Carbon dioxide gas accelerates solventless synthesis

Article abstract of DOI:10.1039/a909703a

Some solventless reactions involving solid reactants or products can be accelerated by the influence of subcritical gaseous CO₂.

Full text of DOI:10.1039/a909703a

Carbon dioxide gas accelerates solventless synthesis†
Philip Jessop,* Dolores C. Wynne, Scott DeHaai and Denise Nakawatase
Department of Chemistry, University of California, Davis, CA, 95616 USA. E-mail: jessop@chem.ucdavis.edu
Received (in Covallis, OR, USA) 8th December 1999, Accepted 8th March 2000
Published on the Web 30th March 2000
Some solventless reactions involving solid reactants or
products can be accelerated by the influence of subcritical
gaseous CO2.
generating a small quantity of the product, 2-ethylnaphthalene
(mp 27.4 °C),7 which acts as a liquid solvent for further
reaction. Thus one would expect the reaction to proceed very
slowly until enough liquid product has formed, at which point
the system would accelerate. However, at 33 °C no conversion
is observed in 30 min (entry 1).
Although the utility of supercritical carbon dioxide for solvent-
less synthesis has been well publicized,1 it is less well known
that subcritical gaseous CO2 can promote such reactions. At
temperatures above 31 °C and pressures below 74 bar, carbon
dioxide exists as a gas and not a supercritical fluid. It therefore
does not dissolve organic solids or liquids to a significant
extent. However, it is capable of dissolving into organics, and in
so doing can affect the properties of the organic material such as
its viscosity, diffusion coefficient,2 dielectric constant, melting
point,3 glass transition temperature,4 or ability to dissolve
hydrogen.5 In each of these ways, the presence of gaseous CO2
could affect a reaction taking place in the organic phase. These
possibilities have not been much explored. We now present
examples of the effect of gaseous CO2 on reactions involving
organic solids.
Three reactions are offered as examples of the accelerating
effect of CO2. In each case, the reaction performance under
gaseous CO2 is compared to the performance in the absence of
CO2. In the first example, the starting material is a solid at the
reaction temperature, while the product is a liquid. In the second
example, the product is the solid. In the last example, both the
starting material and the product are solids.
In the presence of subcritical gaseous CO2, the reaction
proceeds more rapidly.† At 36 °C, the enhancement of the rate
over that observed in the absence of CO2 is at least an order of
magnitude (entries 4 and 5). Complete conversion is obtained
within 90 min (entry 6). At 33 °C, the yield after 30 min is
obviously superior to that in the absence of CO2 (compare
entries 1 and 2). Hydrogen gas seems to be less effective than
CO2 in enhancing the rate (entry 3). Note that in all of the
experiments, the pressure of CO2 (56 bar) is below the critical
pressure of CO2 (73.9 bar),8 while the total pressure of the H2/
CO2 mixture (Ptot = 66 bar) is below the mixture critical
pressure.‡ Thus one can be certain that the gaseous phase is not
supercritical. It is unlikely that the reaction takes place in the
gaseous phase because: (a) at the pressure used, the density of
the gaseous CO2 is so low (ca. 0.14 g mL21 at 36 °C and 56 bar
of CO2)8 that it can dissolve neither the substrate nor catalyst to
any significant extent, (b) the catalyst used here has not been
modified to render it soluble in CO2, and (c) all of the product
mixture was found inside the glass liner at the end of the
reaction. The most likely explanation of the rate enhancement is
the effect of gaseous CO2 on the melting point of 2-vinyl-
naphthalene.
This explanation involves the phenomenon of gas-induced
melting (or gas-induced melting point lowering) of organic
solids. Melting of a solid compound A can not normally be
achieved below the temperature of its triple point. However, in
the presence of a gas B, the melting temperature of A becomes
a function of the pressure of B. The melting temperature of A
can be significantly lowered from its triple point, especially at
pressures approaching the critical pressure of gas B. The degree
of melting point lowering depends on the solubility of B in
molten A, and can be limited by the occurrence of a critical end
point in the binary phase diagram.§ The end result of this phase
behaviour is that solid organic compounds can be made to melt
even at temperatures well below their normal melting points.
This phenomenon is the basis of the recently-developed PGSS
(Particles from Gas Saturated Solution) process for the
micronization of solids.9–11 Note, however, that this effect alone
can not bring the melting point of 2-vinylnaphthalene to as low
as 36 or 33 °C; a combination of the induced-melting effect and
some initial surface reaction is required to explain the rapid rate
at these temperatures.
The first example is the hydrogenation of 2-vinylnaphthalene
(lit.6 mp 65–66 °C, mp of commercially available sample
62–65 °C) catalysed by RhCl(PPh3)3 (Scheme 1). In the absence
of CO2, this reaction can be performed at temperatures of 36 °C
or higher (entry 4 in Table 1). The fact that it can proceed at
temperatures somewhat below the melting point of 2-vinyl-
naphthalene is probably a result of a slow surface reaction
Scheme 1
Table 1 Hydrogenation of 2-vinylnaphthalene in the absence of added
solventa
Conv.
(%)
Entry
T/°C
PCO b/bar t/h
TOFc/h21
2
1
2
3
4
5
6
33
33
33
36
36
36
0
56
0e
0
56
56
0.5
0.5
0.5
0.5
0.5
1.5
0
0
310
96
33
430
200
52d
16
5d
73d
100
The hydrogenation of oleic acid to stearic acid, catalysed by
5% Pt/C (Scheme 2), is also affected by the induced-melting
effect. Oleic acid is a model compound for the unsaturated fatty
acids in vegetable oils.12 While oleic acid is a liquid (mp
13–16 °C),13 the hydrogenation product stearic acid (mp
69–70 °C)14 is a solid at room temperature. Therefore, one
would expect that the solventless hydrogenation would proceed
a 7 mg (7.6 mmol) RhCl(PPh3)3, 350 mg (2.27 mmol) 2-vinylnaphthalene,
10 bar H2, vessel size 160 mL. b Calculated as Ptotal 2 PH c Turnover
.
2
frequency = mol product per mol catalyst per hour. d Average of two runs.
e H2 pressure 66 bar.
† Electronic supplementary information (ESI) available: experimental
procedures for the hydrogenation of 2-vinylnaphthalene and oleic acid, and
the hydroformylation of 2-vinylnaphthalene See http://www.rsc.org/
suppdata/cc/a9/a909703a/
Scheme 2
DOI: 10.1039/a909703a
Chem. Commun., 2000, 693–694
This journal is © The Royal Society of Chemistry 2000
693

View Article Online
Table 2 Hydrogenation of oleic acid in the absence of added solventa
Table 3 Hydroformylation of 2-vinylnaphthalene in the absence of added
solvent.a
Conv. (%)
Conv. (%)
Entry
T/°C
t/h
PCO = 0 bar
PCO = 60 bar
2
2
Entry
T/°C
t/h
0.5
2
16
2
1
2
4
11
PCO = 0 bar
PCO = 55 bar
2
2
1
2
3
4
5
6
35
35
35
50
50
50
1
4
25
1
4
25
90
90
90
94
95
95
97
97
—
98
99
—
1
2
3
4
5
6
7
8
33
33
33
36
43
43
43
43
0
9.6
—
14
—
93
—
98
8.5
24
88
44
74
96
100
—
a 100 mg (0.35 mmol) oleic acid, 3.5 mmol Pt (as 5% Pt/C), 10 bar H2, vessel
size 160 mL, vial diameter 12 mm. PCO calculated as Ptotal 2 PH
.
2
2
a 12 mg (13 mmol) RhH(CO)(PPh3)3, 400 mg (2.6 mmol) 2-vinyl-
naphthalene, 10 bar each of CO and H2, vessel size 160 mL, vial diameter
22 mm. PCO calculated as Ptotal 2 PH 2 PCO. The selectivity of the
readily at temperatures above 4 °C, but the reaction should stop
short of completion because at high conversions the melting
point of the reaction mixture would climb above the ambient
temperature; the conversion at which the reaction ‘stalls’ should
be a function of the reaction temperature. At 50 °C, for example,
the conversion climbs no higher than 95% even after 25 h (Table
2). The reaction in the presence of subcritical gaseous CO2
attains 99% conversion within 4 h.† At 35 °C, the reaction in the
absence of CO2 climbs no higher than 90% even after 25 h,
while it reaches 97% after 1 h in the presence of CO2. Thus
gaseous CO2 has a strong effect on this reaction at high
conversions.
2
2
reaction is high (14+1) for the desired17 branched aldehyde.
formylation reactions of 2-vinylnaphthalene are accelerated by
CO2 pressure, while the conversions obtained from the
hydrogenation of oleic acid and hydroformylation of 2-vinyl-
naphthalene are improved by CO2 pressure. The use of gaseous
CO2 can thus extend the range of temperatures at which ‘neat’
reactions can be performed. Future work will include the testing
of other gases and other reactions.
This material is based upon work supported by the EPA/NSF
Partnership for Environmental Research under NSF Grant No.
9815320. We also acknowledge useful discussions with Dr
Charles Eckert of the Georgia Institute of Technology and
experimental assistance from Mr Philip Stalcup.
The third reaction to be described is the solventless
hydroformylation of 2-vinylnaphthalene catalysed by
RhH(CO)(PPh3)3 (Scheme 3). This reaction generates
2-(2-naphthyl)propanal (mp 53 °C),15 plus traces of 3-(2-naph-
thyl)propanal (mp 42 °C).16 In the absence of CO2, the melting
point of the reaction mixture starts at 65 °C (the melting point
of 2-vinylnaphthalene), drops after partial conversion (because
a mixture forms), and rises up towards the melting point of the
product as the reaction approaches completion. Thus in the
absence of CO2 and at temperatures below 50 °C, one would
predict that the reaction would start slowly, accelerate after
some product is formed, and become slow again at high
conversions. Indeed, the presence of subcritical gaseous CO2
was found to make the reaction start more quickly (Table 3,
entries 1, 2, and 4) and reach completion more readily (entries
6–8).†
Notes and references
‡ The mixture critical pressure curve starts at 73.9 bar for pure CO2 and rises
with increasing H2 mol fraction.18
§ Not all binary solid/SCF mixtures have such an end point. For example,
the systems p-dichlorobenzene/ethene and menthol/ethene do not. In these
systems, the induced lowering of the melting temperature can be
significantly greater: G. A. M. Diepen and S. E. C. Scheffer, J. Am. Chem.
Soc., 1948, 70, 4081.
There are many advantages to performing a reaction by
induced-melting rather than in a SCF: (1) the pressure of the
SCF/gas is lower, (2) the volume of vessel required is lower, (3)
the concentration of reagent in the reaction phase is much
higher, which could lead to greater rates, (4) homogeneous
catalysts do not have to be designed to be CO2-soluble, and (5)
depending on the substrate, the polarity of the reaction phase
may be much higher than the very low polarity of scCO2. The
disadvantage of the induced-melting option is that it is only
effective for organic solids which have melting points within
30–40 °C of the reaction temperature. Even for those reactions,
one could perform the reaction by melting the reagent in the
usual manner of raising the temperature, but this may not always
be an option, depending on the temperature dependence of the
selectivity (particularly for enantioselective reactions).
In conclusion, we have demonstrated, with three examples,
the acceleration of solventless synthesis by the application of
subcritical (gaseous) CO2. The hydrogenation and hydro-
1 Chemical Synthesis using Supercritical Fluids, ed. P. G. Jessop and W.
Leitner, VCH/Wiley, Weinheim, 1999.
2 C. Dariva, L. A. F. Coelho and J. V. Oliveira, Fluid Phase Equilib.,
1999, 160, 1045.
3 M. A. McHugh and T. J. Yogan, J. Chem. Eng. Data, 1984, 29, 112.
4 R. G. Wissinger and M. E. Paulaitis, J. Polym. Sci., Part B: Polym.
Phys., 1987, 25, 2497.
5 N. P. Freitag and D. B. Robinson, Fluid Phase Equilib., 1986, 31,
183.
6 D. T. Mowry, M. Renoll and W. F. Huber, J. Am. Chem. Soc., 1946, 68,
1105.
7 T. Ohno and S. Kato, Bull. Chem. Soc. Jpn., 1981, 54, 1517.
8 International Thermodynamic Tables of the Fluid State: Carbon
Dioxide, ed. S. Angus, B. Armstrong and K. M. de Reuck, IUPAC,
Pergamon Press, Oxford, 1976.
9 E. Weidner, R. Steiner and Z. Knez, in High Pressure Chemical
Engineering, ed. P. R. von Rohr and C. Trepp, Elsevier, Amsterdam,
1996.
10 A. Bertucco, in Chemical Synthesis using Supercritical Fluids, ed. P. G.
Jessop and W. Leitner, Wiley–VCH, Weinheim, 1999.
11 I. Kikic, M. Lora and A. Bertucco, Ind. Eng. Chem. Res., 1997, 36,
5507.
12 H. B. W. Patterson, Hydrogenation of Fats and Oils, Applied Science,
London, 1983.
13 N. Yoshimoto, T. Nakamura, M. Suzuki and K. Sato, J. Phys. Chem.,
1991, 95, 3384.
14 Merck Index, ed. S. Budavari, M. J. O’Neil, A. Smith, P. E. Heckelman
and J. F. Kinneary, Merck & Co., Inc., Whitehouse Station, NJ, 1996.
15 G. Darzens, C. R. Hebd. Seances Acad. Sci., 1907, 145, 1342.
16 T. Lee and J. B. Jones, J. Am. Chem. Soc., 1997, 119, 10260.
17 D. Neibecker, R. Réau and S. Lecolier, J. Org. Chem., 1989, 54,
5208.
Scheme 3
18 C. Y. Tsang and W. B. Streett, Chem. Eng. Sci., 1981, 36, 993.
694
Chem. Commun., 2000, 693–694