Liquid and supercritical carbon dioxide as organic solvents (original) (raw)
The Journal of Organic Chemistry, 1993
Elimination reactions are one of the most studied transformations in all of chemistry. Numerous aspects of the mechanism have been probed, and a tremendous wealth of information has been 0btained.l The selectivity in these processes i s often influenced by solvation, aggregation, and counterion effects. The intrinsic reactivity, therefore, is of special interest. Ab initio molecular orbital calculations and gas-phase ion molecule investigations are noteworthy in this regard. In this paper the first stereochemical information on 1,4-eliminations in the gas phase is presented. Strong bases (amide and hydroxide) are found to be relatively nonselective whereas weaker bases (tert-butoxide and fluoride) display a strong preference for the syn pathway. Elimination reactions, somewhat surprisingly, have only recently been examined with high-level computations,2 but they have been the subject of numerous gas-phase studies.3 Many questions remain unanswered, however, in part because substitutions and eliminations both afford the same ionic products (which are what is detected). One method for overcoming this difficult is to design substrates so that the ions "tell" how they are formed. For example, 1-methoxy-2-cyclohexene (1) reacts with a number of bases (B-) to afford cyclohexadienide (21, methoxide clusters (CHBO-~BH, 3), and free methoxide (4, eq 1).4 The former two species must result from an elimination reaction and cannot be due to substitution. We have previously examined the regiochemistry in this system, 1,2-vs 1,4elimination, by labeling 1 with deuterium at either C4 or C6. Strong bases were found to induce l,4-eliminations, ~~ (1) (a) Gandler,
Journal of Physical Organic Chemistry, 2003
The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323–435°C and pressure range 28.5–242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z=substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH)=−(0.57±0.19)σα+(1.34±0.49)σR− (r=0.9256; SD=0.16) at 400°C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β-hydrogen of the Cβ—H bond by the oxygen carbonyl. Because ρα is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH) versus ESC for CH3OCOOCH2CH2R at 400°C, R=alkyl, δ=−0.17 (r=0.9993, SD=0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the β-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright © 2003 John Wiley & Sons, Ltd.
Carbon Dioxide, a Solvent and Synthon for Green Chemistry
Springer eBooks, 2005
Carbon dioxide is a renewable resource of carbon when we consider the reuse of existing CO 2 as a carbon source for producing chemicals. The development of new applications is of major interest from the point of view of carbon dioxide sequestration and within the scope of green chemistry. For example, using CO 2 instead of CO or COCl 2 for chemical synthesis constitutes an attractive alternative avoiding hazardous and toxic reactants. However, it has the lowest chemical reactivity, which is a serious drawback for its transformation. Supercritical CO 2 as a reaction medium offers the opportunity to replace conventional organic solvents. Its benign nature, easy handling and availability, non volatile emitting, and the relatively low critical point (P c = 73.8 bar, T c = 31 °C) are particularly interesting for catalytic applications in chemical synthesis, over a wide range of temperatures and pressures. The benefits of coupling catalysis and supercritical fluids are both environmental and commercial: less waste and VOCs emission, improved separation and recycling, and enhanced productivity and selectivity. The case study described in this paper concerns the reaction of carbon dioxide with alcohols to afford dialkyl carbonates with special emphasis on dimethyl carbonate. It is of significant interest because the industrial production of this class of compounds, including polycarbonates, carbamates, and polyurethanes, involves phosgene with strong concerns on environmental impact, transport, safety and waste elimination. The future of carbon dioxide in green chemistry, including supercritical applications, is highly linked to the development of basic knowledge, know-how, and tools for the design of catalyst precursors and reactors.
ACS Sustainable Chemistry & Engineering
The selectivity of a catalytic alkane functionalization process can be modified just changing the reaction medium from neat alkane to supercritical carbon dioxide (scCO2). A silica supported copper complex bearing an Nheterocyclic carbene ligand promotes the functionalization of carbon-hydrogen bonds of alkanes by transferring the CHCO2Et group from N2=CHCO2Et (ethyl diazoacetate, EDA). In neat hexane only 3% of the primary C-H bonds (ethyl heptanoate being the product) are functionalized in that manner, whereas the same reaction carried out in scCO2 provides a 30% yield in this linear ester. Such effect seems to be induced by an electronic density flux from the NHC ligand to the surrounding carbon dioxide molecules.
The Facile Reduction of Carbon Dioxide to Carbon Monoxide with an Amido-Digermyne
Angewandte Chemie International Edition, 2012
The steady increase in the atmospheric concentration of the greenhouse gas, CO 2 , since the industrial revolution is thought to be the main cause of recent increases in global temperatures. The future implications of this phenomenon are driving significant current efforts to develop efficient methods for the sequestration of CO 2 , and for its use as a C 1 feedstock in the formation of useful chemicals. With regard to the latter, the reduction of CO 2 to CO is of particular interest as carbon monoxide can be used as a fuel or as a chemical feedstock in its own right. However, CO 2 reduction is problematic because of the considerable strength of its O=C(O) bond (532 kJ mol À1 ), and for kinetic reasons. With that said, CO 2 reduction has been achieved, either stoichiometrically or catalytically, in nature (e.g. with CO dehydrogenase/acetyl-coenzyme A synthase) and in the laboratory by use of photolytic, electrochemical, or metalbased oxygen abstraction protocols. While the vast majority of methodologies for the reduction of CO 2 to CO require d-or f-block metal-containing materials to proceed, a handful of low oxidation state p-block compounds have recently been shown to effect CO 2 reductions at room temperature. These include three-coordinate, intramolecularly donor-stabilized (D!) silylenes (R 2 (D! )Si:), [9, 10] a disilyne (R(D!)SiSi( ! D)R), and N-heterocyclic carbenes. Moreover, a small number of normal oxidation state p-block systems, for example, Al/P-based "frustrated Lewis pairs", have been reported to reduce CO 2 to CO. The reduction reactions involving these compounds exemplify the rapidly emerging interest in the "transitionmetal-like" reactivity of main group compounds. We have become involved in this area with, for example, the preparation of the bulky amido-substituted two-coordinate diger-myne,
Journal of Physical Organic Chemistry, 2010
The gas-phase elimination of 1,1-dimethoxycyclohexane yielded 1-methoxy-1-cyclohexene and methanol. The kinetics were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor cyclohexene. The working temperature was 310-360 -C and the pressure was 25-85 Torr. The reaction was found to be homogeneous, unimolecular, and follows a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equation: log k(s S1 ) ¼ [(13.82 W 0.07) -(193.9 W 1.0)(kJ mol S1 )](2.303RT) S1 ; r ¼ 0.9995. Theoretical calculations were carried out using density functional theory (DFT) functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31GRR(d,p). The calculated values for the energy of activation and enthalpy of activation are in reasonably good agreement with the experimental values using the PBE/6-31G (d,p) level of theory. Both experimental results and theoretical calculations suggest a molecular mechanism involving a concerted polar four-membered cyclic transition state. The transition state structure of methanol elimination from 1,1-dimethoxycyclohexane is characterized by a significantly elongated C-O bond, while the C b -H bond is stretched to a smaller extent, as compared to the reactant. The process can be described as moderately asynchronic with some charge separation in the TS.
The Journal of Chemical Thermodynamics, 2005
The equilibrium constants K for the ketoreductase-catalyzed reduction reactions of 1-benzyl-3-pyrrolidinone, ethyl 2-oxo-4-phenylbutyrate, ethyl 4-chloroacetoacetate, 1-benzyl-4-piperidone, and 1-benzyl-3-piperidone were measured in n-hexane at T = 298.15 K by using gas chromatography. The equilibrium constants for the reaction involving 1-benzyl-4-piperidone were also measured as a function of temperature (288.15 to 308.05) K. The calculated thermodynamic quantities for the reaction (1-benzyl-4-piperidone + 2-propanol = 1-benzyl-4-hydroxypiperidine + acetone) reaction carried out in n-hexane at T = 298.15 K are: K = (26.2 ± 1.7); D r G m ¼ Àð8:10 AE 0:16Þ kJ Á mol À1 ; D r H m ¼ Àð3:44 AE 0:42Þ kJ Á mol À1 ; and D r S m ¼ ð15:6 AE 1:4Þ J Á K À1 Á mol À1 . The chirality of the hydroxyl products of the reactions (1)-(3) and (5)has also been investigated. The results showed that the stereoselectivity of the hydroxyl products formed can be controlled by the selection of the solvent and enzyme used in these reactions. The thermochemical results for these reactions are compared with the results for reactions that have analogous structural features as well as with the results of quantum chemical calculations. Published by Elsevier Ltd.
Journal of the …, 1995
The reactions of the allylic ethers CHs-CH%H-CH2-OEt (l), CH3-CH%H-CH2-OMe (2), and CH2%H-CH2-OEt (3) with a variety of anionic first-row (carbon, nitrogen, oxygen, and fluorine) bases have been investigated with use of FT-ICR mass spectrometry and density-functional theory (D E). Base-induced 1,4-elimination is an extremely facile process which competes effectively with simple proton transfer 1 ,Zelimination, and vinylic 1,2elimination as well as aliphatic (SN2) and allylic (SN2') substitution. Overall bimolecular rate constants for baseinduced reactions of 1 range from 6 x (OH-+ 1) cm3 molecule-' s-l. Oxygen bases are the most reactive amongst the employed bases. The ionic products of base-induced 1,4-elimination are either the bare leaving group, RO-, or the leaving group solvated by the conjugate acid of the base, [BH, RO-1. The former reaction channel prevails for strong bases (e.g., NHz-). The latter pathway becomes dominant for weaker bases (e.g., F-), because the complexation energy compensates for the reduced exothermicity. This makes the reaction an efficient tool for the preparation of solvated anions under low-pressure conditions. The stereochemistry (i.e., E or Z) around the P,y-double bond of the substrate has no detectable influence on the course of base-induced 1,4eliminations. Deuterium labeling experiments with 2 reveal &proton transfer only. The absence of product ions from a-proton transfer is ascribed to a facile electron detachment from the a-allyl anions. The base-induced 1,4eliminations studied proceed via an Elcb mechanism, as indicated by experiment and shown by theory. This mechanism exists in various modifications amongst which are single-, double-, and triple-well Elcb elimination. To our knowledge, the single-well Elcb mechanism is conceptually unprecedented.