Biphasic Systems with Supercritical Carbon Dioxide

Biphasic Systems with Supercritical Carbon Dioxide 

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Biphasic Systems with Supercritical Carbon Dioxide... 

Ionic liquids (IL) can be used as solvents for nucleophilic substitution reactions of alkyl halides or tosylates with NaN3. ° The authors studied three ionic liquids (84 and 85), [bmim][PF6], [bmim][N(Tf)2]> [hpyr][N(Tf)2] (where bmim = l-butyl-3-methyl-imidazo-lium, hpyr = 1-hexylpyridinium, PFg = hexafluorophosphate, N(Tf)2 = bis(trifluoromethy lsulfonyl)imide). It was observed that nucleofugacity scales for this reaction are similar to those reported for the same process in cyclohexane. It was also observed that elimination reaction does not compete with substitution even in cases with sterically hindered substrates such as the triflate ester of diacetone-D-glucose 81. The nucleophilic displacement on n-octyl mesylate (86) with potassium azide in a biphase system of supercritical carbon dioxide (SCCO2) and water, in the presence of catalyst Bu4PBr is also an adequate medium for the synthesis of the corresponding azide 87 ° (Scheme 3.11). 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

As a unique medium for asymmetric hydroformylation, supercritical carbon dioxide has recently been examined, which can be carried out in an extremely low catalyst concentration. The reactions of styrene (16a) and pentafluorostyrene (16e) catalyzed by Rh-BINAPHOS appear to give mixed results that are highly dependent on the reaction conditions [77,78], Enantioselectivity up to 92-95% ee for 16a or 85 % ee for 16e has been observed [78]. A biphasic reaction system has also been examined for the reaction using Rh(acac)(CO)2 with a sulfonated diphosphine ligand BINAS [79], The reaction proceeds smoothly at 40°C and 100 atm in high conversion with excellent branched aldehyde selectivity (95%), but enantioselectivity is very low (18% ee). The use of these newer reaction conditions is still in the very early stage and further development is expected in the next decade. 

As evident from Fig. 8.4, an increase in the selectivity has been observed in IL/ scCOj biphasic systems media (>99.5%) with respect to scCO assayed alone (95%). These results could be explained by the use of water-immiscible ILs which have a specific ability to reduce water activity in the enzyme microenvironment. The synthetic activity of the immobilized lipase in IL/scCO biphasic systems is lower than that in scCO assayed alone. Similar results were found by Mori et al. [40] in IL/ hexane biphasic systems. These authors reported that the enzymatic membranes prepared by simple adsorption of CaLB onto the surface were more reactive than membranes prepared with ILs. As can be observed in Fig. 8.4, the initial reaction rate in the assayed IL/scCO biphasic systems increased in the following sequence [bdimim ][PF ]<[bmim ][PFg ]<[bmim ][NTfj ]<[omim ] [PF ], which was practically in agreement with flie activity sequence reported by these authors using free Candida antarctica lipase B in homogeneous ionic liquid systems ([bmim ] [PF ]<[bdmim+][PFg ]<[bmim+][NTfj ]<[omim ][PF ]), with the exception of [bmim [PF ] and [bdimim+][PFg ]. These results were explained taking into account that biotransformation occurs within the ionic liquid phase, so substrates have to be transported from scCOj to the ionic liquid phase. The mechanism of substrate transport between the ionic liquid and the supercritical carbon dioxide could be by three consecutive steps diffusion of the substrates through the diffusion... 

To sum up, the appropriate selection of the IL will greatly improve the enzymatic activity, selectivity and the mass-transfer phenomena, permitting better design of IL/scCOj biphasic systems, while the use of supercritical carbon dioxide as a second phase will facilitate recovery of the products without any cross-contamination of the extract with the ILs. 

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). 

In terms of green syntheses , the community expects ongoing intensive research work with aqueous biphasic operations. One of the reasons is that water is the only green solvent being used industrially. Aspiring users of the others, such as supercritical carbon dioxide or ionic liquids, suffer from lack of experience, and with all the other biphasic variants they have to cope with the problem that the demanding catalyst systems need to be tolerated by the second phase - mostly as far as the extremely complicated and costly ligands are concerned. 

In conjunction with the work being done on fluorous biphasic systems, the development of fluorinated ligands has also made an impact in the area of supercritical carbon dioxide (SCCO2) catalysis. Supercritical CO2 has been heavily studied as a reaction medium for organometallic chemistry due to its unique properties. Its... 

Using supercritical carbon dioxide (scCOi), the polymerisation of L-lactide by immobilised CALB was achieved at 65 "C in a biphasic media system, where the supercritical phase coexists with a liquid organic phase (melted monomer), and the growing poly(L-lactide) (PLLA) chains are soluble. A semicrystalline polymer with MW up to 12,900 g/mol was obtained, with a monomer conversion rate dependent on enzyme concentration and its initial water activity [90]. 

Acid zeolites have also been tested for the racemisation of alcohols under biphasic conditions.Their scope was found, however, to be limited to benzylic alcohols, since electron-rich benzylic alcohols were not suitable substrates because of the formation of dimers. Under optimised conditions, based on the use of H-Beta zeolite, CALB lipase, and an excess of vinyl octanoate at 60 °C, enantiopure (R)-l-phenylethyl octanoate (>99% ee) was obtained in 90% yield from 1-phenylethanol. In addition, Lozano et al. have recently performed the DKR of this alcohol in the presence of acidic zeolite catalysts (CBV400) in an ionic liquid-supercritical carbon dioxide system with a continuous reaction system. Therefore, when Novozym 435 was employed at 50 °C and 100 bars in the presence of vinylpropanoate as the acyl donor, the expected (R)-phenylethylpropionate was produced in excellent yield of 98% with enantioselectivity of 97% ee and without any activity loss during 14 days of operation. 

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