Glycerol as an efficient promoting medium for organic reactions

Article abstract of DOI:10.1002/adsc.200800328

Whereas the beneficial effect of water on reaction rate is decreased with an increase of the reactant's hydrophobicity, we report here that the use of glycerol as solvent was able to considerably accelerate the reaction rate of an organic reaction even st

Full text of DOI:10.1002/adsc.200800328

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DOI: 10.1002/adsc.200800328
Glycerol as An Efficient Promoting Medium for Organic
Reactions
Yanlong Gu,a Jol Barrault,a and FranÅois JØrômea,*
a
Laboratoire de Catalyse en Chimie Organique, UniversitØ de Poitiers-CNRS, 40 Avenue du recteur Pineau, Poitiers
Cedex, 86022, France
Fax : (+33)-5-4945-3349; e-mail: francois.jerome@univ-poitiers.fr
Received: April 29, 2008; Revised: June 30, 2008; Published online: August 13, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800328.
Abstract: Whereas the beneficial effect of water on
reaction rate is decreased with an increase of the
reactant’s hydrophobicity, we report here that the
use of glycerol as solvent was able to considerably
accelerate the reaction rate of an organic reaction
even starting from more hydrophobic substrates
than those usually used on water. Moreover, the
possibility of directly using crude glycerol generated
by the biodiesel industry economically and environ-
mentally improves the interest of using glycerol as
solvent.
In seeking further improvements on catalysis on
water, we found that glycerol could behave similarly
as water. Indeed, like water, glycerol is abundant, bio-
degradable, cheap (sometimes even cheaper than
water!) and non-toxic. Moreover, like water, glycerol
is highly hydrophilic, poorly soluble with most of or-
ganic substrates and its cohesion is also maintained by
a strong hydrogen bond network. Based on the close
similarity of its solvent properties with those of water,
it occurred to us that the development of organic re-
actions on glycerol could be a great means to over-
come the high hydrophobicity of organic substrates
which are now the limiting factor for the development
of on-water reactions. Compared to water, it has to be
pointed out that glycerol exhibits a very high boiling
point (2908C) making also the development of organ-
ic reactions at high temperature technically easier.
From a social and economical point of view, utilisa-
Keywords: catalysts; glycerol; green solvents; rate
acceleration
Since the 1980s, the use of water as solvent has at- tion of glycerol takes a place as one of the most
tracted a lot of attention not only because it offers en- urgent topic of this beginning century.[9] Indeed, the
vironmentally friendlier pathways but also because spectacular and rapid development of the vegetable
water itself is able to considerably increase the reac- oil industries generates a tremendous amount of
tion rate of an organic reaction. Indeed, thanks to a crude glycerol as by-product (1.5 million tons expect-
strong hydrogen bond network, it was clearly demon- ed in 2008) which is now in urgent need of chemical
strated that the water interface can lower the activa- utilisation. Up to now, most solutions deal with the
tion barrier of a process, by solvation of the transition transformation of glycerol to more valuable chemicals
state, and consequently act as a potential catalyst.[1] such as monoglycerides,[10] glycerol ethers,[11] acro-
Based on this particular property of water, numerous
C
dation[6] and many others[7] were successfully per-
formed on water without addition of any catalyst. Fas- for organic transformations would be conceptually
cinating reviews about this topic may be found in the highly interesting since it would offer (i) a sustainable
literature.[8] However, even if some spectacular rate medium able to drive some organic transformations
improvements were observed on water, these on with more hydrophobic substrates than those com-
water-based processes are still subject to strict limita- monly used on water and (ii) an economically attrac-
tions when highly hydrophobic substrates are used.
tive alternative use for the growing production of
glycerol.
Adv. Synth. Catal. 2008, 350, 2007 – 2012
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2007

Yanlong Gu et al.
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Table 1. The aza-Michael reaction in different solvents
Recently, Wolfsonꢁs group in Israel showed that
some selected organic reactions such as Pd-catalyzed
under catalyst-free conditions.[a]
À
Heck C C coupling and Suzuki reactions could be
performed in glycerol.[14] However, compared to other
organic solvents, no significant improvement in terms
of reaction behaviour and catalytic performance was
observed in glycerol. As a result, glycerol has not
been recognised yet as a necessary solvent for organic
reactions.
In this communication, we report that, like water,
the glycerol interface can also act as a potential cata-
lyst and drive many organic transformations such as
Michael additions of amines, anilines and indoles, ring
opening of styrene oxide with p-anisidine, and acid-
catalysed dehydrative dimerisation of tertiary alcohol-
in a friendly manner. Moreover, whereas on-water re-
actions are limited when more hydrophobic substrates
are involved, we show here that on-glycerol reactions
tolerate the presence of more hydrophobic reactants.
The first reaction we examined is an aza-Michael
reaction of amines on a,b-unsaturated compounds,
which is one of the most effective ways to prepare b-
amino acids and their ester derivatives.[15] Many cata-
lytic systems, such as Pd and Cu complexes,[16] Lewis
acids,[17] Brønsted acids,[18] zeolites[19] and organic
bases including DBU[20] and [BMIm]OH,[21] were re-
ported to be effective for aza-Michael reactions of
amines. In some cases, assistance of microwave irradi-
ation was found to be helpful for a better catalytic
Entry
Solvent
Yield [%]
1
2
3
4
glycerol
no solvent
toluene
DMF
DMSO
ClCH2CH2Cl
water
1,2-propanediol
crude glycerol
82 (82,[b] 80[c])
<5d
0
0
0
0
5
6[d]
7
<5[e]
30
8
9
81 (80)[b]
[a]
p-Anisidine: 1.0 mmol, butyl acrylate: 1.0 mmol, solvent:
1 mL.
[b]
[c]
[d]
[e]
Reused in the second time.
Reused in the third time.
908C.
GC yield.
performance.[22] Some researchers have also examined compared to what was observed on water. Theoretical
the reaction in water,[23] however, acceleration effect calculations are currently underway in our group and
of water on the reaction rate was only observed in the preliminary results will be reported in due course. In-
reaction of aliphatic amines. Indeed, starting from terestingly, compared to other high boiling point alco-
aniline derivatives, assistance of a catalyst was found hols such as 1,2-propanediol, a higher reaction yield
to be necessary.
was obtained in glycerol, further indicating the great
With the aim of showing the great contribution of contribution of glycerol for driving an aza-Michael re-
glycerol as reaction solvent, we investigated the aza- action (Table 1, entry 8).
Michael addition of p-anisidine (1a) to butyl acrylate
Besides the beneficial effects exerted by pure glyc-
(2a). When this reaction was performed at 1008C erol on the aza-Michael reaction, we also investigated
under solvent-free conditions or in the presence of or- the possibility of conducting the same reaction using
ganic solvents such as toluene, DMF, DMSO and 1,2- industrial grade glycerol (80%) which will be econom-
dichloroethane, no reaction occurred (Table 1, en- ically and environmentally more attractive. Glycerol
tries 2–6). As reported above, the reaction can pro- generated by the biodiesel industry is not pure and
ceed on water but a very low reaction rate was ob- generally contains about 15 wt% of water and 5 wt%
served since only trace amounts of product (<5%) of soap. Fortunately, the impurities present in crude
were detected after 20 h of reaction at 1008C glycerol do not significantly change the physical prop-
(Table 1, entry 7). Remarkably, when the reaction was erties of crude glycerol. These aspects led us to direct-
now performed on glycerol, a considerable improve- ly use crude glycerol in our investigation. Table 1
ment of the reaction rate was observed compared to shows that, using crude glycerol, nearly the same
water and the resulting aza-Michael adduct was isolat- result of using pure glycerol are obtained (81%), in-
ed with 82% yield after 20 h of reaction (Table 1, creasing thus the interest of using glycerol as reaction
entry 1). This example clearly shows that, as expected, media (entry 9).
the glycerol interface is also able to directly “cata-
The recyclability of glycerol was also investigated.
lyse” an organic reaction. The difference of behaviour At the end of the reaction, the aza-Michael adduct is
observed here between water and glycerol might not soluble in the glycerol phase and can be directly
come from the better affinity of p-anisidine for the extracted after decantation. However, as in the case
glycerol interface inducing thus a faster reaction rate of water, this technique is not so easy to handle and
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Table 2. Substrate scope for aza-Michael reactions in glycerol.[a]
Entry
Amines
Olefin
Product
Temp. [oC]
Time [h]
Yield [%]
Glycerol
Crude glycerol
Neat conditions
1
2
3
4
1a
1b
1b
1c
1c
1d
1d
1d
1e
1f
1g
1h
1h
1i
2b
2a
2b
2a
2b
2c
2d
2e
2d
2a
2d
2d
2e
2c
2d
2e
2d
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
100
100
100
100
100
100
80
80
80
100
80
80
80
100
80
80
80
38
20
20
20
38
5
14
14
14
14
4.5
4.5
14
5
77
78
72
66
60
85
92
93
77
83
72
74
65
65
91
90
80
76
74
70
64
60
77
88
90
80
80
62
67
66
55
90
87
72
<5[b]
<5[b]
<5[b]
<5[b]
<5[b]
29
5
6[c]
7[d]
8[d]
9[d]
10[c]
11[d]
12[d]
13[d]
14[c]
15[d]
16[d]
17[d]
<5[b]
<5[b]
<5[b]
<5[b]
<5[b]
<5[b]
41
<5[b]
<5[b]
<5[b]
<5[b]
1i
1i
1j
4.5
14
14
[a]
Ratio of amine/olefin=1/1.2.
GC yield.
Ratio of amine/olefin=1/1.
Ratio of amine/olefin=1/2.
[b]
[c]
[d]
an important loss of product is observed during the taining catalysts. Substituted anilines with electron-
extraction stage. With the aim of showing the possible donating groups gave better reaction yields than that
recyclability of the glycerol phase, complete and of aniline itself (entries 1 to 5).[24] Primary and secon-
easier extraction of the reaction product was per- dary amines, such as 4-chlorobenzylamine, phenyl-
formed with ethyl acetate. Before the next run, the
A
recovered glycerol was maintained under vacuum (14 benzylmethylamine and 1,2,3,4-tetrahydroisoquinoline
mmHg) at 608C for 30 min in order to remove all re- reacted smoothly with methacrylonitrile at 808C in
sidual traces of ethyl acetate. The yields obtained glycerol or crude glycerol affording the corresponding
after three cycles were comparable to that of fresh mono-adducts in good to excellent yields (entries 7, 9,
glycerol or crude glycerol, indicating a good stability 11, 12 and 15). Some less reactive a,b-unsaturated
of glycerol (Table 1, entry 1).
carbonyl compounds, such as benzyl methacrylate and
Having these results in hand, we then investigated crotononitrile, could also react in the presence of
the substrate scope of the present aza-Michael reac- glycerol (entries 6, 8, 13 and 16). Finally, a selective
tion in glycerol or crude glycerol, and the results are Michael addition of tryptamine was examined using
listed in Table 2. In order to confirm the promoting methacrylonitrile as substrate, and it was observed
effect of glycerol solvent on the reaction rate, results that the reaction occurs selectively on the NH2 posi-
under neat conditions were also collected. As sum- tion in glycerol leading to the corresponding monoad-
marized in Table 2, whatever the reactant considered, duct with 80% yield (entry 17). From these results, it
use of glycerol as solvent significantly boosted the re- is evident that our methodology is reasonably general
action rate of the aza-Michael reactions offering thus and can be applied to many anilines, amines and a,b-
a great alternative to the use of expensive metal-con- unsaturated carbonyl compounds.
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Encouraged by the above-mentioned results, we performance as pure glycerol for the model reaction
then investigated the Michael reaction of indole in (entry 6). Other substrates such as 1-methylindole, 2-
glycerol. This type of reaction was generally carried phenylindole, 1-methyl-2-phenylindole and methyl
out in the presence of either Brønsted[25] or Lewis vinyl ketone were also successfully used in this system
acid catalysts.[26] Water has shown a beneficial effect (entries 7 to 10). To the best of our knowledge, it is
on this reaction as solvent but always with the assis- the first example of catalyst-free Michael reactions of
tance of acidic or neutral catalysts.[27] As shown in indoles.
Table 3, the Michael reaction of indole with b-nitro-
Taking advantage of this remarkable property of
glycerol, we then moved on to the ring opening of sty-
rene oxide with p-anisidine in order to show the great
versatility of these glycerol-based processes.[28] In this
reaction, water and glycerol both exhibited a great
advantage compared to solvent-free conditions since,
in these cases, the resulting amino alcohol was still
obtained with more than 85% yield without addition
of any catalyst (Scheme 1).
Table 3. Michael reaction of indoles in different solvents at
catalyst-free conditions.[a]
Entry Indole Olefin Product Solvent
Yield [%]
1
2
3
4
5
6
7
8
4a
4a
4a
4a
4a
4a
4b
4c
4d
4b
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
6a
6a
6a
6a
6a
6a
6b
6c
6d
6e
glycerol
no solvent
toluene
DMF
80
<5[b]
<5[b]
<5[b]
55
H2O
crude glycerol 78
Scheme 1. Ring-opening of styrene oxide with p-anisidine.
glycerol
glycerol
glycerol
glycerol
75
70
62
84
9
10[c]
Having now demonstrated that glycerol was able to
act as a promoting medium for some organic reac-
tions, we finally tested whether it would be possible
to develop much more complex reactions in/on glycer-
ol. Following this goal, we decided to check if it was
possible to observe a kind of synergistic effect be-
[a]
Indole/olefin: 1/1.
GC yield.
Indole/olefin: 1/2, 808C.
[b]
[c]
styrene proceeded very well in pure glycerol at 808C, tween the glycerol phase and a solid catalyst. Recent-
and 80% yield was obtained after 24 h (entry 1). ly, we have reported that, compared to conventional
Under similar conditions, we found that water can homogeneous or heterogeneous catalysts, silica-sup-
also drive this reaction without assistance of any cata- ported sulfonic acid coated with ionic liquid (IL-SiO2-
lyst but the reaction rate was still lower than that ob- SO3H) was able to catalyse various highly selective
served in glycerol since the reaction product was ob- reactions in water.[28]
tained with only 55% yield after 24 h of reaction
Based on this observation, we investigated the acid-
(entry 5). As mentioned above, we assume that this catalysed tandem dehydration/dimerisation of 2-(4-bi-
better improvement of the reaction rate observed phenylyl)-2-propanol in glycerol. Interestingly, where-
with glycerol could be ascribed to a better affinity of as in neat conditions or in the presence of water, ex-
indole derivatives for glycerol than for water. Here tensive polymerisation of 2-(4-biphenylyl)-2-propanol
again, only a trace amount of product was detected was observed over IL-SiO2-SO3H[29], we found that in
using organic solvents such as toluene, DMF or sol- glycerol, the IL-SiO2-SO3H solid catalyst was able to
vent-free conditions (entries 2 to 4) confirming, as in selectively drive a tandem dehydrative/dimerisation
the case of water, the key role played by the glycerol reaction affording 2,4-bis(4-biphenylyl)-1-pentene
hydrogen bond network on the reaction progress. As with 94% yield (Scheme 2). This last example clearly
observed above, crude glycerol also showed a similar shows that the use of glycerol as solvent offers a sus-
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Glycerol as An Efficient Promoting Medium for Organic Reactions
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Scheme 2. Acid-catalysed dehydrative dimerization of alcohol.
tainable and efficient alternative to the use water Acknowledgements
when more hydrophobic substrates are used.
The authors thank the French Ministry of Research and
CNRS for their financial support for both research and
CNRS associated researcher position of YG.
In conclusion, we have found that the use of glycer-
ol and crude glycerol as solvent can behave similary
to water in organic synthesis and could be familiarly
considered as “organic water”. Indeed, like water, we
showed that glycerol was able to promote an organic
reaction without addition of any catalyst. In addition,
like water, glycerol is cheap, safe, biodegradable and
reusable. Moreover, whereas with more hydrophobic
substrates the beneficial effect of water on reaction
rate becomes limited, we showed here that glycerol is
still able to efficiently drive the selected organic
transformation. In particular, glycerol and crude glyc-
erol were found to be highly efficient for conducting
many organic transformations such as aza-Michael re-
actions of amines or anilines, Michael reaction of in-
doles and ring opening of styrene oxide with p-anisi-
dine without addition of any catalyst as is generally
the case. In a last example, we showed that associa-
tion of a solid catalyst with glycerol as solvent al-
lowed us to selectively control the mechanism path-
way of more complex reaction such as a tandem dehy-
drative/dimerisation of a tertiary alcohol.
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2012
asc.wiley-vch.de
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 2007 – 2012