Cross-Coupling Reactions Catalyzed by Iron Group Metals and N-Heterocyclic Carbenes via Nonconventional Reaction Mechanisms

抄録

New cross-coupling reactions catalyzed by iron or iron group metals (IGMs), consisting of Fe, Co, and Ni with N-heterocyclic carbenes (NHCs), are described in this report. Highly selective biaryl cross-coupling reactions between aryl halides and aryl Grignard reagents were achieved by using a combination of fluoride salts of IGMs and NHCs. In the course of the study, an unexpected alkenylative cross-coupling between alkyl aryl sulfides and aryl Grignard reagents was found, in which a typical Ni/NHC catalyst displayed unprecedented reactivity toward sulfide substrates. Theoretical studies suggest that the biaryl coupling and the alkenylative coupling reactions proceed via two nonconventional mechanisms, which are substantially different from the widely-accepted cross-coupling mechanism: In the biaryl cross-coupling, treatment of the catalyst mixtures of IGM fluorides and NHCs with an excess amount of the aryl Grignard reagent results in the generation of organometalate complexes, [Ar¹M^IIF₂]MgBr (M=Fe, Co, and Ni). These organometalate species undergo oxidative addition of aryl halide substrates to form intermediates in a high oxidation state (most likely a+IV state) possessing two different aryl groups, Ar¹Ar²M^IVF₂. The heteroleptic diaryl organometallic intermediates collapse easily to afford unsymmetrical biaryls in a highly selective manner. On the other hand, the alkenylative coupling reaction using a Ni/NHC catalyst involves formation of a low-oxidation-state Ni(0)-thioaldehyde complex, which is transformed to an alkenylnickel species via α-deprotonation of the thioaldehyde and subsequent C-S bond cleavage of the resulting enethiolate intermediate. The alkenylnickel species undergoes transmetalation with an aryl Grignard reagent to form alkenyl/aryl coupling products via reductive elimination. The present cross-coupling reactions catalyzed by IGMs with NHC ligands provide highly selective C_sp²-C_sp² coupling methods for the synthesis of unsymmetrical biaryls and styrene derivatives, offering an opportunity to gain new mechanistic insights into IGM-catalyzed cross-coupling reactions.