Ring Enlargement of Three‐Membered Boron Heterocycles upon Reaction with Organic π Systems: Implications for the Trapping of Borylenes

Abstract

New low-energy pathways for the reaction between substituted boriranes and borirenes with unsaturated hydrocarbons (ethyne or ethene) were discovered using density functional and coupled cluster theory. The interaction between the π bond of the hydrocarbon and the empty p orbital of the boron center leads to ring expansion of the three-membered to a five-membered boron heterocycle. The reactions are strongly exothermic and have low or even no barriers. They involve intermediates with a pentacoordinate boron center with two hydrocarbon molecules coordinating to boron akin to metal-olefin complexes. These borylene complexes are shallow minima on the potential energy surfaces. But significantly higher barriers for ring formation are computed for 1,5-cyclooctadiene and dibenzocyclooctatetraene complexes of borylenes, making these complexes likely detectable under appropriate experimental conditions. Our computational findings have implications for the interpretation of trapping experiments of thermally generated small borylenes with excess of small π systems. Because of very low barriers for reactions of three-membered boron heterocycles with π systems and the at least locally large excess of the latter under such conditions, formation of five-membered boron heterocycles should be considered.

Borylenes: An Emerging Class of Compounds

“…[22] Scheme 1. Generation and chemical trapping of putative transient borylenes These results [23] clearly demonstrate that free borylenes are so highly reactive that the quest for stable versions will be a difficult task. This conclusion prompted us and others to attempt taming borylenes, using one or two Lewis bases.…”

Reactions of Dihaloboranes with Electron-Rich 1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadienes

“…1 H and 13 C{ 1 H} NMR spectra were referenced to an external tetramethylsilane (TMS) via the residual protons of the solvent ( 1 H) or the solvent itself ( 13 C{ 1 H}). 11 B NMR spectra were referenced to the external BF 3 •OEt 2 . High-resolution mass spectrometry (HMRS) was performed with a Thermo Fisher Scientific Q Exactive Mass Spectrometer (MS) system.…”

“…The progress in this research area is highlighted by the very recent results in terms of borylene-mediated N 2 activation [ 9 ] and N 2 coupling [ 10 ]. Nonetheless, the synthetic approach to low-valent boron species is severely limited [ 11 , 12 , 13 ]. Almost all of the reported synthetic strategies require a strong metallic reducing agent (e.g., Li, K, Na, KC 8 ) [ 3 , 4 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ], harsh reaction conditions, and a strict moisture- and oxygen-free atmosphere.…”

Borylene: eine aufstrebende Verbindungsklasse

“…Diese Ergebnisse belegen zweifelsfrei, dass freie Borylene hochreaktiv sind. Daher ist die Suche nach einer stabilen Version eine große Herausforderung.…”