Tripod Immobilization of Triphenylphosphane on a Silica‐Gel Surface to Enable Selective Mono‐Ligation to Palladium: Application to Suzuki–Miyaura Cross‐Coupling Reactions with Chloroarenes

Abstract

A silica-supported triphenylphosphane (Silica-3p-TPP) with a Ph3P-type core, immobilized on a silica surface, was synthesized and characterized by nitrogen-absorption measurements and solid-state NMR spectroscopy. The tripodal immobilization constrains the mobility of the phosphane molecule and causes the lone pair on the phosphorus atom to face in the direction perpendicular to the support, resulting in the selective formation of a 1:1 metal-phosphane species that is free from unfavorable steric repulsions caused by the silica surface. Heterogeneous Pd catalysts created in this manner enabled room-temperature Suzuki-Miyaura cross-coupling reactions with unactivated chloroarenes, despite the moderate electronic and steric nature of the Ph3P-based ligands. These catalysts also showed potential in reactions with more challenging substrates under mild conditions. Tripodally immobilized and well-dispersed phosphanes on the silica surface were crucial for high catalytic activity.

Silica‐Supported Tripod Triarylphosphane: Application to Transition Metal‐Catalyzed C(sp3)H Borylations

“…Furthermore, the other silica‐supported phosphane Silica‐1p‐EtTPP, which has a longer and more flexible linker chain, was even less effective (3% yield, entry 6). No reaction occurred with the corresponding soluble ligand [4‐( i‐ PrO)Me 2 Si‐C 6 H 4 ] 3 P (3p‐TPP)6a in accordance with our previous observations3k in the experiments with Ph 3 P, bulky biarylphosphane X‐Phos,14 and bulky NHC ligand SIPr15 (entries 7–10). Thus, the catalytic function of the Rh catalyst based on Silica‐TRIP could mostly be reproduced by immobilizing a non‐caged triarylphosphane in tripodal or bipodal modes.…”

“…Surprisingly, this was significantly lower than that expected from the [P] value of the starting Silica‐3p‐TPP 16. This strongly suggests that some P atoms in Silica‐3p‐TPP were not used for metal coordination, in contrast to the case of previous Pd coordination 6a. Dimeric [RhCl(cod)] 2 may be more inaccessible than the sterically less demanding monomeric PdCl 2 (PhCN) 2 17…”

“…In contrast, the reaction of [RhCl(cod)] 2 with monopod‐type silica‐supported phosphane Silica‐1p‐TPP (1:4 Rh/P) at 60 °C for 1 h afforded two overlapping 31 P signals at 30 and 23 ppm, which were assigned to a 1:1 Rh/P complex RhCl(cod)(Silica‐1p‐TPP) and a 1:2 Rh/P complex [Rh(cod)(Silica‐1p‐TPP) 2 ] + , along with the signal for free phosphane at −5 ppm (Figure 2c). The 1:2 complex was assigned on the basis of the homogeneous reaction between [Rh(cod) 2 ]BF 4 and the corresponding soluble ligand [4‐( i‐ PrO)Me 2 Si‐C 6 H 4 ]PPh 2 (1p‐TPP)6a (1:3 Rh/P) in benzene, which gave a 31 P NMR (CDCl 3 ) signal for bis(phosphane)‐Rh complex [Rh(cod)(1p‐TPP) 2 ]BF 4 at 26.4 ppm 18. The bidentate P‐coordination by the monopod silica‐supported phosphane indicated that the P center of the monopod phosphane Silica‐1p‐TPP was flexible and the monopodal immobilization via a dimethylsilylene (‐SiMe 2 ‐) linker was not an effective method for inhibiting bidentate P‐coordination on the silica gel surface 19,20…”

Immobilization of an Aminobisphosphine–PdII Complex over Graphene Oxide: An Efficient and Reusable Catalyst for Suzuki–Miyaura, Ullmann Coupling and Cyanation Reactions

“…Different monophosphines and chelated bisphosphines having alkoxysilane groups easily react with silanol groups of silica during immobilization . On the other hand tripodal‐linker systems have been connected on silica through three silicon atoms . The major drawback of these immobilized catalyst systems is the detachment of active species and the most common way of detachment is the hydrolysis of the Si–O–E bond (E = Si, Al or Ti) …”

Decorated‐magnetic‐nanoparticle‐supported bromine as a recyclable catalyst for the oxidation of sulfides

“…The easy separation and reusability of heterogeneous catalysts systems make it possible to conduct the oxidation process continuously and to recycle the catalyst without a loss in activity for further use . Motivated by this concept, many inorganic or organic materials, such as silica gels, sepiolites, zeolites, carbon nanotubes, and others, have been used to support catalysts. However, the control of the catalyst contents is still a challenge because the catalysts are usually supported by weaker physical absorption.…”