Synthesis of new chiral ionic liquids based on (-)-menthol and (-)-borneol

Article abstract of DOI:10.1016/j.tetlet.2008.01.011

New chiral ionic liquids (CILs) based on (-)-menthol and (-)-borneol were designed and synthesized in very good yields using a simple and efficient 3-step strategy. The properties and characterization of these compounds are discussed.

Full text of DOI:10.1016/j.tetlet.2008.01.011

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1652–1655
Synthesis of new chiral ionic liquids based on (ꢀ)-menthol
and (ꢀ)-borneol
Ricardo Alexandre F. Matos, Carlos Kleber Z. Andrade *
ˆ
´
Universidade de Brası´lia, Instituto de Quı´mica, Laborato´rio de Quı´mica Metodolo´gica e Organica Sintetica (LaQMOS),
C.P. 4478, 70910-970 Brası´lia, DF, Brazil
Received 8 August 2007; revised 25 December 2007; accepted 3 January 2008
Available online 8 January 2008
Abstract
New chiral ionic liquids (CILs) based on (ꢀ)-menthol and (ꢀ)-borneol were designed and synthesized in very good yields using a sim-
ple and efficient 3-step strategy. The properties and characterization of these compounds are discussed.
Ó 2008 Elsevier Ltd. All rights reserved.
Nowadays, the search for green reaction media for
organic reactions has triggered the development of several
alternatives to volatile and toxic organic solvents.1 Among
these, room temperature ionic liquids (RTILs) have
emerged as promising candidates due to characteristics
such as extremely low vapor pressure, tunability of their
properties by varying the structures of the cations or
anions, and recyclability, just to mention a few.2 For these
reasons, many compounds of this type have been synthe-
sized and successfully used as solvents in organic synthe-
sis.1,3 In many cases, reaction yields and/or selectivities
were improved when compared to the same reactions in
usual organic solvents. Furthermore, being ionic in nature,
they have a polar characteristic thus dissolving a wide
range of organic and inorganic reagents.
comprise an ether functionality.6a,b The introduction of
an ester function, besides being an easy task, could perhaps
improve some of their properties especially those related to
the stabilization of polymers, which is one of our goals.
The synthesis of the ionic liquids involved a general
three-step strategy, as shown in Scheme 1.8 In the first step,
chloroacetic acid was esterified with (ꢀ)-menthol and (ꢀ)-
borneol, via DCC/DMAP coupling, furnishing chloro-
acetylmenthol and chloroacetylborneol in 81% and 96%
yields, respectively.
The corresponding esters were then reacted with 1-butyl-
imidazole or 1-methylpyrrolidine under solvent-free condi-
tions, to form the chloride ILs (80–97% yields). In this step,
a higher temperature (130 °C) had to be used in the
More recently, chiral ionic liquids (CILs) have been
designed and used as chiral environments aiming the induc-
tion of chirality in reactions involving non-chiral sub-
strates.4 Nevertheless, so far only a few have proved to
be efficient in this task.5 Therefore, the search for new CILs
seems to be demanding. Herein, we wish to report the syn-
thesis and characterization of several CILs based on (ꢀ)-
menthol6 and (ꢀ)-borneol7 (Fig. 1). Although many other
menthol-based CILs have been reported, most of them
O
O
X-
Bu
X-
N
N
N
O
O
O
O
Bu
N
N
N
X-
X-
O
O
X = Cl, BF4 or PF6
Fig. 1. Synthesis of chiral ionic liquids.
*
Corresponding author. Tel.: +55 61 3307 2155; fax: +55 61 3273 4149.
E-mail address: ckleber@unb.br (C. K. Z. Andrade).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.011

R. A. F. Matos, C. K. Z. Andrade / Tetrahedron Letters 49 (2008) 1652–1655
1653
O
O
or
DCC/DMAP
CH2Cl2, r.t.
Cl
Cl
+
OH
R1O
HO
R1 =
or
N
OH
X-
O
R2 =
or
Cl-
R2
N
N
KX
H2O, r.t. 3 h
R2
O
or
N
N
N
R1O
Bu
Bu
R1O
heat , 4 h
X = BF4 or PF6
Scheme 1. General strategy for the synthesis of CILs based on (ꢀ)-menthol and (ꢀ)-borneol.
Table 1
Structures and some properties of the CILs synthesized
20
Entry
1
Name11
Structure
Yields (%)
96
½aꢁD (c 5.0, MeOH)
Mp (°C)
[amebim]Cl
ꢀ37.4
Liquid
O
Cl-
N
N
N
N
O
O
Bu
1
O
-
BF4
2
3
4
[amebim]BF4
[amebim]PF6
[amempyr]Cl
97
86
81
ꢀ28.0
ꢀ40.0
ꢀ44.2
Liquid
Liquid
63
Bu
2
O
-
PF6
N
N
O
Bu
3
O
Cl-
N
O
4
O
-
BF4
5
6
[amempyr]BF4
[amempyr]PF6
90
72
ꢀ45.1
ꢀ53.8
64
66
N
N
O
O
5
O
-
PF6
6
7
8
[abobim]Cl
97
79
ꢀ37.4
ꢀ3.2
Liquid
Liquid
O
Bu
N
N
Cl-
7
O
[abobim]BF4
O
O
Bu
BF4
N
N
8
-
O
9
[abobim]PF6
86
ꢀ12.2
Liquid
Bu
PF6
N
N
9
-
O
(continued on next page)

1654
R. A. F. Matos, C. K. Z. Andrade / Tetrahedron Letters 49 (2008) 1652–1655
Table 1 (continued)
20
Entry
Name11
Structure
Yields (%)
80
½aꢁD (c 5.0, MeOH)
Mp (°C)
10
[abompyr]Cl
ꢀ31.3
67
O
N
N
10
Cl-
O
11
12
[abompyr]BF4
[abompyr]PF6
86
74
ꢀ11.1
ꢀ12.5
88
84
O
11
-
BF4
O
O
N
12
-
PF6
O
reactions of the borneol series as compared to the menthol
series (80 °C) to get a more homogenous stirring. The last
step was the anion exchange, which was easily accomplished
in aqueous media using either KBF4 or KPF6. All imidazol-
ium-derived ILs were obtained as viscous brownish liquids9
(79–96% yields), whereas those derived from the N-methyl-
pyrrolidine cation were low melting solids (mp <90 °C, 72–
90% yields). The structures and some main characteristics
of the compounds synthesized are shown in Table 1. All
compounds had their structures confirmed by IR, mass
(2 ppm) anions, showing a strict relation between the IL
structure and the size of the anion. The 13C NMR spectra
completed the analysis of the compounds, showing peaks
corresponding to the imidazolic carbons (d 120–140), to
the acyl groups (d 164–168), and to the carbinolic carbons
(d 76–84).
In conclusion, 12 chiral ionic liquids based on (ꢀ)-men-
thol or (ꢀ)-borneol were synthesized in a straightforward
3-step strategy under very mild conditions and in very good
yields. The application of these compounds as polymers
stabilizers and as chiral solvents in some organic reactions
is currently underway and will be published in due course.
1
spectra, H NMR, and 13C NMR analysis.10 The main IR
bands were the stretch of the carbonyl groups which ranged
from 1740 to 1751 cmꢀ1 and those of the C–O stretch in the
region between 1020 and 1230 cmꢀ1
.
Acknowledgments
Structural investigation using NMR spectroscopy
unequivocally confirmed the structures of the CILs. The
appearance of peaks of the imidazolic hydrogens (com-
pounds 1–3 and 7–9) and of the attached butyl group con-
firmed the coupling of the N-butylimidazole into the
molecule. The diastereomeric methylenic peaks between
the nitrogen and the acyl groups appear between d 4.21
and 5.50 as singlets for the ILs of the borneol series and as
two doublets (J = 17–18 Hz) for the menthol series (except
for [amebim]PF6 (entry 3) and [amempyr]PF6 (entry 6), in
which they appear as singlets). The CHs linked to the oxygen
ester appear as triplets of doublets (menthol series, d 4.77–
4.82) or as multiplets (borneol series, d 4.92–5.04). For the
ILs based on the pyrrolidinic nucleus, the N-methyl group
appears at d 3.27–3.51. Interestingly, the specific rotations
for all compounds in both series showed negative values12
(Table 1).
The authors would like to acknowledge Otilie E. Vercillo
and Jurgen Schmidt from Institute of Plant Biochemistry
¨
(IPB, Germany) for ESI-MS spectra, FINEP CT/INFRA
0970/01, CNPq, CAPES and IQ/UnB for financial support.
References and notes
1. Andrade, C. Z. K.; Alves, L. M. Curr. Org. Chem. 2005, 9, 195–
218.
2. (a) Dupont, J.; Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102,
3667–3692; (b) Freemantle, M. Chem. Eng. News 2004, November, 44–
49.
3. Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-
VCH: Weinheim, 2003.
4. For reviews on CILs, see: (a) Baudequin, C.; Baudoux, J.; Levillain,
J.; Cahard, D.; Gaumont, A.-C.; Plaquevent, J.-C. Tetrahedron:
´
Asymmetry 2003, 14, 3081–3093; (b) Baudequin, C.; Bregeon, D.;
Levillain, J.; Guillen, F.; Plaquevent, J.-C.; Gaumont, A.-C. Tetra-
hedron: Asymmetry 2005, 16, 3921–3945; (c) Ding, J.; Armstrong, D.
W. Chirality 2005, 17, 281–292.
1
However, the most important characteristic of the H
NMR spectra is the d shift of the acidic hydrogen of the
imidazolic group. A significant upfield variation was
observed when the chloride anion was exchanged with the
tetrafluoroborate (0.9 ppm) or hexafluorophosphate
5. (a) Ni, B.; Zhang, Q.; Headley, A. D. Green Chem. 2007, 9, 737–739;
(b) Luo, S.; Mi, X.; Zhang, L.; Liu, S.; Xu, H.; Cheng, J.-P. Angew.
Chem., Int. Ed. 2006, 45, 3093–3097; (c) Malhotra, S. V.; Wang, Y.
Tetrahedron: Asymmetry 2006, 17, 1032–1035.

R. A. F. Matos, C. K. Z. Andrade / Tetrahedron Letters 49 (2008) 1652–1655
1655
6. For the synthesis of other ionic liquids based on (ꢀ)-menthol, see: (a)
Ding, J.; Desikan, V.; Han, X.; Xiao, T. L.; Ding, R.; Jenks, W. S.;
Armstrong, D. W. Org. Lett. 2005, 7, 335–337; (b) Pernak, J.; Feder-
Kubis, J. Chem. Eur. J. 2005, 11, 4441–4449; (c) Ma, H.-Y.; Wan,
X.-H.; Chen, X.-F.; Zhou, Q.-F. Chin. J. Polym. Sci. 2003, 21, 265–
270.
7. A borneol sulfonamide CIL has been reported: Gadenne, B.;
Hesemann, P.; Moreau, J. J. E. Tetrahedron Lett. 2004, 45, 8157–
8160.
164.7, 77.1, 65.0, 64.9, 62.0, 48.8, 46.4, 40.3, 33.6, 31.2, 25.9, 22.8,
21.6, 21.2, 21.1, 20.6, 15.8.
Acetylmentholmethylpyrrolidine tetrafluoroborate, [amempyr]BF4 (7):
20
yield = 90%; ½aꢁD ꢀ45.1 (c 5.0, MeOH); mp = 64 °C; IR (KBr): mmax
1742, 1219, 1062 cmꢀ1 1H NMR (CDCl3): d 4.81 (td, J = 11.2 and
;
4.7 Hz, 1H), 4.66 (d, J = 17.0 Hz, 1H), 4.52 (d, J = 17.0 Hz, 1H),
4.08–3.85 (m, 4H), 3.40 (s, 3H), 2.40–0.80 (m, 13H), 0.92 (d,
J = 6.7 Hz, 3H), 0.90 (d, J = 6.9 Hz, 3H), 0.75 (d, J = 7.2 Hz, 3H);
13C NMR (CDCl3): d 164.5, 77.4, 65.3, 62.0, 55.1, 49.1, 46.5, 40.3,
33.7, 31.3, 26.0, 23.0, 21.7, 21.4, 21.3, 20.6, 15.9.
8. General experimental procedure for the synthesis of the CILs: To a
solution of 64.0 mmol of (ꢀ)-menthol (10.00 g) or (ꢀ)-borneol
(9.86 g) in dry dichloromethane (50 mL) were added, under stirring
and at 0 °C, 83.2 mmol of chloroacetic acid (7.82 g), 83.2 mmol of
DCC (17.2 g) and 6.4 mmol of DMAP (0.78 g). Stirring was contin-
ued for 12 h at room temperature. The suspension was filtered under
vacuum and washed with dry dichloromethane. The organic phase
was then washed with 10% HCl (50 mL), satd NaHCO3 (50 mL) and
brine (50 mL), dried over Na2SO4, filtered and concentrated. The
residue was heated at 80 °C (130 °C for (ꢀ)-borneol) under stirring in
the presence of 1.0 equiv of butylimidazole for 4 h, giving a brown oil
or a solid. For the ion exchange, the residue was stirred in water
(10 mL) at rt for 3 h with 1.0 equiv of either potassium tetrafluoro-
borate or potassium hexafluorophosphate. After water evaporation,
the residue was diluted in CH2Cl2 (10 mL), filtered under vacuum
over a pad of CeliteÒ, dried over Na2SO4 and concentrated to give the
corresponding chiral ionic liquids, which were further dried under
vacuum at 80 °C for several hours. This procedure yielded all
compounds in high purity without the need for further purification.
Analytical data reported in Ref. 10 were obtained after submitting the
samples to simple column chromatography eluting with hexanes/
EtOAc (7:3).
Acetylmentholmethylpyrrolidine hexafluorophosphate, [amempyr]PF6
20
(8): yield = 72%; ½aꢁD ꢀ53.8 (c 5.0, MeOH); mp = 66 °C; IR (KBr):
mmax 1744, 1220, 1021, 840, 558 cmꢀ1 1H NMR (CDCl3): d 4.82 (td,
;
J = 11.0 and 5.0 Hz, 1H), 4.21 (s, 2H), 3.87–3.76 (m, 4H), 3.27 (s,
3H), 2.36–2.21 (m, 2H), 2.05–0.87 (m, 11H), 0.95 (d, J = 6.1 Hz, 3H),
0.92 (d, J = 7.2 Hz, 3H), 0.76 (d, J = 6.6 Hz, 3H); 13C NMR (CDCl3):
d 164.1, 77.8, 65.6, 62.2, 56.2, 49.5, 46.6, 40.2, 33.8, 31.4, 26.0, 23.1,
21.8, 21.5, 21.4, 20.6, 15.9.
Acetylborneolbutylimidazole chloride, [abobim]Cl (9): yield = 97%;
20
½aꢁD ꢀ37.4 (c 5.0, MeOH); IR (film): mmax 1744, 1168, 1085 cmꢀ1;1H
NMR (CDCl3): d 10.51 (s, 1H); 7.51 (s, 1H), 7.35 (s, 1H), 5.58–5.42
(m, 2H), 5.03–4.93 (m, 1H), 4.30 (t, J = 6.9 Hz, 2H), 2.42–1.05 (m,
11H), 0.95 (t, J = 7.1 Hz, 3H), 0.89 (s, 3H), 0.88 (s, 3H), 0.85 (s, 3H);
13C NMR (CDCl3): d 168.0, 134.6, 120.7, 119.6, 83.7, 49.2, 49.0, 46.7,
44.9, 44.6, 38.3, 33.3, 31.8, 26.6, 19.8, 19.5, 19.0, 13.0, 11.1.
Acetylborneolbutylimidazole tetrafluoroborate, [abobim]BF4 (10):
20
yield = 79%; ½aꢁD ꢀ3.2 (c 5.0, MeOH); IR (film): mmax 1751, 1171,
1021, 841 cmꢀ1 1H NMR (CDCl3): d 9.62 (s, 1H), 7.56 (s, 1H), 7.45
;
(s, 1H), 5.33 (s, 2H), 5.04–4.97 (m, 1H), 4.32 (t, J = 7.3 Hz, 2H), 2.40–
1.05 (m, 11H), 1.01 (t, J = 7.1 Hz, 3H), 0.93 (s, 3H), 0.92 (s, 3H), 0.89
(s, 3H); 13C NMR (CDCl3): d 165.3, 136.8, 123.9, 121.9, 83.9, 48.7,
47.8, 46.8, 44.8, 44.6, 38.2, 33.4, 31.7, 26.8, 19.8, 19.5, 19.2, 13.3, 11.2.
Acetylborneolbutylimidazole hexafluorophosphate, [abobim]PF6 (11):
9. Two structurally similar CILs based on (ꢀ)-menthol have been
reported but were solids at room temperature (see Ref. 6c).
10. Selected spectroscopic data for compounds 3–14: Acetylmentholbutyl-
20
yield = 86%; ½aꢁD ꢀ12.2 (c 5.0, MeOH); IR (film): mmax 1749, 1285,
20
imidazolium chloride, [amebim]Cl (3): yield = 96%; ½aꢁD ꢀ37.4 (c 5.0,
1170, 840 cmꢀ1 1H NMR (CDCl3): d 8.58 (s, 1H), 7.38 (s, 1H), 7.32
;
MeOH); IR (film): mmax 3391, 1745, 1227, 1172 cmꢀ1
;
1H NMR
(s, 1H), 5.02 (s, 2H), 4.98–4.92 (m, 1H), 4.17 (t, J = 7.3 Hz, 2H), 2.40–
2.23 (m, 2H), 1.90–1.60 (m, 9H), 0.95 (t, J = 7.6 Hz, 3H), 0.88 (s, 3H),
0.87 (s, 3H), 0.84 (s, 3H); 13C NMR (CDCl3): d 166.1, 136.4, 123.8,
122.9, 83.2, 48.7, 47.7, 44.9, 44.6, 38.8, 36.0, 31.8, 31.5, 26.7, 19.5,
19.1, 19.0, 18.6, 13.1.
(CDCl3): d 10.40 (s, 1H), 7.50 (s, 1H), 7.40 (s, 1H), 5.50 (d,
J = 18.0 Hz, 1H), 5.40 (d, J = 18.0 Hz, 1H), 4.77 (td, J = 10.7 and
4.7 Hz, 1H), 4.40–4.25 (m, 2H), 2.22–1.30 (m, 9H), 1.10–0.80 (m, 4H),
0.97 (t, J = 7.6 Hz, 3H), 0.92 (d, J = 6.8 Hz, 3H), 0.90 (d, J = 7.4 Hz,
3H), 0.75 (d, J = 6.8 Hz, 3H); 13C NMR (CDCl3): d 165.6, 137.7,
123.2, 121.3, 76.7, 49.8, 49.5, 48.9, 40.1, 33.5, 31.5, 30.9, 25.6, 22.3,
21.4, 20.3, 18.8, 15.7, 13.0.
Acetylborneolmethylpyrrolidine chloride, [abompyr]Cl (12): yield =
20
80%; ½aꢁD ꢀ31.3 (c 5.0, MeOH); mp = 67 °C; IR (KBr): mmax 1746,
1219, 1108, 1058 cmꢀ1 1H NMR (CDCl3): d 5.08 (s, 2H), 5.02–4.96
;
Acetylmentholbutylimidazolium tetrafluoroborate, [amebim]BF4 (4):
(m, 1H), 4.18–4.08 (m, 4H), 3.51 (s, 3H), 2.42–1.00 (m, 11H), 0.90 (s,
3H), 0.88 (s, 3H), 0.85 (s, 3H); 13C NMR (CDCl3): d 165.4, 82.7, 64.8,
61.9, 48.6, 47.6, 44.3, 38.6, 36.0, 27.5, 26.7, 25.6, 21.1, 19.9, 19.3, 18.4,
13.1.
20
yield = 97%; ½aꢁD ꢀ28.0 (c 5.0, MeOH); IR (film): mmax 1745, 1173,
1
1063, 912, 734 cmꢀ1; H NMR (CDCl3): d 9.95 (s, 1H), 7.46 (s, 1H),
7.37 (s, 1H), 5.35 (d, J = 18.0 Hz, 1H), 5.26 (d, J = 18.0 Hz, 1H), 4.79
(td, J = 11.2 and 4.6 Hz, 1H), 4.29 (t, J = 8.0 Hz, 2H), 2.10–1.10 (m,
9H), 1.10–0.90 (m, 4H), 0.96 (t, J = 7.5 Hz, 3H), 0.92 (d, J = 7.0 Hz,
3H), 0.90 (d, J = 7.4 Hz, 3H), 0.75 (d, J = 7.0 Hz, 3H); 13C NMR
(CDCl3): d 165.7, 137.5, 123.4, 121.6, 77.1, 49.9, 49.6, 46.4, 40.2, 33.6,
31.7, 31.1, 25.7, 22.8, 21.6, 20.4, 19.1, 15.8, 13.2.
Acetylborneolmethylpyrrolidine tetrafluoroborate, [abompyr]BF4 (13):
20
yield = 86%; ½aꢁD ꢀ11.1 (c 5.0, MeOH); mp = 88 °C; IR (KBr): mmax
1
1747, 1220, 1059 cmꢀ1; H NMR (CDCl3): d 5.02–4.96 (m, 1H), 4.76
(s, 2H), 4.18–4.08 (m, 4H), 3.42 (s, 3H), 2.43–1.05 (m, 11H), 0.90 (s,
3H), 0.88 (s, 3H), 0.85 (s, 3H); 13C NMR (CDCl3): d 165.0, 82.7, 64.8,
61.8, 48.5, 47.4, 44.2, 35.8, 27.3, 26.5, 23.1, 21.1, 19.3, 19.1, 18.3, 18.2,
13.1.
Acetylmentholbutylimidazolium hexafluorophosphate, [amebim]PF6
20
(5): yield = 86%; ½aꢁD ꢀ40.0 (c 5.0, MeOH); IR (film): mmax 1746,
1173, 1040, 840 cmꢀ1; 1H NMR (CDCl3): d 8.66 (s, 1H), 7.37 (s, 1H),
7.32 (s, 1H), 4.99 (s, 2H), 4.79 (td, J = 10.5 and 5.0 Hz, 1H), 4.19 (t,
J = 7.0 Hz, 2H), 2.10–1.20 (m, 9H), 1.12–0.80 (m, 4H), 0.96 (t,
J = 7.0 Hz, 3H), 0.92 (d, J = 7.0 Hz, 3H), 0.90 (d, J = 7.5 Hz, 3H),
0.75 (t, J = 6.9 Hz, 3H); 13C NMR (CDCl3): d 165.4, 136.4, 123.8,
121.9, 76.6, 50.0, 49.7, 46.7, 40.2, 33.9, 31.5, 31.3, 25.9, 23.0, 21.7,
20.6, 19.1, 15.9,13.1.
Acetylborneolmethylpyrrolidine hexafluorophosphate, [abompyr]PF6
20
(14): yield = 74%; ½aꢁD ꢀ12.5 (c 5.0, MeOH); mp = 84 °C; IR
(KBr): mmax 1746, 1231, 840 cmꢀ1 1H NMR (CDCl3): d 5.03–4.97
;
(m, 1H), 4.28 (s, 2H), 3.90–3.65 (m, 4H), 3.27 (s, 3H), 2.41–1.05 (m,
11H), 0.89 (s, 3H), 0.88 (s, 3H), 0.85 (s, 3H); 13C NMR (CDCl3): d
164.9, 83.4, 65.6, 62.2, 48.8, 47.9, 44.7, 36.1, 27.6, 26.7, 25.8, 21.5,
20.1, 19.6, 18.7, 18.6, 13.2.
Acetylmentholmethylpyrrolidine chloride, [amempyr]Cl (6): yield =
11. The names of the CILs synthesized herein were proposed based on
their constituents and not on the IUPAC rules. In this way,
reasonable abbreviations could be proposed for each one (see Ref.
10).
12. This same trend has also been observed for all other CILs based on
(ꢀ)-menthol reported in Refs. 6a–c.
20
81%; ½aꢁD ꢀ44.2 (c 5.0, MeOH); mp = 63 °C; IR (KBr): mmax 1740,
1181, 1098 cmꢀ1 1H NMR (CDCl3): 5.11 (d, J = 17.0 Hz, 1H), 4.89
;
(d, J = 17.0 Hz, 1H), 4.81 (td, J = 11.4 and 5.4 Hz, 1H), 4.17–4.09 (m,
4H), 3.51 (s, 3H), 2.42–0.80 (m, 13H), 0.93 (d, J = 7.5 Hz, 3H), 0.90
(d, J = 7.6 Hz, 3H), 0.75 (d, J = 7.3 Hz, 3H); 13C NMR (CDCl3): d