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Phase-transfer catalyst
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<h2 id="types">Types</h2> <p>Phase-transfer catalysts for anionic reactants are often <a href="/facts/Quaternary_ammonium_salt/bNpF1goc">quaternary ammonium salts</a>. Commercially important catalysts include benzyltriethylammonium chloride, <a href="/facts/Aliquat_336/MbDJE2mu">methyltricaprylammonium chloride</a> and methyltributylammonium chloride. Organic <a href="/facts/Phosphonium_salt/uS2PCt7X">phosphonium salts</a> are also used, e.g., hexadecyltributylphosphonium bromide. The phosphonium salts tolerate higher temperatures. </p><p>An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. <a href="/facts/Crown_ether/UNfhBQs5">Crown ethers</a> are used for this purpose on the laboratory scale. <a href="/facts/Polyethylene_glycol/tC7xBFwr">Polyethylene glycols</a> and their amine derivatives are common in practical applications. One such catalyst is <a href="/facts/Tris(2-(2-methoxyethoxy)ethyl)amine/JAIh3gye">tris(2-(2-methoxyethoxy)ethyl)amine</a>. These ligands encapsulate alkali metal cations (typically Na+ and K+), affording lipophilic cations. Polyethers have a <a href="/facts/Hydrophilic/k8p95WYu">hydrophilic</a> "interiors" containing the ion and a <a href="/facts/Hydrophobic/Ifya0CXQ">hydrophobic</a> exterior. </p><p><a href="/facts/Chirality_(chemistry)/p5a9pftB">Chiral</a> phase-transfer catalysts have also been demonstrated.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Asymmetric alkylations are catalyzed by chiral quaternary ammonium salts derived from <a href="/facts/Cinchona_alkaloid/yTWWW2ao">cinchona alkaloids</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>A variety of functionalized catalysts have been evaluated for PTC. One example is the Janus interphase catalyst, applicable to organic reactions on the interface of two phases via the formation of Pickering emulsion.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h3>Limitations</h3> <p>Quaternary ammonium cations degrade by <a href="/facts/Hofmann_degradation/ScW4RDIc">Hofmann degradation</a> to amines, especially at higher temperatures preferred by process chemists. The resulting amines can be difficult to remove from the product. Phosphonium salt are unstable toward base, degrading to <a href="/facts/Phosphine_oxide/Tq9TMtn4">phosphine oxide</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h3>Laboratory examples</h3> <p>For example, the <a href="/facts/Nucleophilic_substitution/MLLeSsnr">nucleophilic substitution</a> reaction of an <a href="/facts/Aqueous/eRNr3MH2">aqueous</a> <a href="/facts/Sodium_cyanide/CQfl206o">sodium cyanide</a> solution with an <a href="/facts/Ether/jwDg0IKr">ethereal</a> solution of 1-bromooctane does not readily occur. The 1-bromooctane is poorly soluble in the aqueous <a href="/facts/Cyanide/t0o8FMKI">cyanide</a> solution, and the sodium cyanide does not dissolve well in the ether. Upon the addition of small amounts of hexadecyltributylphosphonium bromide, a rapid reaction ensues to give nonyl nitrile: </p> C 8 H 17 Br ( org ) + NaCN ( aq ) → R 4 P + Br − C 8 H 17 CN ( org ) + NaBr ( aq ) {\displaystyle {\ce {C8H17Br_{(org)}{}+ NaCN_{(aq)}->[{\ce {R4P+Br-}}] C8H17CN_{(org)}{}+ NaBr_{(aq)}}}} <p>By the quaternary phosphonium cation, cyanide ions are "ferried" from the aqueous phase into the organic phase.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p><p>Subsequent work demonstrated that many such reactions can be performed rapidly at around room temperature using catalysts such as <a href="/facts/Tetra-n-butylammonium_bromide/tyzkJkXE">tetra-n-butylammonium bromide</a> and <a href="/facts/Methyltrioctylammonium_chloride/MbDJE2mu">methyltrioctylammonium chloride</a> in benzene/water systems.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p> <h2 id="design-of-phase-boundary-catalyst">Design of phase-boundary catalyst</h2> <p>Phase-boundary catalytic (PBC) systems can be contrasted with conventional catalytic systems. PBC is primarily applicable to reactions at the <a href="/facts/Interface_(chemistry)/HnIAndVA">interface</a> of an aqueous phase and organic phase. In these cases, an approach such as PBC is needed due to the <a href="/facts/Miscible/HwQaolbQ">immiscibility</a> of aqueous phases with most organic substrate. In PBC, the catalyst acts at the interface between the aqueous and organic phases. The reaction medium of phase boundary catalysis systems for the catalytic reaction of immiscible aqueous and organic phases consists of three phases; an organic liquid phase, containing most of the substrate, an aqueous liquid phase containing most of the substrate in <a href="/facts/Aqueous_phase/eRNr3MH2">aqueous phase</a> and the solid catalyst. </p><p>In case of conventional catalytic system; </p> <ul><li>When the reaction mixture is vigorously stirred, an apparently homogeneous emulsion is obtained, which segregates very rapidly into two liquid phases when the agitation ceases. Segregation occurs by formation of organic bubbles in the emulsion which move downwards to form the aqueous phase, indicating that emulsion consists of dispersed particles of the aqueous phase in the organic phase.</li> <li>Due to the triphasic reactions conditions, the overall reaction between aqueous phase and organic phase substrates on solid catalyst requires different transfer processes. The following steps are involved: <ol><li>transfer of aqueous phase from organic phase to the external surface of solid catalyst;</li> <li>transfer of aqueous phase inside the pore volume of solid catalyst;</li> <li>transfer of the substrate from aqueous phase to the interphase between aqueous and organic phases</li> <li>transfer of the substrate from the interphase to the aqueous phase;</li> <li>mixing and diffusion of the substrate in the aqueous phase;</li> <li>transfer of the substrate from the aqueous phase to the external surface of solid catalyst;</li> <li>transfer of the substrate inside the pore volume of the solid catalyst;</li> <li>catalytic reaction (<a href="/facts/Adsorption/ARH9peYM">adsorption</a>, <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reaction</a> and <a href="/facts/Desorption/rWdeJjxn">desorption</a>).</li></ol></li></ul> <p>In some systems, without vigorous stirring, no reactivity of the catalyst is observed in conventional catalytic system.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Stirring and mass transfer from the organic to the aqueous phase and vice versa are required for conventional catalytic system. Conversely, in PBC, stirring is not required because the mass transfer is not the rate determining step in this catalytic system. It is already demonstrated that this system works for alkene epoxidation without stirring or the addition of a co-solvent to drive liquid–liquid phase transfer.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> The active site located on the external surface of the zeolite particle were dominantly effective for the observed phase boundary catalytic system.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> <a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p> <h2 id="process-of-synthesis">Process of synthesis</h2> <p>Modified zeolite on which the external surface was partly covered with <a href="/facts/Organosilicon_chemistry/JvUwzDQ4">alkylsilane</a>, called <i>phase-boundary catalyst</i> was prepared in two steps.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a><a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a><a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> First, <a href="/facts/Titanium_dioxide/T0KWmKcz">titanium dioxide</a> made from <a href="/facts/Titanium_isopropoxide/QjnVqFse">titanium isopropoxide</a> was impregnated into NaY zeolite powder to give sample W-Ti-NaY. In the second step, alkysilane from n-octadecyltrichlorosilane (OTS) was impregnated into the W-Ti-NaY powder containing water. Due to the <a href="/facts/Hydrophilicity/k8p95WYu">hydrophilicity</a> of the w-Ti-NaY surface, addition of a small amount of water led to aggregation owing to the <a href="/facts/Capillary_force/aa60yQ32">capillary force</a> of water between particles. Under these conditions, it is expected that only the outer surface of aggregates, in contact with the organic phase can be modified with OTS, and indeed almost all of the particles were located at the phase boundary when added to an immiscible water–organic solvent (W/O) mixture. The partly modified sample is denoted w/o-Ti-NaY. Fully modified Ti-NaY (o-Ti-NaY), prepared without the addition of water in the above second step, is readily suspended in an organic solvent as expected. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Ionic_transfer/UJqtv1Nz">Ionic transfer</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Marc Halpern "Phase-Transfer Catalysis" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_293 <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Katole DO, Yadav GD. Process intensification and waste minimization using liquid-liquid-liquid triphase transfer catalysis for the synthesis of 2-((benzyloxy)methyl)furan. Molecular Catalysis 2019;466:112–21. https://doi.org/10.1016/j.mcat.2019.01.004 <a href="https://doi.org/10.1016/j.mcat.2019.01.004" target="_blank">https://doi.org/10.1016/j.mcat.2019.01.004</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>J. O. Metzger (1998). "Solvent-Free Organic Syntheses". Angewandte Chemie International Edition. 37 (21): 2975–2978. doi:10.1002/(SICI)1521-3773(19981116)37:21<2975::AID-ANIE2975>3.0.CO;2-A. PMID 29711128. <a href="/wiki/Angewandte_Chemie_International_Edition" target="_blank">/wiki/Angewandte_Chemie_International_Edition</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Mieczyslaw Makosza (2000). "Phase-transfer catalysis. A general green methodology in organic synthesis". Pure Appl. Chem. 72 (7): 1399–1403. doi:10.1351/pac200072071399. <a href="https://doi.org/10.1351%2Fpac200072071399" target="_blank">https://doi.org/10.1351%2Fpac200072071399</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide, Chemical Communications, 2000, 2235 – 2236. Abstract <a href="/wiki/Chemical_Communications" target="_blank">/wiki/Chemical_Communications</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide, Journal of Catalysis, 2001, (204) 402 – 408. Abstract <a href="http://www.elsevier.com/locate/issn/00219517" target="_blank">http://www.elsevier.com/locate/issn/00219517</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Direct observation of bimodal amphiphilic surface structures of zeolite particles for a novel liquid-liquid phase boundary catalysis, Langmuir, 2001, (17) 7976 – 7979. doi:10.1021/la011088c <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysts for acid-catalyzed reactions: the role of bimodal amphiphilic structure and location of active sites, Journal of Brazilian Chemical Society, 2004, (15) 719–724 – 2236. Paper <a href="http://jbcs.sbq.org.br/" target="_blank">http://jbcs.sbq.org.br/</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>H. Nur, S. Ikeda, and B. Ohtani, Amphiphilic NaY zeolite particles loaded with niobic acid: Materials with applications for catalysis in immiscible liquid-liquid system, Reaction Kinetics and Catalysis Letters, 2004, (17) 255 – 261. Abstract <a href="https://archive.today/20070502034321/http://springerlink.com/content/1588-2837/" target="_blank">https://archive.today/20070502034321/http://springerlink.com/content/1588-2837/</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Phipps, Robert J.; Hamilton, Gregory L.; Toste, F. Dean (2012). "The progression of chiral anions from concepts to applications in asymmetric catalysis". Nature Chemistry. 4 (8): 603–614. Bibcode:2012NatCh...4..603P. doi:10.1038/nchem.1405. PMID 22824891. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Takuya Hashimoto and Keiji Maruoka "Recent Development and Application of Chiral Phase-Transfer Catalysts" Chem. Rev. 2007, 107, 5656-5682. doi:10.1021/cr068368n <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>M. Vafaeezadeh, W. R. Thiel (2020). "Janus interphase catalysts for interfacial organic reactions". J. Mol. Liq. 315: 113735. doi:10.1016/j.molliq.2020.113735. S2CID 225004256. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Marc Halpern "Phase-Transfer Catalysis" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_293 <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Starks, C.M. (1971). "Phase-transfer catalysis. I. Heterogeneous reactions involving anion transfer by quaternary ammonium and phosphonium salts". J. Am. Chem. Soc. 93 (1): 195–199. Bibcode:1971JAChS..93..195S. doi:10.1021/ja00730a033. <a href="/wiki/J._Am._Chem._Soc." target="_blank">/wiki/J._Am._Chem._Soc.</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Herriott, A.W.; Picker, D. (1975). "phase-transfer catalysis. Evaluation of catalysis". J. Am. Chem. Soc. 97 (9): 2345–2349. Bibcode:1975JAChS..97.2345H. doi:10.1021/ja00842a006. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide, Chemical Communications, 2000, 2235 – 2236. Abstract <a href="/wiki/Chemical_Communications" target="_blank">/wiki/Chemical_Communications</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide, Journal of Catalysis, 2001, (204) 402 – 408. Abstract <a href="http://www.elsevier.com/locate/issn/00219517" target="_blank">http://www.elsevier.com/locate/issn/00219517</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Direct observation of bimodal amphiphilic surface structures of zeolite particles for a novel liquid-liquid phase boundary catalysis, Langmuir, 2001, (17) 7976 – 7979. doi:10.1021/la011088c <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysts for acid-catalyzed reactions: the role of bimodal amphiphilic structure and location of active sites, Journal of Brazilian Chemical Society, 2004, (15) 719–724 – 2236. Paper <a href="http://jbcs.sbq.org.br/" target="_blank">http://jbcs.sbq.org.br/</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>H. Nur, S. Ikeda, and B. Ohtani, Amphiphilic NaY zeolite particles loaded with niobic acid: Materials with applications for catalysis in immiscible liquid-liquid system, Reaction Kinetics and Catalysis Letters, 2004, (17) 255 – 261. Abstract <a href="https://archive.today/20070502034321/http://springerlink.com/content/1588-2837/" target="_blank">https://archive.today/20070502034321/http://springerlink.com/content/1588-2837/</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide, Chemical Communications, 2000, 2235 – 2236. Abstract <a href="/wiki/Chemical_Communications" target="_blank">/wiki/Chemical_Communications</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide, Journal of Catalysis, 2001, (204) 402 – 408. Abstract <a href="http://www.elsevier.com/locate/issn/00219517" target="_blank">http://www.elsevier.com/locate/issn/00219517</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Direct observation of bimodal amphiphilic surface structures of zeolite particles for a novel liquid-liquid phase boundary catalysis, Langmuir, 2001, (17) 7976 – 7979. doi:10.1021/la011088c <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysts for acid-catalyzed reactions: the role of bimodal amphiphilic structure and location of active sites, Journal of Brazilian Chemical Society, 2004, (15) 719–724 – 2236. Paper <a href="http://jbcs.sbq.org.br/" target="_blank">http://jbcs.sbq.org.br/</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>S. Ikeda, H. Nur, P. Wu, T. Tatsumi and B. Ohtani, Effect of titanium active site location on activity of phase boundary catalyst particle for alkene epoxidation with aqueous hydrogen peroxide, Studies in Surface Science and Catalysis Archived 2006-12-01 at the Wayback Machine, 2003, (145) 251–254. <a href="http://www.elsevier.com/wps/find/bookseriesdescription.cws_home/BS_SSC/description" target="_blank">http://www.elsevier.com/wps/find/bookseriesdescription.cws_home/BS_SSC/description</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li> <li id="fn:26"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide, Chemical Communications, 2000, 2235 – 2236. Abstract <a href="/wiki/Chemical_Communications" target="_blank">/wiki/Chemical_Communications</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> <li id="fn:27"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide, Journal of Catalysis, 2001, (204) 402 – 408. Abstract <a href="http://www.elsevier.com/locate/issn/00219517" target="_blank">http://www.elsevier.com/locate/issn/00219517</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Direct observation of bimodal amphiphilic surface structures of zeolite particles for a novel liquid-liquid phase boundary catalysis, Langmuir, 2001, (17) 7976 – 7979. doi:10.1021/la011088c <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li> <li id="fn:29"><p>H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysts for acid-catalyzed reactions: the role of bimodal amphiphilic structure and location of active sites, Journal of Brazilian Chemical Society, 2004, (15) 719–724 – 2236. Paper <a href="http://jbcs.sbq.org.br/" target="_blank">http://jbcs.sbq.org.br/</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></p></li> <li id="fn:30"><p>H. Nur, S. Ikeda, and B. Ohtani, Amphiphilic NaY zeolite particles loaded with niobic acid: Materials with applications for catalysis in immiscible liquid-liquid system, Reaction Kinetics and Catalysis Letters, 2004, (17) 255 – 261. Abstract <a href="https://archive.today/20070502034321/http://springerlink.com/content/1588-2837/" target="_blank">https://archive.today/20070502034321/http://springerlink.com/content/1588-2837/</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li> </ol>