Concerted Catalysis in Tight Spaces: Palladium‐Catalyzed Allylation Reactions Accelerated by Accumulated Active Sites in Mesoporous Silica

Abstract

The surface of mesoporous silica was modified with a Pd–bisphosphine complex and/or a tertiary amine group for concerted acceleration of allylation reactions. Mesoporous‐silica‐supported catalysts with a 1.6 nm pore diameter showed higher performance than nonporous or larger mesoporous silica‐supported catalysts owing to the accumulation of active sites into a confined space. For the case in which allyl alcohol was used in the reaction, the presence of a silanol group on the surface was quite effective: the turnover number of Pd was nine times greater than that of the homogeneous Pd complex.

Effects of Mesopore Internal Surfaces on the Structure of Immobilized Pd-Bisphosphine Complexes Analyzed by Variable-Temperature XAFS and Their Catalytic Performances

“…The silane coupling reaction of PP-Pd to the respective MS afforded the desired MS/PP-Pd. Solid-state 13 C and 31 P MAS NMR analysis of the synthesized MS/PP-Pd and SiO 2 /PP-Pd complexes indicated the preservation of the carbon chain in the Pd complex and the coordination structure of the Pd bisphosphine complex, respectively [11,12]. Elemental analysis measurements of MS(C12)/PP-Pd indicated 0.26 mmol/g of Pd loading.…”

“…The detailed analysis results of synthesized MS were reported previously [12]. The surface areas and average pore diameters of the synthesized MS structures are shown in Table 1.…”

“…Additionally, SiO 2 , without mesopores, was used as a support material for the PP-Pd immobilization (SiO 2 /PP-Pd) (Scheme 1B). The Pd-K edge XAFS analysis of the prepared samples strongly suggested the preservation of the PP-Pd complex structure after immobilization [12]. Variable-temperature XAFS measurements were carried out in order to elucidate in detail the effect that the morphology of the silica support has on the PP-Pd complex local structure.…”

“…Recently, we reported a curve-fitting analysis of Pd K-edge extended X-ray absorption fine structure (EXAFS) spectra of SiO 2 -supported Pd complexes [11,12]. The study indicated that the Debye-Waller (DW) factors (σ 2 ) become greater with an increase in the size of the co-immobilized organic molecules.…”

Silica Support‐Enhanced Pd‐Catalyzed Allylation Using Allylic Alcohols

“…The hydrogen‐bonding interaction between epoxide and silanol enhances the ring‐opening step of the epoxide, as shown in Scheme (A),, which is a crucial step for the overall reaction. These studies encouraged us to examine a silica‐supported Pd complex catalyst for the allylation reaction of nucleophiles with allylic alcohols as allylating agents . It can be expected that the hydroxyl group of allylic alcohol interacts with Si−OH (Scheme (B)) similar to those of other hydrogen sources reported in ref .…”