Efficient nucleophilic substitution of -aryl alcohols with 1,3-dicarbonyl compounds catalyzed by tin ion-exchanged montmorillonite

Article abstract of DOI:10.1055/s-0030-1258567

Tin ion-exchanged montmorillonite demonstrated the high catalytic activity for the direct nucleophilic substitution of a hydroxyl group in -aryl alcohols with various 1,3-dicarbonyl compounds including less acidic 1,3-diesters in crude solvents to afford

Full text of DOI:10.1055/s-0030-1258567

LETTER
2493
Efficient Nucleophilic Substitution of a-Aryl Alcohols with 1,3-Dicarbonyl
Compounds Catalyzed by Tin Ion-Exchanged Montmorillonite
N
ucleophi
i
lic Sub
a
stitution of
c
a
-Aryl
A
lc
h
ohols
w
ith
1
e
,3-Dicarbon
n
yls g Wang, Yoichi Masui, Makoto Onaka*
Department of Chemistry, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan
E-mail: conaka@mail.ecc.u-tokyo.ac.jp
Received 29 June 2010
tution of alcohols with 1,3-dicarbonyl compounds,
homogeneous Lewis and Brønsted acid catalysts, such as
MXn (M = Cu, Zn, Ta, Au, In, Bi, Fe, or In; X = I, Cl, Br,
ClO4 or triflate),1,4,6,14–16 AgSbF6,6 p-toluenesulfonic ac-
id,13,17 trifluoromethanesulfonic acid,13,17,18 heteropoly ac-
id,19 have been applied. However, these catalysts
generally have drawbacks such as low activity, low selec-
tivity, and difficulty in their reuse, and require long reac-
tion times. While the direct substitution of alcohols with
typical active methylenes was reported to proceed in the
presence of a solid acid, proton ion-exchanged montmo-
rillonite (H-Mont), H-Mont failed to catalyze the reaction
with 1,3-diesters which have relatively higher pKa values
of a-protons among active methylenes.3,20 Therefore, it is
still required to discover an efficient solid catalyst for the
reaction of alcohols with 1,3-diesters.
Abstract: Tin ion-exchanged montmorillonite demonstrated the
high catalytic activity for the direct nucleophilic substitution of a
hydroxyl group in a-aryl alcohols with various 1,3-dicarbonyl com-
pounds including less acidic 1,3-diesters in crude solvents to afford
the benzylated products accompanied by water.
Key words: tin ion-exchanged montmorillonite, heterogeneous ca-
talysis, nucleophilic substitution, a-aryl alcohols, 1,3-dicarbonyl
compounds
Benzylation at the active methylenes of 1,3-dicarbonyl
compounds with a-aryl alcohols has aroused considerable
interests because this protocol leads to the straightforward
construction of carbon–carbon bonds. However, due to
the poor leaving ability of a hydroxyl group, the alcohols
are normally transformed into the corresponding halides,
carbonates, or phosphates, followed by the substitution
with nucleophiles. The reactions of those electrophiles
having good leaving groups with typical nucleophiles re-
quire equimolar amounts of bases, thereby causing the
stoichiometric production of waste salts (Scheme 1b). In
light of atom economy as well as easy availability of alco-
hols, the direct substitution of the alcohols with nucleo-
philes like 1,3-dicarbonyls is a more preferable process
(Scheme 1a) than the indirect one (Scheme 1b).
Montmorillonite, a naturally occurring clay, is composed
of stacked, negatively charged two-dimensional alumino-
silicate layers that hold exchangeable cationic species,
mostly sodium ions in the interlamellar space.21 We have
been interested in various metal ion-exchanged montmo-
rillonites as solid acid catalysts.22–24 Among the various
metal ion-exchanged montmorillonites, tin ion-exchanged
montmorillonite (Sn-Mont) was considered the most acid-
ic for the silylation of alcohols.23 More recently, we found
that the mesoporous Sn-Mont, which was a composite of
SnO2 nanoparticles and aluminosilicate layers,25 was
completely different from those of other ion-exchanged
montmorillonites, such as copper ion-exchanged mont-
morillonite (Cu-Mont) including monomeric copper aqua
complexes26 and iron or titanium ion-exchanged montmo-
rillonite (Fe-Mont, Ti-Mont) intercalating chain-like iron
or titanium hydroxide polymers.27 Sn-Mont demonstrated
the unique high catalytic activity for the cyanosilylation of
highly congested ketones,24 the one-pot Strecker synthesis
of a-amino nitriles, especially from ketones with
amines,28 and the direct allylation of a-aryl alcohols.8 In
order to further unveil the high potential of Sn-Mont for
organic synthesis, we demonstrate the usefulness of Sn-
Mont for the direct substitution of alcohols with 1,3-dicar-
bonyl compounds, especially less acidic 1,3-diesters in
the present study.29 The detailed synthesis and character-
ization of Sn-Mont (Sn content: 1.9 mmol/mg; BET sur-
face area 280 m2g–1) can be found in our previous
reports.24,28
OH
R2
Nu
direct substitution
Nu-H, cat.
R1
R1
R2
(a)
(b)
H2O
base (1 equiv)
Nu-H, cat.
X
substitution
waste
R1
R2
HX-base (salt)
Scheme 1 (a) The direct substitution process of an alcohol with a
nucleophile accompanied by only water; (b) and the indirect one of
the alcohol involving waste salt
A variety of nucleophiles, such as indoles,1,2 1,3-dicarbo-
nyls,1,3–6 allylsilane,7,8 amines,9 amides,10 trialkylsilyl cy-
anide,11 enolsilane,12 silyl ketene acetal,12 and thiol,13
have been found to react directly with alcohols in the pres-
ence of various catalysts. Especially for the direct substi-
SYNLETT 2010, No. 16, pp 2493–2497
Advanced online publication: 10.09.2010
DOI: 10.1055/s-0030-1258567; Art ID: U06110ST
x
x
.x
x
.2
0
1
0
To begin with, the substitution of benzhydrol (1a, 1 mmol)
with dimethyl malonate (2a, 1.5 mmol) in the presence of
© Georg Thieme Verlag Stuttgart · New York

2494
J. Wang et al.
LETTER
Sn-Mont (25 mg, 4.75 mol%) at 100 °C in 1.5 hours was silylation of ketones24 and the Strecker synthesis of a-
selected as a model reaction to investigate solvent effects amino nitriles28 by use of ion-exchanged montmorillo-
on the conversion and selectivity as shown in Figure 1. In nites. The reaction hardly proceeded in the presence of
our study, we used crude solvents without any dehydra- Sn(OH)4 prepared from SnCl4 with aqueous NH3, crystal-
tion treatment prior to use. In tetrahydrofuran was ob- line SnO2, and Sn- or Al-containing ordered mesoporous
tained only a symmetric ether 4a derived from the self- silica with high specific surface areas30 (entries 9–12).
condensation of 1a without any desired alkylated 1,3-di- The use of the proton ion-exchanged zeolites, such as H-
methyl ester 3aa, while the conversion of 1a was 70%. In Beta and H-Y, evidently consumed 1a, but no 3aa was
1,4-dioxane, toluene, and heptane, 3aa was produced in produced (entries 13 and 14). It is surprising that homoge-
unsatisfactory selectivities (8–49%) even with good con- neous Brønsted acids, p-toluenesulfonic acid monohy-
versions of 80–96%. The reaction without a solvent re- drate (p-TsOH·H2O) and H2SO4 gave 3aa in very poor
sulted in a moderate conversion (81%) and selectivity yields of 3–5% together with large amounts of side prod-
(74%). The obvious improvement in the conversion ucts (entries 15 and 16), while p-TsOH·H2O was reported
(97%) and the selectivity (94%) was achieved for the to efficiently promote the reactions with 1,3-diketones in
reaction in 1,2-dichloroethane (DCE) at 100 °C. Lower up to 96% yields.13,17
reaction temperature (80 °C) sharply decreased the selec-
tivity of 3aa to 66% even in DCE. Accordingly, perform-
ing the reaction in DCE at 100 °C is optimal for the
substitution of alcohols in the presence of Sn-Mont.
Table 1 Comparison of Different Catalysts for a-Benzylation of 2a
with 1aa
cat.
1a
+
2a
3aa
DCE, 100 °C, 1.5 h
Entry
1
Catalyst
no catalyst
Na-Mont
Sn-Mont
Al-Mont
Ti-Mont
Fe-Mont
Cu-Mont
Mont K10
Sn(OH)4
SnO2
Conversion (%)b Yield (%)b
0
0
2
0
0
3
97 (98c)
91 (90c)
4
98
97
93
83
96
0
69
28e
19e
tracee
16e
0
5
6
7
8
9
Figure 1 Solvent effects on a-diphenylmethylation of dimethyl ma-
lonate (2a) with 1a catalyzed by Sn-Mont. Reagents and conditions:
Sn-Mont (25 mg, 4.75 mol%), benzhydrol (1a; 1 mmol), dimethyl
malonate (2a; 1.5 mmol), 100 °C, 1.5 h, solvent (2 mL). The conver-
sions and selectivities were determined by GC using n-dodecane as an
internal standard based on 1a; the formation of bis(diphenylmethyl)
ether (4a) accounts for the differences between the conversion of 1a
and the yield of 3aa. a The reaction was conducted under reflux con-
ditions. b The reaction was conducted at 80 °C.
10
11
12
13
14
15
16
0
0
Sn-MCM-41
Al-MCM-41
H-Beta
0
0
0
0
100
0e
H-Y
90
95
94
0e
p-TsOH·H2Od
0e
A comparison of catalytic activity between solid acids and
homogeneous Brønsted acids was conducted in the reac-
tion of 1a with 2a under the optimal reaction conditions,
showing the highest catalytic activity of Sn-Mont as
shown in Table 1. No reaction took place in the absence
of a catalyst or in the presence of Na-Mont (entries 1 and
2). Among various ion-exchanged montmorillonites, a
catalytic amount of Sn-Mont (25 mg, 4.75 mol%) showed
the best activity and gave a 97% conversion of 1a and a
91% yield of 3aa (entry 3). In contrast, other montmoril-
lonites such as Al-Mont, Ti-Mont, Cu-Mont and commer-
cially available K10 gave unsatisfactory yields of 3aa,
while good conversions (83–98%) of 1a were attained in
all cases (entries 3–8). Similar trends in the catalysis were
also observed for the silylation of alcohols,23 the cyano-
H2SO4
0e
d
a Reaction conditions: catalyst (25 mg), 1a (1 mmol), 2a (1.5 mmol),
DCE (2 mL), 100 °C, 1.5 h.
b Determined by GC using n-dodecane as an internal standard based
on 1a.
c The conversion and yield in parentheses were obtained for the reac-
tions using the recovered catalyst.
d 5 mol%.
e The main product was 4a in these reactions.
The high catalytic performance of Sn-Mont was also con-
firmed for the large-scale reaction of 1a (10 mmol) with
2a (15 mmol) in the presence of 50 mg of Sn-Mont in 1.5
Synlett 2010, No. 16, 2493–2497 © Thieme Stuttgart · New York

LETTER
Nucleophilic Substitution of a-Aryl Alcohols with 1,3-Dicarbonyls
2495
hours. After the reaction, the used catalyst was easily sep-
arated by simple filtration from the reaction mixture, and
3aa was obtained in a 91% isolated yield after silica gel
chromatography, indicating a high turnover number
(TON) of 95.8. These values are evidently an order of
magnitude higher than those in the preceding studies us-
ing the homogeneous or heterogeneous catalysts such as
FeCl3 (8.2),15 and FeCl3–SiO2 under microwave irradia-
tion (9.3).16 The recovered Sn-Mont could also be reused
without any significant loss of its high catalytic perfor-
mance and selectivity (Table 1, entry 3).
Table 3 Reactions of Alcohols with 1,3-Diesters Catalyzed by Sn-
Monta
O
O
O
O
OH
Sn-Mont
+
R2
R2
3
Ar
R1
R2 DCE, 100 °C, 1.5 h
R2
1
2
Ar
R1
Entry Alcohol 1
Diester Product Yield
2
3
(%)b
OH
1
1b
2a
3ba
89
To further demonstrate the practical applicability of Sn-
Mont, a series of 1,3-dicarbonyl compounds (2a–f) were
applied to the substitution of 1a under the optimal reaction
conditions.31 As presented in Table 2, Sn-Mont could ef-
ficiently catalyze the reactions of 1a with not only diesters
(2a,b), but also diketones (2c–e) as well as a keto ester
(2f), and the yields in all cases were more than 90%. For
the reaction of 1a with diethyl malonate (2b; entry 6), the
desired alkylated product 3ab was obtained in a 93% iso-
lated yield with a TON of 19.6. The TON is much higher
than that of InCl3 (12.2).1 In sharp contrast, it was reported
that the reaction of 1a with 2b failed in the presence of H-
Mont.20
Cl
OH
2
3
4
1c
1d
1e
2a
2a
2a
3ca
3da
3ea
83
93
90
NO2
OH
OH
OMe
5
6
7
1b
1d
1e
2b
2b
2b
3bb
3db
3eb
85
91
82
Table 2 Reactions of 1a with Different 1,3-Dicarbonyl Compounds
Catalyzed by Sn-Monta
Sn-Mont
O
O
1a +
2
3
R1
R2
DCE, 100 °C
OH
Entry 1,3-Dicarbonyl compound 2
Time
(h)
Product Yield
3
(%)b
8
1f
2a
3fa
66
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
R1 = R2 = OMe
R1 = R2 = OEt
R1 = R2 = Me
1.5
1.5
1
3aa
3ab
3ac
3ad
3ae
3af
90
93
a Reaction conditions: Sn-Mont (25 mg, 4.75 mol%), 1 (1 mmol), 1,3-
diesters 2 (1.5 mmol), DCE (2 mL), 100 °C, 1.5 h.
b Isolated yields.
93
R1 = Me, R2 = Ph
R1 = R2 = Ph
1
91
with alcohols were examined (Table 4). a-Aryl alkanols
and their derivatives with an electron-donating or elec-
tron-withdrawing group afforded the corresponding prod-
ucts 3 in good to excellent yields in 1–1.5 hours (entries
1–7). Allylic alcohol 1f as well as a-diaryl methanols (1a,
1b, 1d, 1e) gave good yields (entries 8–12). To our de-
light, a more sterically congested alcohol, 9-fluorenol
(1n) was also applicable to afford 3nc with a moderate
yield of 70% in a little prolonged reaction time (3 h; en-
tries 13), and the product was confirmed as a mixture of
keto and enol forms by NMR.32 Moreover, the reaction of
a primary alcohol, benzyl alcohol (1o) with 2c smoothly
proceeded in 65% yield (entry 14). In all cases, the selec-
tive monoalkylation took place without dialkylation.
Thus, it is concluded that the Sn-Mont catalytic system
has the unique high activity for the nucleophilic substitu-
tion of a-aryl alcohols with a wide range of 1,3-dicarbonyl
compounds, as summarized in Tables 2– 4.
1
98
R1 = Me, R2 = OEt
1
92
a Reaction conditions: Sn-Mont (25 mg, 4.75 mol%), 1a (1 mmol),
1,3-dicarbonyl compounds 2 (1.5 mmol), DCE (2 mL), 100 °C.
b Isolated yields.
Various benzhydrol derivatives (1b–e) with different sub-
stituents on the benzene ring other than 1a were also test-
ed for the substitution reactions with a 1,3-diester 2a or 2b
under the optimal conditions. The excellent yields (83–
95%) of the alkylated diesters 3 were obtained regardless
of having an electron-withdrawing or electron-donating
group in 1 (Table 3, entries 1–7). Allylic alcohol 1f also
reacted with 2a to give 3fa in a moderate yield of 66%
(entry 8).
To unveil the generality of the Sn-Mont-catalyzed reac-
tions, the reactions of a more active acetylacetone (2c)
Sn-Mont has Brønsted acidity confirmed by the FT-IR
spectrum of Sn-Mont after pyridine adsorption.25,28 Thus,
the reaction pathway in the nucleophilic substitution of al-
Synlett 2010, No. 16, 2493–2497 © Thieme Stuttgart · New York

2496
J. Wang et al.
LETTER
even in the crude solvent without any formation of waste
salts. In all cases, the desired alkylated products were ob-
tained in moderate to excellent yields in short reaction
times. Therefore, easily prepared, water-tolerant, reus-
able, highly active, and nontoxic Sn-Mont has great po-
tential in organic synthesis by replacing the previously-
reported homogeneous catalysts, most of which are
hydrolyzable, expensive, or toxic.
Table 4 Reactions of Alcohols with Acetylacetone (2c) Catalyzed
by Sn-Monta
Sn-Mont
1 + 2c
3
DCE, 100 °C, 1–1.5 h
Entry Alcohol 1
X
Product Yield
3
(%)b
1
2
3
1g
1h
1i
H
3gc
93
91
82
OMe 3hc
Cl
HO
HO
3ic
Acknowledgment
X
The present work was partially supported by a Grant-in-Aid for Sci-
entific Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. J.W. thanks the Japan Society for
the Promotion of Science for the financial support (P10082).
4
5
1j
3jc
92
79
OH
References and Notes
1k
3kc
3lc
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OH
9608.
6
7
1l
1m
H
77
80
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8
1f
3fc
88
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11
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1a
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93
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86
85
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OH
OH
13c
14
1n
1o
3nc
3oc
70d
65e
a Reaction conditions: Sn-Mont (25 mg, 4.75 mol%), 1 (1 mmol), 2c
(1.5 mmol), DCE (2 mL), 100 °C, 1–1.5 h.
b Isolated yields.
c Sn-Mont (20 mg), 1m (0.5 mmol), 3 h.
d Product 3nc existed as a mixture of keto and enol forms.
e Determined by GC.
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carbocation intermediate derived from the dehydration of
the protonated alcohol with the aid of Sn-Mont,20 fol-
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to form the alkylated 1,3-dicarbonyl derivatives.
In conclusion, we have developed an efficient and envi-
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substitution of benzylic alcohols with various 1,3-dicar-
bonyl compounds including 1,3-diesters in 1,2-dichloro-
ethane using a catalytic amount of a solid Brønsted
catalyst, Sn-Mont. The reactions smoothly proceeded
Synlett 2010, No. 16, 2493–2497 © Thieme Stuttgart · New York

LETTER
Nucleophilic Substitution of a-Aryl Alcohols with 1,3-Dicarbonyls
2497
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malonate (2a; 1.5 mmol). The resultant mixture was stirred
vigorously at 100 °C under reflux. After the completion of
the reaction monitored with TLC, the catalyst was removed
by filtration through a Celite plug, followed by being washed
with CH2Cl2. The combined solution was evaporated under
reduced pressure to give the crude product. Further
purification was carried out by silica gel chromatography
(eluting solvent: hexane–EtOAc, 8:1) to give the desired 3aa
as a white solid in 90% yield.
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(31) Spectroscopic data for selected products: 3-(Diphenylmethyl)-
pentane-2,4-dione (3aa): white solid; mp 112–114 °C. 1H
NMR (500 MHz, CDCl3): d = 2.00 (s, 6 H), 4.73 (d, J = 12.3
Hz, 1 H), 4.81 (d, J = 12.3 Hz, 1 H), 7.10–7.20 (m, 2 H),
7.25–7.30 (m, 8 H). 13C NMR (126 MHz, CDCl3): d = 29.7,
51.2, 74.5, 127.0, 127.7, 128.9, 141.2, 203.0. HRMS: m/z
calcd for C18H18O2: 266.1307; found: 266.1312. Dimethyl 2-
(Diphenylmethyl)malonate (3ac): white solid; mp 88–90 °C.
1H NMR (500 MHz, CDCl3): d = 3.54 (s, 6 H), 4.36 (d, J =
12.3 Hz, 1 H), 4.77 (d, J = 12.3 Hz, 1 H), 7.15–7.20 (m, 2 H),
7.24–7.30 (m, 8 H). 13C NMR (126 MHz, CDCl3): d = 51.1,
52.6, 57.2, 126.9, 127.7, 128.6, 141.1, 168.0. HRMS: m/z
calcd for C18H18O4: 298.1205; found: 298.1199. Ethyl 3-
Oxo-2-(diphenylmethyl)butanoate (3af): white solid; mp
87–89 °C. 1H NMR (500 MHz, CDCl3): d = 1.00 (t, J = 7.1
Hz, 3 H), 2.09 (s, 3 H), 4.00 (q, J = 7.1 Hz, 2 H), 4.53 (d,
J = 12.3 Hz, 1 H), 4.76 (d, J = 12.3 Hz, 1 H), 7.15–7.25 (m,
2 H), 7.24–7.35 (m, 8 H). 13C NMR (126 MHz, CDCl3): d =
13.7, 30.0, 50.9, 61.5, 65.2, 126.8, 126.9, 127.7, 127.8,
128.6, 128.8, 141.2, 141.5, 167.6, 201.8. HRMS: m/z calcd
for C19H20O3: 296.1412; found: 296.1423.
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(29) General Procedure for Sn-Mont-Catalyzed Nucleophilic
Substitution of Alcohols with 1,3-Dicarbonyl
Compounds: In a flask were placed Sn-Mont (25 mg, 4.75
mol%) which was activated at 120 °C for 1 h in a vacuum,
1,2-dichloroethane (2 mL), which was not required to be
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Synlett 2010, No. 16, 2493–2497 © Thieme Stuttgart · New York