The Reductive Carbonylation of Organic Nitro Compounds

The Reductive Carbonylation of Organic Nitro Compounds Introduction: The development of environmentally more benign and efficient synthetic methods has stimulated the evolution of new strategies and technologies for the synthesis of useful products in a safe, compact, and energy efficient manner. In this respect our preliminary focus to investigate typically efficient and complementary methodology with palladium-catalyzed reductive carbonylation of nitro arenes employing CO gas. CO represents the most important C1 building block molecule to introduce a carbonyl group into parent molecules. In the last fifty years, the reductive carbonylation of organic 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 are isocyanates, ureas and carbamates but also a number of heterocycles and other non-cyclic compounds. Isocyanates 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 life 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 for more than seven million metric tons per year. Synthesis of Isocyanetes: Currently, industrial synthesis of isocyanates is carried out with two step reaction process using nitroarenes with very high yield and selectivity. Nitroarenes is usually reduced to corresponding aniline using heterogeneous transition metal catalyst, and subsequently the amine is reacted with phosgene to give corresponding isocyanate along with hydrochloric acid. Phosgene based synthetic route seemed to be effective and well established technology employed. Major drawbacks of the phosgene based synthesis: There are essentially four major drawbacks with the synthesis of MDI and TDI via phosgene route. The first and most prominent observation are its extreme toxicity and flammability of phosgene and isocyanates, which make these chemicals extremely difficult to handle in bulk synthesis. Phosgene was used as a chemical weapon during the World War I, and around million people were injured and got killed by the use of poisonous gases. The second major drawback is the production of corrosive hydrochloric acid, rendering the medium very aggressive with time, thus allowing other side reaction to occur and to result in reactor degradation. The third limiting factor is the dilution of reaction medium as the high dilution is required to avoid recycling 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 final product leading to detrimental to the further processing of the isocyanate. Considering above drawbacks, any process design incorporating phosgene will get extra costs to ensure a safe environment. Industrial requirements for an alternative isocyanate synthesis: In order to replace the phosgene route, a number of requirements can be made in the ideal scenario. First of all, readily accessible chemicals should be used and second, they should be as harmless as possible. Finally, a one-step synthetic procedure will be the route par excellence. A high overall yield, purity and selectivity, a temperature close to about 25 oC and the absence of over and/or under pressures in the plant should be preserved ideally. In principle, most of the requirements could be met by an efficient catalytic system.The additional requirements such as Turn Over Frequency (TOF) in the order of 104 h-1 or higher, 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 acceptable but still economically competitive phosgene-free route to isocyanates synthesis is most demanding in near future. Alternative routes to TDI and MDI: Various synthetic pathways to isocyanates: Reductive 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 easily during the reaction and high tendency to form complexes with carbamoyl groups.A palladium strongly prefers the oxidation states 0 and +2, which are separated by a relatively narrow energy gap, making palladium an excellent catalyst for both 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 â€Å"soft†, with a high tendency for concerted reaction as well as a high affinity to â€Å"soft† and -donors, 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 previous 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 supported by variable bidentate N- or P- donor ligands. Methyl phenyl carbamate (MPC) was synthesized including other side products. Isocyanates and catalysis: Within the vast variety of synthetic strategy 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 studied with various catalytic systems. However, aniline have to be synthesized by hydrogenation of nitrobenzene, thus considering industrial application, the most attractive strategy would be direct synthesis of isocyanates from nitro compounds which is also thermodynamically favorable. Reactivity of isocyanates and carbamates: Phosphorus and Nitrogen as donor atoms: Both 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 Ï€ acid with their ÏÆ'* orbitals, so they can be involved in Ï€ backbonding (providing the metal has available 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 ÏÆ' donation and Ï€ 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 â€Å"soft-base† ligands like diphosphines in combination with strongly coordinating anions, or â€Å"hard-base† electron-donating ligands such as phenanthroline, combined with non- or weakly-coordinating anions, resulted in relatively active and selective catalytic systems (maximal TOF’s (h-1)/ carbamate selectivities (%) = 150/80 and 1600/98 for diphosphines and phenanthroline respectively). General remarks on P and N based systems: In general, the statement made by Drent already in 1990 that: ‘Pd with chelating â€Å"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 â€Å"soft-base†Ã¢â‚¬â„¢ 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, there 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 recent 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)2][Y]2). 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 addition 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 a nd the reaction times are differed considerably. Methanol is used as a typical solvent , which is partially consumed to yield the carbamate, but occasionally toluene/methanol or other alcohols are used. Finally, the 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 Mechanism The palladium catalyzed reductive carbonylation of nitrobenzene in methanol has been investigated in great extent. All catalytic reactions were performed using both catalyst precursor 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 which 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 palladacycle complex C2 becomes the major product-releasing intermediate, resulting the nitrobenzene carbonylation product MPC with high selectivity. Different side products: In all catalytic reactions performed using both catalyst precursor and pre-formed complex resulted carbamate as well as side products. The side product are Conclusion and future prospects: In 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-aromatic 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. f or 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 urgent requirement to produce effective palladium based reductive carbonylation of nitro-aromatic compounds considering industrial background. Thus, the current research project 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 system: The active species in the catalytic cycle as Pd(0) are seem to be as stable 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 steric 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 coordinate tightly to the Pd to prevent the formation of Pd black, and 3) thei r steric bulk favors a 12 electron, monocarbene–Pd [L-Pd] species and increases the rate of reductive elimination. Variation in carbamate synthesis: The formation of carbamates in the catalytic system utilizing PdII, phenantroline, acid and various 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 straightforward, as the use of another nucleophile as a solvent may be too 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:

College Admissions Essay: Moving Beyond Pastry :: College Admissions Essays

Moving Beyond Pastry A few days ago, I innocently happened upon what, according to the bakery sign, was an almond croissant. Delighted, I ordered one, and dreamily handed over my two dollars as memories of bustling Parisian streets and morning bakery smells drifted back to me. However, as I took my first bite a record screeched in my head, violently thrusting me out of my daydream and landing me back into the reality that I was not in Paris, but in the middle of the USA, eating what amounted to a dry piece of wonder bread with two barely distinguishable almond bits on top. Ah, Paris! If you were to ask me why one should live, visit, or return to Paris my answer would undoubtedly be, "Pastry." But on a more serious note, as much as I love pastries and sweets, I didn't take out student loans, search for scholarships and cross the Atlantic Ocean so I could eat a crepe or a pain au chocolat as shamelessly thin, stylish people wearing black walked by. I thought I was going to France to study French. And this I certainly did. My classes were all in French, including a religious studies class at the graduate level (funny how no one mentioned this to me before it was too late to drop it!). But the true benefits of my studies abroad continue to become more and more apparent the longer I am home in the United States. In short, I understand that the world is great big place with all kinds of places and people not in an abstract sense, but as a result of experience. When I see the Mona Lisa on television I think of my first visit to the Louvre as I stared awestruck at her small, mischievous face. When I heard that 200,000 Germans gathered in solidarity at the Brandenburg Gate to express their sympathy for the US citizens in the aftermath of the recent terrorist attacks I think about the German people I met this summer and the day that I walked through that gate myself. And when I heard that the Paris traffic and metro stopped as a display of sympathy and grief, I felt my eyes sting with tears.

Literature research Essay

I found a lot of valuable information from doing my literature research. I was able to see that many people have different answers on how to combat bullying. I also found out very interesting stats on bullying over the past 3 years. They showed me that each year there was an increasing amount of children calling child line about bullying issues compared to the previous years. This is statistical evidence to support my hypothesis, â€Å"bullying is spiralling out of control†. If I had been able to conduct my interview my results would have been much more valid. Valid data is a true picture of what is being measured. From the interview that I did conduct I was able to look at the views of a head teacher who had been in education for many years so was able to see if bullying really was getting worse. She believes that bullying hasn’t got any worse it is just society now has a greater awareness of it. She also says as there is now much more media coverage that gives young people ideas to do different things to people to get attention. This would refute my hypothesis but as this is of only one persons view it is not as valid as I would like. When conducting my questionnaires I had 10 participants for the adult questionnaire and 10 participants for the children’s questionnaire. This was fairly small scale therefore making it lass valid. If I had a longer period of time I would be able to get much more respondents and get them from a wider area making my results more valid and more reliable My results from the adults questionnaire showed me that all the respondents believe that bullying isn’t spiralling out of control its just society has a greater awareness of it. 70% of my respondents were also aware of bullying when they were at school. My questionnaires that I received from the children showed me that they have a lot of knowledge on the subject. All of the children said that bullying had been around for a long time and 0ver half of them thought that it was getting worse. This could be due to the fact that as the get older they are starting to experience more bullying situations and see this as it is due to the fact that bullying is getting worse. In conclusion my results both support my hypothesis ( bullying is spiralling out of control) and refute it. I found supporting evidence for both of these so I cant say which one I believe is true. My questionnaires and interview are supporting evidence for my hypothesis but the literature research shows me strong evidence to refute it especially the statistical evidence I found. Therefore I cant conclude from my research that bullying is spiralling out of control but it is a strong possibility that society is now more aware of it which may make bullying look like it is spiralling out of control with all the recent media attention there has been. If I repeated my research project to make it more valid and reliable I would do a longitudinal survey. With these I would be able to analyse the changes of bullying and make comparisons over time making the results I already have more reliable as I would be able to back them up with these repeated results. I would be able to gain a lot of quantitative data which would enable me to produce more reliable stats and help me to analyse statistical data over a longer period of time. I could also use supervised questionnaires as I would then know that the person that I wanted to fill in the questionnaire actually did making them more valid. A way of getting more respondents to fill my questionnaire within the short space of time that I would have would be to use postal questionnaires. I could cheaply reach a large amount of people making my results more valid. The problem with these is that they have a low response rate (less than 50%). I could also look at bullying from a different angle, to gain a better understanding of it. I could look more at the types of bullying that happen and the different effects it has on the victims. This could give me a deeper insight into the different aspects of bullying and could possibly show me ways in which bullying could be stopped. Bibliography   http://news.bbc.co.uk

Syrian The Refugee Crisis During The Syrian Civil War

I’m sure in the past few months and years if you have turned on the news this topic has been talked about in high regard and with deep concern. A country at war, civilians pried from their homes and forced into ridicule and danger while in a search for a better life. If you haven’t caught on by now I am speaking about the Syrian refugee crisis in Europe. To really understand what should happen and what is happening you must know all things that are beyond that. First you should know that this is all a result of the Syrian Civil War started by the refusal of the Assad regime to step down during the Peaceful Arab Spring demonstration. During this protest in Syria, protesters were fired upon, some were killed, and many were injured. After this act, Syrian civilians started opening fire against government militia, and having violent protests of the government. Intern from all this, rebels leave the Syrian army to form the free Syrian army. Kurds also defect from the Assad R egime and make it a three-way battle. In 2012 Iran, Assads biggest aid, starts to lend arms and troops to his war effort. The Gulf States, Saudi Arabia, Kuwait, Bahrain, Qatar, and United Arab Emirates help to fund the rebels in an attempt to end this war and have order amongst its neighbors. After photos surface up in 2013 of Assad using chemical weapons on his own civilians the United States steps in and helps fund the rebels or free Syrian army as well. This is where it gets complicated because now youShow MoreRelatedSyrian Refugees Crisis And The Syrian Refugee Crisis Essay1346 Words   |  6 PagesLatifa HassanJan Syrian Refugee University of Kansas On September of 2015, the image of 3-year-old Aylan Kurdi went viral. â€Å"The toddler’s lifeless body on a Turkish beach had reverberated across the globe† (Parkinson George-Cosh, 2015). 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