The Reductive Carbonylation of Organic Nitro Compounds

The Reductive Carbonylation of Organic Nitro CompoundsIntroductionThe development of environmentally more benign and in effect(p) synthetic methods has stimulated the evolution of new st aimgies and technologies for the price reduction of useful ingatherings in a safe, comp cultivate, and energy efficient manner. In this respect our preliminary focus to investigate typically efficient and complementary methodology with palladium-catalyzed subtractive carbonylation of nitro argonnes employing CO gas. CO represents the most important C1 building block iota to introduce a carbonyl group into p bent molecules.In the last fifty years, the subtractive carbonylation of complete nitro compounds has been the subject of intense research due to the fact that industrially important chemicals can be obtained in a single step. Among these, the most relevant atomic number 18 isocyanates, carbamides and carbamates but also a pattern of heterocycles and former(a) non-cyclic compounds.Isocya nates are commodity chemicals mostly employed in polyurethane synthesis but also intermediates in the production of carbamates and ureas. Polyurethanes are widely applied in almost every part of modern animation in the form of plastic foams, coatings, adhesives, sealants and elastomers and binders. The most widely employed aromatic isocyanates are especially toluenediisocyanate (TDI) and 4,4-methylene diphenyl diisocyanate (MDI) that account statement for more than seven million metric tons per year.Synthesis of IsocyanetesCurrently, industrial synthesis of isocyanates is carried divulge with two step reply process using nitroarenes with very high yield and selectivity. Nitroarenes is usually reduced to match aminobenzine using heterogeneous enactment admixture gas, and subsequently the amine is reacted with phosgene to give corresponding isocyanate along with hydrochloric acid.Phosgene establish synthetic route seemed to be effective and well established technology empl oyed.Major drawbacks of the phosgene based synthesisThere are essentially four major drawbacks with the synthesis of MDI and TDI via phosgene route. The first and most full-grown comment are its extreme toxicity and flammability of phosgene and isocyanates, which make these chemicals extremely difficult to handle in protrude synthesis. Phosgene was used as a chemical weapon during the World War I, and around million citizenry were injured and got killed by the use of poisonous gases.The second major drawback is the production of corrosive hydrochloric acid, rendering the middling very aggressive with time, thus allowing other side reaction to occur and to resolvent in nuclear reactor degradation.The third limiting factor is the dilution of reaction medium as the high dilution is required to avoid recycle and concentration costs. The final drawback is the isolation of pure isocyanate from reaction mixture. The chloride containing side products are difficult-to-remove from the f inal product leading to detrimental to the march on processing of the isocyanate. Considering above drawbacks, any process design incorporating phosgene ordain get otiose costs to ensure a safe environment.Industrial requirements for an alternative isocyanate synthesisIn rules of order to convert the phosgene route, a number of requirements can be make in the ideal scenario. First of all, readily reachable chemicals should be used and second, they should be as harmless as possible. Finally, a one-step synthetic surgery will be the route par excellence. A high general yield, purity and selectivity, a temperature occlude to about 25 oC and the absence of over and/or under pressures in the plant should be preserve ideally.In principle, most of the requirements could be met by an efficient catalytic administration.The accessional requirements such as unloosen Over Frequency (TOF) in the order of 104 h-1 or high, Turn Over Number (TON) in the order of 106 or above and easy recycling of catalyst would be maintained.On this account, the necessity of environmentally satisfying but take over economically competitive phosgene-free route to isocyanates synthesis is most demanding in near succeeding(a). resource routes to TDI and MDIVarious synthetic pathways to isocyanatesReductive carbonylation with Palladium metal catalyzed system has proved to be an effective transition metal catalytic system due to its ability to be oxidized or reduced advantageously during the reaction and high tendency to form complexes with carbamoyl groups.A palladium strongly prefers the oxidation states 0 and +2, which are unconnected by a comparatively narrow energy gap, making palladium an excellent catalyst for both(prenominal) oxidation and reduction reactions. Secondly, the moderately large van der Waals radius of palladium together with the high number of delectrons (favorable d10 and d8 complexes) means that the organometal is classified as rather soft, with a high t endency for concerted reaction as well as a high affinity to soft and - bestowers, leading to useful chemoselectivity. Finally, Pd is relatively electronegative, resulting in a rather nonpolar Pd-C bond, suppressing the reactivity towards polar functional groups.In our preceding(prenominal) report, nitrobenzene was typically used as a model substrate, and the use of palladium proved to result in the most effective catalytic systems. The carbonylation of nitrobenzene is generally performed in methanol using homogeneous palladium complexes support by variable bidentate N- or P- donor ligands. Methyl phenyl carbamate (MPC) was synthesized including other side products.Isocyanates and catalysis at bottom the vast variety of synthetic system for isocyanates, one of the approaches is to catalytically convert a nitro or amine compound to the corresponding isocyanate (see Figure 4). One of the approach using oxidative carbonylation and carboalkoxylation of aniline has been analyze with unhomogeneous catalytic systems. However, aniline create to be synthesized by hydrogenation of nitrobenzene, thus considering industrial application, the most attr dynamic strategy would be direct synthesis of isocyanates from nitro compounds which is also thermodynamically favorable. Reactivity of isocyanates and carbamatesPhosphorus and Nitrogen as donor atomsBoth phosphorus and nitrogen ligands of the YR3 (Y = P, N) type (called phosphines and amines respectively) can be described as sp3 hybrids in a (close to) tetrahedral geometry, having a lone pair on the central atom, capable of donating its electron density to an empty (transition) metal d-orbital. Amines are more electronegative than their phosphine analogues, so it could bind strongly with metal centre. However, unlike amines, phosphines can act as a acid with their * orbitals, so they can be involved in backbonding (providing the metal has operational d-electrons), rendering the overall bond strength larger than would be expected intuitively (see Figure 6). So, the overall bond strength is determined by an interplay of donation and backbonding, the first having an increasing contribution when electropositive/donating substituents are employed, the latter when electronegative/withdrawing substituents are used.In 1990, E. Drent et. al. reported the palladium catalyze reductive carbonylation of nitroarene introducing soft-base ligands like diphosphines in combination with strongly coordinating anions, or expectant-base electron-donating ligands such as phenanthroline, combined with non- or weakly-coordinating anions, resulted in relatively active and discriminating catalytic systems (maximal TOFs (h-1)/ carbamate selectivities (%) = 150/80 and 1600/98 for diphosphines and phenanthroline respectively).General remarks on P and N based systemsIn general, the statement made by Drent already in 1990 that Pd with chelating hard base electron donating ligands such as phenanthroline, combined with non or weakly coordinating anions, can result in relatively active and selective catalytic systems.Has been generally accepted and indeed thoroughly studied in the past decades by several people.On the other hand, the statement that Pd with chelating soft-base ligands like diphosphines in combination with strongly coordinating anions , can result in respectively active and selective catalytic systems.Has not yet been the subject of intense academic studies, most likely due to the poor results that were obtained initially, when compared to the N-donor systems. However, in principle, in that respect is no reason why N-donor systems should be superior to P-donor systems, except that phosphine ligands are known to be easily oxidized, thus troubling both the preparation and use of such systems.On the basis of new-made observations on chelating N-donor systems like 1,10-phenanthroline with electron donating substituents (R) in combination with Pd(II) and a weakly or non-coordinating anions (Y), comprises the most active systems to date (i.e. Pd(Rxphen)2Y2). Since, 1, 10- phenanthroline (unfettered of substituents ) is relatively cheap and readily available, this ligand ligand is most frequently used. Furthermore, the performed catalyst is more active and selective than its in situ formed analogues and in almost all cases a slight excess of free ligand is added. The gain of a BrOnsted acid (with bulky, non-coordinating anion) as a co catalyst , as well as the addition of a substrate related aniline is known to improve both reactivity and selectivity. The experiment is carried out in inert atmosphere and addition of reactive drying agent is also known to be beneficial. The temperatures are mostly around 120 170 oC, and both the concentration of all components and the reaction times are differed considerably. Methanol is used as a typical solvent , which is partially consumed to yield the carbamate, but at times toluene/methanol or other alcohols are used. Finally, th e influence of varying CO pressure results in different reactivity. Most reactions were performed at pCO = 40 80 bar, the best results were obtained at pCO = 100 bar, and the elevated pressures are believed to enhance the reactivity even further.The MechanismThe palladium catalyzed reductive carbonylation of nitrobenzene in methanol has been investigated in gigantic extent. All catalytic reactions were performed using both catalyst harbinger and pre-formed complex resulted carbamate as well as side products. There are frequently reported (side-) products of this reaction are shown in Figure. azobenzene(azo), azoxybenzene (Azoxy), aniline and N.N-diphenylurea (DPU). Azo and Azoxy are resulted as coupling product of nitrobenzene. Aniline and DPU are hydrogenation products which are indicating the presence of moisture in the reaction mixture as well as methanol could be source of H-atom for hydrogenation reaction. DPU is the carbamate analogue of isocyanate reacting with aniline whi ch is the better nucleophile than methanol.The mechanism of reductive carbonylation of nitro compounds into carbamates has been extensively studied with palladium, (substituted) phenanthroline ligands, MeOH as solvent and an acid cocatalyst.The mechanism proposed by Mooibroek et al. seems to be the most complete as it also explains the formation of all side products Scheme . The palladium-imido species L2Pd=NPh (C3) and the palladacycle L2PdC(O)N(Ph)OC(O) (C2) were considered as possible carbonylation product-releasing species for both (substituted) phenanthroline and diphosphine ligated catalytic system. The result of catalytic experiments , supported by spectroscopic (ESI-MS and NMR) compound C2 is not the major product- releasing intermediate in reactions performed in the absence of acid. In the absence of acid, Pd-imido complex C3 is the proposed reaction intermediate releasing PhN-containing (Azoxy, MPC, PhNH2) products. On the other hand, in the presence of acid the palladacyc le complex C2 becomes the major product-releasing intermediate, resulting the nitrobenzene carbonylation product MPC with high selectivity.Different side productsIn all catalytic reactions performed using both catalyst precursor and pre-formed complex resulted carbamate as well as side products. The side product areConclusion and future prospectsIn conclusion, the incentive research output from the last decade facilitated us to understand the catalytic reductive carbonylation reaction of nitroarenes to produce relevant carbamates. The reactivity of PdII compounds supported by 1, 10-phenanthroline (phen) or the bidentate diaryl phosphane has been studied in the reaction of nitrobenzene with CO in methanol. The nitrobenzene reduction chemistry in the Pd/phen/CH3OH/H+ system resulted higher in selectivity but lower in activity wherein Pd/diphospane/CH3OH/H+ system shows higher in activity but lower in selectivity. Based on our continuing interest in reductive carbonylations of nitro-ar omatic compounds and considering the importance towards sustainable synthesis of isocyanates, herein our further approach to develop a ultimate catalytic composition of reactants which can produce highly selective product with high yield having tiny or no side-products. However, despite rationalized molecular mechanism developed by Tiddo et. al. for above catalytic system is still not efficient enough, nor is it exactly clear how the catalyst works. In recent years, Great progress has been made in extending the scope of palladium-catalyzed synthetic organic reactions introducing in-situ reductive carbonylation reaction with CO generated from the solvent molecules. Manirul et. al. have reported polymer anchored ruthenium based catalyst for reductive carbonylation. Raquel et. al. have investigated that gold nanoparticles are efficient in N-carbamoylation of aromatic amines. In this respect, further elucidation of the overall mechanism and research in new directions is pressing require ment to produce effective palladium based reductive carbonylation of nitro-aromatic compounds considering industrial background. Thus, the current research projection will address several issues which are envisaged to be worthwhile to explore. Firstly, the modification of ligand system, secondly the designing of catalyst system, thirdly the mechanism, and finally the use of other additives in the catalyst system to be fine-tuned.Modification of ligand systemThe active species in the catalytic cycle as Pd(0) are seem to be as stalls as possible to prevent inactive metallic Pd(black) formation. Our previous research work were illustrated the sharp observation of the bidentate P or N ligand based Pd-catalyzed system to achieve maximum activity and selectivity. Our further observations are envisaged based on tripodal P and N system to stabilize Pd(0) species as well as Pd(II) species during catalytic transformations. The concept of monolegated active species stabilizing Pd(0) by steri c bulk ligand is rationalized to enhance the reactivity. On the basis of the palladium catalyzed coupling reaction of aryl halides system employed with electron rich N-heterocycle carbene ligand which exhibits flexible steric bulk environment, Several factors will be assumed to be execute in these system 1) electron-rich nature enhances the rate of oxidative addition, 2) the ligands engineer tightly to the Pd to prevent the formation of Pd black, and 3) their steric bulk favors a 12 electron, monocarbenePd L-Pd species and increases the rate of reductive elimination.Variation in carbamate synthesisThe formation of carbamates in the catalytic system utilizing PdII, phenantroline, acid and discordant nucleophile has been studied extensively to find an alternative for MeOH as nucleophile. The objective is to yield a carbamate or urea which can be pyrolised at lower temperatures to save energy and prevent degradation of the formed isocyanate. The changing the nucleophile is not strai ghtforward, as the use of another nucleophile as a solvent whitethorn be as well expensive or not possible (if the nucleophile is a solid), hence use of a solvent may be necessary. Therefore, non nucleophilic solvents have to be tested in combination with potential nucleophiles in the catalytic synthesis of carbamates and ureas. Despite the use of common known compound used as a nucleophile, still there are broad spectrum of nucleophiles to be tested with various combinations.Isolation of Palladium-imido complex