Gas-Phase Investigations on the Transmetalation Step in Sonogashira Reactions

Abstract

The microscopic reverse of the transmetalation step in the Pd/M (M = Cu, Ag, Au) catalyzed Sonogashiratype reactions has been observed in the gas phase upon collision-induced dissociation (CID) of the heterobimetallic complexes. Measuring the activation energies by quantitative energy-resolved CID experiments provides an upper bound for the internal rearrangement energies. The potential-energy surface is investigated by density functional theory calculations and compared to the experimental values.

Dynamic PdII/CuIMultimetallic Assemblies as Molecular Models to Study Metal–Metal Cooperation in Sonogashira Coupling

“…Alkyne C−H deprotonation by other basic centers of the ligand such as the O and N atoms in Naph has higher Gibbs energy barriers (22.1 and 28.9 kcal mol −1 , respectively). Previous mechanistic studies of the transmetalation step in Sonogashira coupling have pointed out that transmetalation begins with π‐coordination of the Pd II to the Cu I ‐acetylide fragment and proceeds to a Pd acetylide with Cu coordinated to the triple bond . In I3 the palladium atom is far away from the C≡C bond (Pd1⋅⋅⋅C1 3.65 Å; Pd1⋅⋅⋅C2 4.67 Å).…”

“…Interestingly, the N‐donor ligand itself also acts as an internal base that facilitates the deprotonation of a terminal alkyne even in the absence of external base additives typically used in Sonogashira coupling. Notably, in previously studied Sonogashira model systems, these three steps had not been observed with the same complex . The combination of experimental and DFT studies of these systems allowed for a detailed understanding of these steps.…”

Pt–M Complexes (M=Ag, Au) as Models for Intermediates in Transmetalation Processes

“…For example, complexes with PtÀAu, PtÀCu, or PtÀAg bonds have been identified in the gas phase or in solution as intermediates in transmetalation reactions involving methylg roups, [5a, 11] whereas PdÀAu and PdÀCu speciesh ave been proposed as transition states in aryl ligand interchange processes. [13] Espineta nd co-workers have studied the mechanism of Pd/Ni-catalyzed Negishi coupling reactions by meanso fD FT calculations and have found "the existence, in both transition states and in some intermediates, of Pd···Zn bonding interactions betweent he electron-rich Pd center and the fairly positive Zn center". [13] Espineta nd co-workers have studied the mechanism of Pd/Ni-catalyzed Negishi coupling reactions by meanso fD FT calculations and have found "the existence, in both transition states and in some intermediates, of Pd···Zn bonding interactions betweent he electron-rich Pd center and the fairly positive Zn center".…”

“…For example, complexes with Pt−Au, Pt−Cu, or Pt−Ag bonds have been identified in the gas phase or in solution as intermediates in transmetalation reactions involving methyl groups, whereas Pd−Au and Pd−Cu species have been proposed as transition states in aryl ligand interchange processes . Also, the microscopic reverse of the transmetalation step in Pd/M‐catalyzed (M=Cu, Ag, Au) Sonogashira‐type reactions has been observed in the gas phase . Espinet and co‐workers have studied the mechanism of Pd/Ni‐catalyzed Negishi coupling reactions by means of DFT calculations and have found “the existence, in both transition states and in some intermediates, of Pd ⋅⋅⋅ Zn bonding interactions between the electron‐rich Pd center and the fairly positive Zn center” .…”

Organocopper cross-coupling reaction for C–C bond formation on highly sterically hindered structures

“…Thus, the unique observed reactivity of the copper complex, in contrast to the boron or zinc complex, is supported by the theoretical calculations. Although the possibility of a similar metal–metal interaction in transmetalation has been suggested previously,22–24 our DFT calculations directly compare the transmetalation transition-state structures of an arylpalladium complex with these organometallic reagents. Energy decomposition analysis (EDA)25 of these transition-state structures clearly indicates that small values of the deformation energy (DEF) are mainly responsible for the low activation barrier in the case of the copper reagent (Fig.…”