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Low‐Valent Ate Complexes Formed in Cobalt‐Catalyzed Cross‐Coupling Reactions with 1,3‐Dienes as Additives
Author(s) -
Kreyenschmidt Friedrich,
Koszinowski Konrad
Publication year - 2018
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.201704547
Subject(s) - chemistry , electrophile , alkyl , cobalt , nucleophile , diene , catalysis , coupling reaction , ligand (biochemistry) , reactive intermediate , halide , medicinal chemistry , electrospray ionization , migratory insertion , reagent , photochemistry , stereochemistry , combinatorial chemistry , organic chemistry , ion , biochemistry , natural rubber , receptor
The combination of CoCl 2 and 1,3‐dienes is known to catalyze challenging alkyl–alkyl cross‐coupling reactions between Grignard reagents and alkyl halides, but the mechanism of these valuable transformations remains speculative. Herein, electrospray‐ionization mass spectrometry is used to identify and characterize the elusive intermediates of these and related reactions. The vast majority of detected species contain low‐valent cobalt(I) centers and diene molecules. Charge tagging, deuterium labeling, and gas‐phase fragmentation experiments elucidate the likely origin of these species and show that the diene not only binds to Co as a π ligand, but also undergoes migratory insertion reactions into Co−H and Co−R bonds. The resulting species have a strong tendency to form anionic cobalt(I) ate complexes, the superior nucleophilicity of which should render them highly reactive toward electrophilic substrates and, thus, presumably is the key to the high catalytic efficiency of the system under investigation. Upon the reaction of the in situ formed cobalt(I) ate complexes with organyl halides, only the final cross‐coupling product could be detected, but no cobalt(III) species. This finding implies that this reaction step proceeds in a direct manner without any intermediate or, alternatively, that it involves an intermediate with a very short lifetime.