5 years ago

Reversible Oxidative Addition at Carbon

Reversible Oxidative Addition at Carbon
Udo Radius, Marco Flock, Todd B. Marder, Sonja Fuchs, Antonius F. Eichhorn
The reactivity of N-heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs) with arylboronate esters is reported. The reaction with NHCs leads to the reversible formation of thermally stable Lewis acid/base adducts Ar-B(OR)2⋅NHC (Add1–Add6). Addition of cAACMe to the catecholboronate esters 4-R-C6H4-Bcat (R=Me, OMe) also afforded the adducts 4-R-C6H4Bcat⋅cAACMe (Add7, R=Me and Add8, R=OMe), which react further at room temperature to give the cAACMe ring-expanded products RER1 and RER2. The boronate esters Ar-B(OR)2 of pinacol, neopentylglycol, and ethyleneglycol react with cAAC at RT via reversible B−C oxidative addition to the carbene carbon atom to afford cAACMe(B{OR}2)(Ar) (BCA1–BCA6). NMR studies of cAACMe(Bneop)(4-Me-C6H4) (BCA4) demonstrate the reversible nature of this oxidative addition process. Is carbon the better metal? Just like the isolobal complex fragments C2v-d8-ML4 or C2v-d10-ML2, the carbene cAACMe reacts with selected aryl boronate esters through reversible oxidative addition at room temperature. This reversible, metal-free B−C bond activation at a carbon center might lead to the development of novel carbon atom based catalysts.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201701679

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