Conjugate hydrostannation of unsaturated esters by iodotin hydride ate complex

Full text of DOI:10.1021/jo016081e

8
690
J . Org. Chem. 2001, 66, 8690-8692
Con ju ga te Hyd r osta n n a tion of Un sa tu r a ted
Ester s by Iod otin Hyd r id e Ate Com p lex
Sch em e 1
Ikuya Shibata, Toshihiro Suwa, Kyoichiro Ryu, and
Akio Baba*
Department of Molecular Chemistry, Graduate School of
Engineering, Osaka University, 2-1 Yamadaoka, Suita,
Osaka 565-0871, J apan
shibata@ap.chem.eng.osaka-u.ac.jp
Received September 3, 2001
Sch em e 2
Stannylketene acetals bear high reactivity to afford
effective carbon-carbon bond formation with electro-
1
philes. Although the conjugatge hydrostannation of R,â-
unsaturated esters with tin hydrides could provide the
reagents, no such examples have been reported so far.
Until now, all the hydrostannation of R,â-unsaturated
esters has been performed under radical conditions using
2
2
3
elevated temperatures, UV irradiation, ultrasound, and
4
Et B and gave only the R- and â-stannylated adducts
3
Ta ble 1. Red u ction of 1a by Va r iou s Tin Hyd r id esa
where the R/â-selectivity is dependent on the stability of
the radicals (Scheme 1).5
An ionic promotion could allow the conjugate hy-
drostannation to form stannylketene acetals. However,
no ionic method such as metal-catalyzed hydrostannation
of R,â-unsaturated esters has been known so far. Fur-
thermore, it is known that tri-n-butylstannylketene
entry
tin hydride
n-Bu3SnH
time/h
yield/%
6
1
2
3
4
5
6
7
22
0.5
7
0
n-Bu2SnIH
Li [n-Bu2SnI2H]
trace
0
80
0
0
17
acetals are labile and easily isomerize to less reactive
+
-
1
R-stannylesters at temperatures over -78 °C. Hence, to
n-Bu2SnIH/MgBr2‚OEt2
1
achieve the conjugate hydrostannation and prevent the
isomerization of the resulting stannylketene acetals,
novel tin hydride reagents must be applied other than
n-Bu SnIH/BF ‚OEt
22
20
22
2
3
2
n-Bu2SnIH/ZnCl2
n-Bu3SnH/MgBr2‚OEt2
7
tri-n-butylstannyl hydride reagents. We have already
a Tin hydride (1 mmol), 1a (1 mmol), THF (1 mL), room
reported iododi-n-butyltin hydride and its ate complex
such as n-Bu SnIH and Li [n-Bu SnI H] , which pro-
2 2 2
temperature.
8
+
- 9
only by mixing n-Bu
as a solvent is particularly effective for the formation of
the complete 1:1 complex. This solvent efffect indicates
2
SnIH and MgBr
2
2
‚OEt . Using EtOAc
mote effective conjugate hydrostannation of R,â-unsatur-
ated ketones and aldehydes, respectively. However, these
tin hydrides have no reducing ability for R,â-unsaturated
esters. Quite recently, we have reported the novel tin
+
-
2
that the magnesium cation of [MgBr] [n-Bu SnBrIH]
+
-
would interact with ester functionalities. We found here
that the complex effectively reacted with R,â-unsaturated
esters 1, and the conjugate hydrostannation could be
accomplished for the first time (Scheme 2).
hydride ate complex [MgBr] [n-Bu
2
SnBrIH] to promote
addition to aliphatic alkynes.10 The complex was prepared
*
To whom correspondence should be addressed. Phone: 81-6-6879-
386. Fax: 81-6-6879-7387.
1) Shimada, E.; Inomata, K.; Mukaiyama, T. Chem. Lett. 1974,
7
As shown in Table 1, the reduction of ethyl crotonate
(
(1a ) was examined by using various tin hydride systems.
6
89-690.
No reaction proceeded with n-Bu SnH (entry 1). As
3
+
(
2) Pereyre, M.; Colin, G.; Valade, J . Tetrahedron Lett. 1967, 4805-
-
4
808.
described above, n-Bu
not effective (entries 2 and 3). However, when MgBr
OEt was added to n-Bu SnIH, saturated ester 2a was
2 2 2
SnIH and Li [n-Bu SnI H] were
(
3) Nakamura, E.; Imanishi, Y.; Machii, D. J . Org. Chem. 1994, 59,
2
‚
8
178-8186.
(4) Nozaki, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. J pn. 1991,
2
2
6
4, 2585-2587.
obtained in 80% yield (entry 4). Thus, the reaction could
be performed effectively at room temperature. Here,
(
5) For example, the hydrostannation of n-hexyl crotonate with
SnH/Et B gives the â-adduct, whereas the reaction of methyl
Ph
3
3
4
MgBr
because other types of Lewis acids such as BF
and ZnCl did not promote the reactions (entries 5
and 6). Moreover, the combination of MgBr ‚OEt with
n-Bu SnH gave little effect (entry 7). The formation of
ate complex [MgBr] [Bu
‚OEt
does not simply work as a Lewis acid
crotonate under the same conditions gives the R-adduct.
6) Smith, N. D.; Mancuso, J .; Lautens, M. Chem. Rev. 2000, 100,
257-3282.
7) Tin hydride chemistry; see, for example: Pereyre, M.; Quintard,
2
2
(
3
‚OEt
2
3
2
(
J . P.; Rahm, A. Tin in Organic Synthesis; Butterworth: London, 1987;
2
2
p 35.
3
(8) (a) Kawakami, T.; Miyatake, M.; Shibata, I.; Baba, A. J . Org.
+
-
2
SnBrIH] is essential for con-
Chem. 1996, 61, 376-379. (b) Suwa, T.; Nishino, K.; Miyatake, M.;
Shibata, I.; Baba, A. Tetrahedron Lett. 2000, 41, 3403-3406.
jugate hydrostannation. Thus, an anion in the complex
has a trigonal bypiramidal structure in which two
halogens occupy the apical positions and a hydrogen and
two butyl groups are located at the equatorial positions
(
9) Suwa, T.; Shibata, I.; Baba, A. Organometallics 1999, 18, 3965-
3
967.
(
10) Shibata, I.; Suwa, T.; Ryu, K.; Baba, A. J . Am. Chem. Soc. 2001,
1
23, 4101-4102.
1
0.1021/jo016081e CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/03/2001

Notes
J . Org. Chem., Vol. 66, No. 25, 2001 8691
Ta ble 2. Hyd r osta n n a tion of r,â-Un sa tu r a ted Ester s by Tin Hyd r id e Ate Com p lexa
entry
R1
R2
R3
R4
product
time/h
yield/%
1
2
3
4
5
6
7
8
9
0
1
2
3
Ph
Ph
Ph
H
H
H
H
H
H
Me
H
H
H
H
Me
Me
H
H
H
H
H
H
H
Me
H
H
H
H
H
Et
1b
1c
1c
1d
1e
1f
1g
1h
1i
1j
1j
1k
1k
2b
2c
2c
2d
2e
2f
2g
2h
2i
2j
2j
2k
2k
5
5
5
6
1
51
90
87
76
62
67
61
75
78
90
Bn
Bn
Me
Me
Me
Me
b
p-MeC6H4
p-NO2C6H4
p-ClC6H4
1
5
d
X
t
H
H
H
H
Me
Me
Bu
0.5
0.5
0.5
18
24
18
e
Y
1
1
1
1
Et
Et
Et
Et
c
0
10
18
c
a
MgBr] [Bu2SnBrIH] (1 mmol), 1 (1 mmol), THF (1 mL). b Galvinoxyl (0.1 mmol) was added. c Yields are described in ref 2 using
+
-
[
Bu3SnH under UV irradiation at 70 °C.
d
e
Sch em e 3
Sch em e 5
Sch em e 4
(
Scheme 3).10 In this ate complex, it is assumed that the
9
nucleophilicity of the iodine group is increased. Namely,
the preferential attack of iodine at the â-carbon is
followed by rapid reduction of the carbon-iodine bond
by tin hydride species to give the stannylketene acetal,
which was hydrolyzed to saturated ester 2 (Scheme 4).
(entry 7). A bulky O-substituent in 1h was not problem
and allowed the production of 2h in good yield (entry 8).
A reducible alkyl halide in 1i was not affected (entry 9).
It could be concluded that ionic processes determine the
ate complex promoted hydrostannation. The yield of
product scarcely changed in the presence of a radical
inhibitor such as Galvinoxyl (entry 3). Of interest is that
terminally nonsubstituted alkenes 1j were more reactive
than the substituted alkenes 1k (entries 10 and 12)
because the reactivity order is the reverse of that in the
radical reactions (entries 11 and 13) where the nonsub-
+
-
2
Although the reason [MgBr] [n-Bu SnBrIH] have a
preference effect compared with Li [n-Bu
+
-
2 2
SnI H] is
unclear as yet, unsaturated ester 1 would be activated
by a countercation [MgBr] .
+
11
Table 2 shows the conjugate reduction of various R,â-
unsaturated esters with the ate tin hydride complex in
THF. As in the case of 1a , terminally monosubstituted
alkenes 1b-1f were reduced effectively to the corre-
sponding saturated esters 2b-2f (entries 1-6). The
reducible functionalities such as nitro and halogen groups
in 1e and 1f did not react (entries 5 and 6).12 In the case
of enamine type of substrate 1g, good yield was obtained,
and the amine group did not disturb the reactions at all
2
stituted unsaturated esters do not react. The reactions
of terminally nonsubstituted unsaturated esters with
+
-
[
2
MgBr] [n-Bu SnBrIH] went to completion within 30
min (entries 8-10).
This hydrostannation could develop the further reac-
tion of the generated metal ketene acetal with an elec-
trophile (Scheme 5). Thus, after completing the hy-
drostannation of unsaturated ester 1j, the addition of the
generated stannylketene acetal to p-chlorobenzaldehyde
+
(
11) It has been already cleared that the 1:1 tin ate complex [MgBr] -
2
SnBrIH]- is formed more effectively in EtOAC than in THF+.
8
[
n-Bu
However, no hydrostannation of R,â-unsaturated esters with [MgBr] -
n-Bu
SnBrIH]- proceeded in EtOAc. This would be because of the
[
2
3
a gave â-hydroxy ester 4a in 40% yield. On the other
coordination of EtOAc to the tin complex. In THF solvent, we consider
hand, the hydrostannation of 1j could be performed in
the coexistence of cyclic ketone 3b or 3c to promote
hydroxy ester 4b or 4c as the sole product, respectively,
that effective coordination to R,â-unsaturated esters is possible.
(12) (a) Kabalka, G. W.; Varma, R. S. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford,
1
991; Vol. 8, Chapter 2, pp 363-380. (b) Imamoto, T. In Comprehensive
+
-
in which [MgBr] [n-Bu
2
SnBrIH] predominantly reduces
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press:
Oxford, 1991; Vol. 8, Chapter 4, pp 793-810.
1j beyond cyclic ketones. Although the yields of hydroxy

8
692 J . Org. Chem., Vol. 66, No. 25, 2001
Notes
esters are not yet satisfactory, it should be noted that
the result is the first generation of stannylketene acetal
by the conjugate hydrostannation of unsaturated esters.
In conclusion, conjugate hydrostannation of various
R,â-unsaturated esters could be performed with high
chemoselectivity by using ate tin hydride complex,
mmol) was added. After the mixture was stirred at 0 °C for 1.5
h, p-chlorobenzaldehyde (0.141 g) was added, and stirring was
continued at 0 °C for 7 h. CHCl (5 mL) was added to decompose
3
the remaining tin hydride, and volatiles were removed under
reduced pressure. The residue was subjected to column chro-
matography eluting with 1:1 hexanes-EtOAc to give product
4a in 40% yield (0.097 g). The diastereomeric ratio was deter-
+
-
1
[MgBr] [n-Bu
2
SnBrIH] . The resulting stannylketene
mined by H NMR (syn:anti ) 63:37).
Rea ction of Sta n n ylk eten e Aceta l Gen er a ted by th e 1,4-
acetals could be used for the aldol reactions to give
â-hydroxy esters.
Hyd r osta n n a tion of a n r,â-Un sa tu r a ted Ester w ith
a
+
-
Cyclic Keton e. A THF solution of [MgBr] [n-Bu
2
SnBrIH] (1
mmol) was prepared by the above-described method. The solu-
tion was cooled to -78 °C, and ethyl acrylate (1j) (0.100 g, 1
mmol) and cyclohexanone (0.252 g, 3 mmol) were added. After
the mixture was stirred at from -78 °C to room temperature
Exp er im en ta l Section
Rep r esen ta tive P r oced u r e for Hyd r osta n n a tion of a n
r,â-Un sa tu r a ted Ester 1c. To 1 mL of dry THF in a round-
3
for 5 h, CHCl (5 mL) was added to decompose the remaining
bottomed flask under dry N
.5 mmol) and n-Bu SnI
room temperature, the formation of n-Bu
confirmed by the shift of an IR band (ν(Sn-H)) from 1837 to
2 2 2
were added n-Bu SnH (0.118 g,
tin hydride and volatiles were removed under reduced pres-
sure. The residue was subjected to column chromatography
eluting with 1:1 hexanes-EtOAc to give product 4b in 50% yield
0
2
2
(0.244 g, 0.5 mmol). After 10 min at
SnIH (1 mmol) was
2
(0.093 g).
-
1
1
846 cm . Then, MgBr
stirring was continued for 10 min. IR absorption was observed
at the same value, 1846 cm . To the solution was added
2
‚OEt
2
(0.258 g, 1 mmol) was added and
Ack n ow led gm en t. This work was supported finan-
-
1
cially by the Grant-in-Aid for Scientific Research from
the Ministry of Education, Science and Culture. Thanks
are due to Mr. H. Moriguchi, Faculty of Engineering,
Osaka University, for assistance in obtaining HRMS
spectra.
unsaturated ester 1c (0.238 g, 1 mmol), and the resulting
mixture was stirred at room temperature for 5 h. CHCl (5 mL)
3
was added to decompose the remaining tin hydride, and volatiles
were removed under reduced pressure. The residue was sub-
jected to column chromatography eluting with 1:1 hexanes-
EtOAc to give product 2c in 90% yield (0.216 g), which was
consistent with the authentic sample.
Other reactions of unsaturated esters were performed in
similar ways for the corresponding reaction time (Table 2).
Rea ction of Sta n n ylk eten e Aceta l Gen er a ted by th e 1,4-
Hyd r osta n n a tion of a n r,â-Un sa tu r a ted Ester w ith a n
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
1
13
cedures, IR, H NMR, C NMR, and HRMS spectra fo
13
119
r 2c-2g, 2i, and 4a -c, and C and Sn NMR data for the
tin hydride ate complex [MgBr] [n-Bu SnBrIH] . This mate-
2
+
-
rial is available free of charge via the Internet at http://
pubs.acs.org.
+
-
Ald eh yd e. A THF solution (1 mL) of [MgBr] [n-Bu
1 mmol) was prepared by the above-described method. The
solution was cooled to 0 °C, and ethyl acrylate (1j) (0.100 g, 1
2
SnBrIH]
(
J O016081E