Indium triflate: A mild and efficient Lewis acid catalyst for O-H insertion reactions of α-diazo ketones

Article abstract of DOI:10.1016/S0040-4039(02)00489-6

Facile O-H insertion reactions of α-diazo ketones with aliphatic/aromatic alcohols or benzenethiol have been developed in the presence of indium triflate as a catalyst. These reactions provided good yields of α-alkoxy ketones. A comparative study with other Lewis acids establishes the reactivity of indium triflate in O-H insertion reactions of α-diazo ketones.

Full text of DOI:10.1016/S0040-4039(02)00489-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3133–3136
Indium triflate: a mild and efficient Lewis acid catalyst for O–H
insertion reactions of a-diazo ketones
Sengodagounder Muthusamy,* Srinivasarao Arulananda Babu and Chidambaram Gunanathan
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
Received 14 January 2002; revised 25 February 2002; accepted 8 March 2002
Abstract—Facile O–H insertion reactions of a-diazo ketones with aliphatic/aromatic alcohols or benzenethiol have been developed
in the presence of indium triflate as a catalyst. These reactions provided good yields of a-alkoxy ketones. A comparative study
with other Lewis acids establishes the reactivity of indium triflate in O–H insertion reactions of a-diazo ketones. © 2002 Elsevier
Science Ltd. All rights reserved.
The inter- and intramolecular formation of ether link-
ages through O–H insertion reactions of a-diazo
ketones with aliphatic/aromatic alcohols catalyzed by
copper or rhodium complexes has been well docu-
mented.1 These reactions can also be carried out using
protic acids2 and strong Lewis acids.1a,3 For this pur-
pose, the frequently encountered acid catalysts are
dilute H2SO4 and AlCl3 or BF3·Et2O, respectively. Cop-
per or its chelate-catalyzed insertion reactions of diazo
ketones with alcohols produces a-alkoxy ketones in
competition with the formation of esters via the Wolff
rearrangement.4 Recently, we reported5 our study on
the decomposition of various a-diazo ketones to afford
a-hydroxy ketones, bicycloalkane-1,3-diones and 3-
furanones in the presence of Amberlyst-15.® Many of
the above insertion processes suffered limitations such
as the use of expensive catalysts, moderate product
yields, competitive or slow reactions and the need for
skillful work-up. Consequently, there is a need and
scope for an improved procedure involving mild reac-
tion conditions.
Lewis acid-catalyzed reactions furnish versatile and
selective routes to many natural/unnatural products in
synthetic organic chemistry since this path rules out the
use of corrosive Brønsted acid catalysts.6,7 Often tradi-
tional strong Lewis acids such as AlCl3,7a BF3·Et2O,8
TiCl4 and SnCl4 are employed as catalysts in organic
synthesis. Although their role is well exploited in
organic reactions, the use of Lewis acids such as the
indium halides and indium triflate have been exploited
as catalysts only in recent years.6d,e The use of water
tolerant Lewis acids has attracted some attention in
recent years. In particular, the chemistry of metal
triflates11 has gained considerable attention as they
provide selectivity and good yields of products.
9
10
It is evident from the literature12 that indium triflate is
a mild Lewis acid and it has received considerable
attention in recent years as a catalyst. To the best of
our knowledge, indium triflate has not been used in the
decomposition of a-diazo ketones. As a part of our
ongoing synthetic program13 on the reactions of a-
diazo ketones, we herein report O–H insertion reactions
of a-diazo ketones with aliphatic/aromatic alcohols or
benzenethiol using In(OTf)3 as the catalyst.
O
R3 COCHN2
( )n
R3
OR4
R2
R1
R2
R1
( )n
In(OTf)3
R4OH/r.t.
To a neat solution of diazoacetophenone in dry 1-
butanol was added 10 mol% of indium triflate and the
decomposition started with a slow evolution of nitrogen
gas. The reaction mixture was stirred at room tempera-
ture under a nitrogen atmosphere and was followed by
TLC until the disappearance of the starting material. In
fact, diazoacetophenone was decomposed within 2 min,
but the reaction was stirred for a further 3 min. The
reaction mixture was then concentrated under reduced
pressure and the crude reaction mixture was purified
1
2
Scheme 1.
Keywords: alkoxy ketones; diazo ketones; indium triflate; Lewis acid
catalyst; O–H insertion.
* Corresponding author. Tel.: +91 278 567760; fax: +91 278 567562;
e-mail: salt@csir.res.in
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00489-6

3134
S. Muthusamy et al. / Tetrahedron Letters 43 (2002) 3133–3136
Table 1. In(OTf)3-catalyzed synthesis of a-alkoxy ketones
Product 2
R1
R2
R3
R4
n
Time (min)
Yielda (%)
a
b
c
d
e
f
g
h
i
H
H
H
H
H
H
H
Me
OMe
OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Bu
Pr
Et
Me
iPr
iBu
tBu
Me
Bu
Pr
0
0
0
0
0
0
0
0
0
0
1
1
1
2
2
2
2
4
3
180
8
10
10
3
93
92
90
90
92
93
60
89
90
87
87
89
83
j
k
l
Pr
iPr
Et
5
3
m
-CHꢀCH-CHꢀCH-
a Refers to yields (unoptimized) of isolated products 2.
We decided to carry out a comparative study of O–H
insertion reactions of an a-diazo ketone with an alcohol
using other strong Lewis acids. To this end, we carried
out the reaction of diazoacetophenone in neat dry
1-butanol using various Lewis acids under similar
experimental conditions to afford product 2a. The
results are summarized in Table 2.
using neutral alumina column chromatography to fur-
nish the a-alkoxy ketone 2a in 93% yield (Scheme 1).
This reaction encouraged us to study further the
decomposition of a variety of a-diazo ketones with
indium triflate. We studied this reaction with various
aromatic/aliphatic diazo compounds, all of which
afforded the respective a-alkoxy ketones 2b–m (Table 1)
as a result of O–H insertion. It may be pointed out that
all the reactions proceeded smoothly and in the case of
both primary and secondary alcohols, they were com-
pleted in a short time to afford good yields of products,
but the tertiary alcohol needed 3 h to proceed to
completion. a-Diazo ketones involved in this work were
prepared14a according to literature precedent.1a
In a different experiment the O–H insertion reaction of
diazoacetophenone with 1-propanol or 1-butanol using
ytterbium triflate (12 mol%) afforded the corresponding
insertion product in 3 h. Similarly, we performed the
O–H insertion of diazoacetophenone with 2-propanol
to afford 69% of product 2e in 20 h. It may be noted
that these reactions took longer reaction times.
It was interesting to investigate the indium triflate-cata-
lyzed O–H insertion reaction using an allyl alcohol.
Thus, we carried out the reaction of diazoacetophenone
and geraniol in the presence of indium triflate (10
mol%) using dry benzene as the solvent. The reaction
was stirred at room temperature for 0.5 h and purifica-
tion of the crude reaction mixture afforded the ether
3a14b in 74% yield (Scheme 2) as the sole product.
Similarly, the reaction of diazo-4-methoxyacetophenone
with geraniol furnished product 3b in 71% yield.
Encouraged by the above results, we were also inter-
ested to carry out S–H insertion reactions of a-diazo
ketones. We carried out the reaction of diazoacetophe-
none with benzenethiol in the presence of 10 mol% of
In(OTf)3 at room temperature for 15 min. Purification
of the crude reaction mixture through a neutral alu-
O
COCHN2
O
In(OTf)3
Further, we were eager to carry out O–H insertion
reactions of a-diazo ketones with phenols using indium
triflate. To a solution of diazoacetophenone and phenol
was added 10 mol% of indium triflate at room temper-
ature and the decomposition started with slow evolu-
tion of nitrogen gas. The reaction mixture was
concentrated under reduced pressure and the crude
reaction mixture was purified using neutral alumina
column chromatography to furnish the a-alkoxy ketone
4a in 51% yield (Scheme 3). Subsequently, we per-
formed a similar experiment with 1- or 2-naphthol to
afford products 4b and 4c14b in 48 and 45% yields,
respectively. It may be noted that Lewis acid catalysts
often fail to afford insertion products with phenols as a
result of the non-availability of the Lewis acid in the
reaction mixture due to complex formation with the
phenol.15
geraniol
r.t.
R
R
1
3a; R = H (74%)
3b; R = OMe (71%)
Scheme 2.
COCHN2
COCH2OR
10 mol%
In(OTf)3
phenol (or)
1-naphthol (or)
2-naphthol
1
4a; R = phenyl (51%)
4b; R = 1-naphthyl (48%)
4c; R = 2-naphthyl (45%)
Scheme 3.

S. Muthusamy et al. / Tetrahedron Letters 43 (2002) 3133–3136
3135
Table 2. O–H Insertion reaction of diazoacetophenone to
1-butanol using various Lewis acids
since it will be an alternative to conventional insertion
reaction processes.
Lewis acids
Product 2a
Acknowledgements
Time (h)/mol%
Yielda (%)
ZnBr2
36/50
50/100
5/5
10b
45b
55b
79
This research was supported by the Young Scientist
Scheme, CSIR, New Delhi. We thank Dr. P. K. Ghosh,
Director, Dr. R. V. Jasra, Head of the Division, for
their encouragement of this work. S.A.B. and C.G.
thank CSIR, New Delhi, for a Fellowship.
Yb(OTf)3
3/12
FeCl3
InCl3
TiCl4
AlCl3
SnCl4
BF3·OEt2
0.5/10
12/10
0.5/1.5
0.5/5
0.5/5
0.5/5
67c
d
–
69
71c
76
74
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COCHN2
COCH2SPh
10 mol%
In(OTf)3
benzenethiol
R
R
1
5a; R = H (91%)
5b; R = OMe (86%)
Scheme 4.

3136
S. Muthusamy et al. / Tetrahedron Letters 43 (2002) 3133–3136
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In(OTf)3: To
a
solution containing 2-diazo-1-
phenylethanone (100 mg), geraniol (140 mg) in dry ben-
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1-phenylethanone (3a): colorless liquid, IR (neat) 2969,
2924, 2858, 1701, 1599, 1449, 1380, 1227, 1127, 911, 733
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1
cm−1; H NMR (200 MHz, CDCl3) l 7.94 (d, 2H, J=7.6
Hz, Arom-H), 7.62–7.28 (m, 3H, Arom-H), 5.41 (t, 1H,
J=7.0 Hz, ꢀCH), 5.09 (t, 1H, J=7.0 Hz, ꢀCH), 4.73 (s,
2H, OCH2), 4.17 (d, 2H, J=7.0 Hz, OCH2), 2.16–2.02
(m, 4H), 1.67 (s, 6H, CH3), 1.60 (s, 3H, CH3); 13C NMR
(50.3 MHz, CDCl3) l 197.1 (CꢀO), 142.1 (quat-C), 135.5
(quat-C), 138.8 (ꢀCH), 132.1 (quat-C), 129.1 (ꢀCH),
128.3 (ꢀCH), 124.3 (ꢀCH), 120.4 (ꢀCH), 72.8 (OCH2),
68.1 (OCH2), 40.0 (CH2), 26.7 (CH2), 26.1 (CH3), 18.1
(CH3), 16.9 (CH3); mass m/z: 272 (M+). Anal. calcd for
C18H24O2: C, 79.37; H, 8.88. Found: C, 79.31; H, 8.82.
2-(Naphthalen-2-yloxy)-1-phenylethanone (4c): orange–red
solid, mp 134–136°C; IR (KBr) 2362, 1705, 1631, 1600,
1469, 1215, 1178, 909, 737 cm−1 1H NMR (200 MHz,
;
CDCl3) l 8.05–8.01 (m, 2H, Arom-H), 7.79–7.67 (m, 3H,
Arom-H), 7.62–7.23 (m, 6H, Arom-H), 7.12 (d, 1H,
J=1.8 Hz, Arom-H), 5.37 (s, 2H, OCH2); 13C NMR
(50.3 MHz, CDCl3) l 194.8 (CꢀO), 153.3 (quat-C), 135.1
(quat-C), 134.7 (quat-C), 134.3 (ꢀCH), 130.2 (ꢀCH),
129.8 (quat-C), 129.3 (ꢀCH), 128.6 (ꢀCH), 128.1 (ꢀCH),
127.3 (ꢀCH), 126.9 (ꢀCH), 124.5 (ꢀCH), 119.1 (ꢀCH),
107.8 (ꢀCH), 71.3 (OCH2); mass m/z: 262 (M+). Anal.
calcd for C18H14O2: C, 82.42; H, 5.38. Found: C, 82.34;
H, 5.41.
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14. (a) Typical experimental procedure for the preparation
2-diazo-1-phenylethanone: A solution of benzoyl chloride
(21 mmol) diluted with 40 mL of anhydrous ether was
added dropwise to a solution of freshly prepared ethereal
diazomethane (300 mL, 53 mmol) at 0°C over a 1 h
period under an argon atmosphere. The resulting reaction