Binding the\nPower of Cycloaddition and Cross-Coupling\nin a Single Mechanism: An Unexpected Bending Journey to Radical Chemistry\nof Butadiynyl with Conjugated Dienes

What if an experiment could combine the power of cycloaddition\nand cross-coupling with the <i>in situ</i> formation of\nan aromatic molecule in a single collision? Crossed molecular beam\nexperiments augmented with electronic structure and statistical calculations\nprovided compelling evidence on a novel radical route involving 1,3-butadiynyl\n(HCCCC; X<sup>2</sup>∑<sup>+</sup>) radicals synthesizing (substituted)\narylacetylenes in the gas phase upon reactions with 1,3-butadiene\n(CH<sub>2</sub>CHCHCH<sub>2</sub>; X<sup>1</sup>A<sub>g</sub>) and\n2-methyl-1,3-butadiene (isoprene; CH<sub>2</sub>C(CH<sub>3</sub>)CHCH<sub>2</sub>; X<sup>1</sup>A’). This elegant mechanism <i>de facto</i> merges two previously disconnected concepts of\ncross-coupling and cycloaddition–aromatization in a single\ncollision event via the formation of two new C(sp<sup>2</sup>)–C(sp<sup>2</sup>) bonds and bending the 180° moiety of the linear 1,3-butadiynyl\nradical out of the ordinary by 60° to 120°. In addition\nto its importance to fundamental organic chemistry, this unconventional\nmechanism links two previously separated routes of gas-phase molecular\nmass growth processes of polyacetylenes and polycyclic aromatic hydrocarbons\n(PAHs), respectively, in low-temperature environments such as in cold\nmolecular clouds like the Taurus Molecular Cloud (TMC-1) and in hydrocarbon-rich\natmospheres of planets and their moons such as Titan, which revises\nthe established understanding of low-temperature molecular mass growth\nprocesses in the Universe.