Amino Ether-Phenolato Precatalysts of Divalent Rare Earths and Alkaline Earths for the Single and Double Hydrophosphination of Activated Alkenes

Article abstract of DOI:10.1021/acs.organomet.6b00252

A range of stable ytterbium(II) amino ether-phenolato amido complexes of the type {LO^NxOy}Yb{N(SiMe₃)₂}, together with a congeneric samarium(II) and two calcium and strontium amido and alkyl derivatives, have been synthesized. They constitute very active, fully regioselective (anti-Markovnikov), and chemoselective precatalysts for the intermolecular hydrophosphination of styrene derivatives with PhPH₂, achieving TONs up to 2150, TOFs up to 30 h^-1 and chemoselectivities in the region of 95-99%. The ytterbium(II) precatalysts, among which is most prominently {LO^NO4}Yb{N(SiMe₃)₂} (3), where the ligand possesses a 15-c-5-aza-crown ether side arm, outperform their related calcium analogues, and the activity of the catalyst increases substantially with the denticity of the ligand. The rate law rate = k[p-^tBu-styrene][3] was established following kinetic monitoring of the hydrophosphination of p-^tBu-styrene and PhPH₂ catalyzed by 3. The kinetic studies also revealed that the reactions are entropically driven and that reaction rates increase when electron-withdrawing groups are introduced at the para position of the styrenic substrate. It is proposed that these catalyzed hydrophosphination reactions proceed by rate-limiting olefin insertion into the [Yb]-phosphide bond. The chemoselective one-pot, two-step double hydrophosphination of PhPH₂ with 2 equiv of styrene yields tertiary phosphines. It obeys an unusual kinetic profile where formation of the tertiary phosphine starts only when complete consumption of PhPH₂ is first ensured. This sequence was used to obtain asymmetric tertiary phosphines with excellent selectivity.

Full text of DOI:10.1021/acs.organomet.6b00252

Article
pubs.acs.org/Organometallics
Amino Ether−Phenolato Precatalysts of Divalent Rare Earths and
Alkaline Earths for the Single and Double Hydrophosphination of
Activated Alkenes
Ivan V. Basalov,† Bo Liu,‡ Thierry Roisnel,‡ Anton V. Cherkasov,† Georgy K. Fukin,†
Jean-Franco̧
is Carpentier,*,‡ Yann Sarazin,*,‡ and Alexander A. Trifonov*,†
†G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinina 49, GSP-445, 603950 Nizhny
Novgorod, Russia
‡Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Organometallics: Materials and Catalysis Group, Universite
Rennes 1, Campus de Beaulieu, 35042 Rennes, France
́
de
S
* Supporting Information
ABSTRACT: A range of stable ytterbium(II) amino ether−
phenolato amido complexes of the type {LONxOy}Yb{N(SiMe3)2},
together with a congeneric samarium(II) and two calcium and
strontium amido and alkyl derivatives, have been synthesized.
They constitute very active, fully regioselective (anti-Markovni-
kov), and chemoselective precatalysts for the intermolecular
hydrophosphination of styrene derivatives with PhPH2, achieving
TONs up to 2150, TOFs up to 30 h−1 and chemoselectivities in
the region of 95−99%. The ytterbium(II) precatalysts, among
which is most prominently {LONO4}Yb{N(SiMe3)2} (3), where
the ligand possesses a 15-c-5-aza-crown ether side arm, outper-
form their related calcium analogues, and the activity of the
catalyst increases substantially with the denticity of the ligand. The rate law rate = k[p-tBu-styrene][3] was established following
kinetic monitoring of the hydrophosphination of p-tBu-styrene and PhPH2 catalyzed by 3. The kinetic studies also revealed that
the reactions are entropically driven and that reaction rates increase when electron-withdrawing groups are introduced at the para
position of the styrenic substrate. It is proposed that these catalyzed hydrophosphination reactions proceed by rate-limiting olefin
insertion into the [Yb]−phosphide bond. The chemoselective one-pot, two-step double hydrophosphination of PhPH2 with 2
equiv of styrene yields tertiary phosphines. It obeys an unusual kinetic profile where formation of the tertiary phosphine starts
only when complete consumption of PhPH2 is first ensured. This sequence was used to obtain asymmetric tertiary phosphines
with excellent selectivity.
catalysis remains rather limited, in fact, and chiefly involves the
benchmark hydrophosphination of activated olefins with the
ubiquitous diphenylphosphine. Reactions with primary phos-
phines such as phenylphosphine remain rather rare,8a,9b,c,11
although the resulting secondary phosphine products are of
obvious interest to organic and coordination chemists alike.
Even if trivalent lanthanide (Ln) complexes catalyze the
polymerization,13 hydrogenation,14 hydrosilylation,15 hydro-
amination,16 hydrophosphination,5,17 hydroalkoxylation,18 hy-
drothiolation,18 and hydroboration19 of olefins, little has been
said of the ability of divalent lanthanide complexes to promote
these reactions. This is somewhat peculiar, as the larger ionic
radii and the redox-active nature of Ln(II) ions (effective ionic
radii for CN = 6, Yb2+ 1.02 Å, Eu2+ 1.17 Å, Sm2+ 1.22 Å;
standard oxidation potentials, Yb3+/Yb2+ −1.05 V, Eu3+/Eu2+
−0.35 V, Sm3+/Sm2+ −1.55 V) indicate exciting properties in
INTRODUCTION
■
The creation of C−P bonds by catalyzed intermolecular
hydrophosphination of activated alkenes (acrylonitrile, acryl-
ates, dienes, and styrene) has been the focus of increasing
attention in recent years, because it affords a clean, atom-
efficient access to valuable phosphine derivatives.1 This catalysis
originally relied prominently on catalysts designed around
nickel,2 palladium,3 and platinum4 with Michael-type substrates
or, at the opposite end of the d block, lanthanides.5 The
development of these earlier catalysts has been covered in many
comprehensive reviews.1e,6 More recently, systems based on
copper,7 iron,1f tin,8 or more oxophilic metals, e.g. titanium/
zirconium,9 alkaline earths,10 and divalent lanthanides,10c,d,11
have also proved very competent. A case of FLP-mediated
hydrophosphination of alkenes has also been reported.12
A
substantial breakthrough was Waterman and co-workers’
account of a zirconium precatalyst for the hydrophosphination
of unactivated alkenes such as 1-hexene, a remarkable result for
such notoriously reluctant substrates.9b Substrate scope in this
Received: March 30, 2016
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of the Divalent Lanthanide (1−4) and Alkaline-Earth Complexes (5−8) Supported by Aminoether-
a
Phenolato Ligands
a
For complexes 5 and 6, see ref 24. The bonding patterns are structurally established for 2−7, but they are hypothetical for 1 and 8.
catalysis; the situation in part stems from inherent synthetic
difficulties associated with these reactive elements. As for their
Ln(II) ytterbium, europium, and samarium congeners,
complexes of the large, oxophilic alkaline earths (Ae) calcium,
strontium, and barium (rionic = 1.00, 1.18, and 1.35 Å,
respectively) have d0 outer-shell electronic configurations.
Ln(II) and Ae compounds bearing the same bulky ligand
often exhibit analogous structural features. Although Ae
complexes display greater Lewis acidity and ionic character,20
the filling of low-lying f orbitals (Sm(II), 4f6; Eu(II), 4f7;
Yb(II), 4f14) of limited radial extension is presumed to not alter
fundamentally the reactivity of Ln(II) complexes with respect
to that of their Ae congeners. In fact, their reactivities often
match closely,21 but they can in some cases show marked
differences: e.g., in polymerization catalysis.22 Our research
groups have been involved for a few years in the design of
isoelectronic Ln(II) and Ae precatalysts to mediate organic
transformations, in particular with a view to devising efficient
tools for the production of C−P bonds. Although bulky ligands
were shown to be redundant in the hydrophosphonylation of
carbonyls,23 nitrogen-based amidinates and iminoanilides
proved to be formidable assets in the hydrophosphination of
alkenes.10d,11a,c In a preliminary communication, we have
described the utilization of Yb(II) and Sm(II) complexes
supported by multidentate, monoanionic amino ether−
phenolates to promote the chemoselective and regioselective
(anti-Markovnikov) addition of phenylphosphine to styrene.11b
The full extent of this study, which includes structural
characterization, kinetic investigations, and a comparison of
Ln(II) and Ae precatalysts, is now reported.
B
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RESULTS AND DISCUSSION
■
Synthesis and Characterization. The amino ether−
phenolato ytterbium(II) complexes {LO(NO)2}Yb{N(SiMe3)2}
(1, dark red), {LONO2}Yb{N(SiMe3)2} (2, orange), and
{LONO4}Yb{N(SiMe3)2} (3, bright yellow) were obtained
free of coordinated THF in 80−90% isolated yields upon
amine elimination in the stoichiometric reactions of Yb{N-
(SiMe3)2}2·THF with the protio ligands {LO(NO)2}H, {LONO2
}
-
H, and {LONO4}H in toluene (Scheme 1). The Sm(II)
derivative {LONO4}Sm{N(SiMe3)2} (4, black) was obtained
in 40% yield by the same protocol. The synthesis of these
complexes and the molecular solid-state structures of 2 and 4
were described in our initial communication.11b The known
calcium amido complexes {LO(NO)2}Ca{N(SiMe3)2} (5) and
{LONO2}Ca{N(SiMe3)2} (6), the syntheses and crystallo-
graphic structures of which were reported independently,24
were included in the catalytic study for comparative purposes.
The new colorless alkyl complexes {LONO2}Ca{CH(SiMe3)2}·
THF (7) and {LONO2}Sr{CH(SiMe3)2} (8) were obtained in
55−70% yield upon equimolar treatment of the corresponding
precursors Ae{CH(SiMe3)2}2·2THF (Ae = Ca, Sr) with
{LONO2}H in pentane. Peculiarly, the calcium complex 7
could not be obtained free of THF, even upon gentle heating
under dynamic vacuum, although its strontium derivative 8 was
devoid of any coordinated solvent, despite the much larger
metallic center. Other examples of stable heteroleptic Ae−alkyl
complexes are rather rare and are restricted to the use of bulky
alkyl groups such as bis(trimethylsilyl)methyl10b,c,25 and
Figure 1. ORTEP representation of the molecular solid-state structure
of {LONO2}Ca{CH(SiMe3)2}·THF (7). Ellipsoids are drawn at the
30% probability level. H atoms are omitted for clarity. Selected bond
lengths (Å): Ca1−O10 = 2.221(2), Ca1−O1 = 2.438(3), Ca1−O31 =
2.470(3), Ca1−O8 = 2.486(3), Ca1−N5 = 2.543(3), Ca1−C41 =
2.570(4).
geometry and κ4 coordination of the amino ether−phenolate.
All Ca−O and Ca−N bond distances in 7 are regular and are
for instance comparable to those in the related amido complex
{LONO2}Ca{N(SiMe2H)2}·THF.27 The Ca−C(41) bond
length is substantially longer in 7 (2.570(4) Å) than in its
precursor Ca{CH(SiMe3)2}2·2THF (2.493(2) Å)28 or in
Ca{CH(SiMe3)2}2·2(dioxane) (2.483(5) Å)29 and compares
better with that measured in the β-diketiminato complex
substituted benzyl moieties.26 The H NMR data for 7 (in
1
benzene-d6) and 8 (in THF-d8) are characterized by very high
field resonances for the methyl [Ae]−CH hydrogen atoms, at
1
δ H −1.87 and −2.20 ppm, respectively. The presence of
coordinated THF in 7 is confirmed by multiplets at 3.60 and
1.40 ppm integrating for 4H each.
{BDIiPr}Ca{η2-CH2-2-(Me2N)C6H4} (2.535(4) Å; BDIiPr
=
CH[CMeN(2,6-iPr2-C6H3)]2).25b
The heteroleptic calcium complex 7 is stable in aromatic
hydrocarbons, without signs of decomposition or ligand
scrambling after 2 days at room temperature. However, the
strontium complex 8 is more kinetically labile, and its 1H NMR
spectrum at room temperature always shows contamination by
ca. 3−5 mol % of the homoleptic species {LO6}2Sr and an
unidentified strontium−alkyl species; the proportion of
homoleptic complexes increased with temperature. The action
of Sm{N(SiMe3)2}2·2THF with an equimolar amount of
{LONO2}H only returned crystals of the dark green, homoleptic
{LONO2}2Sm and of the colorless, trivalent {LONO2}Sm{N-
(SiMe3)2}2 complexes, which were characterized by XRD
crystallography; it eventually proved impossible to obtain the
desired {LONO2}Sm{N(SiMe3)2} complex. Multiple attempts
to obtain complexes akin to 3 and 4 by using protio ligands
having macrocyclic side arms derived from 1-aza-12-c-4 or 1-
aza-18-c-6 crown ethers also failed; the protio ligands could be
made readily on multigram scales, but the macrocycles
decomposed with detectable release of ethylene upon exposure
to the very basic Ln(II) or Ae amido precursors. Note also that
the clean synthesis of {LONO4}Ca{N(SiMe3)2}, i.e. the calcium
derivative of 3, could not be achieved due to systematic ligand
redistribution in solution, giving mixtures of the target
compound together with [Ca{N(SiMe3)2}2]2 and {LONO4}2Ca.
The molecular solid-state structure of 7 depicted in Figure 1
features one of the four equivalent molecules found in the
asymmetric unit. It shows a monometallic complex with a six-
coordinate calcium atom sitting in a greatly distorted octahedral
The structure of 3 is displayed in Figure 2. It depicts a THF-
free, monometallic complex with the ytterbium lying in a seven-
coordinate environment. Among the main structural features,
the Yb1−O1 distance of 2.277(2) Å to the Ophenolate atom
matches that in 2 (2.245(12 Å)), and it is slightly longer than
that in the calcium congener {LONO4}Ca{N(SiMe3)2}
(2.211(2) Å).27 However, the Yb1−N2 amido bond length in
3 (2.466(2) Å) is greater than that in 2 (2.326(16) Å), the only
other pertinent Yb(II)−phenolate complex reported to date.11b
The elongated Yb−Namide in 3 certainly reflects the greater
coordination number of this complex, with a more electron rich
Yb(II) center resulting in a looser bond to the reactive amido
group. The distances to the other nitrogen and oxygen atoms in
3 are unexceptional. The Yb2+ ion is too large to fit inside the
crown ether; instead, it rests 1.33 Å above the mean plane
defined by the five heteroatoms in the side-arm macrocycle.
Hydrophosphination Catalysis. The performances of the
precatalysts 1−8 in the hydrophosphination of styrene with
PhPH2 ([St]0/[PhPH2]0/[precat]0 = 50/50/1) were assessed
in a preliminary screening. The data for Yb{N(SiMe3)2}2·THF
and Sm{N(SiMe3)2}2·2THF were added to evaluate the
importance of the amino ether−phenolates (Table 1, entries
1 and 2; no conversion was observed in either case after 1 h at
25 °C).30 Representative data for experiments carried out at
25−60 °C in benzene-d6 are collected in Table 1. Under the
chosen experimental conditions, the reactions were very
chemoselective, affording the secondary phosphine (sec-P) in
>95% selectivity: that is, less than 5% of tertiary phosphine
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sively the anti-Markovnikov product sec-P, with no indication
for the formation of its regioisomer.
Several trends emerge from entries 1−12 in Table 1. (i) The
best two precatalysts are by a great margin the Ln(II)
complexes 3 (Yb) and 4 (Sm) supported by the ligand
{LONO4}− of highest denticity, the ytterbium species actually
faring better (entries 5−8). (ii) For a given metal, the activity
increases with the denticity of the ligand; compare entries 3−5
for ytterbium or entries 9 and 10 for calcium. (iii) With the
tetradentate {LOON2}− ligand, the ytterbium(II) complexes are
more efficient than their calcium analogues; compare entries 4
and 10−12. Note that 8 is not particularly efficient at 60 °C,
presumably owing to its propensity for kinetic lability. The poor
efficiency of the ytterbium(II) complex 1 (entry 3) in
comparison with that of its calcium derivative 5 (entry 9) can
be attributed to its limited solubility. (iv) The calcium alkyl and
amido precatalysts 6 and 7 are equally competent. (v) The
presence of the ancillary amino ether−phenolato ligand is
required, as the bis(amido) compounds showed little catalytic
activity under the chosen experimental conditions (entries 1
and 2). Note that issues linked to substrate diffusion might in
some cases, e.g. entries 6 and 13, have limited the final
conversion to ca. 70−75%; when the reaction was performed in
a Schlenk flask, complete conversion was observed (entry 20),
whereas only 72% of 100 equiv of each substrate had been
converted in an NMR-scale reaction (entry 13).
Figure 2. ORTEP representation of the molecular solid-state structure
of {LONO4}Yb{N(SiMe3)2} (3). Ellipsoids are drawn at the 50%
probability level. H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Yb1−O1 = 2.277(2), Yb1−N2 =
2.466(2), Yb1−O2 = 2.481(2), Yb1−O3 = 2.615(2), Yb1−N1 =
2.633(3), Yb1−O5 = 2.705(2), Yb1−O4 = 2.712(2); Si2−N2−Yb1 =
134.18(14), Si1−N2−Yb1 = 100.98(11).
Further reactions were carried out with the best precatalyst, 3
(Table 1, entries 13−19). It proved robust enough to withstand
as much as 2500 equiv of each substrate (entry 18), achieving
86% conversion (overall turnover TON = 2150; apparent
formed upon double (stepwise) hydrophosphination. The
monohydrophosphination was regiospecific, generating exclu-
a
Table 1. Hydrophosphination of Styrene with PhPH2 Catalyzed by 1−8
b
c
run
precat
[St]0/[PhPH2]0/[precat]0
T (°C)
t (h)
conversion (%)
sec-P/tert-P (%)
1
Yb{N(SiMe3)2}2·THF
Sm{(N(SiMe3)2}2·2THF
50/50/1
60
60
60
60
60
25
60
25
60
60
60
60
25
60
60
60
60
60
60
25
24
24
24
24
0.5
1
8
18
1
99/1
99/1
n.d.
2
50/50/1
3
1
2
3
3
4
4
5
6
7
8
3
3
3
3
3
3
3
3
50:50:1
4
50/50/1
52
100
74
80
63
24
40
34
15
72
91
82
66
100
86
100
100
99/1
94/6
97/3
97/3
97/3
97/3
99/1
99/1
100/0
99/1
95/5
96/4
93/7
93/7
80/20
0/100
99/1
5
50/50/1
6
50/50/1
7
50/50/1
0.5
1
8
50/50/1
9
50/50/1
24
24
24
24
3
10
11
12
13
50/50/1
50/50/1
50/50/1
100/100/1
100/100/1
200/200/1
500/500/1
500/500/1
2500/2500/1
100/50/1
100/100/1
d
14
3
e
15
3
f
16
0.5
15
72
24
3
f
17
g
18
19
h
20
a
Reactions were carried out in benzene-d6 (0.4−0.5 mL) with [precat]0 = 11.5 mM unless otherwise specified. The formation of sec-P was 100% anti-
b
c
Markovnikov regiospecific, as established by NMR spectroscopy. Conversion of styrene, determined by NMR spectroscopy. Product
d
e
f
g
chemoselectivity determined by 31P NMR spectroscopy. [precat]0 = 9.35 mM. [precat]0 = 7.95 mM. [precat]0 = 6.99 mM. [precat]0 = 1.62 mM.
h
Reaction performed in a Schlenk flask with continuous stirring.
D
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Scheme 2. Synthesis of Tertiary Phosphines by a One-Pot, Two-Step Reaction Catalyzed by 3
Figure 3. Conversion of PhPH2 vs reaction time for the hydrophosphination of p-tBu-styrene catalyzed by {LONO4}Yb{N(SiMe3)2} (3) in benzene-
d6. Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 30/100/1, with [3]0 = 18.0 mM; T = 25 °C; total volume 0.6 mL.
Figure 4. Semilogarithmic plot of ln([p-tBu-styrene]0/[p-tBu-styrene]t) vs reaction time for the hydrophosphination of p-tBu-styrene with PhPH2
catalyzed by {LONO4}Yb{N(SiMe3)2} (3) in benzene-d6. Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 100/X/2.5, with [3]0 = 45.0 mM;
◆
◆
●
▲
T = 25 °C; total volume 0.6 mL; X = 10 (magenta ), 15 (blue ), 25 (red ), 30 (green ) equiv.
turnover frequency TOF = 30 h−1) after 72 h, although the final
chemoselectivity was lower under such demanding conditions
requiring longer reaction times. With up to 500 equiv of
substrates, excellent conversions and chemoselectivity were
observed, and the reaction times remained reasonably short
(entries 13−17). Interestingly, with a [styrene]0/[PhPH2]0/
[3]0 = 100/50/1 initial ratio, the formation of the tertiary
phosphine tert-P was quantitative and selective after 24 h (entry
19).
atom (Scheme 2). The set of experiments was performed in
benzene-d6 at 25 °C. A reaction time of 2 h for the equimolar
hydrophosphination of PhPH2 with styrene ([styrene]0/
[PhPH2]0/[3]0 = 50/50/1) enabled quantitative conversion
of the first load of substrates. A second batch of styrene
derivative was then added to this reaction mixture (50 equiv vs
3). A series of such experiments were conducted with different
t
substituents in the para position (X = Cl, F, Bu, OMe) of the
styrenic substrate. As with styrene (X = H), for the p-Cl and p-
F derivatives the second phase of the process was complete
within 3 h, neatly affording original asymmetric phosphines in
quantitative yields. The reaction was slower with the p-tBu
To take advantage of this last feature, 3 was employed for the
one-pot, two-step synthesis of unsymmetrical tertiary phos-
phines having three different substituents on the phosphorus
E
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Figure 5. Plot of the observed rate constant vs concentration in p-tBu-styrene for the hydrophosphination of p-tBu-styrene with PhPH2 catalyzed by
{LONO4}Yb{N(SiMe3)2} (3) in benzene-d6. Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 100/X/2.5, with [3]0 = 45.0 mM; T = 25 °C;
total volume 0.6 mL; X = 10, 15, 25, 30 equiv.
Figure 6. Plot of ln kobs vs ln [3] for the hydrophosphination of p-tBu-styrene with PhPH2 catalyzed by {LONO4}Yb{N(SiMe3)2} (3) in benzene-d6.
Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 100/25/X; T = 25 °C; [PhPH2]0 = 1.08 mM; [p-tBu-styrene]0 = 0.27 mM; total volume 0.6
mL; [3]0 = 10.8, 18.9, 27.0, 35.5, 43.3 mM.
Figure 7. Plot of kobs vs [3]0 for the hydrophosphination of p-tBu-styrene with PhPH2 catalyzed by {LONO4}Yb{N(SiMe3)2} (3) in benzene-d6.
Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 100/25/X; T = 25 °C; [PhPH2]0 = 1.08 mM; [p-tBu-styrene]0 = 0.27 mM; total volume 0.6
mL; [3]0 = 10.8, 18.9, 27.0, 35.5, 43.3 mM.
substituent, the conversion reaching 56% in 20 h, whereas it did
not proceed further with the very electron donating p-OMe
substituent. The reaction rate is hence sensitive to electronic
effects in the styrenic substrate and varied according to OMe <
tBu < H, F, Cl.
d6, using precatalyst 3. The hydrophosphination between
PhPH2 and p-tBu-styrene, a substrate which lends itself well
to the monitoring of substrate conversion, was first examined.
At 25 °C, in the presence of a 3−5-fold excess of p-tBu-styrene
vs PhPH2, the plot of the conversion of PhPH2 vs reaction time
was linear (Figure 3), indicating a zeroth-order dependence in
[PhPH2]. Conversely, with a 3−10-fold excess of PhPH2, the
Kinetic Analysis. Kinetic monitoring was performed by 1H
NMR spectroscopy. The reactions were carried out in benzene-
F
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Figure 8. Eyring plot for the hydrophosphination of p-tBu-styrene with PhPH2 catalyzed by {LONO4}Yb{N(SiMe3)2} (3) in benzene-d6, in the
temperature range 25−75 °C. Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 25/25/1; total volume 0.6 mL; [3]0 = 36.0 mM.
Figure 9. Plot of PhPH2 conversion vs reaction time for the hydrophosphination of p-X-styrene catalyzed by {LONO4}Yb{N(SiMe3)2} (3) in
benzene-d6. Reaction conditions: [PhPH2]0/[p-X-styrene]0/[3]0 = 25/25/1; T = 25 °C; total volume 0.6 mL; [3]0 = 36.0 mM; X = Cl, H, F, Me,
tBu, OMe.
semilogarithmic plots of the concentration in p-tBu-styrene vs
reaction time were linear (Figure 4), showing a first-order
rate = k[p‐tBu‐styrene]1[3]1
(1)
dependence in [p-tBu-styrene]. On the other hand, the
corresponding observed rate constants kobs decreased linearly
from [4.3170(1)] × 10−5 to [2.9830(1)] × 10−5 s−1 with
increasing p-tBu-styrene concentration (Figure 5). This is
indicative of catalyst inhibition by this substrate, probably by
formation of an adduct as an off-cycle species; this suggests that
the rate-limiting insertion of the CC bond into the Yb−P
bond (vide infra) first requires the dissociation of molecules of
p-tBu-styrene from the Lewis acidic ytterbium(II) cation.
Relevant occurrences of catalyst inhibition by the substrate
have been reported for the cyclohydroamination of amino-
alkenes31 and for the hydroalkoxylation/cyclization of alkynyl
alcohol32 catalyzed by alkaline-earth complexes.
A partial kinetic order of 1 in precatalyst concentration [3]
was determined from the plot of ln kobs vs ln [3], when the
concentration in 3 was increased from 10.8 to 43.3 mM while
the concentrations in the substrates were kept constant (Figure
6); this was confirmed by plotting the variation of the observed
first-order rate constant kobs vs [3]0, which was also a straight
line (Figure 7). The rate law for the hydrophosphination of
p-tBu-styrene with PhPH2 catalyzed by 3 therefore agrees with
the idealized eq 1:
The activation parameters ΔH⧧ = 7.2 kcal mol−1 and ΔS⧧ =
−46.4 cal mol−1 K−1 were determined for the reaction by
Eyring analysis performed in the temperature range 25−75 °C
(Figure 8). This corresponds to a ΔG⧧ value of 21.0 kcal mol−1
at 25 °C. These values indicate that the reaction is essentially
controlled by a high entropic barrier, with an associative
mechanism involving a very ordered transition state. Occur-
rences of large activation entropy such as that observed here
have been mentioned before. This has been reported in other
pioneering computational and kinetic works, e.g. in thiourea
catalysis33 or in the metal-templated synthesis of [3]-
rotaxanes.34 In our case, it may perhaps be linked to substantial
stiffening of the otherwise very flexible ligand aza-crown ether
side arm in the transition state, although we cannot provide
evidence to support this claim.
The influence of the substituent on the styrene derivatives
mentioned above was probed in more detail by monitoring the
kinetics for the hydrophosphination between PhPH2 and
substituted para-X-styrene derivatives (p-X-styrene, with X
t
chosen among CF3, F, Cl, Br, H, Bu, Me, and OMe) or meta-
OMe-styrene. With p-CF3-styrene, the immediate formation of
a black precipitate indicative of catalyst decomposition was
G
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Scheme 3. Proposed Mechanism for the Hydrophosphination of Styrene Derivatives with PhPH2 Catalyzed by Ln(II)
Precatalysts, Illustrated Here with 3
Figure 10. Plot of reagent conversion vs reaction time for the hydrophosphination of p-tBu-styrene with PhPH2 catalyzed by
{LONO4}Yb{N(SiMe3)2} (3) in benzene-d6 at 25 °C. Reaction conditions: [PhPH2]0/[p-tBu-styrene]0/[3]0 = 25/50/1; total volume 0.6 mL;
[3]0 = 72.2 mM.
seen, presumably by reaction between the complex and the
fluorinated group. From the plots given in Figure 9, the
observed zero-order rate constants kobs can be deduced
immediately. The reactions with p-Cl-styrene and p-Br-styrene
were very fast, and only a few data points could be recorded
before full conversion. Figure 9 confirms that reaction rates
globally increase with the Hammett σ constant of the
substituents (except for the peculiarly low rate observed for
p-F) in the sequence p-OMe (σp = −0.27) < p-tBu (σp = −0.20)
< p-Me (σp = −0.17) < p-F (σp = 0.06) < p-H (σp = 0) < m-
OMe (σm = 0.12) < p-Br (σp = 0.23) ≈ p-Cl (σp = 0.23).35 The
lower reaction rates observed for p-F-styrene in comparison
with styrene are surprising, but the reproducibility between
independent tests was excellent; this must reflect a concrete
experimental fact or mechanistic feature which we cannot
rationalize at this time.36 That the reaction rates increase overall
with the Hammett σ constants of the substituents on the
aromatic ring in the styrene derivatives suggests that bond
polarization occurs, with a transition state having a negative
charge developing on the carbon atom in a position α to the
aromatic ring. This replicates the scenario known for alkaline-
earth catalysts.10 On the basis of this analogy with Ae catalysis
and the rate law in eq 1, we propose that these hydro-
phosphination reactions proceed by turnover-limiting olefin
insertion into the [Yb]−phosphide bond (Scheme 3). The
tailing off observed in the reactions with styrene (X = H) may
be due to a concentration gradient which slows reactivity in the
J. Young NMR tube, as mentioned previously (Table 1, entries
6, 13, and 20).
Consecutive Hydrophosphinations. The kinetics of the
catalyzed reaction between p-tBu-styrene and 2 equiv of PhPH2
leading to the formation of the secondary (sec-P) and tertiary
(tert-P) phosphines by mono- and dihydrophosphination,
respectively, were monitored to inform ourselves about the
chemoselectivity of this process. The reaction was carried out in
benzene-d6 at 25 °C, with [PhPH2]0/[p-X-styrene]0/[3]0 = 25/
50/1 and [3]0 = 72.2 mM. The plot displayed in Figure 10 is
rich with information. The kinetic profile for the concentrations
H
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Article
in the various phosphines is not typical of consecutive
reactions.37
archetypal PhPH2. Future efforts should focus on rendering this
process enantioselective.
The formation of sec-P is fully chemospecific, and it is only
when the whole of PhPH2 has been converted into sec-P that
the formation of tert-P begins, after nearly 5 h. As expected
owing to the zeroth-order dependence in [PhPH2], the rate of
consumption of PhPH2 is linear for at least 120 min, at which
point a regime of substrate depletion is reached. The rate of
consumption of styrene in the first phase (corresponding to the
formation of sec-P, t < 300 min) and in its second phase
(formation of tert-P, t > 300 min) shows the expected first-
order dependence in [p-tBu-styrene]. Remarkably, rate
EXPERIMENTAL SECTION
■
General Procedures. NMR spectra were recorded on Bruker AC-
1
400 and AM-500 spectrometers. All H and 13C chemical shifts were
determined using residual signals of the deuterated solvents and were
calibrated vs SiMe4. Assignment of the signals was carried out using
1D (1H, 13C{1H}) and 2D (COSY, HMBC, HMQC) NMR
experiments. 31P NMR spectra were calibrated vs phosphoric acid.
All coupling constants are given in hertz. Elemental analyses were
performed on a Carlo Erba 1108 Elemental Analyzer instrument at the
London Metropolitan University by Stephen Boyer and were the
average of a minimum of two independent measurements. HN-
(SiMe3)2 (Acros) was dried over activated 3 Å molecular sieves and
distilled under reduced pressure prior to use. Solvents (THF, Et2O,
CH2Cl2, pentane, and toluene) were purified and dried (water
contents all below 6 ppm) over alumina columns (MBraun SPS). THF
was further distilled under argon from sodium mirror/benzophenone
ketyl prior to use. All deuterated solvents (Eurisotop, Saclay, France)
were stored in sealed ampules over activated 3 Å molecular sieves and
were thoroughly degassed by several freeze−thaw cycles. The
precursors Ln{N(SiMe3)2}2(THF)x (Ln = Yb, x = 1; Ln = Sm, x =
2),39 Ca{CH(SiMe3)2}2·2THF,28 and the amino ether−phenol protio
ligands24 were prepared as described elsewhere. The syntheses of the
divalent lanthanide complexes 1−411b and alkaline-earth complexes 5
and 624 were reported previously, and the readers are referred to the
relevant articles for all synthetic and characterization details. Despite
repeated attempts performed on crystalline samples from different
batches, satisfactory elemental analysis could not be obtained for
compounds 7 and 8, most probably because of their extreme moisture
sensitivity. Styrenic substrates (Acros/Aldrich) were vacuum-distilled
over CaH2 and then degassed by freeze−pump−thaw methods.
Phenylphosphine was purchased from Aldrich and used as received.
X-ray Diffraction Crystallography. Diffraction data for complex
7 (CCDC 1468793) were collected at 150(2) K using a Bruker APEX
CCD diffractometer with graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å). A combination of ω and Φ scans was carried out to
obtain at least a unique data set. The crystal structures were solved by
direct methods; the remaining atoms were located from difference
Fourier synthesis followed by full-matrix least-squares refinement
based on F2 (programs SIR97 and SHELXL-97).40 Many hydrogen
atoms could be found from the Fourier difference analysis. Carbon-
bound hydrogen atoms were placed at calculated positions and forced
to ride on the attached atom. The hydrogen atom contributions were
calculated but not refined. All non-hydrogen atoms were refined with
anisotropic displacement parameters. The locations of the largest
peaks in the final difference Fourier map calculation and the
magnitudes of the residual electron densities were of no chemical
significance. The crystallographic data for 3 (CCDC 1045763) were
collected on a Xcalibur Eos diffractometer (graphite monochromated,
Mo Kα radiation, ω-scan technique, λ = 0.71073 Å, T = 100(2) K). Its
structure was solved by direct methods and was refined on F2 using the
SHELXTL v. 6.12 package.41 All non-hydrogen atoms were found
from Fourier syntheses of electron density and were refined
anisotropically. All hydrogen atoms were placed in calculated positions
and were refined in the riding model. SCALE3 ABSPACK42 was used
to perform area detector scaling and absorption corrections. Relevant
collection and refinement data for all compounds are given in the
Supporting Information. All data can be obtained from the Cambridge
Structural Database via www.ccdc.cam.ac.uk/data_request/cif.
constants are observed that are commensurate, kobs,1
=
[4.667(1)] × 10−5 s−1 and kobs,2 = [11.500(1)] × 10−5 s−1,
respectively. Similarly, the rate of consumption of sec-P in the
second phase (k2 = [6.815(1)] × 10−3 mol L−1 s−1) is
comparable to that measured during the first phase for the
conversion of PhPH2 (k1 = [8.275(1)] × 10−3 mol L−1 s−1). In
other words, the rates of the reactions of a primary (PhPH2)
and a secondary (sec-P) phosphine with p-tBu-styrene are very
similar. On this basis, it is remarkable that the formation of tert-
P only starts occurring when complete consumption of PhPH2
and quantitative formation of sec-P are ensured, after ca. 300
min. These events are also concomitant with a reacceleration of
the rate of consumption of p-tBu-styrene (blue trace in Figure
10). These observations strongly suggest that, although the
secondary phosphine is competitive in terms of kinetics, the σ-
bond metathesis of the reaction intermediate with the
secondary phosphine is much slower than that with the
primary phosphine.
CONCLUSION
■
Ytterbium(II) amino ether−phenolato complexes constitute
very competent precatalysts for the intermolecular hydro-
phosphination of activated alkenes, illustrated here in the case
of styrenic substrates. The catalyzed reactions also work for
conjugated dienes, e.g. isoprene, but the reactivity of the
catalyst is not sufficient to warrant the hydrophosphination of
unactivated alkenes. For instance, the hydrophosphination of 1-
nonene with PhPH2 catalyzed by 3 only reaches 3% conversion
after 70 h at 60 °C. Perhaps this situation could be improved by
preparing alkyl derivatives of 3, and we are working toward this
aim. In the meantime, Waterman’s zirconium alkyl−amido
complex remains unique in this respect.9b
The differences between alkaline-earth metals and divalent
lanthanides in this very catalysis are plain, with ytterbium(II) in
particular clearly surpassing the more common Ae, and
especially calcium, precatalysts. With these ytterbium(II)
amino ether−phenolato species, the catalytic activity increases
substantially with the denticity of the ligand. The opposite was
found in the Ae-mediated hydroamination of aminoalkenes,38
whereas there was no clear trend in Ae-promoted intermo-
lecular alkene hydrophosphination.10b,c
One of the most salient properties of these ytterbium(II)
hydrophosphination precatalysts, and among these most
prominently the kinetically stabilized {LONO4}Yb{N(SiMe3)2}
(3), is their chemoselectivity toward the production of
secondary or tertiary phosphines. The unusual reactivity and
kinetic profile which characterize the double hydrophosphina-
tion are remarkable features that enable the preparation of a
virtually boundless range of valuable asymmetric tertiary
phosphines starting from primary phosphines such as the
Hydrophosphination Experiments. In a typical reaction with
neat substrates, the precatalyst was loaded in an NMR tube in the
glovebox. p-tBu-styrene and PhPH2 were then placed in the tube and
the reaction time started after quickly placing the NMR tube in an oil
bath preheated at the desired temperature. After the desired reaction
time, the NMR tube was opened to air, nondried benzene-d6 was
added to the reaction mixture, and the 1H NMR spectrum was
recorded. Conversion was determined by integrating the remaining
I
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p-tBu-styrene and the newly formed addition product. Reactions in dry
benzene-d6 were carried out in a similar manner, but with the solvent
added at the same time as the substrates to the NMR tube.
Catalysis for Sustainable Development”, and by the European
Union (grant FP7-People-2010-IIF ChemCatSusDe to B.L.).
{LONO2}CaCH(SiMe3)2·THF (7). At room temperature, a solution of
{LONO2}H (0.35 g, 1.00 mmol) in pentane (10 mL) was added slowly
to a solution of Ca{CH(SiMe3)2}2·2THF (0.52 g, 1.03 mmol) in
pentane (10 mL). After 6 h, the reaction solution was concentrated to
5 mL, giving a white precipitate which was isolated by filtration. The
resulting powder was dried in vacuo to constant weight to afford 7 as a
colorless powder (0.40 g, 70%). Single crystals suitable for X-ray
diffraction crystallography were obtained by recrystallization from a
concentrated pentane/benzene mixture. 1H NMR (benzene-d6, 298 K,
500.13 MHz): δ 7.58 (d, 4JHH = 2.4 Hz, 1H, m-H), 6.96 (d, 4JHH = 2.4
Hz, 1H, m-H), 3.60 (m, 4H, THF), 3.16 (br s, 2H, Ar-CH2N), 2.97 (s,
6H, OCH3), 2.73 (m, 2H, CH2C(H)HO), 2.63 (m, 2H, CH2C(H)-
HO), 2.02 (m, 2H, NCH2CH2), 1.83 (m, 2H, NCH2CH2, overlapping
with s, 9H, o-C(CH3)3), 1.48 (s, 9H, p−C(CH3)3), 1.40 (m, 4H,
THF), 0.47 (s, 18H, Si(CH3)3), −1.87 (s, 1H, CHSi(CH3)3) ppm.
13C{1H} NMR (benzene-d6, 298 K, 125.76 MHz): δ 164.8 (i-C), 137.6
(o-C), 126.2 (p-C), 124.9 (m-C), 70.4 (CH2CH2O), 68.7 (THF), 60.4
(OCH3), 58.2 (ArCH2N), 54.3 (NCH2CH2), 36.3 (o-C(CH3)3), 34.9
(p-C(CH3)3), 32.9 (p-C(CH3)3), 31.0 (o-C(CH3)3), 26.1 (THF), 6.8
(Si(CH3)3), 1.9 (CHSi(CH3)3) ppm.
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1
powder (0.38 g, 55%). H NMR (THF-d8, 298 K, 500.13 MHz): δ
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ArCH2N + s, 2H, CH2O), 3.52 (br s, 2H, CH2O), 3.47 (s, 6H,
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126.8 (o-C), 124.0 (m-C), 123.6 (m-C), 71.7 (CH2O), 69.9 (CH2O),
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
added as CIF files. H NMR spectra for monitored catalyzed
reactions. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.organomet.6b00252.
NMR data for 7 and 8, NMR monitoring of substrate
conversion in hydrophosphination reactions, character-
ization of new phosphines, and crystallographic data for 3
and 7 (PDF)
XRD structure of 3 (CIF)
XRD structure of 7 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
(8) (a) Erickson, K. A.; Dixon, L. S. H.; Wright, D. S.; Waterman, R.
Inorg. Chim. Acta 2014, 422, 141−145. (b) Stelmach, J. P. W.; Bange,
C. A.; Waterman, R. Dalton Trans. 2016, 45, 6204.
(9) (a) Perrier, A.; Comte, V.; Moise, C.; Le Gendre, P. Chem. - Eur.
J. 2010, 16, 64−67. (b) Ghebreab, M. B.; Bange, Ch. A.; Waterman, R.
J. Am. Chem. Soc. 2014, 136, 9240−9243. (c) Bange, C. A.; Ghebreab,
M. B.; Ficks, A.; Mucha, N. T.; Higham, L.; Waterman, R. Dalton
Trans. 2016, 45, 1863−1867.
*E-mail for J.-F.C.: jean-francois.carpentier@univ-rennes1.fr.
*E-mail for Y.S.: yann.sarazin@univ-rennes1.fr.
*E-mail for A.A.T.: trif@iomc.ras.ru.
Notes
The authors declare no competing financial interest.
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J. 2013, 19, 13445−13462.
ACKNOWLEDGMENTS
■
This work was sponsored by the Russian Foundation for Basic
Research (Grant 15-33-20285), by a Grant of the President of
the Russian Federation for young scientists (Grant No. MK-
5702.2015.3), by the GDRI CNRS-RAS “Homogeneous
J
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K
DOI: 10.1021/acs.organomet.6b00252
Organometallics XXXX, XXX, XXX−XXX