Highly regioselective rearrangement of 2,3-diaryl-oxiranes into 2,2-diarylacetaldehydes catalyzed by ytterbium porphyrin complex, Yb(TPP)OTf

Article abstract of DOI:10.3987/COM-08-S(F)27

[5,10,15,20-Tetraphenylporphynato]ytterbium(III) triflate, Yb(TPP)-OTf, is an efficient Lewis acid catalyst for the highly regioselective rearrangement of 2,3-diaryloxiranes into 2,2-diarylacetaldehydes. The catalytic process can be easily performed not only in dichloroethane but also in a more environmentally friendly solvent, benzotrifluoride.

Full text of DOI:10.3987/COM-08-S(F)27

HETEROCYCLES, Vol. 77, No. 1, 2009
365
HETEROCYCLES, Vol. 77, No. 1, 2008, pp. 365 - 374. © The Japan Institute of Heterocyclic Chemistry
Received, 11th June, 2008, Accepted, 25th July, 2008, Published online, 28th July, 2008.
DOI: 10.3987/COM-08-S(F)27
HIGHLY REGIOSELECTIVE REARRANGEMENT OF 2,3-DIARYL-
OXIRANES INTO 2,2-DIARYLACETALDEHYDES CATALYZED BY
YTTERBIUM PORPHYRIN COMPLEX, Yb(TPP)OTf
Toshikatsu Takanami,* Shin-ichiro Nakajima, Shoichi Nakadai,
Fumio Hino, and Kohji Suda*
Meiji Pharmaceutical University, 2-522-1, Noshio, Kiyose, Tokyo 204-8588,
Japan
e-mail: takanami@my-pharm.ac.jp; suda@my-pharm.ac.jp
Abstract – [5,10,15,20-Tetraphenylporphynato]ytterbium(III) triflate, Yb(TPP)-
OTf, is an efficient Lewis acid catalyst for the highly regioselective rearrangement
of 2,3-diaryloxiranes into 2,2-diarylacetaldehydes. The catalytic process can be
easily performed not only in dichloroethane but also in a more environmentally
friendly solvent, benzotrifluoride.
INTRODUCTION
2,2-Diarylacetaldehydes are useful synthetic precursors of many biologically active natural and synthetic
products that contain diarylmethane structural motifs.1 Among numerous methods for the preparation of
2,2-diarylacetaldehydes,1,2 acid-promoted rearrangement of 2,3-diaryloxiranes constitutes one of the most
straightforward strategies, and hence a number of different Lewis acid-catalyzed methods have been
developed for this purpose.3,4 However, most of these developed methods appear to suffer from lack of
regioselectivity, usually affording a mixture of 2,2-diarylacetaldehydes and their regioisomeric products,
1,2-diarylethanones, and reduced yields of the desired aldehydes are therefore often obtained. Thus, there
is a need to develop an alternative catalyst to promote such rearrangement with higher efficiency and
regioselectivity.
Our laboratory has recently developed iron(III) and chromium(III) porphyrin complexes (Figure 1) as a
new family of highly regio- and stereoselective Lewis acid catalysts for the ring-opening rearrangement
of various epoxides, including α,β-epoxy silyl ethers, α,β-epoxy enones, and monoalkyl-substituted
epoxides, to the corresponding carbonyl compounds.5,6 This paper describes the efficient rearrangement
of 2,3-diaryloxiranes 1 to 2,2-diarylacetaldehydes 2 that proceeds with high levels of yields and regio-
This paper is dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75th birthday.

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selectivity using a new porphyrin-based Lewis acid catalyst, [5,10,15,20-tetraphenylporphynato]-
ytterbium(III) triflate, Yb(TPP)OTf. Another advantage of the method is that the reported rearrangements
using the Yb(TPP)OTf catalyst system can be easily performed in benzotrifluoride (BTF),7 which has
recently emerged as a useful alternative solvent for organic reactions currently conducted in
dichloromethane (DCM) and related solvents, without deterioration in yields and regioselectivity. This is
considered of increasing importance as greener processes are sought because of environmental concerns.
X
Ph
Fe(TPP)OTf: M = Fe, X = OTf
N
Ph
N
Cr(TPP)OTf: M = Cr, X = OTf
Yb(TPP)Cl: M = Yb, X = Cl
Yb(TPP)OTf: M = Yb, X = OTf
M
N
Ph
N
Ph
Figure 1
. Structures of porphyrin-based Lewis acid catalysts
RESULTS AND DISCUSSION
cis-Stilbene oxide cis-1a was chosen as a test substrate in the initial experiments to optimize the reaction
conditions, because cis-2,3-diaryloxiranes often provide lower regioselectivity than the trans isomers in
Lewis acid-promoted rearrangements.3a,3c Thus, we evaluated various Lewis acid catalysts in the
rearrangement of model substrate cis-1a, and the results are illustrated in Table 1. When we attempted the
rearrangement of cis-1a with 1 mol% of porphyrin-based Lewis acid catalysts, Fe(TPP)OTf and
Cr(TPP)OTf, in dichloroethane (DCE) at 83 °C, we obtained the corresponding aldehyde 2a in only
moderate yields along with substantial amounts of undesired isomeric ketone 3a (entries 1 and 2). Other
Lewis acids, such as Er(OTf)3,3a Yb(OTf)3, Bi(O)ClO4,3b Bi(OTf)3,3c and [(η5-C5H5)Fe(CO)2(THF)]BF4,3d
most of which are known as effective Lewis acid catalysts for the rearrangement of epoxides to carbonyl
compounds, also gave unsatisfactory results to obtain moderate yields of the desired aldehyde 2a (entries
7–11). In the case with InCl3,3e no reaction occurred to recover the starting epoxide cis-1a (entries 12 and
13).8 Despite these discouraging results, we recognized that lanthanide- and bismuth-based Lewis acids
can afford slightly higher regioselectivity compared to the porphyrin-based Lewis acid catalysts (cf.
entries 1, 2, and 7–10), and hence we assumed that the use of metalloporphyrin complexes possessing
such central metal ions as Er(III), Yb(III), and Bi(III) as catalysts might improve the regioselectivity of
the reaction. Among these porphyrin complexes, Yb(III)-containing complexes have been well-charac-
terized with regard to their structures and physical properties, and several efficient methods for their
preparation are available in the literature.9 Therefore, we chose Yb(TPP)Cl and Yb(TPP)OTf as
porphyrin-based Lewis acid catalysts, and assessed their catalytic ability toward the rearrangement of

HETEROCYCLES, Vol. 77, No. 1, 2009
367
cis-1a. Indeed, while Yb(TPP)Cl could not promote the rearrangement (entry 3), treatment of epoxide
cis-1a with 1 mol% of Yb(TPP)OTf in DCE at 83 °C resulted in a highly regioselective rearrangement to
afford 89% yield of the desired aldehyde 2a along with only a trace amount (<5%) of ketone 3a (entry 4).
Further optimization revealed that DCE was the optimal solvent (entries 4–6).
Using the optimal conditions, we next explored the scope and generality of the procedure (Table 2). As a
whole, the Yb(TPP)OTf catalyst system could easily be applied to various 2,3-diaryloxiranes such as
those having phenyl, naphthyl, and biphenyl moieties. Regardless of whether the stereochemistry of a
substrate is cis or trans, excellent yields and high regioselectivity are typically obtained (entries 1 and 2).
Thus, this catalytic protocol can obviate the need for careful separation of the cis and trans isomers of
Table 1. Lewis acid-catalyzed rearrangement of cis-stilbene oxide cis-1a
O
Ph
O
catalyst (1 mol%)
+
Ph
cis-
O
Ph
Ph
Ph
Ph
1a
2a
3a
yield (%)a
of 2a
yield (%)a
of 3a
conditions
entry
catalyst
solvent/temp/time
1
2
3
4
5
Fe(TPP)OTf
Cr(TPP)OTf
Yb(TPP)Cl
DCE/83 ºC/1 h
DCE/83 ºC/3 h
DCE/83 ºC/12 h
DCE/83 ºC/4 h
dioxane/100 ºC/2 h
toluene/110 ºC/6 h
DCM/rt/2 h
73
23
32
66
no reaction
89
Yb(TPP)OTf
Yb(TPP)OTf
<5
55
21
21
15
16
13
13
6
7
Yb(TPP)OTf
Er(OTf)3
44
73
62
69
74
Yb(OTf)3
Bi(O)ClO4
Bi(OTf)3
8
DCM/rt/1 h
9
DCM/rt/10 h
10
DCM/rt/1 h
BF4
11
OC
OC
DCM/rt/15 h
75
15
Fe+
THF
InCl3
InCl3
12
13
THF/50 °C/24h
DCM/40 °C/24h
no reaction
no reaction
a) Isolated yield.

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HETEROCYCLES, Vol. 77, No. 1, 2009
2,3-diaryloxiranes prior to using these epoxides as the substrates in the rearrangement. For example,
subjecting 1 mol% of Yb(TPP)OTf to a 1:1 mixture of cis- and trans-1a led to the formation of aldehyde
2a as the major product, the yield of which (87%) was comparable to that obtained from the
corresponding single isomers (entry 3). Similar results were obtained for epoxides 1b and 1c that consist
of their cis and trans isomers (entries 4 and 5). As expected, biphenyl-substituted derivative trans-1d
could easily be converted to the corresponding aldehyde 2d, a useful synthetic intermediate for a clay
mimic organic host,10 in a high yield (entry 6). The asymmetrical epoxide 1e that possesses different aryl
substituents on the oxirane ring also proved to be a suitable substrate for this catalytic rearrangement, fur-
Table 2. Yb(TPP)OTf-catalyzed rearrangement of 2,3-diaryloxirane to 2,2-diaryl-
acetaldehydea
entry
1
substrate
time (h)
4
product
Ph
yield (%)b
89
O
Ph
O
Ph
Ph
2a
2a
cis-1a
O
Ph
2
3
4
4
94
87
Ph
trans-1a
O
Ph
2a
Ph
1a (cis/trans = 1:1)
O
Cl
O
Cl
4
5
12
4
90
87
Cl
Cl
2b
1b (cis/trans = 1:1)
O
O
O
1c (cis/trans = 1:1)
2c
O
6
7
5
85
92
trans-1d
2d
O
O
O
O
O
12
O
Br
Br
2e
1e (cis/trans = 1:1)
a) Conditions: Yb(TPP)OTf (1 mol%), DCE, 83 ºC. b) Isolated yield.

HETEROCYCLES, Vol. 77, No. 1, 2009
369
nishing the desired aldehyde 2e in 92% yield (entry 7). In all cases, only trace amounts (<5%) of
undesired isomeric ketones could be detected in the 1H-NMR spectra of the crude reaction mixtures.
We next turned our attention to the use of BTF as a solvent in place of DCE. As briefly mentioned above,
BTF is a potential substitute for DCM and related solvents mainly because of its higher boiling point and
lower toxicity, and its use as reaction media in preparative organic chemistry and homogeneous catalysis
is a topic of ongoing interest.7 However, Lewis acid-mediated reactions in BTF are still limited since BTF
is somewhat labile toward Lewis acidic conditions; for example, the CF3 group of BTF is known to react
with strong Lewis acids such as AlCl3 even at ambient temperature.7 Thus, development of a new, milder
Lewis acid catalyst that can be used in BTF is highly desirable. As illustrated in Table 3, the
Yb(TPP)OTf-catalyzed rearrangement of cis-1a occurred in refluxing BTF with yield and regioselectivity
fully comparable to those obtained from the counterpart reaction in DCE (entry 1, cf. entry 4 in Table 1),
whereas other catalysts, such as Er(OTf)3, Yb(OTf)3, Bi(OTf)3, Bi(O)ClO4, and [(η5-C5H5)Fe-
(CO)2(THF)]BF4, all of which did not work well in BTF, produced significant erosion of yields and
regioselectivity as well as prolonged reaction time (entries 2–6). The present conditions using the
Yb(TPP)OTf catalyst and BTF as a solvent translated smoothly to other 2,3-diaryloxiranes (Table 4). As
expected, all the substrates examined readily undergo the rearrangement to afford the corresponding 2,2-
diarylacetaldehydes in high yields. More noteworthy was the observation that the yields of these reactions
Table 3. Lewis acid-catalyzed rearrangement of cis-stilbene oxide cis-1a in BTF
O
O
Ph
catalyst (1 mol%)
+
Ph
cis-
O
Ph
Ph
Ph
Ph
1a
CF3
BTF
2a
3a
yield (%)a yield (%)a
of 2a of 3a
entry
catalyst
temp (ºC) time (h)
1
2
3
4
5
Yb(TPP)OTf
Er(OTf)3
102
rt
3
10
9
87
<5
51
50
40
51
20
20
12
17
Yb(OTf)3
Bi(O)ClO4
Bi(OTf)3
rt
rt
24
5
rt
BF4
OC
6
rt
46
18
24
Fe+
OC
THF
a) Isolated yield.

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HETEROCYCLES, Vol. 77, No. 1, 2009
in BTF are generally higher than those of the parallel reactions performed in DCE (cf. Table 2). These
results clearly show that BTF can be employed as an excellent replacement for DCE in
Yb(TPP)OTf-catalyzed rearrangements.
Table 4. Yb(TPP)OTf-catalyzed rearrangement of 2,3-diaryloxirane to 2,2-di-
arylacetaldehyde in BTFa
yield (%)b
entry
substrate
1a
time (h)
product
2a
1
2
3
4
5
6
cis-
1a (cis/ trans = 1:1)
3
3
87
91
2a
1b
1c
2b
(cis/trans = 1:1)
(cis/ trans = 1:1)
12
5
93
90
88
93
2c
1d
2d
trans-
6
1e (cis/ trans = 1:1)
2e
12
a) Conditions: Yb(TPP)OTf (1 mol%), BTF, 102 ºC. b) Isolated yield.
CONCLUSION
In summary, we have developed a new class of porphyrin-based Lewis acid catalyst, Yb(TPP)OTf, that
can efficiently promote highly regioselective rearrangement of 2,3-diaryloxiranes to 2,2-diaryl-
acetaldehydes. This rearrangement was successfully carried out in DCM but could also be performed in
the more environmentally friendly solvent BTF. Mechanistic investigations and applications of this class
of catalysts to new transformations are currently underway.
EXPERIMENTAL
1
13
General H- and C-NMR spectra were recorded on a JEOL JNM-AL300 spectrometer. The chemical
shifts were reported in ppm relative to CHCl3 (δ = 7.24) for 1H-NMR and relative to the central resonance
of CDCl3 (δ = 77.0) for 13C-NMR. IR spectra were recorded on a JASCO FT/IR-4100 spectrophotometer.
The mass spectroscopic data were obtained on a JEOL JNM-DX302 spectrometer. UV-visible absorption
spectra were measured with a Hitachi U-3210 spectrophotometer. Dichloroethane (DCE) and
dichloromethane (DCM) were distilled under argon from CaH2. Benzotrifluoride (BTF) was distilled
under argon from P2O5. Tetrahydrofuran was distilled under argon from sodium benzophenone ketyl.
Kanto Kagaku Silica Gel 60 (spherical) and Merck kiesel-gel 60 F254 were employed for silica gel
column and thin layer chromatography, respectively. Both cis- and trans-stilbene oxides, cis-1a and
trans-1a, were purchased from Aldrich. Other 2,3-diaryloxiranes 1b–e were prepared as described in the

HETEROCYCLES, Vol. 77, No. 1, 2009
371
literature.1d,4a,11 Porphyrin-based catalysts, Cr(TPP)OTf and Fe(TPP)OTf, were prepared by the literature
methods.5d Unless otherwise stated, all commercially available chemicals were used without further
purification.
Preparation of [5,10,15,20-tetraphenylporphynato]ytterbium(III) chloride [Yb(TPP)Cl]9
A 300 mL three necked flask equipped with a magnetic stirring bar, a reflux condenser, and rubber septa
was charged with 5,10,15,20-tetraphenyl-21H,23H-porphine [H2TPP] (1.0 g, 1.6 mmol) and 1-chloro-
naphtalene (150 mL). The mixture was stirred at 150 °C for 30 min to dissolve the porphyrin. To the
solution was added YbCl3·6H2O (1.0 g, 2.6 mmol), and the mixture was refluxed until the Soret band of
the free base porphyrin was replaced by that of the ytterbium porphyrin complex at 416 nm. The reaction
mixture was allowed to cool to rt and then concentrated under a reduced pressure. The resulting residue
was dissolved in 10 mL of pyridine, and the solution was poured into ice-cooled water (300 mL). The
resulting precipitate was collected by filtration and washed with copious amounts of water to give the
pyridine-coordinated ytterbium porphyrin complex. The complex was recrystallized from CH2Cl2/hexane
(twice times) and dried in a vacuum oven for 48 h at 150 °C under a reduced pressure (1 mmHg) to give
the pyridine-free Yb(TPP)Cl complex (806 mg, 60%). UV/vis (CH2Cl2): λmax (log ε): 416 (5.4), 552 (4.2)
nm; Anal. Calcd for C44H28ClN4Yb·H2O: C, 62.97; H, 3.60; N, 6.68. Found: C, 63.52; H, 3.49; N, 6.66.
Preparation of [5,10,15,20-tetraphenylporphynato]ytterbium(III) triflate [Yb(TPP)OTf]
A flame-dried 50 mL two necked flask equipped with a magnetic stirring bar, a reflux condenser, and
rubber septa was charged with Yb(TPP)Cl (100 mg, 0.12 mmol) and AgOTf (30 mg, 0.15 mmol). The
reaction vessel was evacuated and flushed with argon (three times), and absolute THF (25 mL) was added.
The mixture was refluxed under argon for 10 h, allowed to reach rt, and concentrated under a reduced
pressure. The residue was dissolved in CH2Cl2 (ca. 30 mL), and filtered through a pad of Celite. The
filtrate was concentrated under a reduced pressure. The resulting solid was recrystallized from
CH2Cl2/hexane (twice times) and dried in a vacuum oven for 24 h at 100 °C under a reduced pressure (1
mmHg) to give Yb(TPP)OTf (97 mg, 85%). UV/vis (CH2Cl2): λmax (log ε): 418 (5.5), 549 (4.2) nm; Anal.
Calcd for C45H28F3N4O3SYb·H2O: C, 56.72; H, 3.17; N, 5.88. Found: C, 56.94; H, 3.25; N, 5.60.
General procedure for the Yb(TPP)OTf-catalyzed rearrangement of 2,3-diaryloxiranes to
2,2-diarylacetaldehydes
To a solution of 2,3-diaryloxirane 3 (0.5 mmol) in DCE or BTF (5 mL) was added Yb(TPP)OTf (4.8 mg,
0.005 mmol). The mixture was refluxed under argon. After completion of the reaction (monitored by
TLC), the reaction mixture was concentrated under a reduced pressure. The residue was passed through a
silica gel short column (1:3 AcOEt−hexane) to remove the catalyst. The eluate was concentrated in vacuo,
and the residue was chromatographed on silica-gel (1:9 AcOEt−hexane) to give 2,2-diarylacetaldehyde 2.

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HETEROCYCLES, Vol. 77, No. 1, 2009
The aldehydes, 2a,12 2c,4a and 2e,1d are known compounds and easily characterized by comparison with
authentic samples.
2,2-Diphenylacetaldehyde (2a) 1H-NMR (CDCl3, 300 MHz) δ: 9.94 (1H, d, J = 2.5 Hz), 7.40-7.14 (10H,
m), 4.88 (1H, d, J = 2.5 Hz).
1
2,2-Bis(2-chlorophenyl)acetaldehyde (2b) H-NMR (CDCl3, 300 MHz) 9.93 (1H, d, J = 0.5 Hz),
7.48-7.45 (2H, m), 7.31-7.22 (4H, m), 7.04-6.97 (2H, m), 5.84 (1H, brs); 13C NMR (CDCl3, 100 MHz): δ
197.1, 135.1, 133.4, 130.5, 130.2, 129.2, 127.1, 58.3; IR (KBr): 3737, 1720, 1651, 1516, 1469, 1045, 748,
671 cm-1; HRMS (EI) m/z: calcd for C14H10Cl2O: 264.0109, found: 264.0107.
1
2,2-Di(naphthalen-1-yl)acetaldehyde (2c) H-NMR (CDCl3) δ: 10.22 (1H, d, J = 2.1 Hz), 7.92-7.7.89
(4H, m), 7.53-7.38 (6H, m), 7.24-7.19 (4H, m), 6.37 (1H, d, J = 2.1 Hz).
2,2-Di(biphenyl-4-yl)acetaldehyde (2d) 1H-NMR (CDCl3) δ: 10.01 (1H, d, J = 2.2 Hz), 7.73-7.31 (18H,
13
m), 4.97 (1H, d, J = 2.2 Hz); C NMR (CDCl3, 100 MHz): δ 198.3, 140.7, 140.5, 135.2, 130.7, 129.6,
128.8, 127.8, 127.1, 63.5; IR (KBr): 3737, 1701, 1647, 1516, 1115, 675, 671 cm-1; HRMS (EI) m/z: calcd
for C26H20O: 348.1514, found: 348.1516.
1
2-(6-Bromobenzo[d][1,3]dioxol-5-yl)-2-phenylacetaldehyde (2e) H-NMR (CDCl3) δ: 9.91 (1H, d, J =
0.9 Hz), 7.40-7.19 (5H, m), 7.06 (1H, s), 6.57 (1H, s), 5.95 (1H, d, J = 3.9 Hz), 5.94 (1H, d, J = 3.9 Hz),
5.39 (1H, brs).
ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Scientific Research (KAKENHI) from JSPS and a Special
Grant (GAKUCHO-GRANT) from Meiji Pharmaceutical University.
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