Low‐Valent Ate Complexes Formed in Cobalt‐Catalyzed Cross‐Coupling Reactions with 1,3‐Dienes as Additives

Abstract

The combination of CoCl 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.

Mechanisms of Cobalt/Phosphine‐Catalyzed Cross‐Coupling Reactions

“…[13] To provide the missing mechanistic information and confirm or refute the suggested mechanisticc ycles, we here turn to electrospray-ionization (ESI)m ass spectrometry.A sw ea nd others have shown previously,t his method is well-suited for identifying elusive organometallic speciesi ng eneral [16,17] and organocobalt intermediates in particular. [18] AlthoughE SI mass spectrometry cannot detect neutral analytes, it may nonetheless afford valuable information on their chemistry if chargetagged substrates are used. [16a, 19] By complementing ESI mass spectrometry (MS 1 experiments) with gas-phase fragmentation experiments on mass-selected ions (MS 2 experiments), we moreover gain insight into their unimolecular reactivity.W ith this approach, individual elementary steps can be probedwithout the interference from equilibration processes or bimolecular consecutive reactions, whichs everelyc omplicate mechanistic analyses in solution.…”

Sulfonated salicylidene thiadiazole complexes with Co (II) and Ni (II) ions as sustainable corrosion inhibitors and catalysts for cross coupling reaction

“…N^O‐bidentate 2‐(hydroxyimino)‐1‐phenylpropan‐1‐one coordinated to Co (II) ion in an octahedral geometry as an effective catalyst for Sonogashira Hagihara cross coupling of electron‐rich and/or electron‐poor substituted aryl halides of with alkyne derivatives . Cobalt chloride catalyzed the cross coupling of 1,3‐dienes (additives) within homogeneous complexation, as reported elsewhere . Hu et al .…”

Synthesis of 1,4‐Cyclohexadiene Carboxylates through a Formal [2+4]‐Cycloaddition of Propiolates under Cobalt Catalysis

“…[29] In a next step, coordination of the diene takes place producing intermediate diene complex B, as already described by Schäfer and co-workers for a similar system. [20] [30] Further coordination of the alkyne moiety delivers C, which undergoes a metallocyclization forming complex D which in turn undergoes olefin insertion to E, followed by reductive elimination to the final cycloaddition product 6. Alternatively, intermediate C can undergo a direct [2 + 4]-cycloaddition delivering cyclohexadiene complex D' which can then release 6 and the active catalyst A as described through quantum chemical calculations in the gas phase by Schäfer and co-workers.…”