The bis(triphenylphosphine)immium (PPN) cation is
well-known for imparting a high degree of stability to
air-sensitive anions and therefore has been extensively
used as counterion for anionic mono- and polynuclear
metal carbonyls.4 It is readily available as halide salts
and salts containing other simple inorganic anions such
as cyanate, thiocyanate, azide, and nitrate. X-ray pho-
toelectron spectroscopic (ESCA) study on PPN salts
implied that the phosphorus atoms in PPN+ carry a
degree of positive charge, whereas the nitrogen atom is
largely negative in character, suggesting that the ionic
structure I is the more appropriate description of PPN+.5
We study the catalytic activity of some PPN salts in the
CO2/epoxide coupling reactions for the formation of cyclic
carbonates because we are of the opinion that proximity
of acidic and basic sites on PPN+ renders the PPN salts
potential catalysts for the reactions.
Coupling Reactions of CO2 with Neat
Epoxides Catalyzed by PPN Salts To Yield
Cyclic Carbonates
Wing Nga Sit, Siu Man Ng, Kar Yan Kwong, and
Chak Po Lau*
Department of Applied Biology & Chemical Technology,
The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong
Received May 30, 2005
The readily available PPN+Cl- was found to be active
for the catalytic coupling reactions of CO2 (5 bar) with
epoxides at 100 °C to form cyclic carbonates, the results
of which are shown in Table 1. However, the tetrafluo-
roborate and the triflate salts, PPN+BF4- and PPN+OTf-,
respectively, show no catalytic activity. It is therefore
probably true that the catalytic activity of PPN+Cl- does
not originate from the cooperative actions of the basic
and acidic sites on the PPN ion shown in I, but the anion
might also play an important role in the catalysis. The
nucleophilicity of the chloride anion in PPN+Cl- is much
higher than those of the tetrafluoroborate and triflate
The off-the-shelf reagent PPN+Cl- and PPN-manganese
carbonylates [PPN]+[Mn(CO)4L]- (L ) CO, PPh3) are good
catalysts for the coupling reactions of CO2 with neat epoxides
without the use of organic solvents to afford cyclic carbon-
ates. PPN salts with weak nucleophilic anions such as
-
PPN+BF4 and PPN+OTf- are, however, inactive for the
coupling reactions.
The utilization of carbon dioxide as a feedstock for the
production of chemical products has attracted much
attention owing to its economic and environmental
benefits.1 One promising methodology in chemical CO2
fixation is the reaction of carbon dioxide with epoxides
to synthesize cyclic carbonates, which are valuable as
aprotic polar solvents, fine chemical intermediates, and
sources for polymer and engineering plastic syntheses.2
Various catalysts have been explored for the CO2/epoxide
coupling reactions.3 It has been suggested that parallel
Lewis base activation of CO2 and Lewis acid activation
of epoxide are important for the success of the coupling
reactions.3a-d For example, it has been proposed that in
the (salen)Cr(III) complex/(4-dimethylamino)pyridine
(DMAP)-catalyzed CO2/epoxide coupling reactions the
starting (salen)Cr(III) complex acts as a Lewis acid to
activate the epoxide, and the (salen)Cr(III)‚DMAP com-
plex, in which the Cr(III) center is rendered more
electron-rich by coordination of the DMAP molecule,
activates the CO2 by forming a metallocarboxylate
intermediate.3a It is also likely that the catalytic activity
of the Mg-Al mixed oxides in the coupling of carbon
dioxide with epoxides originates from the cooperative
actions of the neighboring basic and acidic sites on the
surface.3b
anions in PPN+BF4 and PPN+OTf-, respectively. We
-
have also studied the catalytic activity of the PPN salts
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10.1021/jo051077e CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/14/2005
J. Org. Chem. 2005, 70, 8583-8586
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