Catalytic Organic Transformations with nanostructured metals, alloys and metal Oxides, mesoporous MCM-41 supported metal oxide and Organic-inorganic hybrid materials

Abstract

Chapter 1 Chapter 1 presents a general introduction to porous materials and, in particular, mesoporous materials. The mechanism of formation of MCM 41 involves supramolecular self assembly of surfactants (e.g., CTAB). Mesoporous silica MCM 41 has found profound applications owing to its extremely high surface area, regular 1 D porous structure (pore size 20–40 Å), and high mechanical and thermal stability. Of particular interest is the extensive use of MCM 41 as a support in heterogeneous catalysis. It can immobilize nanoparticles of metals, alloys and metal oxides inside its pores and thereby act as a catalyst. There is negligible leaching of the nanomaterials into solution because of strong interactions between the metal or metal oxide and the silica surface. Covalent functionalization of mesoporous silica MCM 41 with an organic ligand can be carried out either by:

Grafting a trialkoxysilyl derivative of the organic moiety onto previously synthesized MCM 41, or Direct co condensation of the trialkoxysilyl derivative with tetraethoxysilane (TEOS).

Characterization techniques such as XRD, TEM and XPS are used to study nanostructured metals and metal oxides immobilized on MCM 41. If an organic ligand is covalently attached, FT IR and solid state NMR (^13C CP MAS) serve as powerful tools for characterizing the resulting organic–inorganic hybrid materials.

Chapter 2 Chapter 2 is divided into two parts.

Part A Part A describes the synthesis and characterization of CoO–MCM 41, prepared by ultrasonically controlled deposition–precipitation of cobalt tricarbonyl nitrosyl in the presence of MCM 41. The cobalt oxide formed a close packed monolayer (0.4 nm) inside the pores of MCM 41.

XRD showed the composite to be amorphous. HRTEM revealed the regular channel structure characteristic of mesoporous materials. XPS confirmed strong CoO–silica surface interactions, forming surface silicate species and firmly anchoring the monolayer.

Catalysis: CoO–MCM 41 proved to be an efficient catalyst for epoxidation of olefins under aerobic conditions (1 atm O , i PrCHO, 28 °C).

Epoxidation of geranyl acetate (1) yielded the monoepoxide 1a regioselectively, with no 2,3 epoxide formation. Epoxidation of cholesteryl acetate (2) gave the epoxide 2a predominantly, suggesting involvement of an oxo cobalt(IV) intermediate.

A variety of olefins was studied in this chapter.

Part B Part B deals with the sonochemical synthesis of highly reactive amorphous iron oxide catalysts, both supported and unsupported. Amorphous Fe O particles were synthesized by ultrasonic decomposition of Fe(CO) in decalin. Particle size could be controlled by decreasing precursor concentration. Additional catalysts studied included:

Fe O supported on mesoporous titania Mesoporous Fe O synthesized using CTAB as template Trimetallic Fe–Co–Ni oxide synthesized by ultrasonic decomposition of the respective organometallic precursors

Catalytic studies showed:

Only amorphous metal oxides were active in cyclohexane oxidation. Crystalline oxides (obtained by calcination) were inactive. Mesoporous Fe O gave the best result (35.6% conversion, alcohol:ketone = 5:1).

The Fe–Co–Ni trimetallic oxide composition was 2Ni:2Fe:1Co (EDAX). Higher pressure and temperature increased conversion of cyclohexane to alcohol 3a and ketone 3b. Oxidation of adamantane (4) with mesoporous Fe O gave 1 adamantanol (4a) exclusively (100% selectivity, 52–58% conversion).

Chapter 3 Chapter 3 presents the synthesis, characterization and catalytic studies of novel organic–inorganic hybrid catalysts for direct aldol reactions. Proline Derived Catalyst

A chiral proline derivative (compound 12) was grafted onto MCM 41. Synthesis involved benzyl protection, Appel chlorination, azide formation, and Staudinger reduction. The resulting amine was reacted with 3 isocyanatopropyltriethoxysilane to form a silane anchorable derivative. This was covalently tethered to MCM 41 and deprotected to yield the active catalyst 12.

Penicillin Derived Catalyst

Benzylpenicillin derivative 15 was prepared by electrophilic lactam ring opening using aminopropyl modified MCM 41.

Both catalysts were tested for direct aldol reactions between acetone and aromatic aldehydes:

Catalyst 12 (proline derivative) gave:

45% yield and 36% ee for condensation with 4 nitrobenzaldehyde 42% yield and 59% ee with 4 fluorobenzaldehyde

Catalyst 15 showed lower enantioselectivity (<5% ee).

Increasing hydrophobicity of catalyst 12 using HMDS silylation did not improve ee.

Chapter 4 Chapter 4 describes tartaric acid derived sol–gel interphase catalysts for asymmetric epoxidation of geraniol (20). Hybrid polysilsesquioxanes 18 and 19 were synthesized via sol–gel hydrolytic polycondensation and characterized by FT IR and ^13C CP MAS NMR. Catalytic studies were performed under Sharpless epoxidation conditions (Ti(O^iPr) , TBHP, CH Cl , –20 °C):

Catalyst 18: 53% yield, <5% ee Catalyst 19: 78% yield, <5% ee

These preliminary results suggest further optimization is required to achieve efficient asymmetric epoxidation.