Fully conjugated tetraborylethylene: selenium mediated C-C double bond formation from diborylcarbenoid

Heteroatom-substituted ethylenes have long been studied owing to their potential application to electronic devices. In contrast to well-studied π-donor substituted ethylene, the π-acceptor substituted one has only been limitedly reported. While boron can be a candidate of π-acceptors, there has still been no example of fully conjugated tetraborylethylene (TBE). Herein, we synthesized the first fully conjugated TBE 2 by selenium-mediated C-C double bond formation from diborylcarbenoid 1, a synthetic equivalent of diborylcarbene (DBC). An intermediate of bis(diborylmethylene)-λ⁴-selane 3_Se, wherein two DBC fragments were bound to one selenium atom, was confirmed. TBE 2 has a longer C-C bond length of 1.368(2) Å than typical C-C double bonds (1.34 Å) owing to π-electron deficiency. By density functional theory calculations, the LUMO was found to be low-lying at -1.75 eV by the contribution of vacant p-orbitals on the boron atoms adjacent to the C-C double bond.