Studies on the Heck Reaction Catalyzed by Nickel Complexes

Abstract

The Heck-Mizoroki reaction is a powerful process to make C-C bonds through the coupling between alkenes and aryl electrophiles, and less substituted alkenes. Palladium-based catalysts proved to be extremely efficient for this process, but in recent years there has been a growing interest in replacing the latter for a cheaper and more abundant transition metal. Nickel is the most obvious replacement for palladium in the Heck reaction, since both elements share similar chemical features, which are critical for the process Hybrid P,N ligand complexes provide an excellent playground for the study of the basic processes involved in the Heck reaction, which imply the ability to produce relatively stable catalytic intermediates. Thus, we focused on the isolation and characterization of the different species that could be involved as intermediates in the catalytic reaction, namely: oxidative addition (OA) of aryl halides, migratory insertion (MI) of alkene, and base induced b-hydrogen elimination (BHE), and then proceed to investigate if these would behave as intended in the proposed cycle. The two first steps (OA and MI) proved to be selective to the formation of the respective products, which were isolated and fully characterized, albeit BHE resulted to be a major challenge worthy of study. Thus a model was proposed, in which the P,N ligands had a role beyond being mere spectators. We discovered that during this last step, the enolizable P,N ligands might act as proton shuttle, due to their ability to be deprotonated in the presence of suitable bases such as NaH or LiPiv. A catalytic system comprised of either in situ generated or isolated Ni(0) species, stabilized by P,N ligands, in combination with a tertiary imine (NEt3) and MA, proved effective in the olefination of 4-acetylphenyl triflate. Those catalyst containing the bulkiest DiPP substituent on the imino donor group prevents effective catalytic cycling, whereas those containing mesityl or ligand PhPCH2Py proves to improve the catalytic yields (TON ~ 4). Given these results, migratory insertion reactions of MA were performed, allowing to isolate cationic products with h3-oxaallyl structures arising from 2,1 insertion. The latter can also be obtained by protonation of the Ni(0) species containing the Heck “alkene”, but in this case, a mixture was obtained in which the benzyl-type isomers were present, in fixed proportions. These complexes tend to decompose through several competing mechanisms such as BHE. This decomposition can be accelerated by the addition of bases such as LiPiv. The latter gives rise to the selective formation of Ni(0)-alkene complexes, which strongly supports a Heck reaction mediated by the ligand (MLC).