Influence of -SO3H functionalization (N-SO3H or N-R-SO3H, where R = alkyl/benzyl) on the activity of Br?nsted acidic ionic liquids in the hydration reaction

Article abstract of DOI:10.1016/j.tetlet.2012.04.066

Sulfonic acid group functionalized imidazole based Br?nsted acidic ionic liquids (BAILs) were synthesized and their activities were investigated in the hydration reaction of alkynes. The Hammett acidity order determined from UV-visible spectroscopy of BAILs is consistent with their activity order observed in hydration reactions. Theoretical studies further help to establish the structure-activity relationship. Recycling experiments suggest that these novel BAILs can be reused without significant loss in activity. Applicability of BAILs in hydration reaction opens a non-toxic, economical, and eco-friendly route to synthesize alkyl ketones from alkynes.

Full text of DOI:10.1016/j.tetlet.2012.04.066

Tetrahedron Letters 53 (2012) 3245–3249
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Influence of –SO3H functionalization (N-SO3H or N-R-SO3H, where
R = alkyl/benzyl) on the activity of Brönsted acidic ionic liquids
in the hydration reaction
⇑
Rajkumar Kore, Rajendra Srivastava
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Sulfonic acid group functionalized imidazole based Brönsted acidic ionic liquids (BAILs) were synthesized
and their activities were investigated in the hydration reaction of alkynes. The Hammett acidity order
determined from UV–visible spectroscopy of BAILs is consistent with their activity order observed in
hydration reactions. Theoretical studies further help to establish the structure–activity relationship.
Recycling experiments suggest that these novel BAILs can be reused without significant loss in activity.
Applicability of BAILs in hydration reaction opens a non-toxic, economical, and eco-friendly route to syn-
thesize alkyl ketones from alkynes.
Received 30 January 2012
Revised 5 April 2012
Accepted 6 April 2012
Available online 21 April 2012
Keywords:
Brönsted acidic ionic liquids
Hydration reaction
DFT calculations
Ó 2012 Elsevier Ltd. All rights reserved.
Hammett acidity
Ionic liquids have been initially introduced as green reaction
media due to their negligible volatility, excellent thermal stability,
and the variety of available structures.1–3 Ionic liquids are being
widely used in the place of classical organic solvents. They offer
a new and environmentally benign approach toward modern syn-
thetic chemistry.4,5 Ionic liquids find applications in many areas of
interdisciplinary research. However, our interest is to use ionic liq-
uids as catalysts and structure directing agents for material syn-
thesis.6–9 Last decade is the witness for the development of
Brönsted acidic and Lewis acidic ionic liquids as environmental-
friendly acid catalysts.10,11 Very recently, we have reported the
assessment of the catalytic activity of mono-sulfonic and multi-
sulfonic acid group functionalized imidazole/benzimidazole based
Brönsted acidic ionic liquids in a variety of chemical reactions.8,9
Although various studies have been made for Brönsted acidic ionic
liquids, to the best of our knowledge, there is no report in the lit-
erature which studies the influence of direct –SO3H functionaliza-
tion (N-SO3H) or functionalization through spacer (N-R-SO3H
where R = alkyl/benzyl) on their activity. In this study, a variety
of –SO3H functionalized Brönsted acidic ionic liquids (BAILs) were
prepared (Scheme 1) and their activity was investigated in the
hydration reaction of alkynes (Scheme 2). The hydration reaction
of alkynes involves the simple addition of a water molecule with
100% atom efficiency and is regarded as a convenient and efficient
method for the production of ketones and has been revealed as a
useful tool in total synthesis.12
Traditionally Hg(II) catalysts are used for the hydration reaction
of terminal alkynes to form methyl ketones.13,14 Much research has
been devoted to find catalysts based on less toxic metals or Noble
metal free hydration of alkynes.15–21 Very recently, H2SO4 (0.5–
8 mol equiv to substrate) catalyzed hydration reaction of alkynes
to ketones in excess amount of ionic liquid has been reported.22
The objective of this study is to develop, non-toxic and economical
route for the hydration reaction of alkynes using task-specific ionic
liquids. Another objective of this study is to understand the influ-
ence of –SO3H functionalization (N-SO3H and N-R-SO3H, where
R = alkyl/benzyl) on the acidity and activity of resultant BAILs.
BAILs were synthesized by one-step or multi-step synthesis
route (Scheme 1).23,24 BAILs were characterized by using FT-IR,
NMR, elemental, and thermo gravimetric analyses. Acidity of BAILs
was measured using Analytikjena Specord 250 PLUS, UV–visible
spectrophotometer with 4-nitroaniline as a basic indicator by fol-
lowing the concept reported in literature.25,26 With the increase
of acidity of the BAILs, the absorbance of the unprotonated form
of the basic indicator decreased, whereas the protonated form of
the indicator could not be observed because of its small molar
absorptivity and its location, so the [I]/[IH+] (I represents indicator)
ratio can be determined from the differences of measured absor-
bance after the addition of BAILs and Hammett function, H0, can
be calculated using Eq. 1. This value can be regarded as the relative
acidity of the BAILs.
⇑
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.066

3246
R. Kore, R. Srivastava / Tetrahedron Letters 53 (2012) 3245–3249
SO3H
N
Cl
Cl-SO3H, CH2Cl2
298 K, 30 min
[SO3Hmim][Cl]
N
H
(Yield = 88 %)
CH3
N
HCl
[Hmim][Cl]
(Yield = 94 %)
298 K, 30 min
N
Cl
N
CH3
SO3
SO3H
298 K, 2 h
N
N
HCl/H2SO4
[C3SO3Hmim][Cl]
O
O
O
Yield = 88 % X= Cl
X
(
)
S
N
298 K, 30 min
N
N
[C3SO3Hmim][HSO4]
(Yield = 84 %) X= HSO4
CH3
CH3
CH3
CH3
C
H3
N
N
Toluene
Cl-SO3H, CH2Cl2
343 K, 12 h
383 K, 24 h
[Benz-SO3Hmim][Cl]
Yield = 81 %
Cl
N
N
PhCH2Cl
Cl
(
)
SO3H
SO3H
N
N
Cl-SO3H, CH2Cl2
298 K, 12 h
Cl
[(SO3H)2im][Cl]
(Yield = 83 %)
N
H
N
SO3H
Scheme 1. Schematic representation for the synthesis of BAILs investigated in this study.
O
BAILs
R
R'
H2O
+
R'
333 K, 10 h
R
3
2
1
0
Scheme 2. Hydration reaction of alkynes using BAILs.
Reference
[Hmim][Cl]
[C3SO3Hmim][Cl]
[Benz-SO3Hmim][Cl]
[SO3Hmim][Cl]
[(SO3H)2im][Cl]
[C3SO3Hmim][HSO4]
H0 ¼ pKðIÞaq þ log½ðIÞ=ðIHþÞꢀ
ð1Þ
Under the same concentration of 4-nitroanline (10 mg/L,
pK(I)aq = pKa = 0.99) and BAILs (50 mmol/L) in ethanol, H0 values
of all BAILs were determined. The maximal absorbance of the
unprotonated form of the indicator was observed at 373 nm in eth-
anol. When the BAIL was added, the absorbance of the unprotonat-
ed form of the basic indicator decreased ( Fig. 1 and Table 1).
Hammett acidity (H0) of these BAILs was calculated using Eq. 1.
As shown in Figure 1, the absorbance of the unprotonated form
of the indicator on addition of BAILs decreased as follows:
[Hmim][Cl] > [C3SO3Hmim][Cl]ꢁ[Benz-SO3Hmim][Cl] > [SO3Hmim]
[Cl] > [C3SO3Hmim][HSO4] > [(SO3H)2im][Cl]. Calculations suggest
that the Hammett acidity (H0) of these ionic liquids follows the or-
der: [(SO3H)2im][Cl] > [C3SO3Hmim][HSO4] > [SO3Hmim][Cl] >
[C3SO3Hmim][Cl]ꢁ[Benz-SO3Hmim][Cl] > [Hmim][Cl] (Table 1).
Activities of BAILs were assessed by hydration reaction of al-
kynes in the absence of H2SO4/Noble metal catalysts. No product
was obtained in the absence of BAILs. In the initial investigation,
phenyl acetylene was chosen as a model substrate for hydration
reaction. Influence of temperature, amount of BAILs, solvents,
and reaction time were investigated. Highest yield of the product
was obtained when the reaction was performed in the neat condi-
tion. After evaluating several reaction parameters, an optimum
condition (Reaction temp. = 333 K, BAILs = 1 equiv with respect to
300
350
400
450
500
550
Wavelength (nm)
Figure 1. Absorbance spectra of 4-nitroaniline in ethanol after addition of BAILs.

R. Kore, R. Srivastava / Tetrahedron Letters 53 (2012) 3245–3249
3247
Table 1
[Benz-SO3Hmim][Cl] > [C3SO3Hmim][Cl] > [C3SO3Hmim][HSO4] >
[Hmim][Cl] (Table 2), which is consistent with the acidity order ob-
tained from UV–visible spectroscopy, except for [C3SO3Hmim][H-
SO4]. Acidity of BAILs is due to the presence of acidic [(N2)C-H]
proton and –SO3H group. However, the pKa of [(N2)C-H] is ex-
pected to be significantly higher than the –SO3H group.27 The
apparent changes in the activity of BAILs may also be due to the
thermodynamic and kinetic parameters, geometrical and/or solu-
bility aspects of BAILs. Clꢂ is a small and coordinating anion and
forms interionic hydrogen bonds in imidazolium chloride ILs.28,29
However, these interactions are very weak compared to hydrogen
Calculation and comparison of H0 values of different BAILs in ethanol at 298 K
BAILs
Amax
I(%)
IH+(%)
H0
None
[Hmim][Cl]
2.71
2.70
2.65
2.35
2.65
2.45
2.25
100
—
99.6
97.8
86.7
97.8
90.4
82.1
0.4
2.2
13.3
2.2
9.6
17.9
3.39
2.64
1.8
2.64
1.96
1.65
[C3SO3Hmim][Cl]
[C3SO3Hmim][HSO4]
[Benz-SO3Hmim][Cl]
[SO3Hmim][Cl]
[(SO3H)2im][Cl]
Indicator: 4-nitroaniline.
bonding between HSOꢂ4 and –SO3H group in BAILs containing HSOꢂ
4
and –SO3H groups. In our previous study, we found that the activ-
ity of –SO3H functionalized BAILs having HSOꢂ anion is greatly
Table 2
4
Activity of BAILs in the hydration reaction of phenyl acetylene
influenced by the hydrogen bonding between –SO3H group and
HSOꢂ4 anion.9 To confirm these observations, catalytic activities of
Entry
BAILs
Yield (%)
-
[C3SO3Hmim][Cl] and [C3SO3Hmim][HSO4 ] were compared. Cata-
1
2
3
4
5
6
[Hmim][Cl]
6
[C3SO3Hmim][ HSO4]
[C3SO3Hmim][Cl]
[Benz-SO3Hmim][Cl]
[SO3Hmim][Cl]
30
39
41
65
82
lytic activity data clearly demonstrate that the activity is reduced
ꢂ
-
due to the presence of HSO4 . It was found that HSO4 strongly
influences the –SO3H group, which is explained in the following
section by using theoretical calculations. One of the objectives of
this study to understand the influence of –SO3H functionalization,
hence Clꢂ was chosen as anion, which seems to be having negligi-
ble influence on –SO3H group (because of inability to form hydro-
gen bonding). N-SO3H functionalized BAIL ([SO3Hmim][Cl]) was
found to be more active than the N-R-SO3H functionalized BAILs.
It is known that the water content appreciably changes the
structure of the ILs through the participation of water in C(sp2)-H
mediated hydrogen bonds.30 Hence, theoretical studies have been
performed by considering H2O molecules and polarization effect
in the calculation. The minimum-energy geometries of BAILs were
determined by performing DFT geometry optimizations at the
[(SO3H)2im][Cl]
Reaction condition: Phenyl acetylene (1.0 mmol), de-ionized water (3.0 mmol),
BAILs (1 mmol), temperature (333 K), reaction time (10 h). Experimental errors =
2.5.
Table 3
The geometry parameters of BAILs calculated using B3LYP/6-31++G(d,p)
Observed Parameters
[C3SO3Hmim][Cl] [C3SO3Hmim][HSO4]
H–O bond distance of –SO3H (Å)
H23–O22 = 0.975
H22–O21 = 1.024
32–O31 = 0.974
H
(N)2C–H
C3–H7 = 1.08
H7ꢃ ꢃ ꢃCl12 = 2.38
—
C3–H7 = 1.082
H7ꢃ ꢃ ꢃO28 = 2.25
H10ꢃ ꢃ ꢃO28 = 2.63
H22ꢃ ꢃ ꢃO29 = 1.546
ꢂ1707.6
B3LYP/6-31++G(d,p) level using the Gaussian 09 program.31
A
N)2C–Hꢃ ꢃ ꢃX
vibrational analysis was performed to ensure the absence of nega-
tive frequencies and verify the existence of a true minimum. For
comparison, studies were also performed without considering
H2O molecules and the polarization effect (Supplementary data).
To confirm, why the activity of [C3SO3Hmim][HSO4] is less com-
pared to the activity of [C3SO3Hmim][Cl], theoretical studies were
performed. The optimized structure and related parameters of
[C3SO3Hmim][HSO4] and [C3SO3Hmim][Cl] are provided in this
manuscript; whereas the optimized structures and related param-
eters of other BAILs investigated in this study are provided in the
Other hydrogen bonds in BAILs
(Å)
Energy (a.u.)
Dipole Moment (Debye)
ꢂ1468.2
19.15
26.01
alkynes, and solvent = none, reaction time = 10 h) was obtained to
investigate the influence of a variety of BAILs for this reaction. All
BAILs investigated in this study were found to be active. The activ-
ity of BAILs follows the order: [(SO3H)2im][Cl] > [SO3Hmim][Cl] >
Figure 2. Optimized molecular structure of BAILs using B3LYP/6-31++G (d,p).

3248
R. Kore, R. Srivastava / Tetrahedron Letters 53 (2012) 3245–3249
70
60
50
40
30
20
10
0
no interaction, whereas H22 of –SO3H group in [C3SO3Hmim][HSO4]
interacts with O29 of HSOꢂ4 (H22ꢃ ꢃ ꢃO29 = 1.546) and inhibits the
accessibility of H22 to the reactant molecules for the hydration
reaction and hence [C3SO3Hmim][HSO4] activity was found to be
less compared to the activity of [C3SO3Hmim][Cl]. N-Me
substitution provides some electron density to the imidazole ring
due the electron donating nature of –CH3 group, whereas in
[(SO3H)2im][Cl], imidazole ring will become more electropositive
because of the two electron withdrawing –SO3H functionalizations.
Due to this reason, the O–H bond length in –SO3H group of these
two BAILs was found to be different (Table 1, Supplementary data).
O–H bond length in [SO3Hmim][Cl] was found to be 1.721 Å,
whereas O–H bond lengths in [(SO3H)2im][Cl] were found to be
1.031 Å and 1.038 Å, which makes –SO3H group in [[(SO3H)2im][Cl]
less active and hence [[(SO3H)2im][Cl] activity is not as signifi-
cantly high as it should be expected because of the presence of
two -SO3H functionalizations.
0
1
2
3
4
5
To demonstrate the robustness and durability of BAILs, [SO3H-
mim][Cl] was recycled 5 times. At the end of each cycle, the reac-
tion product was extracted and yield was determination by using
gas chromatography.32 Throughout all the recycling, comparable
yields (on average 60%) were observed (Fig. 3). The results strongly
suggest that the BAILs are a robust and recyclable system that can
constantly promote the transformation of phenyl acetylene to
acetophenone without losing its activity. Hydration reaction
proceeded well, delivered good product yields, and accommodated
Number of recyles
Figure 3. Activity of [SO3Hmim][Cl] in recycling experiments.
Supplementary data. Optimized structure and related parameters
clearly show that a significant interaction exists between HSO4ꢂ
and –SO3H, whereas such interaction is absent in Clꢂ and –SO3H
(Table 3 and Fig. 2). H23 of –SO3H group in [C3SO3Hmim][Cl] has
Table 4
Hydration reaction of alkynes using [SO3Hmim][Cl]
Entry
Alkyne
Product
Yield (%)
65(100)a
O
1
O
2
64(100)a
H3C
H3C
O
3
4
56(88)a
45(82)a
F
F
CH3
CH3
O
6
4
H3C
O
CH
6
4
3
5
6
43 (79)a
35 (64)a
H3C
CH
3
O
O
7
22(40)a
Reaction condition: Alkyne (1.0 mmol), de-ionized water (3.0 mmol), [SO3Hmim][Cl] (1 mmol), temperature (333 K), reaction time
(10 h).
a
Parenthesis represent the yield obtained after 24 h of reaction.

R. Kore, R. Srivastava / Tetrahedron Letters 53 (2012) 3245–3249
3249
13. Kutscheroff, M. Chem. Ber. 1881, 14, 1540–1542.
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4563–4565.
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2179.
a wide range of alkynes (Table 4). Both aromatic and aliphatic al-
kynes could be converted to ketones under the given conditions.
Compared to aromatic terminal alkynes, aliphatic terminal alkynes
are less reactive. Internal alkynes are less reactive than terminal al-
kynes. To demonstrate the practicability of this synthesis protocol,
hydration reaction of phenyl acetylene was performed on a large
scale (100 mmol) to afford the isolated yield of 95% of acetophe-
none after 24 h of the reaction.
In conclusion, efficient, non-toxic, Noble metal free, and Brön-
sted acidic ionic liquid based economical route was developed
for the hydration reaction of alkynes. N-SO3H functionalized BAIL
([SO3Hmim][Cl]) was found to be more active than the N-R-SO3H
(R = alkyl/benzyl) functionalized BAILs. These reactions are easy
to perform and the purification protocol is simple. Apart from
the experimental simplicity, the advantage of this methodology
is to use mild and efficient BAILs for hydration reaction which
makes them interesting candidates for commercial use. Based on
the results, one can conclude that to establish a structure–activity
relationship, theoretical studies along with UV–visible study were
very helpful.
21. Allen, A. D.; Chiang, Y.; Kresge, A. J.; Tidwell, T. T. J. Org. Chem. 1982, 47, 775–
779.
22. Wong, W. L.; Ho, K. P.; Lee, L. Y. S.; Lam, K. M.; Zhou, Z. Y.; Chan, T. H.; Wong, K.
Y. ACS Catal. 2011, 1, 116–119.
23. Synthesis
of
BAILs-[Hmim][Cl],4
[SO3Hmim][Cl],5
[(SO3H)2im][Cl],5
[C3SO3Hmim][Cl]24 and [C3SO3Hmim][HSO4]24 were synthesized by
following the reported procedure.4,5,24 For the synthesis of [Benz-
SO3Hmim][Cl], first 1-benzyl-3-methylimidazolium chloride was prepared by
following the reported procedure.9 It was then sulfonated using stoichiometric
amount of chlorosulfonic acid in CH2Cl2 to obtain [Benz-SO3Hmim][Cl]
(Yield = 81%).
[Benz-SO3Hmim][Cl]: IR (KBr, t
, cmꢂ1) = 3415, 3150, 2960, 1655, 1455, 1255,
1060, 880, 775, 665. 1H NMR (400 MHz, D2O): d (ppm) 8.56 (s, 1H), 7.35–7.22
(m, 6H), 5.22 (s, 2H), 3.71 (s, 3H); 13C NMR (D2O): d (ppm) 135.99, 133.49,
129.22, 129.17, 128.47, 123.66, 122.15, 52.69, 35.56. Elemental analysis for
C11H13N2SO3Cl: Theoretical (%): C 45.75, H 4.51, N 9.71; Experimental (%): C
45.32, H 4.62, N 9.46.
24. Zhang, L.; Xian, M.; He, Y.; Li, L.; Yang, J.; Yu, S.; Xu, X. Bioresour. Technol. 2009,
100, 4368–4373.
Acknowledgments
25. Thomazeau, C.; Bourbigou, H. O.; Magna, L.; Luts, S.; Gilbert, B. J. Am. Chem. Soc.
2003, 125, 5264–5265.
26. Gu, Y.; Zhang, J.; Duan, Z.; Deng, Y. Adv. Synth. Catal. 2005, 347, 512–516.
27. Subbiah, S.; Venkatesan, S.; Ming-Chung, T.; Yen-Ho, C. Molecules 2009, 14,
3780–3813.
28. Fabio, R.; Douglas, G.; Gustavo, M. d. N.; Paulo, S. S. J. Phys. Chem. B 2012, 116,
1491–1498.
29. Richard, C. R.; Jayme, L. W.; Ashleigh, L. R.; Guillermo, M. J. Phys. Chem. B 2007,
111, 11619–11621.
Authors thank the Ministry of Human Resource and Develop-
ment, New Delhi and the Director, IIT Ropar for financial assis-
tance. We are grateful to Dr. T.J. Dhilip Kumar, Chemistry
Department, IIT Ropar for providing Gaussian 09 software for
DFT calculation. Authors acknowledge Dr. Snehlata Jaswal, IIT
Ropar, for proof reading the manuscript.
30. Andrea, M.; Chieu, D. T.; Silvia, H. D. P. L. Angew. Chem., Int. Ed. 2003, 42, 4364–
4366.
Supplementary data
31. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.;
Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.;
Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven,
T.; Montgomery J. A. Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.;
Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas,
O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian 09, Revision
A.1, Gaussian, Inc., Wallingford CT 2009.
Supplementary data (1H, 13C NMR spectra, and TGA of [Benz-
SO3Hmim][Cl]; optimized structures (Fig. 1 and related parameters
(Table 1) of BAILs calculated using B3LYP/6-31++ G(d,p); observed
parameters of BAILs calculated using B3LYP/6-31G (Tables 2))
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2012.04.066.
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32. Hydration Reaction-In
a typical procedure, a solution of phenyl acetylene
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(Yonglin 6100; BP-5; 30 m ꢄ 0.25 mm ꢄ 0.25
identified by GC–MS (Shimadzu QP-5000; 30 m long, 0.25 mm i.d., with a
0.25- m-thick capillary column, DB-1) and authentic samples obtained from
lm). The products were
l
Aldrich.
Aqueous portion of the reaction mixture was evaporated to remove the water.
Residue was washed three-four times with diethyl ether to remove any organic
impurity. Finally, ionic liquid portion was dried under vacuum at 353 K for 4 h.
The recovered ionic liquid was used in the recycling experiments.
11. Harjani, J. R.; Nara, S. J.; Salunkhe, M. M. Tetrahedron Lett. 2002, 43, 1127–1130.
12. Trost, B. M. Science 1991, 254, 1471–1477.