S. M. A. Hakim Siddiki

Sustainable heterogeneous platinum catalyst for direct methylation of secondary amines by carbon dioxide and hydrogen.

Pt and MoO(x) co-loaded TiO2 is found to be highly effective for direct methylation of aliphatic and aromatic secondary amines by CO2 and H2 under solvent-free conditions. This is the first additive-free and reusable heterogeneous catalytic system with acceptable turnover number.

General and selective C-3 alkylation of indoles with primary alcohols by a reusable Pt nanocluster catalyst.

The platinum rule: Heterogeneous, additive-free C-3 selective alkylation of indoles by aliphatic and aromatic alcohols proceeded under transfer hydrogenation conditions with the reusable Pt/θ-Al2 O3 catalyst (see scheme; TON=turnover number).

Direct synthesis of quinazolinones by acceptorless dehydrogenative coupling of o-aminobenzamide and alcohols by heterogeneous Pt catalysts

HBEA zeolite-supported Pt metal nanoclusters (Pt/HBEA) effectively catalyze direct dehydrogenative synthesis of quinazolinones from o-aminobenzamide and alcohols under promoter-free conditions. This is the first heterogeneous catalytic system for this reaction, which has a turnover number (TON) more than 25 times higher than previous homogeneous catalysts as well as wide scope for aliphatic and aromatic alcohols.

C-3 alkylation of oxindole with alcohols by Pt/CeO2 catalyst in additive-free conditions

In a series of transition metal-loaded CeO2 catalysts and Pt-loaded catalysts on various supports, Pt-loaded CeO2 shows the highest activity for the selective C-3 alkylation of oxindole with octanol. The catalyst is effective for alkylation of oxindole and N-substituted oxindole with a series of substituted benzyl, linear, hetero-aryl alcohols under additive-free conditions and is recyclable. Our results demonstrate the first additive-free catalytic system for this reaction. Mechanistic studies show that this system is driven by the borrowing-hydrogen pathway. Structure–activity relationship studies show that co-presence of surface Pt0 species on Pt metal clusters and basic support is indispensable for this catalytic system.

Acceptorless dehydrogenative coupling of primary alcohols to esters by heterogeneous Pt catalysts

Supported platinum catalysts have been studied for the acceptor-free dehydrogenative coupling of primary alcohols to esters in the liquid phase under solvent-free conditions in N2 at 180 °C. The activity depends on the support material, and Pt-loaded SnO2 (Pt/SnO2) gives the highest activity. Pt/SnO2 shows higher activity than various transition metals (Ir, Re, Ru, Rh, Pd, Ag, Co, Ni, Cu) loaded on SnO2. The Pt/SnO2 catalyst (1 mol%) selectively converted various primary alcohols to their corresponding esters in moderate to high isolated yield (53–91%). This is the first example of reusable heterogeneous catalysts for the acceptor-free dehydrogenative coupling of primary alcohols to esters under additive-free and solvent-free conditions. Mechanistic and infrared (IR) studies are also shown to discuss the reaction pathway and a possible role of the SnO2 support as Lewis acid sites that activate carbonyl groups of adsorbed aldehyde intermediates.

CeO2 as a versatile and reusable catalyst for transesterification of esters with alcohols under solvent-free conditions

CeO2 acted as an efficient and reusable heterogeneous catalyst for transesterification of esters with alcohols under the solvent-free conditions at 160 °C. Among the 11 kinds of metal oxides, CeO2 is the most suitable catalyst in terms of catalytic activity, leaching-resistance and reusability. This catalytic system tolerates various esters and alcohols, and valuable esters such as heteroaromatic esters and benzyl benzoates are produced, demonstrating a practical utility of the system. On the basis of kinetic analysis and in situ IR studies of adsorbed species, a reaction mechanism is proposed, in which proton abstraction of alcohol by a Lewis base site of CeO2 to yield alkoxide species is the rate-limiting step.

Versatile and Sustainable Synthesis of Cyclic Imides from Dicarboxylic Acids and Amines by Nb2O5 as a Base-Tolerant Heterogeneous Lewis Acid Catalyst

Catalytic condensation of dicarboxylics acid and amines without excess amount of activating reagents is the most atom-efficient but unprecedented synthetic method of cyclic imides. Here we present the first general catalytic method, proceeding selectively and efficiently in the presence of a commercial Nb2 O5 as a reusable and base-tolerant heterogeneous Lewis acid catalyst. The method is effective for the direct synthesis of pharmaceutically or industrially important cyclic imides, such as phensuximide, N-hydroxyphthalimide (NHPI), and unsubstituted cyclic imides from dicarboxylic acid or anhydrides with amines, hydroxylamine, or ammonia.

A Heterogeneous Niobium(V) Oxide Catalyst for the Direct Amidation of Esters

This study reports the first example of a heterogeneous catalytic system for the direct amidation of various esters with amines. Of 25 types of catalyst, Nb2O5 shows the highest activity in the amidation of methyl benzoate with aniline. Nb2O5 gives high yields in the amidation of various esters and amines under solvent‐free conditions, is reusable, and shows higher turnover numbers than previously reported homogeneous catalysts such as La(OTf)3. IR spectroscopic studies of ethyl acetate adsorbed on the catalysts show a strong acid–base interaction between the Nb5+ Lewis acid site and carbonyl oxygen, which can result in high reactivity of the ester with a nucleophile (amine) and, thus, high activity of Nb2O5. Kinetic results show that the activity of Nb2O5 does not markedly decrease with increasing aniline concentration, in contrast to reference catalysts TiO2 and La(OTf)3. The relatively low negative impact of basic molecules on the Lewis acid catalysis of Nb2O5 also enables its high activity.

Versatile and sustainable alcoholysis of amides by a reusable CeO2 catalyst

CeO2 catalyzed the esterification between an equivalent molar ratio of primary amides and alcohols under neutral conditions, which provides the first versatile reusable catalytic system for direct alcoholysis of amides to esters with wider scope and 67 times higher turnover number (TON) than previous catalytic systems.

Hydrodeoxygenation of sulfoxides to sulfides by a Pt and MoOx co-loaded TiO2 catalyst

Supported metal nanoparticle catalysts were studied for the hydrodeoxygenation of sulfoxides to sulfides under solvent-free and mild conditions (50–155 °C, 1 or 7 atm H2). The catalytic activity for the model reaction of diphenyl sulfoxide depended on the type of metals, support materials and co-loaded oxides of transition metals (V, Nb, Mo, W, Re). Pt and MoOx co-loaded TiO2 (Pt–MoOx/TiO2) showed the highest activity. Pt–MoOx/TiO2 was reusable after the reaction and was effective for the reduction of various sulfoxides and showed a higher turnover number (TON) than previously reported catalysts. Using Pt–MoOx/TiO2, benzylphenylsulfone was reduced by H2 to give phenylbenzyl sulfide via benzylphenyl sulfoxides, which represented the first example of catalytic conversion of a sulfone to a sulfide by H2. Characterization studies of Pt–MoOx/TiO2 show that the surface of TiO2 is covered by small (or thin layer) Mo oxide species with exposed Mo cations as Lewis acid sites, and 4–5 nm sized Pt metal nanoparticles are supported on the Mo oxide-covered TiO2.