J . Org. Chem. 2001, 66, 4047-4049
4047
bility problem. In fact, under solid-liquid phase-transfer
catalysis conditions (SL-PTC), nucleophilic displacements
have been reported in scCO2, providing that acetone is
present as a cosolvent for the PT-catalyst.12 Yet, reactions
proceed at moderate rates (50 °C: after 48 h, 15-60%
conversions). Similarly, the Glaser coupling of alkynes
has been claimed in scCO2 in the presence of MeOH as a
“polar modifier” which enhances the solubility of the
reactive salts (CuCl2 and AcONa).13
Nu cleop h ilic Disp la cem en ts in
Su p er cr itica l Ca r bon Dioxid e Usin g
Silica -Su p p or ted P h a se-Tr a n sfer Agen ts
J oseph DeSimone,† Maurizio Selva,*,‡,§ and
Pietro Tundo*,‡,
Department of Chemistry, University of North Carolina at
Chapel Hill, Chapel Hill, North Carolina 27599-3290, and
Dipartimento di Scienze Ambientali dell’Universita’ Ca’
Foscari, Calle Larga S. Marta, 2137-30123 Venezia, Italy
As a phase-transfer solvent, scCO2 has been described
in the extraction/methylation of chlorinated benzoates
-
(ArCO2
: Ar ) C6H3Cl2 and C6H2Cl3) from aqueous
selva@unive.it
solutions:14 in this case, no cosolvents are required
because of the high hydrophobic character of the nucleo-
philes.
Received September 8, 2000
Nucleophilic displacements have also been reported in
water/scCO2 microemulsions (w/c-me) prepared with
fluorinated surfactants (perfluoropolyether ammonium
carboxylates: PFPECO2NH4).15a,b More recently, electro-
generated radicals and cations have been stabilized in
the water core of similar w/c-me (PFPE-phosphate).16
In scCO2, efficient Diels-Alder reactions and enzy-
matic esterifications of polar substrates have been claimed
with the use of amorphous silica gel,17,18 whose promoting
role, although not well understood, is explained through
the adsorption of the reactants which come into closer
contact to each other, on the silica surface.
A related, but conceptually different approach can be
conceived with supported onium salts as reagents: as
reported by us,19,20different anions (I-, Br-, SCN-) paired
to bulky onium cations immobilized on alumina or silica
gel, undergo fast nucleophilic substitutions in the pres-
ence of nonpolar media (n-heptane or toluene).
In this paper, we report for the first time that im-
mobilized PT-agents provide also efficient anionic nu-
cleophiles for reactions in scCO2.
In tr od u ction
Carbon dioxide in both its liquid and supercritical
states is receiving more and more interest for organic
reactions as an attractive environmentally friendly sol-
vent.1 Accordingly, several different reactions such as
Diels-Alder cycloadditions,2 reductions and couplings of
organic halides,3 cracking and rearrangements,4 hydro-
genations, hydroformylation, and oxidation processes,1,5
and photochemical reactions6 are claimed to proceed in
supercritical CO2 (scCO2).
However, the poor solubilizing power of the scCO2 for
many polar compounds may represent a severe limita-
tion, especially for reactions involving ionic moieties.
In the case of catalytic processes carried out with
neutral or ionic complexes of transition metals, elegant
solutions have been achieved following three different
approaches: (i) the incorporation of “CO2-philic” frag-
ments (e.g., perfluorinated groups) into the catalysts,
either as ligands of the metal or in the counteranions,5a,7-9
(ii) the use of biphasic systems (scCO2-ethylene glycol,
scCO2-water),10 and (iii) the use of surfactant-stabilized
water/CO2 emulsions.11
Resu lts a n d Discu ssion
When ionic or highly polar reagents are used, the
introduction of polar cosolvents may overcome the solu-
The SN2 displacement on n-octyl methanesulfonate (1)
by bromide and iodide anions was chosen as a model
reaction. All experiments were performed in scCO2 as the
solvent, at 50-70 °C and 130 bar (see Experimental
Section). Silica-supported onium salts (2a -e) were used
either as reagents or as catalysts (Scheme 1). The results
are reported in Table 1.
† University of North Carolina at Chapel Hill. E-mail: desimone@
unc.edu. Fax: +1919 962 5467.
‡ Universita’ Ca’ Foscari.
§ Fax: +39 041 257 8620.
E-mail: tundop@unive.it.
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10.1021/jo001337m CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/04/2001