Syntheses of a novel fluorinated trisphosphinoborate ligand and its copper and silver complexes. Catalytic activity toward nitrene transfer reactions

Article abstract of DOI:10.1021/ic4028223

A novel fluorinated ligand, the anionic PhB(CH₂P(p-CF ₃C₆H₄)₂)₃ (PhBP ₃ ^p-CF3Ph), has been synthesized and characterized, as well as its corresponding thallium, copper

Full text of DOI:10.1021/ic4028223

Article
pubs.acs.org/IC
Syntheses of a Novel Fluorinated Trisphosphinoborate Ligand
and Its Copper and Silver Complexes. Catalytic Activity
toward Nitrene Transfer Reactions
†
‡
§
,†
,‡
́
́
Ismael Arenas, M. Angeles Fuentes, Eleuterio Alvarez, Yolanda Díaz,* Ana Caballero,*
Sergio Castillon,*,† and Pedro J. Perez*,‡
́
́
†Departament de Química Analítica i Química Organ
Spain
‡Laboratorio de Catal
̀
ica, Universitat Rovira i Virgili, C/Marcel·lí Domingo, s/n, 43007 Tarragona,
́
isis Homogen
́
́
ea, Unidad Asociada al CSIC, CIQSOCentro de Investigacion en Química Sostenible and
Departamento de Química y Ciencia de los Materiales, Campus de El Carmen s/n, Universidad de Huelva, 21007-Huelva, Spain
§Instituto de Investigaciones Químicas Isla de la Cartuja, CSICUniversidad de Sevilla, Avenida de Amer
́
ico Vespucio 49,
41092-Sevilla, Spain
S
* Supporting Information
ABSTRACT: A novel fluorinated ligand, the anionic PhB(CH2P(p-
CF3C6H4)2)3 (PhBP3 p‑CF3Ph), has been synthesized and characterized, as
well as its corresponding thallium, copper, and silver derivatives. The
presence of fluorine atoms in the ligand structure induced the desired
effect of enhancing electrophilic character at the metal center, without
promoting substantial changes in the ligand skeleton compared with the
−
parent ligand PhB(CH2PPh2)3 (PhBP3). Olefin aziridination and C−H
amidation reactions have been induced with those complexes as catalyst
precursors. The copper derivative catalyzed the olefin aziridination of an
array of olefins bearing either electron-donating or electron-withdrawing
groups. The silver analogue was found to promote the C−H amidation of
a series of substrates in moderate to high yields.
complex was found to be a good catalyst for reactions that did not
require a somewhat electron deficient metal center, such as
carbene or nitrene transfer reactions to unsaturated substrates,
insertion into X−H bonds, or reactions in which a relatively
electron-rich metal center is preferred, such as the Atom Transfer
Radical Addition (ATRA) or Atom Transfer Radical Polymer-
ization (ATRP) (Scheme 1).5
INTRODUCTION
■
Trispyrazolylborate ligands (Tpx, Figure 1) have contributed
with great achievements to organometallic chemistry and
Unfortunately, complex 1 did not catalyze reactions in which a
more electrophilic metal center is desired.6,7 We wondered
whether the attachment of electron-withdrawing groups to the
aryl moieties of the ligand could induce such an effect. Since such
ligands have not been reported yet, we focused on a synthetic
route of the derivative containing a trifluoromethyl group in the
para position (Scheme 2). To verify if such incorporation
originates the desired effect at the metal center, we have chosen
metal-catalyzed nitrene transfer reactions as probes (Scheme 2):
the electron-poor olefin aziridination for which the use of
complex 1 proved to be unsuccessful and the more challenging
C−H bond amidation reaction, that could not be observed in the
presence of 1 as the catalyst. In this contribution we describe the
synthesis and characterization of a novel trisphosphinoborate
Figure 1. Trispyrazolylborate and trisphosphinoborate ligands.
homogeneous catalysis.1 These are formed by three pyrazolyl
rings bonded to a tetracoordinated boron atom. Those
heterocycles may bear up to three different substituents,
providing a great number of Tpx ligands with variable steric
and electronic properties. Usually, these ligands form very stable
metal complexes.2 However, their syntheses are laborious and
require high temperatures, and often yields are low to moderate.
Because of this, we have recently focused on another family of
3,4
−
ligands, the trisphosphinoborate PhB(CH2PPh2)3 . We re-
cently described5 the synthesis and the catalytic properties of the
copper complex [PhB(CH2PPh2)3]Cu(PPh3) (1) bearing the
tridentate ligand PhB(CH2PPh2)3− (PhBP3 from now on). This
Received: November 13, 2013
Published: April 4, 2014
© 2014 American Chemical Society
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Deprotection of the phosphine−borane complex was readily
accomplished upon heating the ligand with an excess of
diethylamine overnight.9 Examination of an aliquot by 31P{1H}
NMR showed a clean conversion, as inferred from the
observation of a single resonance at −7.2 ppm, at variance with
that of 19.5 ppm of the BH3-protected species (Scheme 3). A
transmetalation reaction with TlNO3 was carried out with the
previous crude product,10 affording the Tl[PhB(CH2P(p-
CF3C6H4)2)3] as a white solid in 65% yield. The 31P{1H}
NMR consisted of a broad signal centered at 1.4 ppm. Although
multinuclear NMR data were in accord with the expected
product, an X-ray diffraction study, carried out with single
crystals grown from slow diffusion of hexane into a dichloro-
methane solution of the complex at −20 °C, was performed in
order to verify the structure. Figure 2 shows the ORTEP view of
Scheme 1. Catalytic Activity Reported for
[(PhBP3)Cu(PPh3)] (1)
Scheme 2. Targeted New Ligand and Probe Catalytic
Reactions in This Work
Figure 2. Molecular structure of Tl[PhBP3p‑CF3Ph]. Thermal ellipsoids
are drawn at the 50% probability level. H atoms have been omitted for
clarity. Selected bond lengths (Å) and bond angles (deg) for
p‑CF3Ph
PhBP3
ligand: Tl(1)−P(1) = 2.932(2), Tl(1)−P(2) =
2.9626(18), Tl(1)−P(3) = 2.9647(18), P(1)−Tl(1)−P(2) =
73.97(5), P(1)−Tl(1)−P(3) = 74.57(5), P(2)−Tl(1)−P(3) =
73.15(5).
ligand containing fluorinated, electron-withdrawing groups in
the P-substituents and the corresponding copper and silver
complexes. The presence of those groups affected the electron
density at metals in such a way that catalytic properties not
observed with the parent ligand are now described.
the molecules of Tl[PhB(CH2P(p-CF3C6H4)2)3], corroborating
the coordination of the novel PhBP3p‑CF3Ph ligand to the thallium
center in a tridentate fashion. When compared with the parent
PhBP3− ligand, the presence of the CF3 groups in the ligand did
not significantly modify the Tl−P distances (average 2.95 Å for
PhBP3p‑CF3Ph and 2.92 Å for PhBP3) or the P−Tl−P bond angles
(73.89° for PhBP3p‑CF3Ph and 75.02° for PhBP3).
RESULTS AND DISCUSSION
■
Synthesis of Tl[PhB(CH2P(p-CF3C6H4)2)3]. The synthesis
of Tl[PhB(CH2P(p-CF3C6H4)2)3] (PhBP3p‑CF3Ph from now on)
is illustrated in Scheme 3, following a reaction sequence similar to
that reported for the parent Tl[PhB(CH2PPh2)3] compound.3,4
First, dichloromethyl phosphine was reacted with 2 equiv of
p-CF3C6H4MgBr in Et2O, followed by protection of the phos-
phine with BH3·THF. The boronate group plays an important
role, since it protects the phosphine from oxidation and increases
the acidity of the methyl group, thus favoring the next step.
Attempts to carry out deprotonation of unprotected phosphine
led to complex mixtures, as previously reported by Peters and co-
workers.8 Consequently, MeP(p-CF3C6H4)2(BH3) was reacted
with sec-BuLi at −78 °C for 3 h, and then PhBCl2 was added,
allowing the mixture to slowly warm to room temperature
overnight. After workup, the lithium salt of BH3-protected
trisphosphine was obtained in 80% yield. The 1H NMR spectrum
showed a unique set of resonances for the aryl substituents,
consistent with the existence of PhB(CH2P(BH3)(p-CF3C6H4)2)3¯
anions. The 11B{1H} and 19F{1H} NMR spectra were also in
agreement with such a formulation (see Experimental Section).
Synthesis of Copper and Silver Complexes Bearing the
PhP(CH2PAr2)3¯ Ligands (Ar = Ph; p-CF3C6H4). The new
copper and silver complexes were obtained following a
procedure similar to that employed for complex 1.5 Therefore,
complex [PhP(CH2P(p-CF3C6H4)2)3]Cu(PPh3) (2) was pre-
pared upon reacting Tl[PhBP3p‑CF3Ph], copper iodide, and PPh3
in tetrahydrofuran as the solvent, at room temperature, and
isolated in 80% yield (eq 1). The 31P{1H} NMR spectrum
showed two broad signals centered at 11.1 and −5.7 ppm
corresponding to the P(p-CF3Ph)2 and PPh3 ligands, respec-
tively. VT NMR experiments at −80 °C only showed a slight
sharpening of the resonance, with no observation of defined
lines. The resonances of the P nuclei of the tripodal ligand shifted
to lower field compared with that of complex 1 (−5.8 ppm),5 due
to the presence of the fluorine atoms in the ligand framework.
Multinuclear NMR data collected were also in agreement with
the existence of a C3 symmetry axis in solution, located along the
B−Cu axis (see Experimental Section). The effect of the
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Scheme 3. Synthesis of Tl[PhB(CH2P(p-CF3C6H4)2)3]
fluorinated groups on the electronic properties at the metal
center was evaluated upon bubbling carbon monoxide through a
solution of 2 in THF, and the concomitant generation of the
carbonyl derivative [PhB{CH2P(p-CF3C6H4)2}3]Cu(CO). The
solution IR spectrum showed one absorption centered at
2068 cm−1, corresponding to ν(CO), assessing a lower electronic
density at the metal center in 2 compared with that of 1
(2055 cm−1).5 In our related investigations with TpxM (M = Cu,
Ag), we have observed that the attachment of ligands containing
electron-withdrawing groups induces an increase in the ν(CO)
values of the TpxM(CO) complexes, as well as a parallel augment
of the catalytic activity toward carbene or nitrene transfer
reactions.6,7 Thus, the observed increase from 1 to 2 seems to
point toward the targeted goal, as discussed below.
that described for 1, although some comments have to be
addressed. The P−Cu−P average angle for the tripodal donor
atoms is 94.6°, slightly lower than that observed for 1 (95.7°).
The PPh3 ligand occupies the fourth coordination site, being
displaced from the ideal position along the B−Cu axis (B−Cu−P
angle of 174.71°). This effect has also been observed in related
complexes of formula TpxCu(NCMe).11Overall, the presence of
CF3 groups in the para position of the phenyl groups of the
tripodal ligand has little effect on the structural parameters.
AgCl or AgOTf
PPh3
M[PhB(CH2PAr2)3] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [PhB(CH2PAr2)3]Ag(PPh3)
THF, rt
Ar=Ph, 3
M=TI, Li
Ar=p‐CF3C6H4, 4
(2)
TI[PhB(CH2P(p‐CFC6H4)2)3]
3
The silver analogues of 1 and 2 were prepared following a
related procedure. The corresponding ligand was reacted with
the silver source (see the Experimental Section) and 1 equiv of
PPh3 in tetrahydrofuran as the solvent (eq 2) to afford the
complexes [PhP(CH2PPh2)3]Ag(PPh3) (3) and [PhP(CH2P-
(p-CF3C6H4)2)3]Ag(PPh3) (4), in 70% and 75% yield, respectively.
In contrast with the copper analogues (which gave broad signals),
CuI,PPh3
⎯⎯⎯⎯⎯⎯⎯⎯→ [PhB(CH2P(p‐CFC6H4)2)3]Cu(PPh3)
3
(2)
THF
(1)
rt, 20 min
The solid state structure of complex 2 has been elucidated
(Figure 3). The copper ion is located in a distorted tetrahedron
formed by the four phosphorus atoms. The structure is similar to
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splitting due to both metal isotopes (1J(P−109Ag) = 215 Hz,
2
1J(P−107Ag) = 192.3 Hz, J(P−P) = 30 Hz). Complex 4 showed
similar NMR spectra (see Experimental Section).
Single crystals of 3 were obtained, and an X-ray diffraction
study was carried out, with the structure of the molecules being
shown in Figure 5. Marks and co-workers have described12
Figure 3. Molecular structure of 2. Thermal ellipsoids are drawn at the
50% probability level. H atoms have been omitted for clarity. Selected
bond lengths (Å) and bond angles (deg) for 2: Cu(1)−P(1) =
2.3399(10), Cu(1)−P(2) = 2.3640(10), Cu(1)−P(3) = 2.3315(10),
Cu(1)−P(4) = 2.2823(10), P(1)−Cu(1)−P(2) = 94.11(4), P(1)−
Cu(1)−P(3) = 95.69(3), P(1)−Cu(1)−P(4) = 117.47(4), P(2)−
Cu(1)−P(3) = 94.03(4), P(2)−Cu(1)−P(4) = 126.25(4), P(3)−
Cu(1)−P(4) = 121.96(4).
the 31P{1H}NMR spectrum was highly informative. Figure 4 shows
that of 3, which displayed one doublet of pseudoquintets at
16.2 ppm and a doublet of doublets of doublets, centered at
−2.7 ppm, with relative integrals of 1:3, therefore corresponding to
PPh3 and the tripodal ligand. The observed pattern of lines is
explained as the result of the coupling of the P nuclei to the two
NMR active isotopes of silver, 107Ag, and 109Ag. Thus, both
pseudoquintets must be considered as four quartets with partial
overlap of lines (Figure 4). The higher coupling constants correspond
to those induced by the heavier isotope 109Ag (1J(P−109Ag) =
397.4 Hz, 1J(P−107Ag) = 338.6 Hz, 2J(P−P) = 30 Hz). The high
field resonance of the coordinated PPh2 moiety also shows the
Figure 5. Molecular structure of 3. Thermal ellipsoids are drawn at the
50% probability level. H atoms have been omitted for clarity. Selected
bond lengths (Å) and bond angles (deg) for 3: Ag(1)−P(1) =
2.5432(6), Ag(1)−P(2) = 2.5522(6), Ag(1)−P(3) = 2.5210(6),
Ag(1)−P(4) = 2.4380(6), P(1)−Ag(1)−P(2) = 92.660(18), P(1)−
Ag(1)−P(3) = 88.164(18), P(1)−Ag(1)−P(4) = 128.166(19), P(2)−
Ag(1)−P(3) = 90.802(18), P(2)−Ag(1)−P(4) = 120.89(2), P(3)−
Ag(1)−P(4) = 125.559(19).
related complexes such as [PhB{CH2P(Ph)2}3]Ag(PEt3). The
structure of 3 is similar to that, with a distorted-tetrahedral
geometry consisting of four P donors (from the tridentate PhBP3
Figure 4. 31P{1H} NMR spectrum of complex 3, showing the different P−P, P−107Ag, and P−109Ag coupling constants.
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and the PPh3 ligands) around the silver atom. The distances and
angles also compared well: the P−Ag−P average value angle for
the tripodal donor atoms of 90.5° is almost identical to that found
for [PhB{CH2P(Ph)2}3]Ag(PEt3) (89.3°), and the Ag−P
distances averaged 2.44 Å (2.40 for [PhB{CH2P(Ph)2}3]-
Ag(PEt3)). The PPh3 ligand deviates only slightly from the
B−Ag axis (B−Ag−P angle of 178.24°) when compared with the
copper analogue.
Scheme 5. Aziridination of Styrenes Using Complex 2 as
Catalyst
a b
,
A final comment on the hydroscopic behavior of the
compounds containing this new fluorinated ligand has to be
noted. The three compounds with thallium, copper, or silver
are isolated as crystalline materials, with NMR spectra showing
the expected resonances. However, in all cases those spectra (see
Supporting Information) showed the presence of water due to
that hydroscopic nature. Even under extreme dried and
anhydrous handling, water was incorporated to the samples,
precluding the achievement of good elemental analyses. We
believe that the presence of the fluorinated groups must be
related to this behavior, since the copper and silver complexes 1
and 3 with the parent BP3 ligand provided such data within the
appropriate experimental error.
Nitrene Transfer Reactions Catalyzed by (PhBP3p‑CF3Ph)-
M(PPh3) (M = Cu, 2; Ag, 4). (a) Olefin Aziridination
Reactions. We first screened the catalytic activity of complexes
2 and 4 toward the aziridination of styrene using PhI=NTs
as the nitrene source (Scheme 4).13 The copper-based catalyst
Scheme 4. Styrene Aziridination Catalyzed by Complexes 2
and 4
a
General conditions: 5 mmol % Cu-complex, 0.1 mmol of PhINTs,
b
0.5 mmol of alkene, 5 mL of dichloromethane, 5 h, rt. Isolated yields,
average of at least two independent runs. One mmol of alkene
employed. Only trans-aziridine observed.
c
d
In addition, electron-donating styrenes also provided moderate
to good yields (5k−5m). Sterically hindered alkenes, such as
2-vinylnaphthalene and 4-vinylbiphenyl, were also aziridinated
(products 5n−5o).
Aziridines from indene and 1,2-dihydronaphthalene, however,
were obtained in moderate yields, probably as a result of an
increase in bulkiness (5p−5q). Interestingly, trans-β-methyl
styrene and trans-stilbene (5r−5s) were stereoselectively
aziridinated, with only trans-aziridines being observed in the
1H NMR spectra of the reaction crudes. An experiment with cis-
stilbene gave the trans-aziridine as the major product (trans:cis
ratio = 4:1). Finally, substitution at the α-position of the alkene
did not affect the reaction (5t).
We have recently proposed a general mechanism for the olefin
aziridination reaction catalyzed by TpxCu complexes.16 On the
basis of an experimental and theoretical study, we proposed that
this transformation is triggered by the formation of a triplet
metallonitrene intermediate that reacts with the olefin to give a
radical intermediate, also in the triplet state (Scheme 6). For aryl-
or alkyl-substituted olefins, the pathway continues by spin
crossing with the open shell singlet surface and ring closing to
yield aziridine and revert TpCuI to restart the catalytic cycle.
Thus, the observance of retention of the initial geometry of the
olefin in the final aziridines is explained on the basis of that
crossing, that takes place before rotation around the C−C bond
might occur. The radical nature of the initial metallonitrene
intermediate was assessed by the fitting of experimental data
provided high yields of the expected aziridine whereas the
silver analogue gave poor yields. This is in contrast with
recent results from our laboratories with TpxM (M = Cu, Ag)
complexes where both metals exhibited quite good catalytic
activities for the aziridination of carbon−carbon double
bonds, although with the assistance of directing groups in
that case.13d
Therefore, we screened the use of catalyst 2 in the aziridination
of alkenes (20 examples, Scheme 5). The standard conditions
used were a 1:20:100 ratio of [2]:[PhINTs]:[olefin] stirred at
room temperature for 5 h. A wide range of styrenes were
aziridinated in good to excellent yields, including some with
electron-withdrawing groups (5a−5f). Interestingly, there are very
few examples in the literature of catalytic systems based on
copper that induce the aziridination of electron-poor olefins to a
similar extent compared to those found with electron-rich
olefins. And this is in spite of the seminal work by Evans and
co-workers, nearly 20 years ago, that demonstrated that the
bis(oxazoline)-copper catalyst could convert cinnamates into the
aziridines derivatives.14 Lebel and co-workers have recently
applied a similar system to p-nitrostyrene with good results.15
Different substitution patterns along the ring also provided
good yields (5g−5i), with even the pentafluorostyrene derivative
being converted with an 82% yield (5j). This feature is exclusive
of complex 2, since 1 does not provide such a reaction outcome.
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Scheme 6. Mechanism for the Olefin Aziridination Proposed
with Trispyrazolylborate-Containing Copper Catalysts
Involving Singlet and Triplet Pathways
Figure 6. Plot of Experimental log (KX/KH) vs Calculated σ+ρ+ + σ•ρ•.
of these groups of stabilizing benzyl-radicals when located in the
para position explains the observed decrease of the catalytic
activities of the corresponding meta-isomers.
from competition experiments with p-substituted styrenes to a
double-parameter Hammett equation (eq 3).17
We have also carried out two additional experiments. First, a
well-known radical trap such as BHT (2,6-bis(tert-butyl)-
4-hydroxytoluene) was added to the reaction mixture, in an
attempt to inhibit the reaction. However, we did not observe
such inhibition, just a decrease from 80 to 50%, in line with our
previous work with Tpx ligands,16 probably due to fast ring
closing before BHT can trap the C-centered radical similar to 3RI
in Scheme 5. A second experiment consisted of the use of 1,1-
dicyclopropylethylene, a radical clock, as the olefin, and 2 as the
catalyst (eq 5). If C-centered radicals survive enough, then
log(KX/KH) = σ+ρ+ + σ•ρ•
(3)
Since most authors have frequently invoked that the
mechanism of the metal-catalyzed aziridination reaction is
system-dependent,18 we have focused on providing data to
extend our previous proposal to this catalytic system. A series of
competition experiments with p-substituted styrenes (eq 4) were
carried out. In a typical procedure, equimolar mixtures of styrene
and the p-substituted styrene were reacted with PhINTs in the
presence of 2 (1:20:100:100 ratio of 2:PhINTs:p-substituted
styrene:styrene). The relative ratios of the corresponding
aziridines were determined by H NMR spectroscopy. Data
shown in Table 1 only fit into the dual-parameter Hammett
1
cyclopropyl-opening products should be observed to a certain
extent in the reaction mixture. However, we did not detect any
ring-opening products, but an imine derived from the copper-
nitrene derivative, as also observed for the Tpx-based system. The
formation of this compound can be inferred to result from high
steric induction by both cyclopropyl groups, thus preventing ring
closing and aziridine formation.17 Both results are in accord with
a short-lived radical intermediate that undergoes ring-closing
prior to C−C rotation.
Therefore, data collected supports that this trisphosphinoborate
copper(I) catalytic system seems to operate through similar stepwise
pathways than those proposed for the related TpxCu-based system, in
which both the triplet and the singlet states are involved.
(b) C−H Amidation Reactions. We have also studied the
catalytic capabilities of complexes 2 and 4 toward the insertion of
nitrene moieties into carbon−hydrogen bonds. This reaction has
been reported in a yet reduced number of cases.20,21 In a probe
reaction, we employed tetrahydrofuran as the substrate. As
shown in Scheme 7, only complex 4 exhibited a successful
catalytic performance. It is worth mentioning that the copper
complex 1 did not catalyze this reaction, in contrast with other
copper-based catalysts that we have previously reported to
induce this transformation.7,21,22
The capabilities of the silver complex toward this reaction were
further exploited with a series of substrates bearing C−H bonds
at benzylic positions or vicinal to O-atoms (Scheme 8). These
substrates were functionalized in moderate to good yields.
Table 1. Competition Experiments with p-Substituted
Styrenes Using 2 as Catalyst
X
KX/KH
σ+
σ•
OMe
Me
Cl
1.86
1.38
1.19
1.41
1.17
1
−0.78
−0.31
0.11
0.42
0.39
0.18
0.42
0.12
0
Ph
−0.18
−0.07
0
F
H
NO2
0.83
0.79
0.76
equation (eq 3).17 This equation has been previously employed
to account for reactions involving both polar and radical effects of
substituents.13b,16,19 A good fit with eq 3 (and Jackson’s
constants)17b was obtained with ρ+ = −0.24 and ρ• = 0.11
(Table 1 and Figure 6). The negative value of ρ+ is consistent
with the existence of an electrophilic transition state whereas the
positive value of ρ• indicates the intermediacy of radical
contributions. The ρ+/ρ• value of 2.2 is higher than any of
those recently reported for TpxCu.16 Moreover, the proposal of
radical species is in agreement with the observance of the
different reactivity of the meta- and para- substituted styrenes
bearing nitro or bromo substituents. The well-known capabilities
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purchased from Aldrich and Alfa-Aesar and were used as received.
Flash column chromatography was carried out using a forced flow of the
indicated solvent on Merck silica gel 60 (230−400 mesh). Reactions
were monitored by TLC carried out on 0.25 mm E. Merck silica gel
60 F254 glass or aluminum plates. Multinuclear NMR spectra were
recorded on a Varian Mercury 400 MHz instrument. Chemical shift
values for 1H and 13C were referred to internal SiMe4 (0.0 ppm), and for
31P, they were referred to H3PO4 (85% solution in D2O, 0 ppm).
Synthesis of MeP(p-CF3C6H4)2(BH3). The Grignard reagent
p-CF3PhMgBr was generated from p-CF3PhBr and Mg in Et2O. In a
separate flask MePCl2 (1.80 mL, 18 mmol) was dissolved in Et2O
(23 mL) and cooled to 0 °C. The solution of the aryl Grignard reagent
was slowly added via cannula over the cold MePCl2 solution. The
reaction mixture was then stirred at rt for 2 h, before the reaction mixture
was quenched with a saturated aqueous solution of NH4Cl (10 mL). The
organic layer was washed with water (2 × 20 mL) and dried over
magnesium sulfate. Removal of the volatiles provided the MeP(p-CF3Ph)2
phosphine as a colorless oil. The crude was then dissolved in THF
(4 mL), and BH3·THF (23.5 mmol, 23.5 mL of a 1 M solution) was
slowly added. The mixture was stirred overnight before adding EtOH
(15 mL), with the resulting solution being concentrated under reduced
pressure to give an oily residue that was purified by column chromato-
graphy applying hexanes:ethyl acetate (25:1). MeP(p-CF3C6H4)2(BH3)
was isolated as a colorless oil (3.27 g, 52% yield (over 2 steps)). 1H NMR
(CDCl3, 400 MHz): δ = 7.81−7.76 (m, 4H, p-CF3Ph), 7.73−7.71 (m,
4H, p-CF3Ph), 1.94 (d, 3H, J = 9.6 Hz, CH3-P), 1.50−0.50 (br, 3H, P−
BH3). 13C{1H} NMR (CDCl3, 100.6 MHz): δ = 134.4 (d, J = 53 Hz,
ipso-PPh), 133.6 (d, J = 33 Hz, o-PPh), 132.4 (d, J = 10 Hz, m-PPh),
126.0 (m, p-PPh), 123.5 (q, J =272Hz, CF3), 11.5 (d, J = 39.5 Hz, CH3−P).
31P{1H} NMR (CDCl3, 162 MHz): δ = 12.5 (d, J = 62 Hz). 11B{1H}
NMR (CDCl3, 128 MHz): δ = −38.2 (d, J = 45 Hz). 19F{1H} NMR
(CDCl3, 376 MHz, δ in ppm): δ = −63.2 (s). ESI-HRMS (m/z)
(2M-BH3)+ calcd for C30H25BF12P2: 686.13, found: 686.13.
Scheme 7. Nitrene Insertion into C−H Bonds of
Tetrahydrofuran with PhINTs Induced by Complexes 2 and 4
Scheme 8. Nitrene Insertion into C−H Bonds with PhINTs
a
Catalyzed by 4
a
General conditions: 5 mmol % Ag-complex, 0.1 mmol of PhINTs,
b
5 mL of substrate, 5 h, room temperature. Isolated yields. Yield was
calculated by adding pentafluorostyrene as an internal standard.
Synthesis of [PhB{CH2P(p-CF3C6H4)2(BH3)}3]Li(Et2O). A diethyl
ether solution (100 mL) of MeP(p-CF3C6H4)2(BH3) (1.41 g, 4.04
mmol) was placed into a Schlenk flask at −78 °C (dry ice/acetone bath).
A solution of sec-BuLi (3.20 mL, 1.4 M, 3.20 mmol) was added by
syringe, and the reaction mixture was stirred at that temperature for 3 h.
Then, a toluene solution (5 mL) of PhBCl2 (172 μL, 1.33 mmol) was
added via cannula into the reaction flask at −78 °C. The resulting
mixture was stirred and allowed to gradually warm up to room
temperature over 14 h. Volatiles were removed under reduced pressure,
and the residue was redissolved in Et2O (20 mL). Filtration through a
plug of Celite was carried out to eliminate LiCl. The filtrate was
concentrated in vacuo, and the resulting foam was washed with hexanes
(2 × 10 mL), affording the title compound as a colorless solid (3.93 g,
The use of silver as catalyst was first described by He and-
co-workers,23 followed by work developed in our laboratories,6,24
the catalytic activity displayed by 4 comparing well with those
previous examples.
CONCLUSION
■
We have synthesized and characterized the novel triphosphino-
borate ligand [PhB(CH2P(p-CF3C6H4)2)3]¯, containing fluori-
nated electron-withdrawing groups, as well as the corresponding
Tl-, Cu-, and Ag-complexes, with the coinage metal derivatives
containing PPh3 as coligand. Our goal of enhancing electro-
philicity at the metal center to improve the catalytic performance
of the corresponding copper and silver derivatives has been
achieved. Thus, these compounds have been used as catalysts in
nitrene transfer reactions to saturated and unsaturated substrates
with good results. Aziridination of both electron-rich and
electron-deficient olefins has been achieved with the copper
derivative, with the latter substrates being unaffected with parent
complex 1. Further, C−H bonds of several cyclic ethers were
amidated with the silver complex as the catalyst, providing
another example to the yet scarce number of catalytic systems
which verify this transformation. We have demonstrated that
substitution at distal positions in the P-aryl enhances the
electrophilicity at the metal center, with virtually no incidence in
the sterics of the metal complex, but improves significantly the
catalytic properties.
1
80% yield, 95% purity). H NMR ((CD3)2CO, 400 MHz): δ = 7.72−
7.68 (m, 12H, p-CF3Ph), 7.56−7.54 (m, 12H, p-CF3Ph), 6.85 (d, 2H, J =
6.8 Hz, o-PhB), 6.25 (t, 1H, J = 7.4 Hz, p-PhB), 6.10 (m, 2H, m-PhB),
3.39 (q, 4H, J = 7.2 Hz, Li(CH3CH2O)2), 1.94 (br d, 6H, J = 16.4 Hz,
-BCH2P-), 1.14 (t, 6H, J = 7.2 Hz, Li(CH3CH2O)2), 1.5−0.5 (br, 9H,
(P−BH3)3). 13C{1H} NMR ((CD3)2CO, 100.6 MHz): δ = 141.9, 134.4,
126.5, 123.0 (PhB-), 133.4 (d, J = 8.7 Hz, ipso-PPh-CF3), 131.4 (dq, J =
32, 2 Hz, p-PPh-CF3), 125.3 (d, J = 4.0 Hz, o-PPh-CF3), 125.2 (s, m-PPh-
CF3), 125.0 (q, J = 271 Hz, -CF3), 66.0 (Et2O), 15.5 (Et2O),
21.1 (PhB{CH2P(p-CF3Ph)2(BH3)}3). 31P{1H} NMR ((CD3)2CO,
162 MHz): δ = 19.5 (br s). 11B{1H} NMR ((CD3)2CO, 128 MHz, δ in
ppm): δ = −16.6 (br s, PhBCH2P), −40.2 (br s, P-BH3). 19F{1H} NMR
((CD3)2CO, 376 MHz): δ = −63.4 (s). ESI-HRMS (m/z) (M)− (negative
mode) calcd for C51H44B4F18P3: 1135.27, found: 1135.33.
Synthesis of Tl[PhB{CH2P(p-CF3C6H4)2}3]. A Schlenk tube was
charged with [PhB{CH2P(p-CF3C6H4)2(BH3)}3]Li(Et2O) (1.21 g,
1 mmol), degassed diethylamine (4 mL), and 2 mL of THF. The reaction
mixture was heated to 55 °C during 14 h. Examination of an aliquot by
31P{1H}NMR spectroscopy showed full conversion toward the free-
borane ligand (singlet at −7.2 ppm). Volatiles were removed under
reduced pressure, and the resulting foam was washed with hexanes
(3 × 20 mL). Then, the crudematerialwas dissolved in30mL of THF and
slowly added with a cannula to another flask containing TlNO3
(266 mg, 1 mmol). The mixture was stirred at room temperature during
EXPERIMENTAL SECTION
■
General Methods. All preparations and manipulations were carried
out under an oxygen-free nitrogen atmosphere using conventional
Schlenk techniques or inside a glovebox. The solvents were dried,
distilled, and degassed using a standard procedure. Reagents were
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Article
2 h when volatiles were removed under reduced pressure and the resulting
brownish foam was washed with hexanes (3 × 20 mL) to give
[PhB{CH2P(p-CF3Ph)2}3] as a white solid (843 mg, 65% yield (over 2
steps)). Single crystals were obtained by slow diffusion of hexane into a
dichloromethane solution of the complex at −20 °C. 1H NMR
((CD3)2CO, 400 MHz): δ = 7.49−7.45 (m, 12H, p-CF3Ph), 7.43−7.41
(m, 12H, p-CF3Ph), 7.36 (d, 2H, J = 7.2 Hz, o-PhB), 6.84 (t, 2H, J = 7.2
Hz, m-PhB), 6.75 (t, 1H, J = 7.2 Hz, p-PhB), 1.50 (br s, 6H, B-CH2-P).
13C{1H} NMR ((CD3)2CO, 100.6 MHz): δ = 149.0 (br s, ipso-PhB-),
133.3, 127.1, 123.5 (PhB-), 133.8 (d, J = 18 Hz, ipso-PPh-CF3), 133.8 (d,
J = 3.9 Hz, o-PPh-CF3), 129.9 (q, J = 32 Hz, p-PPh-CF3), 125.3 (d, J = 1.8
Hz, m-PPh-CF3), 125.3 (q, J = 271 Hz, -CF3), 22.9 (m, PhB{CH2P(p-
CF3Ph)2}3). 31P{1H} NMR ((CD3)2CO, 162 MHz): δ = 1.4 (br s).
11B{1H} NMR ((CD3)2CO, 128 MHz): δ = −12.6 (br s).19F{1H} NMR
((CD3)2CO, 376 MHz): δ = −63.2 (s). The crystalline material is highly
hydroscopic, precluding appropriate values for elemental analysis.
Synthesis of [PhB{CH2P(p-CF3C6H4)2}3]Cu(PPh3) (2). A Schlenk
flask was charged with the Tl complex (150 mg, 0.115 mmol), CuI
(22 mg, 0.115 mmol), PPh3 (30 mg, 0.115 mmol), and 10 mL of THF.
The mixture was stirred at room temperature for 20 min, and then it was
filtered through a plug of Celite. The resulting solid was washed with
hexanes (2 × 20 mL), affording the Cu-complex as an off-white solid
(130 mg, 80% yield). Single crystals were obtained by cooling at −20 °C
(m, 10H, AgPPh3), 7.46−7.36 (m, 31H, AgPPh3, o-PhB, p-CF3Ph), 7.20
(t, 2H, J = 7.4 Hz, m-PhB), 7.01 (t, 1H, J = 7.4 Hz, p-PhB), 1.55
(br s, -BCH2P-). 13C{1H} NMR ((CD3)2CO, 100.6 MHz): δ = 143.7
(br s, ipso-PhB), 127.0, 125.04, 125.02 (PhB), 132.6 (d, J = 17.0 Hz, ipso-
PPh-CF3), 132.6 (d, J = 5.1 Hz, o-PPh-CF3), 130.8 (s, m-PPh-CF3),
130.2 (q, J = 32.3 Hz, p-PPh-CF3), 124.0 (q, J = 271 Hz, CF3), 133.7 (d,
J = 16.2 Hz, Ag−PPh3), 130.9 (d, J = 1.4 Hz, AgPPh3), 129.2 (d, J = 9.5
Hz, Ag−PPh3), 123.2 (s, AgPPh3), 18.8 (m, PhB{CH2P(p-CF3Ph)2}3).
31P{1H} NMR ((CD3)2CO, 162 MHz): δ = 16.7 (doublet of pseudo-
quintets, 1J(P-109Ag) = 416.5 Hz, 1J(P-107Ag) = 357.2 Hz, 2J(P−P) = 29.4
Hz, Ag-PPh3), −2.6 (ddd, 1J(P-109Ag) = 210.2 Hz, 1J(P-107Ag) = 182.5
Hz, 2J(P−P) = 29.4 Hz, PhB{CH2P(p-CF3Ph)2}3). 11B{1H} NMR
((CD3)2CO, 128 MHz): δ = −14.6 (s). 19F{1H} NMR ((CD3)2CO, 376
MHz): δ = −63.4 (s). The crystalline material is highly hydroscopic,
precluding appropriate values for elemental analysis.
General Procedure for Aziridination Reaction. A Schlenk flask
was charged, under an argon atmosphere, with the catalyst (0.005 mmol,
5 mmol % catalyst loading), 5 mL of anhydrous CH2Cl2, and the
substrate (0.5 mmol). The mixture was stirred for 5 min, and freshly
prepared PhINTs (0.1 mmol) was added in two portions over 1 h, with
the mixture being further stirred for an additional 4 h. Then, the reaction
mixture was filtered through a plug of silica gel to remove the catalyst,
eluted with CH2Cl2, and evaporated to dryness under vacuum.
1
1
Conversions were calculated by H NMR of the reaction crude, and
a concentrated THF solution of the complex. H NMR ((CD3)2CO,
purification was carried out by flash chromatography to provide isolated
yields.
400 MHz): δ = 7.63−7.58 (m, 6H, CuPPh3), 7.48−7.43 (m, 6H,
CuPPh3), 7.37−7.26 (m, 29H, p-CF3Ph, o-PhB and CuPPh3), 7.20 (t,
2H, J = 7.6 Hz, m-PhB), 7.04 (t, 1H, J = 7.2 Hz, p-PhB), 1.63 (br s, 6H,
-BCH2P-). 13C{1H} NMR ((CD3)2CO, 100.6 MHz): δ = 143.8 (br s,
ipso-PhB), 127.0, 124.8, 124.1 (PhB), 132.7 (d, J = 14.4 Hz, ipso-PPh-
CF3), 132.7 (d, J = 4.4 Hz, o-PPh-CF3), 130.6 (d, J = 1.4 Hz, m-PPh-
CF3), 130.2 (q, J = 32.1 Hz, p-PPh-CF3), 124.0 (q, J = 271 Hz, -CF3),
133.7 (d, J = 14.1 Hz, CuPPh3), 131.0 (CuPPh3), 129.0 (d, J = 8.9 Hz,
CuPPh3), 128.5 (d, J = 7.4 Hz, CuPPh3), 18.8 (m, PhB{CH2P-
(p-CF3Ph)2}3). 31P{1H} NMR ((CD3)2CO, 162 MHz): δ = −5.7 (br s,
CuPPh3), 11.1 (br s, PhB{CH2P(p-CF3Ph)2}3). 11B{1H} NMR
((CD3)2CO, 128 MHz): δ = −15.1 (s). 19F{1H} NMR ((CD3)2CO,
376 MHz): δ = −63.4 (s). The crystalline material is highly hydroscopic,
precluding appropriate values for elemental analysis.
General Procedure for Competition Experiments. Following
the above procedure, a mixture of styrene (0.5 mmol) and the
corresponding para-substituted styrene (0.5 mmol) was employed.
After 5 h of stirring, the catalyst was removed by filtration through a plug
of silica gel. Elution with CH2Cl2 and concentration by rotary
1
evaporation led to a residue that was investigated by H NMR to
provide the relative ratio of aziridines.
General Procedure for C−H Activation with PhINTs. A Schlenk
flask was charged, under an argon atmosphere, with the catalyst
(0.005 mmol, 5 mmol % catalyst loading) and the substrate (5 mL) as
the solvent. Freshly prepared PhINTs (0.1 mmol) were added in one
portion and the mixture stirred at room temperature for 5 h. Volatiles
were removed under reduced pressure, and the crude was investigated
by 1H NMR. Purification was carried out by flash chromatography.
X-ray Crystal Determinations. A single crystal of PhBP3p‑CF3PhTl
(2 or 3) suitable for X-ray diffraction studies was coated with dry
perfluoropolyether and mounted on a glass fiber and fixed in a cold
nitrogen stream [T = 173(2) K] to the goniometer head. Data
collection25 was carried out on a Bruker-Nonius X8 kappa APEX II
CCD area-detector diffractometer using graphite-monochromatic
radiation λ(Mo Kα) = 0.71073 Å, by means of ω and φ scans with
narrow frames. Data reduction was performed using SAINT,25 and
corrected for Lorentz polarization effects and absorption by the
multiscan method applied by SADABS.26 The structure was solved by
direct methods (SIR-2002)27 and refined against al F2 data by full-matrix
least-squares techniques with SHELXTL.28 All non-hydrogen atoms
were refined with anisotropic displacement parameters. The hydrogen
atoms were included from calculated positions and refined riding on
their respective carbon atoms with isotropic displacement parameters.
Data associated with the crystal structures of PhBP3p‑CF3PhTl (2 and 3)
are provided in the Supporting Information as cif files.
Synthesis of [PhB{CH2PPh2}3]Ag(PPh3) (3). Silver(I) chloride
(57.3 mg, 0.4 mmol) was added to a solution of [PhB{CH2PPh2}3]-
Li(TMEDA) (323.5 mg, 0.4 mmol) in tetrahydrofuran (10 mL). The
flask was covered with aluminum foil, and the mixture was stirred for
2.5 h. Then, 1 equiv of PPh3 (105.0 mg, 0.4 mmol) was added and the
reaction was stirred for 2.5 h. The solvent was removed under reduced
pressure to give a white powder that was washed with petroleum ether
(2 × 15 mL). The solid was dissolved in tetrahydrofuran (10 mL), and
3 equiv of PPh3 was added to favor crystallization. After concentration
and cooling overnight at −20 °C, [PhB{CH2PPh2}3]Ag(PPh3) was
obtained as colorless crystals, some of them suitable for X-ray studies
1
(0.29 g, 70% yield). H NMR (CD2Cl2, 400 MHz): δ 7.61 (d, 2H,
o-PhB-), 7.58 (m, 6H, AgPPh3), 7.45 (t, 3H, AgPPh3), 7.24 (m, 8H,
m-PhB-, AgPPh3), 7.15 (t, 6H, PPh2), 7.05 (m, 12H, PPh2), 7.02 (t, 1H,
p-PhB-), 6.96 (t, 12H, PPh2), 1.43 (br s, 6 H, PhB{CH2PPh2}3).
13C{1H} NMR (CD2Cl2, 100.6 MHz): δ 140.2 (br s, ipso-PhB), 127.9
(m), 126.7, 122.9 (PhB), 133.6 (d), 132.3 (m), 131.1, 130.0 (d), 128.7
(d), 127.9 (m), AgPPh3 and PhB{CH2PPh2}3, 18.8 (m, PhB-
{CH2PPh2}3). 31P{1H} NMR (CD2Cl2, 162 MHz): δ 16.2 (doublet
1
1
of pseudoquintets, J(P−109Ag) = 397.4 Hz, J(P−107Ag) = 338.6 Hz,
2J(P−P) = 30 Hz, Ag−PPh3), −2.7 (ddd, 1J(P−109Ag) = 215 Hz,
1J(P−107Ag) = 192.3 Hz, 2J(P−P) = 30 Hz, PhB{CH2PPh2}3). 11B{1H}
NMR (CD2Cl2, 128 MHz): δ −14.7. Anal. Calcd for C63H56BAgP4: C,
71.68; H, 5.35. Found: C, 70.87; H, 5.65.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectroscopic data as a single pdf file as well as crystallographic
information files in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
Synthesis of [PhB{CH2P(p-CF3C6H4)2}3]Ag(PPh3) (4). A Schlenk
flask was charged with the Tl complex (200 mg, 0.154 mmol), AgOTf
(40 mg, 0.154 mmol), PPh3 (40 mg, 0.154 mmol), and 20 mL of THF.
The mixture was stirred at room temperature for 20 min and then
filtered with a plug of Celite. The resulting solid was washed with
hexanes (2 × 20 mL), affording the title compound as an off-white solid
(0.17 g, 75% yield). 1H NMR ((CD3)2CO, 400 MHz): δ = 7.71−7.61
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: perez@dqcm.uhu.es.
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Inorganic Chemistry
Article
Mathieu, M. I.; Castillon
7092−7095.
́
, S.; Per
́
ez, P. J. Angew. Chem., Int. Ed. 2010, 49,
Notes
The authors declare no competing financial interest.
(14) Evans, D. A.; Faul, M. M.; Bilodeau, M. T.; Anderson, B. A.;
Barnes, D. M. J. Am. Chem. Soc. 1993, 115, 5328−5329.
(15) Lebel, H.; Parmentier, M.; Leogane, O.; Ross, K.; Spitz, C.
Tetrahedron 2012, 68, 3396−3409.
ACKNOWLEDGMENTS
■
We thank MICINN (Proyectos CTQ2011-28942-C02-01 and
CTQ-2011-22872) and Junta de Andalucia (Proyecto P10-
(16) Maestre, L.; Sameera, W. M. C.; Díaz-Requejo, M. M.; Maseras,
́
F.; Per
(17) (a) Jiang, X.-K.; Ji, G.-Z. J. Org. Chem. 1992, 57, 6051−6056.
(b) Dincturk, S.; Jackson, R. A. J. Chem. Soc., Perkin Trans. 2 1981,
́
ez, P. J. J. Am. Chem. Soc. 2013, 135, 1338−1348.
FQM-06292 and P12-FQM-01765) for financial support. I.A.
thanks MICINN, and M.A.F thanks MEC for a research fellowship.
́
̧
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1127−1131. (c) Fisher, T. H.; Meierhoefer, A. W. J. Org. Chem. 1978,
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