Maurizio Selva

Green chemistry metrics: a comparative evaluation of dimethyl carbonate, methyl iodide, dimethyl sulfate and methanol as methylating agents

The methylating efficiency of dimethyl carbonate (DMC), dimethyl sulfate (DMS), methyl iodide (MeI), and methanol (MeOH) was assessed based on atom economy and mass index. These parameters were calculated for three model reactions: the O-methylation of phenol, the mono-C-methylation of phenylacetonitrile, and the mono-N-methylation of aniline. The analysis was carried out over a total of 33 different procedures selected from the literature. Methanol and, in particular, DMC yielded very favourable mass indexes (in the range 3–6) indicating a significant decrease of the overall flow of materials (reagents, catalysts, solvents, etc.), thereby providing safer greener catalytic reactions with no waste.

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.

Ionic Liquids Made with Dimethyl Carbonate: Solvents as well as Boosted Basic Catalysts for the Michael Reaction

This article describes 1) a methodology for the green synthesis of a class of methylammonium and methylphosphonium ionic liquids (ILs), 2) how to tune their acid-base properties by anion exchange, 3) complete neat-phase NMR spectroscopic characterisation of these materials and 4) their application as active organocatalysts for base-promoted carbon-carbon bond-forming reactions. Methylation of tertiary amines or phosphines with dimethyl carbonate leads to the formation of the halogen-free methyl-onium methyl carbonate salts, and these can be easily anion-exchanged to yield a range of derivatives with different melting points, solubility, acid-base properties, stability and viscosity. Treatment with water, in particular, yields bicarbonate-exchanged liquid onium salts. These proved strongly basic, enough to efficiently catalyse the Michael reaction; experiments suggest that in these systems the bicarbonate basicity is boosted by two orders of magnitude with respect to inorganic bicarbonate salts. These basic ionic liquids used in catalytic amounts are better even than traditional strong organic bases. The present work also introduces neat NMR spectroscopy of the ionic liquids as a probe for solute-solvent interactions as well as a tool for characterisation. Our studies show that high catalytic efficacy of functional ionic liquids can be achieved by integrating their green synthesis, along with a fine-tuning of their structure. Demonstrating that ionic liquid solvents can be made by a truly green procedure, and that their properties and reactivity can be tailored to the point of bridging the gap between their use as solvents and as catalysts.

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.

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.

Selective catalytic etherification of glycerol formal and solketal with dialkyl carbonates and K2CO3

At T ≥ 200 °C, in the presence of K2CO3 as a catalyst, an original etherification procedure of non-toxic acetals such as glycerol formal (GlyF) and solketal has been investigated by using dialkyl carbonates as safe alkylating agents. The effects of parameters including the temperature, the reaction time, and the loading of both the catalyst and the dialkyl carbonate have been detailed for the model case of dimethyl carbonate (DMC). Both GlyF and solketal were efficiently alkylated by DMC to produce the corresponding O-methyl ethers with selectivity up to 99% and excellent yields (86–99%, by GC). The high selectivity could be accounted for by a mechanistic study involving a combined sequence of methylation, carboxymethylation, decarboxylation and hydrolysis processes. The O-methylation of GlyF and solketal could be successfully scaled up for multigram synthesis even operating with a moderate excess (5 molar equiv.) of DMC and in the absence of additional solvent. Notwithstanding the advantageous reduction of the process mass index, scale up experiments provided evidence that prolonged reaction times may induce the decomposition of DMC mainly by the loss of CO2. The K2CO3-catalyzed etherification of solketal with other carbonates such as dibenzyl and diethyl carbonate (DBnC and DEC, respectively), proceeded with the same good selectivity observed for DMC. However, at 220 °C, the solketal conversion did not exceed 81% since both DBnC and DEC were extensively consumed in competitive decarboxylation and hydrolysis reactions.

The methylation of benzyl -type alcohols with dimethyl carbonate in the presence of Y- and X-faujasites: selective synthesis of methyl ethers

At 165–200 °C, in the presence of sodium-exchanged faujasites (NaX or NaY) as catalysts, the reaction of dimethyl carbonate with benzyl-, o- and p-methoxybenzyl-, p-hydroxybenzyl-, diphenylmethyl-, and triphenylmethyl-alcohols (1a, 2a,b, 3a, 4a, and 4c, respectively), produces the corresponding methyl ethers in up to 98% yields. A peculiar chemoselectivity is observed for hydroxybenzyl alcohols (compounds 3a and 3b, para- and ortho-isomers) whose etherification takes place without affecting the OH aromatic groups. Acid–base interactions of alcohols and DMC over the faujasite surface offer a plausible explanation for the catalytic effect of zeolites NaY and NaX, as well as for the trend of reactivity shown by the different alcohols (primary > secondary > tertiary). However, in the case of substrates with mobile protons in the β-position (i.e.1-phenylethanol and 1,1-diphenylethanol), the dehydration reaction to olefins is the major, if not the exclusive, process.

Sequential coupling of the transesterification of cyclic carbonates with the selective N-methylation of anilines catalysed by faujasites

Anilines (R′C6H4NH2: R′ = H, p-MeO, p-Me; p-Cl, and p-NO2) react with a mixture of ethylene carbonate and methanol at 180 °C in the presence of alkali metal exchanged faujasites—preferably of the X-type—to give the corresponding N,N-dimethyl derivatives (R′C6H4NMe2) in isolated yields up to 98%. Evidence proves that methanol is not the methylating agent. The reaction instead takes place through two sequential transformations, both catalyzed by faujasites: first transesterification of ethylene carbonate with MeOH to yield dimethyl carbonate, followed by the selective N-methylation of anilines by dimethyl carbonate. Propylene carbonate, is less reactive than ethylene carbonate, but it can be used under the same conditions. The overall process is highly chemoselective since the competitive reactions between the anilines and the cyclic carbonates is efficiently ruled out. Ethanol and propanol form the corresponding diethyl- and dipropyl- carbonates in the first step, but these compounds are not successful for the domino alkylation of anilines.

Reactions of p-coumaryl alcohol model compounds with dimethyl carbonate. Towards the upgrading of lignin building blocks

Cinnamyl alcohol 1 and 4-(3-hydroxypropyl)phenol 2, two compounds resembling the lignin building block p-coumaryl alcohol, can be selectively transformed into different products by catalytic methodologies based on dimethyl carbonate (DMC) as a green solvent/reagent. Selectivity can be tuned as a function of the reaction temperature and of the nature of the catalyst. Basic catalysts such as K2CO3, trioctylmethylphosphonium methylcarbonate ([P8881][CH3OCOO]), and CsF/αAl2O3 promote selective transesterification of the aliphatic hydroxyl group at 90 °C. However, amphoteric solids such as alkali metal-exchanged faujasites, NaX and NaY, selectively yield the corresponding alkyl ethers at higher temperatures (165–180 °C). The phenolic hydroxyl group of 2 can be methylated similarly with the faujasites at high temperatures. This preliminary screening for selectivity illustrates reactivity trends and delineates some of what might be among the most promising synthetic pathways to upgrade lignin-derived chemical building blocks.