Shaheen M. Sarkar

Self-assembled poly(imidazole-palladium): highly active, reusable catalyst at parts per million to parts per billion levels.

Metalloenzymes are essential proteins with vital activity that promote high-efficiency enzymatic reactions. To ensure catalytic activity, stability, and reusability for safe, nontoxic, sustainable chemistry, and green organic synthesis, it is important to develop metalloenzyme-inspired polymer-supported metal catalysts. Here, we present a highly active, reusable, self-assembled catalyst of poly(imidazole-acrylamide) and palladium species inspired by metalloenzymes and apply our convolution methodology to the preparation of polymeric metal catalysts. Thus, a metalloenzyme-inspired polymeric imidazole Pd catalyst (MEPI-Pd) was readily prepared by the coordinative convolution of (NH(4))(2)PdCl(4) and poly[(N-vinylimidazole)-co-(N-isopropylacrylamide)(5)] in a methanol-water solution at 80 °C for 30 min. SEM observation revealed that MEPI-Pd has a globular-aggregated, self-assembled structure. TEM observation and XPS and EDX analyses indicated that PdCl(2) and Pd(0) nanoparticles were uniformly dispersed in MEPI-Pd. MEPI-Pd was utilized for the allylic arylation/alkenylation/vinylation of allylic esters and carbonates with aryl/alkenylboronic acids, vinylboronic acid esters, and tetraaryl borates. Even 0.8-40 mol ppm Pd of MEPI-Pd efficiently promoted allylic arylation/alkenylation/vinylation in alcohol and/or water with a catalytic turnover number (TON) of 20,000-1,250,000. Furthermore, MEPI-Pd efficiently promoted the Suzuki-Miyaura reaction of a variety of inactivated aryl chlorides as well as aryl bromides and iodides in water with a TON of up to 3,570,000. MEPI-Pd was reused for the allylic arylation and Suzuki-Miyaura reaction of an aryl chloride without loss of catalytic activity.

Amphiphilic Self-Assembled Polymeric Copper Catalyst to Parts per Million Levels: Click Chemistry

Self-assembly of copper sulfate and a poly(imidazole-acrylamide) amphiphile provided a highly active, reusable, globular, solid-phase catalyst for click chemistry. The self-assembled polymeric Cu catalyst was readily prepared from poly(N-isopropylacrylamide-co-N-vinylimidazole) and CuSO(4) via coordinative convolution. The surface of the catalyst was covered with globular particles tens of nanometers in diameter, and those sheetlike composites were layered to build an aggregated structure. Moreover, the imidazole units in the polymeric ligand coordinate to CuSO(4) to give a self-assembled, layered, polymeric copper complex. The insoluble amphiphilic polymeric imidazole Cu catalyst with even 4.5-45 mol ppm drove the Huisgen 1,3-dipolar cycloaddition of a variety of alkynes and organic azides, including the three-component cyclization of a variety of alkynes, organic halides, and sodium azide. The catalytic turnover number and frequency were up to 209000 and 6740 h(-1), respectively. The catalyst was readily reused without loss of catalytic activity to give the corresponding triazoles quantitatively.

A Highly Active and Reusable Self‐Assembled Poly(Imidazole/Palladium) Catalyst: Allylic Arylation/Alkenylation

Metalloproteins, supramolecular composites of polymeric peptides and metal species, are essential organic transformation systems for maintaining vital activity to promote highly efficient enzymatic reactions. For example, the metalloprotein catalase provides extremely high turnover efficiencies of 40000 000 sec . However, metalloproteins are easily disassimilated and exhibit substrate specificity. Therefore, the development of a metalloprotein-inspired polymeric metal catalyst is an important objective for organic, organometallic, and supramolecular chemistry, as well as sustainable and industrial process chemistry. These catalysts are expected to provide highly active and selective organic transformation systems with high reusability, safety, cleanness, ease of use, and substrate tolerance. In metalloproteins, the basic imidazole unit within histidine plays an important role for binding with metal species, thus forming catalytic sites within a supramolecular structure; therefore imidazole ligands are widely utilized as the building blocks of artificial metal– organic self-assembled supramolecules for functional materials including catalysts. We believe that some insoluble selfassembled complexes of amphiphilic polymeric imidazoles and metal species could offer catalytic activities as high as that of metalloproteins, but with much greater reusability. We recently reported the preparation of highly active, reusable, heterogeneous polymeric metal catalysts for organic transformations, also known as molecular convolution, where a soluble linear polymer having multiple ligand groups is convoluted (noncovalently cross-linked) with transition metals through coordinative or ionic complexation. We envisioned applying this concept to the preparation of metalloprotein-inspired polymeric imidazole metal catalysts to produce highly active, reusable, heterogeneous, selfassembled catalysts. Herein we report the development of a novel polymeric imidazole/acrylamide palladium catalyst that was utilized for the allylic arylation/alkenylation of allylic esters with aryl/alkenylboronic acids and tetraaryl borates. Even 0.8–40 ppm of the catalyst efficiently promoted the allylic arylation/alkenylation in alcohol or water with a catalytic turnover number (TON) of 20 000–1250 000, and the catalyst was reusable without loss of catalytic activity. We found that our molecular convolution methodology provided the globular-aggregated, self-assembled structure of the catalyst. The metalloprotein-inspired polymeric imidazole/palladium catalyst 3 (MPPI-Pd) was readily prepared as follows. When the coordinative convolution of [(NH4)2PdCl4] (2 ; 1 mol equiv Pd) and poly[(N-vinylimidazole)-co-(N-isopropylacrylamide)5] [5] (1; 2 molequiv imidazole) was carried out in a methanol/water (1:1) solution at 80 8C for 30 minutes, the resulting compound 3 (brown powder) was precipitated out (Scheme 1). The precipitates were hardly soluble in water, methanol, DMF, EtOAc, CH2Cl2, or n-hexane. As shown in the top left panel of Figure 1, scanning electron microscopy

Highly active thiol-functionalized SBA-15 supported palladium catalyst for Sonogashira and Suzuki–Miyaura cross-coupling reactions

Highly ordered mesoporous silica SBA-15 with pendent 3-mercaptopropyl groups has been prepared by condensation of surface silanols and (3-mercaptopropyl)trimethoxysilane. Treatment of the mercaptopropylated SBA-15 with (CH3CN)2PdCl2 gave a heterogeneous Pd-catalyst. The immobilized Pd-catalyst served as an efficient heterogeneous catalyst for Sonogashira and Suzuki–Miyaura cross coupling reactions of aryl halides. Furthermore, the SBA-15 supported Pd-catalyst was recovered by a simple filtration from the reaction mixture and reused five times without significant loss of its catalytic activity.

Efficient removal of transition metal ions using poly(amidoxime) ligand from polymer grafted kenaf cellulose

A desired copolymer, cellulose-graft-polyacrylonitrile was synthesized by a free radical initiating process and optimum reaction conditions were determined for maximum grafting yield (125%). The nitrile functionalities of the grafted copolymers were converted into the amidoxime ligand by oximation reaction. The kenaf cellulose, cellulose grafted copolymer and poly(amidoxime) ligand were characterized by infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The pH of the solution acts as a key factor to achieve the optical detection of metal ions using complexation of this ligand with some transition metal ions. The reflectance spectra of the [M–ligand]n+ complex was found to have a highest absorbance ranging from 97 to 99.9% at pH 6. The reflectance spectra were increased by increasing Cu2+ ion concentration and a broad peak at 700 nm was observed which indicates the charge transfer (π–π transition) complex. The adsorption capacity with copper was found to be superior; 326.6 mg g−1. The adsorption capacities for other transition metal ions were also found to be strong; Fe3+, Co3+, Mn2+, Cr3+, Ni2+ and Zn2+ were 273.6, 271.6, 241.7, 228.2, 204.2 and 224.3 mg g−1, respectively at pH 6. The experimental data of all metal ions fitted significantly with the pseudo-second-order rate equation. The data proved that the transition metal ion sorption onto ligands was well fitted with the Langmuir isotherm model (R2 > 0.99), which suggest that the cellulose-based adsorbent known as poly(amidoxime) ligand surface is homogenous and monolayer. The reusability was checked by the sorption/desorption process for seven cycles and the sorption and extraction efficiency in each cycle was determined. The new type of adsorbent can be reused in many cycles without any significant loss in its original sensing and removal performances.

Heck, Suzuki and Sonogashira cross-coupling reactions using ppm level of SBA-16 supported Pd-complex

SBA-16 supported 1,2-diaminocyclohexane Pd-complex (100–500 mol ppm of Pd) was found to be an efficient catalyst in the palladium-catalyzed Mizoroki–Heck, Suzuki–Miyaura and copper-free Sonogashira cross-coupling reactions of aryl halides under mild reaction conditions. The Pd-complex efficiently promoted all of these coupling reactions with appropriate coupling partners to afford the corresponding coupling products with almost quantitative yield. The supported Pd-complex was readily recovered and used five times without a significant loss of its catalytic activity.

Preparation of mesoporous SBA-16 silica-supported biscinchona alkaloid ligand for the asymmetric dihydroxylation of olefins

Optically active cinchona alkaloid was anchored onto mesoporous SBA-16 silica and the as-prepared complex was used as a heterogeneous chiral ligand of osmium tetraoxide for the asymmetric dihydroxylation of olefins. The prepared catalytic system provided 90-93% yield of vicinal diol with 92-99% enantioselectivity. The ordered mesoporous SBA-16 silica was found to be a valuable support for the cinchona alkaloid liganded osmium catalyst system which is frequently used in chemical industries and research laboratories for olefin functionalization.

Waste Corn-Cob Cellulose Supported Bio-Heterogeneous Copper Nanoparticles for Aza-Michael Reactions

Bio-heterogeneous poly(amidoxime) copper nanoparticles were prepared on the modified surface of waste corn-cob cellulose through a graft copolymerization process. The Cu-nanoparticles (0.05 mol% to 50 mol ppm) selectively promoted the aza-Michael reaction of aliphatic amines to give the corresponding alkylated products at room temperature in methanol. The supported nanoparticles were easy to recover and reused eight times without a significant loss of their activity.

Pyridinyl Functionalized MCM-48 Supported Highly Active Heterogeneous Palladium Catalyst for Cross-coupling Reactions

An MCM-48 supported 2-pyridinylmethanimine Pd-catalyst was found to be a highly efficient catalyst in the Mizoroki–Heck, Suzuki–Miyaura and copper-free Sonogashira cross-coupling reactions of aryl halides under aqueous reaction conditions. The catalyst efficiently promoted these coupling reactions with ppm levels of palladium to afford the corresponding coupling products in up to 98% yield. The supported Pd-catalyst was readily recovered and reused several times without significant loss of its catalytic activity.

Synthesis of ion imprinted polymers for selective recognition and separation of rare earth metals

Lanthanide-ion imprinted polymers (L-IIPs) were synthesized by stoichiometric amounts of rare earth ions and the cavities in the polymers were created for the corresponding lanthanide ions. The maximum sorption capacities were estimated to be 125.3, 126.5, 127.6, 128.2 and 129.1 mg/g for Pr, Nd, Sm, Eu and Gd, respectively at pH 6. In the selectivity study, the L-IIPs exhibited good selectivity to the specific rare earth ions in the presence of coexisting cations. The imprinting results were found to be excellent with some rare earth ions over other competitor rare earth ions with the same charges and close ionic radius.