Olefin cyclopropanation catalyzed by new diiminophosphorane and triiminophosphorane complexes of copper and palladium

Article abstract of DOI:10.1016/j.tet.2005.07.044

Chelating diiminophosphorane and tripodal iminophosphorane copper and palladium complexes are found to efficiently catalyze the cyclopropanation of activated monosubstituted olefins with ethyl diazoacetate. Cycloolefins, and linear α-olefins are somewhat

Full text of DOI:10.1016/j.tet.2005.07.044

Tetrahedron 61 (2005) 9025–9030
Olefin cyclopropanation catalyzed by new diiminophosphorane
and triiminophosphorane complexes of copper and palladium
*
Laurence Beaufort, Albert Demonceau and Alfred F. Noels
` `
Laboratory of Macromolecular Chemistry and Organic Catalysis (CERM), University of Liege, Sart-Tilman (B6a), 4000 Liege, Belgium
Received 7 June 2005; revised 14 July 2005; accepted 14 July 2005
Available online 3 August 2005
Abstract—Chelating diiminophosphorane and tripodal iminophosphorane copper and palladium complexes are found to efficiently catalyze
the cyclopropanation of activated monosubstituted olefins with ethyl diazoacetate. Cycloolefins, and linear a-olefins are somewhat less
reactive. The diastereoselectivities of the reactions are moderate and no major differences were seen when comparing the bidentate chelating
ligand to the tripodal ligands.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
nation, more bulky ester groups favoring the formation of
the trans product. Recent developments have highlighted the
use of nitrogen ligands in homogeneous catalysis, including
enantioselective cyclopropanation catalyzed by transition
metals.6,7 This resulted in the emergence of copper
complexes containing a large variety of ligands8 that
include inter alia Schiff bases,9,10 oxazolines,11,12 semi-
corrin,13 and tris(pyrazolyl)14,15 ligands for the cyclo-
propanation reaction.
Organic chemists have always been fascinated by cyclo-
propanes because the smallest cycloalkane is present as a
basic structural element in a great variety of natural and
synthetic components of notable interest.1,2 Cyclopropanes,
besides their utility as such, can also be used as synthons in
the molecular construction. They are versatile intermediates
and can be converted into a variety of useful products by
cleavage of the strained three-membered ring. This has led
to the development of many methods for cyclopropanation
reactions, including the Simmons–Smith reaction3,4 and the
now quite popular transition metal-catalyzed reactions of
diazo compounds with olefins (Scheme 1).2,5
The present article reports the first results of a work aimed at
assessing the potential utility of a series of transition metal
complexes based on chelating diiminophosphorane ligands
and on triiminophosphorane ligands (their tripodal
analogues) as catalysts for the cyclopropanation of olefins
with ethyl diazoacetate (EDA) as the carbene source
(Scheme 1). Iminophosphoranes are predominantly two
electron s-donors with only minor p-acceptor properties.
They feature a short P–N bond and act as neutral
monodentate ligands via the lone pair at the nitrogen centre.
Further, iminophosphoranes where the carbon backbone
bridges the two or three nitrogen atoms (such as those
reported in this study) are expected to be sterically
demanding ligands since their bulky phosphino substituents
Control of the stereochemistry of this reaction is an
important objective in organic synthesis and to date, the
need for cheap, readily available, and highly diastereo-
selective cyclopropanation catalysts remains, however,
largely unmet and the trans to cis (or syn to anti) selectivities
remain usually modest with most Cu-based catalysts. This
results from the fact that the stereocontrol of the
intermolecular cyclopropanation reaction is most often
controlled by the particular olefin-diazocompound combi-
Scheme 1.
Keywords: Cyclopropanation; Alkenes; Diazoester; Iminophosphorane; Tripodal ligand.
*
Corresponding author. Tel.: C32 4 3663463; fax: C32 4 3663497; e-mail: af.noels@ulg.ac.be
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.07.044

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L. Beaufort et al. / Tetrahedron 61 (2005) 9025–9030
are close to the metal centre. The triiminophosphorane
ligands utilized in this work constitute a new family of
tripodal ligands, the synthesis of which will be reported
elsewhere (Scheme 2).26
diamine in the presence of triethylamine.16 The correspond-
ing air stable metallic complexes 5–8 were formed by
reacting the four different ligands 1–4 with the appropriate
metallic salt, respectively, CuOTf, Cu(OTf)2 (OTfZ
trifluoromethanesulfonate anion) or PdCl2 in dichloro-
methane (Scheme 3).
Scheme 2.
The iminophosphorane ligand being quite flexible, the
symmetry induced by the ligand seems to strongly depend
on the coordination sphere of the metal.16 The diimino-
phosphorane ligands formally form 16 electron complexes
with Cu(I)- and Pd(II) whereas tripodal iminophosphoranes
are expected to form 18 electron complexes provided
ligation to all three nitrogen atoms takes place. Because
these ligands can act in different coordination modes, the
different occupancy of the metal coordination sphere might
thus, bring about different geometry and have an influence
on the catalytic behavior of the metal complexes. Indeed, if
complexes of four-coordinate copper are most often
tetrahedral, they can also be square-planar whereas six-
coordinate copper complexes are octahedrally disposed.
The coordination chemistry of iminophosphoranes and of
imino-aza-P(V) ligands has recently been reviewed.17 To
date, however, there is only one report in the literature on
the use of diiminophosphorane ligands in olefin
cyclopropanation.16
Scheme 3.
Screening cyclopropanation experiments with the
complexes were performed under typical reaction con-
ditions as already reported,18 using a syringe pump for
diazoester addition (EDA) and with styrene or cyclooctene
as model substrates for activated and non-activated olefins,
respectively. In order to minimize carbene coupling, a large
excess of styrene has been used; the molar ratio
Cu:EDA:styrene being 1:100:1000 and its addition occurs
very slowly by means of a drip. The tests have been realized
at two different temperatures, 60 8C and rt and the reaction
products (cis and trans cyclopropanes, diethyl maleate, and
fumarate) identified by comparison with authentic samples
and quantified by VPC. Table 1 summarizes the results
obtained with styrene.
It follows that, as usual, various parameters that include the
catalyst structure, the substrate and the reaction temperature
affect to some extent the rates and the yields of the reactions,
as well as their diastereoselectivities (cis/trans or exo/endo
ratio).
2. Results and discussion
2.1. Diiminophosphorane complexes of Cu and Pd
The diiminophosphorane ligands 1–4 were prepared as
previously reported by reacting a dibromotriaryl-
phosphorane (Ar3PBr2, prepared in situ) with o-phenylene-
As a rule, the cyclopropanation reactions take place with
Table 1. Styrene cyclopropanation with Ph(N]PR3)2–Cu and Pd catalysts
Cyclopropanationb
cis/trans
Dimerisation
Maleate/fumarate
a
T (8C)
Ligand PR3
Metal
Yield (%)
Yield (%)
60 8C
PCp3
Cu(II)
Cu(I)
Pd(II)
Cu(II)
Cu(I)
Cu(II)
Cu(I)
Pd(II)
Cu(II)
Cu(I)
Cu(II)
Pd(II)
Cu(II)
Cu(II)
Pd(II)
Cu(II)
Cu(I)
99
95
93
95
98
96
99
98
98
87
79
82
90
89
74
90
88
0.65
0.63
0.50
0.61
0.66
0.54
0.45
0.55
0.70
0.73
0.92
0.46
0.31
0.42
0.62
0.62
0.85
0.5
5
7
5
2
4
0.5
2
0.94
1.49
0.15
1.30
1.22
0.72
0.27
0.45
0.63
0.71
1.01
0.13
1.29
3.89
0.67
1.07
1.72
Ph2Cy
PPh3
PCy3
PCp3
2
12
18
18
7
9
25
8
rtc
Ph2Cy
PPh3
PCy3
12
a Abbreviations: Cp, cyclopentyl; Cy, cyclohexyl.
b Experimental conditions: Cu complex, 0.01 mmol; styrene, 2 mL; ethyl diazoacetate (EDA), 1 mmol diluted by the olefin up to 1 mL; addition time, 3 h.
Yields based on EDA.
c rt, room temperature.

L. Beaufort et al. / Tetrahedron 61 (2005) 9025–9030
9027
high yields (O95%) at 60 8C, whatever the initial oxidation
state of copper. There is a general agreement that EDA
reduces Cu(II) to Cu(I) species and that in both cases, the
active form of copper is the C1 oxidation state.8,19 Only a
small amount of carbene dimers (ethyl maleate and ethyl
fumarate) is formed as by-products. On the whole, the
alteration of the phosphine part of the ligand does not much
influence the cyclopropanation yields, although it does
affect the diastereoselectivity of the reaction. This is mostly
noticeable with the copper(I) catalysts for which the ratio
cis/trans varies from 0.45 to 0.73 at 60 8C (i.e., about a 30–
40% fraction of cis isomers), depending on the selected
phosphine. Such values come within the range of results
obtained with the vast majority of Cu-based catalysts
reported up to now.9 Even if the stereoselectivity of the
cycloaddition remains modest, there seems to exist a direct
relationship between the steric bulk of the phosphine and the
relative amount of cis cyclopropane synthesized, which
increases following the series PPh3!PPh2CyzPCp3!
PCy3. No such trend is seen for carbene dimers.
duration of the reaction time needs again to be considerably
increased. The cyclopropanation yields fall from 90% for a
reaction performed at 60 8C to 60% at rt with the less
efficient Cu-based system. The simultaneous increase of
maleate and fumarate formation that is observed is
concomitant with a fall of the cyclopropanation yields.
Moreover, the issue of the cyclopropanation reaction in term
of selectivity is similar to that obtained at higher
temperature.
It appears from these preliminary results that the structural
features of the ligand used in the cyclooctene or styrene
cyclopropanation reactions do not modify the outcome of
the reactions to a large extent, neither in terms of
stereoselectivity nor of yield. On the other hand, replacing
copper for palladium, mostly brings about some modifi-
cation of the ratio of the isomers formed. In this context, the
diiminophosphoranes complexes of palladium even if they
are slightly more selective than the corresponding copper
triflate based complexes, are also slightly less efficient
cyclopropanation catalysts, especially with cyclooctene.
The reactions also take place at rt with the copper and
palladium complexes though the reaction time for diazo-
ester decomposition needs then to be lengthened, and it
takes around 20 h for the reaction to be complete instead of
3 h at 60 8C. The yields remain very good, although slightly
lower than those obtained at higher temperature.
2.2. Triiminophosphorane complexes of Cu and Pd
Recent advances in olefin cyclopropanation catalyzed by
metal complexes have highlighted the interest of tridentate
ligands for obtaining highly active and selective catalysts. It
is the case namely for complexes based on pyridine-
bisoxazoline (pybox) and on homoscorpionate ligands
(pyrazolylborates, Tp, see Scheme 5), some of which are
capable of inducing large enantiomeric excesses in the
conversion of alkenes into cyclopropanes while also
favoring cis-isomer formation.15,20,21 The necessity of
being able to promote reactions of highly enantioselective
and cis-selective cyclopropanation is illustrated inter alia by
the recent synthesis of a powerful inhibitor of the inverse
transcriptase (HIV-1).22
The same copper- and palladium-based catalysts have been
applied to the cyclopropanation of cyclooctene, a non
activated olefin (Scheme 4). The results of the cyclopropa-
nation reactions at two different temperatures are reported in
Table 2.
Scheme 4.
At 60 8C, good cyclopropanation yields are obtained with
both metal complexes, though slightly inferior to those
observed with styrene. With both metals, the exo-
cyclopropane is preferentially formed, especially with
Pd-based catalysts, the copper(II) complexes forming
relatively more endo-isomer. Here also all metal complexes
remain active for EDA decomposition at rt, although the
Scheme 5.
Table 2. Cyclooctene cyclopropanation with Ph(N]PR3)2–Cu and –Pd catalysts
T (8C)
60 8C
rtb
Liganda
Metal
Cyclopropanation
Dimerisation
Maleate/
fumarate
Time (h)
Yield (%)
endo/exo
Yield (%)
Ph(N]PCp3)2
Ph(N]PPh3)2
Ph(N]PCp3)2
Ph(N]PPh3)2
Cu(II)
Pd(II)
Cu(II)
Pd(II)
Cu(II)
Pd(II)
Cu(II)
92
83
89
89
73
68
60
0.41
0.16
0.50
0.19
0.39
0.20
0.47
7
1.36
0.38
1.41
0.94
1.46
0.38
1.60
5
5
4
48
22
22
22
17
10
11
27
32
39
a Experimental conditions: complex, 0.01 mmol; cyclooctene, 2 mL; ethyl diazoacetate, 1 mmol diluted by the olefin up to 1 mL; addition time, 3 h.
b rt, room temperature.

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L. Beaufort et al. / Tetrahedron 61 (2005) 9025–9030
Table 3. Styrene cyclopropanation with tripodal ligand-based catalysts
T (8C)
Liganda
Metal
Cyclopropanationb
cis/trans
Dimerisation
Maleate/fumarate
Yield (%)
Yield (%)
60
CH3–C–(CH2NPPh3)3
Cu(II)
Pd(II)
Cu(II)
Cu(II)
Pd(II)
Cu(II)
Cu(II)
Cu(II)
97
81
88
97
86
93
84
86
0.57
0.56
0.47
0.54
0.52
0.50
0.60
0.12
3
1.33
0.59
1.95
1.40
0.73
1.32
1.76
0.90
19
12
3
14
7
CH3–C–(CH2NPCp3)3
Ph–C–(CH2NPPh3)3
Ph–C–(CH2NPCp3)3
CH3–C–(CH2NPPh3)3
CH3–C–(CH2NPCp3)3
rtc
15
12
a Abbreviations: Cp, cyclopentyl; Cy, cyclohexyl.
b Experimental conditions: complex, 0.01 mmol; styrene, 2 mL; ethyl diazoacetate, 1 mmol diluted by the olefin up to 1 mL; addition time, 3 h.
c rt, room temperature.
There are some coordinating similarities between the
pyrazolyborate 1 ligands shown in Scheme 5 and the
triiminophosphoranes 2. Each of these ligands looks like a
hand formed by three ‘fingers’ relatively independent from
each other, although one might expect a somewhat greater
flexibility for the triiminophosphoranes. Each ‘fingers’
contains a sp2 nitrogen atom that will coordinate to the
transition metal through its lone electron pair. The proposed
tridentate chelation is based on their spectroscopic data as
no suitable crystals of any complexes have been obtained so
far.
significantly different to that observed with the correspond-
ing bis-iminophosphorane complexes. This concurs with
15,
´
what has already been reported by Perez and co-workers,
20 and according to these authors, the catalyst structure has
little influence on the stereoselectivity of the reaction. This
fact has been rationalized assuming that the high reactivity
of the metal–carbene complex results from a transition state
(A) (Scheme 6) in which the olefin remains always at a
relatively important distance from the metal centre, which
prevents it from being significantly affected by the steric
bulk of the ligand. For that reason, the steric influences are
not decisive in the induction of the diastereoselectivity.
14,15,20,23,24
´
Perez and co-workers,
and Penoni and
co-workers25 have demonstrated the potential of copper
complexes bearing homoscorpionate (Tp) ligands in
carbene reactions. Hence, this justifies the interest of
exploring the catalytic activity of the new triimino-
phosphorane ligand system in olefin cyclopropanation.
The triiminophosphorane ligands were prepared as reported
and the corresponding metallic complexes were formed by
reaction of the ligand with the appropriate metallic salt in a
dichloromethane solution.22 Screening cyclopropanation
experiments were performed under the same reaction
conditions as reported for the diiminophosphorane
complexes18 and an overview of the activity/selectivity
pattern of Cu- and Pd-complexes ligated to such tripodal
ligands is reported hereafter.
Scheme 6.
At rt, the results are slightly less satisfactory regarding the
yield of the cyclopropanation. On the other hand, in the case
of the CH3–C–(CH2NPCp3)3 ligand, the copper complex
tends to form predominantly the trans-isomer in a more
significant way than at higher temperature. Furthermore, the
stereoselectivity is opposite to that, which is observed with
the corresponding bis-Cp-iminophosphorane (cis/trans
ratiosZ0.12 vs 0.92, see Table 1).
The results for styrene cyclopropanation at two different
temperatures are summarized in Table 3. It comes out that
these triiminophosphorane-based complexes present a good
catalytic activity, the yields vary from 81 and 99% and the
secondary products (ethyl maleate and ethyl fumarate) are
formed in low amounts when the reactions are performed
with copper at 60 8C. The yields obtained with the Pd-based
complexes are somewhat lower and from that on, an
increased formation of carbene dimers can be observed. No
significant influence of the substituent (methyl or phenyl
group) at the sp3 carbon bearing the three functionalized
tethers is seen.
In order to study the influence of the substrate and of the
stereo-electronic effects in these cyclopropanation
reactions, differently substituted styrene derivatives and
some aliphatic or aromatic olefins have been tested with the
metal complex [CH3–C–(CH2NPCp3)3Cu(OTf)2]. The
results are reported in Table 4.
As a whole, the yields of the cyclopropanation reaction of
the styrene derivatives are good to excellent (entries 1–7),
with the exception of 4-t-butylstyrene and to a lesser extent
of 4-methylstyrene. It should be noted that 4-methoxy as
well as 4-chloro, and 4-trifluoromethylstyrenes give high
yields of cyclopropanation products. Electron-rich and
electron-depleted styrenes are thus, suitable targets for
the carbene (or carbenoid) species formed in situ. The
The diastereoselectivity of the reaction at 60 8C is not
exceptional and does not vary much from one ligand to the
other. In the whole, the cis- and trans-cyclopropanes are
formed in an average ratio 35:65 whatever the ligand
utilized. The ligand structure seems thus, to have relatively
little influence on the outcome of the cyclopropanation
reaction even if the stereoselectivity of the reactions can be

L. Beaufort et al. / Tetrahedron 61 (2005) 9025–9030
9029
Table 4. Cyclopropanation of various olefins with EDA and CH3–C–(CH2NPCp3)3Cu(OTf)2
Entry
Substrate
Cyclopropanationa
cis/trans ratio
Dimerisation (%)
Yield (%)
1
2
3
4
5
6
7
8
Styrene
88
93
98
62
78
89
91
52
77
68
2
0.54
0.45
0.50
0.47
0.49
0.80
0.49
0.51
0.65
0.61
—
12
7
2
4-Chlorostyrene
4-Methoxystyrene
4-t-Butylstyrene
4-Methylstyrene
a-Methylstyrene
4-Trifluoromethylstyrene
1-Octene
Cyclooctene
1-Decene
Diethyl fumarate
Phenylacetylene
16
21
10
9
48
23
32
ndb
79
9
10
11
12
21
—
a Experimental conditions: complex, 0.005 mmol; olefin, 1 mL; ethyl diazoacetate, 1 mmol diluted by the olefin up to 1 mL; addition time, 3 h; temperature,
60 8C.
b nd, not determined.
copper-catalyzed reactions of ethyl diazoacetate with linear
alkenes or cycloalkenes are more difficult but proceed in
relatively good yields, although as expected, the activated
alkenes (styrenes) are always more reactive than the non-
activated ones (1-octene, cyclooctene, 1-decene).
precatalyst in the competitive cyclopropanation of an
equimolar mixture of two styrenes, the non-substituted
C6H5CH]CH2 and a para-substituted styrene of general
formula XC6H4CH]CH2 (Scheme 7). The ratio of the
resulting cyclopropanes 11a,b:12a,b were determined by
gas chromatography. The results are summarized in Table 5.
Globally, all the diasteroselectivities of the cyclopropanations
obtained with the different olefins remain more or less in the
same range of cis/trans ratio but for a-methylstyrene where it
reaches the value of 0.8. This styrene derivative is known to
give frequently ratio of isomeric cyclopropanes close to 1.
In all cases, the cyclopropanation reaction favors the
electron rich derivatives, whatever the kind of copper
complex used (bearing a di- or a triiminophosphorane type
ligand), that is, the p-methoxy- and p-tertbutylstyrenes,
whereas the electron-withdrawing groups such as Cl and
CF3 clearly disfavor the cycloaddition. This confirms the
electrophilic character of carbenes (carbenoids) formed
during the cyclopropanation reaction and is consistent with
the observation that the carbene addition on electron poor
olefins such as diethyl maleate or diethyl fumarate is only
poorly effective, as depicted in Table 4.
Despite the fact that the steric requirements for di- and
triiminophosphorane complexes are quite different, the
resulting stereoselectivities remain relatively close
(although sometimes significantly different, however), and
are difficult to rationalize. We have then performed
competition experiments with para-substituted styrenes to
obtain some further informations on the electronic effects of
these ligands in the cyclopropanation reaction.
Furthermore, the complexes ligated to cyclopentyl-tri-
iminophosphoranes always appear to be more selective
than their bis-iminophosphorane counterparts, the reverse
Four different complexes have been employed as the
Scheme 7.
Table 5. Competition experimentsa with para-substituted styrenes
Catalyst
Ratio of products [(12aC12b)/(11aC11b)]
p-OMe/H
p-tBu/H
p-Cl/H
p-CF3/H
Ph-1,2-(N]PPh3)2Cu(OTf)2
Ph-1,2-(N]PCp3)2Cu(OTf)2
CH3–C(CH2N]PCp3)3Cu(OTf)2
Ph-1,2-(N]PPh3)3Cu(OTf)2
1.66
1.63
2.07
1.38
1.67
1.54
1.86
1.46
0.72
0.79
0.84
0.83
0.32
0.49
0.52
0.21
a Experimental conditions: complex, 0.005 mmol; olefin, 1 mL of an equimolar mixture; ethyl diazoacetate, 1 mmol diluted by the equimolar mixture of olefins
up to 1 mL; addition time, 3 h; temperature, 60 8C.

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L. Beaufort et al. / Tetrahedron 61 (2005) 9025–9030
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On the other hand, the phenyl-bisiminophosphoranes show
the reverse trend and select preferentially the electron-rich
olefins when compared to their triiminophosphorane
analogs. This constitutes a further indication that the
metal complexes of bis- and triiminophosphoranes yield
different active species in situ.
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excellent catalysts for olefin cyclopropanation. However,
despite the bulkiness of the iminophosphoranes used in this
study, the stereoselectivities of the cyclopropanation
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two necked flasks fitted by a three way stopcock and by a
septum. In this flask, 1!10K5 mol of the copper complex
are introduced. The flask is then placed under inert
atmosphere by three consecutives vacuum–argon cycles.
To the catalyst, 2 mL of the dried, distillated, and degassed
olefin are then added. The flask is heated to 60 8C in an oil
bath (excepted for the reactions carried out at rt). In a 1 mL
seringe, 0.125 g of the diazocompound are weighted and
diluted up to 1 mL with the olefin. The diazoester is slowly
added to the alkene solution via a syringe pump, the
duration of the addition is 3 h at 60 8C. The kinetics of ethyl
diazoacetate decomposition is followed by volumetry
through N2 evolution. To this end, the reaction flask is
connected via the three way stopcock to a water column
through a metallic canula. At the end of the reaction, the
reaction mixture is analyzed by gas chromatography and the
reaction products identified and quantified by comparison
with authentic samples.
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26. Manuscript in preparation.