Palladium-catalyzed [2+1+1] annulation of norbornenes with (z)-bromostyrenes: Synthesis of bismethylenecyclobutanes via twofold C(sp2)-H bond activation

Article abstract of DOI:10.1039/c4cc06545j

The first Pd-catalyzed intermolecular [2+1+1] annulation reaction of (Z)-bromostyrene derivatives and norbornenes has been realized. Bismethylenecyclobutane derivatives were obtained with high yields. The domino coupling reaction involves a double Heck-type coupling process, twofold C(sp²)-H bond activation and formation of three carbon-carbon bonds.

Full text of DOI:10.1039/c4cc06545j

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ARTICLE TYPE
Palladium-Catalyzed [2+1+1] Annulation of Norbornenes with (Z)-
Bromostyrenes: Synthesis of Bismethylenecyclobutanes via Twofold
C(sp2)-H Bond Activation
Jiangang Maoab and Weiliang Bao*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
The first Pd-catalyzed intermolecular [2+1+1] annulation
de Meijere's work
reaction of (Z)-bromostyrene derivatives and norbornenes
has been realized. Bismethylenecyclobutane derivatives were
10 obtained with high yields. The domino coupling reaction
involves in double Heck-type coupling process, twofold
C(sp2)–H bond activation and three carbon-carbon bonds
formation.
H
H
H
Ar
R2
R3
R2
R3
Ar
Ar
R2
R3
H
[Pd]
+
R1
+
Br
H
Ar
5
6
1
2
our previous work
H
H
R2
R3
R2
R3
H
[Pd]
R1
R1
+
Br
1
2
4
Synthesis of small cycloalkanes such as cyclobutanes has
15 captivated organic chemists for several decades, for both
theoretical and practical purposes.1 Cyclobutanes represent an
important structural motif frequently found in biological active
natural products,2 pharmaceuticals,3 and organic synthesis.4 Its
synthesis has greatly attracted the interest of chemists and many
20 effective methodologies have been developed. The most
straightforward synthetic routes to cyclobutanes are
photochemically initiated [2+2] pericyclic reactions5 and other
ionic or free radical stepwise cycloaddition strategies.6 The ring
expansion of cyclopropylcarbinyl precursors,7 intramolecular 1,4ꢀ
25 cyclization of acyclic substrates,8 intermolecular [3+1]
cycloaddition9 and transition metal catalyzed CꢀH bond activation
were also used to construct cyclobutane derivatives.10 However,
to our knowledge, no [2+1+1] annulation reaction has been
reported yet.
This work, [2+1+1] Cycloaddition via twofold C(sp2)-H bond activation
R1
Br
R1
H
R2
R3
R2
R3
Pd(OAc)2/PPh3
1
Cs2CO3, toluene
110 oC, 12 h
R1
H
R1
Br
2
1
3
Scheme 1. Selective synthesis of small cycloalkanes.
50
To reach the best results, (Z)ꢀ1ꢀ(2ꢀbromovinyl)ꢀ3ꢀ
chlorobenzene 1f was initially employed to react with endoꢀNꢀ(pꢀ
tolyl)norbornenesuccinimide 2a using palladium catalyst system
under various conditions (Table 1). The desired
55 bismethenylcyclobutane derivative 3fa was obtained selectively
as the final products instead of compounds 4, 5 or 6 (Scheme 1).
Then, the reaction conditions were screened to achieve the best
selectivity and yield of bismethenylcyclobutane 3fa. The results
were summarized in Table 1.
30
Prospect of devising catalytic processes to convert selectively
simple substrates into two or more products in a controlled
fashion is always a fascinating challenge to organic chemists,[11]
due to its fundamental scientific interest and potential usage in
organic synthesis. Selectivity (chemoꢀ, regioꢀ, stereoꢀ and
60
At the outset, metal catalysts were screened in the [2+1+1]
cycloaddition reactions. Pd(OAc)2 was better than PdCl2,
Pd(PPh3)4 and Pd(dppf)Cl2 (Table 1, entries 1ꢀ4, 91% yield
compared with 66%ꢀ84%). The effect of ligand was then
investigated, and PPh3 was found to be the optimal, affording 3fa
65 in 91% yield. Other screened ligands (dppf, dppe, TMOPP,
DCHPP and TCHP) could also promote the reaction with
decreased yields, possibly due to either steric or electronic effects
(65%ꢀ89%, entries 5ꢀ9). Varying the types and load of the bases
(K2CO3, KOAc, CsOAc, tBuOK and Cs2CO3) influenced the
70 outcome considerably (Table 1, entries 10ꢀ17). The basicity had a
significant effect on reaction selectivity and product yield. The
usage of tBuOK yielded bismethylenecyclobutane 3fa (79%,
entry 17), while K2CO3, KOAc and CsOAc gave
methylenecyclopropane derivatives 4fa in good to high yields
35 enantioꢀ) strategy as a versatile handle can afford diverse and
industrially important chemicals.11,12 de Meijere and coꢀworkers
reported a domino coupling of bromostyrene derivatives with
norbornenes in 1994, which gave a mixture of compounds 5 and
6, or produce 6 (26% yield) as exclusive product by changing
40 reaction conditions, but only one example13 (Scheme 1). Recently
we found that Pd(0)ꢀcatalyzed C−H bond activation reactions of
bromostyrene derivatives and benzyl bromides with norbornenes
could provide cyclopropane derivatives 4 via [2+1] cycloaddition
processes (Scheme 1).14 In the experiment we found the synthesis
45 of cyclobutanes 3 via [2+1+1] annulation of (Z)ꢀbromostyrene
derivatives with norbornenes was possible if suitable reaction
conditions were sought.
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Table 1. Optimization of reaction conditions and chemoselectiviꢀ
Scheme 2. Substrate scope of Pd(0)ꢀcatalyzed bismethylenecyꢀ
ty under various conditions.a
clobutanation.a
3fa or 4fa
Entry [Pd]
Ligand
Base(mmol)
Solvent
Yield [%]b
1
2
3
4
5
6
7
8
PdCl2
Pd(PPh3)4
Pd(OAc)2
PPh3
PPh3
PPh3
Cs2CO3
Cs2CO3
Cs2CO3(1.5)
Cs2CO3
Cs2CO3
toluene 3fa (84)
toluene 3fa (82)
toluene 3fa (91)
toluene 3fa (66)
toluene 3fa (67)
toluene 3fa (63)
toluene 3fa (86)
toluene 3fa (89)
toluene 3fa (65)
toluene 4fa (86)
toluene 4fa (72)
toluene 4fa (80)
toluene 4fa (77)
toluene 4fa (81)
toluene 4fa (84)
toluene 3fa (31)/
4fa (56)
Pd(dppf)Cl2 PPh3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
dppe
dppf
TMOPP Cs2CO3
DCHPP Cs2CO3
TCHP
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
Cs2CO3
9
Cs2CO3
10
11
12
13
14
15
16c
K2CO3(1.5)
KOAc(3.0)
CsOAc(1.0)
CsOAc(2.0)
CsOAc(3.0)
Cs2CO3(0.5)
Cs2CO3(1.0)
17
18
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PPh3
PPh3
PPh3
tBuOK(1.5)
Cs2CO3
Cs2CO3
toluene 3fa (79)
dioxane 3fa (78)d
DEDM 3fa (81)
19
a
Reaction conditions unless otherwise noted: 1a (0.55 mmol), 2a (0.5
5
mmol)，catalyst (0.05 mmol), ligand (0.11 mmol), base (1.5 mmol),
solvent (2.0 mL), 110 °C, 12 h in sealed tube. b Yield of isolated product.
c The ratio of 3 and 4 was determined by 1H NMR analysis. d 100 oC. dppe
= 1,2ꢀBis(diphenyl phosphino)ethane. dppf = 1,1'ꢀBis(diphenylphosphino)
ferrocene. TMOPP
= tris(4ꢀmethoxylphenyl)phosphine. DCHPP =
10 dicyclohexylphenyl phosphine. TCHP = tricyclohexyl phosphine. DEDM
= diethylene glycol dimethyl ether.
(72%ꢀ86%, entries 10ꢀ14). Compared with tBuOK, Cs2CO3 (3.0
equiv) was the best in promoting this reaction (91%, Table 1,
15 entry 3). The varying load of Cs2CO3 (0.5mmol, 1.0 mmol and
1.5mmol) controlled successfully the chemoselectivity of the
domino cycloaddition processes, and thus yielded exclusively
[2+1] or [2+1+1] cycloaddition products (entries 3, 15ꢀ16).
However, intrinsic detailed mechanism of the selectivity is still
20 quite dubious at the moment.13,15 Next, the effect of solvent was
also examined. The reaction in toluene gave the best result,
affording 3fa in 91% yield (entry 3). Other polar aprotic solvents
(dioxane, DEDM) also gave good yields (Table 1, entries 18ꢀ19).
Under the optimized reaction conditions with the palladium (0)
25 catalyst, formed in situ from Pd(OAc)2 and PPh3, various (Z)ꢀ
bromostyrene derivatives were tested to investigate the generality
of the [2+1+1] cycloaddition reaction, as shown in Scheme 2. (Z)ꢀ
2ꢀbromovinylarene substrates bearing electronꢀwithdrawing
group (3ꢀCl, 4ꢀCl, 4ꢀF) on the benzene ring led to the
30 corresponding bismethenylcyclobutane derivatives 3 in excellent
yields (Scheme 2, 3ea-3ga, 3eb-3gb, 3ec), while substrates
bearing electronꢀdonating (3ꢀMe, 4ꢀMe, 4ꢀMeO) group on the
Reaction conditions unless otherwise noted: 1a (1.1 mmol), 2a (0.5
40 mmol)，catalyst (0.05 mmol), ligand (0.11 mmol), base (1.5 mmol),
a
solvent (2.0 mL), 110 °C, 12 h in sealed tube. The yield is that of the
isolated product. b 1a (1.0 mmol), 2a (0.6 mmol).
yields may root in electronic effect of the substituents. The paraꢀ
45 substituted vinylarenes produced annulation products 3ba, 3ea,
3eb and 3bc in higher yield (83%, 94%, 90% and 70% yield) due
to much smaller steric hindrance compared with the
corresponding metaꢀones (3ca, 3fa, 3fb and 3cc; 77%, 91%, 83%
and 67% yield). Orthoꢀsubstituted products can not be isolated
50 because of its strong steric hindrance and the corresponding low
yield. We extended the method to (E)ꢀ(2ꢀbromovinyl)benzene
benzene
core
also
afforded
the
desired
and
(E)ꢀ1ꢀ(2ꢀbromovinyl)ꢀ4ꢀfluorobenzene
to
prepare
bismethenylcyclobutanation products 3 in good to high yields
35 (Scheme 2, 3ba-3da, 3bb, 3bc, 3cc, 3bd, 3cd). The difference of
bismethenylcyclobutane compounds under the same reaction
2
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DOI: 10.1039/C4CC06545J
condition. Unfortunately, no desired corresponding product was
obtained. Maybe the aromatic ring of (Z)ꢀ2ꢀbromovinylarene
stabilized the intermediate of norbornenyl palladium via
favorable remote coordination of the aromatic π system with the
Palladium center.14a,16 However, the spatial configuration of (E)ꢀ
2ꢀbromovinylarenes is difficult to exert the same stabilizing
function. Heterocyclic and aliphatic bromostyrene derivatives
5
such
as
(Z)ꢀ3ꢀ(2ꢀbromovinyl)pyridine
and
(Z)ꢀ(2ꢀ
bromovinyl)cyclohexane were also employed to examine the
10 scope of substrates, but only very low yield products were
discovered and couldn’t be isolated.
The optimized [2+1+1] cycloaddition procedure was
subsequently applied to a range of norbornene derivatives. Endoꢀ
Nꢀ(pꢀtolyl)norbornenesuccinimide 2a and endoꢀNꢀ(isobutyl)
15 norbornenesuccinimide 2b underwent the bismethenylcyclobutꢀ
anation reaction smoothly with (Z)ꢀbromostyrene derivatives to
provide the corresponding products 3aa−3ga, 3bb and 3ebꢀ3fb in
good to excellent yields. Dicyclopentadiene 2d also yielded
desired annulation products 3ac, 3bc, 3cc and 3ec in 78%, 74%,
20 69% and 80% yields respectively. Only one of the carbonꢀcarbon
double bonds of dicyclopentadiene was selectively cyclized by
(Z)ꢀbromostyrene derivatives. Norbornene itself 2d showed lower
reactivity and gave the corresponding products in moderate yields
(3ad, 3bd and 3cd; 73%, 70% and 65%). For norbornene
25 substrates, the electronic effect influenced the yields more than its
steric hindrance. Nonꢀrigid alkene, such as cyclohexene, was also
tested to carry out the [2+1+1] cycloaddition procedure, however,
the reaction did not proceed smoothly. The structure of the
product was further confirmed unambiguously by an Xꢀray
30 crystallographic analysis of a single crystal of 3ba (see the
Scheme 3. Proposed reaction mechanism for the Pd(0)ꢀcatalyzed
bismethenylcyclobutanation.
60
In summary, a new methodology for the synthesis of
cyclobutane is developed. The Pdꢀcatalyzed selective [2+1+1]
annulation of (Z)ꢀbromostyrene derivatives with norbornene
derivatives is realized under extremely concise reaction condition.
Highly functionalized bismethenylcyclobutane deirvatives were
65 obtained selectively in good to excellent yields in the presence of
Cs2CO3. The annulation is an oneꢀpot, mutipleꢀstep domino
reaction, which involves a double Heckꢀtype coupling, twofold
C(sp2)–H bond activation and three carbonꢀcarbon bonds
formation. The reaction is interesting both for theoretical and
70 organic synthetic chemistry.
Supporting
Information,
Figure
S1).
The
formed
bismethenylcyclobutane moiety took the exoꢀface of norbomene.
The stereochemistry of all the compounds is the same as that of
3ba based on the analysis of the chemical shifts and coupling
35 constants (J) of their NMR.14 In addition, only one diastereomer
was isolated in all cases.
We gratefully acknowledge the Natural Science Foundation of
China (No. 21072168) for financial support of this research.
Notes and references
a Department of Chemistry, Zhejiang University (Xixi Campus),
75 Hangzhou 310028, People’s Republic of China. Fax: +86ꢀ571ꢀ
88273814; Tel: +86ꢀ571ꢀ88273814; Eꢀmail: wlbao@zju.edu.cn
b School of Metallurgy and Chemical Engineering, Jiangxi University of
Science and Technology, 86 Hongqi Avenue, Ganzhou, 341000, Jiangxi,
China
80 † Electronic Supplementary Information (ESI) available: Experimental
procedures, NMR spectra and characterizations for all new compounds.
See DOI: 10.1039/b000000x/
Based on the current experiment results and related
precedents,14,17 a postulated mechanism was proposed (Scheme 3).
The oxidative addition of palladium (0) species to (Z)ꢀ
40 bromostyrene derivative 1 produced a (Z)ꢀstyrylpalladium (II)
bromide II. Subsequently the syn addition of II to norbornene 2
generated norbornenylpalladium complexe III at the exoꢀface of
2, which underwent a selective C(sp2)−H bond activation on the
(Z)ꢀbromostyrene derivative, rather than C(sp2)−H bond on
1
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Greenberg, J. F. Liebman), Academic Press: New York, 1978; (b) E.
L. Ruff in Methods of Organic Chemistry (HoubenꢀWeyl).
Carbocyclic FourꢀMembered Ring Compounds (Eds.: A. de.
Meijere), Thieme Verlag, Stuttgart, 1997, Vol. E17e, pp 118–140; (c)
The chemistry of cyclobutanes (Eds.: Z. Rappoport, J. F. Liebman),
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Tantillo, Chem. Soc. Rev., 2014, 43, 5042.
45 benzene ring and gave
a
regioselective fourꢀmembered
palladacycle IV in the presence of Cs2CO3. The selectivity might
result from the energy advantage of intermediate IV,14 which
could undergo oxidative addition to another equivalent of 1 again
and generated the intermediate V with octahedrally coordinated
50 palIadium(IV).17a,18 The selective reductive elimination of the
intermediate V produced another (Z)ꢀstyrylpalladium (II) bromide
intermediate VI,19 which underwent once again C(sp2)−H bond
activation and afforded palladacyclopentadiene complex VII in
the presence of Cs2CO3. Following reductive elimination of the
55 intermediate VII, the desired bismethenylcyclobutane compound
3 was produced and the Pd(0) I was regenerated.
85
90
2
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