Reactions of diborenes with terminal alkynes: mechanisms of ligand-controlled <i>anti</i>-selective hydroalkynylation, cycloaddition and CC triple bond scission
Lukas Englert, Uwe Schmidt, Michael Dömling, Max Passargus, Tom E. Stennett, Alexander Hermann, Merle Arrowsmith, Marcel Härterich, Jonas H. Müssig, Alexandra Phillipps, Dominic Prieschl, Anna Rempel, Felix Rohm, Krzysztof Radacki, Fabian Schorr, Torsten Thiess, J. Óscar C. Jiménez‐Halla, Holger Braunschweig
Abstract
-selective hydroalkynylation at room temperature, whereas [2 + 2] cycloaddition was observed at higher temperatures, invariably followed by a C-N bond activation at one NHC ligand, leading to the ring-expansion of the initially formed BCBC ring and formation of novel boron-containing heterocycles. For phosphine-stabilised diborenes only [2 + 2] cycloaddition was observed, followed by a rearrangement of the resulting 1,2-dihydro-1,2-diborete to the corresponding 1,3-isomer, which amounts to complete scission of both the B[double bond, length as m-dash]B double and C[triple bond, length as m-dash]C triple bonds of the reactants. The elusive 1,2-isomer was finally trapped by using a cyclic phosphine-stabilised diborene, which prevented rearrangement to the 1,3-isomer. Extensive density functional theory (DFT) calculations provide a rationale for the selectivity observed.