Straightforward iron-catalyzed synthesis of vinylboronates by the hydroboration of alkynes

Article abstract of DOI:10.1002/asia.201200931

An iron-catalyzed hydroboration of alkynes to produce vinylboronates has been examined. With a straightforward system composed of iron carbonyls and pinacolborane, good to excellent yields and chemoselectivities were achieved for a variety of alkynes. Cop

Full text of DOI:10.1002/asia.201200931

COMMUNICATION
DOI: 10.1002/asia.201200931
Straightforward Iron-Catalyzed Synthesis of Vinylboronates by the
Hydroboration of Alkynes
Michael Haberberger and Stephan Enthaler*[a]
Olefins have a wide range of applications including, as
building blocks for bulk chemicals, pharmaceuticals, agro-
chemicals, polymers, in the syntheses of natural products,
and as key intermediates in organic syntheses.[1] During the
last decades, a number of methodologies have been estab-
lished to access alkenes; of these transformations, the reduc-
tion of alkynes to produce alkenes is of relevance, because
of the availability and straightforward synthesis of the start-
ing materials.[2] In this regard, the application of transition
metal catalysts has been demonstrated as a useful tool to
access olefins through, for example, hydrogenation, hydrosi-
lylation, or hydroboration.[3] Especially, the construction of
functionalized olefins, such as vinylsilanes or vinylboranes,
allow for further straightforward transformations, such as
coupling reactions (Scheme 1).[4] Up to now, manifold sys-
silylation by the group of Chirik, who applied well-defined
iron-complexes modified by tridentate nitrogen ligands to
reduce alkynes with silanes to the corresponding alkene at
ambient temperature.[8] Furthermore, Plietker et al. realized
the hydrosilylation of internal alkynes with [FeH(CO)(NO)-
AHCTUNGTRENNUNG
(PPh3)2] as a precatalyst.[9] Interestingly, depending on the
kind of silane employed the stereochemistry of the products
can be easily tuned. Recently, we reported the application
of easily accessible iron phosphane complexes in the hydro-
silylation of disubstituted alkynes under mild reaction condi-
tions.[10] In contrast, the iron-catalyzed hydroboration of al-
kynes has been not reported so far. However, the feasibility
of the addition of boranes to an unsaturated system such as
olefins in the presence of iron catalysts has been recently re-
alized.[11] Based on these results and our initial results in the
iron-catalyzed hydrosilylation
of alkynes, we wish to portray
the application of a robust and
easy-to-adopt iron-based cata-
lyst for the highly selective hy-
droboration of alkynes to pro-
duce vinylboronates.
Preliminary studies on the
influence of the reaction con-
Scheme 1. Hydroboration of substituted alkynes.
ditions were carried out with
diphenylacetylene
1
as the
tems rely on precious metals, such as rhodium, ruthenium,
iridium, or palladium. However, owing to the high price and
sometimes toxicity, less-expensive metal-based catalysts are
highly desirable. In this regard, the potential of iron has
been recently proven.[5] Moreover, one important aspect of
the protocol should be to address the selectivity issues, as
various isomers are feasible depending on the substitution
pattern of the starting material (Scheme 1).[6] Indeed, iron
catalysts have been applied in the catalytic reduction of al-
kynes using molecular hydrogen as a reductant.[7] On the
other hand, promising efforts have been reported on hydro-
model substrate by using 1.67 mol% of triiron dodecacar-
bonyl and pinacolborane 2 as reducing reagent in toluene at
1008C (Table 1). After 24 h the reaction mixture was ana-
lyzed. Full conversion of the starting material was observed
and the desired products were found in a ratio of 72:28
((Z)-3/(E)-3; Table 1, entry 2). Noteworthy, the catalyst was
highly selective towards the formation of the vinylboronates,
while no over-reduction was observed (Table 1, entry 2).
Moreover, catecholborane was applied as a hydroborane
source. However, no product formation was achieved
(Table 1, entry 3). Besides Fe3(CO)12, Fe2(CO)9 and Fe(CO)5
were also tested as precatalysts (Table 1, entries 5–8). Excel-
lent performance was found for Fe2(CO)9 with >99% con-
[a] M. Haberberger, Dr. S. Enthaler
Technische Universitꢀt Berlin
version and
a ((Z)-3/(E)-3) ratio of 93:7, while with
Department of Chemistry
Fe(CO)5 lower yields and selectivities were attained. Fur-
thermore, various iron salts were investigated, but resulted
in no product formation (Table 1, entries 9–11), whilst in the
absence of any iron salts no product was detected (Table 1,
entry 1). However, no progress was realized with respect to
yield and selectivity. Decreasing the reaction temperature
Cluster of Excellence “Unifying Concepts in Catalysis”
Straße des 17. Juni 115/C2, 10623 Berlin (Germany)
Fax : (+49)3031429732
E-mail: stephan.enthaler@tu-berlin.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200931.
Chem. Asian J. 2013, 8, 50 – 54
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Stephan Enthaler and Michael Haberberger
Table 1. Iron-catalyzed hydroboration of diphenylacetylene 1.
Table 2. Scope and limitations of the iron-catalyzed hydroboration of al-
kynes.
Entry[a] Iron source
(mol%)
t [h] Conv. [%][b] Yield [%][b] Yield [%][b]
(Z)-3
(E)-3
1
2
–
24
24
<1
>99
<1
72
<1
28
Fe3(CO)12
(1.67)
Fe3(CO)12
(1.67)
Fe3(CO)12
(1.67)
3[c]
4
24
1
<1
<1
<1
Entry[a] Substrate
1
Conv. [%][b] Yield [%][b]
37
35
2
4
5
>99
>99
4a: >99 (E)
5a: >99 (E)
5
6
7
8
9
Fe2(CO)9 (2.5)
1
41
>99
30
86
<1
41
93
25
83
<1
<1
7
5
3
<1
Fe2(CO)9 (2.5) 24
Fe(CO)5 (5.0)
Fe(CO)5 (5.0) 24
FeCl2·4H2O
(5.0)
2
3
1
1
6
7
>99
6a: >99 (E)
10
11
FeCl3 (5.0)
1
1
3
<1
3
<1
<1
<1
Fe
(ClO4)3
7a: 71 (E): 6
(Z)
4
78
(5.0)
12[d]
13[e]
14[f]
15[g]
16[h]
Fe2(CO)9 (2.5) 24
Fe2(CO)9 (2.5) 24
Fe2(CO)9 (2.5) 24
Fe2(CO)9 (2.5) 24
Fe2(CO)9 (2.5) 24
34
<1
68
98
>99
33
<1
67
91
88
1
<1
1
7
12
5
6
8
9
>99
>99
8a: 74 (E)[c]
9a: >99 (E)
[a] Reactions were carried out with an iron source (0.036 mmol,
5 mol%), diphenylacetylene (0.72 mmol), borane (0.79 mmol,
7
10 98
10a: 98 (E)
1.25 equiv), in toluene (2.0 mL) at 1008C. [b] The conversion and yield
were determined by GC-MS (30 m Rxi-5 ms column, 40–3008C) by ap-
plying n-dodecane as an internal standard. [c] Catecholborane was used
as the borane source. [d] 708C. [e] room temperature. [f] THF, 708C. [g] 2
(2.0 equiv). [h] 100 mmol scale.
8
9
11 >99
12 >99
11a: 40[d]
12a: >99 (E)
towards 708C or room temperature (Table 1, entries 12 and
13), respectively, did not improve the reaction outcome. By
using tetrahydrofuran (THF) as solvent at 708C resulted in
68% yield and 99% Z-selectivity. Moreover, the influence
of various nitrogen- (e.g., N,N,N’,N’-tetramethylethylenedia-
mine (TMEDA), 2,2’-bipyridine, phenanthroline, imidazole)
and phosphorous-containing ligands (e.g., PPh3, PBu3, 1,1’-
10
11
12
13 >99
14 43
13a: >99 (E)
14a: 43
15 >99
15a: 61[d]
bis(diphenylphosphino)ferrocene,
diphenylphosphino-
ethane) were studied. However, in comparison to the un-
modified iron carbonyl complexes no improvement was ob-
served. Noteworthy, the system was easily scaled up to
100 mmol, and showed excellent conversion and good selec-
tivity under non-inert conditions (Table 1, entry 16). The
product was easily obtained by recrystallization from n-
hexane–after removing the catalyst by filtration over a short
plug of silica–in 82% yield.
To explore the scope and limitations of the iron catalyst
[Fe2(CO)9], a variety of alkyne substrates were tested under
the reaction conditions given in Table 1, entry 6. First, vari-
ous terminal phenylacetylenes were treated with pinacolbor-
ane 2 (Table 2, entries 1–9). Excellent yields and selectivities
were observed for unsubstituted phenylacetylene (Table 2,
16a: 92 (E): 8
(Z)
13
14
15
16 >99
17 >99
18 >99
17a: 68 (E)
18a: 96 (E): 4
(Z)
16
17
18
19 <1
19a: <1
20 >99
20a: >99 (E)
3: 93 (Z): 7 (E)
1
>99
Chem. Asian J. 2013, 8, 50 – 54
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Stephan Enthaler and Michael Haberberger
ficulties were observed for benzylic and propargylic sub-
strates (Table 2, entries 11 and 12). Furthermore, the effect
of alkyl substituents was investigated, and showed excellent
conversions (Table 2, entries 13–15). However, to some
extent the corresponding Z-isomer (4–8%) was detected.
Interestingly, the iron catalyst showed excellent performance
Table 2. (Continued)
Entry[a] Substrate
Conv. [%][b] Yield [%][b]
91[c]
63
21a: 29:57:3:2[g]
19
20
21
21
21a: 21:38:2:2[g]
22 97
23 53
22a: 27:65:4:2[g]
À
for a substrate containing a C C double bond in conjugation
23a: 13:32:4:4[g]
24a: 30:47:1:1[g]
À
to the C C triple bond (Table 2, entry 14). Noteworthy, an
exclusive reaction pathway for the hydroboration of the
alkyne was observed. The hydroboration of substrates con-
taining heteroaryls was successful for thiophene, while the
pyridyl group probably inhibits the catalyst (Table 2, en-
tries 16 and 17). In addition, disubstituted alkynes were also
used (Table 2, entries 18–24). Noteworthy, for unsymmetri-
cal alkynes four potential products are feasible. Indeed, for
most substrates a mixture of the four possible isomers was
observed. Unfortunately, separation and stereochemistry as-
signment of the mixture was not successful, owing to com-
plexity. To study the selectivity of the hydroboration in the
presence of substrates containing functional groups that are
sensitive to reduction different substrates in combination
with diphenylacetylene were treated with 2 in the presence
of Fe2(CO)9 under optimized reaction conditions (Table 2,
entries 26–29). Excellent selectivity (>99%) for the hydro-
boration was observed in the presence of esters (yield: 73%
(Z)-3, 3% (E)-3) and nitriles (yield: 81% (Z)-3, 6% (E)-3).
In contrast, sulfoxide (yield: 8% of the sulfide) and alde-
hyde (98% of the alcohol) functionalities were reduced to
some extent.
22
23
24 79
25a:
90[e]
>99
18:10:12:50[g]
25a:
25
18:27:8:47[g]
26a:
24
25
26 75
27 77
3:20:32:20[g]
27a: 74:3[g]
26
27
28 <1
29 76
3: <1, (98)[f]
3: 73 (Z): 3 (E),
(<1)[f]
3: 75 (Z):1 (E),
28
29
30 76
31 87
(8)[f]
3: 81 (Z):6 (E),
(<1)[f]
Moreover, the developed protocol was used in the synthe-
sis of trisubstituted olefins (Scheme 2). First, the hydrobora-
tion of diphenylacetylene 1 was carried out as described in
Table 2, entry 18. After isolation of the corresponding vinyl-
boronate 3, a palladium-catalyzed cross coupling with differ-
[a] Reactions were carried out with Fe2(CO)9 (0.018 mmol, 2.5 mol%), diphe-
nylacetylene (0.72 mmol), borane (0.79 mmol, 1.25 equiv), in toluene
2
(2.0 mL) at 1008C, 24 h. [b] The conversion and yield were determined by
GC-MS (30 m Rxi-5 ms column, 40–3008C) and 1H NMR spectroscopy.
[c] 16% of undefined side products. [d] Undefined side products.
[e] Fe3(CO)12 (1.67 mol%). [f] 1.0 equiv of substrate 28–31 was added. The
yield of the reduced form of the additional substrate is stated in brackets.
[g] Stereochemistry was not assigned, due to complexity.
ent aryl halides was carried out.[13] Addition of [Pd
ACHTUNGTRENNUNG(PPh3)4]
(5.0 mol%) and K2CO3 as a base afforded olefin 3 in good
to excellent yield after 24 h at 1208C. Noteworthy, in ac-
cordance to literature reports the E-isomer was observed
almost exclusively.[14]
entry 1). Interestingly, the cor-
responding E-isomer was ex-
clusively formed by anti-Mar-
kovnikov and syn-addition of
the borane reagent. Next, the
influence of methyl substitu-
tion on the phenyl group was
studied (Table 2, entries 2–4).
Excellent performance was ob-
served for para- and meta-sub-
stitution, while
a decreased
yield and selectivity was no-
ticed for ortho-substitution.
Moreover, substrates contain-
ing halide groups were reduced
in excellent yields and selectiv-
ities (Table 2, entries 5–7). Dif-
Scheme 2. Follow-up chemistry of vinylboronates (Reaction conditions for the cross coupling: [PdACHTUNGTRENNUNG(PPh3)4]
(5.0 mol%), 3 (0.52 mmol), K2CO3 (0.95 mmol, 1.5 equiv), and the aryl halide (0.63 mmol, 1.25 equiv), in tolu-
ene (2.0 mL) at 1208C for 24 h).[15]
Chem. Asian J. 2013, 8, 50 – 54
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Stephan Enthaler and Michael Haberberger
technical support. Furthermore, Dr. Kathrin Junge (Leibniz Institut fꢂr
Katalyse, Rostock) is thanked for donation of chemicals.
Keywords: alkynes
·
homogeneous
catalysis
·
hydroboration · iron · olefins
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With respect to the reaction mechanism we have pro-
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À
insertion of the alkyne into the Fe H bond (C) can take
À
place or on the other hand into the Fe B bond (D). Finally,
a reductive elimination occurs to form the desired vinylbor-
onate and by coordination of a new alkyne molecule the
starting complex A is reformed.
In summary, we have reported a protocol for the efficient
iron-catalyzed hydroboration of alkynes. With Fe2(CO)9 and
pinacolborane a straightforward system was found, which is
capable of producing a variety of vinylboronates in good to
excellent yields and selectivities. Noteworthy, the reaction
can be easily scaled up to 100 mmol to allow access to the
products under non-inert conditions and this makes it prob-
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Experimental Section
General procedure for the catalytic hydroboration
A Schlenk flask was charged with an appropriate amount of Fe2(CO)9
(2.5 mol%), 2 (1.25 equiv, 1.57 mmol), and the corresponding alkyne
(1.96 mmol). The flask was flushed with nitrogen under vacuum. After-
wards toluene (2.0 mL) was added. The flask was sealed and heated at
1008C for 24 h. After that time, the mixture was cooled, and an aliquot
was taken and dissolved in dichloromethane for GC-MS analysis. The
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Acknowledgements
This work was supported by the Cluster of Excellence “Unifying Con-
cepts in Catalysis” (sponsored by the Deutsche Forschungsgemeinschaft
and administered by the Technische Universitꢀt Berlin). The authors
thank Jçrg Schmidt (TU Berlin) and Priya Anantharajah (TU Berlin) for
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[15] The stereochemistry of 3d was assigned by single-crystal X-ray dif-
fraction analysis (CCDC 909406 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.)
Received: October 4, 2012
Revised: October 19, 2012
Published online: November 12, 2012
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