Ionic liquids as powerful solvent media for improving catalytic performance of silyl borate catalyst to promote Diels-Alder reactions

Article abstract of DOI:10.1021/jo071099w

(Chemical Equation Presented) Use of the acidic chloroaluminate ionic liquid, including the recycled ones, can improve the catalytic activity of the toluene-coordinated silyl borate in enhancing rates, stereo-selectivities, and yields of Diels-Alder reactions.

Full text of DOI:10.1021/jo071099w

Ionic Liquids as Powerful Solvent Media for
Improving Catalytic Performance of Silyl Borate
Catalyst to Promote Diels-Alder Reactions
Anil Kumar* and Sanjay S. Pawar
Physical Chemistry DiVision, National Chemical Laboratory,
Pune 411008, India
a.kumar@ncl.res.in
ReceiVed May 28, 2007
FIGURE 1. Ionic liquids used.
Use of the acidic chloroaluminate ionic liquid, including the
recycled ones, can improve the catalytic activity of the
toluene-coordinated silyl borate in enhancing rates, stereo-
selectivities, and yields of Diels-Alder reactions.
SCHEME 1. Investigated Diels-Alder Reactions
Diels-Alder reactions are accelerated by Lewis acid catalysts.
It has been recently demonstrated that a Lewis acid catalyst,
such as toluene-coordinated silyl borate, [Et3Si(toluene)]B-
(C6F5)4, can offer high catalytic activity for promoting Mu-
kaiyama and Diels-Alder reactions.1 The Diels-Alder reactions
of 1,3-cyclohexadiene with methyl acrylate, methyl crotonate,
methyl vinyl ketone, and crotonaldehyde and of 2,3-dimeth-
ylbutadiene with methyl acrylate proceeded with high yields
and endo:exo ratios, but no rate data were reported. As a part
of our research program on ionic liquids,2 we now demonstrate
that ionic liquids can further improve the catalytic performance
of silyl borate catalyst. The ionic liquids employed in this
investigation include 1-butyl-3-methylimidazolium tetrafluo-
roborate [BMIM][BF4], 1-butyl-3-methylimidazolium hexafluo-
rophosphate [BMIM][PF6], 1-butyl-3-methylimidazolium hexaflu-
oroantimonate [BMIM][SbF6], 1-ethyl-3-methylimidazolium
tetrafluoroborate [EMIM][BF4], 1-butyl-3-methylimidazolium
iodide [BMIM]I, and 1-octyl-3-methylimidazolium tetrafluo-
roborate [OMIM][PF6] (Figure 1). These ionic liquids were
chosen because they were easy to synthesize and handle in
laboratories. The preference to employ ionic liquids over
(1) Hara, K.; Akiyama, R.; Sawamura, M. Org. Lett. 2005, 7, 5621 and
references cited therein.
(2) (a) Tiwari, S. S.; Kumar, A. Angew. Chem., Int. Ed. 2006, 45, 4824.
(b) Kumar, A.; Pawar, S. S. J. Mol. Catal. A 2005, 235, 244. (c) Kumar,
A.; Pawar, S. S. J. Mol. Catal. A 2004, 211, 43. (d) Kumar, A.; Pawar, S.
S. J. Org. Chem. 2004, 69, 1419. (e) Kumar, A.; Pawar, S. S. J. Mol. Catal.
A 2004, 208, 33. (f) Kumar, A.; Sarma, D. Recent Applications of
Chloroaluminate Ionic Liquids in Promoting Organic Reactions. In Ionic
Liquids IIB: Fundamentals, Progress, Challenges, and Opportunities,
Transformations and Processes; Rogers, R. D., Seddon, K. R., Eds.; ACS
Symposium Series 902: American Chemical Society: Washington, DC,
2005; Chapter 24, p 350.
conventional organic solvents in chemical transformations has
been highlighted in several reports.3
(3) For example, see: (a) Anastas, P. T.; Warner, J. C. Green
Chemistry: Theory and Practice; Oxford University Press: Oxford, 1998.
(b) Holbrey, J. D.; Seddon, K. R. Clean Products and Processes; Springer:
Berlin, 1999; Vol. 1, p 223. (c) Welton, T. Chem. ReV. 1999, 99, 2071. (d)
Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391. (e)
10.1021/jo071099w CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/19/2007
J. Org. Chem. 2007, 72, 8111-8114
8111

TABLE 1. Rates,a Yields,b Reaction Times,c and endo:exo Datad for the Reaction of 1 with 2a with and without Silyl Borate Catalyst in
Different Media with Catalytic Loading at 27 °C
105 k2
entry
catalyst/ionic liquid
1 mol % of silyl boratee
(M-1 s-1
)
yieldb (%), timec (h)
endo:exod
1
2
9.00
4.52
5.92
8.00
9.94
11.51
13.22
13.55
13.98
3.92
12.21
3.45
11.01
5.86
85, 2.5
76, 4.5
84, 4
87, 2.5
90, 2
80:20
74:26
79:21
82:18
86:14
89:11
93:7
[BMIM][BF4]
3
4
5
6
7
8
9
0.1 mol % of silyl borate + [BMIM][BF4]
0.25 mol % of silyl borate + [BMIM][BF4]
0.5 mol % of silyl borate + [BMIM][BF4]
0.75 mol % of silyl borate + [BMIM][BF4]
1 mol % of silyl borate + [BMIM][BF4]
1.5 mol % of silyl borate + [BMIM][BF4]
2 mol % of silyl borate + [BMIM][BF4]
[BMIM][PF6]
92, 2
95, 1.5
95, 1.5
95, 1.5
72, 4.5
90, 2
70, 4.5
90, 2.5
73, 4
85, 2
70, 5
70, 5.5
68, 6
72, 4.5
91:9
92:8
10
11
12
13
14
15
16
15
16
17
74:26
87:13
72:28
91:9
73:27
90:10
70:30
72:28
68:32
73:27
1 mol % of silyl borate + [BMIM][PF6]
[BMIM][SbF6]
1 mol % of silyl borate + [BMIM][SbF6]
[EMIM][BF4]
1 mol % of silyl borate + [EMIM][BF4]
[OMIM][PF6]
1 mol % of silyl borate +[OMIM][PF6]
[BMIM]I
1 mol % of silyl borate +[BMIM]I
16.76
3.11
7.34
2.46
5.98
a Standard error of 5%; the reactions were carried out in a 1 mmol scale in 1.5 mL of ionic liquid. b Isolated yields. c No further increase in yields after
the times reported. d Determined by GC analysis. e Toluene ) 1 mL.
The reaction of cyclopentadiene 1 with methyl acrylate 2a
(Scheme 1) was carried out in the presence of the above catalyst
in different media (Table 1). This reaction is faster in water as
compared to that in conventional organic solvents.4,5 This
reaction gave 85% yield in toluene-coordinated silyl borate
catalyst with a rate constant (k2) ∼9 times higher than that
in chloroaluminate ionic liquid. The results are given in Table
2. First, the reaction was carried out in silyl borate with its
concentration starting from 0.1 to 1 mol % in the acidic
chloroaluminate ionic liquid (N-1-butylpyridinium chloride
abbreviated as BPC with AlCl3 with mole fraction, X, of AlCl3
) 0.6; Supporting Information Figure S1). Both the k2 and the
yields were maximized in 0.3 mol % of silyl borate catalyst in
the acidic [BPC][AlCl3]. The reaction was 8 times faster with
20% higher product and endo:exo ratio of 98:2 in 0.3 mol % of
catalyst in the acidic [BPC][AlCl3] as compared to that in ionic
liquid alone. The catalyzed reaction in 0.3 mol % of silyl borate
became slow in the basic chloroaluminate ionic liquid [BPC]-
[AlCl3]. The catalyzed reaction carried in a neutral chloroalu-
minate gave better results than those obtained in basic chloro-
aluminate. This clearly shows that the catalytic activity of silyl
borate is lowered in basic chloroaluminate ionic liquid. It may
be noted that the reaction in 0.3 mol % of silyl borate catalyst
in conjunction with the acidic chloroaluminate was 47 times
faster than in 2,2,4-trimethylpentane. This shows that the Lewis
acid effect imparted by the acidic chloroaluminate ionic liquid
is additive to the Lewis acid catalytic effect of the silyl catalyst.
A combination of the catalyst in a neutral chloroaluminate ionic
liquid offers 86% of cycloadduct with a 78:22 endo:exo product.
The use of AlCl3 with and without silyl borate catalyst did not
give desirable results as obtained with the use of silyl borate
catalyst in ionic liquids.
observed in 2,2,4-trimethylpentane (k2 ) 1.03 × 10-5 M-1 s-1
,
with a yield ) 66% in 7 h, endo-stereoisomer ) 63%). The
use of 1 mol % of the silyl catalyst in [BMIM][BF4] offered
the maximum yield (Table 1, entries 3-9). The reaction was
∼3 times faster in 1 mol % of the catalyst with [BMIM][BF4]
as compared to that in ionic liquid alone. This reaction with
the silyl catalyst in [BMIM][PF6], however, resulted in 90%
yield with endo:exo ) 87:13. The reaction was 3-fold faster in
1 mol % of silyl borate + [BMIM][PF6] with improvement in
yield and with higher endo:exo ratios. The performance of silyl
borate catalyst in another ionic liquid, [BMIM][SbF6], was
recorded to be similar to the one achieved in [BMIM][PF6].
The catalyzed reaction was about 3 times faster in [EMIM]-
[BF4] than in ionic liquid alone. However, in this case, sufficient
improvement was not observed as compared to the reaction
catalyzed in the absence of an ionic liquid.
The catalyzed reaction was suppressed in the presence of
[OMIM][PF6] and [BMIM]I. In our recent work, we have
demonstrated that Diels-Alder reactions are retarded in ionic
liquids with high viscosities due to a diffusion problem.2a This
seems to be also valid in the present investigation.
Considering the role of chloroaluminate ionic liquids in
accelerating the Diels-Alder reaction,2c we decided to inves-
tigate whether the catalytic performance can be further improved
It is clear from Table 2 that the silyl borate catalyst when
used with the acidic chloroaluminate ionic liquid can offer better
catalytic performance than that obtained in silyl borate catalyst
or ionic liquids alone. Another important experimental observa-
tion is that only one-third of the concentration of the catalyst is
required in the acidic chloroaluminate ionic liquid as compared
to that in other ionic liquids for achieving optimum rates and
products.
Wassercheid, P.; Keim, M. Angew. Chem., Int. Ed. 2000, 39, 3772. (f) Poole,
C. F. J. Chromatogr. A 2004, 1037, 49. (g) For catalytic applications of
ionic liquids, see a special issue of J. Mol. Catal. A 2004, 214 (issue no.
1). (h) Chiappe, C.; Pieracinni, D. J. J. Phys. Org. Chem. 2005, 18, 275. (i)
Rogers, R. D.; Seddon, K. R. In Ionic Liquids: Industrial Applications to
Green Chemistry; ACS Symposium Series 818; American Chemical
Society: Washington, DC, 2005.
(4) Berson, J. A.; Hamlet, Z.; Mueller, W. A. J. Am. Chem. Soc. 1962,
84, 297.
(5) Pawar, S. S.; Phalgune, U. D.; Kumar, A. J. Org. Chem. 1999, 64,
7055.
Considering the utility of the acidic chloroaluminate in
improving the performance of the silyl borate catalyst, we
decided not to employ other ionic liquids for carrying out these
reactions. The exo-selective reaction of 1 with methyl meth-
acrylate, 2b, in the acidic chloroaluminate ionic liquid gave 60%
8112 J. Org. Chem., Vol. 72, No. 21, 2007

TABLE 2. Kinetic Data for the Reaction of 1 with 2a with the Catalyst Loading of Silyl Borate in the Most Efficient Chloroaluminate Ionic
Liquids at 27 °C
105 k2
a
entry
catalyst/ionic liquid
(M-1 s-1
)
yieldb (%), timec (h)
endo:exod
e
1
2
3
4
5
6
7
8
9
AlCl3
7.53
10.32
5.81
15.40
31.89
44.65
48.5
48
47.6
47.5
11.08
8.55
77, 7
79:21
82:18
72:28
81:19
87:13
94:6
98:2
97:3
96:4
97:3
e
1 mol % of silyl borate + AlCl3
[BPC][AlCl3]
87, 5.5
80, 6.5
91, 6
93, 4
96, 3
98, 2
97, 2
97, 2
97, 2
0.05 mol % of silyl borate + [BPC][AlCl3]f
0.1 mol % of silyl borate + [BPC][AlCl3]f
0.2 mol % of silyl borate + [BPC][AlCl3]f
0.3 mol % of silyl borate + [BPC][AlCl3]f
0.5 mol % of silyl borate + [BPC][AlCl3]f
0.75 mol % of silyl borate + [BPC][AlCl3]f
1 mol % of silyl borate + [BPC][AlCl3]f
1 mol % of silyl borate + [BPC][AlCl3]g
1 mol % of silyl borate + [BPC][AlCl3]h
10
11
12
86, 3.5
50, 4.5
78:22
65:35
a Standard error of 5%; the reactions were carried out in a 1 mmol scale in 1.5 mL of ionic liquid. b Isolated yields. c No further increase in yields after
the times reported. d Determined by GC analysis. e In toluene 1 mL. f Acidic, AlCl3 X (mole fraction) ) 0.6. g Neutral, AlCl3 X ) 0.5. h Basic, AlCl3 X )
0.45.
TABLE 3. Diels-Alder Reactionsa Catalyzed by Silyl Catalyst in Acidic Chloroaluminate at 27 °C
b
107 k2
entry
catalyst/ionic liquid
(M-1 s-1
)
yieldc (%), timed (h)
endo:exoe
reaction: 1 + 2b
1
2
3
silyl boratef
6.32
9.87
45, 7
47, 5.5
60, 4
40:60
75:25
80:20
[BPC][AlCl3]g
silyl borate + [BPC][AlCl3]g,h
24.15
reaction: 1 + 2c
10.56
4
5
6
silyl boratef
45, 5
57, 5
70, 2.5
80:20
86:14
93:7
[BPC][AlCl3]g,h
18.40
38.44
silyl borate + [BPC][AlCl3]g
reaction: 1 + 6
79.27
7
8
9
silyl boratef
87, 2
90, 2
98, 1
85:15
83:17
98:2
[BPC][AlCl3]g,h
71.52
179.5
silyl borate + [BPC][AlCl3]g
reaction: 8 + 9i
43.21
10
11
12
silyl boratef
95, 30 min
92, 30 min
99, 10 min
-
[BPC][AlCl3]g
37.34
92.54
silyl borate + [BPC][AlCl3]g
reaction: 8 + 2aj
13
14
silyl boratef
2.05k
3.53
91, 13j
96, 8
-
silyl borate + [BPC][AlCl3]g
a Diene ) dienophile ) 8 mmol, silyl borate catalyst ) 0.3 mol % used in all experiments given in this table. b Standard error of 5%, ionic liquid ) 1.5
mL. c Yields determined by NMR spectroscopy. d No further increase in yields after the times reported. e Determined by GC analysis. f Toluene ) 1 mL.
g Acidic XAlCl3 ) 0.6. h Rrom ref 2b. i 8 ) 6 mmol, 9 ) 2 mmol. j From ref 1. k Measured by us, not mentioned in ref 1.
product with 80% endo-stereoisomer. It may be noted that the
reaction of 1 with 2b, which offers predominant exo-stereoi-
somer (60%) in silyl borate (Table 3, entry 1) as in organic
solvents,4 now proceeds with higher endo-stereoselective ste-
reoisomer (80%) in the mixture of silyl borate with the acidic
[BPC][AlCl3]. This is clearly an improvement over our recent
work, in which we reported the same reaction carried out in
chloroaluminate ionic liquids.2d
The reaction of 1 with methyl trans-crotonate 2c proceeded
in silyl borate + acidic chloroaluminate to give 70% of
cycloadduct in 2.5 h with endo:exo ratio of 93:7. This reaction
in silyl borate + acidic chloroaluminate proceeded to give 70%
of cycloadduct in 2.5 h with endo:exo ratio of 93:7.
zolium tetrafluoroborate [HMI][BF4] is reported6 to give 95%
cycloadduct with endo:exo ratio of 93:7. As clear from the data
given in Table 3, the reaction in silyl borate + acidic
chloroaluminate proceeded faster with comparable yields and
the endo/exo ratio as compared to that in the acidic chloroalu-
minate ionic liquid or the catalyst alone.
The reaction of 2,3-dimethylbutadiene 8 with 1,4-naphtho-
quinone 9 proceeded in silyl borate + acidic chloroaluminate
to give >99% cycloadduct in 10 min when compared to the
one catalyzed by silyl borate alone. The use of scandium triflate
+ [BMIM][BF4] has been reported to offer 96% cycloadduct
with 99% as the endo stereoisomer.7
In order to demonstrate the utility of ionic liquids in enhancing
the catalytic performance, we repeated the reported reaction of
8 with 2a under the reported reaction conditions.1 The reaction
proceeded in a mixture of 0.3 mol % of silyl borate with acidic
[BPC][AlCl3] to give 96% yield in just 8 h as compared to 91%
The reaction of 1 with methyl vinyl ketone 6 was noted to
be 2-fold faster in the silyl borate catalyst + acidic chloroalu-
minate as compared to that in the silyl borate catalyst. This
reaction as catalyzed by scandium triflate in 1-hexyl-3-imida-
(6) Silvero, G.; Arevalo, M. J.; Bravo, J. L.; Avalos, M.; Jimenez, J. L.;
Lopez, I. Tetrahedron 2005, 61, 7105.
(7) Song, C. E.; Shim, W. H.; Roh, E. J.; Lee, S.; Choi, J. H. Chem.
Commun. 2001, 1122.
J. Org. Chem, Vol. 72, No. 21, 2007 8113

of ionic liquid) and was allowed to equilibrate at the desired
temperature controlled to (0.01 K. The reaction was initiated by
addition of diene (1 mmol in 1 mL). The reaction progress was
monitored at appropriate time intervals by extraction of aliquots
with ether followed by appropriate dilution and GC analysis. The
reported k2 value is an average of triplicate runs with Fs standard
error of 5%. The endo and exo products for the reactions of 1 with
2a and 2b were characterized by NMR.10,11 However, for the
reaction of 1 with 2c, the stereochemical assignments were made
by condensing crotonic acid with 1 and the resultant endo and exo
bicyclic acids separated by iodolactonization as described by Evans
et al. and us.12,13 The 1H NMR spectrum of endo-ester is also
reported by Ikota.14
yield in 13 h in silyl borate alone, suggesting that a suitable
ionic liquid can further promote the catalyzed reaction with an
increase in yield and reduction in reaction time.
We also used the recycled acidic chloroaluminate by extract-
ing the product with Et2O.2c,5 The catalyzed reaction of 1 with
2a in the recycled condition gave 91-95% yield for up to seven
recycles (see Supporting Information), thus confirming the active
role of the ionic liquid in promoting the reaction.
Above, we have shown that (i) the acidic chloroaluminate
ionic liquid can further enhance the catalytic power of an
expensive silyl borate catalyst for carrying out Diels-Alder
reactions; (ii) less amount of catalyst in the above ionic liquid
is required to obtain optimum results; (iii) this is an improvement
on the recent work of Hara et al.1 as evidenced by repeating
the reaction carried out by them; and (iv) the used ionic liquid
can be recovered and reused in promoting Diels-Alder reactions
to offer comparable yields. This study can be expanded to cover
more Lewis acid sensitive organic reactions.
Acknowledgment. Thanks are due to Dr. M. Heilig for his
help in preparing the catalyst.
Supporting Information Available: Experimental details, data,
1H NMR spectra of products, and GC analysis of sample product.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Experimental Section
JO071099W
We prepared the catalyst by the procedure of Lambert and
Zhang.8 In a typical run, first the dienophile (8 mmol) was added
to the mixture containing the catalyst prepared in toluene (1 mL)
and the ionic liquid (1.5 mL). To this was added the diene (8 mmol),
and the reaction mixture was magnetically stirred at room temper-
ature. For the reaction of 8 with 9, 6 mmol of 8 and 2 mmol of 9
were used for obtaining quantitative yields.9,10 For a standard kinetic
run, the dienophile (1 mmol) was added to the ionic liquid (1 mL
(9) Pawar, S. S.; Phalgune, U. D.; Kumar, A. J. Org. Chem. 1999, 64,
7055.
(10) Nakagawa, K.; Ishii, Y.; Ogawa, M. Tetrahedron 1976, 32, 1427.
(11) Allen, C. F. H.; Bell, A. Organic Syntheses; Wiley: New York,
1955; Collec. Vol. III, p 310. (b) Casashi, A.; Desimomi, G.; Faita, G.;
Ivernizzi, A. G.; Lanati, S.; Righetti, P. P. J. Am. Chem. Soc. 1993, 115,
8002.
(12) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988,
110, 1238.
(13) Sarma, D.; Kumar, A. Org. Lett. 2006, 8, 2199.
(14) Ikoto, N. Chem. Pharm. Bull. 1989, 37, 2219.
(8) Lambert, J. B.; Zhang, S. Chem. Commun. 1993, 383.
8114 J. Org. Chem., Vol. 72, No. 21, 2007