Palladium-catalyzed bromoalkynylation of C-C double bonds: Ring-structure-dependent synthesis of 7-alkynyl norbornanes and cyclobutenyl halides

Article abstract of DOI:10.1002/anie.201100002

Strain versus flexibility: The palladium-catalyzed reaction of haloalkynes with norbornene derivatives leads to 7-alkynyl norbornane products (see scheme). Key to the success of this reaction is the formation of a "bridging" palladium species, which can rearrange to result in a C-7 functionalization. The ring-structure-dependent [2+2] cycloaddition of haloalkynes with cyclooctene has been achieved in moderate to good yields under similar conditions. Copyright

Full text of DOI:10.1002/anie.201100002

DOI: 10.1002/anie.201100002
Synthetic Methods
À
Palladium-Catalyzed Bromoalkynylation of C C Double Bonds:
Ring-Structure-Dependent Synthesis of 7-Alkynyl Norbornanes and
Cyclobutenyl Halides**
Yibiao Li, Xiaohang Liu, Huanfeng Jiang,* Bifu Liu, Zhengwang Chen, and Peng Zhou
1
The development of efficient and sustainable procedures for
the synthesis of complex molecules is an important task in
modern organic chemistry. The direct cleavage of an alkynyl–
halogen bond followed by the reconnection of both the
alkynyl and halogen ions with the two carbon atoms of an
unsaturated carbon–carbon bond provides ready access to
highly functionalized products from simple alkynes with
excellent atom economy.[1] We previously achieved highly
regio- and stereoselective bromoalkynylation of internal
alkynes for the synthesis of conjugated cis-bromoalkenynes.[2]
Subsequent research on this subject revealed a further use of
bromoalkynes in complex molecule synthesis.
by H NMR spectroscopy. This unexpected result attracted
our interest, since, to the best of our knowledge, no example
of a direct 7-alkynyl bromonorbornane formation has been
reported.
From previous reports,[5] we realized that after the
formation of the nonclassical “norbornonium” cation, the
C-7 functionalization can be achieved through a nucleophile
rearrangement (Scheme 2). Our success in synthesizing C-7-
Norbornene derivatives are an appealing group of organic
molecules that are convenient starting materials for the
synthesis of polymers, solar-energy-storage materials, and
bioactive products.[3] In addition, their strained structure and
high potential to coordinate to transition metals, as well as
their possible industrial applications have attracted consid-
erable research interest.[4] Thus, we decided to react phenyl-
ethynyl bromide (2a) with norbornene (1a), expecting to
Scheme 2. Rearrangement of a functional group by nucleophile disso-
ciation.
functionalized norbornyl alkynes proved the compatibility of
this cation with the aforementioned alkynylation reaction
conditions (see retrosynthesis in Scheme 3). The resulting
products, which were formed with high selectivity, are not
otherwise easily accessible and can find potential applications
in both synthetic and materials chemistry.[6]
obtain
2-bromo-3-(2-phenylethynyl)bicyclo[2.2.1]heptane
(3aa; Scheme 1). However, when using the previously
optimized conditions,[2] we did not detect the formation of
3aa, instead we obtained 2-bromo-7-(2-phenylethynyl)-
bicyclo[2.2.1]heptane (3a) in excellent yield, as confirmed
Scheme 3. Retrosynthesis of a 7-alkynyl halonorbornane via a “non-
classical” cation by Pd dissociation. L=CH3CN.
Scheme 1. Bromoalkynylation of norbornene (1a) with phenylethynyl
bromide (2a).
The scope of the C-7 functionalization reaction was
examined for several haloalkynes and norbornene-derivatives
(Scheme 4 and Table 1). Both electron-rich and electron-poor
phenylethynyl bromides were reacted with 2-norbornene (1a)
to give the corresponding alkynylation products 3a–3k in
good to excellent yields (Table 1, entries 1–11). The reaction
tolerated a variety of substituents including Cl, Br, F, NO2,
and OMe groups, and substituents at the ortho position of the
benzyl group did not affect the yield of the reaction (Table 1,
entries 2 and 7). 5-Substituted 2-norbornene 1b (mixture of
endo and exo) was also successfully converted to 2,5,7-
substituted norbornanes 3l–3p in good yields with no obvious
inversion of configuration compared to the corresponding
starting materials (Table 1, entries 12–16). The reaction
showed no regioselectivity when 5-vinyl-2-norbornene 1c
and polycyclic norbonene 1d were used as the substrates
[*] Y. B. Li, X. H. Liu, Prof. H. F. Jiang, B. F. Liu, Z. W. Chen, P. Zhou
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou 510640 (China)
Fax: (+86)20-2223-6337
E-mail: jianghf@scut.edu.cn
[**] This work was supported by the National Natural Science
Foundation of China (20625205, 20772034 and 20932002), the
National Basic Research Program of China (973 Program)
(2011CB808600), the Doctoral Fund of Ministry of Education of
China (20090172110014), and the Guangdong Natural Science
Foundation (10351064101000000).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100002.
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iodoalkynylation took place to give 3y in excellent yields
(Table 1, entry 25). The use of norbornenoic acid 1g resulted
in the exclusive formation of the 3-alkynyl derivative 3x
(Table 1, entry 24), thus indicating that the nucleophilic attack
occurred prior to the rearrangement.[8] Unfortunately, the
reaction of haloalkynes with open-chain alkenes, such as 4-
octene, only afforded a mixture of products, and the reaction
was unsuccessful with terminal alkenes.
The reaction between a bromoalkyne and cyclooctene is
equally interesting, since the latter is more flexible compared
to the strained norbornene derivatives. We predicted that the
reaction should lead to a 2-propynyl bromide derivative
(Scheme 5, path a). However, when the reaction was per-
Scheme 5. Bromoalkynylation of cyclooctene.
Scheme 4. Norbornene derivatives and haloalkynes. Substrates 1b, 1c,
1e, and 1g are mixtures of endo and exo isomers (see the Supporting
Information).
formed under conditions similar to those described above, we
found that this reaction resulted in the formation of a four-
membered ring by a [2+2] cycloaddition reaction (Scheme 5,
path b). To the best of our knowledge, the [2+2] cycloaddition
of alkynes and monocyclic alkenes continues to represent a
challenge.[9] This approach represents another utilization of
haloalkynes for carbocycle formations that employ palladium
catalysis.
(Table 1, entries 17 and 18). On the other hand, the carbonyl
substituent in norbornene 1e led to an obvious selectivity for
2,5,7-substituted norbornane 3s (Table 1, entry 19). To our
surprise, the use of 4-bromo-2-phenylbut-3-yn-2-ol (2n)
resulted in the formation of the dehydration product 3w in
67% yield (Table 1, entry 23). Trimethylsilylethynyl bromide
(2l) also added onto norbornene to afford 2-bromo-7-
alkynylnorbornane 3u in good yield (Table 1, entry 21), thus
showing the mildness and robustness of our method.[7] Alkyl
alkynyl bromides such as n-pentyl bromide (2m) can also
undergo this transformation in good yields (Table 1, entry 22).
In this case, simple filtration through a silica gel plug was
sufficient to remove the residual catalyst and provide product
3v. Interestingly, the use of 1,4-bis(2-bromoethynyl)benzene
(2p) resulted in the formation of the corresponding diyne
product 3z as white crystals in 73% yield (Table 1, entry 26).
The molecular structure of 3z was established by X-ray
crystallography (Figure 1), which enabled us to confirm the
reaction nature as a 2,7-addition process. We also extended
this reaction to phenylethynyl iodide (2o) and found that the
As shown in Scheme 6, aromatic alkynyl bromides with
either electron-donating or electron-withdrawing groups
attached to the benzene rings were able to smoothly undergo
a [2+2] cycloaddition with cyclooctene, and generated the
corresponding products in moderate to good yields. The
reaction tolerated a variety of substituents including Cl, Br, F,
and OMe groups. The steric hindrance associated with the
alkynyl bromide also affected the yield of the reaction, as the
introduction of an o-trifluoromethyl group onto the alkynyl
bromide lowered the conversion of the haloalkyne (Scheme 6,
5o). We also extended this reaction to phenylethynyl iodide
(2o) and found that the [2+2] cycloaddition took place to give
5n in good yield. Other alkenes such as cycloheptene were
also subjected to this reaction, however, the yields decreased
and the main products were inseparable from Alder–ene by-
products.[9e] Furthermore, it was found that reactions with
cyclododecene were completely ineffective. These observa-
tions indicate that the ring size of the alkene plays a major
role in the formation of the desired product. Finally, the
reaction of bromoalkyne 2e with cyclopentene under typical
palladium-catalyzed cross-coupling conditions gave enynes
6a and 6b in 87% yield, thus suggesting that the reaction
involves the insertion of the alkene to an alkynyl palladium
species rather than a palladium cyclopentene intermediate
(Scheme 7a).[9]
To demonstrate the synthetic utility of cyclobutenyl
bromide, we showed that the Pd/Cu-catalyzed coupling of
5b with phenylacetylene gave enyne 7a in 71% yield.
Figure 1. X-ray crystal structure of 3z. Thermal ellipsoids set at 50%
probability.
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Angew. Chem. Int. Ed. 2011, 50, 6341 –6345

Table 1: Addition of haloalkynes to norbornene derivatives.[a]
Entry Alkene Haloalkyne Product
Yield Entry Alkene Haloalkyne Product
[%][b]
Yield
[%][b]
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1a
1a
1a
2a
2b
2c
2d
2e
2 f
2g
2h
2i
3a 95
14
15
16
17
18
19
20
21
22
1b
1b
1b
1c
1d
1e
1 f
2d
2i
3n
3o
3p
87
78
65
3b 89
3c 91
3d 92
3e 96
3 f 95
3g 81
3h 85
2j
3q’: R4 =H, R5 =Br 87,[c]
3q’’: R4 =Br, R5 =H 1:1
2a
2a
2a
2a
2l
3r’: R4 =H, R5 =Br 84,[c]
3r’’: R4 =Br, R5 =H 1:1
3s
3t
68[d]
87
1a
1a
3u
3v
82
91,[e]
1:1
3i
3j
84
83
2m
67,[e]
4:1
10
1a
2j
23
1a
2n
3w
11
12
13
1a
1b
1b
2k
2a
2b
3k 83
24
25
26
1g
1a
1a
2a
2o
2p
3x
3y
3z
95
3l
85
91
3m 83
73[f]
[a] Reaction conditions: Pd(OAc)2 (5 mol%), alkene (1.3 mmol), haloalkyne (1.0 mmol), CH3CN, 358C, 8–12 h. [b] Yields of isolated products. Some
products contained 3–5% of an unidentified isomer that could not be removed by preparative TLC on silica gel or by column chromatography (see the
Supporting Information). [c] The ratio of regioisomers was estimated by 1H NMR analysis or GC-MS of the crude product. [d] exo/endo=1:3. [e] Minor
regioisomers were 2-bromo-3-alkynyl-norbornane derivatives. [f] The reaction was carried out with 3 equivalents of the alkene.
Compound 7a is difficult to obtain through the direct
cycloaddition between a diyne and a cycloalkene (Sche-
me 7b). The molecular structure of complex 7a was estab-
lished by X-ray crystallography (see the Supporting Informa-
tion).
interesting is that the cleavage of chemical bonds also
depends on the cyclic alkenes. Although this distinct reac-
tivity has not been thoroughly understood, it seems that the
ring structure or ring constraints result in the formation of
different products.
As shown in Scheme 8 for the reaction of bromoalkynes,
the structures of the products depend on the employed cyclic
alkenes. Norbornenes, which have strained structures,
afforded noncrowded products, whereas the flexible cyclo-
octene led to strained four-membered-ring products. More
As the bromoalkynylation process involves several bond-
forming and -migrating steps, it is currently difficult to make
strong mechanistic implications. A pathway that involves a
“bridging” palladium center seems most likely based on
the stereoselective formation of cis-norbornyl products
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Communications
Scheme 9. Proposed mechanism for the bromoalkynylation reaction.
complex to the haloalkyne to form a high-valent alkyne–
palladium species. The cis-alkynyl palladium intermediate A
is then formed by the addition of the alkyne–palladium
intermediate to the alkene. Subsequently, the “bridging”
palladium complex B is generated, and the palladium is
delivered to the bridgehead carbon on the same side as the
incoming alkyne. Thus, the alkyl palladium halide intermedi-
ate C is formed with high stereoselectivity. A subsequent
reductive elimination of C generates brominated product 3
and the active catalyst species.[12]
Scheme 6. Cycloaddition of haloalkynes to cyclooctene. Reaction con-
ditions: Pd(OAc)2 (5 mol%), 2 (1.0 mmol), 4 (1.3 mmol), CH3CN,
358C, 12–15 h. Yields of isolated products are reported. [a] GC-MS and
NMR reveals the formation of Alder–ene products as inseparable
mixtures.[9e] p-Am=n-pentyl.
In conclusion, we have successfully developed a ring-
structure-dependent synthesis of 7-alkynyl norbornanes and
cyclobutenyl halides that occurs by the
palladium-catalyzed bromoalkynylation of
À
C C double bonds. The reaction conditions
are extremely mild and tolerate various
functional groups. These novel processes
not only represent the first examples of
bromoalkynylation reactions of alkenes, but
also afford 7-alkynyl norbornane products
and cyclobutenyl bromides selectively with
excellent atom economy. Current efforts are
aimed at further elucidating the detailed
Scheme 7. Heck and Sonogashira cross-coupling reactions. dppp=1,3-bis(diphenylphosphi-
no)propane.
reaction mechanism and applying these
novel methods to the synthesis of complex
and highly-functionalized molecules.
Experimental Section
Typical procedure for the reaction of phenylethynyl bromide and
norbornene (Table 1, 3a): Pd(OAc)2 (12 mg, 0.05 mmol), CH3CN
(2 mL), NBE (122 mg, 1.3 mmol), and phenylethynyl bromide
(180 mg, 1 mmol) were added successively to a Schlenk tube. After
stirring for 10 h at 358C, the solution was filtered though a small
amount of silica gel. The residue was purified by preparative TLC on
silica gel (n-hexane) to furnish 3a (260 mg, 95%) as a pale-yellow oil.
IR (KBr): nmax = 3051, 2965, 2874, 2226, 1597, 1489, 1444, 983, 757,
Scheme 8. Products from reactions of bromoalkynes with cyclooctene
(flexible) and norbornene (strained).
(Scheme 9). At least two mechanisms involving Pd0/PdII and
PdII/PdIV can be envisaged.[10,11] We believe that this reaction
is initialized by an oxidative addition of the Pd0 or PdII
1
692 cmÀ1; H NMR (CDCl3, 400 MHz): d = 7.44–7.47 (m, 2H), 7.25–
7.28 (m, 3H), 3.96 (q, J = 4.8 Hz, 1H), 2.73(d, J=4.4 Hz, 1H), 2.62–
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Angew. Chem. Int. Ed. 2011, 50, 6341 –6345

2.66 (m, 2H), 2.47 (t, J = 4.0 Hz, 1H), 2.18 (q, J = 8.0 Hz, 1H), 1.58–
1.68 (m, 2H), 1.15–1.28 ppm (m, 2H); 13C NMR (CDCl3, 100 MHz):
d = 131.2, 131.2, 128.0, 128.0, 127.5, 123.9, 89.2, 84.4, 50.4, 49.7, 43.9,
42.7, 39.9, 29.4, 26.8 ppm; HRMS EI (m/z): calcd for C15H15Br,
274.0357; found, 274.0351.
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Received: January 1, 2011
Revised: April 21, 2011
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Published online: May 31, 2011
Keywords: bromoalkynylation · cycloaddition · norbornanes ·
.
palladium · rearrangement
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