**1. Introduction**

Environment-friendly organic synthesis is the main challenge of modern science in order to meet the objectives of sustainable development. Among many types of functionalization, insertion of a single carbonyl group into an organic molecule plays a special role due to large-scale demand for carbonyl compounds [1,2]. Excellent examples of products (both commodities and specialty chemicals) with carbonyl groups are urea derivatives, widely used in modern chemical industry— they find application in the production of pesticides (herbicides, fungicides), resin precursors, and fiber dyes [3,4], as well as antiviral and anticancer agents and other pharmaceuticals [4–8]. Numerous derivatives of ureas—including isocyanates and carbamates—are employed in syntheses of adhesives, varnishes, rubbers, paints, and polyurethane foams [9]. Unfortunately, dominating technologies of production of diphenylureas from aromatic amines are based on the phosgene method [9,10], and the real challenge in this sector of industry is to replace them by phosgene-free methods. The most common approaches involve less environmentally harmful carbonylating agents such as CO or alkyl carbonates [1,3,5,11–13], including carbonylation performed in beneficial nonconventional solvents such as ionic liquids [12,13]. The main limitation of CO-based methods is the high pressure applied.

Over the past few years, our studies have been focused on the carbonylation of aromatic nitrocompounds and amines by CO in the presence of the PdCl2(XnPy)2/Fe/I2/XnPy catalytic system, where Py = pyridine, X = Cl or CH3, *n* = 0–2 [14–17]. We have successfully optimized the reaction conditions and proposed detailed mechanisms for carbonylation of aniline (AN) to N,N--diphenylurea (DPU, equation 1), or to ethyl N-phenylcarbamate (EPC, Equation (2)), by CO/O2.

Processes presented by Equations (1) and (2) involve mixture of CO (a common source of carbonyl group) and O2 (oxidizing agent), which is potentially explosive in a wide range of concentrations: 16.7–93.5% (under atmospheric pressure and at 18 ◦C) [18,19]. Therefore, strategies of eliminating or replacing hazardous substrates with safer and less expensive compounds are actively researched [20], and introduction of carbon dioxide to gaseous components is one of such approaches following the promising trends of green chemistry. CO2 is already used as a reagen<sup>t</sup> in relatively few industrial processes such as production of urea, salicylic acid, and some carbonates. Despite limited number of applications of CO2 in organic synthesis caused mainly by high kinetic inertness and thermodynamic stability of CO2 [21–24], every year new approaches involving CO2 have been investigated [25–28]. For many years, carbonylation of aromatic amines by CO2 was poorly represented in the literature, in contrast to carbonylation of ammonia and aliphatic amines by CO2 [29–34]. Perhaps, lower nucleophilicity of nitrogen atom in the aromatic ring of aromatic amines decreases their reactivity towards CO2 [5]. However, in recent years significant progress has been made toward the synthesis of isocyanates, ureas, carbamates, and other compounds using CO2 and aromatic amines [35–41]. Despite promising results, many of these methods suffer from some limitations such as long reaction times required, harsh reaction conditions, low yields, and other difficulties in application of CO2 as a carbonylating agen<sup>t</sup> [42,43]. Therefore, some reports are focused on the use of CO2 as an additive for CO, or as a reaction medium (liquid CO2) [5,27,44]. Gabriele et al. [44] observed that in carbonylation of amines performed in the presence of CO/air and PdI2, the addition of CO2 significantly increased yield of the reaction for aliphatic amines, whereas less satisfactory results were obtained for carbonylation of aromatic amines. Surprisingly, good performance was observed for both aliphatic and aromatic amines when carbonylation was conducted in pure CO2 as a non-polar aprotic solvent. Using an appropriate amount of CO2 resulted in nearly three times higher catalytic activity of PdI2 catalyst [44]. Based on the results obtained by Gabriele, although in many processes it is very difficult to replace CO by CO2 as carbonylating agent, addition of CO2 may increase the yield of the carbonylation of amines by CO. Moreover, carbon dioxide is a byproduct formed during industrial production of CO and its complete removal makes an additional complication in the production process. Furthermore, CO2 exhibits a much stronger suppression effect on the explosion of flammable gases than nitrogen [45], and thus it decreases explosiveness of gases employed in the synthesis (CO and O2) leading to enhanced safety of the process [46]. Last but not least, presence of CO2 in a traditional liquid phase under mild pressures (tens of bar) results in generation of a gas-expanded liquid (GXL) phase. GXL retains the beneficial attributes of a conventional solvent (polarity, catalyst/reactant solubility) with some additional advantages: higher miscibility of permanent gases (O2, CO, etc.) and enhanced transport rates compared to organic solvents at ambient conditions. The enhanced gas solubilities in GXLs may result in reaction rates greater than those achieved in neat organic solvent or supercritical carbon dioxide (sCO2) [47]. In the case of our process, even if CO2cannot serve as a carbonylating agent,

it is interesting to explore other potential benefits of replacing CO with CO2, i.e., decreasing the amount of CO used and introducing CO2 without any preconceived notion regarding its exact function.

Inspired by the promising properties of CO2 and results reported by Gabriele et al. for carbonylation of aniline in the presence of CO2, K2PdI4 as catalyst and without any additives [44], we decided to study the effect of CO2 as one of the components of CO/O2/CO2 mixture on the carbonylation of aniline (model aromatic amine) in the presence of our original catalytic system PdCl2(XnPy)2/Fe/I2/XnPy, at shorter reaction time and under lower total pressure. Also, our goal is to develop our previous catalytic system based on PdCl2(XnPy)2 complexes into nanocatalysts, characterized by unique catalytic properties. Based on the results of our recent studies, we turned our attention to derivatives of pyridine as ligands that optimally stabilize palladium NPs [48–51] i.e., the access to the catalyst surface is not restricted, in contrast to bulky ligands [52–54]. Choosing 4-methylpyridine (model derivative of pyridine) as stabilizing ligand allows nanoparticles (NPs) to effectively interact with ligands and the reacting compound(s). In our previous works, we developed a reduction of aromatic nitrocompounds to aromatic amines in the presence of palladium nanoparticles stabilized by 4-methylpyridine (PdNPs/4MePy) [55,56]. Obtained results encourage us investigation of the catalytic activity of PdNPs/4-MePy in another process i.e., oxidative carbonylation of aniline in the presence of mixture CO2/CO/O2. For the first time, carbonylation of aniline is carried out in the presence of CO2 and palladium nanoparticles stabilized by 4-methylpyridine.
