Synthesis of allenes via Nickel-catalyzed cross-coupling reaction of propargylic bromides with grignard reagents

Article abstract of DOI:10.1055/s-0031-1290365

We describe a convenient method for the synthesis of terminal allenes from cross-coupling of propargylic bromide with Grignard reagent. The reaction of propargylic bromide with 1.2 equivalents of Grignard reagent mediated by Ni(acac)₂ (2 mol%) and Ph₃P (4 mol%) in THF may produce terminal allenes in good yields and high regioselectivities at room temperature. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290365

LETTER
747
Synthesis of Allenes via Nickel-Catalyzed Cross-Coupling Reaction of
Propargylic Bromides with Grignard Reagents
Synthesis of
A
llen
i
es nghan Li,*a Hanmou Gaub
a
College of Chemistry and Environmental Proection Engineering, Southwest University for Nationalities,
Chengdu, 610041, P. R. of China
Fax +86(28)85524382; E-mail: lqhchem@163.com
Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan
b
Received 11 November 2011
ing, and limited substrates. Recent investigations have
demonstrated that the nickel is a good catalyst for many
cross-coupling reactions.21 Herein we report a novel nick-
el(II)-catalyzed cross-coupling of a propargylic halide
Abstract: We describe a convenient method for the synthesis of
terminal allenes from cross-coupling of propargylic bromide with
Grignard reagent. The reaction of propargylic bromide with 1.2
equivalents of Grignard reagent mediated by Ni(acac)2 (2 mol%)
and Ph3P (4 mol%) in THF may produce terminal allenes in good with a Grignard reagent at ambient temperature in a short
yields and high regioselectivities at room temperature.
time with good yield for the synthesis of allenes.
Key words: allenes, nickel, propargylic bromide, Grignard re-
agent, cross-coupling
In a preliminary study, treatment of propargylic bromide
(1a) with phenylmagnesium bromide (2a) in the absence
of metal and ligand at ambient temperature in THF led to
the formation of phenylallene (3aa) and 3-phenylprop-1-
Allenes are useful structural motifs in organic
transformations1 and have been found in many natural
products as well as pharmaceutically related compounds.2
As a result, allenes have inspired ample interest of organic
and medical chemistry,1b–1h,3 and numerous new synthetic
methodologies to access such compounds have been de-
veloped,4 for instance, the organocopper-catalyzed SN2¢-
type displacement of propargyl alcohol derivatives,5,6 the
homologation of 1-alkynes,7 asymmetric allylations,8 the
stereoselective reduction of alkynes,9 b-eliminations by
Horner–Emmons–Wadsworth10 or sulfinyl radical11 reac-
tions, palladium-catalyzed hydrogenolysis12 and hydride-
yne (4aa) in a moderate conversion of 74%, but with a low
regioselectivity of 3aa/4aa (63:37, Table 1, entry 1). To
our delight, when NiBr2/Ph3P was used, the reaction con-
version was signally elevated to 98% with a good regio-
selectivity (3aa/4aa = 78:22, Table 1, entry 2). To further
understand the essence of this catalysis, we tested the
model reaction under various conditions. As shown in
Table 1 (entries 3–5), NiCl2/Ph3P, Ni(NO3)2/Ph3P, and
Ni(OAc)2/Ph3P were also effective for the conversion of
the coupling between propargylic bromide (1a) and phe-
nylmagnesium bromide (2a), but show some lower regio-
selectivity. Further screen of other nickel complex,
Ni(acac)2/Ph3P was found to be the most efficient catalyst
for the coupling between propargylic bromide (1a) and
phenylmagnesium bromide (2a) considering both the con-
version and regioselectivity (Table 1, entry 6).
transfer
reaction,13
coupling
reactions
of
allenylstannanes14 and allenylindiums,15 indium-mediated
reactions of propargylic bromides with aldehydes,16 al-
lene cross metathesis,17 carbene/inylidene cross-cou-
pling,18 and cross-coupling of N-tosylhydrazone with
terminal alkyne.19 Among these methods hitherto devel-
oped, the organocopper-catalyzed SN2¢-type displacement
of propargyl alcohol derivatives is one of the most gener-
ally useful ones.
Next, the effect of other ligands on the conversion and re-
gioselectivity of this reaction using Ni(acac)2 were stud-
ied (Table 1, entries 6–9). When PCy3, (4-MePh)3P and
(C6F5)3P were used, the conversion and the regioselectiv-
ity of the model reaction were not varied appreciably. In
addition, only Ni(acac)2 was used as catalyst, the conver-
sion and the regioselectivity of this reaction were slightly
decreased (Table 1, entry 6 vs. entry 10). Thus the
Ni(acac)2/Ph3P complex was identified as the most effec-
tive catalyst (Table 1, entry 6).
The Grignard reagent is a reactive nucleophilic reagent
and can easily be prepared from the corresponding halide.
Although the synthesis of allene by cross-coupling of
Grignard reagent with propargylic compounds has been
described,20 the synthesis based on direct coupling of pro-
pargylic halide with Grignard reagent is developed rarely.
To the best of our knowledge, to date there are only a few
reports in the literature, and copper is often used as cata-
lyst in these methods.20a,b,d,h However, these methods still
have some drawbacks, such as long reaction time, low
temperature, low regioselectivity or large catalyst load-
A study on the solvent effect showed that THF provided
the best conversion and regioselectivity of the product 3aa
+ 4aa (Table 1, entry 6). In other solvent such as toluene,
hexane, Et2O, and CH2Cl2, good regioselectivity could not
be observed (Table 2, entries 1–4). Changing the molar
ratio between Ni(acac)2/ Ph3P led to a low regioselectivity
(Table 2, entries 5 and 6). Unexpectedly, the highest con-
version and regioselectivity was obtained when changing
the molar ratio of substrates 1a/2a from 1.0:1.1 to 1.0:1.2
SYNLETT 2012, 23, 747–750
x
x.
x
x.
2
0
1
2
Advanced online publication: 24.02.2012
DOI: 10.1055/s-0031-1290365; Art ID: W64011ST
© Georg Thieme Verlag Stuttgart · New York

748
Q. Li, H. Gau
LETTER
(Table 2, entry 7). Further studies indicated that the load-
ing of catalyst exerted a slight influence upon the conver-
sion and regioselectivity. The most favorable loading was
2 mol% Ni(acac)2/4 mol% Ph3P (Table 2, entry 7).
Table 1 Efffect of the Metal and Ligand in the Cross-Coupling of
Propargylic Bromide with Phenylmagnesium Bromidea
MgBr nickel (2 mol%)
•
ligand (4 mol%)
+
+
Br
With the optimized reaction conditions in hand, the scope
of this reaction was studied by using various Grignard re-
agent and propargylic bromide. The cross-coupling of
propargylic bromide 1a with Grignard reagent 2a–o pro-
ceeded smoothly to provide the corresponding products
3aa–ao in moderate to good yields (Table 3, entries 1–
15). The reaction was not significantly affected by the
substituents on the aromatic ring of the Grignard reagent.
Both electron-rich (Table 3, entries 2–8) and electron-de-
ficient substituent on the aromatic ring of the Grignard re-
agent were effective (Table 3, entries 9–11). Notably,
alkoxy, methyl, chloro, fluoro, and trifluoromethyl groups
are all tolerated under the given reaction conditions. The
reaction also worked well with naphthyl Grignard reagent
(Table 3, entries 12 and 13), alkyl Grignard reagent
(Table 3, entries 14 and 15).
THF, r.t., 4 h
1a
2a
3aa
4aa
Entry Nickel source Ligand
Conv. (%)b Ratio 3aa/4aac
1
2
–
–
74
98
98
97
96
98
97
97
98
93
63:37
78:22
60:40
72:28
66:34
86:14
83:17
83:17
83:17
84:16
NiBr2
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
PCy3
(4-MePh)3P
(C6F5)3P
–
3
NiCl2
4
Ni(NO3)2
Ni(OAc)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
5
6
7
8
Then, the reaction was examined by using 1-bromopent-
2-yne with Grignard reagent (Table 3, entries 16–18).
However, whether electron-rich (Table 3, entry 17) or
electron-deficient (Table 3, entry 18) substituent on the
aromatic ring of the Grignard reagent gave only moderate
yields.
9
10
a Reaction conditions: 1a (1.0 mmol), 2a (1.1 mmol), THF (2 mL),
4 h, r.t.
b Conversions of 3aa and 4aa were determined by 1H NMR.
c The ratio of 3aa/4aa was determined by 1H NMR
Table 3 Ni(acac)2/Ph3P-Catalyzed Cross-Coupling Reaction of
Propargylic Bromide with Grignard Reagenta
R1
Table 2 Efffect of the Solvent and the Molar Ratio of Ni(acac)2/
Ph3P in the Cross-Coupling of Propargylic Bromide with Phenylmag-
nesium Bromidea
•
R1
R2
Ni(acac)2 (2 mol%)
Br
3
+
Ph3P (4 mol%)
1
MgBr
+
R2 MgBr
2
Ni(acac)2 (2 mol%)
R1
THF, r.t., 4–6 h
R2
Ph3P (4 mol%)
+
4
+
Br
solvent, r.t., 4 h
Entry
1
2
Yield of 322 Yield of 4
1a
2a
3aa
4aa
(%)b
(%)b
Entry
Ni(acac)2/Ph3P Solvent
(mol%)
Conv.
(%)b
Ratio
3aa/4aac
MgBr
MgBr
3aa 78a
3aa 50c
4aa 10a
4aa 20c
1
2
1a R = H
1
2:4
2:4
2:4
2:4
2:2
2:6
2:4
2:4
1:2
Et2O
90
83
92
91
92
96
98
98
96
54:46
63:37
62:38
82:18
80:20
71:29
90:10
80:20
86:14
2a
2
hexane
toluene
CH2Cl2
THF
3
1a
3ab 80
4ab 8
4
2b
2c
2d
5
MgBr
3
4
1a
1a
3ac 74
3ad 75
4ac 14
4ad 13
6
THF
7d
8e
9d
THF
MgBr
THF
THF
a Reaction conditions: 1a (1.0 mmol), 2a (1.1 mmol), THF (2 mL).
b Conversions of 3aa and 4aa were determined by 1H NMR.
c The ratio of 3aa/4aa was determined by 1H NMR.
OMe
MgBr
5
1a
3ae 70
4ae 11
d Reaction conditions: 1a (1.0 mmol), 2a (1.2 mmol), THF (2 mL).
e Reaction conditions: 1a (1.0 mmol), 2a (1.3 mmol), THF (2 mL).
2e
Synlett 2012, 23, 747–750
© Thieme Stuttgart · New York

LETTER
Synthesis of Allenes
749
Table 3 Ni(acac)2/Ph3P-Catalyzed Cross-Coupling Reaction of
Table 3 Ni(acac)2/Ph3P-Catalyzed Cross-Coupling Reaction of
Propargylic Bromide with Grignard Reagenta (continued)
Propargylic Bromide with Grignard Reagenta (continued)
R1
R1
•
•
R1
R1
R2
R2
Ni(acac)2 (2 mol%)
Ph3P (4 mol%)
Ni(acac)2 (2 mol%)
Ph3P (4 mol%)
Br
Br
3
3
1
1
+
R2 MgBr
2
+
+
R2 MgBr
2
+
R1
R1
THF, r.t., 4–6 h
THF, r.t., 4–6 h
R2
R2
4
4
Entry
1
2
Yield of 322 Yield of 4
Entry
1
2
Yield of 322 Yield of 4
(%)b
(%)b
(%)b
(%)b
MgBr
MgBr
17
18
1b
3bd 56
4bd 33
6
1a
3af 72
4af 15
2d
OMe
MgBr
2f
1b
3bj 53
4bj 33
MgBr
F
2j
7
8
1a
1a
3ag 74
3ah 78
4ag 12
4ah 9
a Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), Ni(acac)2
(2 mol%), Ph3P (4 mol%), THF (2 mL).
OMe
b Isolated yield.
2g
c Ref. 20d: 1 (0.12 mmol), 2 (0.1 mmol), CuBr (7 mol%), LiBr (23
mol%), THF (14.0 mL), the reaction mixture was stirred at –50 °C for
45 min, then the reaction mixture was stirred at 10 °C.
MgBr
2h
On comparing the cross-coupling of propargylic bromide
with Grignard reagent by the nickel-catalyzed method
with that by the copper-catalyzed method (Table 3, entry
1), we found that the former proceeded with comparably
high yields and regioselectivity, low catalyst loading, and
easy manipulation.
MgBr
9
10
11
1a
1a
1a
3ai 83
3aj 73
3ak 78
4ai 8
Cl
2i
MgBr
4aj 15
4ak 12
In conclusion, an improved procedure for the nickel-cata-
lyzed cross-coupling of propargylic bromide with Grig-
nard reagent has been developed, and it is demonstrated
that this methodology is a simple and efficient method for
the preparation of various monosubstituted arylallenes
and alkylallenes with high regioselectivity. Further appli-
cation of these allenes in organic synthesis and detailed
mechanistic studies are in progress.
F
2j
MgBr
F3C
2k
MgBr
12
13
1a
1a
3al 76
4al trace
4am 9
Acknowledgment
2l
We thank the Fundamental Research Funds for the Central Univer-
sities, Southwest University for Nationalities (No. 11NZYTD01)
and the Natural Science Foundation of Southwest University for
Nationalities (No. 381010).
MgBr
3am 80
2m
MgBr
References and Notes
14
15
16
1a
1a
3an 77
3ao 74
3ba 54
4an 10
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© Thieme Stuttgart · New York
Synlett 2012, 23, 747–750

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LETTER
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Under an atmosphere of nitrogen, Ni(acac)2 (5.12 mg, 0.02
mmol), Ph3P (0.04 mmol), and THF (2 mL) were mixed in a
Schlenk flask. Shortly afterwards, aryl or alkyl Grignard
reagent (1.2 mmol) was added, and subsequently the
proparglic bromide was added. Then the reaction mixture
was stirred at r.t. for 4–6 h. After completion of the reaction,
the mixture was diluted with H2O (15 mL) and extracted
with Et2O (3 × 15 mL). The combined organic layers were
dried over anhydrous Na2SO4, filtered, and evaporated in
vacuum. The residue was subjected to flash column
chromatography on silica gel (hexane or EtOAc–hexane,
100:1) to afford the corresponding allene products 3.
Allenylbenzene (3aa):13e colorless oil; 1H NMR (400 MHz,
CDCl3): d = 7.31–7.19 (m, 5 H), 6.17 (t, J = 6.8 Hz, 1 H),
5.16 (d, J = 6.8 Hz, 2 H). 13C {1H} NMR (100 MHz, CDCl3):
d = 209.8, 133.9, 128.6, 126.9, 126.7, 93.9, 78.7. HRMS:
m/z calcd for C9H8: 116.0626; found: 116.0637 [M]+.
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