[2+1] Cycloadditions of terminal alkynes to norbornene derivatives catalyzed by palladium complexes with phosphinous acid ligands

Article abstract of DOI:10.1002/anie.200500879

(Chemical Equation Presented) Two plus one makes three-membered rings in an unusual [2+1] cycloaddition of phenylethyne with norbornadiene as catalyzed by a new palladium(II) complex with phosphinous acid ligands (see scheme). A related complex (cyclohexy

Full text of DOI:10.1002/anie.200500879

Angewandte
Chemie
Palladium-Catalyzed Chemistry
[2+1] Cycloadditions of Terminal Alkynes to
Norbornene Derivatives Catalyzed by Palladium
Complexes with Phosphinous Acid Ligands**
Julie Bigeault, Laurent Giordano, and GØrard Buono*
Secondary phosphine oxides (SPOs) have been widely used
for the synthesis of tertiary phosphine oxides and have found
applications as Wittig–Horner reagents[1–2] and, later, as
effective ligands for transition-metal complexes.[3] Recently,
Li et al. showed that SPOs form air-stable palladium com-
plexes, such as POPd1, when they are
mixed with PdCl2(MeCN)2 and then
treated with Et3N.[4] These complexes
proved efficient as catalysts in sev-
eral cross-coupling reactions[4–5] as
well as in asymmetric allylic alkyla-
tions.[6] Recent reports showed also
that these new ligands are suitable
for other types of catalyzed reactions such as the hydrolysis of
nitriles[7] and the asymmetric hydrogenation of imines[8] and
alkenes.[9] Our continued interest in the metal-catalyzed
cycloaddition reactions between alkynes and norborna-
diene[10] prompted us to investigate the catalytic behavior of
palladium(ii) complexes coordinated by SPOs.
First, we developed an easier way to synthesize the
palladium catalyst 1. Upon treatment of Pd(OAc)2 with tert-
butyl(phenyl)phosphane oxide (L1), dihydrogen di-m-
acetatotetrakis(tert-butylphenylphosphinito-k-P)dipalladate
(1) was quantitatively obtained without further treatment
(Scheme 1).[11] Second, as a model we examined the reaction
of phenylethyne (3a) with norbornadiene (2) in the presence
of 2.5 mol% of 1 in toluene at 508C for 24 h. Unexpectedly,
the palladium(ii) complex 1 coordinated by L1 favored the
formation of benzylidenecyclopropane (4a) as a single
diastereomer in 17% yield (Scheme 2) and contaminated by
an unidentified byproduct (5%). Surprisingly, a similar
reaction using the known chloro-bridged analogue 5[6a–12] as
catalyst did not work. Furthermore, in the reaction catalyzed
by 5, the addition of 10 mol% of AgOAc (4 equiv relative to
[*] J. Bigeault, Dr. L. Giordano, Prof. Dr. G. Buono
Laboratoire de Synth›se AsymØtrique
UMR 6180 CNRS “Chirotechnologies: Catalyse et Biocatalyse”
Case A62, UniversitØd’Aix-Marseille P. CØzanne
FacultØdes Sciences et Techniques de St JØrôme, EGIM
Avenue Escadrille Normandie Niemen
13397 Marseille Cedex 20 (France)
Fax: (+33)4-9128-2742
E-mail: gerard.buono@univ.u-3mrs.fr
[**] The authors thank the CNRS for financial support. J.B. thanks
MENRT for a doctoral fellowship. We also thank Dr. N. Vanthuine for
kind assistance with chiral HPLC separation of L1 and L2, Dr. M.
Giorgi for X-ray analyses, and Dr. L. Charles for ESI-MS measure-
ments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 4753 –4757
DOI: 10.1002/anie.200500879
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Table 1: Benzylidenecyclopropanation of 2 with 3a.
Entry[a]
L
T [8C]
Yield [%][b]
1
2
3
4
5
6
L1
L1
L2
L2
L3
L3
25
50
50
25
25
50
trace
42
76
80
21
Scheme 1. Synthesis of the new air-stable palladium(ii) complex 1 with
phosphinous acid ligands.
58
[a] All reactions were carried out with 2/3a/Pd(OAc)2/L in a ratio of
2:1:0.05:0.1 for 24 h. [b] Yield of isolated product. Cy=cyclohexyl.
Having established the feasibility of the cycloaddition
reaction, we tested various terminal alkynes 3a–k to extend
its applicability. Under optimized conditions, all reactions
proceeded cleanly and various functional groups such as
ethers, esters, alcohols, sulfones, or tertiary amines present in
the alkyne were tolerated (Table 2). Propargylic alcohols 3 f
and 3g, sulfone 3i, and tertiary amine 3j also reacted with 2 to
afford 4 f, 4g, 4i, and 4j, respectively, in 57–75% yields
Scheme 2. Palladium(ii) complexes 1 or 5 catalyze an unusual
[2+1] cycloaddition of 2 with 3a. Conditions: 2/3a/1=2:1:0.025;
2/3a/5/AgOAc=2:1:0.025:0.1.
Table 2: Palladium-catalyzed alkylidenecyclopropanation of 2 with terminal alkynes 3.
catalyst 5) led to the formation of
4a in 15% yield.[13] Even if few
examples of ruthenium-[14] or pal-
ladium-catalyzed[15] cyclopropana-
tions of norbornene derivatives
with alkynes are known, a catalytic
process that is able to favor the
direct formation of alkylidenecy-
clopropanes as illustrated in
Scheme 2has, to our knowledge,
not been reported. Herein, we
show that palladium(ii) complexes
stabilized by secondary phosphine
oxides are active catalysts for an
unusual [2+1] cycloaddition of
norbornene derivatives with termi-
nal alkynes to produce various
alkylidenecyclopropanes.
Entry[a]
Alkyne
R
T [8C]
t [h]
Cycloadduct
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
3a
3b
3c
3d
3e
3 f
3 f
3g
3h
3i
Ph
nBu
25
50
50
25
25
25
50
50
50
25
25
50
25
20
20
48
50
50
50
48
36
50
48
48
36
60
4a
4b
4c
4d
4e
4 f
4 f
4g
4h
4i
80
9
–
CH2TMS
CH2OBn
CH2OAc
CH2OH
CH2OH
C(Me)2OH
CH2CH(CO2Me)2
CH2SO2Ph
CH2Mp
70
73
34
57
75
52
66
34
60
56
3j
3j
3k
4j
4j
6k
CH2Mp
CO2Me
In
a preliminary study, we
[a] Experiments were performed on a 1-mmol scale using 5 mol% of Pd(OAc)2 and 10 mol% of L2 (2/3/
Pd/L2=2:1:0.05:0.1). [b] Yield of isolated product. TMS=trimethylsilyl; Bn=benzyl; Mp=morpho-
linyl (C4H8NO).
found that the addition of
5 mol% of acetic acid was benefi-
cial to the reaction and led to the
formation of 4a as the exclusive
product in 26% yield. This finding revealed that acetate plays
an important role in the reaction. Indeed, AcOH was released
in the reaction medium during the formation of catalyst 1.
This observation prompted us to test the formation of
catalysts in situ. Two other SPOs, L2 and L3, were tested
under similar conditions by generating the catalyst in situ
(Table 1). The best result was observed when Pd(OAc)2 was
associated with L2 (1:2molar ratio), affording 4a in
satisfactory yield at room temperature (entry 4).[16–17] Sim-
ilarly, palladium catalyst systems generated with L1[18] or L3
proved active but less efficient (entries 2and 6).
(Table 2, entries 7, 8, 10, and 12). For an unknown reason, no
product was obtained with propargylsilane 3c (entry 3).[19]
Likewise, with an unfunctionalized alkyne such as 3b, the
reaction was slow and conversion was very low (Table 2,
entry 2). Surprisingly, an acetate group in a propargylic
position such as in 3e afforded the desired cyloadduct 4e
without formation of byproducts (entry 5).[20] A notable
difference in reactivity was observed for tertiary acetates
such as 3l and 3m. In this case, known allenylidenecyclopro-
panes[21] 7l and 7m were obtained in 27% and 64% yield,
respectively (Scheme 3).
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norbornenes gave the products in fair yields (entries 4 and 6).
For cyclopentadiene dimer 11, the cyclopropanation occurred
exclusively at the most strained double bond to yield the
cycloadduct 16 in 51% as a 1:1 mixture of diastereomers
(Table 3, entry 5). Note the exclusive formation of cyclo-
propane 15 from 10 (entry 4). Indeed, diacetate 10 has been
previously used in palladium(0)-catalyzed elimination[25] and
alkylation[26] reactions and proceeds via an intermediate p-
allyl complex.
Scheme 3. Synthesis of allenylidenecyclopropanes 7l and 7m from
tertiary acetates. Conditions: 2/3/Pd/L2=2:1:0.05:0.1.
Although the mechanism of this unusual cyclopropana-
tion remains unclear at the moment, we assume that the
reaction may involve palladium vinylidene species as key
intermediates in a catalytic process that favors the formation
of [2+1] cycloadducts over dimerization[27] products of
alkynes (Scheme 5). Palladium vinylidene complex B may
be generated from 1-alkyne 3,[28,29] and B could allow a
[2+2] cycloaddition with the double bond of the norbornene.
The resulting 2-alkylidene palladacyclobutane (C) would
release cycloadduct 4 after reductive elimination. The for-
mation of this unprecedented Pd vinylidene complex is
supported by results from deuterium-labeling experiments.
Starting from monodeuterophenylacetylene [D1]3a and 2,
With the electron-deficient alkyne 3k (Table 2, entry 13),
the expected cycloadduct 4k and the rearranged (valence
isomerization) product 6k were observed in 1:1 ratio in the
crude reaction mixture. After purification by chromatography
on silica gel, only 6k was isolated in 56% yield. Valence
isomerization proved to be effective on cycloadducts 4a, 4h,
and 4i under thermal conditions in the absence of solvent:
heating these cycloadducts to 1808C at 1 mmHg resulted in
clean and complete conversion after 1 h (Scheme 4).[22–23]
Scheme 4. Thermal rearrangements (valence isomerization)
of 4.
The cycloaddition reaction was extended to other
norbornene derivatives by using phenylethyne (3a) as a
partner (Table 3).[24] The cyclopropanation of alkenes 8–
12 afforded expected benzylidenecyclopropanes 13–17
in moderate to good yields. The best yields were
observed for norbornene (8) or benzonorbornene (9;
entries 2and 3). The cycloaddition with functionalized
Scheme 5. A possible pathway for the palladium-catalyzed [2+1] cycloaddition of
norbornadienes and alkynes. (D) denotes the proton exchanged for deuterium in
labeling experiments.
under similar conditions the reaction led to compound [D1]4a
in 88% yield with 80% incorporation of deuterium on the
external double bond (Scheme 5).[30] For the formation of
Table 3: Cyclopropanation of various norbornenes 8–12 with phenyl-
ethyne (3a).
Entry[a]
Alkene
T [8C] t [h]
Cycloadduct
Yield [%][b]
allenylidenecyclopropanes 7l and 7m, the reaction is thought
1
2
25
50
60
48
51
94
to proceed through a similar mechanism via a palladium
[31]
= = =
allenylidene intermediate (Pd C C CR2).
Finally, we briefly examined the interesting asymmetric
version of this cycloaddition by using chiral secondary
phosphine oxides (À)-L1 and (À)-L2 obtained by separation
with chiral HPLC.[32] Indeed, alkylidenecyclopropanes syn-
thesized from symetrical 2 showed geometrical enantio-
morphic isomerism.[33–34] Preliminary results obtained are
encouraging: an asymmetric induction of 59% ee was
achieved with (À)-L1 without optimization of the reaction
conditions (Table 4, entry 1).
In conclusion, we have shown that palladium(ii) com-
plexes stabilized by secondary phosphine oxide ligands
catalyze a very unique [2+1] cycloaddition of norbornene
derivatives with various terminal alkynes to afford function-
alized alkylidenecyclopropanes.[35–36] Moreover, our results
suggest a Pd vinylidene complex as a key intermediate. A
detailed investigation of the mechanism and the development
3
4
25
50
60
36
84
58
5
6
50
50
72
48
51
56
[a] Experiments were performed on a 1-mmol scale using 5 mol% of
Pd(OAc)2 and 10 mol% of L2. [b] Yield of isolated product.
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[9] X. Jiang, M. Van den Berg, A. J. Minnaard, B. L. Feringa, J. G.
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[11] With enantiopure L1, the complex 1 was clearly identified by
NMR spectroscopy. The acetato-bridged structure was sup-
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(see Supporting Information).
Table 4: Enantioselective benzylidenecyclopropanation of 2 with 3a
using a chiral secondary phosphine oxide ligand.
Entry[a]
L
T [8C]
t [h]
Yield [%][b]
ee [%][c]
1
2
(À)-L1
(À)-L2
50
25
60
20
53
80
59
22
[12] Complex 5 was prepared according to Liꢀs procedure[4] from L1.
As shown by 31P NMR spectra and X-ray crystallography, only
one diastereomeric complex was obtained which exhibits C2
symmetry and retention of configuration at phosphorus (see
Supporting Information). During the course of our work, Leung
and co-workers reported chiral bisphosphinite metalloligands
derived from a P-chiral secondary phosphine oxide: E. Y. Y.
Chan, Q. F. Zhang, Y.-K. Sau, S. M. F. Lo, H. H. Y. Sung, I. D.
Williams, R. K. Haynes, W.-H. Leung, Inorg. Chem. 2004, 43,
4921 – 4926.
[a] Experiments were performed using 5 mol% of Pd(OAc)2 and
10 mol% of ligand L. [b] Yield of isolated product. [c] ee values
determined on a Daicel Chiralcel OJ-H column at l=254 nm using
hexane/iPrOH 99:1 as eluent with a flow rate of 1 mLminÀ1; t1 =6.4 min,
t2 =7.1 min.
of asymmetric cycloaddition reactions are underway in our
laboratory.
[13] Cycloadduct 3a was also contaminated with 5% of an unidenti-
fied byproduct.
Received: March 9, 2005
Revised: April 28, 2005
Published online: June 27, 2005
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Keywords: alkynes · cycloaddition · cyclopropanes · palladium ·
.
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s2s + s2p mechanism, which is a symmetry-forbidden reaction.
4756
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They have therefore been generally interpreted to occur by
cleavage to a diradical followed by rapid intramolecular trapping
of the diradical by the p bond (see reference [21a]).
[23] The structure and stereochemistry of compounds 4i and 6i were
confirmed by X-ray analysis (see reference [36]).
[24] The same reaction carried out with cyclopentene in place of
norbornene derivatives did not work. No oligomers or self-
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À
oxidative addition of the C H bond to Pd.
[30] The position of the deuterium was based on the disappearance of
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[32] Pure enantiomers were obtained by semipreparative chiral
HPLC on chiralpak AD or AS (250 ” 10 mm2) columns using
hexane/ethanol (90:10) or ethanol as eluents. For L2, the
absolute configuration at phosphorus is currently unknown.
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[34] This type of chirality known under the name of “geometric
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[36] See Supporting Information for full synthetic details and
characterization of new compounds. CCDC 265344–265346 (4i,
6i, and 5) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
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