The Palladium-Catalyzed Carbonylation of Nitrobenzene into Phenyl Isocyanate: The Structural Characterization of a Metallacylic Intermediate

Catalysis of Synergistic Reactions by Host-Guest Assemblies: Reductive Carbonylation of Nitrobenzenes

Numerous chemical transformations require two or more catalytically active sites that act in a concerted manner; nevertheless, designing heterogeneous catalysts with such multiple functionalities remains an overwhelming challenge. Herein, it is shown that by the integration of acidic flexible polymers and Pd-metallated covalent organic framework (COF) hosts, the merits of both catalytically active sites can be utilized to realize heterogeneous synergistic catalysis that are active in the conversion of nitrobenzenes to carbamates via reductive carbonylation. The concentrated catalytically active species in the nanospace force two catalytic components into proximity, thereby enhancing the cooperativity between the acidic species and Pd species to facilitate synergistic catalysis. The resulting host-guest assemblies constitute more efficient systems than the corresponding physical mixtures and the homogeneous counterparts. Furthermore, this system enables easy access to a family of important derivatives such as herbicides and polyurethane monomers and can be integrated with other COFs, showing promising results. This study utilizes host-guest assembly as a versatile tool for the fabrication of multifunctional catalysts with enhanced cooperativity between different catalytic species.

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.

Intramolecular Pd-Catalyzed Reductive Amination of Enolizable sp3-C-H Bonds

A palladium-catalyzed reductive cyclization of nitroarenes has been designed to construct sp³-C-NHAr bonds from sp³-C-H bonds by using an enolizable nucleophile to intercept a nitrosoarene intermediate. Exposure of ortho-substituted nitroarenes to 5 mol % of Pd(OAc)₂ and 10 mol % of phenanthroline under 2 atm of CO constructs partially saturated 5-, 6-, or 7-membered N-heterocycles using α-pyridyl carboxylates, malonates, 1,3-dimethylbarbituric acid, 1,3-diones, or difurans as the nucleophile.

Harnessing the Power of the Water-Gas Shift Reaction for Organic Synthesis

Since its original discovery over a century ago, the water-gas shift reaction (WGSR) has played a crucial role in industrial chemistry, providing a source of H2 to feed fundamental industrial transformations such as the Haber-Bosch synthesis of ammonia. Although the production of hydrogen remains nowadays the major application of the WGSR, the advent of homogeneous catalysis in the 1970s marked the beginning of a synergy between WGSR and organic chemistry. Thus, the reducing power provided by the CO/H2 O couple has been exploited in the synthesis of fine chemicals; not only hydrogenation-type reactions, but also catalytic processes that require a reductive step for the turnover of the catalytic cycle. Despite the potential and unique features of the WGSR, its applications in organic synthesis remain largely underdeveloped. The topic will be critically reviewed herein, with the expectation that an increased awareness may stimulate new, creative work in the area.

Copper-catalyzed carbonylation of anilines by diisopropyl azodicarboxylate for the synthesis of carbamates

Herein, a simple Cu-catalyzed system for the synthesis of carbamates has been developed using anilines with diisopropyl azodicarboxylate as a carbonyl source under solvent-free conditions.

Fe3O4@SiO2/Schiff base/Pd complex as an efficient heterogeneous and recyclable nanocatalyst for chemoselective N-arylation of O-alkyl primary carbamates

An Fe₃O₄@SiO₂/Schiff base/Pd complex as an efficient, heterogeneous magnetically recoverable and reusable catalyst for the N-arylation of O-alkyl primary carbamates.

4-Dodecylbenzenesulfonic acid (DBSA) promoted solvent-free diversity-oriented synthesis of primary carbamates, S-thiocarbamates and ureas

A simple and efficient solvent-free preparation of primary carbamates, S-thiocarbamates and ureas from alcohols, phenols, thiols and amines in the presence of 4-dodecylbenzenesulfonic acid, as a cheap and green Brønsted acid, has been described.

Away from phosgene: reductive carbonylation of nitroarenes and oxidative carbonylation of amines, understanding the mechanism to improve performance

Recent progress, with some emphasis on the author's own work, on the understanding of the mechanism of the reductive carbonylation of nitroarenes and of the oxidative carbonylation of amines is discussed, highlighting the close connection between the two reactions and trying to unify scattered data in the literature. Isocyanates, carbamates and ureas can be obtained by either procedure, without the need for the use of the toxic and corrosive phosgene.

Catalysis over zinc-incorporated berlinite (ZnAlPO4) of the methoxycarbonylation of 1,6-hexanediamine with dimethyl carbonate to form dimethylhexane-1,6-dicarbamate

Background

The alkoxycarbonylation of diamines with dialkyl carbonates presents promising route for the synthesis of dicarbamates, one that is potentially 'greener' owing to the lack of a reliance on phosgene. While a few homogeneous catalysts have been reported, no heterogeneous catalyst could be found in the literature for use in the synthesis of dicarbamates from diamines and dialkyl carbonates. Because heterogeneous catalysts are more manageable than homogeneous catalysts as regards separation and recycling, in our study, we hydrothermally synthesized and used pure berlinite (AlPO₄) and zinc-incorporated berlinite (ZnAlPO₄) as heterogeneous catalysts in the production of dimethylhexane-1,6-dicarbamate from 1,6-hexanediamine (HDA) and dimethyl carbonate (DMC). The catalysts were characterized by means of XRD, FT-IR and XPS. Various influencing factors, such as the HDA/DMC molar ratio, reaction temperature, reaction time, and ZnAlPO₄/HDA ratio, were investigated systematically.

Results

The XRD characterization identified a berlinite structure associated with both the AlPO₄ and ZnAlPO₄ catalysts. The FT-IR result confirmed the incorporation of zinc into the berlinite framework for ZnAlPO₄. The XPS measurement revealed that the zinc ions in the ZnAlPO₄ structure possessed a higher binding energy than those in ZnO, and as a result, a greater electron-attracting ability. It was found that ZnAlPO₄ catalyzed the formation of dimethylhexane-1,6-dicarbamate from the methoxycarbonylation of HDA with DMC, while no activity was detected on using AlPO₄. Under optimum reaction conditions (i.e. a DMC/HDA molar ratio of 8:1, reaction temperature of 349 K, reaction time of 8 h, and ZnAlPO₄/HDA ratio of 5 (mg/mmol)), a yield of up to 92.5% of dimethylhexane-1,6-dicarbamate (with almost 100% conversion of HDA) was obtained. Based on these results, a possible mechanism for the methoxycarbonylation over ZnAlPO₄ was also proposed.

Conclusion

As a heterogeneous catalyst ZnAlPO₄ berlinite is highly active and selective for the methoxycarbonylation of HDA with DMC. We propose that dimethylhexane-1,6-dicarbamate is formed via a catalytic cycle, which involves activation of the DMC by a key active intermediate species, formed from the coordination of the carbonyl oxygen with Zn(II), as well as a reaction intermediate formed from the nucleophilic attack of the amino group on the carbonyl carbon.

Preparation of Cationic Dinuclear Hydrido Complexes of Ruthenium, Rhodium, and Iridium with Bridging Thiolato Ligands and Their Reactions with Nitrosobenzene

A series of cationic dinuclear hydrido complexes with bridging thiolato ligands [CpMH(&mgr;-SPr(i))(2)MCp][OTf] (4, M = Ru; 6a, M = Rh; 7a, M = Ir; Cp = eta(5)-C(5)Me(5), OTf = OSO(2)CF(3)) were synthesized by treatment of the corresponding chloro complexes [CpRuCl(&mgr;-SPr(i))(2)Ru(OH(2))Cp][OTf] (1) or [CpM(&mgr;-Cl)(&mgr;-SPr(i))(2)MCp][OTf] (2, M = Rh; 3, M = Ir) with HSiEt(3). The dirhodium and diiridium complexes 6 and 7 have been shown to possess a bridging hydrido ligand by crystallographic analysis, while the diruthenium complex 4 is proposed to have a terminal hydrido ligand that undergoes facile migration between the two ruthenium centers in solution. Complexes 4, 6a, and 7a reacted with nitrosobenzene to give the paramagnetic dinuclear nitrosobenzene complexes [CpM(&mgr;-PhNO)(&mgr;-SPr(i))(2)MCp](+) (M = Ru, Rh, Ir) along with azoxybenzene. The molecular structures of the three nitrosobenzene complexes have been determined by X-ray diffraction study to reveal that in each case nitrosobenzene acts as a &mgr;-eta(1):eta(1)-N,O ligand. Judging from the molecular structures and the ESR spectra, the unpaired electron is considered to be located mainly on the nitrosobenzene ligand, at least in the diruthenium and dirhodium complexes. On the other hand, complex 2 reacted with nitrosobenzene to give the incomplete cubane-type trinuclear cluster [(CpRh)(3)(&mgr;-Cl)(2)(&mgr;(3)-S)(&mgr;-SPr(i))](+), whose molecular structure has also been determined crystallographically.