Phosphine-Catalyzed Regioselective and Stereoselective Hydrohalogenation Reaction of 2-(1-Alkynyl)-2-alken-1-ones: Synthesis of 2-Halo-1,3-dienes

Article abstract of DOI:10.1002/adsc.201500408

A triphenylphosphine (PPh₃)-catalyzed, regioselective and stereoselective hydrohalogenation reaction of 2-(1-alkynyl)-2-alken-1-ones with metal halides in the presence of acetic acid is described, furnishing functionalized 3,4-disubstituted 2-halo-1,3-dienes in moderate to good yields with high stereoselectivity and regiospecificity. The simple execution, mild conditions and the potential utilization of the products make this protocol attractive.

Full text of DOI:10.1002/adsc.201500408

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DOI: 10.1002/adsc.201500408
Phosphine-Catalyzed Regioselective and Stereoselective
Hydrohalogenation Reaction of 2-(1-Alkynyl)-2-alken-1-ones:
Synthesis of 2-Halo-1,3-dienes
Wenbo Li,a Lihua Gao,a Zhenting Yue,a and Junliang Zhanga,b,*
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering,
East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, People’s Republic of China
Fax: (+86)-21-6223-5039; e-mail: jlzhang@chem.ecnu.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, CAS. 345 Lingling Road,
b
Shanghai 200032, People’s Republic of China
Received: April 25, 2015; Revised: June 17, 2015; Published online: August 18, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500408.
Abstract: A triphenylphosphine (PPh3)-catalyzed,
regioselective and stereoselective hydrohalogena-
tion reaction of 2-(1-alkynyl)-2-alken-1-ones with
metal halides in the presence of acetic acid is de-
scribed, furnishing functionalized 3,4-disubstituted
2-halo-1,3-dienes in moderate to good yields with
high stereoselectivity and regiospecificity. The
simple execution, mild conditions and the potential
utilization of the products make this protocol at-
tractive.
gen halides or metal halides with alkylidenecyclopro-
panes proceed to afford homoallylic halides.[8]
Diene units are important building blocks in organ-
ic synthesis, especially in those Diels–Alder reac-
tions[9] and cycloadditions.[10] Surprisingly, to the best
of our knowledge, there are only a few methods to
access such highly substituted halo-dienes and most of
them use allenic acetates or allenols as starting mate-
rial. For example, Bäckvall and co-workers reported
an efficient Pd(OAc)2-catalyzed SN2’ reaction of ace-
tylated a-allenic alcohols with LiBr under acidic con-
ditions, providing 4-monsubstituted 2-bromo-1,3-
dienes in good yields, but they did not show the aryl
substitution pattern of the allenic acetate [Scheme 1,
Eq. (1)].[11] Later, Ma and Wang realized the efficient
synthesis of 3,4-disubstituted 2-halo-1,3-dienes with
moderate E/Z selectivity by the use of 1-aryl-2,3-alle-
Keywords: enynes; 2-halo-1,3-dienes; hydrohaloge-
nation reaction; metal halides; triphenylphosphine
The evidence for a renewed interest in hydrohaloge- nols reacting with LiX in acetic acid. [Scheme 1, Eq.
nation reactions of alkynes, especially electron-defi-
cient alkynes, has been compelling in recent years.[1–4]
A number of routes for hydrohalogenation reactions
of alkynes has been described. In particular, the use
of metal halides in acetic acid has been long exam-
ined as a useful reaction system since Ma and Luꢀs
pioneering work.[2a–e] Even a weak nucleophile such
ꢀ
as fluoride can be added to the C C triple bonds cat-
alyzed by an Au(I) complex.[3a–b] The product vinyl
halides, especially iodides and bromides, are novel
starting materials for radical reactions and transition
metal-catalyzed cross-coupling reactions.[5] The hydro-
halogenation reactions of electron-deficient allenes,
such as 1,2-allenyl carboxylic acids, esters and 1,2-al-
lenyl ketones provided an efficient route to b-halo-
b,g-unsaturated enoic acids, enoates, and enones.[6]
1,2-Allenic sulfones and sulfoxides could also give 2-
haloallylic sulfones and sulfoxides.[7] Recently, Yama-
moto and Shi have shown that the reactions of hydro- Scheme 1. Stereoselective synthesis of 2-halo-1,3-dienes.
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Wenbo Li et al.
Table 1. Optimization of the reaction conditions.[a]
Table 2. Hydrobromination reaction of 2-(1-alkynyl)-2-
alken-1-ones.[a]
Entry
Solvent
Time
[h]
Yield
[%][b]
Z/E
Entry
R1/R2
(1)
Time
[h]
Product
Yield
[%][b]
1[c]
2
3
4
5
6
7
8
9
HOAc
HOAc
17
12
4
4
4
12
4
21
21
4
17
54
60
14
60
56
68
22
trace
56
75
0
n.d.
1
2
3
Me/4-MeC6H4 (1b)
Me/4-MeOC6H4 (1c)
Me/4-ClC6H4 (1d)
Me/4-NCC6H4 (1e)
Me/n-C4H9 (1f)
H/Ph (1g)
4
4
4
5
2
4
2
2
2
2
2
2
2b
2c
2d
2e
2f
2g
2h
2i
64
64
60
57
56
61
53
46
72
46
57
59
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
n.d.
n.d.
>20:1
8:1
HOAc-CF3CO2H(1:1)
HOAc-CF3CO2H(1:3)
HOAc-CF3CO2H(3:1)
HOAc-toluene(1:1)
HOAc(1 M)-toluene
HOAc(1 M)-THF
HOAc(1 M)-CH3CN
HOAc(2 M)-toluene
HOAc(1 M)-toluene
HBr/HOAc
4
5[c]
6
7
8
9
10
11
12
Ph/Ph (1h)
Ph/4-ClC6H4 (1i)
Ph/1-naphthyl (1j)
Ph/styryl (1k)
4-ClC6H4/Ph (1l)
4-MeOC6H4/Ph (1m)
2j
2k
2l
10
11[d]
12[e]
4
4
–
2m
[a]
The reaction was carried out with 1a (0.3 mmol) and
LiBr·H2O (1.5 equiv.) in the given solvent (2 mL) in the
presence of a PPh3 catalyst (20 mol%).
Isolated yield.
The reaction was carried out without PPh3, n.d.=not de-
termined.
LiBr·H2O (2.0 equiv.) was used.
Commercial HBr (33%) in HOAc was used instead of
LiBr·H2O as the reagent and solvent.
[a]
The reaction was carried out with 1 (0.3 mmol) and
LiBr·H2O (1.5 equiv.) in HOAc (1M)-toluene (2 mL) in
the presence of a PPh3 catalyst (20 mol%).
Isolated yield.
[b]
[c]
[b]
[c]
The 3.3:1 mixture of Z/E isomers (2 f) was isolated.
[d]
[e]
of starting material 1a with LiBr·H2O and 20 mol%
PPh3 indeed enhanced the yield to 54% with excellent
stereoselectivity (Z/E> 20:1) (Table 1, entry 2). The
(2)].[12] However, with respect to the atom-economy reaction can also occur in a 1:1 mixture of HOAc and
and stereoselectivity issues, the development of a new CF3CO2H (Table 1, entry 3). Increasing or decreasing
system for preparing multi-functionalized 2-halo-1,3- the amount of CF3CO2H did not improve the reaction
dienes with excellent stereoselectivity is still highly efficiency (Table 1, entries 4 and 5). Similarly, the re-
desirable. In connection with our current research in- action provided the corresponding vinyl bromide 2a
terest in the stereoselective synthesis of dienes in 56% yield with a 1:1 mixture of HOAc and toluene
through phosphine-catalyzed addition reactions,[13,14] as the solvent (Table 1, entry 6). The reaction was
we wish to report herein a novel phosphine-catalyzed best run in 1M HOAc in toluene to give 2a in 68%
hydrohalogenation reaction of electron-deficient yield (Table 1, entry 7). Further studies showed that
enynes with metal halides in the presence of acid, other solvents failed to improve the yield (Table 1, en-
which provides a facile access to multi-functionalized tries 8–10). With the amount of LiBr·H2O increased
2-halo-1,3-dienes in moderate to good yields with to 2 equiv., the yield was improved to 75%, but the
high stereoselectivities [Scheme 1, Eq. (3)].
ratio of Z/E isomers decreased to 8:1 (Table 1,
We initiated our investigation using 3-benzylidene- entry 11). When 1a was treated with commercial HBr
pent-4-yn-2-one (1a)[15] and LiBr·H2O as model sub- (33%) in HOAc solvent, the corresponding product
strates (Table 1). The reaction of 1a with LiBr·H2O was not detected (Table 1, entry 12).
(1.5 equiv.) was carried out at 808C in acetic acid to
With the optimal reaction conditions in hand
give 3-benzylidene-4-bromopent-4-en-2-one 2a in only (Table 1, entry 7), we next explored the scope of the
17% yield along with recovery of 1a in 65% yield method by testing various yne-enones 1 in the reac-
(Table 1, entry 1). Our previous work[16] showed that tion. As shown in Table 2, moderate to good yields of
two molecules of yne-enone could undergo the hydrobromination products 2 can be obtained as es-
hetero-[4+2] cycloaddition under the catalysis of (n- sentially single geometrical isomers (Z/E> 20:1)
Bu)3P, leading to 4H-pyrans, in which the reaction is under the standard conditions, except for 2f. When
believed to be initiated by (n-Bu)3P via nucleophilic substrates containing electron-rich or electron-defi-
addition of the yne-enones. We envisaged that tertiary cient aryl groups on the alkenyl moiety were subject-
phosphines may improve the conversion. Treatment ed to the reaction, the corresponding vinyl bromides
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Phosphine-Catalyzed Regio- and Stereoselective Hydrohalogenation
Table 3. Hydrochlorination reaction of 2-(1-alkynyl)-2-
alken-1-ones.[a]
Entry
R1/R2
(1)
Time
[h]
Product
Yield
[%][b]
1
2
3
Me/Ph (1a)
3
2
2
2
2
2
2
2
2
3a
3b
3c
3d
3e
3f
3g
3h
3i
63
66
71
53
62
48
69
70
60
Me/4-MeC6H4 (1b)
Me/4-MeOC6H4 (1c)
Me/4-NCC6H4 (1e)
H/Ph (1g)
Figure 1. X-ray crystallographic structure of 3d.
4
5[c]
6
Ph/Ph (1h)
electron-donating or electron-withdrawing substitu-
ents on the phenyl rings (Table 3, entries 2–4). Nota-
bly, the formyl substituent on the enyne affords hy-
drochlorination product 3e in 62% yield and a 13:1
ratio of Z and E isomers (Table 3, entry 5). The sub-
strate 1h, which has a phenyl group on the a-position
of ketone, was converted to the vinyl chloride 3f in
a lower yield (Table 3, entry 6). Good yields were also
obtained from those reactions of yne-enones bearing
different electronic-property aryl ketones (Table 3,
entries 7–9). Moreover, the reaction of 1l works much
better in HOAc than under the standard conditions
7
Ph/1-naphthyl (1j)
4-ClC6H4/Ph (1l)
4-MeOC6H4/Ph (1m)
8[d]
9
[a]
The reaction was carried out with 1 (0.3 mmol) and LiCl
(3.0 equiv.) in HOAc (1M)-toluene (2 mL) in the pres-
ence of a PPh3 catalyst (20 mol%).
[b]
[c]
[d]
Isolated yield.
The 13:1 mixture of Z/E isomers (3g) was isolated.
Reaction conditions: yne-enones 1l (0.3 mmol), LiCl
(3 equiv.) and PPh3 (20 mol%) in HOAc (2 mL).
were obtained in moderate to good yields (Table 2, (Table 3, entry 8). The present reaction is highly ste-
entries 1–4 and 8). In the case of 1f bearing an ali- reoselective, except for 3e. The relative stereochemis-
phatic substituent on the alkenyl moiety, 3-(1-bromo- try of 3d was further confirmed by X-ray crystallo-
vinyl)oct-3-en-2-one 2f was obtained in 56% total graphic analysis (Figure 1).[17]
yield with 3.3:1 Z/E (Table 2, entry 5). To our delight,
It is a pity that the established method was not ap-
formyl enyne 1g provided the corresponding 1,3- plicable to the synthesis of vinyl fluorides. To our de-
dienyl bromide 2g in moderate yield as well under the light, 3-benzylidenepent-4-yn-2-one 1a also reacted
same conditions (Table 2, entry 6). In general, aromat- smoothly with NaI and afforded the corresponding 3-
ic ketones were more reactive than aliphatic ketones benzylidene-4-iodopent-4-en-2-one 4 in 69% yield as
and required shorter reaction times. The phenyl-sub- a 12.5:1 mixture of Z and E isomers, which could be
stituted ketone provided a poorer yield, presumably reduced to alcohol 5 in 78% yield as a single isomer
because it is more unstable under the reaction condi- [Scheme 2, Eq. (4)]. When internal alkynes with an
tions and more susceptible to polymerization, leading aryl substituent or an aliphatic substituent were sub-
to undesired side products (Table 2, entry 7). The jected to the standard reaction conditions, the 1,3,4-
naphthyl group on the alkenyl moiety was tolerated trisubtituted 2-bromo-1,3-dienes 7a and 7b were iso-
effectively, giving the product 2j in good yield lated in lower yields, respectively [Scheme 2, Eq. (5)].
(Table 2, entry 9). It is noteworthy that compound 1k Furthermore, the hydrobromination of internal yne-
with a styryl group on the alkenyl moiety afford the enone 6c furnished the corresponding product 7c in
corresponding 2-bromo-1,3,5-triene 2k as a single a 42% yield along with some unidentified by-products
isomer (Table 2, entry 10). Moreover, p-substituted [Scheme 2, Eq. (6)]. The vinyl bromide 2a produced
aryl ketones also reacted well with LiBr·H2O under by this chemistry should be very useful for the synthe-
the optimal conditions (Table 2, entries 11 and 12).
sis of conjugated polyunsaturated structure 8 via tran-
After examination of the hydrobromination, we sition metal-catalyzed cross-coupling reactions
next turned to expand our studies to the hydrochlori- [Scheme 2, Eq. (7)].
nation of 2-(1-alkynyl)-2-alken-1-ones 1 (Table 3).
Based on the above experiments, a plausible mech-
When the amount of LiCl was increased from 1.5 to anism is proposed [Scheme 3]. The nucleophilic addi-
3 equiv. , the yield was improved and vinyl chloride 3a tion of Ph3P to the yne-enone 1[16] in the presence of
was obtained in 63% yield as a single geometrical acid gives three convertible intermediates IA, IB and
isomer (Table 3, entry 1). Variations on the aryl unit IC. The allenyl ketone intermediate IC undergoes an
of the alkenyl moiety were well-tolerated, with either
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Experimental Section
Typical Procedure for the Preparation of 2a
A mixture of enyne 1a (0.3 mmol, 51.0 mg), LiBr·H2O
(0.45 mmol, 1.5 equiv., 46.8 mg) and PPh3 (0.06 mmol,
20 mol%, 15.7 mg) in 1M HOAc-toluene (2 mL) was stirred
at 808C for 4 h. The reaction mixture was concentrated
under reduced pressure. The crude product was purified by
flash column chromatography on silica gel (petroleum/ethyl
acetate=10:1) to afford 2a as a colorless oil; yield: 48.4 mg
1
(68%). H NMR (400 MHz, CDCl3): d=7.74–7.69 (m, 2H),
7.47 (s, 1H), 7.41–7.36 (m, 3H), 5.94 (d, J=1.2 Hz, 1H),
5.82 (d, J=1.2 Hz, 1H), 2.48 (s, 3H); 13C NMR (100 MHz,
CDCl3): d=196.32, 139.83, 138.71, 133.39, 131.08, 130.29,
128.63, 125.22, 123.50, 27.06; MS (70 eV): m/z (%)=250
(M+, 8.95), 252 (M+ +2, 8.79), 43 (100); HR-MS: m/z=
249.9993, calcd. for C12H11O79Br: 249.9993; IR (neat): n=
3061, 1690, 1673, 1357, 1231, 1193, 902, 753, 691 cmÀ1.
Acknowledgements
We are grateful to 973 program (2011CB808600), NSFC
(21172074), Fok Ying Tung Education Foundation (121014)
and the Program of Eastern Scholar at Shanghai Institutions
of Higher Learning for financial support.
Scheme 2. Hydrohalogenation reaction of enynes and prod-
uct transformation.
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