Lewis base adducts of tetra-alkoxy diboron compounds, in particular bis(pinacolato)diboron (B2 pin2 ), have been proposed as the active source of nucleophilic boryl species in metal-free borylation reactions. We report the isolation and detailed structural characterization (by solid-state and solution NMR spectroscopy and X-ray crystallography) of a series of anionic adducts of B2 pin2 with hard Lewis bases, such as alkoxides and fluoride. The study was extended to alternative Lewis bases, such as acetate, and other diboron reagents. The B(sp(2) )-B(sp(3) ) adducts exhibit two distinct boron environments in the solid-state and solution NMR spectra, except for [(4-tBuC6 H4 O)B2 pin2 ](-) , which shows rapid site exchange in solution. DFT calculations were performed to analyze the stability of the adducts with respect to dissociation. Stoichiometric reaction of the isolated adducts with two representative series of organic electrophiles-namely, aryl halides and diazonium salts-demonstrate the relative reactivities of the anionic diboron compounds as nucleophilic boryl anion sources.
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.
Ringerweiterung von NHCs! Die Reaktion N‐heterocyclischer Carbene (NHCs) mit Hydrosilanen Ph4−nSiHn (n=1, 2, 3) führt zu einem vollständigen Aufbrechen des Heterocyclus unter Silyleninsertion in eine der C‐N‐Bindungen des Carbens (siehe Schema; R=Alkyl, Aryl).
Diboron Compounds In their Full Paper on , T. Marder et al. report the isolation and detailed structural characterization of a series of anionic adducts of B2pin2 (bis(pinacolato)diborane) with hard Lewis bases such as alkoxides and fluoride. Such sp2–sp3 adducts have been implicated as intermediates in both metal‐catalyzed and metal‐free borylations. The ability of the isolated adducts to transfer a nucleophilic boryl anion directly to electrophilic aromatic substrates in the absence of a transition metal was demonstrated in several stoichiometric reactions with aryl halides and diazonium salts.
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