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Mechanisms of Cobalt/Phosphine‐Catalyzed Cross‐Coupling Reactions
Author(s) -
Kreyenschmidt Friedrich,
Meurer Selim E.,
Koszinowski Konrad
Publication year - 2019
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201805964
Subject(s) - chemistry , disproportionation , cobalt , radical , phosphine , alkyl , coupling reaction , catalysis , photochemistry , electron transfer , redox , transmetalation , heck reaction , ionic bonding , electrospray ionization , denticity , palladium , organic chemistry , ion , metal
>The combination of CoCl 2 with bidentate phosphines is known to catalyze challenging cross‐coupling and Heck‐type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray‐ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low‐valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br − . In cross‐coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck‐type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single‐electron transfer processes and the involvement of radicals in these transformations.