Oxygenation vs iodonio substitution during the reactions of alkenylsilanes with iodosylbenzene: Participation of the internal oxy group

Article abstract of DOI:10.1039/b615888a

Reaction of 4-acyloxybut-1-enylsilanes with iodosylbenzene in the presence of BF₃·OEt₂ gave 4-acyloxy-2-oxobutylsilane and 3-acyloxytetrahydrofuran-2-ylsilane via a 1,3-dioxan-2-yl cation intermediate, which is generated by participation of the acyloxy group during the electrophilic addition of iodine(iii) to the substrate. The Royal Society of Chemistry.

Full text of DOI:10.1039/b615888a

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Oxygenation vs iodonio substitution during the reactions of
alkenylsilanes with iodosylbenzene: participation of the internal oxy
group{
Morifumi Fujita,* Hee Jin Lee, Takashi Sugimura and Tadashi Okuyama
Received (in Cambridge, UK) 1st November 2006, Accepted 8th December 2006
First published as an Advance Article on the web 10th January 2007
DOI: 10.1039/b615888a
Reaction of 4-acyloxybut-1-enylsilanes with iodosylbenzene in
the presence of BF ?OEt gave 4-acyloxy-2-oxobutylsilane and
-acyloxytetrahydrofuran-2-ylsilane via a 1,3-dioxan-2-yl cation
silanes without any detectable yield of the expected alkenyliodo-
18
1
8
nium salt. The O-tracer experiments using H
2
O show that the
3
2
1
8
3
O atom is incorporated at the ester carbonyl group of the
oxygenated product via 1,3-dioxan-2-yl cation. In this commu-
nication, the novel reaction of alkenylsilanes with iodosylbenzene
is detailed.
intermediate, which is generated by participation of the acyloxy
group during the electrophilic addition of iodine(III) to the
substrate.
(
E)-Alk-1-enyltriethylsilanes (E)-1a–e were stereoselectively pre-
9
pared by rhodium-catalyzed hydrosilylation of alkynes, while the
Hypervalent iodine(III) reagents have been employed as a mild
oxidant toward various substrates such as alcohols, phenols, and
Z isomers (Z)-1c and (Z)-1d were obtained by ruthenium-catalyzed
10
hydrosilylation. These alkenylsilanes contain an oxy function
1,2
alkenes. During the oxidation of 1-arylalkenes by iodine(III)
reagents such as iodosylbenzene, migration of the aryl group takes
3
place via benzenium (phenonium) ion. Participation of the
such as acyloxy (1a–d) and methoxycarbonyl (1e) in the alkenyl
moiety. They were subjected to the reaction with iodosylbenzene in
intramolecular aryl group is synthetically applied to selective
the presence of BF ?OEt (Scheme 2) under the general
3 2
3f
3g
6,7
preparation of lactones and a-aryl ketones. Such a neighboring
group participation provides an opportunity to develop new
reactions with iodine(III) for preparation of various types of
conditions used for preparation of alk-1-enyliodonium salts
from alkenylsilanes. The results are summarized in Table 1.
Reactions of (E)-1a and (E)-1b gave (E)-iodonium salts (E)-2a and
(E)-2b, respectively (runs 1 and 2). In contrast, in the reactions of
(E)-1c–e no iodonium salt was detected even in the crude product
mixtures. The products from (E)-1c contained a-silyl ketone 3c,
2-silyltetrahydrofuran 4c, and a small amount of desilylated ketone
5c (runs 3–6). The tetrahydrofuran product was obtained as a
single diastereomer, and its configuration was determined to be cis
4
oxidation products.
Iodosylbenzene has also been employed for preparation of aryl,
5
alkenyl, and alkynyl iodonium salts. Reaction of alk-1-enylsilanes
with iodosylbenzene in the presence of BF ?OEt gives the alk-1-
3
2
6,7
enyl(phenyl)iodonium salts with retention of the olefin geometry.
We recently found that substitution reaction of alkenylboronates
with iodosylbenzene proceeded with inversion of configuration in
8
the presence of an internal oxy group (Scheme 1). A nucleophilic
1
by NOE observed in H NMR (see ESI{). The reaction of the Z
isomer (Z)-1c gave a similar product distribution of a-silyl ketone
11
interaction of the internal oxy group was proposed as a mechanism
for inversion of configuration. We have now examined the
reaction of acyloxy-substituted alkenylsilanes in comparison with
the boronates. Oxygenated products were obtained from the
3c and 2-silyltetrahydrofuran 4c, but the stereochemistry of the
tetrahydrofuran 4c was trans (run 7). Thus, cyclization to give
2-silyltetrahydrofuran 4c is stereospecific. Similar stereospecific
reaction was observed for the reaction of the acetoxy substrates
(E)-1d and (Z)-1d (runs 8–10), and gave a larger amount of
tetrahydrofuran product 4d in comparison with the reaction of the
benzoyloxy substrates 1c. The yield of tetrahydrofuran 4 increased
when water was intentionally added to the reaction vessel before
Scheme 1 Stereochemical inversion in preparation of iodonium salts
from boronates.
Graduate School of Material Science, Himeji Institute of Technology,
University of Hyogo, 3-2-1 Kohto, Kamigori, Hyogo 678-1297, Japan.
E-mail: fuji@sci.u-hyogo.ac.jp; Fax: +81-791-58-0115;
Tel: +81-791-58-0170
{
Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds. See DOI:
0.1039/b615888a
1
Scheme 2 Reaction of alkenylsilane with iodosylbenzene.
Chem. Commun., 2007, 1139–1141 | 1139
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a
Table 1 Reaction of 1 with iodosylbenzene
Isolated yield (%)
b
Distribution
1/2/3/cis-4/trans-4/5
Run Sub.
2
3
cis-4 trans-4
5
c
d
e
1
2
3
4
5
6
7
8
9
1
1
a
(E)-1a
0/100/0/—/—/0
0/100/0/—/—/0
0/0/85/10/0/5
68
54
0
c
(E)-1b
(E)-1c
(E)-1c
(E)-1c
(E)-1c
(Z)-1c
(E)-1d
(E)-1d
56
6
33 39
14 38
32
21 42
0
13
36
6
4
0
0
0
0
4
0
0
22
3
0
0
0
0
0
0
0
c
f
0
0
0
0
0
0
0
0
0/0/46/54/0/0
3/0/41/56/0/0
17 /0/67/0/10/6
g
h
j
i
0
0/0/38/62/0/0
0/0/0/100/0/0
27 /0/29/0/44/0
f
42
0
k
0
1
(Z)-1d
(E)-1e
15/0/64/—/—/22
11
Reaction was carried out in dichloromethane at 240 uC for 1–2 h.
The concentrations of 1, iodosylbenzene, and BF ?OEt were
typically 0.25, 0.30, and 0.30 M, respectively. Product distribution
3
2
b
1
c
of the crude mixture determined by H NMR. Reaction was
d
e
carried out at 0 uC. (E)-2a : (Z)-2a = 92 : 8. (E)-2b : (Z)-2b =
f
98 : 2. In the presence of H
g
O (1% v/v). (Diacetoxyiodo)
.
benzene was employed instead of iodosylbenzene. The sample is a
2
h
7
3 : 27 mixture of (Z)-1c and 3-(triethylsilyl)but-3-enyl benzoate
i j
1c9). The crude mixture contains 1c9 but no (Z)-1c. The sample is
(
a 67 : 33 mixture of (Z)-1d and 3-(triethylsilyl)but-3-enyl acetate
Scheme 3 A plausible mechanism leading to 3 and 4.
shown in Scheme 4. The reaction of (E)-1c with iodosylbenzene
k
1d9). The crude mixture contains 1d9 but no (Z)-1d.
(
1
8
3 2 2
and BF ?OEt was carried out in the presence of H O (99 atom%
1
8
of O) which was added in advance, and the reaction mixture was
starting the reaction (runs 5 and 9). Product distribution of the
reaction with (diacetoxyiodo)benzene (run 6) was silmilar to that
of the reaction with iodosylbenzene in the presence of water. The
reaction of methoxycarbonyl substrate (E)-1e also gave no
iodonium salt 2e but the corresponding a-silyl ketone 3e (run 11).
Selectivity of the formation of iodonium salt 2 vs. oxygenated
products 3–5 is well controlled by position of the acyloxy group of
quenched with normal water. The oxygenated products 3c and cis-
4c were isolated in 14% and 49% yield, respectively, from the
mixture. In mass analysis (ESI+), the 3c has m/z = 329 (M + Na)
and 331 peaks in 50 and 100% relative intensities in comparison
with 100 and 7% for those of the normal 3c. The cis-4c also has
m/z = 329 and 331 peaks in 48 and 100% relative intensities in
comparison with 100 and 3% for those of the normal cis-4c. These
1
a–d. The reactions of (E)-1a and (E)-1b gave (E)-iodonium salt
E)-2. The stereochemical retention of olefin geometry was also
observed for the reaction of simple alkenylsilane substrates
1
8
data clearly suggest that the O atom is incorporated in
(
1
3
18
18
the oxygenated products. The O contents of both O-3c and
18
6,7
O-4c are roughly evaluated to be 60 and 65%, respectively. To
proceeding via addition-elimination mechanism. The carbonyl
oxygen of 1a,b is separated from the double bond by six bonds,
while that of 1c–e giving oxygenated products is separated by five
bonds. These results suggest that the internal carbonyl oxygen
atom participates in the oxygenation reaction. Survival of the silyl
group in the oxygenated products 3 and 4 excludes a possibility of
13
identify which oxygen in these products is labeled, C NMR
150 MHz) measurements were performed. In the ester carbonyl
18
region, two signals were detected for both O-3c and O-4c (d =
(
18
18
1
66.40 and 166.44 ppm for O-4c; d = 166.31 and 166.35 ppm for
1
8
O-4c) due to isotopic shift (see ESI{). The other carbon signals
were the same as those for the normal products. Thus, the
18
O
1
2
formation of those products via iodonium salt 2. A plausible
mechanism for the oxygenation is illustrated in Scheme 3.
For the tetrahydrofuran product 4, regiochemical relationship
between the acyloxy and triethylsilyl groups of 4 is changed from
that of the substrate 1. The migration can be explained by an
atom is incorporated at the ester carbonyl group of 3c and 4c.
1
4
These results agree well with the mechanism shown in Scheme 3.
18
Incorporation of O atom at the ester carbonyl is consistent with a
mechanism involving nucleophilic addition of water at C2 of the
1 2
1,3-dioxan-2-yl cations I and I .
1
intermediate formation of 1,3-dioxan-2-yl cation I , which is
According to Scheme 3, the selectivity of formation of a-silyl
ketone 3 vs 2-silyltetrahydrofuran 4 is controlled by the step of
bond cleavage of the hemiorthoester moiety of I3 and/or by
generated by participation of the acyloxy group in the electrophilic
addition of the iodine(III) reagent. Stereospecific formation of the
tetrahydrofuran 4 must be resulted from the ring opening of the
competition of nucleophilic addition to I
elimination of I to give I . Difference between the benzoyloxy
substrate 1c and acetoxy one 1d may be attributed to the
1 3
to give I vs 1,2-
3
intermediate I at the hemiorthoester function and the ensuing
participation of the hydroxy group in the departure of phenylio-
1
2
donio group. Formation of a-silyl ketone 3 can also be explained
1
by the pathway via the 1,3-dioxan-2-yl cation I . According to the
reaction mechanism, the oxygen atom of the oxygenated products
comes from water, which is contained in the starting materials.
Indeed, intentional addition of water (runs 5 and 9) affects the
product distribution.
To obtain further insight into the reaction mechanism involving
18
1
8
18
18
water, O-tracer experiments using H
2
O were carried out as
Scheme 4
2
O-Tracer experiments using H O.
1
140 | Chem. Commun., 2007, 1139–1141
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Synthesis, 2002, 2490; (f) A. C. Boye, D. Meyer, C. K. Ingison,
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competition between the nucleophilic addition and the 1,2-
elimination of I ; the phenyl group stabilizes the cationic site of
to retard the nucleophilic addition pathway leading to a
1
I
1
4
5
(a) Participation of internal chloride has been reported in the
3
4b
tetrahydrofuran product but not much affects the elimination. The
consideration is also consistent with the increasing amount of
tetrahydrofuran product 4 in the presence of water as a
nucleophile.
preparation of vinyl-l -bromane (vinyl bromonium salt). The
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Participation of acyloxy groups giving dioxocarbocation inter-
mediates has been utilized for acceleration and stereochemical
control of nucleophilic substitution reactions such as solvolyses
1
5
and glycosylations. The participation during the reaction of
alkenylboronate with iodosylbenzene also controls the stereo-
7
chemistry of vinylic substitution leading to iodonium salts. In
contrast, for the reaction of alkenylsilane reported here, the
reaction mode changes from substitution to oxygenation under
the control of the participation. The triethylsilyl group remained in
the oxygenated product, while the boronate group was eliminated.
The elimination of the boronate and silyl groups should occur with
nucleophilic assistance from some external source, and the higher
8
9
16,17
Lewis acidity of the boronate than the silyl group
may be
10 C. S. Aric o´ and L. R. Cox, Org. Biomol. Chem., 2004, 2, 2558.
1 For the reaction of (Z)-substrates (Z)-1c and (Z)-1d, 3-(triethylsilyl)but-
-enyl carboxylate (19) was included in the starting substrate (see
1
responsible for the different reaction course of the two classes of
the substrates. Participation of the acyloxy group during the
reaction of alkenylsilanes with iodosylbenzene provides a-silyl
ketone and 2-silyltetrahydrofuran, which are of interest in organic
3
footnote for Table 1). The reaction mixture contained 19 but no (Z)-1,
and most amount of the regioisomeric alkenylsilane 19 was recovered
due to its lower reactivity. Thus, the product distribution is not affected
by the contamination of 19.
2 (a) Four electron oxidation of allylsilanes with iodosylbenzene has been
12b
reported. ; (b) M. Ochiai, E. Fujita, M. Arimoto and H. Yamaguchi,
18
synthesis.
1
In summary, a-silyl ketone and 2-silyltetrahydrofuran have been
prepared from 4-acyloxybut-1-enyl(triethyl)silanes without elim-
ination of the silyl group under the general conditions for
transformation to alk-1-enyliodonium salts. The reaction proceeds
via a 1,3-dioxan-2-yl cation intermediate generated by the
participation of internal carbonyl oxygen.
Tetrahedron Lett., 1983, 24, 777.
13 When the silylketone 3c was treated under the reaction conditions
18
containing H
2
O, mass spectrum of the recovered 3c was the same as
that of the starting 3c. Thus, a possibility of exchange of the carbonyl
18
2
oxygens of 3 with H O is excluded.
1
4 The Wacker type oxidation mechanism is excluded from reaction
1
8
pathways to 3c. According to this mechanism, O atom must be
incorporated as a ketone carbonyl oxygen. For example, J. Tsuji,
Synthesis, 1984, 369.
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Chem. Commun., 2007, 1139–1141 | 1141