Carbon dioxide, supercritical coupling

H. Daimon and Y. Hirata, Direct coupling of capillary supercritical fluid chromatography with superaitical fluid extraction using modified carbon dioxide , J. High Resolut. Chromatogr. 17 809-813 (1994). 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

The method based on immunosorbents coupled on-line with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [109], discussed in section 9.4.2.1, has been applied to the determination of substituted urea type herbicides. Supercritical fluid extraction with methanol modified carbon dioxide has been applied to the determinants of sulfonyl urea herbicides in soil [261],... 

More recently, some studies have reported the use of supercritical fluid chromatography (SEC) [479,480], Coupling SEC with SEE, sample extraction, preconcentration, and quantification can be performed in a single step. The mobile phase, carbon dioxide, can be modified by adding different... 

SFC provides complementary quantitative data to the structural information afforded by mass spectrometry. Thermally label materials such as isocyanates can be easily analyzed with minimal sample preparation. Supercritical carbon dioxide is nontoxic and can be obtained in high purity as measured by FID. The easy coupling of SFE with SFC makes the selective isolation and quantification of targeted analytes possible. Furthermore, we are in an age of increased environmental awareness. Solvent disposal is discouraged and has become very expensive. The waste disposal costs associated with supercritical carbon dioxide are negligible when compared to the solvent disposal costs generated by traditional Soxhlet methods. 

Gordon and Holmes32 used a supported triphenylphosphine-Pd(II) complex as an effective catalyst for Heck and Suzuki couplings in supercritical carbon dioxide (entry 27). After optimization of the amine base for the reaction, the final products have been isolated in good yields and high purity with no traces of metal. 

McNally and Wheeler [364] used supercritical fluid extraction coupled to supercritical fluid chromatography to determine sulfonylurea herbicides in soil. Klatterback et al. [365,366] used supercritical fluid extraction with methanol-modified carbon dioxide followed by high-performance liquid chromatography with UV detection to determine sulfonylurea herbicides obtained on a Cis solid-phase extraction disc. Alternatively the determination was carried out by gas chromatography of the dimethyl derivatives of the sulfonylurea herbicides, employing an electron capture or a NP detector on the gas chromatograph. 

Another emerging technology is supercritical fluid chromatography (SFC) that uses supercritical carbon dioxide as the apolar eluent [46]. The main advantage of SFC, which can be applied both in the analytical and in the purification area, is the higher resolution than the traditional HPLC, allowing time reduction and more efficient separations. Also this technique can be advantageously coupled with a MS detector, to further improve the full analytical process. 

Solution polymerization offers improved heat transfer over bulk polymerizations. Proper selection of the solvent is critical to avoid chain transfer reactions. Coupled with environmental concerns over organic solvents, the complete removal of solvents from the polymer also poses a potential problem. Recent work has been performed on the use of supercritical carbon dioxide as a solvent, which is easy to remove and poses less environmental concerns.30... 

Supercritical carbon dioxide can be a solvent for rapid cross-coupling reaction [314]. Fluorinated palladium sources are reported to be surperior to non-fluorinated ones in such a reaction medium (Scheme 88) [315,316]. 

The turnover frequency (TOF = mole of product per mole of catalyst per hour) of this rapid reaction is rather high, with values up to 1400. This reaction, carried out at 50 °C in SC-CO2, is 18 times faster than in conventional tetrahydrofuran under otherwise identical reaction conditions. This formic acid synthesis can be coupled with subsequent reactions by addition of methanol or dimethylamine, this supercritical reaction system provides a highly efficient one-pot route to methyl formate and A,A-dimethylforma-mide, respectively [918]. Another example of a reaction in which carbon dioxide acts as both reactant and reaction medium is the formation of tetraethyl-2-pyranone from hex-3-yne and CO2 in the presence of an Ni(II)-diphosphane catalyst at 102 °C under supercritical reaction conditions [919]. 

Fig. 1 Chromatogram of PAHs, herbicides, and phenols obtained by supercritical carbon dioxide modified with methanol. [Reprinted from L. Toribio, M. J. del Nozal, J. L. Bernal, J. J. Jimenez, and M. L. Serna, Packed-column supercritical fluid chromatography coupled with solid-phase extraction for the determination of organic microcontaminants in water,...

Li, J., Jiang, H. Glaser coupling reaction in supercritical carbon dioxide. Chem. Commun. 1999, 2369-2370. 

Coupling reactions. The Suzuki coupling involving chloroarenes uses (dbajjPdj, /-BujP, and CsjCOj. For other coupling reactions (e.g., Stille coupling) good results are obtained in supercritical carbon dioxide. ... 

S. Sarrade, C. Perre, M. Carles, R. Ve)rre and G.M. Rios, Nanofiltration coupled with supercritical carbon dioxide. Interest and preliminary studies. ICOM 93, 30 August-3 Sept. 1993, Heildelberg, Germany. 

Chlorinated phenolic compounds in air-dried sediments collected downstream of chlorine-bleaching mills were treated with acetic anhydride in the presence of triethylamine. The acetylated derivatives were removed from the matrix by supercritical fluid extraction (SEE) using carbon dioxide. The best overall recovery for the phenolics was obtained at 110°C and 37 MPa pressure. Two SEE steps had to be carried out on the same sample for quantitative recovery of the phenolics in weathered sediments. The SEE unit was coupled downstream with a GC for end analysis . Off-line SEE followed by capillary GC was applied in the determination of phenol in polymeric matrices . The sonication method recommended by EPA for extraction of pollutants from soil is inferior to both MAP and SEE techniques in the case of phenol, o-cresol, m-cresol and p-cresol spiked on soil containing various proportions of activated charcoal. MAP afforded the highest recoveries (>80%), except for o-cresol in a soil containing more than 5% of activated carbon. The SEE method was inefficient for the four phenols tested however, in situ derivatization of the analytes significantly improved the performance . 

Palladium(II) acetate micro-encapsulated in polyurea is an economical and versatile heterogeneous catalyst for a range of phosphine-free cross-coupling reactions in both conventional solvents and supercritical carbon dioxide. The catalyst can be recovered by simple filtration and recycled up to four times [93]. The potential of these materials has been demonstrated by their efficacy in Suzuld-type couplings. Investigations have centered upon carbonylation reactions to prepare aryl esters from commercially available aryl iodides. Treatment of iodo-methyl benzene tvith 3 mol% of catalyst in butanol and triethylamine at 90 °C under an atmosphere of carbon monoxide afforded butyl-methyl benzoate in an excellent yield of 89% in 16 h. 

Suzuki-Miyaura coupling has also been conducted in a capillary reactor (400 i.m inner diameter).A commercial-scale continuous flow system consisting of a 14.5 cm x 25.4 mm column packed with Pd catalyst has also been developed. In this case, supercritical carbon dioxide is used as... 

The mechanics of increasing the pressure on the fluid can he done in a couple of ways. One would he to increase the temperature surrounding the hquid carbon dioxide [14]. As noted above, the pressme exerted on the hquid carbon dioxide in the cyhnder is a function of its ambient temperature. Only at higher ambient temperatures would the resultant system pressures be enough to reach the nearly hquid-hke densities of 0.1 to 1.0 g/mL that are required to use supercritical fluid carbon dioxide as an extraction solvent for extraction temperatures ranging from 40 C to 150°C. This approach is not widely used at this time. Liquid pumps (such as those used in liquid chromatographs) are more commonly used. 

Other biphasic C—C bonding reactions were carried out with fluorous solvents, for instance Suzuki- and Sonogashira-couplings [124] or ethene or propene oligomerizations [125, 126], Further new solvent systems use ionic liquids for the linear dimerisation of 1-butene to octenes [127] or the hydrovinylation of styrene with a combination ionic liquid/supercritical carbon dioxide [128] (cf. Section 7.4). 

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