Direct Catalytic Synthesis of N‐Arylcarbamates from CO₂, Anilines and Alcohols

Abstract

The direct catalytic synthesis of carbamates from CO 2 , amines and methanol was achieved by controlling both the reaction equilibrium and the reactivity of the three components. The combination of CeO 2 and 2-cyanopyridine was an effective catalyst, providing various carbamates including N-arylcarbamates in high selectivities.Transformation of CO 2 to valuable chemicals can contribute not only to CO 2 fixation but also to effective CO 2 utilization as a C1 resource. Carbamates attract much attention because of their many applications to pharmaceuticals, agrochemicals, protecting groups and polyurethanes. [1] Carbamates are traditionally produced by using toxic and hazardous reagents such as phosgene or CO, [2] and environmentally-friendly alternative catalytic methods have been intensively investigated: aminolysis of organic carbonates, [3] alcoholysis of ureas, [4] coupling reaction of dimethyl carbonate (DMC) and N,N'-substituted urea [5] and so on. Recently, carbamate syntheses with CO 2 as a carbonyl source have been demonstrated using reactive reagents such as N-tosylhydrazones, [6] alkylhalides, [7] metal alkoxides [8] and silicate esters. [9] Compared with these approaches, carbamate synthesis from CO 2 , amines and alcohols will be ideal (Scheme 1) because the by-product is theoretically only water and all substrates are easily available. [10] N-Arylcarbamates are the most useful among various carbamates, [11] however there are no effective catalyst systems for the arylcarbamate synthesis from CO 2 , arylamines and alcohols (yield 10 %, Table S1) owing to the inert character of CO 2 , equilibrium limitation and side reactions as well as the low reactivity (low basicity and nucleophilicity) of arylamines. [6] The prospective byproducts are organic carbonates from CO 2 and alcohols, and N,N'-disubstituted ureas from CO 2 and amines (Scheme 1). Therefore, development of catalyst systems which can sophisticatedly control the reactivities of the three components and overcome the equilibrium limitation is desirable. Recently, we found that combination of CeO 2 and 2cyanopyridine was an effective catalyst system for the synthesis of dimethyl carbonate (DMC) from CO 2 + CH 3 OH [12] and cyclic carbonates [13] and polycarbonates [14] from CO 2 + diols. Urakawa and co-workers also applied the combination catalyst system to the DMC synthesis at high CO 2 pressure. [15] The severe equilibrium was overcome by removal of H 2 O from the reaction media through the nitrile hydration over CeO 2 . In addition, 2cyanopyridine can be recovered by dehydration of the produced 2-picolinamide over alkali metal-supported SiO 2 catalysts. [12a,d] We envisioned that the combination catalyst was applicable to the one-pot carbamate synthesis from CO 2 , amines and alcohols.At first, we applied the catalyst of CeO 2 and 2-cyanopyridine to methyl N-phenylcarbamate (MPC) formation from CO 2 , aniline and CH 3 OH as a model reaction (Figure 1 and Table S2). Fortunately, the reaction with CeO 2 and 2-cyanopyridine proceeded smooth...

Zinc Oxide Morphology‐Dependent Pd/ZnO Catalysis in Base‐Free CO2 Hydrogenation into Formic Acid

“…“Greenhouse effect” caused by the emission of CO 2 has led to climate changes and increasing temperatures globally within the past decades and thus a major imperative for the 21 st century will be to reduce the concentration of CO 2 in the atmosphere. A net‐zero‐carbon fuel economy may be realized if it efficiently utilizes CO 2 to produce carbon‐based fuels such as alcohols, hydrocarbons and formic acid (FA) [1–20] . Among these, FA is an attractive product that can be easily generated via CO 2 hydrogenation reaction, which exhibits wide applications in leather tanning, silage preservation and hydrogen storage [21–23] .…”

Reaction of CO2 With Alcohols to Linear-, Cyclic-, and Poly-Carbonates Using CeO2-Based Catalysts

“…Another important behavior is the reaction of CO 2 with secondary alcohols such as isopropanol and phenol ( Table 1, Entries 12 and 13), suggesting higher level of difficulty in the synthesis of di-isopropyl carbonate and diphenyl carbonate. The formation of di-isopropyl carbonate using CeO 2 was unfavorable in the reaction of CO 2 +aniline+2-propanol+2cyanopyridine compared to the formation of DMC in the reaction of CO 2 +aniline+methanol+2-cyanopyridine (Tamura et al, 2018a;Gu et al, 2019a). In fact, the energy change between CO 2 +primary alcohols and CO 2 +2-propanol is not so different, however, the reactivity of 2-propanol seems to be much lower than that of primary alcohols over CeO 2 catalyst, which may not be controlled by thermodynamics but by kinetics.…”

“…The catalytic activity per surface area of this pure CeO 2 was almost comparable to that of CeO 2 -ZrO 2 . Based on these results, we have studied the application of pure CeO 2 for the synthesis of organic carbonates, carbamates, and ureas from CO 2 with alcohols, and/or amines (Honda et al, 2011b;Tamura et al, 2013aTamura et al, ,b, 2016aTamura et al, , 2018aGu et al, 2019a). Table 2 lists the catalytic performance of heterogeneous catalysts for the organic carbonate synthesis from CO 2 and alcohols.…”

Catalytic Conversion of Carbon Dioxide through C-N Bond Formation

“…Alternatively, adding the corresponding alcohols to the system including CO 2 and amines was also an efficient route to obtain N -arylcarbamates. In 2018, the combination of CeO 2 and 2-cyanopyridine was used in the direct synthesis of N -arylcarbamates from CO 2 , amines and alcohols (Scheme 19c) [108]. Various linear and branched alcohols were transformed into the corresponding carbamates in high selectivities.…”