On the regioselectivity of the PIFA-mediated bis(trifluoroacetoxylation) of styrene-type compounds

Article abstract of DOI:10.1016/j.tet.2007.12.045

The addition of the hypervalent iodine reagent PIFA [phenyliodine(III) bis(trifluoroacetate)] to a series of styrene-type compounds results in the bis(trifluoroacetoxylation) of the double bond as two possible 1,2- and 1,1-regioisomers. We found that 1,1-regioisomers resulted to be unstable during chromatographic purification yielding the related arylacetaldehydes. In this paper, we show our efforts to explore the regioselectivity of this reaction, and to rationalize the results with respect to the electronic nature of the corresponding aryl ring through alternative mechanistic pathways.

Full text of DOI:10.1016/j.tet.2007.12.045

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 2465e2470
www.elsevier.com/locate/tet
On the regioselectivity of the PIFA-mediated
bis(trifluoroacetoxylation) of styrene-type compounds
*
Imanol Tellitu , Esther Domınguez
*
´
Departamento de Qu´ımica Orga´nica II, Facultad de Ciencia y Tecnolog´ıa, Universidad del Pa´ıs VascodEuskal Herriko Unibertsitatea,
PO Box 644, 48080 Bilbao, Spain
Received 26 September 2007; received in revised form 13 December 2007; accepted 20 December 2007
Available online 22 January 2008
Abstract
The addition of the hypervalent iodine reagent PIFA [phenyliodine(III) bis(trifluoroacetate)] to a series of styrene-type compounds results in
the bis(trifluoroacetoxylation) of the double bond as two possible 1,2- and 1,1-regioisomers. We found that 1,1-regioisomers resulted to be un-
stable during chromatographic purification yielding the related arylacetaldehydes. In this paper, we show our efforts to explore the regioselec-
tivity of this reaction, and to rationalize the results with respect to the electronic nature of the corresponding aryl ring through alternative
mechanistic pathways.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Hypervalent iodine; Styrenes; PIFA; Phenonium ions
1. Introduction
bis(trifluoroacetate)4 or iodine(III) tris(trifluoroacetate),5 per-
forms the bis(trifluoroacetoxylation) of the CeC double
bond although with limited success. More recently,6 C¸ elik
and Balci have demonstrated the efficiency of PIFA [phenylio-
dine(III) bis(trifluoroacetate)] in the direct transformation of
acyclic as well as cyclic alkenes into 1,2- and/or 1,3-bis(tri-
fluoroacetoxy) derivatives.7 Particularly, under such condi-
tions, styrene was transformed into 1-phenyl-1,2-ethanediol
in a 90% yield after ammonolysis of the bis(trifluoroacetoxy-
lated) intermediate generated. Additionally, in a previous con-
nected study, Koser had reported 2 years earlier, in 2004, on
the oxidative rearrangement of 1,1-disubstituted alkenes with
HTIB [hydroxy(tosyloxy)iodo]benzene] in methanol as a pro-
tocol for the synthesis of a-arylketones.8
At the same time when these works were being published,
we were involved in a related long lasting project on the search
for novel applications of PIFA in organic synthesis, and more
precisely on the intramolecular PIFA-mediated olefin amida-
tion of conveniently substituted styrene-type substrates as
a new entrance to the synthesis of quinoline and isoindoline
derivatives.9 In fact, we consider that the synthetic power of
PIFA has been quantitatively underestimated with respect to
Olefins are probably one of the organic functional groups
with the highest diverse reactivity. Stepwise additions, with
initial attack of a nucleophile, an electrophile, or free radical
species, and concerted processes compile a plethora of syn-
thetic strategies for the formation of a diverse array of organic
compounds clearly dominated by a number of important oxi-
dative transformations.1 Among them, the centennial2 OsO4
mediated cis-dihydroxylation of olefins has gained an enor-
mous popularity over the years for its reliability and, more re-
cently, for its evolution into an effective asymmetric strategy
for the preparation of enantiomerically pure vicinal diols
when used in the presence of chiral ligands.3 Although used
under substoichiometric conditions, the toxicity of the osmium
reagents has led to the exploration of novel systems with the
same synthetic purpose. In particular, and among others, the
use of hypervalent iodine reagents, such as propyliodine(III)
* Corresponding authors. Tel.: þ34 94 601 5438; fax: þ34 94 601 2748.
E-mail address: imanol.tellitu@ehu.es (I. Tellitu).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.12.045

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I. Tellitu, E. Dom´ınguez / Tetrahedron 64 (2008) 2465e2470
other hypervalent iodine reagents (i.e., HTIB or PIDAd
phenyliodine(III)diacetate), and in this particular context, we
expect that the high electrophility of the iodine center in
PIFA, as well as the lability of the trifluoroester groups that
are eventually transferred to the substrates, might have impor-
tant consequences in the outcome of the reaction. Thus, in this
communication we report additional results on the action of
PIFA on such kind of derivatives to show that the regioselec-
tivity of the reaction (1,1- vs 1,2-diester formation) depends
mostly on the activation of the aryl ring.
with PIFA also under solvent-free conditions. In particular, un-
der such circumstances, we found that styrene 1a reacted with
PIFA yielding a mixture of regioisomers 2a and 3a in a 1.7/1
ratio,16 and contrarily, styrene 2b was transformed into aldehyde
4b in the absence of solvent, but with a moderate 51% yield.
Therefore, in order to gain more information about the in-
sights of this transformation, the behavior of a series of styrenes
1ceg with different activation pattern with the iodine(III) re-
agent was studied both in solution and under solvent-free con-
ditions. From the information summarized in Table 1, it can
be deduced that 1,1-addition to the double bond dominates
when starting from activated substrates (1bee), and contrarily,
non-activated styrenes tend to afford the 1,2 adducts 2a,f.17 Be-
sides, in some cases (entries 3, 4, 5, 9e12), the generation of the
corresponding arylacetaldehydes 4 is detected at some extent in
the reaction mixture even before the addition of silica gel. It was
also noticeable that vinylpyridine 1g reacts violently with PIFA
under solvent-free conditions, and extremely quickly in solu-
tion. In both cases, the dark residue that was obtained could
not be analyzed. Finally, in none of these cases under study,
we found a benefit from performing the experiment in the
absence of solvent apart from the shorten reaction times that
were usually needed.18
To the view of these results, the following mechanism can be
postulated (see Scheme 1). Due to its electrophilic character, the
hypervalent iodine reagent reacts with the olefinic double bond
in substrates 1aef to afford the iodonium intermediates I, which
are opened by a nucleophilic attack of the trifluoroacetate group.
Final displacement of iodobenzene by a second trifluoroacetate
group20 (route A) affords the 1,2-bis(trifluoroacetates) 2. Alter-
natively, activated aryl rings (in substrates 1bee)21 can proceed
as a second nucleophilic competitor giving rise, in this cases, to
intermediates III following route B. These phenonium interme-
diates recover their aromaticity by reacting with a trifluoroace-
tate group at their most activated benzylic position furnishing
the 1,1-bis(trifluoroacetates) 3, which, on standing on silica
gel, are transformed into the arylacetaldehydes of type 4. It
has to be mentioned that the generation of phenonium ion inter-
mediates had already been postulated22 and their existence
demonstrated,23 even under the action of hypervalent iodine
species.24
2. Results and discussion
We have previously mentioned that styrene (1a) affords
1-phenyl-1,2-bis(trifluoroacetoxy)ethane (2a) in the presence
of PIFA in almost quantitative yield when the reaction is carried
out in CH2Cl2 as solvent. Considering the importance of the
1-aryl-1,2-ethanodiol skeleton in organic synthesis,10 it is sur-
prising that the study on the action of PIFA has not been extended
to other members of the family of styrene compounds. Alerted
by the absence of research in this area, and continuing with our
interest in discovering novel applications of PIFA in organic
synthesis,11 we started our study by exploring the action of
this iodine(III) reagent on a series of commercially available
styrenes 1aeg (see Chart 1).
Styrenes 1a and 1b were selected to optimize the reaction
conditions. Our preliminary observations led us to conclude
that the nature of the aryl ring, and also the dilution of the reac-
tion at some extent, had a critical effect on the regioselectivity of
the process. Thus, while treatment of styrene 1a with PIFA
(0.3e0.5 M in CH2Cl2 at room temperature)12 rendered exclu-
sively the 1,2-diester 2a in 4 h, styrene 1b afforded the related
1,1-diester 3b with complete regioselectivity in few minutes.
It has to be mentioned that, unlike 2a, diester 3b happened to
be unstable during column chromatography resulting in the for-
mation of 2-(2-methoxyphenyl)acetaldehyde (4b). Thus, in
order to optimize the work up, and prior to its purification, the
non-isolated diester 3b was treated with silica gel and the stir-
ring was continued overnight to afford aldehyde 4b in 74%
yield.13 This completely regioselective one-pot transformation
can be considered as a metal-free alternative to the Wacker pro-
cess for which the highly selective formation of aldehydes
through catalytic oxidation of styrene-type compounds without
CC double bond cleavage remains a challenge.14,15 Later, and
looking for an even simpler way to perform this conversion,
we considered exploring the behavior of this family of substrates
The extension of our research to allylbenzenes 1h,i offers
coherent results, as they perfectly matched our original mech-
anistic proposal. As expected, allylbenzene (1h) was trans-
formed into the 1,2-bis(trifluoroacetate) 2h by the action of
PIFA in CH2Cl2 at room temperature in an excellent 93%
yield, and coherently, p-methoxyallylbenzene (1i) afforded
an inseparable 1:1 mixture of the corresponding 1,2-bis(tri-
fluoroacetate) 2i and 1,3-bis(trifluoroacetate) 5i in a combined
83% yield as the result of the lack of regioselectivity in the
opening of the phenonium ion VI (see Scheme 2).25 Similarly
to our previous routine, the synthesis of diester 2h under sol-
vent-free conditions took place in almost quantitative yield,
but contrarily, a more complex mixture was obtained when
1i was treated with PIFA in the absence of solvent.
R1
R2
N
R3
1g
1a, R1=R2=R3=H
1b, R1=MeO, R2=R3=H
1c, R1=R2=H, R3=MeO
1d, R1=H, R2=R3=MeO
R
1e, R1=R2=H, R3=Me
1f, R1=R2=H,R3=Cl
1h, R=H
1i, R=MeO
It should be noted the different regiochemistry in the ring
opening of intermediates I and IV. We propose that iodonium
Chart 1.

I. Tellitu, E. Dom´ınguez / Tetrahedron 64 (2008) 2465e2470
2467
Table 1
Selected synthetic and spectroscopic information for the action of PIFA on styrenes 1aef
OCOCF3
OCOCF3
OCOCF3
H
PIFA
silica gel
OCOCF3
Ar
Ar
1a–f
Ar
and / or
CH2Cl2
(from 3)
O
2a–f
3a–f
4a–f
Entry
Styrene
Methoda
2 (%)
Ratio 2/3/4b
3 (dH-1, dH-2, JH-1/H-2
)
4c (%)
1
2
1a
A
B
82
50
100/0/0
63/37/0
7.08 ppm, 3.30 ppm, 6.1 Hz
7.22 ppm, 3.31 ppm, 5.7 Hz
7.04 ppm, 3.24 ppm, 5.9 Hz
7.04 ppm, 3.23 ppm, 5.7 Hz
7.04 ppm, 3.25 ppm, 6.0 Hz
23
3
4
1b
1c
1d
1e
1f
A
B
0
0
0/88/12
0/80/20
74
51
5
6
A
B
0
0
0/75/25
0/100/0
85
64
7
8
A
B
0
0
0/100/0
0/100/0
39
dd
9
10
A
B
Traces
Traces
5/73/22
5/90/5
69
63
11
12
a
A
B
8717
7717
95/0/5
95/0/5
Traces
Traces
Method A: styrene/PIFA (1/1.2), 0.40 M in CH2Cl2; method B: styrene/PIFA (1/1.2), under solvent-free conditions.
This selectivity was determined from the 1H NMR spectra of the crude mixture before addition of silica gel.
Arylacetaldehydes 4 are extremely unstable, and they were isolated as oils by column chromatography. Some of them are commercially available and others
b
c
have been described previously. In any case, their spectroscopic data match literature values.19
d
A complex mixture of compounds is obtained.
I is opened in an SN1 type process by localizing the positive
charge at the most stabilized benzylic position and, conversely,
the trifluoroacetate group attacks iodonium IV at its most
accessible position following an SN2 pathway.
obtained, encourage to perform the synthesis of the ketones
4j,k from styrenes 3j,k under solvent-free conditions, although
the discordant result from 1l does not allow rationalizing the
actual effect of structure of the substrate on the reactivity in
the absence of solvent.
Finally, in order to expand the limits of our study, a-methyl-
styrenes 1j,k and trans-anethole 1l were submitted to the
action of PIFA under both diluted and solvent-free reaction
conditions. Three of them followed the general mechanistic
proposal, shown in Scheme 1, giving rise in less than 10 min
to the formation of the carbonyl compounds 4jel in 93%,
89%, and 42% yield, respectively (see Table 2).26 These re-
sults indicate that the preliminary formation of phenonium in-
termediates of type VII and VIII had to occur (see Chart 2) to
render, after 1,2-aryl migration, gem-diesters 3jel, which on
treatment with silica gel furnished the reported compounds
4jel.27 The simplicity of the method (addition of PIFA and
further addition of silica gel), and the equally excellent yields
3. Conclusions
The behavior of a series of styrenes in the presence of PIFA
has been studied to conclude that non-activated substrates suffer
1,2-addition of trifluoroacetate groups across the olefinic double
bound, whilst a related 1,1-addition dominates the reactivity on
activated ones. A mechanistic description of this reaction in-
cludes the generation of a phenonium ion with concomitant
1,2-migration of the aryl ring. Taking advantage of the lability
of such gem-diesters, they can be transformed into the corre-
sponding carbonyl compounds on standing with silica gel.
Ph
I
CF3COO
OCOCF3
a)
OCOCF3
OCOCF3
OCOCF3
route A
-PhI
OOCCF3
1a–f
Ar
2
Ar
Ar
b)
IPh
I
II
route B
OCOCF3
OCOCF3
- PhI
Ar
3
OCOCF3
OCOCF3
OOCCF3
OCOCF3
silica gel
Ar
Ar
Ar
H
O
OOCCF3
OOCCF3
Ar
4
III
Scheme 1. Proposed alternative mechanisms for the action of PIFA on styrenes 1aef.

2468
I. Tellitu, E. Dom´ınguez / Tetrahedron 64 (2008) 2465e2470
Ph
I
b)
IPh
OOCCF3
OCOCF3
OOCCF3
(from 1i)
Ar
Ar
a)
OOCCF3
OCOCF3
OCOCF3
MeO
IV
V
VI
route b)
-PhI
route a)
-PhI
(from 1h)
- PhI
OX
OX
Ar
OX
Ph
OX
OX
OX
Ar
5i, X=COCF3
7i, X=H
2h, X=COCF3
6h, X=H
NaBH4
MeOH
NaBH4
MeOH
NaBH4
MeOH
2i, X=COCF3
6i, X=H
Scheme 2. Proposed explanation for the action of PIFA on olefins 1h (Ar¼Ph) and 1i (Ar¼p-MeOC6H4).
This unprecedented procedure for the direct generation of aryl-
acetaldehydes from 1-arylethenes can be considered a competi-
tive metal-free alternative to the Wacker reaction.
R
MeO
1l
1j, R=H
1k, R=Me
4. Experimental section
CF3COO
Me
OCOCF3
CF3COO
OCOCF3
4.1. General procedure for the synthesis of bis(trifluoro-
acetates) 2
Me
VIII
VII
R
MeO
To a magnetically stirred solution of the corresponding sty-
rene (1 mmol) in CH2Cl2 (5 mL), PIFA (1.2 mmol) was added
in one portion and the stirring continued at room temperature
until total consumption of the starting material. After removal
of the solvent, the residue was chromatographed on silica gel
eluting with hexane/ethyl acetate (80:20) to give the 1,2-di-
esters as colorless oils.
(from 1j, R=H)
(from 1l)
(from 1k, R=Me)
Chart 2.
4.2. General procedure for the synthesis of carbonyl
compounds 4
To a magnetically stirred solution of the corresponding
styrene (1 mmol) in CH2Cl2 (3 mL), PIFA (1.2 mmol) was
added at once. After 15 min, silica gel was added and the stir-
ring continued overnight. The mixture was purified by column
chromatography on silica gel eluting with hexane/ethyl acetate
(90:10) to give the related carbonyl compounds 4 as yellowish
oils.
Table 2
Selected synthetic and spectroscopic information for the action of PIFA on
styrenes 1jel
R
R
PIFA
CH2Cl2
silica gel
OCOCF3
R'
1j–l
Ar
CF3COO
Ar
R'
O
3j–l
4j–l
3/4j, Ar=Ph, R=H, R'=Me
3/4k, Ar=p-MeC6H4, R=H, R'=Me
3/4l, Ar=p-OMeC6H4, R=Me, R'=H
4.3. General procedure for the synthesis of diols 6f, 6i, 6j,
and 7j
Styrene Methoda Ratio 3/4b 3 (dOCHO, dArCH, dMe
)
4 (%)c
1j
A
B
90/10
65/35
d, 3.56 ppm, 2.05 ppm
93
91
Some 1,2-diesters (2f) resulted to be partially unstable and,
therefore, they were identified as the corresponding 1,2-diols.
To a cooled (0 ꢀC) magnetically stirred solution of the corre-
sponding 1,2- or 1,3-diester (1 mmol) in MeOH (10 mL),
NaBH4 (5 mmol) was added slowly. The stirring continued
overnight at room temperature, water was added, and the mix-
ture extracted with CH2Cl2. The corresponding diols were
purified by column chromatography on silica gel eluting
with hexane/ethyl acetate (80:20).
The selected spectroscopic data for new products are shown
below. The characterization data for known products 2a,
4aee, 4jel and copies of 1H and 13C NMR spectra are provided
in Supplementary data.
1k
A
B
85/15
90/10
d, 3.50 ppm, 2.02 ppm
89
91
1l
A
B
100/0
dd
6.98 ppm, 3.30 ppm, 1.44 ppm 42
a
Method A: styrene/PIFA (1/1.2), 0.40 M in CH2Cl2; method B: styrene/
PIFA (1/1.2), under solvent-free conditions.
b
This composition was determined from the 1H NMR spectra of the crude
mixture before addition of silica gel.
c
Carbonyl compounds 4jel were isolated as oils by column chromatogra-
phy. They are all commercially available or have been described previously,
and their spectroscopic data match literature values.
d
A complex mixture of compounds is obtained.

I. Tellitu, E. Dom´ınguez / Tetrahedron 64 (2008) 2465e2470
2469
J. Org. Chem. 2005, 70, 2893e2903; (g) Wirth, T. Angew. Chem., Int.
Ed. 2005, 44, 3656e3665.
8. Justik, M. W.; Koser, G. F. Tetrahedron Lett. 2004, 45, 6159e6163.
4.4. Characterization data for new products
4.4.1. 1,2-Bis(trifluoroacetoxy)-3-phenylpropane (2h)
1H NMR (300 MHz, CDCl3) d 7.41e7.20 (m, 5H), 5.50
(ddd, J¼9.8, 6.9, 2.8 Hz, 1H), 4.58 (dd, J¼12.2, 2.8 Hz,
1H), 4.37 (dd, J¼12.2, 6.9 Hz, 1H), 3.14e2.99 (m, 2H); 13C
NMR (300 MHz, CDCl3) d 157.3e156.4 (m, 2ꢁCO), 133.9,
129.2, 129.1, 127.7, 114.2 (q, 2ꢁCF3), 75.4, 66.2, 36.3; MS
m/z (% rel inten.) 230 (MþꢂCH2OCOCF3, 67), 117 (53), 91
(100); HRMS calcd for C11H10F3O2 (MþꢂCH2OCOCF3)
231.0633, found 231.0628.
´
9. (a) Serna, S.; Tellitu, I.; Domınguez, E.; Moreno, I.; SanMartın, R. Tetra-
´
hedron Lett. 2003, 3483e3486; (b) Tellitu, I.; Urrejola, A.; Serna, S.;
´
Moreno, I.; Herrero, M. T.; Domınguez, E.; SanMartin, R.; Correa, A.
Eur. J. Org. Chem. 2007, 437e444.
10. (a) Hudlicky, T.; Thorpe, A. J. Chem. Commun. 1996, 1993e2000; (b)
Carless, H. A. J. Tetrahedron: Asymmetry 1992, 3, 795e826.
11. For some recent contributions from our group, see: (a) Moreno, I.; Tellitu,
´
I.; Etayo, J.; SanMartin, R.; Domınguez, E. Tetrahedron 2001, 57,
´
5403e5411; (b) Serna, S.; Tellitu, I.; Domınguez, E.; Moreno, I.;
SanMartin, R. Org. Lett. 2005, 7, 3073e3076; (c) Correa, A.; Tellitu, I.;
´
Domınguez, E.; Moreno, I.; SanMartin, R. J. Org. Chem. 2005, 70,
´
2256e2264; (d) Correa, A.; Tellitu, I.; Domınguez, E.; SanMartin, R.
Org. Lett. 2006, 8, 4811e4813.
4.4.2. 1,2-Bis(trifluoroacetoxy)-3-(4-methoxyphenyl)propane
(2i) and 1,3-bis(trifluoroacetoxy)-2-(4-methoxyphenyl)
propane (5i)
12. At higher dilution, the reaction time was prolonged excessively.
13. Diesters 3aee were identified by the aid of selective TOCSY experiments
from the crude reaction prior to the addition of silica gel. See selected
information in Table 1.
1H NMR (500 MHz, CDCl3) d 7.16e7.10 (m, 2H-5i, 2H-2i),
6.91e6.85 (m, 2H-2i, 2H-5i), 5.45 (ddd, J¼9.6, 7.2, 2.6 Hz,1H-
2i), 4.65e4.54 (m, 1H-2i, 4H-5i), 4.36 (dd, J¼12.2, 6.9 Hz,
1H-2i), 3.81 (s, 3H, 2i/5i), 3.80 (s, 3H, 5i/2i), 3.57e3.48 (m,
1H, 5i), 3.07e2.93 (m, 2H, 2i); 13C NMR (300 MHz, CDCl3)
d 158.6, 158.1, 157.0e155.2 (m, 4ꢁCO), 129.2, 127.8, 126.3,
124.8, 119.1, 115.3, 113.5, 113.4 (q, 4ꢁCF3), 113.3, 74.6
(2i), 66.6 (5i), 65.2 (2i), 54.1, 54.0, 41.4 (5i), 34.4 (2i);
HRMS calcd for C14H12F6O5 374.0589, found 374.0591.
14. (a) The oxidation of alkenes to carbonyl compounds catalyzed by
palladium(II) salts (known as the Wacker reaction) normally proceeds
with Markovnikov regioselectivity. For a novel anti-Markovnikov regiose-
lectivity (leading to aldehydes from terminal olefins) in the Wacker
reaction of styrenes, see: Wright, J. A.; Gaunt, M. J.; Spencer, J. B.
Chem.dEur. J. 2005, 12, 949e955 and more recently, Muzart, J. Tetra-
hedron 2007, 63, 7505e7521; (b) The catalytic anti-Markovnikov hydra-
tion of terminal alkynes to aldehydes has been also described. See:
Grotjahn, D. B.; Lev, D. A. J. Am. Chem. Soc. 2004, 126, 12232e12233.
15. Phenylacetaldehyde (4a) is a common byproduct also obtained directly
during the oxidation of styrene into styrene oxide by a number of reaction
Acknowledgements
conditions. For
a recent example, see: Zhang, J.-L.; Che, C.-M.
Chem.dEur. J. 2005, 11, 3899e3914. Additionally, it has been shown
that styrene oxides suffer rearrangement of the epoxide ring with InCl3
to afford the corresponding arylacetaldehydes in good yields. See:
Ranu, B. C.; Jana, U. J. Org. Chem. 1998, 63, 8212e8216.
Financial support from the University of the Basque Country
(UPV 41.310-13656) and the Spanish Ministry of Science and
Technology (CTQ 2004-03706/BQU) is gratefully acknowl-
edged. The authors gratefully acknowledge PETRONOR,
S. A. (Muskiz, Bizkaia) for the generous gift of hexanes.
16. (a) Koser reported a similar alteration of the regioselectivity using HTIB.
See: Koser, G. F. J. Org. Chem. 1981, 46, 4324e4326; (b) Another exam-
ple of the use of hypervalent iodine reagents under solvent-free conditions
can be found in: Yusubov, M. S.; Wirth, T. Org. Lett. 2005, 7, 519e521.
17. Diester 2f resulted to be very unstable on silica gel. Therefore, it was iden-
tified from the crude 1H NMR by the distinctive A2M system of its
aliphatic protons [selected absorptions: 6.20 (dd, J¼8.3, 3.6, 1H, H-1);
4.69 (dd, J¼12.2, 8.3, 1H, H-2a); 4.59 (dd, J¼12.2, 3.6, 1H, H-2b)],
and by its transformation into the corresponding known 1-(p-chloro-
phenyl)-1,2-ethanediol 6f (Tsujigami, T.; Sugai, T.; Ohta, H. Tetrahedron:
Asymmetry 2001, 12, 2543e2549) using NaBH4 in MeOH. The yield
given in Table 1 is referred to the combined steps.
Supplementary data
Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2007.12.045.
References and notes
18. It must be pointed out that a control experiment carried out with styrene
1c using PIDA [phenyliodine(III) diacetate], instead of PIFA, under the
same reaction conditions resulted in the recovery of the starting material
completely unchanged. This result evidences the superior activity of PIFA
over PIDA.
1. (a) Modern Oxidation Methods; Backvall, J.-E., Ed.; Wiley-VCH:
¨
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21. In these cases, donating methoxy groups are placed at 2- and 4-positions,
an electronic feature required to stabilize positively charged species III.
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1d disfavors the stability of intermediate III, which explains the dramatic
decrease in the yield for its transformation into 4d.
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2470
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´
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Del Rio, E.; Menendez, M. I.; Lopez, R.; Sordo, T. L. J. Phys. Chem. A
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26. Carbonyl compounds 4jel were spectroscopically identified by compari-
son with the reported data. For ketone 4j, see: Khanh-Van Tran, K.-V.;
Bickar, D. J. Org. Chem. 2006, 71, 6640e6643; For 4k, see: Molinaro,
C.; Mowat, J.; Gosselin, F.; O’Shea, P. D.; Marcoux, J. F.; Angelaud,
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25. (a) In both cases, olefins 1h,i required 12e14 h for their complete conver-
sion instead of the standard 3e4 h; (b) Aliphatic olefins behave in a similar
way with PIFA to afford 1,2-bistrifluoroacetates. See Ref. 6; (c) These
three diesters were identified by their spectroscopic behavior, but for
additional authentication purposes, they were transformed into the corre-
sponding known 1,2-diols 6h,26 6i,27 and 1,3-diol 7i by reductive treat-
ment with NaBH4 in methanol. For spectroscopic data of diol 6h, see:
4l, see: Baumann, T.; Vogt, H.; Brase, S. Eur. J. Org. Chem. 2007,
266e282.
¨
27. At this stage, there is no clear explanation to the fact that no 1,2-addition
of trifluoroacetate groups to styrene 1j is detected, as it would have been
anticipated considering the lack of activation of the phenyl ring.