Di-heteroarylethyl enes: synthesis and donor properties

Abstract

The Wittig-Horner condensation of N-methylpyrrole-2-, and 2,5-dicarbaldehydes with diethyl heteroarylmethylphosphonates (heteroary =N-methylpyrrol-2-yl or 2-thienyl and the corresponding 2,5-diyl) leads to the (E)-1,2-diheteroarylethylenes (1) and (2), and the triheteroaryl derivatives respectively. A new entry into diethyl pyrrolylmethylphosphonates is described starting from the pyrrol-2-ylmethyltrimethylammonium iodide and the sodium salt of diethyl phosphite: the rate and the regiochemistry of the reaction is solvent dependent. The heteroarylethylenes (1)–(5) are electron-rich substrates affording charge transfer (c.t.) complexes with iodine and TCNQ (tetracyano-1,4-quinodimethane): the d.c. conductivities of the c.t. complexes are relatively low and fall in the range of semiconductors. I.r. frequencies of the CN stretching relative to the TCNQ moiety are used to evaluate the degree of charge transfer between the donor and the acceptor. It is believed that the radical cation of (5) arising by monoelectron transfer to TCNQ is the most stable charged species among the series considered.