Our work is intended to search for new or improved metal-catalyzed synthetic methods, related to C-C or C-X bond forming reactions. Two approaches are followed that are illustrated with some examples.
New ligands to accelerate a catalytic step.
The last step in a C-C or C-X bond forming reaction catalyzed by group 10 metals is a reductive elimination. This is a crucial step since, being generally irreversible, if it is efficient it drives the preceding equilibria to a successful coupling. We have found that ligands designed “ad hoc” bearing σ-donor groups at one end and π-acceptors at the other are very effective at improving the reductive elimination. They have revealed themselves as good as, or even better that many bulky phosphines commonly used for the coupling of difficult substrates. The relative efficiency has been measured using the reductive elimination of C6F5-C6F5 (Pf-Pf) in [Pd(C6F5)2(THF)2].
E. Gioria, J. del Pozo, J. M. Martínez de Ilarduya, P. Espinet. “Promoting Difficult Carbon–Carbon Couplings: Which Ligand Does Best ?” Angew. Chem. Int. Ed. 2016, 55, 13276-13280.
Study of organometallic fundamental reactions.
We are interested in the study of the behaviour of reactive palladium carbenes (no N, N) such as monoamino or alkoxycarbenes. Some time ago, we discovered a migratory insertion of an aryl goup to a carbene in this type of complexes that leads to a new C-C bond and a palladium oganometallic complex that can be further functionalized (Angew. Chem. Int. Ed. 2002, 41, 2363; Chem. Eur. J. 2005, 11, 1565). This reaction, little studied, is important in Pd-catalyzed processes that use diazo derivatives and other carbene precursors as substrates.
Our more recent work in this field is the demonstration that the migratory insertion reaction in palladium carbenes is a general process and occurs, not only for aryl groups, but also for alkyl and acyl moieties.
I. Meana, A. C. Albéniz, P. Espinet “Acyl-Carbene and Methyl-Carbene Coupling via Migratory insertion in Palladium Complexes” Organometallics, 2012, 31, 5494-5499.
We also have characterized of a highly electrophilicpalladium alkoxy carbene obtained by transmetalation. This Pd(II) alcoxycarbene also undergoes comproportionation with Pd(0) to give an unprecedented Pd(I) dimeric carbene.
I. Meana, A. Toledo, A. C. Albéniz, P. Espinet ”Detection and Reactivity of a Palladium Alkoxycarbene” Chem. Eur. J. 2012, 18, 7658.