Synthesis and evaluation of a chiral menthol functionalized silsesquioxane: Application to diastereoselective [2+2] photocycloaddition

Article abstract of DOI:10.1007/s11164-012-0635-5

A novel chiral menthol derivative supported on silsesquioxane was easily synthesized by hydrosilylation of the corresponding arylmenthyl olefin with commercially available silsesquioxane-type hydrosilane. We evaluated the applicability of the silsesquioxane-based compound to diastereoselective [2+2] photocycloaddition reaction. The newly synthesized silsesquioxane-based chiral menthol derivative 9 indicated similar diastereoselectivity to the corresponding p-methoxyphenyl menthol derivative.

Full text of DOI:10.1007/s11164-012-0635-5

Res Chem Intermed (2013) 39:101–110
DOI 10.1007/s11164-012-0635-5
Synthesis and evaluation of a chiral menthol
functionalized silsesquioxane: application
to diastereoselective [2+2] photocycloaddition
•
Yuuki Yanagisawa Hiroki Yamaguchi
•
•
Yasuhiro Nishiyama Tsumoru Morimoto
•
•
Kiyomi Kakiuchi Kenji Tabata Ken Tsutsumi
•
Received: 4 August 2011 / Accepted: 4 October 2011 / Published online: 16 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract A novel chiral menthol derivative supported on silsesquioxane was
easily synthesized by hydrosilylation of the corresponding arylmenthyl olefin with
commercially available silsesquioxane-type hydrosilane. We evaluated the
applicability of the silsesquioxane-based compound to diastereoselective [2?2]
photocycloaddition reaction. The newly synthesized silsesquioxane-based chiral
menthol derivative 9 indicated similar diastereoselectivity to the corresponding
p-methoxyphenyl menthol derivative.
Keywords Menthol auxiliary ꢀ Silsesquioxane ꢀ Organic–inorganic hybrid
material ꢀ Diastereoselectivity ꢀ Photocycloaddition
Introduction
Silsesquioxane is the general term for an organosilicon polymer with the general
formula (RSiO3/2)n. These polymers have one organic substituent group on the Si
atom. In general, silsesquioxanes are soluble in organic solvent and have high
thermal and chemical stability, and their physical properties are well defined for the
introduction of substituent groups [1, 2]. Silsesquioxanes have been widely used in
the electronics industry as resistors, interlayer insulators, optical materials, resins,
and rubbers. Silsesquioxanes have a similar structure to the active sites assumed in
solid catalysts such as zeolites and silica-supported catalysts, and therefore have
Y. Yanagisawa ꢀ H. Yamaguchi ꢀ Y. Nishiyama ꢀ T. Morimoto ꢀ K. Kakiuchi (&)
Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan
e-mail: kakiuchi@ms.naist.jp
K. Tabata ꢀ K. Tsutsumi
Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, 1-1 Gakuen
Kibanadai-Nishi, Miyazaki 889-2155, Japan
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Y. Yanagisawa et al.
been studied as molecular model compounds of silica-supported catalysts or
precursors of heterogeneous catalysts [3]. The structures of silsesquioxanes have
been characterized using the following terms, as illustrated in Fig. 1 cage-type (T8),
partial cage-type, random type, and ladder type.
In particular, the cage structure (T8) is a molecular model for silica-grafted
compounds in heterogeneous materials. Furthermore, there is substantial interest in
cage-type octafunctional silsesquioxanes, where all the side groups (R) on the
silicon atoms are identical and reactive. Although a large number of possible side
groups can be introduced, ranging from alkyls, alcohols, amides, and carboxylates
to halides, nitrates, and phosphanes [4–19], there are few reports regarding
silsesquioxanes containing chiral groups [20].
Thus far, we have systematically investigated asymmetric [2?2] photocycload-
ditions, and clarified the utility of menthol derivatives as a chiral auxiliary on these
reactions [21–23]. Chiral menthyl auxiliaries have been widely applied not only to
photoreactions but also to various thermal reactions [24, 25], such as Michael
additions [26], radical reactions [27], Diels–Alder reactions [28], and Friedel–Crafts
reactions [29].
From these viewpoints, we focused on the combination of silsesquioxane and
chiral menthyl. A novel cage-type silsesquioxane, functionalized with chiral
menthyl, was synthesized and characterized. Moreover, diastereoselective [2?2]
photocycloadditions of ethylene to cyclohexenones modified with the chiral
silsesquioxane were carried out in order to evaluate the nature of the asymmetric
introduction.
R
Si
R
R
O
OH
Si
R
Si
Si
O
O
Si
R
R
O
O
O
O
O
R
R
OH
R
Si
Si
O
O
O
OH
O
O
O
O
Si
O
Si
Si
Si
O
O
R
R
O
O
O
R
Si
Si
Si
Si
R
R
R
cage structure (T8)
partial cage structure
R
R
R
R
R
O
O
O
O
O
O
O
R
R
Si
O
Si
Si
Si
Si
O
R
Si
O
Si
O
Si
O
O
O
O
O
O
O
O
Si
R
Si
Si
Si
Si
R
O
Si
Si
R
R
R
R
random structure
ladder structure
Fig. 1 Structures of silsesquioxanes
123

Synthesis and evaluation of silsesquioxane
103
Experimental
General methods
All reactions were carried out under a nitrogen atmosphere. Solvents were distilled
from appropriate reagents. Reagents were used without further purification. 1H
NMR and 13C NMR spectra were recorded on a JEOL LNM-ECP500, and a JEOL
JNM-ECP600NK. 1H NMR spectra are reported as a chemical shift in ppm based on
the peak of TMS (d = 0.0 ppm), integration, multiplicity (s = singlet, brs = broad
singlet, d = doublet, t = triplet, dd = doublet of doublet, ddd = doublet of doublet
of doublets, m = multiplet), and coupling constant (Hz). 13C NMR spectra are
reported as chemical shifts in ppm based on the middle peak of CDCl3
(d = 77.0 ppm). TOF–MS spectra were performed with a Bruker Daltonics
Autoflex II. Infrared spectra were obtained on a JASCO FTIR-420 spectrometer;
absorption peaks are reported in reciprocal centimeters. Optical rotation was
obtained on a JASCO DIP-1000. UV/Vis and CD spectra were measured in a quartz
cell (10 mm thickness) on a JASCO V-630 and a JASCO J-725, respectively.
Syntheses of materials
(–)-8-[(p-Methoxy)phenyl]menthol 1 was prepared from (R)-(?)-pulegone accord-
ing to the modified Corey’s procedure [30]. After the protection of the secondary
alcohol, demethylation was carried out with boron tribromide to produce (–)-8-[(p-
(hydroxy)phenyl]menthyl acetate 2 [22].
Synthesis of chiral menthyl acetate derivative 4
Menthyl acetate 2 (435 mg, 1.5 mmol), Cs2CO3 (1,950 mg, 6.0 mmol), and
11-bromo-1-undecene 3 (0.7 mL, 3.0 mmol) were added to N, N-dimethylformamide
(DMF, 20.0 mL). The mixture was stirred for 3 h at 80 °C. After addition of water, the
mixture was extracted with ethyl acetate. The organic layer was dried over MgSO4 and
concentrated. The product was purified by flash column chromatography on silica gel
using ethyl acetate–hexane as an eluent (90 %). Colorless oil; 1H NMR (CDCl3, ppm)
d 7.17 (2H, d, J = 8.6 Hz), 6.81 (2H, d, J = 8.6 Hz), 5.85–5.77 (1H, m), 4.99 (1H, dd,
J = 17.4, 1.5 Hz), 4.93 (1H, dd, J = 9.8, 1.5 Hz), 4.78 (1H, ddd, J = 10.5, 10.5,
4.7 Hz), 3.92 (2H, t, J = 6.4 Hz), 2.05–2.03 (2H, m), 1.94–1.91 (1H, m), 1.87–1.85
(1H, m), 1.76–1.74 (2H, m), 1.67–1.64 (2H, m), 1.62 (3H, s), 1.43–1.38 (7H, m), 1.29
(9H, m), 1.19 (3H, s), 1.09–1.04 (1H, m), 0.96–0.93 (1H, m), 0.86 (3H, d, J = 6.7 Hz),
0.83–0.80 (1H, m); 13C NMR (CDCl3, ppm) d 143.49, 139.22, 126.29 (2C), 114.12,
113.80 (2C), 74.18, 67.93, 50.42, 41.78, 39.08, 34.54, 34.41, 33.80, 31.27, 29.51,
29.41, 29.38, 29.36, 29.34, 29.10, 28.91, 27.81, 26.60, 26.05, 25.43, 21.79, 21.32.
Synthesis of chiral menthol derivative 5
Menthyl acetate derivative 4 (1.72 g, 3.9 mmol) and NaOH (0.78 g, 19.5 mmol)
were added to i-PrOH (80 mL). The mixture was stirred for 5 h at 80 °C. After
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Y. Yanagisawa et al.
addition of 1 N HCl, the mixture was extracted with ethyl acetate. The organic layer
was dried over MgSO4 and concentrated. The product was purified by flash column
chromatography on silica gel using ethyl acetate–hexane as an eluent (95 %).
Colorless oil;1H NMR (CDCl3, ppm) d 7.30 (2H, d, J = 8.6 Hz), 6.85 (2H, d,
J = 8.6 Hz), 5.86–5.78 (1H, m), 4.99 (1H, dd, J = 17.0, 2.3 Hz), 4.93 (1H, dd,
J = 10.5, 2.3 Hz), 3.92 (2H, t, J = 6.4 Hz), 3.52 (1H, ddd, J = 10.1, 10.1, 4.3 Hz),
2.05–2.03 (2H, m), 1.84–1.83 (1H, m), 1.77–1.74 (3H, m), 1.68–1.65 (2H, m), 1.56
(1H, brs), 1.44–1.41 (3H, m), 1.38 (3H, s), 1.36–1.28 (9H, m), 1.26 (3H, s),
1.05–1.03 (2H, m), 0.96–0.86 (1H, m), 0.88 (3H, d, J = 6.7 Hz), 0.86–0.80 (1H,
m); 13C NMR (CDCl3, ppm) d 146.87, 139.24, 126.74 (2C), 114.36 (2C), 112.05,
72.92, 67.88, 54.12, 45.15, 39.01, 35.18, 34.91, 33.80, 31.44, 29.51, 29.45, 29.42,
29.37, 29.31, 29.11, 28.91, 26.48, 26.39, 26.06, 22.01; [a]2D4 = -7.4 (c = 0.9,
MeOH).
Synthesis of silsesquioxane-based chiral menthol derivative 7
Octakis(dimethylsilyloxy)silsesquioxane 6 (100 mg, 0.1 mmol), olefin 5 (400 mg,
1.0 mmol), and platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Pt(dvs)) (74
lL, 0.005 mmol) were added to toluene (10 mL). The mixture was stirred for 24 h
at room temperature. The mixture was filtered using Florisil. The product was
purified by HPLC (CHCl3) (73 %). Colorless oil; IR (neat, cm-1) 3551, 2921, 1511,
1455, 1250, 1183, 1097, 831, 558; 1H NMR (CDCl3, ppm) d 7.29 (16H, d,
J = 8.6 Hz), 6.84 (16H, d, J = 8.6 Hz), 3.90 (16H, t, J = 6.7 Hz), 3.51 (8H, ddd,
J = 10.2, 10.2, 4.6 Hz), 1.88–1.79 (8H, m), 1.79–1.70 (24H, m), 1.70–1.58 (16H,
m), 1.39–1.24 (192H, m), 1.05–1.00 (8H, m), 0.90–0.85 (40H, m), 0.59 (16H, t,
J = 7.6 Hz), 0.12 (48H, m); 13C NMR (CDCl3, ppm) d 157.13 (8C), 142.80 (8C),
126.71 (16C), 114.30 (16C), 72.87 (8C), 67.85 (8C), 54.06 (8C), 45.16 (8C), 39.00
(8C), 34.89 (8C), 33.50 (8C), 31.43 (8C), 29.73 (8C), 29.71 (8C), 29.66 (8C), 29.49
(8C), 29.44 (8C), 29.35 (8C), 26.39 (8C), 26.11 (8C), 23.91 (8C), 22.97 (8C), 22.00
(8C), 17.69 (8C), -0.03 (8C), -0.34 (16C); MALDI–TOF MS (m/z): 4243.3
(M ? Na)?; Anal. Calcd for C232H408O36Si16: C, 65.98; H, 9.74. Found: C, 64.79;
H, 9.69.
Synthesis of silsesquioxane-based chiral menthyl cyclohexenecarboxylate
derivative 9
Silsesquioxane-based chiral menthol derivative 7 (41 mg, 0.01 mmol), cyclohexen-
3-one-1-carboxylic acid 8 (14.0 mg, 0.10 mmol), diisopropylcarbodiimide (DIPC,
371 mg, 0.24 mmol), and N, N-dimethyl-4-aminopyridine (DMAP, 9 mg,
0.064 mmol) were added to CH2Cl2 (5 mL). The mixture was stirred for 12 h at
room temperature, and filtered using Florisil. The product was purified by HPLC
(CHCl3) (74 %). Colorless oil; IR (neat, cm-1) 2923, 2853, 1711, 1685, 1512, 1456,
1
1252, 1225, 1184, 1093, 830, 788, 556; H NMR (CDCl3, ppm) d 7.12 (16H, d,
J = 8.6 Hz), 6.71 (16H, d, J = 8.6 Hz), 6.23 (8H, s), 4.98 (8H, ddd, J = 10.5, 10.5,
4.5 Hz), 3.81 (16H, t, J = 6.7 Hz), 2.38–2.20 (24H, m), 2.14–2.13 (8H, m),
2.06–2.01 (8H, m), 1.89–1.84 (32H, m), 1.72–1.70 (24H, m), 1.47–1.12 (192H, m),
123

Synthesis and evaluation of silsesquioxane
105
1.01–0.95 (16H, m), 0.88 (24H, d, J = 6.7 Hz), 0.59 (16H, t, J = 7.6 Hz), 0.12
(48H, s); 13C NMR (CDCl3, ppm) d 200.15 (8C), 165.29 (8C), 156.58 (8C), 148.77
(8C), 143.36 (8C), 132.32 (8C), 126.14 (16C), 113.64 (16C), 74.94 (8C), 67.65
(8C), 50.43 (8C), 41.74 (8C), 38.81 (8C), 37.50 (8C), 34.41 (8C), 33.51 (8C), 31.28
(8C), 29.77 (8C), 29.74 (8C), 29.70 (8C), 29.65 (8C), 29.57 (8C), 29.47 (8C), 26.32
(8C), 26.19 (8C), 24.35 (8C), 23.54 (8C), 23.00 (8C), 21.90 (8C), 21.76 (8C), 17.70
(8C), -0.02 (8C), -0.34 (16C); MALDI–TOF MS (m/z): 5223.5 (M ? Na)?, Anal.
Calcd for C288H456O52Si16: C, 66.52; H, 8.84. Found: C, 66.45; H, 8.77; [a]D22
-286.2 (c = 0.9, MeOH).
=
Diastereoselective [2?2] photocycloaddition
Photoreaction of 9 with ethylene: Irradiations ([280 nm) were carried out in a
Pyrex flask installed in a water-cooled quartz immersion apparatus, using a HALOS
500-W high-pressure Hg lamp as the light source (Scheme 3). A 0.006 M solution
of 9 (0.008 mmol) was purged with ethylene for 5 min at 25 °C and irradiated at a
given temperature under ethylene until the enone was almost completely consumed.
The amount of dissolved ethylene was estimated to be large excess over that of 9
[32, 33]. The reaction was monitored by TLC on silica gel. After the solvent was
evaporated, the residue was purified chromatographically to produce a photoadduct.
The diastereomeric excess (de) value of 10 was determined by 1H NMR
1
spectroscopy. Colorless oil; H NMR (CDCl3, ppm) d 7.04 (16H, d, J = 8.5 Hz),
6.99 (5H, d, J = 8.5 Hz), 6.66 (16H, d, J = 8.5 Hz), 6.59 (5H, d, J = 8.5 Hz), 4.90
(8H, m), 3.76 (16H, m), 3.69 (5H, m), 2.95 (8H, m), 2.82 (3H, m), 2.39 (8H, m),
2.24 (16H, m), 2.11 (16H, m), 2.02–1.81 (48H, m), 1.78–1.72 (16H, m) 1.69–1.65
(8H, m), 1.57 (16H, m), 1.43 (24H, s), 1.32 (144H, m), 1.19 (24H, s) 1.00 (16H, m),
0.84 (32H, m), 0.59 (16H, m), 0.12 (48H, s); 13C NMR (CDCl3, ppm) d 175.38,
175.05, 157.04, 156.92, 143.37, 138.18, 129.35, 128.54, 126.68, 126.49, 125.61,
114.12, 113.98, 75.64, 68.17, 50.77, 50.42, 48.77, 46.24, 46.12, 42.09, 42.05, 39.69,
39.22, 34.79, 33.86, 31.62, 30.68, 30.09, 29.83, 28.77, 28.55, 27.74, 27.27, 26.97,
26.49, 23.33, 22.12, 21.69, 21.45, 21.19, 21.05, 18.04, 0.32 (These NMR spectra
were measured as mixture product of major isomer and minor isomer.); MALDI–
TOF MS (m/z): 5446.6 (M ? Na)?.
Results and discussion
Synthesis and characterization of chiral menthol functionalized silsesquioxane
One versatile method of introducing organic functional groups onto the silsesquiox-
ane molecule is based on platinum-catalyzed hydrosilylation of a Si–H to an
unsaturated compound such as alkene or alkyne [34]. We initially prepared the
menthol auxiliary containing an alkenyl group such as 5 (Scheme 1). The synthesis
of 5 was carried out according to our previous method for the preparation of similar
compounds [22]. First, (–)-8-[(p-methoxy)phenyl]menthol 1 was synthesized from
(R)-(?)-pulegone according to the reported procedure [21]. The protection of the
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Y. Yanagisawa et al.
secondary alcohol 1, followed by demethylation with boron tribromide produced the
known compound 2 [22]. The undecenyl group was introduced into 2, then removal
of the acetyl protecting group produced the desired product 5.
We synthesized a novel chiral menthol derivative supported on a cage-type
silsesquioxane, as shown in Scheme 2. Introduction of a chiral menthol group to
silsesquioxane was carried out by platinum-catalyzed hydrosilylation reaction of the
menthol 5 with octakis(dimethylsilyloxy)silsesquioxane 6. In the 1H NMR spectrum
of 7, a peak of the methine proton (CHOH) of menthol groups appeared at d
3.51 ppm. The MALDI–TOF MS of 7 indicated that eight menthol groups were
attached to a silsesquioxane core. The elemental analysis also showed the expected
composition. The condensation reaction of the silsesquioxane-functionalized
octamenthol 7 with 8 proceeded to produce 9 in a good yield (74 %) (Scheme 2).
Diastereoselective [2?2] photocycloaddition of 9 with ethylene
We experimentally evaluated the performance of the menthol-silsesquioxane
molecule by asymmetric [2?2] photocycloaddition (Table 1). In this reaction,
1) Ac2O, DMAP,
Br
Py, rt
HO
AcO
, Cs2CO3,
HO
AcO
9
1M NaOHaq
(>99%)
3
i-PrOH, 80 °C
2) BBr3, CH2Cl2,
-78 °C ~ -25 °C
(94% conv. based)
DMF, 80 °C
(90%)
(95%)
O
O
OMe
OH
9
9
1
2
4
5
Scheme 1 Synthesis of a novel chiral menthol derivative 5
R
R
R
O
O
Si
Si
Si
O
O
R
R
O
R
Si
Si
HO
O
O
5
, Pt cat.
7
[RSiO3/2
]
8
O
O
toluene, rt
85%
R
Si
CH3
O SiH
CH3
O
O
R
Si
Si
O
R =
O
CH3
O Si
CH3
O
9
OH
R =
8
6
Cage type
Silsesquioxane
O
DIPC, DMAP
CH2Cl2, rt
74%
O
O
9
O
CH3
R = O Si
CH3
O
9
Scheme 2 Synthesis of a novel silsesquioxane-based chiral menthyl cyclohexenecarboxylate derivative
9
123

Synthesis and evaluation of silsesquioxane
107
R
R
R
O
O
Si
Si
Si
O
O
R
R
O
hν (>280 nm)
H2C CH2
R
Si
Si
O
O
(excess)
9
O
O
solvent, 2 h
R
Si
O
O
R
Si
Si
O
10
O
O
O
O
+
O
O
CH3
R = O Si
CH3
CH3
R = O Si
CH3
O
9
O
9
Scheme 3 Diastereoselective [2?2] photocycloaddition
Table 1 Comparison between 9 and 11 of the diastereoselectivity for [2?2] photocycloaddition
Entry
Enone (Conc./M)
Solvent
Temp (°C)
Yield (%)a
De (%)b
1
Methylcyclohexane
CH2Cl2
-78
-78
-78
-60
-40
-20
-78
-78
83
85
84
88
85
90
96
95
71
54
80
77
63
49
81
50
2
3
9 (0.006)
Toluene
4
5
6
7c
8c
Methylcyclohexane
CH2Cl2
O
O
O
OMe
(0.05)
11
9
Toluene
-78
-60
-40
-20
99
92
98
99
75
69
60
47
10
11
12
Excess amount of ethylene were dissolved. See Refs. [31, 32]
a
Isolated yield
Determined by 1H NMR
b
c
Ref. [23]
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Y. Yanagisawa et al.
diastereoselectivity was induced by shielding the attack of an ethylene molecule
from one side due to the phenyl ring of the auxiliary, and it was dependent on the
conformation of the substrate [23]. To enhance the electrostatic and/or dipole–
dipole interactions between the C=O moiety of cyclohexenone and the aromatic
ring, less polar solvent produced good selectivity (entries 1–3). Toluene was the best
solvent for selectivity, due to the p–p stacking interaction between toluene and the
aromatic ring (entry 3). Lower temperature was better for selectivity because of the
thermodynamic stability of the substrate (entries 3–6). This result showed a similar
tendency to the corresponding p-methoxyphenylmenthol derivative as a chiral
auxiliary 11 (entries 7–12). Less polar solvent produced good selectivity (entries
7–9), which was enhanced at lower temperature (entries 9–12).
Moreover, we confirmed the conformation of 9 together with those of 11 (Fig. 2).
Previously, we showed from the CD spectra of 11 that its stacked conformations
A
30
CD spectra
25
20
15
10
5
θ
0
-5
-10
-15
200
220
240
260
280
300
320
340
Wavelength / nm
B
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
UV / vis spectra
200
220
240
260
280
300
320
340
Wavelength / nm
Fig. 2 CD Spectra a and UV/Vis spectra, b of 9 (1.0 9 10-5 M; red line) and 11 (1.0 9 10 -5 M; blue
line) in methylcyclohexane at room temperature. (Color figure online)
123

Synthesis and evaluation of silsesquioxane
109
were essential for high diastereoselectivity [23]. The CD spectra of 9 showed almost
the same Cotton effect as that of 11 except for the peak intensity. These results
suggest that the phenylmenthyl cyclohexenonecarboxylate moiety on silsesquioxane
9 adopts the stacked conformation in solution similar to that of 11, leading to high
diastereoselectivity.
Conclusions
We successfully prepared a novel silsesquioxane-based chiral menthol derivative
and examined the performance of the chiral silsesquioxane as a chiral auxiliary for
[2?2] photocycloaddition. The diastereoselectivity was similar to the corresponding
p-methoxyphenyl menthol derivative. The cage structures (T8) were model
compounds of silica, so we planned for the immobilization of menthols to silica.
Acknowledgments This work was supported partially by a Grant-in Aid for Scientific Research (B) No.
21310081 and for Research on Innovative areas ‘‘Organic Synthesis Based on Reaction Integration.
Development of New Methods and Creation of New Substances’’ (No. 2105) No. 22106532 from the
Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government (MEXT),
and a grant-in-aid for scientific research from Nara Institute of Science and Technology (NAIST). We
thank Mr. F. Asanoma for his assistance in 1H NMR spectroscopy, and Ms. Y. Nishiyama and Ms.
M. Yamamura for their help in TOF–MS measurements.
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