing two dialkylamino or diphenylamino groups at p-position, and their unique optical properties. 

(1) Mitsudo, K.; Kamimoto, N.; Murakami, H.; Mandai, H.; Wakamiya, A.; Murata, Y.; Suga, S. Org Biomol Chem 2012, 10, 9562-9569.