Chiral monodentate trialkylphosphines based on the phospholane architecture

Article abstract of DOI:10.1021/om3008772

The development of efficient chiral monodentate phosphine ligands lags behind that of the bidentate congeners. This holds especially true for highly electron rich chiral phosphine analogues able to replace the ubiquitous tricyclohexylphosphine and tri-tert-butylphosphine in catalytic asymmetric transformations. We present a convenient and modular synthesis of a set of chiral monodentate ligands with different steric demands based on the popular phospholane scaffold. Their steric and electronic properties were determined by their corresponding nickel and palladium complexes. They represent good mimics of the popular tricyclohexylphosphine and tri-tert-butylphosphine ligands. Their potential was subsequently evaluated in palladium-catalyzed asymmetric C(sp³)-H functionalization leading to indolines.

Full text of DOI:10.1021/om3008772

Article
pubs.acs.org/Organometallics
Chiral Monodentate Trialkylphosphines Based on the Phospholane
Architecture
Pavel A. Donets, Tanguy Saget, and Nicolai Cramer*
́ ́
Laboratory of Asymmetric Catalysis and Synthesis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de
Lausanne (EPFL), SB-ISIC, BCH4305, 1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: The development of efficient chiral monodentate
phosphine ligands lags behind that of the bidentate congeners. This
holds especially true for highly electron rich chiral phosphine analogues
able to replace the ubiquitous tricyclohexylphosphine and tri-tert-
butylphosphine in catalytic asymmetric transformations. We present a
convenient and modular synthesis of a set of chiral monodentate ligands
with different steric demands based on the popular phospholane scaffold.
Their steric and electronic properties were determined by their
corresponding nickel and palladium complexes. They represent good
mimics of the popular tricyclohexylphosphine and tri-tert-butylphosphine
ligands. Their potential was subsequently evaluated in palladium-
catalyzed asymmetric C(sp3)−H functionalization leading to indolines.
backbone (1)2,3 and one from the taddol backbone (2),4 largely
dominate this field (Chart 1). They are easily accessible, and
INTRODUCTION
■
Major breakthroughs in transition-metal catalysis have most
often followed advances in ligand design. In particular, the
developments of chiral, tailored phosphines as steering ligands
is of utmost importance for asymmetric catalysis. Knowles, a
pioneer in this field, reported asymmetric rhodium-catalyzed
hydrogenation of dehydroamino acid derivatives en route to L-
Dopa. One of the first successful chiral ligands, CAMP, which
gave for that time a great enantioselectivity of 88% ee, was a
monodentate phosphine (eq 1).1 It was abandoned in favor of
Chart 1. Examples of Monodentate Chiral Phosphines
especially the taddol family exhibits a great modularity, a very
attractive feature in adapting the ligand to the needs of the
transformation. In contrast, more electron-rich phosphorus
ligands, targeting complementary reactions, are less common,5
and in several instances chiral carbenes6 have replaced the
phosphines.
Along our efforts to develop enantioselective C−H bond
functionalizations,7 we required electron-rich monodentate
phosphines. While the majority of the reported phosphines
were not suitable, we reported a class of ligands, of which
especially SagePhos (4; Chart 1) emerged as a very powerful
ligand to induce excellent stereoselectivities in challenging
palladium-catalyzed C(sp3)−H functionalizations.8 This struc-
tural scaffold can be seen as a chiral version of the Buchwald
ligands.9 Similarly, there is as well a need for chiral versions of
tricyclohexylphosphine and tri-tert-butylphosphine, the two
electron-rich phosphines which are most ubiquitously utilized
in catalysis. Those chiral versions should closely mimic the
his famous DIPAMP, which was not only more easily accessible
but gave a better enantioselectivity of 95% ee as well. The field
of chiral bidentate phosphines has literally exploded in the
following decades, and now, bidentate ligands are much more
abundantly available than their monodentate counterparts. This
imbalance holds true as well for the variability of their structural
design.
However, over the past few years, an increasing number of
catalytic transformations have been discovered which tolerate
only a single ligand on the metal, leading to a renaissance of
interest in monodentate phosphines as steering ligands. Despite
their longstanding niche character, powerful and privileged
scaffolds have been developed. For relatively electron poor
phosphorus ligands, the phosphoramidite structures are clearly
dominant. Two main classes, one derived from the binol
Received: September 12, 2012
© XXXX American Chemical Society
A
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electronic and steric properties of the parent phosphines. In this
respect, phosphepine derivatives such as 35c−f are currently the
closest mimics. Surprisingly, trialkyl derivatives incorporating
the good-performing, electron-rich, C2-symmetric phospholane
scaffold10 have received only scarce attention so far.11
(II) complexes 9a−d and the nickel(II) complex 10 were
obtained in 55−82% yield.16
As expected, the complexes 9b−d showed the typical square-
planar geometry for palladium(II) complexes with the
phosphine ligands being trans to each other (Figures 1−3).
RESULTS AND DISCUSSION
■
Synthesis of the Ligands and the Metal Complexes.
Chlorodimethylphospholane (5), which is accessible from the
TMS-phospholane in one step,12 was used as the key precursor.
It was reacted in the presence of a catalytic amount of
CuBr·SMe2 and lithium bromide with a range of alkyl Grignard
reagents (Scheme 1). Using our previously reported extraction
Scheme 1. Syntheses of the Alkylphospholane Ligands
6a·HBF4−6e·HBF4 and 8·HBF4
Figure 1. X-ray structure of complex 9d. The thermal ellipsoids are
displayed at 50% probability, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): P1−Pd,
2.3547(13); P2−Pd, 2.3864(15); Cl1−Pd, 2.291(2); Cl2−Pd,
2.297(2); P1−Pd−P2, 178.53(6); Cl1−Pd−Cl2, 178.00(8); P1−
Pd−Cl1, 89.92(6); P1−Pd−Cl2, 88.09(6); P2−Pd−Cl1, 89.14(7);
P2−Pd−Cl2, 92.85(7).
protocol,13 the phosphines are conveniently isolated directly as
their air-stable tetrafluoroborates 6a·HBF4−6e·HBF4 and can
be recrystallized subsequently. In general, this method provides
the phosphonium salts in reasonable yields (38−73%) and high
purities. For the synthesis of phospholane modules other than
the 2,5-dimethyl derivatives, a route involving double
alkylation14 and subsequent cleavage15 of the resulting
phosphine−borane adduct was followed. For instance,
preparation by double alkylation of the primary cyclo-
hexylphosphine with the cyclic sulfate 7 provided the 2,5-
diisopropyl derivative 8·HBF4 in 60% yield. With these two
methods a variety of trialkylphosphines with different
substitution patterns and gradually increasing steric demands
were prepared.
We have next prepared palladium(II) and nickel(II)
complexes of the phospholanes 6 (Scheme 2). The free
phosphines were obtained by treatment with sodium carbonate
in methanol and directly reacted with potassium tetrachlor-
opalladate and nickel(II) chloride dimethoxyethane complex,
respectively. After recrystallization from ethanol the palladium-
Figure 2. X-ray structure of complex 9c. The thermal ellipsoids are
displayed at 50% probability, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): P1−Pd,
2.3428(14); P2−Pd, 2.3226(15); Cl1−Pd, 2.2899(15); Cl2−Pd,
2.3089(14); P1−Pd−P2, 176.69(5); Cl1−Pd−Cl2, 171.95(6); P1−
Pd−Cl1, 92.04(6); P1−Pd−Cl2, 90.04(5); P2−Pd−Cl1, 90.90(6);
P2−Pd−Cl2, 87.25(4).
Scheme 2. Preparation of the Metal Complexes 9a−d and 10
The influence of the different phosphine substituents (tBu, Cy,
and iPr) can be visualized from the X-ray structures. For
instance, this group occupies the space below the complex
plane. The larger this group becomes with respect to the C2-
symmetric phospholane unit, the more a reaction at the metal
center will be influenced by this chiral environment.
Remarkably, these differences become most apparent with the
smallest congener, methyl-substituted phospholane derivative
6a, which yielded the palladium(II) complex 9a, crystallizing as
its cis isomer (Figure 4). However, in solution either the trans
B
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Figure 5. X-ray structure of nickel(II) complex 10. The thermal
ellipsoids are displayed at 50% probability, and hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni−
P1, 2.230(2); Ni−P2, 2.244(2); Ni−Cl1, 2.178(2); Ni−Cl2, 2.177(2);
P1−Ni−P2, 176.50(8), Cl1−Ni−Cl2, 168.83(9); P1−Ni−Cl1,
89.28(9); P1−Ni−Cl2, 89.95(9); P2−Ni−Cl1, 92.05(9); P2−Ni−
Cl2, 89.28(9).
Figure 3. X-ray structure of complex 9b. The thermal ellipsoids are
displayed at 50% probability, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): P1−Pd,
2.3296(8); P2−Pd, 2.3209(8); Cl1−Pd, 2.3058(10); Cl2−Pd,
2.2929(9); P1−Pd−P2, 176.54(3); Cl1−Pd−Cl2, 174.08(4); P1−
Pd−Cl1, 88.83(4); P1−Pd−Cl2, 91.80(3); P2−Pd−Cl1, 88.27(3);
P2−Pd−Cl2, 91.28(3).
Table 1. Buried Volume (Pd Complexes) and CO Stretching
Frequencies (Rh Complexes) of the Ligands
a
b
entry
ligand L
%Vbur
ν(CORh) (cm−1
)
c
d
1
2
3
4
5
6
7
8
9
PCy3
PtBu3
PBu3
6a
31.8
1940 (1943)
c
c
36.7
25.9
d
1955
25.5; 26.0
1946
6b
1951
e
6c
29.3; 29.8
29.1; 29.2
29.8; 30.9
1939 (1947)
1944
6d
6e
1941
8
1942
a
Calculated for both crystallographically independent ligands accord-
b
ing to ref 17. CO stretching frequencies for the trans-[RhCl(CO)-
(PR3)2] complexes; ATR measurements on a Bruker ALPHA FT-IR
c
spectrometer. Data from ref 18, calculated from the free phosphine.
d
e
Data from ref 19. Data from ref 11a.20
Figure 4. X-ray structure of complex 9a. The thermal ellipsoids are
displayed at 50% probability, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): P1−Pd,
2.2611(15); P2−Pd, 2.2710(15); Cl1−Pd, 2.3624(15); Cl2−Pd,
2.3589(16); P1−Pd−P2, 93.98(5); Cl1−Pd−Cl2, 89.68(6); P1−
Pd−Cl1, 90.96(5); P1−Pd−Cl2, 173.56(6); P2−Pd−Cl1, 174.16(6);
P2−Pd−Cl2, 85.75(6).
stretching frequencies of the corresponding rhodium complexes
[(PR3)2Rh(CO)Cl], which are readily prepared from [Rh-
(CO)2Cl]2, were used (eq 2).
isomer (CDCl3) is more stable or a mixture between cis and
trans isomers (CD3CN) can be observed. In contrast, the
corresponding nickel(II) complex 10, ligated as well with 6a,
restores the expected trans geometry (Figure 5).
The carbonyl stretching frequencies of these complexes are a
commonly used indicator of the donor abilities of the respective
phosphine.19 The measured values match very well with
tricyclohexylphosphine as prototype for an electron-rich
trialkylphosphine (Table 1). Only the benzyl derivative 6b
(entry 5) is slightly less electron rich, placing it in the range of
tributylphosphine.
Steric and Electronic Properties of the Ligands. From
the X-ray crystallographic data of the complexes, the buried
volumes (%Vbur) for the synthesized phosphines were
calculated using SambVca17 to assess the steric bulk of the
ligands 6.18 The smallest derivative, 6a, has a buried volume
between 25.5 and 26% (Table 1, entry 4). The larger iPr and
Cy derivatives are expectably relatively similar and have %Vbur
values of between 29.3 and 29.8 for 6c and between 29.1 and
29.2 for 6d. The value for the tert-butyl derivative 6e was
determined to be between 29.8 and 30.9 (entry 8). We next
aimed to compare the steric and electronic properties of the
free phosphines derived from 6a−e with those of some of their
underlying achiral versions. For this purpose, the carbonyl
Catalytic C(sp3)−H Functionalization. For a brief initial
assessment of this class of ligands for potential application in
asymmetric transition-metal catalysis, we evaluated them in the
challenging C(sp3)−H functionalization of aniline derivative 11
to provide indoline 12 (Table 2). This transformation has
recently been studied by several groups,21 including ours.8 We
reported that L4 works excellently with aryl triflates, whereas
the corresponding aryl bromides were less reactive. In contrast,
C
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based on F2 with all non-hydrogen atoms anisotropically defined.
Hydrogen atoms were placed in calculated positions by means of the
“riding” model.
Table 2. Evaluation of the Phosphines 6a−e and 8 in
a
C(sp3)−H Bond Functionalizations of 11
General Procedure for the Synthesis of the Alkylphospho-
lane·HBF4 Compounds. To a mixture of 5 (301 mg, 2 mmol),
CuBr·SMe2 (20 mg, 0.1 mmol), and LiBr (17 mg, 0.2 mmol) in n-
hexane (4 mL) was added dropwise a solution of the alkyl Grignard
reagent (2.4 mmol) at 0 °C. The reaction mixture was stirred at 23 °C
overnight, cooled to 0 °C, and quenched with degassed aqueous 3 M
HBF4 (6.6 mL, 20 mmol). The biphasic mixture was stirred for 15 min
at 23 °C. The aqueous layer was separated, washed with n-hexane (2 ×
5 mL), and extracted with DCM (3 × 20 mL). Combined DCM
extracts were dried (MgSO4) and concentrated in vacuo, yielding
crude phosphine salts which subsequently were purified as stated
below.
b
c
entry
L·HBF4
yield (%)
er
d
1
2
3
4
5
6
6a·HBF4
6b·HBF4
6c·HBF4
6d·HBF4
6e·HBF4
8·HBF4
68
55.5:44.5
d
traces
97
91
96
89
61:39
55.5:44.5
85.5:14.5
43:57
(2S,5S)-1,2,5-Trimethylphospholan-1-ium Tetrafluoroborate
(6a·HBF4). Prepared from 3.2 M (2-MeTHF) MeMgBr (0.75 mL).
Recrystallized from a minimum amount of boiling MeOH (ca. 0.5
mL), after cooling to 0 °C. Yield: 166 mg (38%), white solid. 1H NMR
(400 MHz, CDCl3, δ): 6.27 (dm, JPH = 501.8 Hz, 1H), 3.06−3.21 (m,
1H), 2.51−2.65 (m, 1H), 2.25−2.50 (m, 2H), 1.96 (dd, J1 = 15.4 Hz,
J2 = 5.7 Hz, 3H), 1.58−1.77 (m, 2H), 1.52 (dd, J1 = 19.8 Hz, J2 = 7.1
Hz, 3H), 1.39 (dd, J1 = 18.6 Hz, J2 = 7.3 Hz, 3H). 13C{1H} NMR (100
MHz, CDCl3, δ): 34.14 (d, J = 7.1 Hz), 33.55 (d, J = 7.9 Hz), 31.48 (d,
J = 49.8 Hz), 27.56 (d, J = 49.1 Hz), 15.03 (d, J = 1.7 Hz), 13.16 (d, J
= 2.5 Hz), −0.22 (d, J = 45.6 Hz). 31P{1H} NMR (162 MHz, CDCl3,
δ): 33.2. IR (ATR, cm−1): 3011, 2934, 2888, 1462, 1418, 1390, 1311,
1286, 1048, 1038, 950, 916, 840, 811, 792, 763. HRMS (ESI): calcd
a
Reaction conditions: 0.06 mmol of 11, 7.20 μmol of L·HBF4, 3.00
μmol of Pd(dba)2, 18 μmol of PivOH, 1.5 equiv of Cs2CO3, 0.3 M in
b
mesitylene at 130 °C for 12 h. Yield of isolated product 12.
c
d
Determined by HPLC using a chiral stationary phase. The reaction
did not reach full conversion.
the developed electron-rich phospholanes 6a−e and 8
successfully promoted this test transformation, displaying a
reverse preference. The benzyl derivative 6b did not catalyze
the reaction (entry 2), presumably undergoing a cyclo-
metalation of the ligand leading to an incompetent species.
Increasing the bulk of the alkyl substituent led to more active
catalysts as well as higher enantioselectivities (entry 1 vs entries
3−5). In this respect, ligand 6e, bearing a bulky tert-butyl group
proved to be the best, affording 12 in 96% yield and 85.5:14.5
enantiomeric ratio (entry 5). In contrast with our previous
report,8 the isopropyl substituent on the phospholane module
was not beneficial for the selectivity (entry 6).
20
for [M − BF4]+ 131.0984, found 131.0985. [α]D = −17.3° (c = 0.5,
MeCN). Mp: 225−227 °C.
(2S,5S)-1-Benzyl-2,5-dimethylphospholan-1-ium Tetrafluorobo-
rate (6b·HBF4). Prepared from 1.5 M (THF) BnMgCl (1.6 mL).
Recrystallized from a minimum amount of boiling EtOH (ca. 7 mL),
1
after cooling to 0 °C. Yield: 267 mg (45%), yellowish solid. H NMR
(400 MHz, CD3CN, δ): 7.38−7.49 (m, 5H), 5.97 (dm, JPH = 488.4
Hz, 1H), 3.83 (t, J = 15.6 Hz, 1H), 3.58 (td, J1 = 14.9 Hz, J2 = 10.9 Hz,
1H), 2.93−3.07 (m, 1H), 2.57−2.72 (m, 1H), 2.20−2.46 (m, 2H),
1.60−1.73 (m, 1H), 1.47−1.59 (m, 1H), 1.39 (dd, J1 = 18.1 Hz, J2 =
7.3 Hz, 3H), 0.88 (dd, J1 = 18.8 Hz, J2 = 7.1 Hz, 3H). 13C{1H} NMR
(100 MHz, CD3CN, δ): 130.72 (d, J = 3.0 Hz), 130.53 (d, J = 5.8 Hz),
129.63 (d, J = 3.6 Hz), 129.47 (d, J = 9.5 Hz), 34.87 (d, J = 6.7 Hz),
34.24 (d, J = 7.6 Hz), 31.11 (d, J = 48.5 Hz), 30.04 (d, J = 43.5 Hz),
22.79 (d, J = 38.7 Hz), 15.45 (d, J = 3.1 Hz), 13.39 (d, J = 2.5 Hz).
31P{1H} NMR (162 MHz, CD3CN, δ): 39.7. IR (ATR, cm−1) 3639,
2972, 2940, 2880, 2112, 1602, 1497, 1456, 1408, 1388, 1284, 1237,
1052, 931, 862, 791, 766, 739, 702.81. HRMS (ESI): calcd for [M −
CONCLUSIONS
■
In summary, we have reported a class of electron-rich
trialkylphosphine ligands which incorporate the chiral phos-
pholane moiety. These ligands have a low molecular weight,
and even the bulkiest congener comes more closely to the steric
and electronic properties of tricyclohexylphosphine and to
those of tri-tert-butylphosphine. We demonstrated that they are
able to induce selectivity in C(sp3)−H bond functionalization
at high reaction temperatures.
20
BF4]+ 207.1297, found 207.1291. [α]D = −72.7° (c = 0.5, MeCN).
Mp: 169−170 °C.
EXPERIMENTAL SECTION
(2S,5S)-1-Isopropyl-2,5-dimethylphospholan-1-ium Tetrafluoro-
borate (6c·HBF4). Prepared from 1.3 M (THF) iPrMgCl (1.85 mL).
Recrystallized from EtOH (ca. 0.5 mL), after cooling to −30 °C. Yield:
■
Chemicals and Reagents. All manipulations were carried out
under an inert N2(g) atmosphere using standard Schlenk or glovebox
techniques. Solvents were purified using a two-column solid-state
purification system (Innovative Technology, Amesbury, MA) and
degassed prior to use with three pump−freeze−thaw cycles. Unless
otherwise noted, all other reagents and starting materials were
purchased from commercial sources and used without further
purification. (S,S)-1-Chloro-2,5-dimethylphospholane (5)12 was pre-
pared as previously reported.
1
361 mg (73%), white solid. H NMR (400 MHz, CD3CN, δ): 5.72
(dq, J1(PH) = 474.1 Hz, J2 = 6.4 Hz, 1H), 2.92−3.07 (m, 1H), 2.67−
2.83 (m, 2H), 2.24−2.42 (m, 2H), 1.52−1.68 (m, 2H), 1.33−1.46 (m,
12H). 13C{1H} NMR (100 MHz, CD3CN, δ): 35.47 (d, J = 6.4 Hz),
34.14 (d, J = 6.1 Hz), 30.16 (d, J = 44.1 Hz), 28.86 (d, J = 45.8 Hz),
19.53 (d, J = 38.6 Hz), 18.28 (d, J = 3.0 Hz), 16.94 (d, J = 3.1 Hz),
16.46 (d, J = 2.5 Hz), 13.07 (d, J = 2.5 Hz). 31P{1H} NMR (162 MHz,
CD3CN, δ): 46.62. IR (ATR, cm−1): 2973, 2940, 2881, 1462, 1391,
1284, 1047, 932, 892, 856, 804, 709.92. HRMS (ESI): calcd for [M −
1
Physical Methods. The H, 13C{1H}, and 31P{1H} NMR spectra
were recorded at 293 K on a Bruker Avance 400 spectrometer. HPLC
analyses were conducted on an Agilent 1260 Analytical HPLC system
equipped with a Daicel Chiralcel (Chiralpak) OZH column. Elemental
analyses were performed on a Carlo Erba EA 1110 CHN instrument at
EPFL. X-ray diffraction studies were carried out at the EPFL
Crystallography Facility. The data collections for the crystal structures
were performed at low temperature using Mo Kα radiation on a
Bruker APEX II CCD diffractometer equipped with a κ geometry
goniometer. The data were reduced by EvalCCD22 and then corrected
for absorption.23 The solution and refinement was performed by
SHELX.24 The structure was refined using full-matrix least squares
20
BF4]+ 159.1297, found 159.1299. [α]D = −11.3° (c = 0.5, MeCN).
Mp: 182−183 °C.
(2S,5S)-1-Cyclohexyl-2,5-dimethylphospholan-1-ium Tetrafluoro-
borate (6d·HBF4). Prepared from 1 M (THF) CyMgBr (2.4 mL).
Dissolved in a minimum amount of DCM (ca. 0.3 mL) and layered
1
with n-pentane (5 mL). Yield: 203 mg (51%), white solid. H NMR
(400 MHz, CD3CN, δ): 5.83 (dq, J1(PH) = 476.1 Hz, J2 = 6.5 Hz, 1H),
3.09−3.24 (m, 1H), 2.59−2.87 (m, 2H), 2.28−2.45 (m, 2H), 2.06−
2.17 (m, 1H), 1.82−2.04 (m, 3H), 1.61−1.81 (m, 3H), 1.39−1.56 (m,
10H), 1.23−1.36 (m, 1H). 13C{1H} NMR (100 MHz, CDCl3, δ):
D
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35.04 (d, J = 6.4 Hz), 33.62 (d, J = 6.1 Hz), 29.36 (d, J = 43.6 Hz),
28.40 (d, J = 3.6 Hz), 28.04 (d, J = 45.8 Hz), 27.57 (d, J = 37.3 Hz),
27.03 (d, J = 3.8 Hz), 25.87 (d, J = 13.4 Hz), 25.72 (d, J = 13.4 Hz),
25.15 (d, J = 1.4 Hz), 16.48 (d, J = 2.0 Hz), 13.09 (d, J = 1.9 Hz).
31P{1H} NMR (162 MHz, CDCl3, δ): 42.50. IR (ATR, cm−1): 3627,
3558, 2936, 2860, 1633, 1451, 1389, 1283, 1053, 908. HRMS (ESI):
The filtrate was added dropwise to a vigorously stirred solution of
K2PdCl4 (65 mg, 0.2 mmol) in degassed H2O (1.5 mL) at 23 °C. After
the mixture was stirred for 30 min, the brown-yellow precipitate was
collected on a filter, washed successively with H2O (2 × 0.5 mL) and
MeOH (2 × 0.2 mL), and dried in vacuo. In order to remove any Pd
metal, the obtained powder was dissolved in DCM (1 mL) and filtered
through Celite. The filtrate was evaporated with EtOH in vacuo,
yielding pure Pd complex.
20
calcd for [M − BF4]+ 199.1610, found 199.1609. [α]D = −9.3° (c =
0.5, MeCN). Mp: 65−66 °C.
Complex 9a. Prepared from 6a·HBF4 (109 mg). Yield: 48 mg
(55%), light yellow needles. 1H NMR (400 MHz, CDCl3;
predominantly the trans isomer, δ): 3.20−3.33 (m, 2H), 2.26−2.37
(m, 2H), 2.07−2.23 (m, 4H), 1.61−1.76 (m, 3H), 1.46−1.57 (m,
13H), 1.25 (dd, J1 = 15.6 Hz, J2 = 7.1 Hz, 6H). 31P{1H} NMR (162
MHz, CDCl3, δ): 45.8 (trans), 28.8 (cis). IR (ATR, cm−1): 2953,
2929, 2862, 1452, 1408, 1377, 1286, 1162, 1076, 1059, 1014, 886,
739.32. Anal. Found (calcd) for C14H30Cl2P2Pd: C, 38.46 (38.42); H,
(2S,5S)-1-(tert-Butyl)-2,5-dimethylphospholan-1-ium Tetrafluor-
oborate (6e·HBF4). Prepared from 2 M (Et2O) tBuMgCl (1.2 mL).
Recrystallized from a minimum amount of boiling EtOH (ca. 1 mL),
after cooling to 0 °C. Yield: 282 mg (54%), white solid. 1H NMR (400
MHz, CD3CN, δ): 5.68 (dm, JPH = 468.7 Hz, 1H), 2.95−3.10 (m,
1H), 2.75−2.90 (m, 1H), 2.24−2.42 (m, 2H), 1.54−1.73 (m, 2H),
1.49 (dd, J1 = 13.5 Hz, J2 = 7.2 Hz, 3H), 1.45 (dd, J1 = 15.2 Hz, J2 =
7.2 Hz, 3H), 1.43 (d, J = 17.1 Hz, 9H). 13C{1H} NMR (100 MHz,
CD3CN, δ): 36.06 (d, J = 6.8 Hz), 33.90 (d, J = 4.8 Hz), 32.51 (d, J =
40.0 Hz), 31.23 (d, J = 36.1 Hz), 29.04 (d, J = 44.6 Hz), 26.12, 17.22
(d, J = 2.9 Hz), 13.48 (d, J = 2.6 Hz). 31P{1H} NMR (162 MHz,
CD3CN, δ): 49.7. IR (ATR, cm−1): 3631, 2972, 1626, 1465, 1408,
1377, 1283, 1203, 1051, 932, 895, 813, 763, 678, 665. HRMS (ESI):
20
6.97 (6.91). [α]D = +134.6° (c = 0.5, CHCl3). Mp: 164−166 °C.
Assignment of the trans/cis isomers of the complex 9a by 31P spectra
in different solvents: 31P{1H} NMR (162 MHz, C6D6, δ) 28.6 (cis
isomer, one peak only, sample is sparingly soluble); 31P{1H} NMR
(162 MHz, CDCl3, δ) 45.8 and 28.8 (trans:cis ratio 92:8); 31P{1H}
(162 MHz, CD3CN, δ) 47.1 and 29.2 (trans:cis ratio: 83:17).
Complex 9b. Prepared from 6c·HBF4 (123 mg). Yield: 67 mg
(68%), bright yellow solid. 1H NMR (400 MHz, CDCl3, δ): 2.98−3.13
(m, 2H), 2.36−2.53 (m, 4H), 1.98−2.15 (m, 4H), 1.28−1.62 (m,
28H). 31P{1H} NMR (162 MHz, CDCl3, δ): 44.7. IR (ATR, cm−1):
2954, 2939, 2922, 2865, 1454, 1387, 1368, 1246, 1158, 1086, 1059,
1039, 883. Anal. Found (calcd) for C18H38Cl2P2Pd: C, 43.54 (43.78);
H, 7.67 (7.76). [α]D20 = +174.6° (c = 0.5, CHCl3). Mp: 152−153 °C.
Complex 9c. Prepared from 6d·HBF4 (143 mg). Yield: 86 mg
(82%), bright yellow solid. 1H NMR (400 MHz, CDCl3, δ): 2.94−3.07
(m, 2H), 2.44−2.56 (m, 2H), 1.95−2.23 (m, 10H), 1.64−1.91 (m,
10H), 1.18−1.61 (m, 22H). 31P{1H} NMR (162 MHz, CDCl3, δ):
41.1. IR (ATR, cm−1): 2926, 2852, 1447, 1374, 1174, 1159, 1076,
1003, 849, 733. Anal. Found (calcd) for C24H46Cl2P2Pd: C, 50.22
(50.23); H, 8.06 (8.08). [α]D20 = +106.5° (c = 0.5, CHCl3). Mp: 123−
124 °C.
20
calcd for [M − BF4]+ 173.1454, found 173.1454. [α]D = −5.3° (c =
0.3, MeCN). Mp: 199−204 °C.
(2R,5R)-1-Cyclohexyl-2,5-diisopropylphospholan-1-ium Tetra-
fluoroborate (8·HBF4). Cyclohexylphosphine (28.4 mg, 0.25 mmol)
was weighed into a glovebox into a flame-dried reaction tube, dissolved
in THF (1 mL), and cooled to −78 °C. A 1.6 M solution (hexane) of
BuLi (0.23 mL, 0.37 mmol) was added dropwise, and the solution was
warmed to 23 °C and stirred for 1 h. The reaction mixture was cooled
to −78 °C, and a solution of (4S,7S)-4,7-diisopropyl-1,3,2-
dioxathiepane 2,2-dioxide (59.1 mg, 0.25 mmol) in THF (0.5 mL)
was added dropwise. The reaction mixture was warmed to 23 °C,
stirred for 4 h, and cooled to −78 °C. A 1.6 M solution (hexane) of
BuLi (0.23 mL, 0.37 mmol) was added dropwise, and then the
reaction mixture was slowly warmed to 23 °C and stirred overnight. A
1 M (THF) BH3·THF (0.750 mL, 0.750 mmol) was added at 0 °C,
and the reaction mixture was stirred for 1 h at room temperature.
Then 1 M HCl was added at 0 °C. The reaction mixture was diluted
with pentane. The layers were separated, and the organic layer was
further washed with water (×x2) and dried over MgSO4. The crude
mixture was purified by chromatography on silica gel column using
hexane/Et2O 30/1 as eluent to afford BH3-protected (2R,5R)-1-
cyclohexyl-2,5-diisopropylphospholane (40.2 mg,0.15 mmol, 61%
yield) as a colorless oil. This oil was dissolved under nitrogen in 4.0
mL of dry and degassed DCM and cooled to 0 °C, and
diethyloxonium tetrafluoroborate (0.31 mL, 2.24 mmol) was added
dropwise. The reaction mixture was stirred at 0 °C for 30 min and
then 30 min at 23 °C. Degassed 8.0 M hydrogen tetrafluoroborate
(1.90 mL, 14.9 mmol) was added at 23 °C, and the reaction mixture
was vigorously stirred for 30 min. The aqueous phase was washed with
DCM (3 × 10 mL). The combined organic layers were dried over
MgSO4, and DCM was removed in vacuo to afford 8·HBF4 as a white
Complex 9d. Prepared from 6e·HBF4 (130 mg). Yield: 85 mg
(74%), deep yellow solid. 1H NMR (400 MHz, CDCl3, δ): 3.06−3.18
(m, 2H), 2.42−2.53 (m, 2H), 1.98−2.15 (m, 4H), 1.51−1.65 (m,
17H), 1.42−1.48 (m, 17H). 31P{1H} NMR (162 MHz, CDCl3, δ):
48.4; IR (ATR, cm−1): 2991, 2948, 2928, 2894, 2865, 1461, 1451,
1365, 1182, 1157, 1072, 1016, 813.41. Anal. Found (calcd) for
C20H42Cl2P2Pd: C, 45.95 (46.03); H, 8.21 (8.11). [α]D20 = +116.0° (c
= 0.5, CHCl3). Mp: 241−243 °C.
Synthesis of the Nickel Complexes 10. 6a·HBF4 (218 mg, 1
mmol) and powdered Na2CO3 (212 mg, 2 mmol) were suspended in
EtOH (1 mL). The mixture was sonicated at 23 °C for 30 min and
filtered in the glovebox. NiCl2·DME (88 mg, 0.4 mmol) was dissolved
with heating in EtOH (1 mL). The slightly turbid solution was filtered
through Celite and added dropwise to the vigorously stirred solution
of the phosphine. The resulting deep purple solution was refluxed for
15 min, cooled to 23 °C, and concentrated to a volume of ca. 0.5 mL.
After standing at 23 °C overnight the mother liquor was removed and
the residual deep purple crystalline solid was washed successively with
cold MeOH (2 × 0.3 mL) and Et2O (2 × 0.3 mL) and dried in vacuo.
1
solid (50.1 mg, 0.15 mmol, 98% yield). H NMR (400 MHz, CDCl3,
δ): 6.51 (d, JPH = 486.7 Hz, 1H), 2.86−2.69 (m, 1H), 2.59−2.32 (m,
4H), 2.28−2.08 (m, 2H), 2.06−1.68 (m, 6H), 1.62−1.38 (m, 5H),
1.35−1.27 (m, 1H), 1.26 (d, J = 6.6 Hz, 3H), 1.14 (d, J = 6.8 Hz, 3H),
1.12 (d, J = 6.8 Hz, 3H), 1.09 (d, J = 6.6 Hz, 3H). 13C{1H} NMR (100
MHz, CDCl3, δ): 44.3 (d, J = 41.5 Hz), 40.2 (d, J = 44.6 Hz), 31.3 (d,
J = 7.8 Hz), 30.2 (d, J = 1.8 Hz), 30.0 (d, J = 5.1 Hz), 28.5 (d, J = 37.5
Hz), 28.5 (d, J = 3.6 Hz), 28.0 (d, J = 3.8 Hz), 27.5 (d, J = 1.7 Hz),
26.3 (d, J = 13.5 Hz), 25.9 (d, J = 12.9 Hz), 25.0 (d, J = 1.7 Hz), 24.0
(d, J = 2.7 Hz), 23.6 (d, J = 5.2 Hz), 21.7 (d, J = 12.1 Hz), 20.9 (d, J =
10.8 Hz). 31P{1H} NMR (162 MHz, CDCl3, δ): 27.0. IR (ATR,
cm−1): 2965, 2938, 2860, 2435, 1449, 1051, 518. HRMS (ESI): calcd
for [M − BF4]+ 255.2236, found 255.2237. [α]D20 = −71.0° (c = 0.35,
CHCl3). Mp: 202−203 °C.
1
Yield: 86 mg (55%). H NMR (400 MHz, C6D6, δ): 2.8−2.94 (m,
2H), 2.04 (d, J = 6.7 Hz, 6H), 1.49−1.71 (m, 6H), 1.14−1.29 (m,
2H), 0.77−1.01 (m, 14H). 31P{1H} NMR (162 MHz, CDCl3, δ): 66.9.
IR (ATR, cm−1): 3369, 2964, 2930, 2862, 1452, 1411, 1376, 1288,
1162, 1075, 1059, 1015, 925, 888, 737. Anal. Found (calcd) for
20
C14H30Cl2P2Ni: C, 43.41 (43.12); H, 8.12 (7.75). [α]D = +60° (c =
0.5, toluene). Mp: 101−102 °C.
Procedure for the Catalytic Reaction with Aryl Bromide 11.
Pd(dba)2 (1.72 mg, 3.00 μmol), 6e·HBF4 (1.87 mg, 12.0 μmol), 11
(20.8 mg, 0.06 mmol), PivOH (1.84 mg, 18.0 μmol), and Cs2CO3
(29.3 mg, 0.09 mmol) were weighed into a vial equipped with a
magnetic stirrer bar and sealed with a rubber septum. The vial was put
under vacuum and then backfilled with nitrogen (×3). Mesitylene
(200 μL, 0.3 M) was added, and the mixture was degassed by three
General Procedure for the Synthesis of the Palladium
Complexes.25 Phosphine salt 6·HBF4 (0.5 mmol) and powdered
Na2CO3 (106 mg, 1 mmol) were suspended in MeOH (1.5 mL). The
mixture was sonicated at 23 °C for 30 min and filtered in the glovebox.
E
dx.doi.org/10.1021/om3008772 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
freeze−pump−thaw cycles. The mixture was stirred at 23 °C for 1 h
and then heated to 135 °C for 12 h, cooled to room temperature, and
then directly purified on silica gel, yielding 12 as a colorless oil. The
absolute configuration was assigned by comparison with the literature
data.8 1H NMR (400 MHz, CDCl3, δ): 7.48 (d, J = 7.7 Hz, 1H), 7.28−
7.23 (m, 2H), 7.20−7.11 (m, 1H), 4.84−4.65 (m, 1H), 3.54 (dd, J =
15.9, 9.2 Hz, 1H), 2.74 (dd, J = 15.9, 1.7 Hz, 1H), 1.46 (d, J = 6.5 Hz,
3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, δ): 138.4, 130.6, 128.1,
125.8, 125.5, 120.2 (q, JC−F = 325.3 Hz), 115.7, 60.9, 36.3, 22.8 ppm.
IR (ATR, cm−1): 2986, 2930, 1597, 1480, 1462, 1395, 1327, 1254,
1224, 1188, 1144, 1102, 1027, 939, 752, 709, 649, 608, 575, 528 cm−1.
HRMS (EI): calcd. for [C10H10F3NO2S]•+ 265.0379, found 265.0379.
22.5786(8) Å, α = 90°, β = 90°, γ = 90°, V = 2510.49(16) Å3, Z = 4,
ρcalcd = 1.380 Mg/m3.
Crystallographic Details for 10. A total of 39 495 reflections
(−15 < h < 15, −16 < k < 16, −31 < l < 31) were collected at T =
100(2) K in the range 3.29−27.50°, 8439 of which were unique (Rint
=
0.1022), with Mo Kα radiation (λ = 0.710 73 Å). The structure was
solved by direct methods. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were placed in calculated
idealized positions. The residual peak and hole electron densities
were 1.433 and −1.346 e Å−3, respectively. The absorption coefficient
was 1.461 mm−1. The least-squares refinement converged normally
with residuals of R(F) = 0.0638 and Rw(F2) = 0.1619 and GOF =
1.065 (I > 2σ(I)). Crystal data for C14H30Cl2NiP2: Mw = 389.93, space
group P212121, orthorhombic, a = 12.373(4) Å, b = 12.529(2) Å, c =
24.461(5) Å, α = 90°, β = 90°, γ = 90°, V = 3791.8(15) Å3, Z = 8, ρcalcd
= 1.366 Mg/m3.
20
[α]D = 25.3 (c = 1.0, CHCl3). Rf = 0.64 (pentane/EtOAc, 15/1).
HPLC separation (Chiralpak OZH, 4.6 × 250 mm; 0.5% i-PrOH/
hexane, 1.0 mL/min, 254 nm; tr(minor) = 8.8 min, tr(major) = 6.7
min): 85.5:14.5 er.
Crystallographic Details for 9a. A total of 7427 reflections (−9
< h < 10, −14 < k < 12, −28 < l < 24) were collected at T = 293(2) K
in the range 3.13−29.28°, 4319 of which were unique (Rint = 0.0379),
with Mo Kα radiation (λ = 0.710 73 Å). The structure was solved by
direct methods. All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were placed in calculated idealized positions. The
residual peak and hole electron densities were 0.611 and −0.741 e Å−3,
respectively. The absorption coefficient was 1.430 mm−1. The least-
squares refinement converged normally with residuals of R(F) =
0.0457 and Rw(F2) = 0.0732 and GOF = 1.049 (I > 2σ(I)). Crystal
data for C14H30Cl2P2Pd: Mw = 437.62, space group P212121,
orthorhombic, a = 8.0418(3) Å, b = 10.5872(4) Å, c = 22.0696(9)
Å, α = 90°, β = 90°, γ = 90°, V = 1879.02(14) Å3, Z = 4, ρcalcd = 1.547
Mg/m3.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures giving NMR spectra of all new compounds and HPLC
traces for 12 and CIF files giving crystallographic data for 9a−d
and 10. This material is available free of charge via the Internet
at http://pubs.acs.org. CCDC 900820−900824 also contain
the supplementary crystallographic data for 9a−d and 10.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nicolai.cramer@epfl.ch.
Notes
The authors declare no competing financial interest.
Biography
■
Crystallographic Details for 9b. A total of 10 844 reflections
(−10 < h < 11, −11 < k < 16, −28 < l < 29) were collected at T =
293(2) K in the range 2.92−29.29°, 5129 of which were unique (Rint
=
0.0234), with Mo Kα radiation (λ = 0.710 73 Å). The structure was
solved by direct methods. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were placed in calculated
idealized positions. The residual peak and hole electron densities
were 0.555 and −0.481 e Å−3, respectively. The absorption coefficient
was 1.169 mm−1. The least-squares refinement converged normally
with residuals of R(F) = 0.0307 and Rw(F2) = 0.0541 and GOF =
1.049 (I > 2σ(I)). Crystal data for C18H38Cl2P2Pd: Mw = 493.72, space
group P212121, orthorhombic, a = 8.5322(2) Å, b = 12.0949(3) Å, c =
22.4379(5) Å, α = 90°, β = 90°, γ = 90°, V = 2315.50(9) Å3, Z = 4,
ρcalcd = 1.416 Mg/m3.
Crystallographic Details for 9c. A total of 6684 reflections (−11
< h < 11, −24 < k < 28, −11 < l < 11) were collected at T = 140(2) K
in the range 2.55−30.08°, 6684 of which were unique (Rint = 0.0000),
with Mo Kα radiation (λ = 0.710 73 Å). The structure was solved by
direct methods. All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were placed in calculated idealized positions. The
residual peak and hole electron densities were 1.504 and −1.999 e Å−3,
respectively. The absorption coefficient was 1.014 mm−1. The least-
squares refinement converged normally with residuals of R(F) =
0.0658 and Rw(F2) = 0.1758 and GOF = 0.976 (I > 2σ(I)). Crystal
data for C24H46Cl2P2Pd: Mw = 573.85, space group P21, monoclinic, a
= 8.298(3) Å, b = 19.970(6) Å, c = 8.468(2) Å, α = 90°, β =
105.993(16)°, γ = 90°, V = 1348.9(7) Å3, Z = 2, ρcalcd = 1.413 Mg/m3.
Crystallographic Details for 9d. A total of 33 297 reflections
(−10 < h < 11, −16 < k < 17, −28 < l < 29) were collected at T =
Nicolai Cramer was born in 1977 in Stuttgart, Germany. He studied
chemistry at the University of Stuttgart (1998−2003) and obtained his
Ph.D. from the same institution (2005; advisor Prof. Sabine Laschat).
After a postdoctoral stint at Stanford (advisor Prof. Barry M. Trost),
he started in 2007 his independent career as a habilitant at the chair of
Prof. Erick M. Carreira at ETH Zurich, Switzerland. In 2010, he
assumed his current position as assistant professor at the Ecole
293(2) K in the range 2.86−29.37°, 6272 of which were unique (Rint
=
0.0345), with Mo Kα radiation (λ = 0.710 73 Å). The structure was
solved by direct methods. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were placed in calculated
idealized positions. The residual peak and hole electron densities
were 1.318 and −1.146 e Å−3, respectively. The absorption coefficient
was 1.082 mm−1. The least-squares refinement converged normally
with residuals of R(F) = 0.0533 and Rw(F2) = 0.1249 and GOF =
1.040 (I > 2σ(I)). Crystal data for C20H42Cl2P2Pd: Mw = 521.78, space
group P212121, orthorhombic, a = 8.5030(3) Å, b = 13.0765(5) Å, c =
́ ́
Polytechnique Federale de Lausanne (EPFL). His research interests
encompass asymmetric catalysis, selective C−H and C−C bond
functionalizations, and synthesis of bioactive natural products.
F
dx.doi.org/10.1021/om3008772 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(14) Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L. J. Am.
Chem. Soc. 1993, 115, 10125−10138.
(15) Denmark, S. E.; Werner, N. S. Org. Lett. 2011, 13, 4596−4599.
(16) Complexation experiments with 6b with palladium(II) salts
were not attempted due to the cyclometalation propensity of the
benzyl substituent.
(17) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.;
Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759−1766.
Parameters used for SambVca calculations: (i) for %Vbur calculations,
3.50 Å was selected as the value for the sphere radius; (ii) 2.28 Å was
considered as the distance for the metal−ligand bond; (iii) hydrogen
atoms were omitted; (iv) scaled Bondi radii were used.
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137.
ACKNOWLEDGMENTS
■
This work was supported by the EPFL and by the European
Research Council under the European Community’s Seventh
Framework Program (FP7 2007−2013)/ERC Grant agreement
No. 257891. We thank Dr. R. Scopelliti for X-ray crystallo-
graphic analyses and Dr. R. Kadyrov (Evonik Industries AG)
for generous donations of 2,5-dimethyl-1-TMS-phospholane.
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dx.doi.org/10.1021/om3008772 | Organometallics XXXX, XXX, XXX−XXX