Stereoselective cross-coupling reaction of 2,4-diaryl-1,1-diboryl-1,3- butadienes: Stereocontrolled approach to 1,3,4,6-tetraarylated 1,3,5-hexatrienes

Article abstract of DOI:10.1055/s-2007-982567

The boryl group cis to the C(3)=C(4) group in 2,4-aryl-1,1- bis(pinacolatoboryl)-1,3-butadienes undergoes Pd-catalyzed cross-coupling reaction with aryl iodides stereoselectively at room temperature, giving rise to the corresponding boronates as a single diastereomer. Subsequent coupling of the boronates with alkenyl iodides allows us to synthesize 1,3,4,6-tetraarylated 1,3,5-hexatrienes stereoselectively. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-982567

LETTER
1969
Stereoselective Cross-Coupling Reaction of 2,4-Diaryl-1,1-diboryl-1,3-buta-
dienes: Stereocontrolled Approach to 1,3,4,6-Tetraarylated 1,3,5-Hexatrienes
Coupling
Reactionof
1
a
,1-Diboryl
s
aienes ki Shimizu,* Katsuhiro Shimono, Michael Schelper, Tamejiro Hiyama
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto University, Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
Fax +81(75)3832445; E-mail: shimizu@npc05.kuic.kyoto-u.ac.jp
Received 7 April 2007
In memory of Professor Yoshihiko Ito
Ar2
Ar2
Abstract: The boryl group cis to the C(3)=C(4) group in 2,4-aryl-
1,1-bis(pinacolatoboryl)-1,3-butadienes undergoes Pd-catalyzed
cross-coupling reaction with aryl iodides stereoselectively at room
temperature, giving rise to the corresponding boronates as a single
diastereomer. Subsequent coupling of the boronates with alkenyl
iodides allows us to synthesize 1,3,4,6-tetraarylated 1,3,5-hexa-
trienes stereoselectively.
Ar3
I
B
B
Ar1
Ar1
Pd cat.
B
Ar3
1
2
(B = pinacolatoboryl)
Ar2
Ar4
I
Ar4
Ar1
Pd cat.
Key words: alkenes, boron, cross-coupling, palladium, stereose-
lectivity
Ar3
3
Scheme 1 Stereocontrolled approach to 1,3,4,6-tetraaryl-1,3,5-
hexatrienes 3
Much attention has been paid on p-conjugated organic
molecules due to their photonic and electronic properties
in such material science as molecular electronics.1 Since
those properties are closely relevant to their molecular
structures, i.e. stereochemistry, synthetic methodologies
for stereocontrolled p-conjugated system are essential for
advance of organic functional materials. We have recently
demonstrated that Pd-catalyzed cross-coupling reaction of
2-aryl-1,1-diboryl-1-alkenes with aryl iodides proceeds
stereoselectively with the boryl group cis to the alkyl
group.2 Perfect discrimination of the two geminal boryl
groups, induced by differences between alkyl and aryl
groups, prompted us further to examine coupling reaction
of alkenyl- and aryl-substituted 1,1-diborylethenes, which
may provide polyfunctional p-conjugated molecules. We
report herein the Pd-catalyzed coupling reaction of 2,4-
aryl-1,1-bis(pinacolatoboryl)-1,3-butadienes 1 with aryl
iodides Ar3–I that gives rise to monocoupled product 2
with Ar1HC=CH and Ar3 groups being cis as a single ste-
reoisomer (Scheme 1). Further coupling reaction of 2 with
alkenyl iodides Ar4CH=CHI allows us to establish a ste-
reocontrolled approach to diverse 1,3,4,6-tetraarylated
1,3,5-hexatrienes 3.
Table 1 Stereoselective Coupling Reaction of 1 with Aryl Iodides
Entry 2
Ar1
Ar2
Ar3
Yield
(%)
1
2
2aa
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
76
53
2ba
2ca
2da
2ea
2fa
2gc
2hc
2ic
4-CF3C6H4
3
4-EtO2CC6H4 49
4
4-BrC6H4
4-PhC6H4
4-MeOC6H4
2-MeOC6H4
3-MeOC6H4
1-Naphthyl
3-Pyridyl
9-Phenane
Ph
64
45b
91
70
65
72
58
75
55
58f
60
5
6
7
8
9
10
11
12
13
14
2jd
2kc
2la
A THF solution of 1a (Ar1, Ar2 = Ph, 1.0 equiv),3 iodo-
benzene (1.1 equiv), and 3 M KOH aq (3.0 equiv) in the
presence of Pd(PPh3)4 (5 mol%) was stirred at room
temperature for eight hours.4 Workup and purification by
column chromatography on silica gel gave monocoupled
4-MeOC6H4 Ph
2ma 4-MeOC6H4 4-MeOC6H4 4-CF3C6H4
2ng
Ph
Ph
1,4-C6H4
h
a Pd(PPh3)4 (5 mol%), 3 M KOH aq (3 equiv), THF, r.t.
b 4-PhC6H4Br was used.
c PdCl2(dppf) (5 mol%), 3 M KOH aq (3 equiv), DME, 40 °C.
d PdCl2(dppf) (5 mol%), 3 M KOH aq (3 equiv), THF, 40 °C.
e 9-Phenanthrenyl.
f 1E,3E:1Z,3E = 95:5.
SYNLETT 2007, No. 12, pp 1969–1971
2
4
.0
7
.2
0
0
7
g Pd(PPh3)4 (5 mol%), 3 M KOH aq (3 equiv), THF, 40 °C.
h 1,4-Phenylene.
Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-982567; Art ID: U03007ST
© Georg Thieme Verlag Stuttgart · New York

1970
M. Shimizu et al.
LETTER
product 2a as a single stereoisomer in 76% yield (Table 1, chemistry of 3a was confirmed by X-ray structure analy-
entry 1). Stereochemistry of 2a was confirmed by X-ray sis of its single crystals.5 Since longer reaction times
crystal structure analysis, which disclosed that the cou- caused lower diastereoselectivity of 3a, 1E,3Z,5E-iso-
pling took place with the boryl group cis to the alkenyl mers were probably produced by isomerization of
group.5 Under the same conditions, 4-substituted aryl 1E,3E,5E-hexatrienes under the conditions. Under the
iodides reacted with 1a smoothly to give 2b–f selectively same conditions, 2e, 2i, 2j, 2l, 2m, and 2n also coupled
(entries 2–6),6 whereas coupling reaction of 2- and 3- smoothly with alkenyl iodides to give 3 in good yields
methoxyphenyl, 1-naphthyl, 3-pyridyl, and 9-phenan- (except for 3g) with high stereoselectivity (entries 2–7).
threnyl iodides were sluggish.
Minor stereoisomers could be removed by recrystalliza-
tion from hexane.
Fortunately, we were delighted to find that the iodides
coupled with 1a smoothly to produce 2g–k in moderate to The present method is conveniently applicable to stereo-
good yields, when PdCl2(dppf) was used as a catalyst in selective synthesis of 3Z-isomer 5 simply by changing the
DME or THF at 40 °C (entries 7–11). In all cases except order of electrophiles employed as demonstrated in
for 2m (1E,3E:1Z,3E = 95:5), monocoupled products 2 Scheme 2. In this case, the best yield and diastereoselec-
were isolated as a single stereoisomer (Figure 1).6
tivity of the first coupling were obtained with a Pd2(dba)3/
phosphine 6 catalyst system.8 The following coupling of 4
with iodobenzene gave 5 in 93% yield with slightly
lowered diastereoselectivity (85:15), which was ascribed
to isomerization of 5 under the conditions.
B
Ph
Ph
Ph
Ph
B
B
Ph
B
Ph
B
a
2n
73%
(B = pinacolatoboryl)
Ph
Ph
Ph
Figure 1
1a
4
(B = pinacolatoboryl)
(EEE:EZE = 95:5)
i-PrO
The stereochemical outcome can be primarily understood
by assuming that the reaction takes place with less-hin-
dered boryl group, although electronic factors of alkenyl
and aryl groups may also affect the differentiation.
Ph
Ph
b
93%
Ph
Ph
(EZE:EEE = 85:15)
PCy2
i-PrO
5
6
Boronate 2a reacted with alkenyl iodides (E)-PhCH=CHI
in the presence of Pd[(t-Bu)3P]2 (8 mol%) and 3 M KOH
aq (3 equiv) upon heating at 60 °C, giving rise to
1E,3E,5E-hexatrienes 3a as a major isomer in excellent
yield with high selectivity (Table 2, entry 1).7 The stereo-
Scheme 2 Synthesis of 3Z-isomer 5. Reagents and conditions:
a) (E)-PhCH=CHI, Pd2(dba)3 (5 mol%), 6 (20 mol%), 3 M KOH aq
(3 equiv), THF, r.t.; b) PhI, PdCl2(dppf) (5 mol%), 3 M KOH aq
(3 equiv), 60 °C.
Table 2 Synthesis of Tetraarylated 1,3,5-Hexatrienes 3
Entry
3
Ar1
Ar2
Ar3
Ar4
Yield (%)
(ratio)a
1
2
3
4
5
6
7
3ab
3bb
3cc
3db
3eb
3fb
3gc
Ph
Ph
Ph
Ph
98
(96:4)
Ph
Ph
4-PhC6H4
1-Naphtyl
3-Pyridyl
Ph
Ph
84
(95:5)
Ph
Ph
Ph
78
(94:6)
Ph
Ph
Ph
70
(90:10)
4-MeOC6H4
4-MeOC6H4
Ph
Ph
4-CF3C6H4
4-CF3C6H4
Ph
98
(96:4)
4-MeOC6H4
Ph
4-CF3C6H4
83
(91:9)
d
1,4-C6H4
39
(>99:<1)
a Ratios of 1E,3E,5E/1E,3Z,5E isomers.
b Pd(t-Bu3P)2 (8 mol%), 3 M KOH aq (3 equiv), THF, 60 °C.
c 20 mol% of Pd(t-Bu3P)2 was used.
d 1,4-Phenylene.
Synlett 2007, No. 12, 1969–1971 © Thieme Stuttgart · New York

LETTER
Coupling Reaction of 1,1-Diboryl-1,3-butadienes
1971
(4) Typical Procedure
Noteworthy is that solid 3a and 3e exhibit blue and yellow
emission, respectively, by irradiation with UV light (365
nm).9 In view that light emission in aggregated state of
p-conjugated molecules is essential for application to
electroluminescent materials, such observation is quite
attractive for exploration of new emitting materials.10
A solution of 1a (0.10 g, 0.22 mmol), iodobenzene (47 mg,
0.23 mmol), Pd(PPh3)4 (13 mg, 0.011 mmol), and 3 M KOH
aq (0.22 mL, 0.65 mmol) in THF (5 mL) was stirred at r.t. for
8 h before quenching with sat. aq NH4Cl (20 mL). The
aqueous layer was extracted with EtOAc (3 × 20 mL),
washed with sat. NaCl aq (20 mL), and dried over anhyd
MgSO4. Removal of the organic solvents in vacuo followed
by PTLC (hexane–EtOAc, 10:1) gave 2a (72 mg, 76% yield)
as a colorless solid; mp 98.0 °C. 1H NMR (400 MHz,
CDCl3): d = 1.00 (s, 12 H), 6.22 (d, J = 16.0 Hz, 1 H), 7.21–
7.41 (m, 15 H). 13C NMR (67.8 MHz, CDCl3): d = 24.4, 83.4,
126.5, 126.5, 126.7, 127.3, 127.6, 127.9, 128.1, 128.5,
128.8, 129.5, 130.2, 134.2, 137.4, 140.4, 141.6, 148.5. IR
(KBr): 3051, 3023, 2976, 2928, 2362, 2343, 1952, 1730,
1595, 1576, 1491, 1442, 1360, 1308, 1267, 1213, 1142,
1074, 1024, 974, 859, 752, 700, 579 cm–1. MS (EI, 70 eV):
m/z (%) = 409 (21) [M+ + 1], 408 (71) [M+], 280 (100).
HRMS: m/z calcd for C28H29BO2 [M+]: 408.2261; found:
408.2268. Anal. Calcd for C28H29BO2: C, 82.36; H, 7.16.
Found: C, 82.34; H, 7.16.
In summary, we have demonstrated that the Pd-catalyzed
cross-coupling reaction of 1,1-diboryl-1,3-butadienes
with aryl iodides proceeded stereoselectively to give
1,2,4-triaryl-1-boryl-1,3-butadienes as a single diastereo-
mer. In conjunction with the subsequent coupling with
alkenyl iodides, the present method provides a convenient
and stereocontrolled route to 1,3,4,6-tetraarylated 1,3,5-
hexatrienes which may find application to light-emitting
materials. Further studies on stereocontrolled preparation
and optical properties of p-conjugated molecules are in
progress in our laboratory.
(5) Crystal data of 2a and 3a are deposited at Cambridge
Crystallographic Data Centre as CCDC-619246 and CCDC-
619193.
(6) Stereochemical assignment of 2b–n is deduced from that of
2a.
Acknowledgment
This work was supported by Grant-in-Aid for Creative Scientific
Research, No 16GS0209, from Ministry of Education, Culture,
Sports, Science and Technology, Japan.
(7) Typical Procedure
A solution of 2a (10 mg, 0.023 mmol), b-iodostyrene (8.0
mg, 0.035 mmol), Pd[P(t-Bu)3]2 (1.0 mg, 1.8 mmol, 8 mol%)
and 3 M aq KOH solution (23 mL, 0.069 mmol) in THF (1
mL) was stirred at 60 °C for 12 h before quenching with sat.
aq NH4Cl solution (10 mL). The aqueous layer was extracted
with EtOAc (3 × 10 mL), washed with sat. aq NaCl solution
(10 mL). The organic layer was separated, dried over anhyd
MgSO4, and concentrated in vacuo. The residue was purified
by preparative TLC (hexane–EtOAc, 10:1) to give 3a (8.5
mg, 96% yield) as a 95:5 mixture of 1E,3E,5E- and
1Z,3Z,5E-isomers as a pale yellow solid. The
recrystallization of the mixture from hexane gave
(1E,3E,5E)-3a; mp 173.1 °C (hexane). Rf = 0.63 (hexane–
EtOAc, 10:1). 1H NMR (400 MHz, CDCl3): d = 6.07 (d,
J = 16.0 Hz, 2 H), 6.92 (d, J = 16.0 Hz, 2 H), 7.10–7.53 (m,
20 H). 13C NMR (100 MHz, CDCl3): d = 126.5, 127.3, 127.4,
128.3, 128.5, 130.4, 130.8, 132.4, 137.6, 138.9, 141,0. IR
(KBr): 3030, 1595, 1574, 1489, 1442, 1315, 1146, 1070,
1024, 961, 908, 750, 704, 689, 577, 556, 525 cm–1. MS (EI,
70 eV): m/z (%) = 386 (7) [M+ + 2], 385 (31) [M+ + 1], 384
(100) [M+]. HRMS: m/z calcd for C30H24 [M+]: 384.1878;
found: 384.1880. Anal. Calcd for C30H24: C, 93.71; H, 6.29.
Found: C, 93.72; H, 6.53.
References and Notes
(1) (a) Electronic Materials: The Oligomer Approach; Müllen,
K.; Wegner, G., Eds.; Wiley-VCH: Weinheim, 1998.
(b) Tour, J. M. Chem. Rev. 1996, 96, 537. (c) Martin, R. E.;
Diederich, F. Angew. Chem. Int. Ed. 1999, 38, 1350.
(d) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev.
2004, 104, 4891. (e) Nielsen, M. B.; Diederich, F. Chem.
Rev. 2005, 105, 1837.
(2) Shimizu, M.; Nakamaki, C.; Shimono, K.; Schelper, M.;
Kurahashi, T.; Hiyama, T. J. Am. Chem. Soc. 2005, 127,
12506.
(3) Diborylalkenes 1 were prepared by 1,1-diborylation of the
corresponding 1,1-dibromo-1-alkenes with
bis(pinacolato)diboron. See: (a) Hata, T.; Kitagawa, H.;
Masai, H.; Kurahashi, T.; Shimizu, M.; Hiyama, T. Angew.
Chem. Int. Ed. 2001, 40, 790. (b) Kurahashi, T.; Hata, T.;
Masai, H.; Kitagawa, H.; Shimizu, M.; Hiyama, T.
Tetrahedron 2002, 58, 6381
Compound 1a: mp 119.9 °C. 1H NMR (400 MHz, CDCl3):
d = 1.00 (s, 12 H), 1.34 (s, 12 H), 6.20 (d, J = 15.6 Hz, 1 H),
7.20–7.36 (m, 10 H), 7.87 (d, J = 15.6 Hz, 1 H). 13C NMR
(67.8 MHz, CDCl3): d = 24.3, 25.0, 83.1, 83.2, 127.0, 127.6,
127.8, 128.5, 129.1, 132.4, 135.0, 137.5, 142.5, 163.6. IR
(KBr): 2976, 2930, 2357, 2340, 1556, 1489, 1447, 1379,
1360, 1296, 1257, 1142, 984, 854, 756, 698 cm–1. MS (EI, 70
eV): m/z (%) = 459 (8) [M+ + 1], 458 (28) [M+], 457 (13)
[M+ – 1], 330 (100). HRMS: m/z calcd for C28H36B2O4 [M+]:
458.2800; found: 458.2794. Anal. Calcd for C28H36B2O4:
C, 73.40; H, 7.92. Found: C, 73.41; H, 7.97.
(8) Milne, J. E.; Buchwald, S. L. J. Am. Chem. Soc. 2004, 126,
13028.
(9) In contrast, a cyclohexane or CHCl3 solution of 3a exhibited
no fluorescence. See: InTong, H.; Dong, Y.; Häusler, M.;
Lam, J. W. Y.; Sung, H. H.-Y.; Williams, I. D.; Sun, J.;
Tang, B. Z. Chem. Commun. 2006, 1133.
(10) (a) Mitschke, U.; Bäuerle, P. J. Mater. Chem. 2000, 10,
1471. (b) Shirota, Y. J. Mater. Chem. 2005, 15, 75.
Synlett 2007, No. 12, 1969–1971 © Thieme Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not
be copied or emailed to multiple sites or posted to a listserv without the copyright holder's
express written permission. However, users may print, download, or email articles for
individual use.