Sodium borohydride-nickel chloride-methanol catalytic system for regioselective reduction of electron-rich conjugated dienes and reductive cleavage of allyl esters involving π-allylnickel intermediates

Article abstract of DOI:10.1002/adsc.201100612

The regioslective reduction of electron-rich dienes to monoolefins and the reductive cleavage of allyl esters were fulfilled by employing a sodium borohydride-nickel chloride-methanol catalytic system with exceedingly simple manipulations and high functional group tolerability. Both of the reductive reactions may involve π-allylnickel intermediates generated from fresh nickel boride. Copyright

Full text of DOI:10.1002/adsc.201100612

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DOI: 10.1002/adsc.201100612
Sodium Borohydride-Nickel Chloride-Methanol Catalytic System
for Regioselective Reduction of Electron-Rich Conjugated
Dienes and Reductive Cleavage of Allyl Esters Involving
p-Allylnickel Intermediates
Biao-Lin Yin,a,* Cong-Bi Cai,a Jin-Qiang Lai,a Ze-Ren Zhang,a Li Huang,a
Li-Wen Xu,b and Huan-Feng Jianga
a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Peopleꢀs
Republic of China
Fax : (+ 86)-20-8711-3735; phone: (+86)-20-8711-3735; e-mail: blyin@scut.edu.cn
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal
b
University, Hangzhou, Peoples Republic of China
Received: August 2, 2010; Published online: December 6, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100612.
Abstract: The regioslective reduction of electron-
rich dienes to monoolefins and the reductive cleav-
age of allyl esters were fulfilled by employing a
sodium borohydride-nickel chloride-methanol cata-
lytic system with exceedingly simple manipulations
and high functional group tolerability. Both of the
reductive reactions may involve p-allylnickel inter-
mediates generated from fresh nickel boride.
of this reducing system in organic synthesis is of great
practical significance. However, to the best of our
knowledge, little attention has been paid to the regio-
selectivity and stereoselectivity of these reductive re-
actions involving the NaBH4-NiCl2 system.
It is well known that p-allylnickels can be obtained
by a hydronickellation of dienes or an oxidative addi-
tion of allyl substrates to Ni(0) species. This complex
can be coupled with both electrophiles as well as nu-
cleophiles in a regioselective or (and) stereoselective
fashion and has been extensively used in organic syn-
thesis.[7] Previous studies demonstrated that the fresh
nickel boride had the composition of (Ni2B)·H3 and it
might be transformed into a nickel hydride during the
reaction process.[8] We envisaged that the reaction of
fresh nickel boride with dienes or allylic substrates
Keywords: allylic esters; p-allylnickel intermediate;
hydrogenation; selective reduction; sodium borohy-
dride-nickel chloride
The combination of NaBH4 with a catalytic amount would also produce a p-allylnickel complex A via a
of NiCl2, which generates a highly reactive nickel hydronickellation of diene or an oxidative addition of
boride species in situ, has been extensively employed allyl substrate (Scheme 1). After the selective attack
to reduce functional groups that are inert to sodium of hydride on the complex, the regioselective hydro-
borohydride alone.[1] For example, several groups genation of dienes or reductive cleavage of allyl
have reported the reduction of the C=C double bonds esters could be fulfilled. Developing alternative meth-
of a,b-unsaturated esters with NaBH4-NiCl2 in the ods for these two reductive reactions and improving
synthesis of numerous natural and unnatural bioactive their selectivities are highly desirable.[9,10] In this com-
molecules.[2] The NaBH4-NiCl2 system has also been munication, we describe our findings on the regiose-
used in the reduction of aliphatic nitro groups or ni- lective reduction of electron-rich conjugated dienes to
troarenes to amines,[3] a-amino acids to 1,2-amino al- monoolefins and the reductive cleavage of allylic
cohols,[4] 4,5-dihydro-2-oxazole to oxazolidine,[5] and esters with the NaBH4-NiCl2-MeOH system.
in desulfurization processes.[6] This system is especial-
Initially, a series of dienes 1a–1g with an electron-
ly attractive for its low cost, simple manipulations (air donating group (alkylamino, alkyloxy, or silyloxy) at
atmosphere and moisture tolerant), non-pyrophoric the 1-position were chosen as substrates. As shown in
nature, short reaction times (generally requiring only Table 1, upon treatment with NaBH4-NiCl2 at room
a few minutes). Therefore, expanding the application temperature in MeOH/DME (1/1, v/v), the evaluated
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dienes afforded high yields of monoolefins within
5 min, and the 3,4-carbon carbon double bonds (in-
cluding mono-, di- and tri-substituted) were selective-
ly hydrogenated (entries 1–7). Furthermore, even
when the amount of NaBH4 was increased to
10 equivalents, no over-reduction was observed.
During the reaction, the isolated double-bond of 1e
was not influenced. Interestingly, when a large excess
of NaBH4 (20 equiv.) was used, reduction of the iso-
lated double bond of 1g was observed, producing 2g
in excellent yield (90%). Notably, the hydrogenation
did not give rise to the cleavage of the protective
groups of Cbz and Bn (entries 2, 4 and 6). For dienes
with an alkyloxy group at the 2-position and a bulky
phenyl group at the 1-position, their 3,4-double bonds
were selectively reduced, providing 2h–2l in fair to
Scheme 1. Possible pathways for the reaction of dienes or al-
lylic substrates with fresh nickel boride resulted from the
system of NaBH4-NiCl2-MeOH.
Table 1. Regioselective reduction of 1 with the NaBH4-NiCl2-MeOH system.[a]
Entry
1
Diene
Product
Yield [%][b]
85
2
3
4
78
76
81
5
6
84
93
7[c]
90
77
8
9[d]
81
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Sodium Borohydride-Nickel Chloride-Methanol Catalytic System for Regioselective Reduction
Table 1. (Continued)
Entry
Diene
Product
Yield [%][b]
10
70
11
75
12
60
84
13[d]
14
15
81[e]
73[f]
16
67 g]
[a]
Unless otherwise noted, the reaction conditions were: 1 mmol of 1, 0.2 mmol of NiCl2, 10 mmol of NaBH4, 5 mL of
MeOH/DME (1/1, v/v), room temperature, and 5 min.
Isolated yield.
20 mmol of NaBH4.
NaBD4 and CD3OD were used.
Z/E=1/1.
[b]
[c]
[d]
[e]
[f]
Z/E=1/1.
Z/E=1/6.
[g]
good yields (entries 8–16) and with the enol ether
ACHTUNGTRENoNGNU xydienes, allowing for the preparation of more-sub-
being left intact. Except for the nitro group, which stituted silyl enol ethers.
was reduced to an amino group, other functional
Encouraged by the above results, we next investi-
groups such as acetals, aryl chlorides, and amides gated the reductive cleavages of allyl esters using
were tolerated under these reaction conditions. Inter- NaBH4-NiCl2 system. As shown in Table 2, in the
estingly, for conjugated 2-silyloxydienes 2m–2p, selec- presence of 10 equivalents of NaBH4 and a catalytic
tive 1,4-reduction was observed, resulting in the cor- amount of NiCl2 (0.2 equiv.) in MeOH at room tem-
responding mono-silyl enol ethers 3a–3d in good perature, the acetyloxy and benzoyloxy groups placed
yields. The conjugated silyloxydiene has a high elec- at the allylic position of variously protected glycals
tron density, and its selective hydrogenation has never were removed successfully in 5 min, producing the
been reported before. The conventional preparation corresponding 3-deoxy glycals with good to excellent
of silyl enol ethers involves silylation of the enolates yields (Table 2, entries 1–7). 3-Acetoxy glycals gener-
generated from the corresponding ketones and a ally led to higher yields than 3-benzoyloxy glycals
strong bulky base. For unsymmetrical ketones, the (5b>5c; 5d>5e; and 5f>5g), which may be due to
less-substituted silyl enol ethers are preferentially the better leaving ability of acetoxy group than that
formed. Importantly, this protocol provided the hith- of benzoyloxy group. Using this protocol, a variety of
erto unknown 1,4-reduction of electron-rich 2-silyl- hydroxy-protecting groups (e.g., Ac, Bz, TBDPS,
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Table 2. Reductive cleavage of allyl esters with the NaBH4-
Table 2. (Continued)
NiCl2-MeOH system.[a]
Entry
Allyl ester
Product
Yield [%][b]
90
12[c]
Entry
1
Allyl ester
Product
Yield [%][b]
88
13
84
77
14
[a]
Unless otherwise noted, the reaction conditions were:
1 mmol of 4, 0.2 mmol of NiCl2, 10 mmol of NaBH4,
5 mL of MeOH, room temperature, 5 min.
Isolated yield.
2
3
4
5
6
7
93
84
89
83
95
85
[b]
[c]
NaBD4 and CD3OD were used.
TBS, and Bn) were tolerated. Known methods for 3-
deoxygenation of glycals usually involve the forma-
tion of p-allyl-Pd complexes that are then attacked by
a hydride under strong acidic conditions.[10] This re-
ductive system provided an alternative access to di-
versified 3-deoxy glycals conveniently under mild
basic conditions. For 1-alkyloxy-2,3-unsaturated mon-
osaccharides 4i and 4j, both the reductive cleavage at
the 3-position and hydrogenation of the C=C double
bond occurred, giving rise to 5i and 5j in good yields
(entries 10 and 11). In addition, 3-phenylallyl acetate
4l and furan-2-yl-phenylmethyl acetate 4m were also
reduced under these reaction conditions (entries 13
and 14). To study the stereochemistry of this cleavage,
the reaction of 4b with NaBD4/NiCl2 in CD3OD was
performed. To our delight, d-5b was produced with
complete inversion of the configuration at the C-3 po-
sition (Table 2, entry 12). The stereochemistry of d-5b
was verified via NOESY experiments and by compari-
son of their NMR spectra with those reported in the
literature.[10a]
Although the NaBH4-NiCl2 system has been exten-
sively used in organic synthesis, its reaction mecha-
nism is still under discussion.[11] Based on the above
outcomes, two preliminary mechanisms are proposed
for the reductions (Scheme 2). For these dienes, a hy-
dronickellation of (Ni2B)2·H3 with dienes gives rise to
a p-allylnickel complexs of 6. When R1 is an electron-
donating group, hydride attacks exclusively at the 3-
postion of the complex, leading to the formation of
2.[12] If R2 is an electron-donating group and R1 is a
phenyl group , due to the steric bulk of R1 and (or) its
conjugated effect with the olefin, hydride also attacks
exclusively at the 3-postion to form 2. But when R1 is
an alkyl group, hydride attacks exclusively at the 1-
postion, giving rise to double bond isomerization and
forming 3.[13] For allyl esters, an oxidative addition of
8
92
84
84
9
10
11
79
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Adv. Synth. Catal. 2011, 353, 3319 – 3324

Sodium Borohydride-Nickel Chloride-Methanol Catalytic System for Regioselective Reduction
nickel-catalyzed coupling reactions involving the
NaBH4-NiCl2 system are ongoing in our lab, and the
results will be reported in due course.
Experimental Section
Typical Procedure for the Reduction of Dienes with
NaBH4-NiCl2 System (to form 2f)
To a solution of 1f (442 mg, 1 mmol) in 5 mL of MeOH/
DME (1/1, v/v) was added NiCl2 (26 mg 0.2 mmol), then
NaBH4 (380 mg, 10 mmol) in portions over 5 min at room
temperature with the formation of a black solid and evolu-
tion of hydrogen. After additional stirring for 5 min, the re-
action was quenched with 10 mL H2O. The resulting mixture
was extracted with ethyl acetate (3ꢂ10 mL). The combined
organic layers were washed with brine and dried over
Na2SO4. Removal of the solvents yielded a crude product,
which was purified by column flash chromatography on
silica gel to afford 2f as colourless oil; yield: 411 mg (93%);
1
[a]D23: À3.6 (c 0.5 gmLÀ1, CH3COCH3). H NMR (400 MHz,
CDCl3): d=7.39–7.32 (m, 15H), 6.30 (s, 1H), 4.79–4.52 (m,
6H), 4.21–4.15 (m, 1H), 4.12–4.11 (d, J=4.8 Hz, 1H), 4.01–
3.98 (dd, J=6.8 Hz, 4.8 Hz, 1H), 3.86–3.82 (dd, J=10.4 Hz,
5.6 Hz, 1H), 3.78–3.74 (dd, J=10.8 Hz, 4.0 Hz, 1H), 2.11–
2.05 (m, 2H), 1.05–1.01 (t, J=7.4 H z, 3H); 13C NMR
(100 MHz, CDCl3): d=139.6, 138.3, 138.0, 128.4, 128.3,
127.8, 127.7, 127.6, 127.5, 113.9, 76.0, 75.7, 73.7, 73.3, 72.8,
71.3, 68.3, 21.9, 12.8; HR-MS (ESI): m/z=445.2371, calcd.
for C29H33O4 [M+H]+: 445.2379.
Typical Procedure for the NiCl2-Catalyzed Reductive
Cleavage of Allyl Esters
Scheme 2. Proposed mechanistic pathway for the reductive
To a solution of 4d (1 mmol) in MeOH (3 mL) was added
NiCl2 (26 mg 0.2 mmol), then NaBH4 (10 mmol) in portions
over 5 min at room temperature. After additional stirring of
5 min, the reaction was quenched with 10 mL H2O. The re-
sulting mixture was extracted with ethyl acetate (3ꢂ10 mL).
The combined organic layers were washed with brine and
dried over Na2SO4. Removal of the solvents yielded a crude
product, which was purified by column flash chromatogra-
phy on silica gel to afford 5d as a colorless oil; yield: 366 mg
reactions.
the allylic substrate 4b to (Ni2B)2·D3 species on the
back side of the 3-acetoxy gives the p-allylnickel com-
plex 7. Then hydride attacks exclusively at the 3-posi-
tion of the complex to liberate the olefin d-5b with
the generation of the (Ni2B)2·D which reacts with D2
and carries on the catalytic cycle.
1
(89%). H NMR (400 MHz, CDCl3): d=7.75–7.71 (m, 4H),
In summary, for the first time, we used fresh nickel
boride, generated in situ from the combination of
NaBH4 with NiCl2, to fulfill the regioselective reduc-
tion of electron-rich conjugated dienes to monoolefins
and reductive cleavage of allyl esters with exceedingly
simple manipulations. Both of these reactions may in-
volve the generation of p-allylnickel complexes via
the hydronickellation of dienes and the oxidative ad-
dition of the ally ester, respectively. The selectivity
strongly depends on the features of the substrates.
The selective attack of hydride on the allylnickel com-
plexes is responsible for the selectivity. The studies
give us hints that the fresh nickel boride is a Ni(0)
species with high catalytic activity and it can catalyze
some useful allylation reactions. Further studies on
7.46–7.42 (m, 6H), 6.36 (d, J=6.4 Hz, 1H), 5.31–5.26 (m,
1H), 4.65–4.62 (m, 1H), 4.06–4.02 (m, 1H), 3.91–3.82 (m,
2H), 2.46–2.40 (m, 1H), 2.10–2.06 (m, 4H), 1.11 (s, 9H);
13C NMR (100 MHz, CDCl3): d=170.1, 142.8, 135.7, 133.3,
129.8, 127.8, 97.1, 76.2, 66.0, 62.6, 26.8, 24.8, 21.2, 19.3; HR-
MS (ESI): m/z=411.1988, calcd for C29H33O4 [M + H]+
411.1992.
Acknowledgements
This work was support by Doctoral Fund of Ministry of Edu-
cation of China (200805611041), Fundamental Research
Funds for the Central Universities (2009M0241), the National
the detailed mechanism and design of some novel Natural Science Foundation of China (No. 21072062), the
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Natural Science Foundation of Guangdong Province, China
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