Ruthenium(II) complexes bearing a ligand derived from P, N - Or P, N, O-diphenylphosphinobenzoxazine: Synthesis, X-ray characterization, and cis diastereoselectivity in styrene cyclopropanation

Article abstract of DOI:10.1021/om300434u

A phosphino-oxazine based ligand (L; 2-(2-(diphenylphosphino)phenyl)-2,4- dihydro-1H-benzo[d][1,3]oxazine) showing a temperature-dependent equilibrium between a closed bidentate (L^PN) and an opened tridentate (L ^PNO) form, has been synthesized and its coordination behavior toward ruthenium(II) centers studied. Under different experimental conditions, two different species bearing the ligand in either its bidentate or tridentate coordination mode were isolated by reaction with Ru(PPh₃) ₃Cl₂. These species, respectively formulated as [Ru(PPh₃)(L^PNO)Cl₂] (1) and [Ru(PPh ₃)(L^PN)Cl₂] (2), were fully characterized via NMR in solution and by an X-ray structural determination. Notably, compound 2 reacts with an excess of ethyl diazoacetate (EDA) in CH₂Cl ₂ to give a stable η³-diethyl maleate complex, [Ru(L^PN)(cis-EtO(O)CCH=CHC(O)OEt)Cl₂] (3). The crystal structure of 3 has also been determined. Substitution reactions with 4-picoline (4-Me-py) performed on 1 led to two new complexes: the neutral complex [Ru(4-Me-py)(L^PNO)Cl₂] (5) and the salt [Ru(4-Me-py) ₂(L^PNO)Cl](Cl) (6a). The latter compound catalyzed the intermolecular cyclopropanation of styrene with EDA in high yields and with elevated cis diastereoselectivity (i.e., cis/trans = 80/20).

Full text of DOI:10.1021/om300434u

Article
pubs.acs.org/Organometallics
Ruthenium(II) Complexes Bearing a Ligand Derived from P,N- or
P,N,O-Diphenylphosphinobenzoxazine: Synthesis, X-ray
Characterization, and cis Diastereoselectivity in Styrene
Cyclopropanation
G. Attilio Ardizzoia,† Stefano Brenna,*,† Sara Durini,† and Bruno Therrien‡
†Dipartimento di Scienza e Alta Tecnologia, Universita
̀
degli Studi dell’Insubria, Via Valleggio, 11, 22100 Como, Italy
‡Institute of Chemistry, Universite
́
de Neuchat
̂
el, Ave de Bellevaux 51, CH-2000 Neuchatel, Switzerland
̂
S
* Supporting Information
ABSTRACT: A phosphino-oxazine based ligand (L; 2-(2-(diphenylphosphino)phenyl)-2,4-dihydro-1H-benzo[d][1,3]oxazine)
showing a temperature-dependent equilibrium between a closed bidentate (LPN) and an opened tridentate (LPNO) form, has
been synthesized and its coordination behavior toward ruthenium(II) centers studied. Under different experimental conditions,
two different species bearing the ligand in either its bidentate or tridentate coordination mode were isolated by reaction with
Ru(PPh3)3Cl2. These species, respectively formulated as [Ru(PPh3)(LPNO)Cl2] (1) and [Ru(PPh3)(LPN)Cl2] (2), were fully
characterized via NMR in solution and by an X-ray structural determination. Notably, compound 2 reacts with an excess of ethyl
diazoacetate (EDA) in CH2Cl2 to give a stable η3-diethyl maleate complex, [Ru(LPN)(cis-EtO(O)CCHCHC(O)OEt)Cl2] (3).
The crystal structure of 3 has also been determined. Substitution reactions with 4-picoline (4-Me-py) performed on 1 led to two
new complexes: the neutral complex [Ru(4-Me-py)(LPNO)Cl2] (5) and the salt [Ru(4-Me-py)2(LPNO)Cl](Cl) (6a). The latter
compound catalyzed the intermolecular cyclopropanation of styrene with EDA in high yields and with elevated cis
diastereoselectivity (i.e., cis/trans = 80/20).
Chart 1. Phosphinooxazine Ligands Employed in Catalytic
Processes
INTRODUCTION
■
It is well-known that the properties of coordination compounds
can be tuned by the appropriate design of multidentate ligands
binding the metal center.1 Special attention has been directed
to the use of hetero-multidentate ligands having both hard and
soft donor atoms (e.g. P,N, P,O, or P,N,O), since the resulting
complexes often showed fascinating coordination chemistry2 or
remarkable properties in catalysis.3 In the realm of P,N
bidentate species,4 five- or six-membered N-containing hetero-
cycles functionalized with a diphenylphosphino pendant arm
have been extensively studied. Since the first synthesis,
independently performed by Pfaltz,5 Helmchen,6 and Wil-
liams,7 phosphino-oxazoline ligands have been widely used in
asymmetric catalysis.8 More recent examples from Braunstein9
and Helmchen10 have also appeared in the literature.
Phosphino-oxazine ligands (L1−L6; Chart 1) have received
much attention as well. Their complexes exhibit high
enantioselectivity when used as catalysts in Pd-catalyzed allylic
substitution (L1−L4)11,12 or in hydrogenation, Heck, or Diels−
Alder reactions (L5 and L6).13
We recently initiated a study on the coordination chemistry
of fully saturated N-containing heterocycles, and we have
Received: May 18, 2012
Published: July 16, 2012
© 2012 American Chemical Society
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1
Table 1. Selected H and 31P{1H} NMR Spectroscopic Data
for Ligands LPN and LPNO and Compounds
[Ru(PPh3)(LPNO)Cl2] (1), [Ru(PPh3)(LPN)Cl2] (2),
[Ru(LPN)(cis-EtO(O)CCHCHC(O)OEt)Cl2] (3), [Ru(4-
Me-py)(LPNO)Cl2] (5), and [Ru(4-Me-py)2(LPNO)Cl](Cl)
(6a)
already reported on pyridine-functionalized dihydrobenzox-
azine-type ligands14,15 and on the characterization and catalytic
activity of nickel(II) and palladium(II) compounds bearing
oxazolidine-based ligands.16 These species revealed a broad
range of coordination modes, giving to the metal centers a
variety of coordination environments (e.g., N,N, N,N,O, and
N,O). Herein, we report the synthesis of a diphenylphosphino-
oxazine based ligand, 2-(2-(diphenylphosphino)phenyl)-2,4-
dihydro-1H-benzo[d][1,3]oxazine (L; Chart 2), offering either
a
LPN
δ(1H) (ppm)
δ(31P{1H}) (ppm)
4.78 (d, Ha), 4.93 (d, Hb), 9.07 (d, NCHO)
−16.60 (s, PPh2)
b
LPNO
Chart 2. Equilibrium between the Closed Bidentate LPN and
Open Tridentate LPNO Forms of the Ligand L
δ(1H) (ppm)
δ(31P{1H}) (ppm)
4.48 (OH), 4.65 (CH2O), 6.29 (HCN)
−16.60 (PPh2)
a
[Ru(PPh3)(LPNO)Cl2] (1)
δ(1H) (ppm)
δ(31P{1H}) (ppm)
2.69 (OH), 4.43 (Hb), 5.27 (Ha), 8.85 (HCN)
39.9 (PPh3), 62.1 (PPh2)
a
[Ru(PPh3)(LPN)Cl2] (2)
δ(1H) (ppm)
4.99 (CH2O), 6.52 (NCHO)
δ
31P{1H} (ppm)
44.0 (PPh3), 75.6 (PPh2)
a
[Ru(LPN)(cis-EtO(O)CCHCHC(O)OEt)Cl2] (3)
bidentate P,N (LPN) or tridentate P,N,O (LPNO) donation. The
coordination behavior of L with Ru(PPh3)3Cl2 under different
conditions has been studied. The reactivity of the resulting
complexes toward ethyl diazoacetate and the cis diastereose-
lectivity in styrene cyclopropanation catalyzed by one of their 4-
picoline derivatives is also described.
δ(1H) (ppm) 1.37 (CH3), 1.43 (CH3), 4.24 (CH2), 4.45 (CH2), 4.84 (Hb),
4.99 (Ha), 6.52 (HCC), 6.60 (NCHO), 6.68 (HCC)
δ(31P{1H})
51.5 (PPh2)
(ppm)
a
[Ru(4-Me-py)(LPNO)Cl2] (5)
δ(1H) (ppm)
2.26 (CH3), 4.49 (OH), 4.92 (Hb), 5.64 (Ha), 8.92
(HCN)
δ(31P{1H})
(ppm)
73.3 (PPh2)
RESULTS AND DISCUSSION
Synthesis of Ligands. The ligand L was prepared by
■
a
[Ru(4-Me-py)2(LPNO)Cl](Cl) (6a)
condensation of 2-(diphenylphosphino)benzaldehyde with 2-
δ(1H) (ppm)
2.23 (CH3), 2.24 (CH3), 2.41 (OH), 2.59 (Hb), 4.22 (Ha),
9.01 (HCN)
aminobenzyl alcohol, in refluxing ethanol.17 In the H NMR
1
δ(31P{1H})
(ppm)
61.0 (PPh2)
spectrum (see Table 1 for selected NMR data of the
compounds) the AB system centered at 4.85 ppm originates
from diastereotopic protons of the methylene group on the
heterocyclic ring, whereas the doublet at 6.29 ppm is attributed
to the CH in position 2 of the oxazine ring (NCHO), showing
a relatively high coupling with the PPh2 moiety (4JPH = 6.6 Hz).
The latter results in a singlet at −16.6 ppm in the 31P{1H}
NMR spectrum (full spectra are given in the Supporting
Information, Figures S1−S4).
a
b
Spectra measured in CD2Cl2, at 25 °C. Spectrum measured in
toluene-d8, at 85 °C.
The different chemical shifts for Ha and Hb are a consequence
of the coordination of the oxygen of the methylene group to
the metal center. The triplet for Ha derives from similar
coupling constants of this nucleus with both Hb and OH:
indeed, the disappearance of coupling with OH after
deuteration causes signal simplification into a doublet (JH −H
To first explore the possible conversion of free LPN into
LPNO, the product was dissolved in toluene-d8 and the solution
heated to 85 °C, measuring different 1H NMR spectra at 10 °C
intervals (Supporting Information, Figure S5). The appearance
and progressive increase of signals attributed to the open iminic
form occur, with a 65% extent of conversion of LPN into LPNO
at 85 °C. At this stage the signals due to the azomethine proton
(9.07 ppm, d, 4JPH = 4.7 Hz), the methylene group (4.65 ppm,
s), and OH (4.48 ppm, s) are easily identified (Figure 1).
Synthesis of Ruthenium(II) Compounds. Depending on
the reaction temperature, it was possible to isolate two different
ruthenium(II) complexes, bearing the ligand in either its
bidentate (LPN) or tridentate form (LPNO) (Scheme 1).
When a suspension of Ru(PPh3)3Cl2 and L (in a 1/1.1 molar
ratio) was refluxed in toluene, a deep red solid was obtained,
later formulated as [Ru(PPh3)(LPNO)Cl2] (1). The presence of
the ligand in its tridentate form is proven by solution NMR
data (Figures S6−S9 in the Supporting Information): in the 1H
NMR (CD2Cl2, 25 °C), the doublet at 2.69 ppm (JHH = 10.9
Hz) is attributed to the OH group and disappears after
treatment with D2O, whereas the Ha and Hb protons of the
CH2OH moiety generate two signals, respectively, at 5.27 ppm
(triplet, JHH = 11.0 Hz) and 4.43 ppm (doublet, JHH = 10.8 Hz).
a
b
= 11.2 Hz). Finally, the iminic proton resonates as a doublet at
δ 8.85 ppm. The JPH value (8.3 Hz) is in the range found for
4
other similar Ru(II) complexes.18 The presence of a residual
PPh3 coordinated to Ru(II) is confirmed by 31P{1H} NMR,
where two doublets are observed respectively at 39.9 ppm (JPP
= 20.8 Hz, PPh3) and 62.1 ppm (JPP = 24.2 Hz, PPh2). These
values for coupling constants suggest a cis disposition between
the phosphorus nuclei, as already reported for other Ru(II)
phosphine complexes.19 The X-ray crystal structure of 1
confirmed these hypotheses (Figure 2).
For complexes bearing P,N-iminophosphine ligands, the
higher trans influence of the phosphorus atom in comparison to
that of the imine donor functionality2b,20 leads to longer
distances for bonds trans to the phosphorus. On the other
hand, in the presence of tridentate P,N,O ligands (with N being
an iminic nitrogen) the reverse situation is usually observed;21
the related bond distances in [Ru(PPh3)(LPNO)Cl2] (namely,
Ru1−O1 = 2.212(3) Å and Ru1−P2 = 2.345(1) Å) follow this
rule.
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1
PNO
Figure 1. H NMR spectrum (toluene-d8) showing the equilibrium between LPN
) and L
■
( ) present at 85 °C. Inset: expansion of the
△
(
aliphatic region.
Scheme 1. Syntheses of [Ru(PPh3)(LPNO)Cl2] (1) and
[Ru(PPh3)(LPN)Cl2] (2)
Figure 2. ORTEP representation of 1 at the 50% probability level.
Selected bond lengths (Å) and angles (deg): Ru(1)−Cl(1) =
2.3924(11), Ru(1)−Cl(2) = 2.4268(11), Ru(1)−P(1) = 2.2253(13),
Ru(1)−N(1) = 2.142(4), Ru(1)−O(1) = 2.212(3), Ru(1)−P(2) =
2.3448(14); Cl(1)−Ru(1)−Cl(2) = 164.06(4), N(1)−Ru(1)−Cl(1) =
85.65(10), N(1)−Ru(1)−Cl(2) = 89.54(11), N(1)−Ru(1)−P(1) =
87.70(11), N(1)−Ru(1)−P(2) = 174.12(12), N(1)−Ru(1)−O(1) =
85.31(13), O(1)−Ru(1)−P(1) = 170.72(9), P(1)−Ru(1)−P(2) =
98.03(5).
Alternatively, when the temperature was maintained at 0 °C
during the reaction of Ru(PPh3)3Cl2 with L (1/1.1 molar ratio)
in toluene, an emerald green compound was formed,
1
subsequently identified as [Ru(PPh3)(LPN)Cl2] (2). In the H
NMR (Figure S10, Supporting Information), the pattern of LPN
appears: a quartet at 4.99 ppm, attributable to the methylene
group, and a doublet at 6.52 ppm, assigned to the methine of
the oxazine ring. This latter resonance does not originate from
a coupling with phosphorus, as evidenced by a 1H−31P HMBC
experiment (Supporting Information). Thus, it is assumed that
in solution complex 2 is present as a mixture of
diastereoisomers22 and the signal relative to NCHO appears
as a doublet. In the 31P{1H} NMR (Figure S12, Supporting
Information) two resonances are seen: one related to the PPh2
moiety of the ligand (75.6 ppm, JPP = 26.4 Hz) and the other
due to PPh3 (44.0 ppm, JPP = 31.4 Hz). These values for JPP
suggest once again a cis disposition of the phosphorus atoms, as
confirmed by an X-ray structural analysis23 (Figure 3), which
shows the Ru(II) center in 2 to be in a slightly distorted square
pyramidal geometry (τ = 0.12).24
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in toluene was next refluxed in the presence of a catalytic
amount of exogenous ligand (10 mol % relative to [Ru]). This
led to a quick change in color from emerald green to deep red,
and after stirring for 4 h, a red solid was isolated. All
spectroscopic features match those of 1. Consequently, both
temperature and the slight excess of free ligand play a key role
in the conversion of 2 into 1. To prove this assumption, a
further reaction of 2 with a slight excess of free L was
performed, this time with the temperature kept at 20 °C: no
transformation was observed, definitely certifying the syner-
gistic influence of temperature and free-ligand excess in the
formation of 1, both in the original preparation from
Ru(PPh3)3Cl2 and L and as a conversion of 2.
In an additional experiment, a suspension of compound 2
was refluxed in toluene in the simultaneous presence of a
catalytic amount of free ligand and 1 equiv (with respect to
ruthenium) of exogenous PPh3. Under these conditions, species
2 did not turn into 1, meaning that the first step of formation of
1 from 2 is presumably the dissociation of PPh3 (Scheme 2, A).
Next, the vacant site in the 14e [Ru(LPN)Cl2] species is
occupied by the PPh2 moiety of the free ligand, which
reasonably coordinates in its iminic form, predominant at high
temperature (B). The subsequent coordination of nitrogen and
oxygen forces the ligand in its hetero-tridentate LPNO binding
mode, and the concomitant extrusion of the hetero-bidentate
LPN form leads to the five-coordinated [Ru(LPNO)Cl2]
intermediate (C), which readily reacts with PPh3 still present
in the bulk to generate 1 (D).
Presumably, the driving force of the reaction is the higher
stability of octahedral [Ru(PPh3)(LPNO)Cl2] with respect to the
square-pyramidal [Ru(PPh3)(LPN)Cl2]. Actually, an evaluation
of relative energies of species 1 and 2 (DFT; see the Supporting
Information) shows complex 1 being 5.69 kcal/mol more stable
with respect to complex 2. As a matter of fact, when it is treated
under the described experimental conditions, compound 2 is
Figure 3. ORTEP representation of 2 at the 50% probability level with
dichloromethane molecules omitted for clarity. Selected bond lengths
(Å) and angles (deg): Ru(1)−Cl(1) = 2.402(3), Ru(1)−Cl(2) =
2.375(3), Ru(1)−P(1) = 2.208(3), Ru(1)−N(1) = 2.176(8), Ru(1)−
P(2) = 2.309(3), O(1)−C(8) = 1.398(11); Cl(1)−Ru(1)−Cl(2) =
159.95(10), N(1)−Ru(1)−Cl(1) = 81.1(2), N(1)−Ru(1)−Cl(2) =
86.7(2), N(1)−Ru(1)−P(1) = 91.4(2), N(1)−Ru(1)−P(2) =
166.9(2), P(1)−Ru(1)−P(2) = 101.67(11).
Equilibrium Studies. To shed light on the mechanism
leading to 1 and 2, we investigated the possible conversion of
one species into the other. Initially, a suspension of
[Ru(PPh3)(LPN)Cl2] in toluene was heated to reflux, but no
structural change was noticed even after a long reaction time
(12 h). Thus, once coordinated, the ligand does not undergo
ring opening and the mere thermal transformation of 2 into 1 is
prevented. As in the original synthesis of 1 a slight excess of L
was used (1/1.1 Ru/ligand ratio), a suspension of compound 2
Scheme 2. Proposed Mechanism for Conversion of [Ru(PPh3)(LPN)Cl2] (2) into [Ru(PPh3)(LPNO)Cl2] (1)
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irreversibly converted into 1. In contrast, when it is suspended
in toluene at either 20 or 0 °C, in the presence of a catalytic
amount of exogenous ligand (10 mol %), compound 1 did not
convert into 2 even after a long reaction time (24 h).
detected in the 31P{1H} NMR. These features were eventually
confirmed by X-ray diffraction (Figure 4).
Reactivity with Ethyl Diazoacetate (EDA). In our
ongoing exploration of transition-metal-catalyzed cyclopropa-
nation of olefins by means of EDA decomposition,25 we tested
the activity of compounds 1 and 2 as cyclopropanation
catalysts. Following an established procedure, [Ru(PPh3)-
(LPNO)Cl2] was first dissolved in dichloromethane (8 mL) in
the presence of styrene as substrate (1/200 [Ru]/olefin molar
ratio). Ethyl diazoacetate was then added dropwise over a 4 h
period (1/100 [Ru]/EDA molar ratio),26 and the progress of
the reaction was monitored via infrared spectroscopy (νNN
)
and GC-MS. Notably, no gas (N2) evolution was noticed and
no cyclopropanation products were detected after 24 h. Even
performing the reaction at 65 °C (in 1,2-dichloroethane) did
not evidence any conversion: indeed, at this temperature the
dissociation of PPh3 should take place, generating the
pentacoordinate species [Ru(LPNO)Cl2] with a free coordina-
tion site which could enter the catalytic cycle. Thus, the
absence of activity of 1 cannot be ascribed to steric hindrance,
because the coordinative saturation of the Ru(II) center in 1 is
presumably lost at 65 °C (due to dissociation of PPh3). Most
likely, electronic contributions made by the diphenylphosphi-
no-imino ligand should be invoked. Probably, the ligand LPNO
reduces electron density on ruthenium and discourages the
formation of the electrophilic carbene [Ru(CHC(O)OEt)-
(LPNO)Cl2]. As a partial proof, also the square-pyramidal 16e
[Ru(PPh3)(LPN)Cl2], having a free coordination site, did not
catalyze styrene cyclopropanation to a significant extent when it
was dissolved in dichloromethane under the same conditions as
for 1. To better understand this conduct, [Ru(PPh3)(LPN)Cl2]
was treated with a 10-fold excess of ethyl diazoacetate, in
dichloromethane, in the absence of olefin. After the reaction
mixture was stirred for 12 h at room temperature, a color
change from emerald green to brown was noticed. The solution
was evaporated to dryness and the crude residue recrystallized
from diethyl ether, affording a light brown solid subsequently
identified as [Ru(LPN)(cis-EtO(O)CCHCHC(O)OEt)Cl2]
(3) (Scheme 3).
Figure 4. ORTEP representation of 3 at the 50% probability level.
Selected bond lengths (Å) and angles (deg): Ru(1)−Cl(1) =
2.4127(14), Ru(1)−Cl(2) = 2.3672(13), Ru(1)−P(1) = 2.2578(13),
Ru(1)−N(1) = 2.216(4), Ru(1)−O(3) = 2.330(4), Ru(1)−C(31) =
2.126(5), Ru(1)−C(30) = 2.205(5); Cl(1)−Ru(1)−Cl(2) =
160.12(5), N(1)−Ru(1)−Cl(1) = 80.05(11), N(1)−Ru(1)−Cl(2) =
81.73(11), N(1)−Ru(1)−P(1) = 90.32(11), N(1)−Ru(1)−O(3) =
100.21(13), N(1)−Ru(1)−C(30) = 166.80(17), N(1)−Ru(1)−C(31)
= 153.72(19), P(1)−Ru(1)−O(3) = 167.19(9).
The formation of such an uncommon Ru−diethyl maleate
complex has been previously described by Lugan and Lavigne27
as a result of the reaction of pyridine-functionalized phosphine
Ru(II) complexes with excess ethyl diazoacetate. Despite
showing features similar to those of the already reported
analogous compound, species 3 presents some relevant
differences in its crystal structure: (i) a shorter Ru−O bond
(2.330(4) vs 2.414(5) Å) and (ii) a trans disposition of the
chlorido ligands, in comparison with the cis-Ru(Cl)2 fragment
in Lavigne’s complex.
The commonly accepted mechanism for the generation of
such a compound first considers the coordination to ruthenium
of the carbene moiety and its coupling with the coordinated
PPh3 molecule, with consequent elimination of the phospho-
nium ylide Ph3PC(H)COOEt.28 The resulting 14e species
readily coordinates two additional carbene units, ultimately
coupling to give the diethyl maleate fragment. We can assume
that this is the working mechanism also involved in the reaction
between 2 and EDA. Actually, the formation of Ph3P
C(H)C(O)OEt was confirmed by a single resonance at about
18 ppm in the 31P{1H} NMR analysis of the crude bulk
product. The stability of the resultant complex 3 is due to
electronic contributions, with the extra Ru−OC bond (Ru−
O = 2.330(4) Å) occupying the sixth coordination site on
Ru(II). Strong donation from the metal center to the π*
orbitals of the oxazine ring has been previously invoked to
justify uncommon behavior of oxazine-containing complexes.29
This is probably the case with compound 3, where the LPN
ligand reduces electron density on the metal, hence favoring
carbonyl oxygen coordination. The formation of the stable
diethyl maleate species intercepted in the reaction with EDA
precludes compound 2 from being an active catalyst in styrene
cyclopropanation.
Scheme 3
The IR spectrum of 3 shows two absorptions, respectively, at
1725 and 1622 cm−1, assigned to CO of two different ethyl
acetate groups. The simultaneous presence of otherwise
1
bonded C(O)OEt groups was also detected in solution by H
and 13C NMR (Table 1; Figures S13−S15, Supporting
Information), showing the concomitant presence in compound
3 of LPN and a coordinated diethyl maleate molecule, together
with the displacement of the original PPh3. Indeed, a single
resonance at 51.5 ppm due to the PPh2 moiety of the ligand is
Substitution Reactions with 4-Picoline. As is known,
nitrogen-containing heterocyclic ligands such as pyridines are σ
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Scheme 4. Suggested Mechanism of Formation of 4-Picoline-Containing Derivatives 5 and 6 Starting from 1
donors with only weak π-acceptor character;30 thus, we
attempted the substitution of PPh3 with 4-picoline (4-Me-py)
with the aim of enhancing electron density on the Ru(II)
center. Initially, an emerald green suspension of the
pentacoordinated [Ru(PPh3)(LPN)Cl2] (2) in diethyl ether
was reacted with a 2-fold excess of 4-picoline, and the color
immediately changed to orange. The product was identified as
[Ru(PPh3)(4-Me-py)(LPN)Cl2] (4) but could not be addition-
ally investigated in solution. Indeed, when 4 is dissolved in
CH2Cl2, acetone, or toluene, the prompt dissociation of
coordinated 4-picoline occurs, restoring an emerald green
solution corresponding to complex 2. Then, an alternative
substitution reaction was performed, this time starting from
[Ru(PPh3)(LPNO)Cl2] (1), which was refluxed in toluene in the
presence of 4-picoline (1/2 molar ratio) for 6 h. The isolated
solid appeared to be a mixture of two products. The 31P{1H}
NMR spectrum (CD2Cl2, 25 °C) showed two single
resonances, respectively, at 73.3 and 61.0 ppm. Such a pattern
without any JPP coupling can only originate from two distinct
products, each having a single phosphorus atom bound to the
metal. In both compounds the ligand is coordinated in the
tridentate P,N,O form, as evidenced by the appearance of
for 6 h. The methylene signals in the H and 13C{1H} NMR
spectra of the product (Supporting Information) show a
pattern quite similar to that for the corresponding protons in 1
(see the Experimental Section for details). Consequently,
compound 5 was identified as [Ru(4-Me-py)(LPNO)Cl2],
deriving from a simple substitution of PPh3 in 1 with a
molecule of 4-picoline. Complex 5 was associated with the
singlet at 73.3 ppm in 31P{1H} NMR.
1
The presence of another species bearing two molecules of 4-
picoline, as evidenced by 1H NMR analysis of the initial mixture
(namely, 6a), prompted us to investigate the reaction with
increasing amounts of 4-picoline. Indeed, when [Ru(PPh3)-
(LPNO)Cl2] was refluxed in the presence of 4 equiv of 4-picoline
for 6 h, the intensity of the signal relative to 6a increased, and a
nearly 5/1 ratio between 6a and 5 was finally obtained by
employing a 1/8 [Ru]/4-picoline molar ratio (Experimental
Section). Worthy of note, in addition to the synthesis from 1,
the same selectivity toward 6a could also be reached by a direct
synthesis starting from the formerly isolated complex 5, in m-
xylene, in the presence of an excess of 4-picoline. Compound
6a was then related to the resonance at 61.0 ppm in the
31P{1H} NMR.
Most probably, treatment of 1 with 4-picoline initially forms
complex 5, which progressively evolves to complex 6a due to
the excess of heterocyclic amine. The introduction of a donor
ligand such as 4-picoline in 5 enhances electron density on
ruthenium, thus favoring chloride dissociation. The vacant
coordination site generated is then occupied by a second
molecule of 4-picoline, giving the cationic 6a. The ionic nature
1
merely its pattern in the H NMR spectrum. Moreover, the
integration of signals allows us to establish different ratios
between the ligand and 4-picoline in the two complexes. Later,
these were formulated as [Ru(4-Me-py)(LPNO)Cl2] (5) and
[Ru(4-Me-py)2(LPNO)Cl](Cl) (6). In principle, two isomers of
complex 6 (6a, mer; 6b, fac) could form in the reaction of 1 or
5 with excess 4-picoline (Scheme 4). Unfortunately, despite
several attempts to grow single crystals of these 4-picoline
derivatives, no suitable crystals for X-ray determination were
obtained. Nevertheless, due to steric constraints the fac
configuration in 6b is presumably less favored than the mer
form encountered in 6a, which is thought to be the preferred
species obtained. Indeed, a comparison of the energies of 6a
and 6b obtained by optimization of the relative geometries
(DFT; see the Supporting Information) reveals species 6a to be
more stable than 6b by about 14.0 kcal/mol.
of 6a is strongly supported by the molar conductivity (ΛM
=
80.8 Ω−1 cm2 mol−1) measured at 20 °C for a 10−3 M solution
in methanol; this value falls into the range (80−115 Ω−1 cm2
mol−1) expected for a 10−3 M methanol solution of a 1/1
electrolyte type complex.31
Compound 6a was characterized via 1H and 13C{1H}
solution NMR (Supporting Information). The main feature
of its pattern in 1H NMR (CD2Cl2, 25 °C) is definitely
represented by the high proton splitting of the bonded
CH2OH: Ha and Hb are observed respectively at 4.22 and
2.59 ppm, with a separation, Δppm, of about 1.63 ppm. This
The formulation of species 5 and 6a could be tentatively
assigned by NMR investigation in solution. Luckily, it has been
possible to prepare pure 5 by boiling a 1/2 suspension of
[Ru(PPh3)(LPNO)Cl2] and 4-picoline, in m-xylene, at 140 °C
value is almost twice the difference observed in both 1 (Δppm
0.84) and 5 (Δppm = 0.72). Both signals appear as doublets of
=
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Table 2. Cyclopropanation of Styrene by Means of EDA Decomposition Catalyzed by the Ru(II) Compounds Studied in This
a
Work
b
b
c
c
entry
[Ru]
substrate
styrene
A + B (%)
C + D (%)
A/B ratio
C/D ratio
1
2
3
4
5
6
7
1
2
styrene
3
styrene
5
styrene
6a
styrene
81
33
19
67
79.9/20.1
69.7/30.3
61.5/38.5
95.1/4.9
98.2/1.8
6a
α-methylstyrene
6a
6a+Cl‑
100
<5
d
8
styrene
<10
a
b
Conditions: [Ru]/EDA/olefin =1/100/200; CH2Cl2 (8 mL), 20 °C. Yields based on EDA consumption (monitored by disappearance of νNN in
c
d
infrared spectroscopy): (A + B) + (C + D) = 100%. Determined by GC-MS analysis after 4 h. Catalyst: 6a in the presence of 1 equiv of Bu4NCl.
doublets, Ha and Hb coupling with each other (2JHH = 11.8 Hz)
and with OH (3JHH = 4.6 Hz for Ha and 2.4 Hz for Hb). As was
the case for 1 and 5, also in the case of 6a the loss of OH
coupling after addition of D2O in the NMR tube induced the
simplification of Ha and Hb signals into two doublets. These
two resonances also experience a significant upfield shift with
respect to 1 and 5, meaning that the CH2 moiety is positioned
within the shielding cone of the pyridine ring cis to it.32 The
OH proton signal is highly shielded as well, appearing as a
singlet at 2.40 ppm.
cis diastereoselectivity has been first reported by Katsuki,35 who
employed a [(salen)Ru(NO)+] complex to obtain a 93/7 cis/
trans ratio and later by Mezzetti36 with cationic complexes
bearing tetradentate PNNP ligands and Kim37 with a catalyst
generated in situ from [Ru(dmso)4Cl2] and ferrocenyl-
diphosphane ligands. Then, the present work adds compound
6a as a further example of the restricted class of cis-
diastereoselective Ru catalysts for styrene cyclopropanation
with EDA. This preference for the cis isomer is noticed also in
EDA self-coupling: indeed, when 6a is dissolved in CH2Cl2, at
20 °C, in the absence of olefin (entry 7), addition of EDA
afforded diethyl maleate as the only product. This conduct
somehow resembles the formation of the stable η3-diethyl
maleate complex 3 intercepted in the reaction of 2 with an
excess of EDA.
As was noted, it was not possible to perform an X-ray
structural determination of these species [Ru(4-Me-py)(LPNO)-
Cl2] and [Ru(4-Me-py)2(LPNO)Cl](Cl), but their catalytic
performance in styrene cyclopropanation (vide infra) gave a
supplementary corroboration of the proposed formulations.
Cyclopropanation of Styrene. The absence of catalytic
activity of species 1−3 (Table 2, entries 1−3) in the
cyclopropanation of styrene, in the presence of ethyl
diazoacetate (EDA) as carbenoid source, has already been
discussed in the text. Complexes 5 and 6a were also tested as
catalysts in the same reaction, showing different behavior, as
evidenced in Table 2 (entries 4−7). When it was dissolved in a
CH2Cl2 solution of styrene, at 20 °C, [Ru(4-Me-py)(LPNO)Cl2]
(5) did not convert the olefin into the corresponding
cyclopropylic esters after addition of EDA. In contrast,
[Ru(4-Me-py)2(LPNO)Cl](Cl) (6a) efficiently catalyzed the
same reaction in high yields (81%, entry 5) and with elevated
cis selectivity (cis/trans ≈ 80/20). In principle, during
ruthenium-catalyzed decomposition of diazo compounds in
the presence of olefins, a competitive metathesis mechanism
could also occur. Notably, when 6a was employed as the
catalyst, no evidence of formation of metathesis products (i.e.,
PhCHC(H)C(O)OEt) was observed by GC-MS analysis.
Despite a lower yield of conversion into cyclopropanes (33%,
entry 6), a similar relatively high cis diastereoselectivity was
encountered in the cyclopropanation of α-methylstyrene
catalyzed by 6a.
The commonly accepted mechanism for olefin cyclo-
propanation by EDA contemplates the formation in the
catalytic cycle of an active intermediate species bearing a
[RuC(H)C(O)OEt] fragment. In our case, this could derive
from the coordination of the CHC(O)OEt fragment to a
vacant site generated from the dissociation of 4-picoline.
However, this is in contrast with the dissimilar activity
possessed by compounds 5 and 6a. Indeed, both complexes
have a 4-picoline molecule coordinated to Ru(II), being equally
predisposed to generate a vacant site on the metal. In addition,
compound 5 does not catalyze styrene cyclopropanation,
meaning that the higher activity shown by the compound
[Ru(4-Me-py)2(LPNO)Cl](Cl) (6a) has to be ascribed mainly
(if not totally) to electronic effects. The replacement of PPh3
and one chloride in 1 with two molecules of 4-picoline in the
inner coordination sphere of ruthenium in 6a facilitates the
progress into the intermediate Ru carbene active species.
Presumably, this latter species derives from the initial
dissociation of the residual chloride bonded to ruthenium
and could be formulated as [Ru(4-Me-py)2(LPNO)(C(H)C-
(O)OEt)]2+(Cl)− . It is known that donor ligands such as
2
pyridines enhance chloride lability in ruthenium complexes,
favoring the dissociation of a chloride ligand coordinated to the
metal.38 This could also be the case for 6a, where the chloride
dissociates due to the presence of two molecules of 4-picoline.
The resultant cationic fragment [Ru(4-Me-py)2(LPNO)]2+
readily evolves to the Ru carbene intermediate, ultimately
As exhaustively recently reviewed by Perez and co-workers33
and by Nishiyama,34 among the numerous Ru complexes which
were used as catalysts in the intermolecular cyclopropanation of
styrene with ethyl diazoacetate, the large majority induced a
certain excess of the thermodynamically preferred trans isomer.
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Organometallics
Article
ether, giving a yellow solid. Yield: 85% (1.16 g). Mp: 68 °C. Anal.
leading to cyclopropanation products. To support this
hypothesis, we performed a supplementary catalytic run in
the presence of exogenous chloride ions, by dissolving 1 equiv
of Bu4NCl in the CH2Cl2 mixture of catalyst and styrene,
before adding EDA (entry 8). As expected, the catalytic activity
of 6a was reduced to a negligible olefin conversion into
products. It is thought that free Cl− in solution depresses the
initial dissociation of chloride, thus preventing the formation of
the Ru carbene intermediate. Additional studies will help to
extend the scope and limitations of this reaction, hopefully
leading to clarify the nature of the complexes involved.
Calcd for C26H22NOP: C, 78.97; H, 5.61; N, 3.54. Found: C, 78.80; H,
1
5.66; N, 3.59. H NMR (400 MHz, CD2Cl2, 25 °C): δ 4.78 (d, JHH
=
14.7 Hz, 1H, Ha part of an AB system, CH2O), 4.93 (d, JHH = 14.7 Hz,
1H, Hb part of an AB system, CH2O), 6.29 (d, JPH = 6.6 Hz, 1H,
NCHO), 6.38 (dd, JHH = 8.0 Hz, JHH = 0.6 Hz, 1H, Ar H), 6.79 (dt,
JHH = 7.4 Hz, JHH = 1.1 Hz, 1H, Ar H), 6.93 (dd, JHH = 7.6 Hz, JHH
=
0.8 Hz, 1H, Ar H), 7.04 (m, 2H, Ar H), 7.31−7.43 (m, 11H, Ar H),
7.48 (dt, JHH = 7.6 Hz, JHH = 1.2 Hz, 1H, Ar H), 7.84 (ddd, JHH = 7.8
Hz, JHH = 3.9 Hz, JHH = 1.3 Hz, 1H, Ar H). 13C NMR (100 MHz,
CD2Cl2, 25 °C): δ 67.59 (CH2O), 82.42 (d, JPC = 26.7 Hz, NCHO),
116.05 (Ar C), 119.14 (Ar C), 121.69 (Ar C), 124.76 (Ar C), 126.55
(d, JPC = 5.0 Hz, Ar C), 127.16 (Ar C), 128.49 (d, JPC = 6.8 Hz, Ar C),
128.69 (Ar C), 128.98 (d, JPC = 6.4 Hz, Ar C), 129.39 (Ar C), 133.68
(d, JPC = 18.6 Hz, Ar C), 134.14 (d, JPC = 20.2 Hz, Ar C), 135.72 (d,
JPC = 17.3 Hz, Ar C), 136.76 (d, JPC = 10.2 Hz, Ar C), 142.20 (Ar C),
143.49 (d, JPC = 22.3 Hz, Ar C). 31P{1H} NMR (162 MHz, CD2Cl2,
25 °C): δ −16.60 (s, PPh2).
CONCLUSION
■
In summary, ruthenium(II) complexes bearing the diphenyl-
phosphinoxazine derived ligand L have been prepared and
characterized. Depending on the experimental conditions, in
the reaction with Ru(PPh3)3Cl2, L displays different coordina-
tion modes, namely a bidentate P,N (LPN) and a tridentate
P,N,O form (LPNO), giving rise respectively to [Ru(PPh3)-
(LPNO)Cl2] (1) and [Ru(PPh3)(LPN)Cl2] (2). The electronic
properties imposed by the diphenyl-phosphinoxazine ligand
strongly influence the reactivity of these species. Actually, in the
reaction of 2 with excess ethyl diazoacetate (EDA), the stable
η3-diethyl maleate complex [Ru(LPN)(cis-EtO(O)CCH
CHC(O)OEt)Cl2] (3) was intercepted. Its structural character-
ization represents the second example known in the literature
of such a Ru(II) compound. Substitution reactions with 4-
picoline (4-Me-py) were then performed on 1, and two novel
complexes, formulated as [Ru(4-Me-py)(LPNO)Cl2] (5) and
[Ru(4-Me-py)2(LPNO)Cl](Cl) (6a), were isolated. Finally,
compound 6a catalyzed the intermolecular cyclopropanation
of styrene with EDA, in good yields and with a high
diastereoselectivity toward the cis isomer.
[Ru(PPh3)(LPNO)Cl2] (1). A suspension of Ru(PPh3)3Cl2 (1 g, 1.04
mmol) in toluene (15 mL) was treated with L (0.45 g, 1.14 mmol) and
the mixture was gently refluxed for 8 h. During this time the color
changed to purple-red. After it was cooled to room temperature, the
suspension was filtered and the red solid was thoroughly washed with
diethyl ether. Yield: 74% (0.64 g). Anal. Calcd for C44H37Cl2NOP2Ru:
1
C, 63.69; H, 4.49; N, 1.69. Found: C, 63.59; H, 4.43; N, 1.72. H
NMR (400 MHz, CD2Cl2, 25 °C): δ 2.69 (d, JHH = 10.9 Hz, 1H, OH),
4.43 (d, JHH = 10.8 Hz, 1H, Hb of CH2O), 5.27 (t, JHH = 11.0 Hz, 1H,
Ha of CH2O), 6.70 (t, JHH = 6.5 Hz, 2H, Ar H), 6.84 (dd, JHH = 7.5
Hz, JHH = 0.7 Hz, 1H, Ar H), 7.00 (t, JHH = 8.4 Hz, 1H, Ar H), 7.05 (t,
JHH = 6.5 Hz, 1H, Ar H), 7.15 (dt, JHH = 7.9 Hz, JHH = 1.8 Hz, 6H, Ar
H), 7.25−7.43 (m, 19H, Ar H), 7.56 (ddd, JHH = 7.5 Hz, JHH = 3.6 Hz,
JHH = 1.0 Hz, 1H, Ar H), 7.78 (t, JHH = 8.7 Hz, 2H, Ar H), 8.85 (d,
4JPH = 8.3 Hz, 1H, HCN). 13C NMR (100 MHz, CD2Cl2, 25 °C): δ
64.33 (CH2O), 123.46 (Ar C), 127.03 (d, JPC = 8.9 Hz, Ar C), 127.52
(d, JPC = 10.0 Hz, Ar C), 128.81 (Ar C), 128.94 (Ar C), 129.12 (Ar
C), 129.50 (Ar C), 129.60 (Ar C), 129.91 (Ar C), 130.82 (Ar C),
131.31 (Ar C), 132.06 (d, JPC = 6.1 Hz, Ar C), 133.84 (d, JPC = 8.5 Hz,
Ar C), 134.27 (Ar C), 134.64 (d, JPC = 11.7 Hz, Ar C), 135.25 (d, JPC
= 8.9 Hz, Ar C), 135.91 (d, JPC = 7.8 Hz, Ar C), 171.28 (d, JPC = 4.2
Hz, HCN) (it was not possible to identify the NMR signal of four
quaternary carbons). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C): δ
39.9 (d, JPP = 20.8 Hz, PPh3), 62.1 (d, JPP = 24.2 Hz, PPh2). Single
crystals suitable for an X-ray determination were grown by slow
diffusion of diethyl ether into a CH2Cl2 solution of 1, at 20 °C.
[Ru(PPh3)(LPN)Cl2] (2). A suspension of Ru(PPh3)3Cl2 (0.1 g, 1.04
mmol) in toluene (15 mL) was treated with L (0.45 g, 1.14 mmol),
and the mixture was stirred at 0 °C for 6 h. During this time the color
changed to emerald green. Then the suspension was filtered and the
green solid was thoroughly washed with diethyl ether. Yield: 91%
(0.79 g). Anal. Calcd for C44H37Cl2NOP2Ru: C, 63.69; H, 4.49; N,
1.69. Found: C, 63.55; H, 4.41; N, 1.74. 1H NMR (400 MHz, CD2Cl2,
25 °C): δ 4.99 (q, JHH = 14.3 Hz, 2H, CH2O), 6.52 (d, 1H, NCHO),
6.66 (dd, JHH = 9.8 Hz, JHH = 3.7 Hz, 1H, Ar H), 6.79 (m, 2H, Ar H),
6.92 (m, JHH = 9.7 Hz, JHH = 8.0 Hz, JHH = 0.8 Hz, 1H, Ar H), 6.99−
7.49 (m, 26H, Ar H), 7.61 (m, 2H, Ar H), 8.17 (dd, JHH = 7.7 Hz, JHH
= 3.5 Hz, 1H, Ar H). 13C NMR (100 MHz, CD2Cl2, 25 °C): δ 67.95
(CH2O), 87.10 (d, JPC = 9.1 Hz, NCHO), 121.81 (Ar C), 125.06 (Ar
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under purified
nitrogen using standard Schlenk techniques. Solvents were dried and
distilled according to standard procedures prior to use. NMR spectra
were recorded with an AVANCE 400 Bruker spectrometer at 400
1
MHz for H NMR, 100 MHz for 13C{1H} NMR, and 162 MHz for
31P{1H} NMR. Chemical shifts are given as δ values in ppm relative to
residual solvent peaks as the internal reference (for 1H and 13C NMR)
and to external H3PO4 (85%) (for 31P NMR). J values are given in Hz.
1
13C NMR spectra are H decoupled, and the determination of the
multiplicities was achieved by the APT pulse sequence. Elemental
analyses were obtained with a Perkin-Elmer CHN Analyzer 2400
Series II. Quantitative analyses of products in the catalytic runs were
performed on a Finningan Trace GC instrument with a DB-5MS UI
capillary column (30 m, 0.25 mm) equipped with a Finningan Trace
mass spectrometer. Conductivity measurements were performed with
a digital conductimeter (Orion Research Model 101) using platinum
39
electrodes. Ru(PPh3)3Cl2
and 2-(diphenylphosphino)-
benzaldehyde40 were prepared according to literature methods; 2-
aminobenzyl alcohol, ethyl diazoacetate, triphenylphosphine (Aldrich),
and 4-picoline (Fluka) were used without further purification. Styrene
and α-methylstyrene (Aldrich) employed in catalytic runs were taken
from sealed bottles.
C), 125.45 (d, JPC = 8.9 Hz, Ar C), 125.63 (Ar C), 126.98 (d, JPC
10.5 Hz, Ar C), 127.69 (d, JPC = 9.3 Hz, Ar C), 127.83 (Ar C), 128.48
(d, JPC = 7.7 Hz, Ar C), 128.94 (Ar C), 129.48 (Ar C), 130.15 (d, JPC
=
=
13.6 Hz, Ar C), 130.52 (Ar C), 130.78 (d, JPC = 16.9 Hz, Ar C),
133.81 (Ar C), 134.23 (Ar C), 134.60 (d, JPC = 9.9 Hz, Ar C), 134.60
(d, JPC = 10.1 Hz, Ar C), 134.99 (d, JPC = 10.1 Hz, Ar C), 140.19 (Ar
C), 140.29 (Ar C). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C): δ 44.0
(d, JPP = 31.4 Hz, PPh3), 75.6 (d, JPP = 26.4 Hz, PPh2). Single crystals
suitable for an X-ray determination were grown by slow diffusion of
hexane into a CH2Cl2 solution of 2, at 20 °C.
2-(2-(Diphenylphosphino)phenyl)-2,4-dihydro-1H-benzo[d]-
[1,3]oxazine (L). To a solution of 2-(diphenylphosphino)-
benzaldehyde (1 g, 3.44 mmol) and 2-aminobenzyl alcohol (0.47 g,
3.81 mmol) in ethanol (20 mL) were added 2−3 drops of glacial acetic
acid, and the mixture was heated at 70 °C for 12 h. The solvent was
removed under reduced pressure, and the residual oil was dissolved in
CH2Cl2 (50 mL). The solution was then filtered first through a pad of
charcoal and then over Celite. The solvent was evaporated to dryness
and the residue was recrystallized with several washings with diethyl
[Ru(LPN)(cis-EtO(O)CCHCHC(O)OEt)Cl2] (3). Complex 2 (0.4
g, 0.48 mmol) was dissolved in CH2Cl2 (10 mL), and a large excess of
ethyl diazoacetate (510 μL, d = 1.085 g/mL, 4.85 mmol) was added in
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Table 3. Crystallographic and Selected Experimental Data for 1, 2·2CH2Cl2, and 3
1
2·2CH2Cl2
3
chem formula
formula wt
cryst syst
space group
cryst color and shape
cryst size (mm)
a (Å)
C44H37Cl2NOP2Ru
829.66
C46H41Cl6NOP2Ru
999.51
C34H34Cl2NO5PRu
739.56
triclinic
monoclinic
P21/c
triclinic
P1
̅
P1
̅
orange block
0.22 × 0.16 × 0.15
11.1458(7)
12.3496(8)
14.1942(10)
86.394(5)
75.615(5)
75.546(5)
1832.6(2)
2
orange block
0.20 × 0.16 × 0.13
11.9642(8)
9.9035(6)
37.701(3)
90
orange block
0.25 × 0.20 × 0.17
10.1641(6)
10.2585(6)
16.5236(10)
74.473(5)
78.171(5)
71.912(4)
1563.94(16)
2
b (Å)
c (Å)
α (deg)
β (deg)
102.818(6)
90
γ (deg)
V (Å3)
4355.8(5)
4
Z
T (K)
173(2)
173(2)
173(2)
Dc (g cm−3
)
1.504
1.524
1.570
μ (mm−1
)
0.698
0.839
0.767
scan range (deg)
3.40 < 2θ < 58.38
9881
3.50 < 2θ < 58.46
11 418
4.26 < 2θ < 58.38
8411
no. of unique rflns
no. of rflns used (I > 2σ(I))
Rint
5671
4090
5519
0.1539
0.3739
0.1557
a
final R1 and wR2 indices (I > 2σ(I))
R1 and wR2 indices (all data)
goodness of fit
0.0671, 0.0752
0.1419, 0.0886
0.925
0.1244, 0.1459
0.2826, 0.1901
0.961
0.0683, 0.1259
0.1192, 0.1423
0.988
max, min Δρ (e Å−3
)
0.527, −0.771
0.840, −1.119
0.921, −1.105
a
Structures were refined on Fo : wR2 = [∑[w(Fo − Fc2)2]/∑w(Fo )2]1/2, where w−1 = [∑(Fo ) + (aP)2 + bP] and P = [max(Fo , 0) + 2Fc2]/3.
2
2
2
2
2
small portions. The resulting mixture was stirred at room temperature
for 12 h, during which time gas evolution (N2) was noticed. Then the
solvent was carefully removed under reduced pressure, and the residue
was treated with diethyl ether, affording a light brown precipitate.
Yield: 75% (0.27 g). Anal. Calcd for C34H34Cl2NO5PRu: C, 55.22; H,
4.63; N, 1.89. Found: C, 55.11; H, 4.69; N, 1.83. IR: νmax/cm−1 1725
[Ru(4-Me-py)(LPNO)Cl2] (Mixture of 5 and 6a). Complex 1 (0.2
g, 0.241 mmol) was suspended in 10 mL of solvent (toluene, 1,2-
dichloroethane, THF, or CH3CN), and 4-picoline (47 μL, 0.482
mmol) was added in one portion. The suspension was refluxed for 6 h,
and then the solid was filtered off and washed with diethyl ether.
31P{1H} NMR revealed the crude product as being a mixture with
different proportions of two compounds, identified as [Ru(4-Me-
py)(LPNO)Cl2] (5) and [Ru(4-Me-py)2(LPNO)Cl](Cl) (6a).
1
(OCOEt), 1622 (Ru−OCOEt). H NMR (400 MHz, CD2Cl2,
25 °C): δ 1.37 (t, JHH = 7.2 Hz, 3H, CH3), 1.43 (t, JHH = 7.2 Hz, 3H,
CH3), 4.24 (m, 2H, CH2), 4.45 (m, JHH = 28.0 Hz, JHH = 10.7 Hz, JHH
= 7.1 Hz, 2H, CH2), 4.84 (d, JHH = 13.8 Hz, 1H, Hb part of an AB
system, CH2O), 4.99 (d, JHH = 13.7 Hz, 1H, Ha part of an AB system,
CH2O), 6.52 (t, JHH = 9.2 Hz, 1H, HCC), 6.60 (d, 1H, NCHO),
6.68 (ddd, JHH = 10.7 Hz, JPH = 7.7 Hz, JHH = 0.9 Hz, 1H, HCC),
7.09 (dd, JHH = 7.2 Hz, JHH = 0.6 Hz, 1H, Ar H), 7.16 (t, JHH = 7.2 Hz,
1H, Ar H), 7.24 (dt, JHH = 7.3 Hz, JHH = 1.2 Hz, 2H, Ar H), 7.37−7.57
(m, 10H, Ar H), 7.67 (tt, JHH = 7.7 Hz, JHH = 1.2 Hz, 2H, Ar H), 8.13
(dd, JHH = 7.8 Hz, JHH = 4.0 Hz, 2H, Ar H). 13C NMR (100 MHz,
CD2Cl2, 25 °C): δ 13.83 (CH3), 14.51 (CH3), 60.43 (CH2CH3), 63.48
(HCC), 63.87 (CH2CH3), 64.09 (HCC), 68.63 (CH2O), 88.37
(d, JPC = 13.6 Hz, NCHO), 124.05 (Ar C), 124.62 (d, JPC = 8.6 Hz, Ar
[Ru(4-Me-py)(LPNO)Cl2] (5). Complex 1 (0.2 g, 0.241 mmol) was
suspended in 10 mL of m-xylene, and 4-picoline (47 μL, 0.482 mmol)
was added in one portion. Refluxing the suspension for 6 h afforded a
deep red solid, which was washed thoroughly with Et2O and dried in
vacuo. Yield: 67% (0.107 g). Anal. Calcd for C32H29Cl2N2OPRu: C,
1
58.19; H, 4.43; N, 4.24. Found: C, 58.31; H, 4.59; N, 4.79. H NMR
(400 MHz, CD2Cl2, 25 °C): δ 2.26 (s, 3H, CH3), 4.49 (d, 3JHH = 11.0
2
3
Hz, 1H, OH), 4.92 (dd, JHH = 11.6 Hz JHH = 1.6 Hz, 1H, Hb of
2
3
CH2O), 5.64 (t, JHH = JHH = 11.4 Hz, 1H, Ha of CH2O), 6.67 (m,
JHH = 8.2 Hz, JHH = 1.5 Hz, 1H, Ar H), 6.76 (d, JHH = 6.1 Hz, 2H,
picoline Hm), 6.97 (t, JHH = 8.4 Hz, 1H, Ar H), 7.08 (t, JHH = 6.9 Hz,
2H, Ar H), 7.25 (t, JHH = 6.9 Hz, 1H, Ar H), 7.30−7.59 (m, 13H, Ar
H), 8.64 (d, JHH = 11.0 Hz, 2H, picoline Ho), 8.92 (s, 1H, HCN).
13C{1H} NMR (100 MHz, CD2Cl2, 25 °C): 20.34 (CH3), 65.84
C), 125.34 (d, JPC = 9.2 Hz, Ar C), 127.24 (Ar C), 127.89 (d, JPC
=
10.4 Hz, Ar C), 128.59 (d, JPC = 10.1 Hz, Ar C), 128.81 (d, JPC = 11.6
Hz, Ar C), 129.06 (d, JPC = 7.7 Hz, Ar C), 129.34 (Ar C), 130.05 (Ar
C), 130.92 (Ar C), 131.15 (Ar C), 131.20 (Ar C), 131.37 (Ar C),
133.17 (d, JPC = 5.3 Hz, Ar C), 134.07 (d, JPC = 12.7 Hz, Ar C), 139.65
(d, JPC = 11.6 Hz, Ar C), 140.19 (Ar C), 171.91 (C(O)OEt), 178.51
(C(O)OEt). 31P{1H} NMR (162 MHz, CD2Cl2, 25 °C): δ 51.5 (s,
PPh2). Single crystals suitable for an X-ray determination were grown
by slow diffusion of diethyl ether into a CH2Cl2 solution of 3, at 20 °C.
[Ru(PPh3)(4-Me-py)(LPN)Cl2] (4). Compound 2 (0.2 g, 0.241
mmol) was suspended in diethyl ether (10 mL), and 4-picoline (47
μL, 0.482 mmol) was added in one portion. The suspension was
stirred for 6 h, and then the orange solid was filtered off, washed with
diethyl ether (10 mL), and then dried under vacuum. Yield: 86%
(0.191 g). Anal. Calcd for C50H44Cl2N2OP2Ru: C, 65.08; H, 4.81; N,
3.04. Found: C, 64.92; H, 4.98; N, 3.19.
(CH2O), 121.88 (Ar C), 123.77 (picoline Cm), 127.19 (Ar C), 127.59
(d, JPC = 9.4 Hz, Ar C), 127.89 (Ar C), 128.35 (Ar C), 129.41 (AR-c),
129.67 (Ar C), 130.61 (Ar C), 130.12 (d, JPC = 13.2 Hz), 131.35 (Ar
C), 131.67 (d, JPC = 5.9 Hz, Ar C), 133.69 (d, JPC = 9.0 Hz, Ar C),
134.05 (d, JPC = 9.2 Hz, Ar C), 134.54 (d, JPC = 7.9 Hz, Ar C), 138.39
(d, JPC = 14.2 Hz, Ar C), 146.72 (Ar C), 154.65 (Ar C), 155.49
(picoline Co), 170.93 (s, HCN). 31P{1H} NMR (162 MHz, CD2Cl2,
25 °C): δ 73.3 (s, PPh2).
[Ru(4-Me-py)2(LPNO)Cl](Cl) (6a) (5/1 Mixture with Compound
5). Compound 1 (0.2 g, 0.241 mmol) was suspended in 10 mL of m-
xylene, and an 8-fold excess of 4-picoline (190 μL, 1.952 mmol) was
added in one portion. Refluxing the suspension for 6 h afforded a deep
red solid, which was washed thoroughly with Et2O and dried in vacuo.
1
Data for compound 6a are as follows. H NMR (400 MHz, CD2Cl2,
3
25 °C): δ 2.23 (s, 3H, CH3), 2.24 (s, 3H, CH3), 2.41 (d, JHH = 3.2
5435
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Organometallics
Article
2
3
Hz, 1H, OH), 2.59 (dd, JHH = 11.8 Hz JHH = 2.4 Hz, 1H, Hb of
ASSOCIATED CONTENT
■
2
3
CH2O), 4.22 (dd, JHH = 11.9 Hz JHH = 4.6 Hz, 1H, Ha of CH2O),
6.77−7.75 (m, 22H, Ar H), 8.44 (d, JHH = 2.9 Hz, 2H, picoline Ho),
8.58 (d, JHH = 3.4 Hz, 2H, picoline Ho), 9.01 (s, 1H, HCN).
13C{1H} NMR (100 MHz, CD2Cl2, 25 °C): 20.46 (CH3), 20.48
(CH3), 62.70 (CH2O), 118.93 (Ar C), 124.34 (picoline Cm), 124.63
(picoline Cm), 127.06 (Ar C), 127.84 (d, JPC = 9.5 Hz, Ar C), 128.89
(d, JPC = 1.8 Hz, Ar C), 129.02 (d, JPC = 2.8 Hz, Ar C), 129.84 (d, JPC
= 2.7 Hz, Ar C), 130.51 (Ar C), 130.70 (Ar C), 131.91 (Ar C), 132.15
S
* Supporting Information
Text, figures, tables, and CIF files giving full NMR spectra (1H,
1
13C{1H}, 31P{1H} and H−31P HMBC) of LPN and complexes
1−3, 5, and 6a, crystallographic data for compounds 1−3,
illustrations of the possible diastereoisomers of 2 and 3,
computational details, and a full list of authors for ref 42. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(d, JPC = 6.3 Hz, Ar C), 132.36 (Ar C), 132.76 (Ar C), 133.26 (d, JPC
=
9.7 Hz, Ar C), 134.29 (d, JPC = 8.0 Hz, Ar C), 138.32 (d, JPC = 13.5
Hz, Ar C), 147.63 (Ar C), 147.83 (Ar C), 153.04 (picoline Co), 155.50
(picoline Co), 171.97 (d, JPC = 4.5 Hz, HCN). 31P{1H} NMR (162
MHz, CD2Cl2, 25 °C): δ 61.0 (s, PPh2).
Synthesis of 6a from 5. Compound 5 (0.15 g, 0.227 mmol) was
suspended in 10 mL of m-xylene, and a 4-fold excess of 4-picoline (90
μL, 0.925 mmol) was added in one portion. Refluxing the suspension
for 6 h afforded 6a as the major product (as evidenced by 31P{1H}
NMR of the bulk product).
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +39-(0)31-2386476. Fax: +39-(0)31-2386119. E-mail:
stefano.brenna@uninsubria.it.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
General Procedure for Cyclopropanation of Styrene and α-
Methylstyrene. In a standard experiment, to a solution of catalyst
(0.02 mmol) in dichloromethane (8 mL) at room temperature and
under an inert atmosphere was added the olefin (4 mmol) in one
portion (catalyst/olefin molar ratio 1/200). Then, ethyl diazoacetate
(2 mmol) was added dropwise over a period of 4 h, and the reaction
mixture was stirred at 20 °C for an additional 4 h. The consumption of
EDA was monitored by infrared spectroscopy (νNN). Yields and
diastereoselectivity (cis/trans ratio) were determined by GC-MS
analysis of the bulk product by comparison with pure samples.25d
Computational Details. Geometries were optimized, without
symmetry constraints, employing the standard B3LYP hybrid
functional as implemented in the ADF 2012.01 program suite.41 AE-
TZ2P basis sets (C, H, N, P, Cl) combined with the AE-QZ4P basis
set (Ru) were used. Scalar relativistic effects on ruthenium atoms were
treated by applying the zeroth-order regular approximation (ZORA).42
Starting geometries for 1 and 2 were taken from experimental
Cartesian coordinates obtained by the X-ray diffraction data, while the
PM6 semiempirical method,43 as implemented in the MOPAC200944
program package, was employed to preoptimize geometries for 6a and
6b cations. All geometry optimizations were performed simulating a
CH2Cl2 solvation using the COSMO dielectric continuum model as
implemented in ADF. Frequency analyses were performed to ensure
all optimized molecular structures were real minima (no imaginary
frequencies) (see the Supporting Information).
X-ray Crystallographic Study. Crystals of 1, 2·2CH2Cl2, and 3
were mounted on a Stoe Mark II-Image Plate Diffraction System, using
Mo Kα graphite-monochromated radiation, image plate distance 135
mm, 2θ range from 2.4 to 51.3°, Dmax−Dmin = 16.029−0.836 Å. The
structures were solved by direct methods using the program SHELXS-
97.45 Refinement and all further calculations were carried out using
SHELXL-97.45 The H atoms were included in calculated positions and
treated as riding atoms using the SHELXL default parameters. The
non-H atoms were refined anisotropically, using weighted full-matrix
least squares on F2. Despite several attempts to get better crystals of
complex 2 and a better data set, only poor-quality data were obtained.
Nevertheless, the molecular structure of 2 confirms the suggested
structure based on the compiled NMR data. Crystallographic details
are summarized in Table 3. Figures 2−4 (ORTEP drawings) were
drawn with Mercury CSD 3.0 software.
■
Dr. E. Alberti is greatly acknowledged for setting up the
1H−31P HMBC experiments.
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