Structure–Activity Relationships of Oligocationic, Ammonium‐Functionalized Triarylphosphines as Ligands in the Pd‐Catalyzed Suzuki–Miyaura Reaction

Abstract

A series of oligocationic triarylphosphine ligands, containing a varying number (from 2 to 6) of meta‐ammoniomethyl substituents, have been synthesized, characterized, and tested as ligands in the Pd‐catalyzed Suzuki–Miyaura cross‐coupling reaction. By systematic catalytic investigations and spectroscopic (NMR and UV/Vis) studies of these ligands in the palladium(0) complexes as well as of their neutral amine analogues and of triphenylphosphine, structure–activity relations have been established. The substitution of a triarylphosphine core with cationic meta‐ammoniomethyl substituents increases the steric demand (cone angle) of the phosphine ligand and decreases its σ‐donation strength. Furthermore, Coulombic inter‐ligand repulsion forces are introduced. The catalytic activity in the Suzuki–Miyaura reaction within this class of ligands is governed by the number of cationic charges in the ligand structure, rather than by their steric properties or their σ‐donating strength. With an increasing number of charges, the preference for the formation of coordinatively unsaturated phosphine–palladium species increases, which leads to a higher catalytic activity through faster catalyst activation.

A Remarkable cis‐ and trans‐Spanning Dibenzylidene Acetone Diphosphine Chelating Ligand (dbaphos)

“…This value sits between those recorded for [Pd- MeOH). [29] 31 P-31 P exchange spectroscopy (EXSY) experiments were used to obtain rate constants for the exchange process by varying the mixing time (d8) at different temperatures ( Figure 5). [30] The exchange processes are affected by temperature to a greater extent than nOe processes, allowing the exchange to be frozen out.…”

Steric, Electronic, and Secondary Effects on the Coordination Chemistry of Ionic Phosphine Ligands and the Catalytic Behavior of Their Metal Complexes

“…Furthermore, an increase of the number of cationic groups in the ligand structure from 10 b to 9 b to 8 b gave a dramatic increase of the catalytic activity. [49] The Coulombic interA C H T U N G T R E N N U N G liA C H T U N G T R E N N U N G gand repulsion is concluded to lead to an increased tendency towards the formation of coordinatively unsaturated complexes with palladium, which facilitates the formation of the catalytically active, monophosphine-Pd 0 species and therefore leads to a higher observed activity. Importantly, tetraA C H T U N G T R E N N U N G (alkyl)ammonium functionalities, such as those in 8-11 and in [PTA(Me)] + , are not able to engage in stabilizing hydrogen-bonding interactions with solvent molecules, as is the case with sulfonate-substituted phosphines (Scheme 3) and with protonated PTA.…”

“…This difference has been attributed to the specific conformation of the aryl rings in the crystal structure of 8 a(S), which is not expected to be retained in solution. [49] Furthermore, molecular modeling studies were used to estimate the cone angles of the dendritic phosphines 8 c and 8 d; in those cases values around 1808 were found. [50] Steric and electronic properties of other ionic phosphine ligands: Alkylphosphines with positively charged groups such as ammonium or phosphonium substituents (Scheme 2) are generally less strongly donating than their neutral alkylphosphine analogues.…”

“…The combination of these ligands with a suitable metal precursor leads to in situ formation of homogeneous catalysts that have found applications in Pd-catalyzed Suzuki-Miyaura reactions as well as in Rh-catalyzed hydroformylation reactions. [49,55,61,62] The electronic properties of all oligocationic ligands in the series have been evaluated through the 1 J P,Se coupling constant of the corresponding phosphine selenides ( Table 2, entries 5-15). [49] Furthermore, for 8 a and 11, the corresponding complexes trans-[RhCl(CO)L 2 ] (L= 8 a, 11) have been synthesized and characterized in terms of the CO stretching frequency (Table 2, entries 28 and 29).…”

“…[49,55,61,62] The electronic properties of all oligocationic ligands in the series have been evaluated through the 1 J P,Se coupling constant of the corresponding phosphine selenides ( Table 2, entries 5-15). [49] Furthermore, for 8 a and 11, the corresponding complexes trans-[RhCl(CO)L 2 ] (L= 8 a, 11) have been synthesized and characterized in terms of the CO stretching frequency (Table 2, entries 28 and 29). [55] Depending on the number of ammoniomethyl substituents, these ligands are progessively less strongly s-donating and/or more strongly p-accepting than TPP, which can be explained by the electron-withdrawing field effect (Hammett substituent constants: s m = + + 0.40; s p = + + 0.44) [45] of an ammoniomethyl Remarkably, when the Tolman cone angle of 8 a was estimated based on the crystal structure of 8 a(S), a much larger value of 1868 was found.…”