Stereodivergent zinc-mediated three-component synthesis of tri- and tetrasubstituted alkenes

Article abstract of DOI:10.1002/chem.201201385

Zinc simple: The loading-dependent zinc-mediated addition of benzyl bromides to alkynes is the key step for the formation of vinyl bromides. In combination with a palladium-catalyzed Suzuki cross-coupling reaction with boronic acids, tri- and tetrasubstit

Full text of DOI:10.1002/chem.201201385

COMMUNICATION
DOI: 10.1002/chem.201201385
Stereodivergent Zinc-Mediated ACTHNUGRTNEUNGThree-Component Synthesis of
Tri- and Tetrasubstituted Alkenes
Anne Miersch and Gerhard Hilt*[a]
Dedicated to Karl-Heinz Dominicus on the occasion of his 70th birthday
The stereoselective synthesis of disubstituted olefins can
zinc powder added is closely related to the stereoselectivity
of the vinyl bromide formed. The use of 150 mol% of zinc
powder generated the E-isomer 3 as main product within
2 h reaction time, whereas utilizing catalytic amounts of zinc
powder (5 mol%) led to the predominant formation of the
corresponding Z-isomer 4 after 48 h reaction time at ambi-
ent temperature. Furthermore, we realized that under di-
verse conditions, utilizing varying amounts of zinc powder
the E-isomer is the primary product (Table 1).
be realized by a vast number of synthetic methods. Several
named reactions for the selective synthesis of E- or Z-con-
figured alkenes are available underlining the high synthetic
value of this functional group.[1] The number of practical
methods for the straightforward generation of tri- and tetra-
substituted double bonds is considerably lower. Among
those transformations are several transition-metal-mediated
additions of organometallic species to alkynes followed by
cross-coupling reactions.[2]
Table 1. Time-dependent generation of E/Z vinyl bromides 3 and 4.
Over the last couple of years, some Lewis acid catalyzed
synthetic methods appeared in the literature, in which ben-
zylic alcohols or halides were reacted with alkynes for the
synthesis of halide-substituted alkenes.[3] In the course of
our ongoing investigations into atom-economic transforma-
tions for the generation of carbon–carbon bonds utilizing
low-valent cobalt complexes, 1-bromoethylbenzene (1) and
phenylacetylene (2) were reacted under reductive conditions
to regioselectively generate vinyl bromides 3 and 4.[4] We
soon realized that these products were formed even in the
absence of the cobalt catalysts, and that zinc powder alone
was sufficient to initiate this transformation (Scheme 1).[5,6]
Herein, we report the regiodiverse synthesis of alkenyl
bromides from benzylic bromides and terminal as well as in-
ternal alkynes mediated by zinc powder and palladium-cata-
lyzed follow-up transformations. Surprisingly, the amount of
Zn [mol%]
t [h]
Yield [%] (ratio 3/4)
150
150
100
100
50
50
10
10
5
2
48
2
48
2
48
2
48
2
87 (88:12)
85 (87:13)
84 (82:18)
78 (51:49)
95 (81:19)
64 (50:50)
83 (75:25)
62 (7:93)
78 (77:23)
70 (7:93)
5
48
Even in the presence of one equivalent zinc powder, the
isomerization to the Z-isomer 4 was observed after 48 h re-
action time. However, when 150 mol% were used, this iso-
merization stopped, indicating that the excess zinc powder
applied removed the initiator for the isomerization from the
reaction mixture (see below). On the other hand, when cata-
lytic amounts of zinc powder were applied, the isomeriza-
tion to the Z-isomer 4 reached the maximum (7:93) and did
not change when the reaction time was prolonged.
Another important parameter for the synthesis of 3 utiliz-
ing 150 mol% of zinc powder is the amount of 1 applied.
The reaction of 2 with one equivalent of 1 led to 3 in only
68%. The application of a slight excess of 1 (1.2 equiv) in-
creased the yield already to 80%, whereas the best results
were obtained when 1.5 equivalents of 1 were used. In this
case, 3 could be isolated in 95% yield. However, larger
excess of 1 did not further improve the yield. Although
most reactions of organozinc species were performed in
THF or other ethereal solvents, dichloromethane was identi-
fied to be the solvent of choice for the desired conversion.[7]
These findings prompted us to investigate the stereodiver-
gent synthesis of vinyl bromides of type 6 and 7 utilizing
Scheme 1. Zinc-initiated addition of benzylic bromide to phenylacety-
lene.
[a] A. Miersch, Prof. Dr. G. Hilt
Fachbereich Chemie
Philipps-Universitꢀt Marburg
Hans-Meerwein-Strasse, 35043 Marburg (Germany)
Fax : (+49)6421-2825677
E-mail: Hilt@chemie.uni-marburg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201385.
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150 mol% zinc powder for the predominant formation of
products of type 6 after 2 h reaction time. Complementary
to these reactions, the application of catalytic amounts of
zinc powder (5 mol%) led to the formation of products of
type 7 after a prolonged reaction time of 48 h (Scheme 2).
tetrasubstituted vinyl bromides 6h,j and 6l,m in good yields
and good E-selectivities. Also, enynes could be applied, and
the 1,3-dienyl bromides 6d and 6l were obtained. The appli-
cation of cyclopropyl acetylene gave product 6 f in a good
yield of 78%, and more importantly, the cyclopropyl moiety
serves as radical probe and remained unchanged, indicating
that radical intermediates are most likely not involved in
the reaction mechanism.
The zinc-mediated synthesis of vinyl bromides was then
applied towards other benzylic bromides (8) to investigate
the scope of the reaction for educts of type 8 (Scheme 3). In
this series of experiments, only the reaction conditions for
the zinc-mediated synthesis of the E-isomer 9 utilizing
150 mol% of zinc powder was applied, because this protocol
gave the higher yields in the previous investigation. The re-
sults for these reactions are summarized in Table 2.
Scheme 3. Zinc-mediated synthesis of vinyl bromides 9 utilizing other
benzylic bromides.
Table 2. Results of the zinc-mediated synthesis of vinyl bromides 9.
Entry
Product
Yield [%] (9:10)
1
44 (96:4)
2
3
60 (90:10)
79 (74:26)
4
83 (56:44)
Benzyl bromide as well as benzyl iodide (R’=H) gave the
desired products of type 9 only in a low yield, whereas a con-
siderable amount of the Wurtz coupling product 1,2-diphe-
nylethane was obtained. Other benzylic bromides could be
applied with better success. Remarkably, the use of (1,2-di-
bromoethyl)benzene (Table 2, entry 2) led to the functional-
ization of the benzylic bromide, whereas the homobenzylic
bromide stayed intact. Overall, the application of other ben-
zylic bromides was possible, but either yields or selectivities
were not as high as with 1.
The vinyl bromides of type 6 and 7 were then applied in
palladium-catalyzed Suzuki cross-coupling reactions utilizing
boronic acids 11 for the formation of the tri- (R1 =H) and
tetrasubstituted alkenes 12 and 13 (Scheme 4). Unlike other
protocols, we were interested to realize this two-step three-
component reaction sequence as a one-pot procedure with-
out isolating the intermediates of type 6 and 7.[8]
Scheme 2. Zinc-initiated addition of benzylic bromide to alkynes.
For terminal alkynes (R1 =H), the complementary proce-
dures gave good-to-excellent yields and E/Z-selectivities
mostly in the range 90:10 in favor of the respective double-
bond configuration. Also, a wide range of functional groups
were tolerated, and electron-donating as well as electron-
withdrawing groups were well accepted and gave good re-
sults. Noteworthy, internal alkynes could be applied with
good success for the synthesis of products of type 6, whereas
the corresponding products of type 7 could not be obtained
with comparable Z-selectivity as for terminal alkynes. Nev-
ertheless, the application of internal alkynes generated the
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Stereodivergent Zinc-Mediated Three-Component Synthesis
COMMUNICATION
Table 3. Results of the zinc-mediated addition/palladium-catalyzed cross-
coupling reaction sequence.
Entry R1; R2; R3
Product
Yield [%]
(12:13)
R1 =H
1
2
R2 =cyclohexenyl
82 (91:9)
81 (95:5)
R3 =C6H5
R1 =H
R2 =3-MeC6H4
R3 =2-BrC6H4
Scheme 4. Zinc-initiated three-component one-pot synthesis of tri- and
tetrasubstituted alkenes.
R1 =H
3
4
99 (98:2)
91 (5:95)
R2 =C6H5
Therefore, the intermediates were subjected to Suzuki
cross-coupling conditions applying boronic acids (11) as
third component. The results of these transformations are
summarized in Table 3.
R3 =4-AcOC6H4
R1 =H
5
6
R2 =3-MeC6H4
R3 =(CH2)3CH=CH2
41 (99:1)
80 (94:6)
In two cases, the zinc-mediated reactions were performed
with 5 mol% of zinc powder (Table 3, entries 4 and 8). In
these reactions, the alternative products 13c and 13 f were
obtained in good yields and excellent E/Z-selectivities.
Noteworthy, the reaction sequence could be realized in
a one-pot procedure and, to our delight, the yields remained
generally good-to-excellent. Many functional groups, hetero-
cycles, and additional double bonds were tolerated enabling
the synthesis of a broad variety of different molecular archi-
tectures. Eventually, the use of internal alkynes (Table 3, en-
tries 9–11) led to the formation of tetrasubstituted alkenes.
Remarkably, not only aromatic, but also aliphatic boronic
acids could be applied in the Suzuki cross-coupling (Table 3,
entries 5 and 10).
From a mechanistic point of view, the addition of the ben-
zylic bromide to the alkyne could follow a pathway involv-
ing an organozinc species leading to the Z-configured prod-
uct. After the cis-addition to the alkyne, the vinyl zinc bro-
mide must undergo regeneration of the benzylic zinc bro-
mide species upon reaction with 1. Quite contrary, we ob-
serve the formation of 3 as the primary product, which
suggests that the reaction proceeds through addition of ben-
zylic cations to the alkyne as proposed by Ji and co-workers
and Liu and co-workers.[3a,e] When the benzylic organozinc
species underwent a Wurtz reaction, zinc bromide was
formed.[9] Indeed, the reaction of 1 and 2 can also be cata-
lyzed by commercially available ZnBr2 (10 mol%). Howev-
er, the reaction was less effective (66% yield), but gave the
Z-isomer 4 in identical selectivity. Zinc bromide was also
proposed to be responsible for the formation of HBr, which
is causing the isomerization of the primarily formed E-vinyl
bromide to the Z-vinyl bromide 4. Indeed, HBr was detect-
ed in the gas phase and could have been formed by ZnBr2-
induced elimination of HBr from 1. The HBr-induced iso-
merization was inhibited in control experiments, in which
only the products of type 6 were obtained in the presence of
NaH or cyclohexene, and no isomerization occurred. Also,
when phenyl silane was added as additive, benzene could be
detected by GC indicating the formation of HBr in situ.
R1 =H
R2 =4-HexC6H4
R3 =2-C4H3S
R1 =H
7
8
91 (93:7)
74 (7:93)
R2 =4-ClC6H4
R3 =5-(2-CHOC4H2)
R1 =nBu
R2 =C6H5
R3 =(3,4,5-
MeO)3C6H2
9
62[a] (98:2)
84 (87:13)
R1 =nBu
R2 =C6H5
R3 =Me
10
R1 =nPr
11
R2 =4-ClC6H4
R3 =5-(2-MeOC5H2N)
96[a] (94:6)
[a] The product contains an unidentified impurity, which could not be re-
moved by simple column chromatography.
When 150 mol% zinc powder was used, HBr was proposed
to react with the excess zinc and was thereby removed from
the reaction mixture, inhibiting the isomerization.
In conclusion, we have presented a zinc-mediated proce-
dure for the synthesis of vinyl bromides, in which the stereo-
chemistry of the double bond was dependent on the amount
of zinc applied. The vinyl bromides of type 6 and 7 could
then be used in a one-pot protocol for the three-component
assembly of tri- and tetrasubstituted alkenes in good yields
and good E/Z-selectivities utilizing a Suzuki cross-coupling
of the vinyl bromides with boronic acids. The high function-
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G. Hilt and A. Miersch
K. Shimizu, Org. Lett. 2005, 7, 195–197; h) T. Kamei, K. Itami, J.
Yoshida, Adv. Synth. Catal. 2004, 346, 1824–1835; i) A. G. Fallis, P.
Forgione, Tetrahedron 2001, 57, 5899–5913.
al-group tolerance and the flexibility of the synthetic ap-
proach are remarkable and allow the synthesis of more com-
plex alkenes in a straightforward fashion.
[3] a) M.-M. Li, Q. Zhang, H.-L. Yue, L. Ma, J.-X. Ji, Tetrahedron Lett.
2012, 53, 317–319; b) S. Biswas, S. Maiti, U. Jana, Eur. J. Org. Chem.
2009, 2354–2359; c) H.-H. Li, Y.-H. Jin, J.-Q. Wang, S.-K. Tian, Org.
Biomol. Chem. 2009, 7, 3219–3221; d) Z. Liu, J. Wang, Y. Zhao, B.
Zhou, Adv. Synth. Catal. 2009, 351, 371–374.
Experimental Section
[4] For the synthesis of alkenyl halides, see: a) A. Pradal, A. Nasr, P. Y.
Toullec, V. Michelet, Org. Lett. 2010, 12, 5222–5225; b) A. Spaggiari,
D. Vaccari, P. Davoli, G. Torre, F. Prati, J. Org. Chem. 2007, 72,
2216–2219; c) J. Barluenga, D. Palomas, E. Rubio, J. M. Gonzꢂlez,
Org. Lett. 2007, 9, 2823–2826; d) S. I. Lee, G.-S. Hwang, D. H. Ryu,
Synlett 2007, 59–62; e) J. S. Yadav, B. V. S. Reddy, M. K. Gupta, S. K.
Pandey, J. Mol. Catal. A: Chem. 2007, 264, 309–312; f) S. A. Worli-
kar, T. Kesharwani, T. L. Yao, R. C. Larock, J. Org. Chem. 2007, 72,
1347–1353; g) X. Fang, X. Y. Yang, X. J. Yang, S. J. Mao, Z. H. Wang,
G. R. Chen, F. H. Wu, Tetrahedron 2007, 63, 10684–10692; h) K.
Kamei, N. Maeda, T. Tatsuoka, Tetrahedron Lett. 2005, 46, 229–232;
i) J. S. Yadav, B. V. S. Reddy, M. K. Gupta, B. Eeshwaraiah, Synthesis
2005, 57–60; j) Z. F. Xi, W. X. Zhang, Z. Y. Song, W. X. Zheng, F. Z.
Kong, T. Takahashi, J. Org. Chem. 2005, 70, 8785–8789; k) J. W. Sun,
S. A. Kozmin, J. Am. Chem. Soc. 2005, 127, 13512–13513; l) Z. F. Xi,
W. X. Zhang, T. Takahashi, Tetrahedron Lett. 2004, 45, 2427–2429;
m) M. E. Furrow, A. G. Myers, J. Am. Chem. Soc. 2004, 126, 5436–
5445; n) K. Tsuchii, M. Imura, N. Kamada, T. Hirao, A. Ogawa, J.
Org. Chem. 2004, 69, 6658–6665; o) R. Nakajima, C. Delas, Y. Ta-
kayama, F. Sato, Angew. Chem. 2002, 114, 3149–3151; Angew. Chem.
Int. Ed. 2002, 41, 3023–3025; p) K. Kamei, N. Maeda, R. Ogino, M.
Koyama, N. Nakajima, T. Tatsuoka, T. Ohno, T. Inoue, Bioorg. Med.
Chem. Lett. 2001, 11, 595–598; q) M. P. Jennings, E. A. Cork, P. V.
Ramachandran, J. Org. Chem. 2000, 65, 8763–8766; Ramachandran,
J. Org. Chem. 2000, 65, 8763–8766; r) T. Takahashi, W. H. Sun, C. J.
Xi, H. Ubayama, Z. F. Xi, Tetrahedron 1998, 54, 715–726; s) T. Stꢃ-
demann, M. Ibrahim-Ouali, P. Knochel, Tetrahedron 1998, 54, 1299–
1316; t) A. Fꢃrstner, N. Y. Shi, J. Am. Chem. Soc. 1996, 118, 12349–
12357; u) T. Takahashi, D. Y. Kondakov, Z. F. Xi, N. Suzuki, J. Am.
Chem. Soc. 1995, 117, 5871–5872.
Representative procedure for the synthesis of 6a and 12c (Scheme 2,
product 6a and Table 3, entry 3): In a Schlenk tube, 1-bromoethylben-
zene (1; 140 mg, 0.75 mmol, 1.5 equiv) and phenylacetylene (2; 51 mg,
0.50 mmol, 1 equiv) were dissolved under argon atmosphere in anhydrous
dichloromethane (1 mL), and zinc powder (48 mg, 0.75 mmol,
150 mol%) were added. The reaction mixture was stirred for 2 h at RT
and filtered over silica gel (pentane/diethyl ether 1:1). The solvent was
removed and the residue was purified by column chromatography on
silica gel (pentane). The product E-(1-bromobut-1-ene-1,3-diyl)dibenzene
(6a) was obtained as colorless oil (137 mg, 0.48 mmol, 95%, E/Z 87:13).
1H NMR (CDCl3, 300 MHz): d=7.39–7.35 (m, 5H), 7.32–7.29 (m, 2H),
7.25–7.16 (m, 3H), 6.37 (d, J=10.7 Hz, 1H), 3.53 (dq, J=6.9, 10.8 Hz,
1H), 1.36 ppm (d, J=6.9 Hz, 3H); 13C NMR (CDCl3, 75 MHz): d=144.6,
138.5, 128.8, 128.7, 128.6, 128.3, 126.8, 126.4, 119.8, 40.7, 22.0 ppm; IR
(film): u˜ =3059, 3027, 2966, 2926, 2869, 1601, 1491, 1444, 1373, 1015, 872,
764, 699, 543 cmÀ1; MS (EI): m/z (%) 286 ([M+], 5), 207 (83), 191 (54),
129 (100), 105 (31), 77 (27); HRMS (EI): m/z calcd for C16H15Br:
286.0357; found: 286.0348.
For the synthesis of 12c, the intermediate 6a was generated in situ as
before. After complete conversion of the starting materials to 6a, anhy-
drous THF (1 mL), anhydrous methanol (1 mL), potassium hydroxide
(56 mg, 1.0 mmol, 2 equiv), 4-acetylphenylboronic acid (122 mg,
0.75 mmol, 1.5 equiv), triphenylphosphine (5 mg, 0.02 mmol, 4 mol%),
and palladium(II) acetate (3 mg, 0.01 mmol, 2 mol%) were added. The
reaction mixture was stirred for 16 h at 408C and then filtered over silica
gel (diethyl ether). The solvent was removed, and the residue was puri-
fied by column chromatography over silica gel (pentane/diethyl ether
2:1). The product E-4-(1,3-diphenylbut-1-enyl)phenyl acetate (12c) was
obtained as pale yellow oil (169 mg, 0.49 mmol, 99%, E/Z 98:2).
1H NMR (CDCl3, 300 MHz): d=7.84 (d, J=8.6 Hz, 2H), 7.45–7.37 (m,
3H), 7.30 (d, J=8.5 Hz, 4H), 7.23–7.17 (m, 5H), 6.34 (d, J=10.4 Hz,
1H), 3.63 (dq, J=6.9, 10.3 Hz, 1H), 2.57 (s, 3H), 1.41 ppm (d, J=6.9 Hz,
3H); 13C NMR (CDCl3, 75 MHz): d=197.6, 146.9, 145.7, 139.4, 139.3,
136.4, 135.6, 129.7, 128.6, 128.5, 128.2, 127.4, 127.3, 126.9, 126.2, 39.4,
26.5, 22.1 ppm; IR (film): u˜ =3026, 2965, 2923, 2869, 1681, 1599, 1447,
1356, 1264, 837, 762, 699, 592 cmÀ1; MS (EI): m/z (%) 342 ([M+], 1), 326
(100), 311 (16), 283 (28), 205 (27), 165 (14); HRMS (EI): m/z calcd for
C24H22O2: 342.1620; found: 342.1611.
[5] For carbozincation of alkynes, see: a) K. Murakami, H. Yorimitsu, K.
Oshima, Chem. Eur. J. 2010, 16, 7688–7691; b) K. Murakami, H. Yor-
imitsu, K. Oshima, Org. Lett. 2009, 11, 2373–2375; c) G. Sklute, C.
Bolm, I. Marek, Org. Lett. 2007, 9, 1259–1261; d) R. Shintani, T. Ya-
magami, T. Hayashi, Org. Lett. 2006, 8, 4799–4801; e) S. Xue, L. He,
Y.-K. Liu, K.-Z. Han, Q.-X. Guo, Synthesis 2006, 666–674; f) H.
Yasui, T. Nishikawa, H. Yorimitsu, K. Oshima, Bull. Chem. Soc. Jpn.
2006, 79, 1271–1274; g) R. Shintani, T. Hayashi, Org. Lett. 2005, 7,
2071–2073.
[6] For benzylmetalation of alkynes, see: a) G. S. Kauffman, P. S. Watson,
W. A. Nugent, J. Org. Chem. 2006, 71, 8975–8977; b) T. Konno, T.
Daitoh, A. Noiri, J. Chae, T. Ishihara, H. Yamanaka, Tetrahedron
2005, 61, 9391–9404; c) P. Sellꢄs, Org. Lett. 2005, 7, 605–608; d) M. P.
Jennings, K. B. Sawant, Eur. J. Org. Chem. 2004, 3201–3204; e) T.
Konno, T. Daitoh, A. Noiri, J. Chae, T. Ishihara, H. Yamanaka, Org.
Lett. 2004, 6, 933–936; f) N. Fujiwara, Y. Yamamoto, J. Org. Chem.
1999, 64, 4095–4101; g) A. Basheer, I. Marek, Beilstein J. Org. Chem.
2010, 6, 77–88.
[7] a) P. Knochel, J. J. A. Perea, P. Jones, Tetrahedron 1998, 54, 8275–
8319; b) P. Knochel, R. D. Singer, Chem. Rev. 1993, 93, 2117–2189.
[8] A. L. Monteiro, C. M. Nunes, D. Steffens, Synlett 2007, 0103–0106.
[9] Small amounts of the Wurtz product were observed by GC/MS analy-
sis.
Keywords: alkenes · alkynes · boronic acids · palladium ·
zinc
[1] For key references, see: a) E.-I. Negishi, Z. Huang, G. W. Wang, S.
Mohan, C. Wang, Acc. Chem. Res. 2008, 41, 1474–1485; b) M. Mori,
Eur. J. Org. Chem. 2007, 4981–4993.
[2] a) K. Nozaki, M. Yamashita, Y. Okuno, Eur. J. Org. Chem. 2011,
3951–3958; b) E. Negishi, S. Xu, C.-T. Lee, H. Rao, Adv. Synth.
Catal. 2011, 353, 2981–2987; c) E. Negishi, G. Wang, H. Rao, Z. Xu,
J. Org. Chem. 2010, 75, 3151–3182; d) A. B. Flynn, W. W. Ogilvie,
Chem. Rev. 2007, 107, 4698–4745; e) E. Negishi, Z. Tan, Angew.
Chem. Int. Ed. 2006, 45, 762–765; f) C. X. Zhou, R. C. Larock, J.
Org. Chem. 2005, 70, 3765–3777; g) M. Mori, Y. Sato, M. Takimoto,
Received: April 23, 2012
Published online: && &&, 0000
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Stereodivergent Zinc-Mediated Three-Component Synthesis
COMMUNICATION
Synthetic Methods
A. Miersch, G. Hilt* ......... &&&& — &&&&
Stereodivergent Zinc-Mediated ACHTUNGTRENNUNGThree-
Component Synthesis of Tri- and
Tetrasubstituted Alkenes
Zinc simple: The loading-dependent
zinc-mediated addition of benzyl bro-
mides to alkynes is the key step for the
formation of vinyl bromides. In combi-
nation with a palladium-catalyzed
Suzuki cross-coupling reaction with
boronic acids tri- and tetrasubstituted
alkenes were obtained in good yields
and stereoselectivities in a one-pot
protocol (see scheme).
Chem. Eur. J. 2012, 00, 0 – 0
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