Room Temperature Ring Expansion of N‐Heterocyclic Carbenes and BB Bond Cleavage of Diboron(4) Compounds

Abstract

We report the isolation and detailed structural characterization, by solid-state and solution NMR spectroscopy, of the neutral mono- and bis-NHC adducts of bis(catecholato)diboron (B2 cat2 ). The bis-NHC adduct undergoes thermally induced rearrangement, forming a six-membered -B-C=N-C=C-N-heterocyclic ring via C-N bond cleavage and ring expansion of the NHC, whereas the mono-NHC adduct is stable. Bis(neopentylglycolato)diboron (B2 neop2 ) is much more reactive than B2 cat2 giving a ring expanded product at room temperature, demonstrating that ring expansion of NHCs can be a very facile process with significant implications for their use in catalysis.

Insertion of Group 12–16 Hydrides into NHCs: A Theoretical Investigation

“…It was shown that the expanded product could be synthesized directly by heating (70 °C) B 2 cat 2 with two equivalents of the NHC for several hours. Interestingly, this degradation pathway was even shown to occur at room temperature when using the more reactive B 2 neop 2 . Franz and Inoue explored steric effects on the insertion pathway of iminoborane dihydride (NHBH 3 ) into NHC i Pr or NHC Mes (Mes=2,4,6‐trimethylphenyl), finding that insertion followed that of the previously reported pathway.…”

“…Rivard reported the first example of boron hydride insertion into NHCs, which has been followed by a number of observations of analogous boron hydride reactivity . As of yet, there are no reports of analogous reactivity for the heavier group 13 elements.…”

“…In 2014, Stephan reported the insertion of 9‐borabicyclo[3.3.1]nonane (9‐BBN), mirroring the four‐step insertion pathway . Analogous boron reactivity includes bis(catecholato)diboron (B 2 cat 2 ) and bis(neopentylglycolato)‐diboron (B 2 neop 2 ) . The reaction of tetramethyl substituted NHC with B 2 cat 2 produced the (NHC)B 2 cat 2 adduct, which was reported as thermally stable in solution.…”

Structurally Diverse Boron–Nitrogen Heterocycles from an N2O23− Formazanate Ligand

“…[26] Upon reaction of 7 with BF 3 ·OEt 2 in the presence of NEt 3 followed by the addition of H 2 O (Scheme 1), the expected product 8 was not detected;instead, the reaction mixture was found to contain formazan 7 and five new compounds (9)(10)(11)(12)(13), which could be separated by column chromatography in typical combined yields of 65-75 %( Figure S1 in the Supporting Information). [26] Upon reaction of 7 with BF 3 ·OEt 2 in the presence of NEt 3 followed by the addition of H 2 O (Scheme 1), the expected product 8 was not detected;instead, the reaction mixture was found to contain formazan 7 and five new compounds (9)(10)(11)(12)(13), which could be separated by column chromatography in typical combined yields of 65-75 %( Figure S1 in the Supporting Information).…”

Synthesis and properties of heteroaromatic carbenes of the imidazole and triazole series and their fused analogues

“…305,306 These result in the formation of borazines of types 67D,E, along with boroxoles 68B (Scheme 68). 307 A beryllium analogue 308 of the silazines 67C, compound 67F (Scheme 67), is also known. Compounds of this type (and their precursor borane carbene complexes) are promising reduction catalysts of the FLP-type.…”