Gas-Phase Synthesis of Boronylallene (H2CCCH(BO)) under Single Collision Conditions: A Crossed Molecular Beams and Computational Study

Abstract

The gas phase reaction between the boron monoxide radical (¹¹BO; X²Σ⁺) and allene (H₂CCCH₂; X¹A₁) was investigated experimentally under single collision conditions using the crossed molecular beam technique and theoretically exploiting ab initio electronic structure and statistical (RRKM) calculations. The reaction was found to follow indirect (complex forming) scattering dynamics and proceeded via the formation of a van der Waals complex (¹¹BOC₃H₄). This complex isomerized via addition of the boron monoxide radical (¹¹BO; X²Σ⁺) with the radical center located at the boron atom to the terminal carbon atom of the allene molecule forming a H₂CCCH₂¹¹BO intermediate on the doublet surface. The chemically activated H₂CCCH₂¹¹BO intermediate underwent unimolecular decomposition via atomic hydrogen elimination from the terminal carbon atom holding the boronyl group through a tight exit transition state to synthesize the boronylallene product (H₂CCCH¹¹BO) in a slightly exoergic reaction (55 ± 11 kJ mol^-1). Statistical (RRKM) calculations suggest that minor reaction channels lead to the products 3-propynyloxoborane (CH₂(¹¹BO)CCH) and 1-propynyloxoborane (CH₃CC¹¹BO) with fractions of 1.5% and 0.2%, respectively. The title reaction was also compared with the cyano (CN; X²Σ⁺)-allene and boronyl-methylacetylene reactions to probe similarities, but also differences of these isoelectronic systems. Our investigation presents a novel gas phase synthesis and characterization of a hitherto elusive organyloxoborane (RBO) monomer-boronylallene-which is inherently tricky to isolate in the condensed phase except in matrix studies; our work further demonstrates that the crossed molecular beams approach presents a useful tool in investigating the chemistry and synthesis of highly reactive organyloxoboranes.