Liquid and supercritical carbon dioxide as organic solvents (original) (raw)
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The Journal of Physical Chemistry, 1979
analogous to those shown in Figure 3 for the decomposition of the hydroxylamines formed in the reactions involving primary and secondary amines. This heuristic model offers explanations for both the unusually great importance of the R-loss route in the 0 + TMA reaction and the fact that the H20 loss route was not observed in the same reaction. The latter route would require the loss of two primary hydrogens in sequential steps, each of which involves a competition with a second pathway which is probably energetically favored. These six studies have begun to reveal details of the mechanism of 0 + amine reactions under essentially collision-free conditions following the formation of an energy-rich adduct. Recognizing that an excited amine N-oxide is the first intermediate in this reaction, we have shown in this study that amine N-oxides with 60-70 kcal/mol of internal energy decompose not only along the path of lowest free-energy increase, but to a very great extent by other routes which have not been observed before. Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation. References and Notes
Synthesis of Amides and Lactams in Supercritical Carbon Dioxide
The Journal of Organic Chemistry, 2009
Supercritical carbon dioxide can be employed as an environmentally friendly alternative to conventional organic solvents for the synthesis of a variety of carboxylic amides. The addition of amines to ketenes generated in situ via the retro-ene reaction of alkynyl ethers provides amides in good yield, in many cases with ethylene or isobutylene as the only byproducts of the reaction. Reactions with ethoxy alkynes are performed at 120-130 °C, while tert-butoxy derivatives undergo the retro-ene reaction at 90 °C. With the exception of primary, unbranched amines, potential side reactions involving addition of the amines to carbon dioxide are not competitive with the desired C-N bondforming reaction. The amide synthesis is applicable to the preparation of β-hydroxy and β-amino amide derivatives, as well as amides bearing isolated carbon-carbon double bonds. Preliminary experiments aimed at developing an intramolecular variant of this process to afford macrolactams suggest that the application of CO 2 /co-solvent mixtures may offer advantages for the synthesis of large-ring compounds.
Solvent and substituent effects on conjugated eliminations in propargylic systems
Tetrahedron Letters, 2000
The deprotonation of 4-methoxy-but-2-ynal diethyl acetal by n-butyllithium induces an acetylenic±allenic isomerization (in diethylether) or a conjugated elimination reaction (in THF), providing the corresponding 1,4-dialkoxycumulene. An allenyllithium, that has been trapped as an allenylstannane, is proposed to be a common intermediate to both pathways. Also, the deprotonation of 4-dialkylamino-but-2-ynal diethyl acetals in the same conditions aords a mixture of (E) and (Z) aminocrotonates of which formation can be explained by a chemioselective removal of the acetalic proton leading to an intermediate allenyllithium that has equally been trapped by stannylation. #
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.
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.