Ene-carbonyl reductive coupling mediated by zinc and ammonia for the synthesis of γ-hydroxybutyric acid derivatives

Article abstract of DOI:10.1002/adsc.201300073

The reductive coupling of an aryl carbonyl group with an activated alkene to give the corresponding γ-hydroxybutyric acid (GHBA) derivative was achieved in the presence of zinc and ammonia. A board scope of GHBA derivatives could be synthesized under ambient conditions by this method. A mechanism involving a Zn-ketyl radical as the key intermediate is proposed. Copyright

Full text of DOI:10.1002/adsc.201300073

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DOI: 10.1002/adsc.201300073
Ene-Carbonyl Reductive Coupling Mediated by Zinc and
Ammonia for the Synthesis of g-Hydroxybutyric Acid
Derivatives
Chien-Hung Yeh,a Rajendra Prasad Korivi,a and Chien-Hong Chenga,*
a
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
E-mail: chcheng@mx.nthu.edu.tw
Received: January 25, 2013; Revised: April 2, 2013; Published online: April 30, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300073.
Abstract: The reductive coupling of an aryl carbonyl mechanism involving a Zn-ketyl radical as the key
group with an activated alkene to give the corre- intermediate is proposed.
sponding g-hydroxybutyric acid (GHBA) derivative
was achieved in the presence of zinc and ammonia.
A board scope of GHBA derivatives could be syn- Keywords: ammonia; GHBA; reductive coupling;
thesized under ambient conditions by this method. A zinc dust
Introduction
Results and Discussion
The regioselective reductive coupling (RRC) reac- Our preliminary investigation of the reaction of 1a
tion[1] of organic carbonyls with alkenes has emerged with 2a in acetonitrile in the presence of zinc dust,
as a powerful tool for the formation of functionalized water and pyridine at room temperature showed no
alcohols. The reactions have been achieved by SmI2 expected reductive coupling product. Other nitrogen
(Kaganꢀs reagent),[2,3] by electoreduction[4] and by bases, 1,10-phenanthroline and Et3N were also not ef-
TiCl4-Mg reagent.[5] In addition to the metal-mediated fective for the reaction, but Et2NH gave the hydro-
coupling reactions, the use of nickel complexes as cat-
ACHTUNGTRENaNGNU mination product 3-(diethylamino)propanenitrile.
alyst for the reductive coupling of organic carbonyls Fortunately, when we used 30% NH4OH(aq) (50 mol%
and dienes was demonstrated by Tamaru and co- to 1a) as additive without additional water, the de-
workers.[6] In these reactions, g-hydroxy derivatives sired product 3a was obtained in 75% yield. If the re-
were obtained. Another type of reductive ene-carbon- action was performed by adding water under one at-
yl coupling is the reductive aldol reaction[7] in which mosphere of ammonia gas (balloon pressure) for 12 h,
À
the C C bond is formed at the a-carbon atom of 3a was obtained in 65% isolated yield (Table 1,
a conjugated alkene.
entry 1). Control experiments confirmed that in the
As part of our ongoing efforts devoted to the devel- absence of either ammonia or zinc dust, no product
opment of metal-catalyzed reductive coupling meth- was obtained (see the Supporting Information for de-
ods, we introduced Zn/H2O as a reducing reagent for tailed studies).
the reductive coupling reactions.[8] In the course of
To evaluate the scope of this zinc-mediated reduc-
these studies, surprisingly, we observed that the Zn/ tive coupling reaction, different carbonyl derivatives
NH3 system could directly couple an aryl carbonyl were treated with an array of alkene derivatives
group and an electron-deficient alkene in the absence under the aforementioned optimized conditions, the
of a transition metal complex. Herein, we present this initial results are shown in Table 1. Benzaldehyde de-
unusual zinc-mediated regioselective reductive cou- rivatives bearing an electron-withdrawing group (1b)
pling reaction including the scope of the two sub- at the para position and electron-donating groups
strates and the possible mechanism of the reaction. It (1c–f) at different positions on the aryl group provid-
is worth noting that the coupling was achieved only ed the corresponding g-hydroxy derivatives 3b–h in
with the combination of Zn metal powder and NH3. 85 to 22% yields (Table 1, entries 2–8). Notably, the
The use of Zn metal powder and other amines or pyr- reaction of 1b with alkene 2g bearing methyl substitu-
idines did not lead to the expected coupling product.
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Ene-Carbonyl Reductive Coupling Mediated by Zinc and Ammonia
Table 1. Results of zinc-mediated reductive coupling of carbonyl derivatives with activated alkenes.[a]
Entry
1[d,h]
1
2
Product 3
Yield [%][b]
65
1a
2a
2[d,h]
3[c]
4
1b
1c
1d
1e
2a
2a
2a
2a
55 (85)[e]
85 (65)[e]
68
5
74
6
1f
1b
1b
2a
2f
2g
82 (57)[e]
7[d]
22
8[f]
43 (dr: 1.2:1)
9[d,h]
1g
1h
2a
2a
68 (43)[e]
46
10
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Table 1. (Continued)
Entry
1
2
Product 3
Yield [%][b]
43[c]
11[d,h]
1i
1j
2a
12[h]
13
2a
2a
2a
44
61
96
1k
14
1m
15
1m
2b
53
16[d]
17
1m
1m
2c
2d
78
72
18
1m
1m
2e
2h
95
19[f]
64 (dr: 1.5:1)
20[g]
1l
2i
46
[a]
All reactions were carried out under ammonia atmosphere using 1 (0.20 mmol), 2 (1.00 mmol), H2O (0.40 mmol) and Zn
(1.00 mmol) in CH3CN (1.2 mL) at room temperature for 20 h.
Isolated yields.
The product contained unseparated impurities; its yield was determined by NMR using methylene as an internal stan-
dard.
Using 2 (0.60 mmol) and Zn (0.60 mmol).
NH3(g) and H2O were replaced with 30% NH4OH(aq) (0.10 mmol).
DMF instead of CH3CN as solvent.
DMSO instead of CH3CN as solvent.
Reaction times required for 3a, 12 h; 3b, 3 h; 3i, 10 h; 3k, 10 h; 3l, 30 h.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
tion at the a-carbon did not occur in CH3CN, but pro- cellent yield. When ethyl vinyl ketone 2b, methyl ac-
ceeded smoothly in DMF (entry 8). rylate 2c, ethyl acrylate 2d and vinyl sulfone 2e were
The scope of the carbonyl substrate was extended used as the alkene substrates, the corresponding cou-
to heterocyclic aldehydes and ketones. Thus, 2-formyl- pling products (3o and 3q) and cyclized coupling
furan 1g, 2-acetylfuran 1h, 2-formylthiophene 1i, 2- product (3p) were obtained in moderate to excellent
acetylthiophene 1j and 2-acetylpyridine 1k also react- yields. Alkenes 2h and 2i having methyl substitution
ed efficiently with 2a to give the corresponding g-hy- at the a-carbon also worked with 1m and 1l, respec-
droxy alcohol derivatives 3i–m in moderate yields tively (entries 19 and 20; similar to entry 8). Notably,
(Table 1, entries 9–13). With 2-benzoylpyridine 1m, products 3o and 3r were obtained as 1,4-ketols which
the reaction gave the g-hydroxy derivative 3n in ex- are in equilibrium with their hemiketals as indicated
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Ene-Carbonyl Reductive Coupling Mediated by Zinc and Ammonia
lactone 3p was obtained in 76% yield. Similarly, the
reaction of fluorenone (1.1 g) and acrylonitrile
(5 equiv.) gave g-hydroxybutyric acid derivative 3t in
78% yield.
The products obtained from this highly regioselec-
tive reaction are g-lactones,[10] g-hydroxy ketones, g-
hydroxy esters, g-hydroxy nitriles and g-hydroxy sul-
fones. They are all g-hydroxybutyric acid derivatives
(GHBA), which are important metabolites of g-ami-
nobutyric acid (GABA).[11] Moreover, GHBA is used
for the treatment of cataplexy, excessive daytime
sleepiness in the patients with narcolepsy, insomnia
and alcoholism.[12]
The formation of pinacol products or alcohols from
the carbonyl compounds was observed as side reac-
tions during the present ene-carbonyl reductive cou-
pling.[13] To understand the role of ammonia, the zinc
metal powder was treated with 1,2-dibromoethane, I2
and TMSCl to activator the surface and was then al-
lowed to react with 1a and 2a in the absence of NH3.
The reaction did not give the desired product 3a indi-
Scheme 1. Unexpected tertiary alcohol reduction and nitrile
hydrolysis.
by their NMR spectra in CDCl3.[9] It is interesting to cating that ammonia is probably not an activator for
note that under the present reaction conditions the zinc surface. A possible role may be for Zn to act
(Table 1, conditions [a]) 4-benzoylpyridine 1n reacted as ligand to enhance the reducing capability of zinc
differently with acrylonitrile to give an amide product powder. A possible mechanism for the zinc-ammonia
4a in 70% yield (Scheme 1). Presumably, the expected mediated ene-carbonyl reductive coupling is shown in
product 3’ further underwent alcohol reduction and Scheme 3. The Zn/NH3 system undergoes a single
hydrolysis of the cyano group. The same functional electron transfer to a carbonyl group to form a Zn-
group transformation also occurred when we chose ketyl radical. Addition of this radical to the alkene
coupling product 3t as the substrate for further reac- substrate followed by further reduction gives a five-
tion in the reducing system.
membered zincacycle. Final protonation affords the
Encouraged by the results obtained with acyclic reductive coupling product 3. This proposed mecha-
carbonyl derivatives, we also tested the cyclic carbon- nism is similar to that of the Sm(II)-mediated ketyl-
yl substrates for the present reductive coupling reac- alkene coupling reaction.[2j]
tion. Pleasingly, all conjugated cyclic ketones on re-
acting with conjugated alkenes afforded the corre-
sponding products in moderate to excellent yields Conclusions
(Table 2). A higher product yield was obtained for
fluorenone 1o than for the seven-membered cyclic In summary, we have developed a new ene-carbonyl
ketone 1p (Table 2, 3t and 3u). Acrylonitrile 2a was reductive coupling method using the relatively inex-
proved to be more efficient with fluorenone than pensive Zn/NH3 reagent. The scope of alkene species
enone 2j and acrylates 2c, 2d and 2k (entries 3, 5–7). is wide and the carbonyl substrates can be aldehydes
Benzoanthracenone 1q is more reactive with acrylate or ketones. The reaction provides a facile route to
2d than with enone 2b (entries 4 and 8). Among the
a
variety of g-hydroxybutyric acid derivatives
ene derivatives tested, dimethylacrylamide 2g appears (GHBA) and an inexpensive alternative to the SmI2-
to be the least reactive (Table 1, entry 7 and Table 2, mediated approach. Further applications to the reduc-
entry 11) and sulfone 2e is most efficient (Table 1, tive coupling of different p-components mediated by
entry 16 and Table 2, entries 9, 10). The reaction of the zinc/ammonia system are under investigation.
methyl acrylate with fluorenone 1o afforded lactone
3x as an exclusive product, but the reaction of ethyl
acrylate with 1o gave the reductive coupling product
3y instead of the cyclic lactone.
Experimental Section
To see whether the present methodology can be ap-
plied to the gram-scale synthesis of the ene-carbonyl
reductive coupling products, we tested (Scheme 2) the
Typical Procedure for Ene-Carbonyl Reductive
Coupling Mediated by Zinc and Ammonia
reaction of 2-pyridinyl phenyl ketone (1.1 g) with
methyl acrylate (5 equiv.), and the corresponding g- with zinc dust [0.6–1.0 mmol, 3.0–5.0 equiv.; purchased from
A glass tube equipped with magnetic stir bar was charged
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Table 2. Results for the formation of g-hydroxy derivatives using cyclic ketones.[a]
Entry
1
1
2
Product 3
Yield [%][b]
95
1o
2a
2
3
1p
1o
2a
2j
34
74
4[c]
1q
2b
48
5[c]
6[c]
7[d]
1o
1o
1o
2c
2d
2k
67
46 + 5
36
8
1q
1o
1q
2d
2e
2e
74
92
93
9
10
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Ene-Carbonyl Reductive Coupling Mediated by Zinc and Ammonia
Table 2. (Continued)
Entry
1
2
Product 3
Yield [%][b]
11[e]
1q
2g
24
[a]
All reaction were carried out under ammonia atmosphere using 1 (0.20 mmol), 2 (1.00 mmol), H2O (0.40 mmol) and Zn
(1.00 mmol) in CH3CN (1.2 mL) at room temperature for 20 h.
Isolated yields.
Using 2 (0.60 mmol) and Zn (0.60 mmol).
DMF instead of CH3CN as solvent.
0.8 mmol of H2O was added.
[b]
[c]
[d]
[e]
to this procedure and their spectral data are given in Sup-
porting Infomation.
Gram-Scale Synthesis of g-Hydroxybutyric Acid
Derivatives: Representative Procedure for the
Synthesis of 3n
A 100-mL round-bottom flask equipped with magnetic stir
bar was charged with ketone 1k (1.1 g, 6.0 mmol, 1.0 equiv.),
zinc dust (2.0 g, 30.0 mmol, 5.0 equiv.), methyl acrylate 2c
(2.6 g, 30.0 mmol, 5.0 equiv.), H2O (0.22 g, 12 mmol,
2.0 equiv.) and dry acetonitrile (30 mL) and was then sealed
with a rubber septum. The solution was saturated with NH3
(a long needle was connected to an NH3 cylinder via tygon
tubing and then was pierced through the septum) by slow
bubbling for 20 min and stirred at room temperature for
24 h. The resulting mixture was quenched with saturated
aqueous NH4Cl (20 mL) and extracted with EtOAc (3ꢂ
10 mL). The combined organic layer was dried over MgSO4,
filtered through Celite and concentrated. Separation on
a column of silica gel using hexane/EtOAc as eluent gave 3n
as a colorless oil; yield: 1.1 g (76%).
Scheme 2. Results of gram-scale syntheses of g-lactone and
g-hydroxybutyric acid derivatives.
Acknowledgements
Scheme 3. A plausible mechanistic pathway for the forma-
tion of g-hydroxy product 3.
We thank the National Science Council of Republic of China
(NSC-101–2628M-007-004) for support of this research.
Aldrich (ꢀ99%) or Alfa Aesar (99.9%); used as received],
aldehyde 1 (0.2 mmol, 1.0 equiv.), conjugated alkene 2 (0.6–
1.0 mmol, 3.0–5.0 equiv.), H2O (0.4 mmol, 2.0 equiv.) and
dry acetonitrile (1.2 mL) and was then sealed with a rubber
septum. The solution was saturated with NH3 in a balloon
by slow bubbling for 5 min and was stirred at room tempera-
ture for 3–30 h (NB: NH3 should only be handled in a chemi-
cal fume hood). The resulting mixture was diluted with
EtOAc, filtered through Celite and the filtrate was then con-
centrated. Separation on a silica gel column using hexane/
EtOAc as eluent gave the reductive coupling product 3.
Compounds 3a–ad, 4a, and 4b were synthesized according
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