A geochemical model for the formation of hydrothermal carbonates on Mars.

The cause of Mars’s loss of surface habitability is unclear, with isotopic data suggesting a ‘missing sink’ of carbonate1. Past climates with surface and shallow-subsurface liquid water are recorded by Mars’s sedimentary rocks, including strata in the approximately 4-km-thick record at Gale Crater2. Those waters were intermittent, spatially patchy and discontinuous, and continued remarkably late in Mars’s history3—attributes that can be understood if, as on Earth, sedimentary-rock formation sequestered carbon dioxide as abundant carbonate (recently confirmed in situ at Gale4). Here we show that a negative feedback among solar luminosity, liquid water and carbonate formation can explain the existence of intermittent Martian oases. In our model, increasing solar luminosity promoted the stability of liquid water, which in turn formed carbonate, reduced the partial pressure of atmospheric carbon dioxide and limited liquid water5. Chaotic orbital forcing modulated wet–dry cycles. The negative feedback restricted liquid water to oases and Mars self-regulated as a desert planet. We model snowmelt as the water source, but the feedback can also work with groundwater as the water source. Model output suggests that Gale faithfully records the expected primary episodes of liquid water stability in the surface and near-surface environment. Eventually, atmospheric thickness approaches water’s triple point, curtailing the sustained stability of liquid water and thus habitability in the surface environment. We assume that the carbonate content found at Gale is representative, and as a result we present a testable idea rather than definitive evidence.

ABSTRACTAn efficient and convenient synthesis of ethylene carbonates was achieved by the reaction of carbon dioxide with 1,2-diols in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), followed by treatment with 1-bromobutane. This DBU-promoted transformation proceeded at an atmospheric pressure of carbon dioxide at 25 °C and gave ethylene carbonates in good yields.

Efficient Fixation of Carbon Dioxide by Electrolysis — Facile Synthesis of Useful Carboxylic Acids —

Since carbon dioxide (CO2) is an abundant, economical, nontoxic and environmentally benign C1 chemical reagent, fixation of carbon dioxide in organic molecules has recently become an attractive project in organic synthesis. An electrochemical method has contributed greatly to this area because it enables an efficient fixation of carbon dioxide in organic molecules even under atmospheric pressure of carbon dioxide when a reactive metal, such as magnesium or aluminum metal, is used as a sacrificial anode. There have been a number of reports on electrochemical fixation of carbon dioxide, and we have also reported synthesis of useful carboxylic acids by electrochemical carboxylation of various organic compounds. On the other hands, it is well known that fluorine-containing organic compounds have unique chemical and physical properties. The introduction of fluorine atoms into biologically-active compounds is also known to cause remarkable modification of their original activities. Therefore, considerable attention has been paid to efficient and selective preparation methods of organofluorine compounds. However, little attention has been paid to electrochemical carboxylation to afford fluorinated carboxylic acids.1-4 During the course of our continuous studies on the synthesis of useful carboxylic acids by electrochemical fixation of carbon dioxide,5 we recently found that electrochemical reduction of polyfluoroarenes in the presence of carbon dioxide resulted in a regioselective cleavage of a C-F bond of the phenyl ring followed by reaction with carbon dioxide to give the corresponding mono-carboxylated products, polyfluorobenzoic acids, in moderate to good yields.6 We report herein the results for synthesis of polyfluorobenzoic acids by regioselective electrochemical carboxylation of polyfluoroarenes.First, we screened reaction conditions for electrochemical carboxylation of hexafluorobenzene (1a) as a substrate. When constant current electrolysis of 1a was carried out in DMF using a one-compartment cell equipped with a Pt cathode and an Mg anode with 3 F/mol of electricity in the presence of carbon dioxide, reductive cleavage of a C-F bond on the phenyl ring followed by reaction with carbon dioxide took place to give pentafluorobenzoic acid (2a). After reaction conditions screening, 2a could be obtained in 73% 19F NMR yield by electrolysis of 1a at –40°C with 5 mA/cm2 of current density. After recrystallization with hexane and acetone, pentafluorobenzoic acid (2a) was obtained in 65% isolated yield as a pure product.We next investigated similar electrochemical carboxylations of several polyfluoroarenes, and the results are shown in Scheme. When pentafluorobenzene (1b) was subjected to the present electrochemical carboxylation under the same conditions for the reaction of 1a, C-F bond cleavage followed by carboxylation also took place efficiently to give 2,3,5,6-tetrafluorobenzoic acid (2b) in 78% 19F NMR yield and 66% isolated yield after recrystallization. It is noteworthy that C-F bond cleavage followed by carboxylation of 1b occurred at the C3 position of 1b predominantly to give 2b as a major product. Similar regioselective electrochemical carboxylation was also achieved when pentafluoroarenes 1c-f were used as substrates. Under the same conditions except for 1d, electrochemical carboxylation of 1c-f gave 4-substituted-2,3,5,6-tetrafluorobenzoic acids 2c-f in 61-84% 19F NMR yields and 49-76% isolated yields after recrystallization. It is also to note that C-F bond cleavage occurred at the para position of the substituents of Me-, AcO-, Me(RO)2C-, and MeS- in pentafluoroethylarenes 1c-f predominantly in all cases to give 4-substituted-2,3,5,6-tetrafluorobenzoic acids 2c-f as major products.<Scheme>Other results of electrochemical carboxylation of polyfluoroarenes and proposed reaction mechanism including regioselectivity of the carboxylation will be presented.

[reaction: see text] Epoxides dissolved in molten tetralkylammonium salts bearing halides as counterions are converted into cyclic carbonates under atmospheric pressure of carbon dioxide. The reaction rate depends on the nucleophilicity of the halide ion as well as the structure of the cation.

For Abstract see ChemInform Abstract in Full Text.

Paleoclimate modelers need estimates of the partial pressure of atmospheric carbon dioxide (pCO2) because CO2 influences the mean global temperature through the greenhouse effect (Arrhenius, 1896). Carbon dioxide enters the atmosphere as a result of high temperature decarbonation reactions and low temperature oxidation of organic C. Carbon dioxide leaves the atmosphere either through Ca, Mg silicate weathering and subsequent burial as carbonate or through photosynthesis and subsequent burial as organic C (Ebelmen, 1845). Changes in these C fluxes, brought about by various biological, oceanographic, and tectonic factors, affect atmospheric pCO2 and, consequently, Earth’s climate (Plass, 1956; Bemer, 1990).

Silica-supported 4-pyrrolidinopyridinium iodide was prepared by quaternization of 4-pyrrolidinopyridine with silica-supported alkyl iodide. The pyrrolidinopyridinium structure on the silica surface was confirmed by solid-state 13C CP MAS NMR. The silica-supported 4-pyrrolidinopyridinium iodide showed excellent catalytic performances for transformations of various epoxides to cyclic carbonates under atmospheric pressure of carbon dioxide (CO2). The reactions took place without any solvents or additives other than the catalyst. The catalyst was reusable with retention of activity and selectivity. 1-n-Hexyl-4-pyrrolidinopyridinium as a homogeneous catalyst showed a lower catalytic performance than the supported catalyst. Bifunctional catalysis involving acidic surface silanol and the basic 4-pyrrolidinopyridinium iodide was proposed.

Ions in carbon dioxide at an atmospheric pressure

A organocatalytic system based on economical and readily available cyanuric acid has been developed for the synthesis of 2-oxazolidinones and quinazoline-2,4(1H,3H)-diones from propargylamines and 2-aminobenzonitriles under atmospheric pressure carbon dioxide. Notably, a low concentration of carbon dioxide in air was directly converted into 2-oxazolidinone in excellent yields without an external base. Through mechanistic investigation by in situ FTIR spectroscopy, cyanuric acid was demonstrated to be an efficient catalyst for carbon dioxide fixation.

A series of nickel‐aluminium, cobalt‐aluminium and zinc‐aluminum layered double hydroxides were synthesized by urea hydrolysis (UH) and reverse micelle (RM) methods and then applied for the cycloaddition of carbon dioxide to propylene oxide (PO) under solvent‐free and ambient conditions. The most efficient catalyst was zinc‐aluminum layered double hydroxide prepared by RM (denoted as ZnAl‐RM) and it showed, in the presence of a co‐catalyst of tetrabutylammonium bromide, 96% propylene carbonate (PC) yield in 12 h at 25 °C and 1 MPa carbon dioxide. It is noteworthy for the same catalyst system that 74% PC yield was achieved in 24 h at room temperature (about 25 °C) and atmospheric carbon dioxide pressure. The high catalytic performance of the ZnAl‐RM and tetrabutylammonium bromide system should result from cooperative effects between zinc ion and bromine ion. Moreover, the ZnAl‐RM and tetrabutylammonium bromide system showed excellent versatility, which was also an active catalyst to the transformation of carbon dioxide and other epoxides into corresponding cyclic carbonates under mild temperature and pressure conditions.magnified image

A simple and straightforward method for the preparation of indole-3-carboxylic acids was discovered through the direct carboxylation of indoles with atmospheric pressure of carbon dioxide (CO(2)) under basic conditions. The key for the reaction was found to be the use of a large excess of LiO(t)Bu as a base to suppress the undesired decarboxylation side reaction.

Extended Pummerer fragmentation mediated by carbon dioxide and cyanide

The physical phenomenon of gas oversolubility in nanoconfined liquids was successfully applied for the catalytic cycloaddition of carbon dioxide to epoxides to generate organic cyclic carbonates. Hybrid adsorbents based on MCM‐41 and SBA‐15 mesoporous silica materials were synthesized, and efficient nucleophile deposition on the surface of the support was achieved through a grafting procedure, which allowed for an effective and durable metal‐free catalytic system. Room‐temperature transformation of styrene and hexene oxides to the corresponding organic carbonates at atmospheric pressure of carbon dioxide was explored.

Oceans play a crucial role in many natural processes, such as oxygen production and climate regulation. Ocean acidification (OA) is a phenomenon caused, among other factors, by the dissolution of certain gases in water, such as carbon dioxide (CO2). It is estimated that by 2100, the partial atmospheric pressure of carbon dioxide (pCO2) will double from pre-industrial levels, aggravating this process. The negative consequences of OA are not yet fully understood, in part due to the limitations of laboratory experiments. However, it is known that they include increased concentrations of CO2 in the atmosphere, structural and functional modifications of ecosystems, changes in richness, diversity and geographic distribution of species, impairment of ecosystem goods and services, and commercial activities, such as fishing and aquaculture, as well as global warming. OA, in principle, is considered irreversible, as it depends on the reduction of emissions of the gases involved in the process, especially CO2, and the slow natural process of neutralization. However, some mitigation measures, allied with adaptation measures, can mitigate the effects of OA. These and other issues related to ocean vulnerability caused by climate change will be raised in this review.