Ruthenium(III) acetylacetonate [Ru(acac)3] - An efficient chemoselective catalyst for the tetrahydropyranylation (THP) of alcohols and phenols under solvent-free conditions

Article abstract of DOI:10.1139/V06-137

A catalytic amount of ruthenium(III) acetylacetonate (2 mol%) [Ru(acac)₃] enables solvent-free tetrahydropyranylation of different types of alcohols and phenols at ambient temperature in moderate to excellent yields. Notably, selective monoprotection of diols can be achieved chemoselectively. Furthermore, the catalyst could be recovered and reused if necessary.

Full text of DOI:10.1139/V06-137

1
174
Ruthenium(III) acetylacetonate [Ru(acac) ] — An
3
efficient chemoselective catalyst for the
tetrahydropyranylation (THP) of alcohols and
phenols under solvent-free conditions
Ravi Varala and Srinivas R. Adapa
Abstract: A catalytic amount of ruthenium(III) acetylacetonate (2 mol%) [Ru(acac) ] enables solvent-free
3
tetrahydropyranylation of different types of alcohols and phenols at ambient temperature in moderate to excellent
yields. Notably, selective monoprotection of diols can be achieved chemoselectively. Furthermore, the catalyst could be
recovered and reused if necessary.
Key words: tetrahydropyranyl ethers, protecting groups, ruthenium(III) acetylacetonate, alcohols, thiols.
Résumé : L’addition de quantités catalytiques (2 mol %) d’acétylacétonate de ruthénium(III) [Ru(acac) ] permet
3
d’effectuer un tétrahydropyranylation sans solvant de divers types d’alcools et de phénols, à la température ambiante,
avec des rendements allant de modérés à excellents. Il est à noter que la monoprotection sélective de diols peut être
réalisée chimiosélectivement. De plus, si cela est nécessaire, le catalyseur peut être récupéré et réutilisé.
Mots clés : oxydes de tétrahydropyranyle, groupes protecteurs, acétylacétonate de ruthénium(III), alcools, thiols.
[
Traduit par la Rédaction] Varala and Adapa 1179
Introduction
drawbacks including long reaction times under reflux, high
catalyst to substrate ratios, the use of large amounts of solid
support (which eventually results in the generation of large
amount of toxic waste, harsh and acidic conditions), the use
of expensive and toxic reagents or solvents, and incompat-
ability with other acid-sensitive functional groups. Despite a
number of precedents, a solvent-free and catalytically effi-
cient practical alternative for this very important transforma-
tion is highly desirable and subject to further improvement.2
Recently, we reported CuBr-catalyzed chemoselective
thioacetalization of carbonyl compounds (5). As part of our
ongoing research program to develop novel synthetic meth-
odologies, particularly carbon–carbon and carbon–heteroatom
bond formations (6), we herein present our preliminary
results using an efficient ruthenium(III) acetylacetonate
The choice of the protection and deprotection strategy in a
synthetic sequence is inevitable owing to the chemoselective
transformations in the presence of various functional groups
(
1). Tetrahydropyran (THP) ethers are one of the most
widely used protecting groups employed during a multistep
organic synthesis because of their low cost, easy removal un-
der mild acidic conditions, and stability in various reaction
conditions such as strong bases, Grignard reagents, hydrides,
redox reagents, alkylating, acylating agents, and catalytic
hydrogenation (2). Furthermore, tetrahydropyranyl ethers are
synthetically useful as building blocks for the synthesis of
biologically important compounds such as nucleosides, car-
bohydrates, steroids,and alkaloids. They can also be used in
perfume formulations (3). A variety of protonic as well as
Lewis acid catalysts, including amberlyst, zeolites, montmo-
[Ru(acac) ]-catalyzed solvent-free tetrahydropyranylation of
3
hydroxy and thiol functions. The procedure, based on the
rillonite K-10, sulfuric acid on silica gel, ZnCl , tetrabutyl-
ammonium bromide, PTSA, BF ·Et O, ZrCl , I , In(OTf) ,
Lewis acid catalytic activity of [Ru(acac) ], represents an en-
2
3
vironmentally benign alternative to current chemical pro-
cesses that use water-intolerant Lewis acids.
The screening of a few acetylacetonates allowed us to se-
3
2
4
2
3
NbCl , InCl , PdCl (CH CN) , lithium perchlorate – diethyl
5
3
2
3
2
ether, TBATB, Bi(OTf) , La(NO ) ·6H O, Fe (SO ) ·xH O,
3
3 3
2
2
4 3
2
VO(OAc) , AlCl ·6H O, and CeCl ·7H O – NaI, have been
lect [Ru(acac) ] as the versatile catalyst to affect this trans-
2
3
2
3
2
3
developed for tetrahydropyranylation of hydroxy functions
4). However, several reported methods are associated with
formation (Table 1).
Among the reagents tested, the addition product of
(
Received 23 May 2006. Accepted 6 September 2006. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on
November 2006.
2
1
R. Varala and S.R. Adapa. Indian Institute of Chemical Technology, Hyderabad-500 007, India.
1
Corresponding author (e-mail: rvarala_iict@yahoo.co.in).
Exothermicity of a large-scale reaction should be controlled by maintaining the temperature and using an appropriate solvent if
2
necessary.
Can. J. Chem. 84: 1174–1179 (2006)
doi:10.1139/V06-137
© 2006 NRC Canada

Varala and Adapa
1175
Table 2. Comparison of reaction rates of DHP with 1 in differ-
Table 1. Comparison of various acetylacetonates for the
tetrahydropyranylation of n-butanol with DHP under solvent-free
conditions.
ent solvents.
Entry
Solvent
MeOH
Time (min)
Yield (%)a
1
2
40
30
70
87
[
Ru(acac)3]
CH CN
3
+
OH
O
O
Solvent-free
RT
3
4
5
6
Toluene
THF
Dioxane
CCl4
45
30
60
30
–
O
88
62
90
1
2
3
Catalyst
Time (min)
Yield (%)a
7
8
9
DCM
DMSO
No solvent
45
45
30
–
72
>99
VO(acac)2
Pd(acac)2
Co(acac)3
30
45
40
30
90
68
80
Note: All reactions were carried out by stirring mixtures of 1 (1 mmol),
[
Ru(acac)3]
>99
DHP (1.1 mmol), and a catalyst for the appropriate times.
a
a
Isolated yields.
Isolated yields.
[
Ru(acac) ] was obtained in >99% yield in 30 min at ambi-
3
tries 10, 28) to give the corresponding THP ethers in good
yields. Furthermore, alcohols bearing the hydroxy group
bonded to a carbon stereogenic centre (Table 3, entries 9,
ent temperature by using solvent-free conditions (Table 1,
entry 4). Next, our efforts focused on achieving the optimum
conditions for tetrahydropyranylation of n-butanol (1) as a
model hydroxy compound in the presence of [Ru(acac) ] as
Lewis acid. The results are summarized in Table 2. From the
results obtained, we noted that the optimum concentration of
1
6, 18) can be protected, giving adducts with a complete re-
3
tention of configuration. However, low diastereoselectivity
is observed.
We also investigated the electronic effects on the phenol
moiety in the THP protection reaction (Table 3, entries 19–
2
mol% of [Ru(acac) ] is sufficient to carry forward the re-
3
action. The reaction is found to be sluggish when carried out
using even stoichiometric – 0.5 equiv. of catalyst amounts.
Among the various solvents tested for the THP protection of
2
7, 29–33). No apparent difference in the rate of reactivity
was observed in the presence of electron donating and with-
drawing groups on the aromatic ring. However, no or less
reaction occurred between entries 20–23 and 3,4-dihydro-
1
(
, toluene and DCM did not furnish the expected product
Table 2, entries 3, 7), whereas other solvents gave moderate
2
H-pyran. Interestingly, p-hydroxy benzaldehyde and o-
to good yields of the addition product. We have also adopted
a solvent-free procedure, and to our surprise, the reaction
can be carried out in a shorter time with almost quantitative
yield (Table 2, entry 9). This allowed us to adopt a simple
work-up procedure by employing ether to dissolve the or-
ganic material and not the catalyst, which could be easily re-
moved by filtration.
With optimized experimental conditions for the reaction
between n-butanol (1) and DHP, this process was investi-
gated for a wide range of aromatic and aliphatic alcohols
and phenols. The results are listed in Table 3.
aminophenol (Table 3, entries 27, 30) did not react as ex-
pected, even after stirring for 2 days.
The efficiency and generality of the present [Ru(acac) ]-
3
catalyzed protocol can be realized at a glance by comparing
our results for the chosen model substrates with those of
some recently developed procedures (Table 4). The reactions
have been compared with respect to the reaction times,
mol% of the catalyst used, and the yields.
In conclusion, we have developed a mild, efficient, and
highly selective methodology for the tetrahydropyranylation
of alcohols using [Ru(acac) ] as a catalyst (2 mol%). Nota-
3
One of the important features of this protocol for the for-
mation of THP ethers is that absolute anhydrous conditions
are not required. The reaction conditions are mild enough
not to induce any isomerization of the double bonds of al-
lylic systems (Table 3, entries 2, 4–6, 8). The reactions are
reasonably fast, which holds well even with hindered alco-
hols like geranial and menthol (Table 3, entries 8–9). We in-
vestigated the substrate scope and tolerance of electronically
divergent alcohols including those that contain sensitive
functionalities, for example, epoxy (Table 3, entry 17), C=C,
ble features of the protocol include clean and simple reac-
tion conditions, nonaqueous work-up, and environmentally
benign reagents. This work widens the scope of using transi-
tion metal salts and complexes in organic synthesis because
of the nontoxic nature of the catalyst. We believe that this
work will be a valuable addition to modern synthetic meth-
odologies.
Typical experimental procedures
CϵC (Table 3, entries 3, 7), NO (Table 3, entry 32), and Cl
The hydroxy compound (1–33, 1.0 mmol), 3,4-dihydro-
2
(
Table 3, entry 33). Another remarkable feature of this
2H-pyran (1.1 mmol), and [Ru(acac) ] (2 mol%) were placed
3
method is the chemoselective monotetrahydropyranylation of
symmetrical 1,n-diols (Table 3, entries 14–16, with 7–8% of
di-THP ethers), which is a difficult transformation to accom-
plish via conventional methods. A primary alcohol has been
selectively protected in the presence of a secondary alcohol
successively in a flask. After stirring, the reaction mixture
was treated with Et O (2 × 10 mL), and the catalyst was re-
2
moved by filtration. The filtrate extracts were concentrated
under reduced pressure, and the crude product was then pu-
rified by silica gel chromatography (hexane – ethyl acetate).
All the obtained tetrahydropyranyl ethers were characterized
(
Table 3, entry 14). Benzylic alcohols are also smoothly
tetrahydropyranylated when reacting with DHP (Table 3, en-
1
by IR, H NMR spectroscopy, and mass spectrometry, and
©
2006 NRC Canada

1
176
Can. J. Chem. Vol. 84, 2006
Table 3. Tetrahydropyranylation of hydroxy compounds in the presence of 2 mol% of
Ru(acac) ] under solvent-free conditions.
[
3
[Ru(acac)3]
R–OH +
R
Solvent-free
RT
O
O
O
a
Entry
Substrate
Time (min/h)
30 min
6 h
Yield (%)
1
OH
OH
OH
>99
82
2
3
4
12 h
74
OH
14 h
72
5
6
7
OH
OH
24 h
22 h
18 h
95
92
68
OH
8
OH
8 h
62
OH
9
1.5 h
90
OH
1
0
2 h
78
92
86
Phth
1
1
24 h
OH
1
2
1.75 h
OH
OH
1
1
1
3
4
5
45 min
>95
>99
HO
8 h
OH
OH
b
30 min
78/08
HO
b
1
6
1.5 h
82/07
HO OH
OH
1
7
8
BnO
24 h
16 h
28
94
O
1
N
H
OH
H N
2
H
OH
1
9
15 h
>99
O
HO
©
2006 NRC Canada

Varala and Adapa
1177
Table 3 (continued).
a
Entry
Substrate
Time (h)
Yield (%)
2
0
4
8 h
8 h
12
–
HO
O
O
4
2
1
N
OH
OH
–
4
8 h
2
2
3
OH
Br
OH
2
4 h
3 h
4
2
2
OH
1
88
2
4
5
OH
OH
o-
m-
p-
14 h
16 h
12 h
85
78
72
2
COCH3
OH
o-
p-
24 h
24 h
48
52
2
6
7
CHO
OH
o-
m-
p-
24 h
24 h
24 h
50
32
2
–
OH
OH
80
1
6 h
2
8
1
2-
-
22 h
8 h
30
56
2
9
0
NH2
–
17
o-
p-
48 h
48 h
3
OH
OH
7
8
2
4 h
3
1
F
OH
2
1
4 h
2 h
32
74
3
2
NO2
OH
Cl
3
3
a
b
Isolated yields.
Ratio of mono-:di-THP ether.
their structures are consistent with the published physical
data.
Entry 4
1
H NMR (300 MHz, CDCl , ppm) δ: 1.49–1.91 (m, 6H),
3
3
4
.52 (m, 1H), 3.86 (m, 1H), 4.01 (m, 1H), 4.25 (m, 1H),
.66 (br s, 1H), 5.24 (dd, 2H), 5.96 (m, 1H).
Spectral data for selected compounds
Entry 3
Entry 9
1
1
H NMR (300 MHz, CDCl , ppm) δ: 1.32–1.38 (m, 6H),
H NMR (300 MHz, CDCl , ppm) δ: 0.75–1.08 (m, 12H),
1.18–1.83 (m, 10H), 2.06–2.35 (m, 2H), 3.30–3.48 (m, 2H),
3
3
2
.25 (t, 1H), 3.29–4.01 (m, 2H), 4.14 (d, 2H), 4.78 (br s, 1H).
©
2006 NRC Canada

1
178
Can. J. Chem. Vol. 84, 2006
Table 4. Comparison of the results of tetrahydropyranylation of alcohols with other catalysts (see ref. 4).
Entry
1
Alcohol (Phenol)
Catalyst
mol%
Time
26 h
Yield (%)
CeCl ·7H O – NaI
2–5
73 (neat)
OH
3
2
NbCl – DCM
10
3 h
85
5
PdCl (CH CN) – THF
10
1 h
49
2
3
2
LiClO – ether
5 mmol/1 mL
12 h
80
4
VO(OAc)2
TBATB
27
2.5
0.1
1
1.5 h
1 h
88
75
Bi(OTf)3
3.25 h
30 min
30 min
40 min
83
AlCl ·6H O
88
3
2
In(OTf)3
0.5
2
85
[
Ru(acac)3]
>95 (neat)
2
3
OH
CeCl ·7H O – NaI
2–5
0.1
2.5
27
24 h
2 h
90 (neat)
74
3
2
Bi(OTf)3
TBATB
1 h
74
VO(OAc)2
Ru(acac)3]
1.5 h
1 h
94
[
2
90 (neat)
OH
VO(OAc)2
27
1
30 min
30 min
30 min
75
HO
AlCl ·6H O
75
3
2
[
Ru(acac)3]
2
78 (neat)
3
.85–3.99 (m, 1H), 4.60–4.81 (m, 1H). EI-MS m/z (%): 139
Entry 32
+
1
(
M – 101).
H NMR (300 MHz, CDCl , ppm) δ: 1.42–2.08 (m, 6H),
.43–3.95 (m, 2H), 5.52 (br s, 1H), 7.11 (d, J = 9.3 Hz, 2H),
.16 (d, J = 9.3 Hz, 2H).
3
3
8
Entry 13
1
H NMR (300 MHz, CDCl , ppm) δ: 4.66 (br s, 1H),
3
Acknowledgments
3
.79–3.94 (m, 1H), 3.41–3.62 (m, 2H), 1.11–1.90 (m, 16H).
+
EI-MS m/z (%): 184 (M ).
We thank Dr. J.S. Yadav, Director, Indian Institute of
Chemical Technology, and the Council of Scientific Indus-
trial Research (CSIR, India) for financial support.
Entry 24
1
H NMR (300 MHz, CDCl , ppm) δ: 1.62–2.07 (m, 6H),
3
3.64 (m, 1H), 3.96 (m, 1H), 5.46 (br s, 1H), 6.99–7.34 (m, 5H).
References
Entry 25 (catechol)
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1
H NMR (300 MHz, CDCl , ppm) δ: 1.21–2.01 (m, 12H),
.02–3.94 (m, 4H), 5.09 (m, 2H), 6.46–7.03 (m, 4H).
3
2
3
4
. C.B. Reese. In Protecting groups in organic chemistry. Edited
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3
Entry 26 (p-hydroxy)
(
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1
H NMR (300 MHz, CDCl , ppm) δ: 1.45–2.05 (m, 6H),
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2
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(
br s, 1H), 7.07 (d, J = 8.8 Hz, 2H), 7.90 (d, J = 8.8 Hz, 2H).
(
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Entry 28
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1
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3
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3
2
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(
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+
MS m/z (%): 192 (M ).
Entry 29 (2-naphthol)
1
H NMR (300 MHz, CDCl , ppm) δ: 1.60–1.73 (m, 3 H),
3
3
2
1
3
1
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4
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( j ) I : N. Deka and J.C. Sarma. Synth. Commun. 30, 4435
2
+
(
m, 3H). EI-MS m/z (%): 228 (M ).
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3
©
2006 NRC Canada

Varala and Adapa
1179
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7
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©
2006 NRC Canada

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. Congde Huo, Tak Hang Chan. 2009. Carbon Tetrabromide/Sodium Triphenylphosphine-m-sulfonate (TPPMS) as an Efficient
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