Recent Advances of Modified TiO 2 Nanostructure as Heterogeneous Catalyst in Organic Transformations

: This review highlights the different strategies of modified TiO 2 nanostructure as heterogeneous catalyst in organic transformations. The modification in TiO 2 nanostructure have been achieved by doping with metal and non-metal, composing with other material such as metals, metal oxides, nonoxides, semiconductor and nanostructure carbon materials. The influence of modification in TiO 2 nanostructure on catalytic properties in organic synthesis also discussed. Different modifications of TiO 2 extend the catalyst selectivity and reusability over unmodified TiO 2 nanoparticles. Recent investigations have shown that modified TiO 2 nanostructures utilised as active catalysts or catalyst support in organic transformations including C-C, C-N, C-S, C-O bond formation reactions, multicomponent reactions (MCR)


I. INTRODUCTION
This A rapid industrialization and urbanization have produced many serious environmental issues during past few decades. 1,2Therefore researchers have been faced with a new challenge of finding the environment-friendly processes that can reduce or eliminate the dependence of hazardous reagents and solvents.'Heterogeneous catalysis' is one of most powerful approach to replace or eliminate polluting processes. 3Heterogeneous catalysis long established in commercial production of fuels, polymers and fibers.In recent years considerable interest has been developed towards the use of the heterogeneous catalyst in the synthesis of advanced intermediate, fine chemicals, and bioactive heterocyclic compounds. 4,57] A variety of conventional catalysts were used for synthesis of heterocyclic compounds.However, these protocols suffered one of the drawbacks such as a necessity of excess organic solvent, needs longer reaction time, tedious work up procedures and recovery of catalyst. 8The solid catalyst was used as a heterogeneous catalyst due to their easy separation and reusability.
0][11] In recent years, (TiO2) have been paying much more attention due to high chemo-selectivity, environmental compatibility, thermal stability and low cost. 124] It is widely demonstrated that the physical and chemical modification in TiO2 were achieved by controlling the particle size to nanometer scale. 15anocrystalline materials are polycrystalline solids with a grain size of a few nanometers, typically < 100 nm.Nanocrystalline materials are a new class of materials that display distinct electrical, optical, magnetic, catalytic properties over their bulk counterparts.These features of nanocrystalline material have great impacts in various fields. 168][19] The properties nanocrystalline materials can be rationalized by adopting various synthesis and post-synthesis routes. 20,21Furthermore, it is possible to confirm these modifications by the available characterization techniques.Therefore, development of a morphologically controllable synthesis of nanocrystalline metal oxides by a simple and economical method is an important research 1.1 TiO2 Titanium dioxide (TiO2) is naturally occurring oxides, was discovered in 1795 and commercial production started in the 1920s. 22TiO2 powders possess distinctive optical, electrical and catalytic properties.Hence, it is extensively used in paints, paper, textiles, plastics, inks, anti-bacterial agents, corrosion-resistant coatings, water and air purification, self-cleaning surfaces, rechargeable batteries, food additives sensor devices, etc .23,25In 1972, Fujishima and Honda revealed the photochemical splitting of water into hydrogen and oxygen with TiO2, that led to great attention towards TiO2 as heterogeneous photo-catalyst. 26Its properties in the visible and UV portions of the electromagnetic spectrum are especially significant.

Nanocrystalline TiO2
The interest in nanocrystalline TiO2 have been grown extensively due to their outstanding chemical and physical properties, which furnished wide their applications such as optics, sensors, catalysts, pigments photovoltaic cells, photocatalysts.TiO2 is known for its easy availability, low cost, prolonged chemical stability and nontoxic nature .The use of nanocrystalline TiO2 in variety of applications was attained by fulfilling requirements in terms of particle size, size distribution, morphology, crystallinity and phase. 13The desired property of TiO2 can be achieved by adapting proper synthetic method and reaction condition. 14II.SIGNIFICANCE OF NANOCRYSTALLINE TIO2 IN ORGANIC TRANSFORMATION 2.1 Bare TiO2 Nanoparticle as Heterogeneous Catalyst 2.1.1.Beckmann Rearrangement Beckmann rearrangement involves conversion of ketoximes or aldoximes into corresponding amides.It is significant route particularly for manufacturing ε-caprolactam in the chemical industry.Beckmann rearrangement are usually carried out in presence of strong Lewis or Bronsted acid such as sulfuric acid, phosphorus penta chloride, hydrochloric acid in presence of organic solvents.These routes leads to formation of variety of by-product.
Sharghi, H. and et al. examined the TiO2 as solid catalyst for Beckmann rearrangement of several ketones and aldehydes.It was found ketones and aldehydes reacts NH2OH.HCl to give corresponding amides in single step with quantitative yield under solvent free condition. 26Solvent free condition, simple work-up, use of commercial, available and inexpensive catalyst and high yields, can make this procedure a useful and attractive alternative to the currently available methods.

Synthesis of Benzimidazole
Bahram, K. and et al. described an oxidative coupling of o-phenylendiamine /o-aminothiophenol and aromatic aldehydes were carried out in presence of H2O2/TiO2 P-25 NPs (particle size ≈ 21 nm) afford 2-substituted benzimidazole and benzothiazole derivatives (Scheme 3). 29It was observed that the absence of TiO2-P25 NPs reaction proceeded very slowly and reaction stopped to formation of imine derivatives, In contrast, the presence of catalytic amount of TiO2-P25 NPs leading to formation of desired products.The catalyst can be activate the decomposition of H2O2.The reaction condition tolerates to both aldehydes bearing electron-withdrawing and electron-donating substituents in excellent yield the desired product.However, the same optimum reaction condition did not give desired 2-substituted benzoxazoles by reaction between 2-aminophenol and aldehydes.The catalyst TiO2-P25 NPs is stable and reused upto the 5 consecutive runs without considerable loss of catalytic activity.Scheme 5: Preparation of β-aminocarbonyl compounds 2.4.6.Hantzsch Condensation 1,4-Dihydropyridine and polyhydroquinoline derivatives have been prepared efficiently in a one-pot synthesis via Hantzsch condensation using nanosized titanium dioxide as a heterogeneous catalyst 32 .The present methodology offers several advantages such as excellent yields, short reaction times (30-120 min) environmentally benign, and mild reaction conditions.The catalyst can be readily separated from the reaction products and recovered in excellent purity for direct reuse.acridinone derivatives in aqueous media using a catalytic amount of titanium dioxide nanoparticles (TiO2 NPs).(Scheme).The advantages of this novel protocol include the excellent yields, operational simplicity, short reaction time, easy work-up, reusability of the catalyst and an environmentally friendly procedure 36 .

28 R
Scheme 1: Beckmann rearrangement 2.1.2Strecker Reaction Seyed Meysam Baghbanian et al. developed a simple and efficient for the synthesis of α-amino nitriles from aldehydes, amines and trimethylsilyl cyanide (Me3SiCN) in the presence of a catalytic amount of cyanuric acid at room temperature.28 Scheme 3: Synthesis of benzimidazoles Scheme 6a: One-pot synthesis of 1,4-DHPs.NPs-nanoparticles Scheme 11: Synthesis of Tetrahydrobenzo[c]acridines Scheme 11: Synthesis of α-aminophosponates

Table 2 :
The efficiency of several classical solvents Synthesis of substituted 2-oxo dihydropyrroles Sunil Rana and et al. demonstrated site-selective multicomponent synthesis of substituted 2-oxo dihydropyrroles catalyzed by heterogeneous TiO2 nanopowder 34 .The reaction is site-selective with respect to aromatic and aliphatic amines.Environmentally benign reaction procedure, excellent yields, tolerance of varieties of functionalities in the reactants, a wide variety of products and reusability of the catalyst make the methodology highly beneficial for the synthesis of polyfunctional dihydropyrroles.