Reductive Carbonylation of Nitroarenes Using a Heterogenized Phen-Pd Catalyst

Abstract

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 phencontaining 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 AlCl 3 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.

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

“…The heterogeneous solid support was metalated with Pd precursor to afford (Phen-POP)-supported Pd catalyst Pd@(Phen-POP) (2). 29 Both catalysts 1 and 2 were characterized, and the characterization results are shown in Figures S1−S4, Supporting Information (SI). Inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis of 2 revealed that the Pd content is 4.99 wt %.…”

“…The band at 2925–2955 cm –1 corresponding to the stretching vibration of the alkyl group confirms the presence of −CH 2 – linkage in Phen-POP, which is absent in the related homogeneous complex 1 . The heterogeneous solid support was metalated with Pd precursor to afford (Phen-POP)-supported Pd catalyst Pd@(Phen-POP) ( 2 ) . Both catalysts 1 and 2 were characterized, and the characterization results are shown in Figures S1–S4, Supporting Information (SI).…”

“…This result demonstrates that the filtrate without solid catalyst 2 does not contain an appreciable amount of catalytically active species. 28,29 Second, a blank reaction was also carried out using POP-Phen catalyst support; no conversion of IB was found. These experimental results suggest that the methoxycarbonylation using 2 has a heterogeneous nature and occurs only by Pd@(Phen-POP) species present in solid catalyst 2.…”

“…28 Upon analysis of the used catalyst, it was found that under carbonylation reaction conditions the Pd(II) is converted to Pd(0) and cycles between the two species. 29 In addition, excess metal binding sites having a bidentate phenanthroline ligand in the solid support can function as an additional ligand site for stabilizing Pd(0) species, which is necessary for the oxidative addition of aryl iodides for alkoxycarbonylation. 29 We envision that such stabilization would help to improve the alkoxycarbonylation of aryl iodides using such Phen-POP-supported heterogeneous Pd catalyst.…”

“…29 In addition, excess metal binding sites having a bidentate phenanthroline ligand in the solid support can function as an additional ligand site for stabilizing Pd(0) species, which is necessary for the oxidative addition of aryl iodides for alkoxycarbonylation. 29 We envision that such stabilization would help to improve the alkoxycarbonylation of aryl iodides using such Phen-POP-supported heterogeneous Pd catalyst. In this respect, here, we report a highly active phenanthroline-containing porous organic polymer-supported Pd catalyst (Pd@Phen-POP) for the base-and additive-free alkoxycarbonylation of aryl and cyclic alkyl iodides.…”