Heterogenized Phenanthroline-Pd(2+)-Catalyzed Alkoxycarbonylation of Aryl Iodides in Base-Free Conditions.

A phenanthroline-based porous organic polymer-supported heterogeneous Pd catalyst (Pd@Phen-POP) is facilely synthesized by the solvent knitting of a Phen scaffold via the Lewis-acid-catalyzed Friedel-Crafts reaction using dichloromethane as a source for linker in the presence of AlCl3. The catalyst very effectively catalyzes the alkoxycarbonylation of various substituted aryl iodides with various alcohols to give corresponding products in good to excellent yields. Owing to the heterotic nature of the catalyst, it can be easily separated by simple filtration from the reaction mixture and recycled.

Efficient Nicotinamide Adenine Dinucleotide Regeneration with a Rhodium-Carbene Catalyst and Isolation of a Hydride Intermediate.

Regeneration of nicotinamide adenine dinucleotide (NADH) has been the primary interest in the field of enzymatic transformation, especially associating oxidoreductases given the stoichiometric consumption. The synthesized carbene-ligated rhodium complex [(η5-Cp*)Rh(MDI)Cl]+ [Cp* = pentamethylcyclopentadienyl; MDI = 1,1'-methylenebis(3,3'-dimethylimidazolium)] acts as an exceptional catalyst in the reduction of NAD+ to NADH with a turnover frequency of 1730 h-1, which is over twice that of the higher catalytic activity of the commercially available catalyst [Cp*Rh(bpy)Cl]+ (bpy = 2,2'-bipyridine). Offsetting the contentious atmosphere currently taking place over the specific intermediate of the NADH regeneration, this study presents pivotal evidence of a metal hydride intermediate with a bis(carbene) ligand: a stable form of the rhodium hydride intermediate, [(η5-Cp*)Rh(MDI)H]+, was isolated and fully characterized. This enables thorough insight into the possible mechanism and exact intermediate structure in the NAD+ reduction process.

Reductive Carbonylation of Nitroarenes Using a Heterogenized Phen-Pd Catalyst.

The reductive carbonylation of nitroarenes in the presence of MeOH and CO(g) is one of the interesting alternative routes without utilizing toxic phosgene and corrosive HCl generation for the synthesis of industrially useful carbamate compounds that serve as important intermediates for polyurethane production. Since homogeneous palladium catalysts supported by phen (phen = 1,10-phenanthroline) are known to be effective for this catalysis, the heterogenized Pd catalyst was developed using the phen-containing solid support. In this study, we report the synthesis of a phen-based heterogeneous Pd catalyst, Pd@phen-POP, which involves the solvent knitting of a phen scaffold via the Lewis-acid-catalyzed Friedel-Crafts reaction using dichloromethane as a source for linker in the presence of AlCl3 as a catalyst. The resulting solid material has been thoroughly characterized by various physical methods revealing high porosity and surface area. Similar to the homogeneous pallidum catalyst, this heterogeneous catalyst shows efficient reductive carbonylation of various nitroarenes. The catalytic reaction using nitrobenzene as a model compound presents a high turnover number (TON = 530) and a reasonable turnover frequency (TOF = 45 h-1), with a high selectivity (92%) for the carbamate formation. According to the recycling study, the heterogeneous catalyst is recyclable and retains ∼90% of the original reactivity in each cycle.

Direct Heterogenization of Salphen Coordination Complexes to Porous Organic Polymers: Catalysts for Ring-Expansion Carbonylation of Epoxides.

Salen and salphens are important ligands in coordination chemistry due to their ability to form various metal complexes that can be used for a variety of organic transformations. However, salen/salphen complexes are difficult to separate from the reaction mixture, thereby limiting their application to homogeneous systems. Accordingly, considerable effort has been spent to heterogenize the metallosalen/salphen complexes; however, this has resulted in compromised activities and selectivities. Direct heterogenization of metallosalens to form porous organic polymers (POPs) shows promise for heterogeneous catalysis, because it would allow easy separation while retaining catalytic function. Thus, a facile synthetic strategy for preparing metallosalen/salphen-based porous organic polymers through direct molecular knitting using a Friedel-Crafts reaction is presented herein for the first time. As representative candidates, salphenM(III)Cl (M = Al3+ and Cr3+) complexes are knitted by covalent cross-linking using this facile, scalable, one-pot method to synthesize highly POPs in high yields. When incorporated with [Co(CO)4]- anions, the resulting heterogeneous Lewis acidic metal (Al3+ and Cr3+) POPs exhibit propylene oxide ring-expansion carbonylation activity on par with those of their homogeneous counterparts.

Hyper-Cross-Linked Porous Porphyrin Aluminum(III) Tetracarbonylcobaltate as a Highly Active Heterogeneous Bimetallic Catalyst for the Ring-Expansion Carbonylation of Epoxides.

Development of an industrially viable catalyst for the ring-expansion carbonylation of epoxides remains challenging in the view of facile product separation and recyclability. Herein, we report a heterogenized porous porphyrin Al(III) tetracarbonylcobaltate bimetallic catalyst for the ring-expansion carbonylation of epoxides. The catalyst was synthesized using a hyper-cross-linking strategy involving methylene bridges introduced by the Friedel-Crafts reaction and incorporated with cobaltate anions. The catalyst effectively converts epoxides into the corresponding ß-lactones with an excellent site time yield of 360 h-1, which is comparable to that of the corresponding homogeneous catalysts and is the highest of any heterogeneous catalyst reported so far for this reaction.

Balancing between Heterogeneity and Reactivity in Porphyrin Chromium-Cobaltate Catalyzed Ring Expansion Carbonylation of Epoxide into ß-Lactone.

The synthesis of a unique heterogeneous catalyst that combines the functionality of a homogeneous catalyst and the advantages of a heterogeneous catalytic process is a continuing goal in various industrially applicable reactions. Here, we report heterogenization of homogeneous catalyst for lactone production from epoxide carbonylation through a facile polymerization using Friedel-Crafts reaction. A correlation between reactivity and degree of heterogeneity has been deduced by synthesizing different sized polymeric catalysts. The partially polymerized catalyst showed a remarkable initial turnover frequency of 400 h-1, and the fully polymerized catalyst displayed excellent selectivity during recycling with a total turnover number of 4100.

Correlation between the Structure and Catalytic Activity of [Cp*Rh(Substituted Bipyridine)] Complexes for NADH Regeneration.

A series of water-soluble half-sandwich [Cp*RhIII(N^N)Cl]+ (Cp* = pentamethylcyclopentadiene, N^N-substituted 2,2'-bipyridine) complexes containing electron-donating substituents around the 2,2'-bipyridyl ligand were synthesized and fully characterized for the regioselective reduction of nicotinamide coenzyme (NAD+). The influence of the positional effect of the substituents on the structural, electrochemical, and catalytic properties of the catalyst was systematically studied in detail. The catalytic efficiency of the substituted bipyridine Cp*RhIII complexes are inversely correlated with their redox potentials. The 5,5'-substituted bipyridine Cp*RhIII complex, which had the lowest reduction potential, most effectively regenerated NADH with a turnover frequency of 1100 h-1. Detailed kinetic studies on the generation of intermediate(s) provide valuable mechanistic insight into this catalytic cycle and help to direct the future design strategy of corresponding catalysts.