Co2(CO)8-mediated Selective Reductions of Propargyl Alcohol Derivatives to Alkenes

Article abstract of DOI:10.1002/cjoc.201400396

In the presence of Co₂(CO)₈ and additives, propargyl alcohol derivatives could be reduced to alkenes in moderate to good yield. The selectivity of this reaction could be controlled by adding different additives: with H₂O as the additive, the major configuration of product is Z-alkene; with CF₃COOH as the additive, the major configuration of product is E-alkene.

Full text of DOI:10.1002/cjoc.201400396

COMMUNICATION
DOI: 10.1002/cjoc.201400396
Co2(CO)8-mediated Selective Reductions of Propargyl Alcohol
Derivatives to Alkenes†
Ying Dou,a Ping Xing,a Zuogang Huang,b and Biao Jiang*,a,b
a CAS Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
b Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
In the presence of Co2(CO)8 and additives, propargyl alcohol derivatives could be reduced to alkenes in moder-
ate to good yield. The selectivity of this reaction could be controlled by adding different additives: with H2O as the
additive, the major configuration of product is Z-alkene; with CF3COOH as the additive, the major configuration of
product is E-alkene.
Keywords Co2(CO)8, propargyl alcohol, reduction, alkene
Introduction
Reductive decomplexation of biscobalthexacarbonyl
acetylenes into olefins has been rarely reported. In
endo-cyclic complexes, the method for reductive de-
complexation was high pressure hydrogenation using
Rh-catalyst to provide olefinic ethers.[10] Except for
Wilkinson catalyst, Isobe also found two new reductive
decomplexation of acetylene biscobalthexacarbonyl
complexes with tin hydride or silicon hydride.[11] Addi-
tional reductive reagents were necessary for these
transformations. The example without any reductive
additives was reported by Periasamy.[12] The similar
transformation occurred only when the substituted
group of alkyne contains silane. Herein we reported a
conversion from propargyl alcohol derivatives to al-
kenes mediated by Co2(CO)8 and additives, in which the
Z/E selectivity of reaction could be controlled by adding
different additives.
Co2(CO)8 has attracted great interests from or-
ganometallic and synthetic chemists since its discovery
by Mond in 1910.[1] As a well known reagent of versa-
tile use, a key property of Co2(CO)8 is to form a moder-
ately air stable co-alkyne complex with an alkyne sub-
strate. Complexation of alkyne with Co2(CO)8 can be
used to decrease the reactivity of the triple bond. In
some cases, cobalt complex protects the alkynes from
addition reactions such as reduction and hydrobora-
tion.[2] Also it can prevent sensitive endiynes from un-
dergoing undesired Bergman cycloaromatization.[3]
Perhaps the best known cobalt-mediated cycloaddi-
tion reaction is the Pauson-Khand reaction discovered in
1971,[4] representing a formal [2＋2＋1] cycloaddition
between an alkyne, alkene and a carbon monoxide as its
simplest form. Although the traditional Pauson-Khand
reactions usually required high pressure and high tem-
perature conditions, recent advances have allowed sig-
nificantly milder reaction conditions.[5] In the presence
of certain additives, such as N-oxides or primary amines,
the reaction could proceed at ambient temperature under
atmospheric pressure.[6] Also the use of dicobalt hexa-
carbonyl alkyne unit to stabilize propargylic cations has
been noted for almost forty years, and it is commonly
referred as the Nicholas reaction when further reacting
with a nucleophile to give an alkylated alkyne.[7] Com-
binational use of Nicholas and Pauson-Khand reactions
has been successfully applied in natural product[8] and
polycyclic structure[9] synthesis.
Experimental
General procedure for the reductive of propargyl
alcohol derivatives using H2O as additive
Co2(CO)8 (1.6 mmol) was added to a solution of
propargyl alcohol (1 mmol) in 4 mL CH3CN and stirred
at r.t. for 30 min. H2O (6.0 mmol) was added to the so-
lution and stirred at r.t. for 15 min. Then the solution
was stirred at reflux temperature until no co-alkyne
complex was detected by TLC. The mixture was filled
through Büchner funnel, concentrated and purified by
flash column chromatography to gain the product.
*
E-mail: jiangb@mail.sioc.ac.cn; Tel.: 0086-021-54925566; Fax: 0086-021-64166128
Received June 16, 2014; accepted September 17, 2014; published online September 29, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400396 or from the author.
Dedicated to Professor Chengye Yuan and Professor Li-Xin Dai on the occasion of their 90th birthdays.
†
Chin. J. Chem. 2014, 32, 999—1002
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
999

Dou et al.
COMMUNICATION
General procedure for the reductive of propargyl
alcohol derivatives using CF3COOH as additive
cyclohexylamine or trimethylamine N-oxide as additive
(Table 1, entries 1, 2). Trace of alkene 7a could be
detected when using H2O as additive (Table 1, entry 3),
and alkene 7a could be separated in 38% yield (Z∶E＝
26∶74) when using CF3COOH as additive (Table 1,
entry 4). In order to improve the yield, we investigated
the reactivity of hydroxyl protected substrates using
H2O or CF3COOH as additive. For the Ac protected
substrate 2a, the reaction gave mainly Z-7a (Z∶E＝
86∶14) with moderate yield (51%) when using H2O as
additive (Table 1, entry 5), and for the Bz protected
substrate 3a, the reaction gave mainly E-7a (Z∶E＝
21 ∶ 79) with moderate yield (50%) when using
CF3COOH as additive (Table 1, entry 8). For other
substrates, the reaction only gave Z/E-7 with low yields
and low selectivities when using H2O or CF3COOH as
additive (Table 1, entries 6, 7, 9－14).
Co2(CO)8 (2.2 mmol) was added to a solution of
propargyl alcohol (1.0 mmol) in 4 mL CH3CN and
stirred at r.t. for 30 min. CF3COOH (6.0 mmol) was
added to the solution and stirred at r.t. for 15 min. Then
the solution was stirred at reflux temperature until no
co-alkyne complex was detected by TLC. The mixture
was filled through Büchner funnel, concentrated and
purified by flash column chromatography to gain the
product.
1
1-(4-(Prop-1-enyl)phenyl)ethanone (7a) Z-7a: H
NMR (300 MHz, CDCl3) δ: 1.93 (dd, J＝7.2, 1.8 Hz,
3H), 2.60 (s, 3H), 5.87－5.95 (m, 1H), 6.46 (d, J＝11.7,
1H), 7.38 (d, J＝8.3 Hz, 2H), 7.93 (d, J＝8.3 Hz, 2H);
E-7a: 1H NMR (300 MHz, CDCl3) δ: 1.92 (d, J＝5.1 Hz,
3H), 2.58 (s, 3H), 6.55－6.32 (m, 2H), 7.40 (d, J＝8.3
Hz, 2H), 7.89 (d, J＝8.3 Hz, 2H).
Table 1 The influences of additives and protecting groups for
the reduction of propargyl alcohol derivatives
1-Methoxy-4-(prop-1-enyl)benzene (7b)
Z-7b:
1H NMR (300 MHz, CDCl3) δ: 1.89 (dd, J＝1.8, 7.2 Hz,
3H), 3,81 (s, 3H), 5.71 (qd, J＝7.2, 11.6 Hz, 1H), 6.38
(d, J＝11.6, 1H), 6.87 (d, J＝8.7 Hz, 2H), 7.24 (d, J＝
8.7 Hz, 2H); E-7b: 1H NMR (300 MHz, CDCl3) δ: 1.87
(dd, J＝1.6, 7.3 Hz, 3H), 3.77 (s, 3H), 6.09 (qd, J＝7.2,
16.0 Hz, 1H), 6.33 (d, J＝16.0 Hz, 1H), 6.82 (d, J＝7.5
Hz, 2H), 7.24 (d, J＝8.7 Hz, 2H).
Co2(CO)8 (1.2 equiv.)
O
OR
additive
CH3CN, reflux
O
1a, 2a, 3a, 4, 5 or 6
Entry Substrate R
7a
Additive (equiv.) Yielda/% Z/Eb
1
2-(Prop-1-enyl)naphthalene (7c) Z-7c: H NMR
1
1a
1a
1a
1a
2a
2a
3a
3a
4
H
CyNH2 (3.5)
TMANO (6.0)
H2O (3.0)
－
－
(300 MHz, CDCl3) δ: 1.99 (d, J＝6.8 Hz, 3H), 5.89 (dq,
J＝11.7, 6.8 Hz, 1H), 6.57 (d, J＝11.7 Hz, 1H), 7.47－
7.54 (m, 3H), 7.70－7.87 (m, 4H); E-7c：1H NMR (300
MHz, CDCl3) δ: 1.95 (d, J＝6.8 Hz, 3H), 6.37 (dq, J＝
17.0, 6.8 Hz, 1H), 6.57 (d, J＝17.0 Hz, 1H), 7.39－7.84
(m, 7H).
2
H
－
－
3
H
trace
－
4
H
CF3COOH (3.0) 38
H2O (3.0) 51
CF3COOH (3.0) 47
H2O (3.0) 26
CF3COOH (3.0) 50
26∶74
86∶14
29∶71
71∶29
21∶79
－
5
Ac
Ac
Bz
Bz
Ts
Ts
6
Prop-1-ene-1,3-diyldibenzene (7e) Z-7e: 1H NMR
(300 MHz, CDCl3) δ: 3.71 (d, J＝7.5 Hz, 2H), 5.85－
5.94 (m, 1H), 6.62 (d, J＝11.1 Hz, 1H), 7.21－7.40 (m,
10H); E-7e: 1H NMR (300 MHz, CDCl3) δ: 3.58 (d, J＝
6.4 Hz, 2H), 6.27－6.51 (m, 1H), 6.46 (d, J＝16.4 Hz,
1H), 7.21－7.40 (m, 10H).
7
8
9
H2O (3.0)
trace
10
11
12
13
14
4
CF3COOH (3.0) 34
50∶50
50∶50
50∶50
83∶17
26∶74
5
COOEt H2O (3.0)
COOEt CF3COOH (3.0) 28
COtBu H2O (3.0)
49
COtBu CF3COOH (3.0) 37
5
(4-(Benzyloxy)but-1-enyl)benzene (7f) Z-7f: 1H
NMR (300 MHz, CDCl3) δ: 2.67 (q, J＝6.3 Hz, 2H),
3.57 (t, J＝6.6 Hz, 2H), 4.52 (s, 2H), 5.71 (dt, J＝7.2,
10.8 Hz, 1H), 6.51 (d, J＝10.8 Hz, 1H), 6.98－7.52 (m,
10H); E-7f: 1H NMR (300 MHz, CDCl3) δ: 2.53 (q, J＝
6.6 Hz, 2H), 3.58 (t, J＝6.6 Hz, 2H), 4.54 (s, 2H), 6.24
(dt, J＝6.9, 15.6 Hz, 1H), 6.51 (d, J＝15.6 Hz, 1H),
7.17－7.35 (m, 10H).
5
6
6
a Isolated yield. b Determined by 1H NMR.
To further improve the yield, we screened the
equivalents of the Co2(CO)8 and the additive. For the Ac
protected substrate 2a using H2O as additive, the
equivalents of H2O had no remarkable effect on the se-
lectivity (Table 2, entries 1－4), but the equivalents of
Co2(CO)8 had great influence on the selectivity and
yield. With increasing equivalents of Co2(CO)8, the se-
lectivity of the reaction decreases albeit yield increases
(Table 2, entries 5－7). A yield of 82% (Z∶E＝85∶15)
could be gained by using 1.6 equiv. of Co2(CO)8 and 6.0
equiv. of H2O (Table 2, entry 5). For the Bz protected
substrate 3a using CF3COOH as additive, a yield of
80% (Z∶E＝12∶88) could be gained by using 2.2
(1,2,3-Trideuteroprop-1-ene-1,3-diyl)dibenzene
(7e') 1H NMR (300 MHz, CDCl3) δ: 3.66 (s, 1H), 7.20
－7.37 (m, 10H); EIMS m/z (%): 197 (M＋); HRMS (EI)
＋
calcd for C15H11D3 : 197.1284, found: 197.1279; IR
(KBr) ν: 2934, 1764, 1106, 703 cm−1.
Results and Discussion
Firstly, we investigated the reduction reaction of
propargyl alcohol 1a under different conditions. We
found that the reaction could not proceed when using
1000
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Co2(CO)8-mediated Selective Reductions of Propargyl Alcohol Derivatives to Alkenes
equiv. of Co2(CO)8 and 6.0 equiv. of CF3COOH (Table
2, entry 10).
Table 3 Selective hydrogenation and reduction of propargyl
alcohol derivatives
R2
R3
R1
R2
Co2(CO)8
R1
Table 2 The influences of equivalents of Co2(CO)8 and addi-
tives
additive, CH 3CN, reflux
7a-7g
2a-2g or 3a-3e
O
OR
Entry Substrate
Product Yielda/% Z/Eb
CH3CN, reflux
2a: R1＝4-Ac-phenyl,
O
1c
7a
7b
7b
7c
7d
7e
7f
82
61
35
58
－
97
53
－
80
36
71
－
11
85∶15
33∶67
78∶22
84∶16
－
R2＝H, R3＝Ac
2a or 3a
7
2b: R1＝4-MeO-phenyl,
R2＝H, R3＝Ac
Co2(CO)8
(equiv.)
Additive (equiv.) Yielda/% Z/Eb
2c
EntrySubstrate
2b: R1＝4-MeO-phenyl,
R2＝H, R3＝Ac
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2a
2a
2a
3a
3a
1.2
1.2
1.2
1.2
1.6
2.2
3.0
1.2
2.2
2.2
2.2
H2O (3.0)
H2O (5.0)
H2O (6.0)
H2O (8.0)
H2O (6.0)
H2O (6.0)
H2O (6.0)
51%
52%
54%
29%
82%
86%
85%
86∶14
86∶14
86∶14
86∶14
85∶15
38∶62
26∶74
21∶79
19:81
3d
2c: R1＝2-naphthyl,
R2＝H, R3＝Ac
4c
2d: R1＝2-Br-phenyl l,
R2＝H, R3＝Ac
5c
2e: R1＝phenyl,
6c
86∶14
91∶9
－
R2＝phenyl, R3＝Ac
2f: R1＝phenyl,
CF3COOH (3.0) 50%
CF3COOH (3.0) 52%
CF3COOH (6.0) 80%
CF3COOH (13.0) 40%
7c
R2＝BnOCH2, R3＝Ac
2g: R1＝phenethyl,
R2＝phenyl, R3＝Ac
3a: R1＝4-Ac-phenyl,
R2＝H, R3＝Bz
8c
7g
7a
7b
7c
7d
7e
10 3a
11 3a
12∶88
11∶89
9e
12∶88
16∶84
24∶76
－
a Isolated yield. b Determined by 1H NMR.
3b: R1＝4-MeO-phenyl,
R2＝H, R3＝Bz
10e
11e
12e
13e
Then the scope of the selective reduction reaction was
examined for a series of propargyl alcohol derivatives 2a
－2g and 3a－3e (Table 3). For the Ac protected substrates
2a－2g, the reaction was carried out using 1.6 equiv. of
Co2(CO)8 and 6.0 equiv. of H2O. For 2b we mainly got the
E-7b in moderate yield (61%, Z∶E＝33∶67) (Table 3,
entry 2), but we could maily gain the Z-7b in moderate
yield by using 1.2 equiv. of Co2(CO)8 and 3.0 equiv. of
H2O (35%, Z∶E＝78∶22) (Table 3, entry 3); for 2c, we
mainly got the Z-7c in moderate yield (58%, Z∶E＝84∶
16) (Table 3, entry 4); for 2d, we mainly got the
bromide-removed product (Table 3, entry 5). For the
secondary alcohol acetate esters 2e and 2f, we mainly got
the Z-7e (97%, Z∶E＝86∶14) and Z-7f (53%, Z∶E＝
91∶9) in moderate to high yield (Table 3, entries 6, 7).
The reduction of alkyl substituted propargyl alcohol
acetate ester 2g was accompanied with alkene re-
arrangement product (Table 3, entry 8). For the Bz
protected substrates 3a－3e, the reaction was carried out
using 2.0 equiv. of Co2(CO)8 and 6.0 equiv. of CF3COOH.
For 3b and 3c, we mainly got the E-7b (36%, Z∶E＝16∶
84) and E-7c (71%, Z∶E＝24∶76) in mederate to high
yield (Table 3, entries 10, 11); for 3d, we maily got the
bromide-removed product (Table 3, entry 12). For the
secondary alcohol benzoate easter 3e, we got the E-7e in
only low yield (11%, Z∶E＝39∶61) (Table 3, entry 13).
In effort to understand the reaction mechanism, we
carried out some other experiments. We found that the
reduction reaction could not procceed for simple alkyne
1-(4-((trimethylsilyl)ethynyl)phenyl)ethanone instead of
propargyl alcohol derivatives when using H2O as additive.
For the 2e, we only got product 7e' in 94% yield con-
3c: R1＝2-naphthyl,
R2＝H, R3＝Bz
3d: R1＝2-Br-phenyl l,
R2＝H, R3＝Bz
3e: R1＝phenyl,
39∶61
R2＝phenyl, R3＝Bz
a Isolated yield. b Determined by H NMR. c The reaction was
carried out using 1.6 equiv. of Co2(CO)8 and 6.0 equiv. of H2O.
d The reaction was carried out using 1.2 equiv. of Co2(CO)8 and
3.0 equiv. of H2O. The reaction was carried out using 2.2 equiv.
of Co2(CO)8 and 6.0 equiv. of CF3COOH.
1
e
taining three deutium atoms when using D2O as the addi-
tive, which could prove that the water is the hydrogen
source (Scheme 1). The research of the reaction mecha-
nism is still in process.
Scheme 1 Reduction of the propargyl alcohol derivatives using
D2O as additive
D
Ph
Co2(CO)8
Ph
Ph
Ph
D2O, CH3CN, reflux
OAc
D
D
94%
2e
7e'
Conclusions
In conclusion, the conversion of propargyl alcohol
derivatives to alkenes mediated by Co2(CO)8 and different
additive with good yield and Z/E ratio was demonstrated.
When using H2O as additive, Z-alkene was the main-
Chin. J. Chem. 2014, 32, 999—1002
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1001

Dou et al.
COMMUNICATION
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Tetrahedron Lett. 1999, 40, 9177.
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was the main-product.
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Acknowledgement
This work was supported by the National Basic
Research Program of China (973-2010CB833200 and
973-2010CB833300), the National Natural Science
Foundation of China (21102167), the Science and
Technology Commission of Shanghai Municipality
(12DZ1930902), Shanghai Green Chemical Engineering
Technology Research Center and the Knowledge
Innovation Program of the Chinese Academy of
Sciences.
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Chin. J. Chem. 2014, 32, 999—1002