cis-Semihydrogenation of alkynes with amine borane complexes catalyzed by gold nanoparticles under mild conditions

Article abstract of DOI:10.1039/c4cc08163c

Supported gold nanoparticles catalyze the semihydrogenation of alkynes to alkenes with ammonia borane or amine borane complexes in excellent yields and under mild conditions. Internal alkynes provide cis-alkenes, making this protocol an attractive alternative of the classical Lindlar's hydrogenation.

Full text of DOI:10.1039/c4cc08163c

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ARTICLE TYPE
cis-Semihydrogenation of Alkynes With Amine Borane Complexes
Catalyzed by Gold Nanoparticles Under Mild Conditions
Eleni Vasilikogiannaki,¶ Ioannis Titilas,¶ Georgios Vassilikogiannakis and Manolis Stratakis*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
Table 1. Reduction of pꢀmethoxyphenylacetylene (1) with amine
55 borane complexes catalyzed by supported Au nanoparticles.
Supported gold nanoparticles catalyze the semihydrogenation of
alkynes to alkenes with ammonia borane or amine borane
complexes in excellent yields and under mild conditions. Internal
10 alkynes provide cis-alkenes, making this protocol an attractive
alternative of the classical Lindlar’s hydrogenation.
Au NPs (1%)
amine borane
MeO
MeO
solvent, 25 o
C
1
1a
Nanogoldꢀcatalyzed reductive processes using either H2 or
transfer hydrogenation pathways have recently received
considerable attention.1 In this context, little progress has
15 been achieved in the reduction of πꢀsystems by direct
hydrogenation2 due to the difficulty of formation of the labile
gold hydride reducing species3 on the surface of
nanoparticulated Au catalysts (Au NPs). On the other hand,
promising examples of heterogenized Auꢀcatalyzed transfer
20 hydrogenation protocols for πꢀsystems are gradually starting
to appear,4 especially in the semireduction of alkynes using
hydrosilanes/H2O,4a or CO/H2O,4c as reductants.
Our recent promising results in the transfer hydrogenation
of nitro compounds using ammonia borane complex catalyzed
25 by supported Au NPs5 drove us to examine the potency of
ammonia borane and other amine borane complexes in the
reduction of π systems and especially alkynes, given that the
semihydrogenation of alkynes into cisꢀalkenes is of significant
interest in synthetic organic chemistry.6 To achieve this
30 transformation, organic chemists mainly hydrogenate alkynes
in the presence of suitable catalysts (e.g. Lindlar’s catalyst or
frustrated Lewis pairs).7 Yet, these processes require special
apparatuses operating with flammable H2 gas. More practical
semireduction procedures would certainly be welcomed by
35 researchers on the bench.8
Molar
equiv
Sel.
Time Conv.
Entry Catalyst Solvent
Reductant
(amine borane)
(%)a
(h)
(%)
Au/TiO2
Au/TiO2
1
2
EtOH
EtOH
NH3BH3
NH3BH3
1.0
0.5
0.2
0.5
100
100
94
98
Au/TiO2
Au/TiO2
Au/TiO2
0.35
2.0
2.0
8.0
1.0
85
8
>99
>99
>99
3
4
5
EtOH
DCM
NH3BH3
NH3BH3
NH3BH3
2.0
16
EtOAc
Au/TiO2
Au/TiO2
Au/Al2O3
Au/ZnO
2.0
2.0
1.0
1.0
1.0
1.0
0.5
0.5
11
100
100
10
>99
93
6
7
8
9
THF
NH3BH3
THF/H2Ob NH3BH3
95
EtOH
EtOH
EtOH
EtOH
NH3BH3
>99
NH3BH3
Au/TiO2
Au/TiO2
10
11
NH2NH2BH3
t-BuNH2BH3
1.5
3.0
0.2
1.0
100
15
98
>99
Au/TiO2 EtOH
Au/TiO2 EtOH
Au/TiO2 EtOH
Au/TiO2 EtOH
12
13
14
15
Me2NHBH3
Me2NHBH3
MeNH2BH3
Me3NBH3
0.5
1.0
0.5
3.0
0.5
0.2
0.5
1.0
100
100
>99
99
100
0
>99
aRelative percent ratio of alkene 1a to the corresponding alkane.
bTHF/H2O=20/1.
60 conditions (80 C, 24ꢀ72 h).12 We found that using Au/TiO2 (1
mol%) as catalyst and ethanol as solvent, smooth reduction of
1 to pꢀmethoxystyrene (1a) took place with 0.5 molar equiv of
ammonia borane (AB), or dimethylamine borane (DMAB),
o
The reduction of pꢀmethoxyphenylacetylene (1) was
examined as a model substrate in the presence of different
supported Au NPs catalysts,9 amine borane complexes10 and
solvents. Note that the use of boron hydride substances as
40 reductants of πꢀsystems and especially alkynes is rather
o
within 30 min at 25 C and under nonꢀinert atmosphere (Table
65 1). Using 1 molar equiv of AB, overreduction to the
corresponding alkane was observed in 6% relative yield, while
in the presence of 1 molar equiv of DMAB, the alkane sideꢀ
product is formed in ~1% yield. The reduction proceeds
sluggishly in nonꢀprotic solvents such as THF, ethyl acetate,
70 or DCM, yet when adding 5% v/v H2O in THF complete
reduction occurs within 1 h with 2.0 equiv of AB, or DMAB.
tertꢀButylamine borane is less efficient, while in the presence
of trimethylamine borane (Me3NBH3), no reduction was seen.
Hydrazine borane on the other hand is exceptionaly reactive,
limited. The Pdꢀcatalyzed reduction of akynes with NaBH4,11a
11b
or NaBHEt3
provides alkanes, as the intermediate formed
alkenes are also rapidly reduced. Ammonia borane has been
used as reducing agent for transforming aryl alkynes into
45 alkenes catalyzed by a Ni(0) complex under rather forcing
Department of Chemistry, University of Crete, Voutes 71003 Iraklion,
Greece. Fax: +30 2810 545001; Tel: +30 2810 545087. E-mail:
stratakis@chemistry.uoc.gr
50 † Electronic Supplementary Information (ESI) available: Spectroscopic
data for all products and copies of 1H and 13C NMR. See DOI:
10.1039/b000000x/
75 however, it suffers from
a relatively fast competing
dehydrogenative decomposition,10b,13 requiring thus 1.5 molar
equiv excess to reduce 1 into 1a. Hydrazine borane had been
anticipated as a highly promising reductant, given the high
¶ These authors contributed equally to this work
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DOI: 10.1039/C4CC08163C
reactivity of ammonia borane and the fact that hydrazine itself
is a potent reductant14 in the presence of supported Au NPs. In
addition, reduction of 1 with dimethyl sulfide borane complex
or pinacol borane, in the presence of Au NPs does not take
place. The reduction process is heterogeneous as under the
reaction conditions Au leaking into the solution is below ppm
level (ICP analysis). Au NPs are recyclable and resuable for 3
concecutive runs regarding the reduction of 1 with AB (0.5
equiv of AB in each run), without any deterioration of their
Table 2 cisꢀSemireduction of alkynes with ammonia borane
or dimethylamine borane catalyzed by Au/TiO2.
amine borane
Au/TiO2 (1%)
R
R'
5
EtOH, 25 o
C
R
R'
amine borane
(molar equiv)
time
(h)
yield
(%)a
product
Ph
(Z/E)/alkane
AB (1.0)
DMAB (1.0)
0.5
0.5
>99%b
>99%b
(98)/2
(100)/0
2a
10 activity. The recycling process consists of filtration, washing
AB (1.0)
DMAB (1.0)
0.5
0.5
86%
95%
o
(95)/5
(99)/1
with ethanol and finally drying at 80 C for 2 h.
3a
Me2N
TBSO
Most importantly, even less than 0.5 molar equiv of AB, or
DMAB, are necessary for a quantitative reaction (entry 3,
Table 1),15 indicating that the possible origins of the two new
15 hydrogen atoms in product 1a are the BꢀH of borane (hydride)
and the NꢀH of amine (proton). Highly supportive of this
proposal is the fact that trimethylamine borane, which lacks of
NꢀH bond is completely unreactive, in sharp contrast to
dimethylamine borane. In addition, the reduction of labelled
20 1-D17 with AB affords stereoselectively 1a-D17 (Scheme 1)
indicative of a cisꢀaddition pattern for the two hydrogen
atoms.
AB (0.6)
DMAB (0.6)
1.0
0.8
85%
95%
(96)/4
(100)/0
4a
MeO
AB (1.0)
DMAB (1.0)
1.5
1.0
97%b
97%b
(90)/10
(100)/0
5a
N
AB (0.5)
DMAB (1.0)
0.5
1.0
78%
93%
(98)/2
(100)/0
6a
PhO
AB (0.7)
DMAB (0.7)
0.5
0.5
96%
97%
(99)/1
(100)/0
TBDPSO
THPO
7a
AB (1.0)
DMAB (1.5)
0.5
1.0
83%
88%
(98)/2
(100)/0
8a
AB (0.7)
DMAB (0.7)
0.5
0.5
92%
97%
(96)/4
(100)/0
BocHN
9a
AB (0.8)
DMAB (0.8)
0.5
0.5
90%
91%
(99)/1
(100)/0
Scheme 1. Stereoselective cisꢀsemihydrogenation of deuterium labelled
25 pꢀmethoxyphenylacetylene 1-D with NH3BH3.
AcO
10a
11a
AB (1.5)
DMAB (0.7)
1.0
0.5
90%
95%
(96)/4
(100)/0
HO
The potency of the Auꢀcatalyzed triple bond semiꢀreduction
was examined in a series of terminal and internal alkynes. Based
on the optimum results from Table 1, ammonia borane (AB) and
30 dimethylamine borane (DMAB) were chosen as reductants,
ethanol as solvent, and the commercially available Au/TiO2 (1
mol%) as catalyst. The results are summarized in Table 2 and
reveal that the cisꢀsemihydrogenation pathway observed in 1-D is
general. In most of the cases, no chromatographic purification of
AB (1.0)
DMAB (0.7)
0.5
1.0
91%
95%
(97/2)/1
(99/1)/0
OH
12a
AB (1.5)
DMAB (1.5)
1.5
3.0
82%
93%
(91/5)/4
(99/1)/0
Pr
13a
HO
OH
AB (5.0)
DMAB (4.0)
3.0
5.0
79%
86%
(95/2)/3
(98/2)/0
14a
35 products is required. The reaction tolerates
a series of
functionalities such as a chloride, free alcohol, arylꢀ, silylꢀ or
THPꢀprotected alcohols, nitrogen containing alkynes, esters and
nitrile. The highly sensitive under typical hydrogenation
conditions benzyl ethers remain intact (product 18a). The only
40 functional group that does not tolerate the reaction conditions are
aldehydes or ketones, which are known to be reduced smoothly
by AB to alcohols under nonꢀcatalyzed conditions.5,17 Thus,
alkynal PhꢀC≡CꢀCHO (26, not shown in Table 2) affords with 1.5
equiv of AB cisꢀalkenol 15a in 85% yield, via intermediate
45 formation of the corresponding alkynol 15 (GCꢀMS). Among all
reactions examined, it was found that only α,βꢀunsaturated esters
(e.g. PhꢀC≡CꢀCOOEt, 20) exhibited a slow E/Zꢀunselective and
uncatalyzed reduction. This competing reduction did not exceed
15ꢀ20% conversion after 12 h and was seen when large excess (5
50 molar equiv) of AB or DMAB were used. In certain cases of α,βꢀ
unsaturated esters (20, 22) and nitrile 23, the cisꢀreduction with
DMAB proceeds in low yield, as competing addition of Me2NH
to the triple bond in a Michael fashion is also taking place,
forming chromatographically labile adducts. Sterically hindered
55 alkynes require a suitable excess of reducing agent, as while the
reduction rate slows down, at the same time amine boranes
AB (0.5)
DMAB (0.5)
2.0
0.5
84%
96%
(95/2)/3
(99/1)/0
Ph
Ph
OH
15a
AB (1.0)
DMAB (1.0)
1.0
1.0
87%
95%
(94/4)/2
(99/1)/0
16a
HO
AB (3.0)
DMAB (1.5)
4.0
2.0
76%
90%
(88/2)/10
(97/2)/1
Ph
17a
OH
Ph
AB (1.5)
DMAB (1.0)
1.0
1.0
82%
95%
(95)/5
(100)/0
O
18a
AB (1.5)
DMAB (1.5)
4.0
3.0
15%b
92%b
(99)/1
(100)/0
Cl
19a
AB (1.0)
DMAB (1.5)
1.0
2.0
78%
51%c
(91/6)/3
(94/6)/0
Ph
COOEt
20a
21a
82%
92%
AB (1.0)
DMAB (1.0)
0.5
1.0
(94/2)/4
(98/2)/0
COOEt
(continued)
60
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|
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DOI: 10.1039/C4CC08163C
mechanism. In Scheme 3, the ideal case of reduction of 3
molecules of alkyne by 1 molecule of AB is depicted.15 We
propose that the reduction involves formation of Auꢀhydrides
40 from insertion of BꢀH bond on Au,22 that nucleophilically
attack the triple bond, while the second hydrogen atom on the
produced alkene derives from the NꢀH moiety, through an
innerꢀsphere23 addition mechanism. The first round of
reduction delivers alkene and H2B=NH2. Then, H2B=NH2
45 reacts with the protic solvent (ROH) forming (RO)H2BNH3.
The new ammonia alkoxyborane complex, which is expected
to be more reactive relative to parent H3BNH3 due to the
electron donating alkoxy substituent, undergoes a second
round of alkyne reduction, forming as byꢀproduct
50 (RO)HB=NH2. Addition of ROH and third round of alkyne
reduction, finally forms the identified NH4B(OR)4. We
believe that the whole process resembles the metalꢀcatalyzed
alcoholysis or hydrolysis of AB,21 with the only difference
that the hydrogen atoms of AB are delivered on the alkyne
55 instead of eliminated as H2. We also point out that, although
AB and related substances are in general sources of H2,24 in
our case it is highly unlikely that reduction takes place via
hydrogenation of the alkyne by the produced H2, because Au
NPs are inefficient catalysts for such transformation under
60 mild conditions.2 Supportive of the mechanism shown in
Scheme 3 is the fact that when performing the reduction of 1
aIsolated yield. bConversion yields by GC analysis. cThe low yield is
due to competing Michaelꢀtype addition of Me2NH to triple bond. d75%
conversion of alkyne 25.
5
gradually undergo competing solvolytic destruction.15 Also, the
reduction of nonꢀpolar alkynes proceeds slowly (e.g. 25),
possibly due to their difficulty accessing the active catalytic sites
of Au NPs on the polar support.18
The current stereoselective semireduction protocol was
applied in the efficient synthesis of combretastatin Aꢀ4, a
potent pharmaceutical stilbenoid isolated from Combretum
caffrum (Scheme 2). Thus, the Au/TiO2ꢀcatalyzed reaction of
diaryl alkyne 3119 with DMAB (1.0 equiv) afforded
15 combretastatin Aꢀ4 (32) in 90% yield and >96% geometrical
purity. The analogous reduction with AB provided marginally
inferior results in terms of stereoselectivity and yield. Alkyne
31 was easily prepared by the Sonogashira coupling of
commercially available 5ꢀiodoꢀ1,2,3ꢀtrimethoxybenzene (29)
20 and acetylene 30, itself obtained in 2 steps from isovaniline.20
10
with AB in CD3OD, or THF/D2O, around 60ꢀ65%
D
incorporation was seen in 1a on both carbon atoms of styrene
(see Supporting Information), as based on our proposal,
65 CD3OD introduces D atoms on the intermediate borazineꢀtype
compounds (X2B=NH2, X = H or OR). Finally, our proposed
mechanism nicely explains the high efficiency of Me2NHBH3
(Tables 1 and 2), although having only one NꢀH functionality.
Scheme 2 A short stereoselective synthesis of combretastatin Aꢀ4 (32)
employing as key step the catalytic reduction of alkyne 27 with DMAB.
Regarding the reaction mechanism, significant information
25 was gained from the fate of ammonia borane during reduction.
Based on direct 11B NMR studies from the liquid phase of the
Auꢀcatalyzed reaction among ammonia borane and 1, carried
out in methanolꢀd4 as solvent, revealed that NH3BH3 (ꢀ24.4
ppm) transforms into NH4B(OCD3)4. The borate salt exhibits
30 a broad peak at 9.0 ppm (see Supporting Information) in
accordance to literature data from the RuCl3ꢀcatalyzed
dehydrogenative methanolysis of AB.21 Even at ~50%
progress of reaction only the specific borate salt was seen; no
other intermediate boronꢀbearing substances were detected.
35 The formation of tetraꢀalkoxy borate salt indicates that a
protic solvent is essential, participating into the reaction
70 Scheme 3 A mechanistic rational for the cisꢀsemihydrogenation of
alkynes by ammonia borane complex in the presence of Au NP, showing
the fate of ammonia borane.
In conclusion, we present herein a highly efficent and
simple protocol for the stereoselective cisꢀsemireduction of
75 functionalized alkynes with amine borane complexes
(especially ammonia or dimethylamine borane), catalyzed by
supported Au nanoparticles under very mild conditions. This
protocol is a compelling alternative to the direct catalytic
hydrogenation process. Given that supported Au nanoparticles
80 are recyclable and resuable as described above, the value of
this reduction protocol becomes even more apparent.
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Yet AB and DMAB undergo a slow Au/TiO2ꢀcatalyzed alcoholytic
decomposion with evolution of H2, which can be followed by 1H
NMR in methanolꢀd4 using an internal standard. The decomposition
of AB, or DMAB, proceeds much slower than the case of hydrazine
borane.
Acknowledgment: The authors acknowledge coꢀfunding of
this research by the European Regional Development Fund of the
EU and national fundsꢀ Greek Ministry of Education and
Religious Affairs, Sport and Culture/GGET – ΕΥDΕꢀΕΤΑΚ,
through the Operational Program Competitiveness and
Entrepreneurship (OPC II), NSRF 2007ꢀ2013, Action
70
5
16 Y. Yabe, Y. Sawama, Y. Monguchi and H. Sajiki, Chem.-Eur. J.,
2013, 19, 484.
75 17 L. Shi, Y. Liu, Q. Liu, B. Wei and G. Zhang, Green Chem., 2012, 14,
1372.
18 The Au/TiO2ꢀcatalyzed reduction of the highly unpolar 1ꢀtetradecyne
(27) into 1ꢀtetradecene (27a), reaches merely 40% conversion after 6
h, even in the presence of 6 molar equiv of ammonia borane. The
“SYNERGASIA 2011” Project: THERAꢀCAN
11ΣΥΝ_1_485.
ꢀ
No.
Notes and references
80
85
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conditions cisꢀ5ꢀdecene (28a) in merely 15% relative yield (GCꢀMS).
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8
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9
Au/TiO2, Au/Al2O3, Au/ZnO (1 wt% in Au) are commercially
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10 Ammonia borane (NH3BH3), dimethylamine borane (Me2NHBH3),
tertꢀbutylamine borane (tꢀBuNH2BH3) and trimethylamine borane
(Me3NBH3) complexes are commercially available. Methylamine
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65 15 In the presence of 0.35 molar equiv of AB, 85% conversion of 1 to 1a
was seen. Based on our mechanistic suggestions, the reaction
stoichiometry requires 1 mol of AB or DMAB per 3 mol of alkyne.
4
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