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New developments in dimethyl carbonate chemistry

Dimethylcarbonate (DMC) is a valuable methylating reagent which can replace methyl halides and dimethylsulfate in the methylation of a variety of nucleophiles. It couples tunable reactivity and unprecedented selectivity toward mono-C- and mono-N-methylation in the reactions of acidic CH2 and primary aromatic amines, respectively. In addition, it is a prototype example of a green reagent, since it is nontoxic, made by a clean process, and biodegradable, and it reacts in the presence of a catalytic amount of base thereby avoiding the formation of undesirable inorganic salts as by-products. Other remarkable reactions are those where DMC behaves as an oxidant: cyclic ketones are transformed into a,w-dimethyl esters with a reaction of atom efficiency of 1.0.

Methods and Reagents for Green Chemistry

Methods and reagents for green chemistry , Methods and reagents for green chemistry , کتابخانه دیجیتال جندی شاپور اهواز

Dimethylcarbonate for eco-friendly methylation reactions

Dimethylcarbonate (DMC), an environmentally friendly substitute for dimethylsulfate and methyl halides in methylation reactions, is a very selective reagent. Both under gas-liquid phase transfer catalysis (GL-PTC) and under batch conditions, with potassium carbonate as the catalyst, the reactions of DMC with methylene-active compounds (arylacetonitriles and arylacetoesters, aroxyacetonitriles and methyl aroxyacetates, benzylaryl- and alkylarylsulphones) produce monomethylated derivatives, with a selectivity not previously observed (i.e., >99%). The highly selective O-methylation of phenols and p-cresols by DMC is also attained by a new methodology using a continuous fed stirred tank reactor (CSTR) filled with a catalytic bed of polyethyleneglycol (PEG) and potassium carbonate.

The synthesis of alkyl carbamates from primary aliphatic amines and dialkyl carbonates in supercritical carbon dioxide

Abstract At 130°C and in the presence of compressed CO 2 , primary aliphatic amines [RNH 2 , R=C 10 H 21 , C 8 H 17 , cHex, 1-(C 10 H 7 )CH 2 ] react with organic carbonates (R′OCO 2 R; R′=Me, Et) to give alkyl carbamates (RNHCO 2 R′, 1 ). Although CO 2 promotes the reaction also at a low pressure, good yields (∼80%) of 1 are achievable only with supercritical carbon dioxide (scCO 2 ) at 90 bar, which inhibits the formation of N -methylated by-products.

Multiphasic heterogeneous catalysis mediated by catalyst-philic liquid phases

This critical review addresses heterogeneous catalysis in systems where multiple liquid phases coexist and where one of the phases is catalyst-philic. This technique provides built-in catalyst separation, and product recovery for organic reactions. Focus is placed on the components of the multiphasic systems with emphasis on the constituents of the catalyst-philic phases (PEGs, onium salts, ionic liquids) that incorporate the catalysts, as well as on the effects on catalytic efficiency. It collects a wide body of scattered information that is often labelled with different terms.

Green synthesis of dimethyl isosorbide.

In the last twenty years, new categories of solvents have been investigated in order to deal with safety and environmental issues; that is, water, supercritical fluids, ionic liquids, solvents derived from CO2 [1b, 4] or from renewables. The advantage of using solvents derived from renewables is that natural products are present in large amounts, although only a small fraction (ca. 4 %) is used for this purpose. d-sorbitol is a good example of biofeedstock and has several applications in food and non-food industries. Besides, its cyclic derivate, isosorbide, is widely used in the pharmaceutical and hygiene industries. The methyl derivative of isosorbide, dimethyl isosorbide (DMI), is also extensively used in cosmetics and as a thinning agent. Furthermore, due to its renewable starting materials and high boiling point (246 8C), DMI is also a suitable substitute of the more toxic currently used solvents, such as dimethylsulfoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAc). However, currently DMI is synthesized by common methylation reactions, which employ dimethyl sulfate (DMS) or methyl halides. Herein, it is reported an improved green synthesis of DMI by reaction of isosorbide with dimethyl carbonate (DMC) as reagent and solvent, in the presence of a base at reflux temperature of 90 8C. (Scheme 1). DMC, nowadays produced by a clean and halogen-free process, is an environmentally benign substitute of phosgene, DMS, and methyl halides and it is a well-known nontoxic solvent and reagent. 10] In general DMC, as a methylating agent (bimolecular, base-catalyzed, alkyl cleavage, nucleophilic substitution, BAl2, mechanism) [4c] requires temperatures higher than 150 8C in the presence of a base. However, we previously reported that under similar conditions the reaction of hard alkoxides and DMC gave exclusively the transesterified methylcarbonates derivatives (via BAc2 mechanism), also at high temperatures. On the contrary, other softer nucleophiles such as anilines, phenols and methylene-active compounds were easily methylated by DMC via a BAl2 mechanism. [4, 12] Methyl ethers of primary alcohols can be obtained through two steps: a BAc2 transesterification followed by the decarboxylation of the resulting methylcarbonate. However, methylation of secondary alcohols was never obtained quantitatively due to the formation of elimination products. On the other hand, this is not the case in the present work; the secondary hydroxyls groups of isosorbide are efficiently methylated at reflux temperature (90 8C) by reaction with DMC in the presence of a range of bases (Table 1). This is quite surprising, especially in consideration of all the possible products that could be formed by reacting isosorbide with DMC, which include three classes of compounds (Figure 1): carboxymethyl derivates (MC-1, MC-2, dicarboxymethyl isosorbide (DC)), carboxymethyl methyl derivates (MCE-1, MCE-2), and methyl derivates (MMI-1, MMI-2 and DMI). In particular, it was possible to isolate all these isosorbide derivatives as pure compounds except MMI-2 and MC-2 due to their low amount in the reaction mixture. The isolated compounds were identified by GS–MS analysis and NMR spectroscopy (see the Experimental Section). Table 1 shows that the reaction of isosorbide with DMC in the presence of weak bases (entries 1–2), which led mostly to the formation of carboxymethylated products MC-1, MC-2, and DC. However, reactions conducted in the presence of a strong base led to the formation of DMI with yields up to 40 % (Table 1, entries 3–4). Increasing the amount of base (Table 1, entries 5–6), enhanced the yield of DMI. Quantitative conversion to DMI could be achieved using 3 equivalents of sodium methoxide. It is also interesting to point out that the methyl carboxymethyl derivative of isosorbide MCE-1 was formed in higher yields compared to MCE-2 (Table 1, entries 3–5). This result shows that the OH in endo position is more reactive towards methylation compared to the OH in exo position. Scheme 1. Synthesis of dimethyl isosorbide.

Surface activity and micelle formation of alkyl-substituted aza-crown ethers and their metal ion complexes

Reference LPI-ARTICLE-1979-003doi:10.1016/0021-9797(79)90165-6View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Selectivity in hydrodehalogenation of polychloro- and polybromobenzenes under multiphase conditions

Abstract The competitive hydrodehalogenation of isomeric o-, m- and p -dichloro (and dibromo) benzenes with H 2 at atmospheric pressure and Pd/C or NiRaney carried out in a multiphase system (organic solvent and 50% KOH aq. solution) is influenced by the presence of a bulky quaternary onium salt where both the catalytic activity and the selectivity change in relation to the halogen to be removed and the metal catalyst used. In particular, the NiRaney catalyst becomes effective in the reduction only if an onium salt is added. Similarly, the onium salt effects the catalytic hydrodehalogenation of 1,2,4-trichloro- and 1,2,4-tribromobenzene.

Multiphase Heterogeneous Catalytic Enantioselective Hydrogenation of Acetophenone Over Cinchona-Modified Pt/C

The multiphase heterogeneous enantioselective hydrogenation of acetophenone in the presence of cinchona-modified Pt/C was investigated. The system demonstrated the feasibility of this reaction on non-activated ketones. The reaction proceeded selectively, at room temperature and atmospheric pressure, towards the formation of 1-phenylethanol, with up to 20% ee (enantiomeric excess) of either enantiomer depending on the modifier used. A mode of action of the modifier is proposed to account for the mechanism. A comparison with other systems indicates that the investigated system likely acts by a different mechanism, and that it is quite specific for acetophenone.

Reaction of the Ambident Electrophile Dimethyl Carbonate with the Ambident Nucleophile Phenylhydrazine

To explore the ambident electrophilic reactivity of dimethyl carbonate (DMC), reactions with the ambident nucleophile phenylhydrazine were investigated. When a Brönsted base was used, selective carboxymethylation occurred at N-1, after that several other compounds were produced selectively utilizing various conditions. Formation of these compounds was explained by using the Hard-Soft Acid-Base (HSAB) theory. Catalysis by some metal salts altered the reactivity of phenylhydrazine, which effected selective carboxymethylation at N-2 of phenylhydrazine instead.