E / Z -Selectivity Controlled by Participation of Internal Oxy Group during Electrophilic Substitution of Alk-1-enylboronate with Bis(2,4,6-trimethylpyridine)iodonium Salt

Article abstract of DOI:10.1055/s-0036-1558967

Stereospecific inversion of configuration was observed in electrophilic iodo-substitution of 4-benzoyloxybut-1-enylboronate using bis(2,4,6-trimethylpyridine)iodine(I) salt, rationalized by participation of the internal benzoyloxy group.

Full text of DOI:10.1055/s-0036-1558967

SYNTHESIS
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
© Georg Thieme Verlag Stuttgart · New York
2017, 49, A–D
A
special topic
M. Fujita et al.
Special Topic
Synthesis
E/Z-Selectivity Controlled by Participation of Internal Oxy Group
During Electrophilic Substitution of Alk-1-enylboronate with
Bis(2,4,6-trimethylpyridine)iodonium Salt
Morifumi Fujita*
+
N
+
N
Hee Jin Lee
I–
I
Chisaki Ichihara
Takashi Sugimura
–
PF6
Ph
O
n
Ph
O
O
H
O
+
X
Graduate School of Material Science, University of Hyogo,
Kohto, Kamigori, Hyogo 678-1297, Japan
fuji@sci.u-hyogo.ac.jp
Ph
O
X
H
I
n
n
n = 0, 1
O
X = B(OR)2, SiEt3
n = 2
Ph
O
I
n
O
Received: 26.12.2016
Accepted after revision: 16.02.2017
Published online: 14.03.2017
mechanism. Boronates and boronic acids with less reactivi-
ty towards the elimination lead to anti-addition–anti-elim-
ination, resulting in inversion of configuration.
DOI: 10.1055/s-0036-1558967; Art ID: ss-2016-c0884-st
In this communication, we report neighboring group
participation control of the E/Z selectivity in iodo-substitu-
tion of alk-1-enylboronate pinacol ester using bis(2,4,6-
trimethylpyridine)iodine(I) hexafluorophosphate (BTMPI).6
BTMPI is a commercially available iodination reagent with
high level of electrophilicity owing to the cationic character,
and liberates weak nucleophiles during iodination.6a These
properties may be suitable for highlighting the intramolec-
ular nucleophilic participation.
The E-isomers of alkenylboronate pinacol ester, (E)-1a–e,
were prepared by conventional hydroboration of alkynes,7
while the Z-isomers (Z)-1b–e were prepared by rhodium-
catalyzed hydroboration.8 The pinacol esters are relatively
stable against hydrolysis, and tolerate purification by silica
gel column chromatography.
Abstract Stereospecific inversion of configuration was observed in
electrophilic iodo-substitution of 4-benzoyloxybut-1-enylboronate us-
ing bis(2,4,6-trimethylpyridine)iodine(I) salt, rationalized by participa-
tion of the internal benzoyloxy group.
Key words alkenylboronate, iodoalkene, neighboring group partici-
pation, E/Z-selectivity, iodonium
Alk-1-enyl iodides are important synthetic intermedi-
ates, especially for stereoselective alkene and polyene syn-
thesis.1 Alkenylboronic acids and alkenylboronates are ste-
reoselectively converted to alkenyl iodides via electrophilic
substitution.2 The E/Z stereoselectivity in iodo-substitution
is controlled by the order of addition of the base and iodine
source (I2 or ICl), as shown in Scheme 1.3–5
The reaction of (E)-1a–e with BTMPI was carried out in
dichloromethane at room temperature and gave alkenyl io-
dide 2, as summarized in Table 1. The reactions of (E)-3-
benzoyloxyprop-1-enylboronate (E)-1a and (E)-4-benzoyl-
oxybut-1-enylboronate (E)-1b preferentially yield the cor-
responding (Z)-alk-1-enyl iodide, (Z)-2 (Table 1, entries 1
and 2). In contrast, substrate (E)-1c with a longer pentenyl
group led to a Z/E mixture of alkenyl iodide (entry 4). High
Z-selectivity was observed in the reaction of (E)-1d, with
the same pentenyl chain as 1c, however, the oxy functional
group is benzyloxy (entry 7). The Z-selectivity in reactions
of (E)-1a, (E)-1b, and (E)-1d is in contrast with the Z/E mix-
ture obtained from simple dec-1-enylboronate (E)-1e (en-
try 9). The reactions in diethy ether/dichloromethane (en-
tries 3 and 5) or under dilute conditions (entries 6 and 8)
slightly affected the stereoselectivity and improved the
chemical yield.
1) NaOH
R
2) I2 or ICl
I
R
B(OR)2
I
R
1) I2 or ICl
2) NaOH
Scheme 1 Iodo-substitution of alkenylboronate
Iodine-substitution in the presence of NaOH proceeds
with retention of configuration. In contrast, inversion of
configuration takes place in the absence of NaOH, when it is
added after the reaction. Sodium hydroxide has been con-
sidered to react at the boron atom position to make a tetra-
hedral boronate complex. Electrophilic substitution of the
tetrahedral boronate complex proceeds rapidly with reten-
tion of configuration via a simple addition–elimination
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–D

B
M. Fujita et al.
Special Topic
Synthesis
The Z-isomers of alkenylboronates were also employed
for the iodo-substitution reaction with BTMPI (Table 2). The
reaction of (Z)-4-benzoyloxybut-1-enylboronate (Z)-1b se-
lectively gave (E)-2b with inversion of configuration (Table
2, entry 1). Taking the result of (E)-1b into consideration,
the substitution of 4-benzoyloxybut-1-enylboronate (1b)
stereospecifically proceeds with inversion of configuration.
A decrease in the E/Z selectivity was observed in the reac-
tion of (Z)-5-benzoyloxypent-1-enylboronate (Z)-1c (en-
tries 2 and 3), as with (E)-1c. The benzyloxy substrate of 1d
led to stereospecific substitution with inversion of configu-
ration (entry 4).
Stereospecific inversion of configuration was observed
in the reactions of 1a, 1b, and 1d. The stereospecifity de-
creased in the reaction of 1c and 1e. These results are ratio-
nalized with a reaction mechanism that involves neighbor-
ing group participation, as illustrated in Scheme 2. The ben-
zoyloxy group of 1a and 1b nucleophilically attacks the
olefin moiety giving five- and six-membered oxonium cat-
ion intermediate, respectively, during the electrophilic sub-
stitution. Initial anti-addition is likely followed by anti-
elimination to yield alkenyl iodide 2 with inversion of con-
figuration. The decrease in E/Z selectivity in the reaction of
1c may be attributed to the less favorable seven-membered
oxonium cation. The benzyloxy substrate 1d is able to form
the five-membered oxolanyl cation during the electrophilic
substitution. The five-membered oxolanyl cation generated
from the benzoyloxy substrate 1c must be unstable owing
to the electron-withdrawing benzoyl group on the oxoni-
um.
Table 1 Reaction of (E)-1 with BTMPIa
+
N
+
N
I–
O
–
I
R
PF6
B
R
O
CH2Cl2
r.t.
(E)-1
(Z)-2
Z/E
Entry
Substrate
R
Yield of 2 (%)
1
(E)-1a
(E)-1b
(E)-1b
(E)-1c
(E)-1c
(E)-1c
(E)-1d
(E)-1d
(E)-1e
PhCO2CH2
60
59
71
39
46
61
55
68
63
89:11
2
PhCO2(CH2)2
PhCO2(CH2)2
PhCO2(CH2)3
PhCO2(CH2)3
PhCO2(CH2)3
PhCH2O(CH2)3
PhCH2O(CH2)3
Me(CH2)7
91:9
96:4
3b
4
Ph
55:45
71:29
40:60
93:7
H
O
Ph
O
+
B(OR)2
5b
6c
7
(Z)-2
B(OR)2
O
n
O
n = 0, 1
H
I
n
(E)-1a: n = 0
(E)-1b: n = 1
(E)-1c: n = 2
+(Z)-2
(E)-2
8c
9
>98:2
68:32
n = 2
CH2Ph
Ph
O
B(OR)2
H
n
O+
B(OR)2
(Z)-2
a The reaction was typically carried out in CH2Cl2 (1 mL) containing 1 (0.17
mmol) and BTMPI (0.22 mmol) at 25 °C for 2–6 h.
(E)-1d: n = 2
H
I
b Reaction was carried out in Et2O/CH2Cl2 = 1:1 (v/v).
c The reaction was carried out in dilute solution. The concentrations of the
substrate and reagent were one fifth of those under the typical conditions.
Scheme 2 Plausible mechanism for inversion of configuration
Table 2 Reaction of (Z)-1 with BTMPIa
Participation of the carbonyl oxygen in iodo-substitu-
tion has been discussed for the reaction of acyloxyprop-1-
enyl(trimethyl)silane with N-iodosuccinimide (NIS).9 For
comparison, 4-benzoyloxybut-1-enylsilane (3b) and 5-ben-
zoyloxypent-1-enylsilane (3c) were subjected to iodo-sub-
stitution with BTMPI as summarized in Table 3. In the liter-
ature,9 the reaction of both (E)- and (Z)-3-acyloxyprop-1-
enyl(trimethyl)silane with NIS preferentially yielded Z-io-
dide resulting from the thermodynamic stability of the cat-
ionic intermediate. A similar tendency was observed in the
reaction of 3b with BTMPI (Table 3, entries 1 and 2). The Z-
isomer of iodide (Z)-2b was preferentially obtained. The
difference in the selectivity between the boronates 1 and
the silanes 3 can be explained by the difference of reactivity
of the elimination step. A difference in the elimination reac-
tivity was also observed in reactions with (diacetoxyio-
do)benzene under acidic conditions.10
+
N
+
N
I–
–
O
PF6
I
B
O
R
CH2Cl2
r.t.
R
(Z)-1
(E)-2
Entry
Substrate
R
Yield of 2 (%)
E/Z
1
(Z)-1b
(Z)-1c
(Z)-1c
(Z)-1d
(Z)-1ec
PhCO2(CH2)2
65
57
69
69
71
>98:2
2
PhCO2(CH2)3
PhCO2(CH2)3
PhCH2O(CH2)3
Me(CH2)7
84:16
66:34
92:8
3b
4b
5b
82:18
a The reaction was typically carried out in CH2Cl2 (1 mL) containing 1 (0.17
mmol) and BTMPI (0.22 mmol) at 25 °C for 2–6 h.
Retention of configuration was observed for the iodo-
b The reaction was carried out in dilute solution. The concentrations of the
substrate and reagent were one fifth of those under the typical conditions.
c The sample was a mixture of (Z)-1e and (E)-1e (87:13).
substitution of alkenylzirconium1a,11 and alkenylindium12
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–D

C
M. Fujita et al.
Special Topic
Synthesis
Table 3 Reaction of Alk-1-enylsilane 3 with BTMPIa
(Z)-5-Benzoyloxypent-1-enylboronate Pinacol Ester [(Z)-1c)]
Bis(1,5-cyclooctadiene)-μ,μ′-dichlorodirhodium (10.8 mg, 0.022
mmol), tricylopentylphosphine (25 μL, 0.086 mmol), Et3N (1.0 mL, 7.2
mmol), and pinacolborane (0.2 mL, 1.4 mmol) were successively
mixed in cyclohexane (3 mL) under N2. After stirring for 30 min, 5-
benzoyloxypent-1-yne (0.50 g, 2.9 mmol) was added. The mixture
was stirred overnight at r.t., and then purified by silica gel column
chromatography (eluent: 10% EtOAc in hexane) to give (Z)-1c (100
mg, 0.32 mmol, 23%); colorless oil.
+
N
+
N
I–
–
I
PF6
R
R
SiEt3
CH2Cl2
r.t.
(E)-3
(Z)-2
Entry
Substrate
R
Yield of 2 (%)
Z/E
>98:2
IR (film): 2979, 1720, 1274, 1145, 713 cm–1
.
1H NMR (CDCl3, 600 MHz): δ = 8.04 (d, J = 7.6 Hz, 2 H), 7.53 (t, J = 7.6
Hz, 1 H), 7.41 (t, J = 7.6 Hz, 2 H), 6.45 (dt, J = 13.1, 6.9 Hz, 1 H), 5.38 (d,
J = 13.1 Hz, 1 H), 4.32 (t, J = 6.9 Hz, 2 H), 2.56 (q, J = 6.9 Hz, 2 H), 1.85
(quint, J = 6.9 Hz, 2 H), 1.21 (s, 12 H).
1
2
3
(E)-3b
(Z)-3b
(E)-3c
PhCO2(CH2)2
PhCO2(CH2)2
PhCO2(CH2)3
45
88
61
69:31
35:65
13C NMR (CDCl3, 150 MHz): δ = 166.57, 153.30, 132.73, 130.54,
129.55, 128.24, 82.86, 64.61, 28.86, 28.63, 24.76.
a Reaction was typically carried out in CH2Cl2 (1 mL) containing 3 (0.17
mmol) and BTMPI (0.34 mmol) at 25 °C for 7–17 h.
HRMS (ESI+): m/z calcd for C18H25BO4Na (M + Na): 339.1744; found:
intermediates, even in the case of 4-benzoyloxybut-1-enyl
and 5-benzyloxypent-1-enyl species. These results suggest
that the internal oxy group does not participate in the reac-
tion of a strong electrofuge.
In summary, we have demonstrated that stereospecific
inversion of configuration is achieved in idodo-substitution
of alk-1-enylboronate through participation of an internal
benzoyloxy group.
339.1782.
(E)-Dec-1-enylboronate Pinacol Ester [(E)-1e]14
Following the literature procedure,14 the reaction of dec-1-yne (0.9
mL, 5 mmol) with pinacolborane (0.8 mL, 5.5 mmol) in the presence
of dicyclohexylborane (0.62 mmol) for 1 h at r.t. gave (E)-1e (0.98 g,
3.7 mmol, 73%) after column chromatography (SiO2, eluent: 30%
EtOAc in hexane). 1H NMR spectrum of the product agreed well with
the reported values.14
Iodo-Substitution; (E)- and (Z)-3-Iodoprop-2-enyl Benzoate (2a);1b
Typical Procedure
Major reagents and solvents were obtained from commercial sources
and were used without further purification, unless otherwise noted.
CH2Cl2 was purified by distillation over CaH2. Column chromatogra-
phy was performed on silica gel 60 (0.063–0.200 mm) from Merck. 1H
and 13C NMR spectra were recorded on a Jeol ECA-600 spectrometer
as solutions in CDCl3. 1H NMR spectra were recorded using the residu-
al CHCl3 as an internal reference (7.24 ppm) and 13C NMR using CDCl3
as an internal reference (77.00 ppm). Jeol JMS-T100LC spectrometer
was used for mass spectra measurements. IR spectra were recorded
on a Jasco FT/IR-410 spectrometer.
Alkenylboronate pinacol esters, (E)-1b,10a (Z)-1b,10a (E)-1c,10a (E)-
1d,10a (Z)-1d,10a and (Z)-1e8 were prepared as reported previously.
Preparation of alkenylsilanes, (E)-3b, (Z)-3b, and (E)-3c were reported
previously.10b
To a solution of (E)-1a (50 mg, 0.17 mmol) in CH2Cl2 (1 mL) was added
BTMPI (113 mg, 0.22 mmol) at r.t. After stirring at r.t. for 3.5 h, the
reaction mixture was quenched with H2O, and extracted with CH2Cl2.
The organic phase was dried (Na2SO4) and concentrated in vacuo. The
residue was purified by column chromatography (SiO2, eluent: 10%
EtOAc in hexane) to yield 2a (31 mg, 0.11 mmol, 60%) as an 89:11
mixture of Z/E; Rf = 0.5 (hexane/EtOAc, 9:1); oil.
1H NMR (CDCl3, 600 MHz): δ (Z-isomer) = 8.04 (d, J = 7.6 Hz, 2 H), 7.55
(t, J = 7.6 Hz, 1 H), 7.43 (t, J = 7.6 Hz, 2 H), 6.60–6.53 (m, 2 H), 4.88 (d,
J = 4.8 Hz, 2 H).
1H NMR (CDCl3, 600 MHz): δ (E-isomer) = 8.03 (d, J = 7.6 Hz, 2 H), 7.55
(t, J = 7.6 Hz, 1 H), 7.43 (t, J = 7.6 Hz, 2 H), 6.74 (dt, J = 14.4, 6.2 Hz, 1
H), 6.60–6.53 (m, 1 H), 4.72 (d, J = 6.2 Hz, 2 H).
(E)-3-Benzoyloxyprop-1-enylboronate Pinacol Ester [(E)-1a]13
(E)-4-Iodobut-3-enyl Benzoate [(E)-2b]11
Following the literature procedure,13 the reaction of 3-benzoyloxy-
prop-1-yne (2.5 g, 15.8 mmol) with pinacolborane (3.4 mL, 24 mmol)
in the presence of dicyclohexylborane (5.0 mmol) for 20 h at r.t. gave
(E)-1a (3.05 g, 10.6 mmol, 67%) after column chromatography (SiO2,
eluent: 30% EtOAc in hexane); colorless oil.
Yield: 32.5 mg, 0.11 mmol (65%); oil; from (Z)-1b (50 mg, 0.17 mmol).
IR (film): 2956, 1718, 1274, 1115, 711 cm–1
1H NMR (CDCl3, 600 MHz): δ = 8.01 (d, J = 7.6 Hz, 2 H), 7.55 (t, J = 7.6
Hz, 1 H), 7.43 (t, J = 7.6 Hz, 2 H), 6.58 (dt, J = 14.4, 6.9 Hz, 1 H), 6.21 (d,
J = 14.4 Hz, 1 H), 4.34 (t, J = 6.9 Hz, 2 H), 2.51 (q, J = 6.9 Hz, 2 H).
.
IR (film): 2979, 1722, 1270, 1145, 712 cm–1
.
13C NMR (CDCl3, 150 MHz): δ = 166.39, 141.66, 133.04, 130.07,
129.58, 128.40, 77.51, 62.85, 35.32.
1H NMR (CDCl3, 600 MHz): δ = 8.06 (d, J = 7.6 Hz, 2 H), 7.54 (t, J = 7.6
Hz, 1 H), 7.42 (t, J = 7.6 Hz, 2 H), 6.71 (dt, J = 18.6, 4.8 Hz, 1 H), 5.78 (d,
J = 18.6 Hz, 1 H), 4.90 (m, 2 H), 1.25 (s, 12 H).
HRMS (ESI+): m/z calcd for C11H11IO2Na (M + Na): 324.9701; found:
324.9726.
13C NMR (CDCl3, 150 MHz): δ = 166.04, 145.94, 132.99, 130.03,
129.67, 128.33, 83.43, 65.71, 24.75.
(Z)-4-Iodobut-3-enyl Benzoate [(Z)-2b]
HRMS (ESI+): m/z calcd for C16H21BO4Na (M + Na): 311.1431; found:
Yield: 71 mg, 0.235 mmol (71%); oil; from (E)-1b (100 mg, 0.33
311.1465.
mmol).
IR (film): 2955, 1718, 1274, 1114, 711 cm–1
.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–D

D
M. Fujita et al.
Special Topic
Synthesis
1H NMR (CDCl3, 600 MHz): δ = 8.03 (d, J = 7.6 Hz, 2 H), 7.55 (t, J = 7.6
Hz, 1 H), 7.43 (t, J = 7.6 Hz, 2 H), 6.39 (d, J = 6.9 Hz, 1 H), 6.31 (q, J = 6.9
Hz, 1 H), 4.39 (t, J = 6.9 Hz, 2 H), 2.62 (q, J = 6.9 Hz, 2 H).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1558967.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
13C NMR (CDCl3, 150 MHz): δ = 166.51, 136.88, 132.99, 130.10,
129.65, 128.36, 85.15, 62.69, 34.49.
HRMS (ESI+): m/z calcd for C11H11IO2Na (M + Na): 324.9701; found:
324.9688.
References
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5-Iodopent-4-enyl Benzoate (2c)
Yield: 29 mg, 0.092 mmol (57%); oil; from (Z)-1c (50 mg, 0.16 mmol).
1H NMR (CDCl3, 600 MHz): δ (Z-isomer) = 8.05 (d, J = 7.6 Hz, 2 H), 7.55
(t, J = 7.6 Hz, 1 H), 7.43 (t, J = 7.6 Hz, 2 H), 6.27 (d, J = 6.9 Hz, 1 H), 6.22
(q, J = 6.9 Hz, 1 H), 4.33 (t, J = 6.9 Hz, 2 H), 2.32 (q, J = 6.9 Hz, 2 H), 1.90
(quint, J = 6.9 Hz, 2 H).
1H NMR (CDCl3, 600 MHz): δ (E-isomer) = 8.02 (d, J = 7.6 Hz, 2 H), 7.55
(t, J = 7.6 Hz, 1 H), 7.43 (t, J = 7.6 Hz, 2 H), 6.54 (dt, J = 14.4, 7.6 Hz, 1
H), 6.07 (d, J = 14.4 Hz, 1 H), 4.31 (t, J = 6.9 Hz, 2 H), 2.22 (q, J = 7.6 Hz,
2 H), 1.87 (quint, J = 6.9 Hz, 2 H).
(Z)-5-Benzyloxy-1-iodopent-1-ene [(Z)-2d]12
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Yield: 68 mg, 0.225 mmol (68%); oil; from (E)-1d (100 mg, 0.33
mmol).
1H NMR (CDCl3, 600 MHz): δ = 7.35–7.31 (m, 4 H), 7.26 (m, 1 H), 6.21–
6.15 (m, 2 H), 4.50 (s, 2 H), 3.49 (t, J = 6.9 Hz, 2 H), 2.23 (q, J = 6.9 Hz, 2
H), 1.74 (quint, J = 6.9 Hz, 2 H).
13C NMR (CDCl3, 150 MHz): δ = 140.73, 138.48, 128.32, 127.59,
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Yield: 48 mg, 0.16 mmol (69%); oil; from (Z)-1d (70 mg, 0.23 mmol).
1H NMR (CDCl3, 600 MHz): δ = 7.35–7.25 (m, 5 H), 6.49 (dt, J = 14.4,
6.9 Hz, 1 H), 5.97 (d, J = 14.4 Hz, 1 H), 4.47 (s, 2 H), 3.45 (t, J = 6.9 Hz, 2
H), 2.14 (q, J = 6.9 Hz, 2 H), 1.69 (quint, J = 6.9 Hz, 2 H).
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(14) Takagi, J.; Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Am. Chem.
Soc. 2002, 124, 8001.
Yield: 50 mg, 0.19 mmol (71%); oil; from (Z)-1e (70 mg, 0.26 mmol).
The analytical and spectral data of the products were in accordance
with the reported values.15,16
(15) Farhat, S.; Zouev, I.; Marek, I. Tetrahedron 2004, 60, 1329.
(16) Ren, G.-B.; Wu, Y. Tetrahedron 2008, 64, 4408.
Acknowledgment
This research was partially supported by the Japan Society for the
Promotion of Science (JSPS) through Grant-in-Aids for Scientific Re-
search (C) (26410057).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–D