Cyclopentadienyl–Silsesquioxane Titanium Catalysts: Factors Affecting Their Formation and Activity in Olefin Epoxidation with Aqueous Hydrogen Peroxide

Abstract

The reaction of titanium chlorosilyl‐substituted cyclopentadienyl (Cp) complexes, Ti(η5‐C5H3R′SiMe2Cl)Cl3 (R′ = H, 1b; SiMe3, 1c), with 1 equiv. of various silsesquioxane trisilanols, R7Si7O9(OH)3 (R = iBu, 2a; Ph, 2b), affords either corner‐capped Cp derivatives, Ti(η5‐C5H3R′SiMe2Cl)(R7Si7O12‐κ3O3) (R′ = H, R = iBu, 5a, Ph, 5b; R′ = SiMe3, R = iBu, 7a, Ph, 7b), or cyclopentadienyl–silsesquioxane complexes, Ti(η5‐C5H4SiMe2OR7Si7O11‐κ2O2)Cl (R′ = H, R = iBu, 6a, Ph, 6b; R′ = SiMe3, R = iBu, 8a, Ph, 8b), depending on the reaction conditions. In any case, upon heating, kinetic products 5 and 7 are transformed into the corresponding thermodynamic products 6 and 8, respectively. The electron‐donating ability of the Cp ring is a relevant controlling parameter: a strong π‐donor character facilitates the isomerization process. In addition, the nature of the silicon substituents in the silsesquioxane framework, the type of solvent, and the reaction temperature are also factors that significantly affect this process. Cyclopentadienyl–silsesquioxane complexes 6b and 8b are efficient and selective catalysts for epoxidation with aqueous hydrogen peroxide under mild conditions. Such a catalytic efficiency is attributed to the hydrophobic environment generated about the titanium atom by the Cp ring incorporated into the cyclopentadienyl–silsesquioxane ligand.

Heptanuclear Cage CuII‐Silsesquioxanes: Synthesis, Structure and Catalytic Activity

“…Specific representatives of cage metallacomplexes, namely cagelike metallasilsesquioxanes (CLMSs), are being intensely studied as (pre)catalysts for numerous chemical processes of high relevance . Cage metallasilsesquioxanes were found to be active in alkene polymerization, epoxidation or epoxide ring opening, metathesis, hydroformylation, Ullmann–Goldberg condensation and amidation . Recently, significant attention was drawn to the potential of CLMS application in the homogeneous oxidation of C–H compounds with activity confirmed for Fe‐,, Cr‐, Ni‐, and Co‐based compounds.…”

Chiral Titanium(IV) Complexes Containing Polydentate Ligands Based on α-Pinene. Catalytic Activity in Sulfoxidation with Hydrogen Peroxide

“…Recently, we disclosed and demonstrated the applicability of H 2 O 2 as oxidant in different oxidation reactions using titanium cyclopentadienyl-silsesquioxane derivatives . Encouraged by these exciting results, we decided to investigate the catalytic activity of complexes 1 – 6 , 8 , and 9 in sulfoxidation reactions using H 2 O 2 as oxidant (Scheme ).…”

Stereospecific Synthesis of Chiral Titanium Complexes Bearing a Bifunctionalized Cyclopentadienyl-Terpenoid Ligand Derived from α-Pinene

“…17−19 The study of their reactivity with proton-containing reagents (for example, water, amines, alcohols, and silanols) has constituted a highly efficient method to obtain a wide range of new complexes bearing a bifunctionalized Cp ligand. 20,21 In this new synthetic protocol, the assembly of both functionalities, the Cp ring and the pendant group, takes place within the coordination sphere of the metal atom, an approach patently different from those reported in the literature, where the bifunctionalized ligand precursor is generated first and subsequently reacted with the metal. Currently, a topic of increasing interest is the use of naturally occurring feedstocks: 22−24 among these there are terpenes, which have notable stereochemical features naturally included.…”

“…The cyclopentadienyl ring is widely used as an ancillary ligand due to the high stability that it confers to the organometallic complexes and, additionally, its ease and potential to be modified, allowing the modulation of the steric and electronic properties around the metal center and adjusting them to the requirements of each specific situation. In this regard, our research group has focused on studying the synthesis and reactivity of early transition--metal complexes bearing a chlorosilyl-substituted cyclopentadienyl ligand, which corresponds to the general formula [M­(η 5 -C 5 R 4 SiMe 2 Cl)­Cl n ] ( n = 3, M = Ti, Zr; n = 4, M = Nb, Ta). − The study of their reactivity with proton-containing reagents (for example, water, amines, alcohols, and silanols) has constituted a highly efficient method to obtain a wide range of new complexes bearing a bifunctionalized Cp ligand. , In this new synthetic protocol, the assembly of both functionalities, the Cp ring and the pendant group, takes place within the coordination sphere of the metal atom, an approach patently different from those reported in the literature, where the bifunctionalized ligand precursor is generated first and subsequently reacted with the metal.…”