Gold(I) complexes of conformationally constricted chiral ferrocenyl phosphines

Article abstract of DOI:10.1021/om300222k

The preparation of two new chiral, enantiopure, and conformationally constrained phosphocin and 1,5-diphosphocin, incorporating two ferrocenyl units, is described. The gold(I) chloride complexes of these ligands and (S)-(R)-PPF-OMe were prepared, and their structures, in solution and solid states, are compared. Abstraction of the chloride anion by the addition of silver salt of either toluenesulfonate or chiral BINOL-phosphates generates active catalysts for the intramolecular cyclization of 6-methyl-1,1- diphenylhepta-4,5-dien-1-ol, where up to 47% ee can be obtained. Match and mismatch effects between chiral ligands and counteranions are highlighted.

Full text of DOI:10.1021/om300222k

Article
pubs.acs.org/Organometallics
Gold(I) Complexes of Conformationally Constricted Chiral Ferrocenyl
Phosphines
Elena M. Barreiro,† Diego F. D. Broggini,‡ Luis A. Adrio,† Andrew J. P. White,† Rino Schwenk,‡
Antonio Togni,*,‡ and King Kuok (Mimi) Hii*,†
†Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, U.K.
‡Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Zurich, CH-8093, Zurich,
̈
̈
Switzerland
S
* Supporting Information
ABSTRACT: The preparation of two new chiral, enantiopure,
and conformationally constrained phosphocin and 1,5-diphos-
phocin, incorporating two ferrocenyl units, is described. The
gold(I) chloride complexes of these ligands and (S)-(R)-PPF-
OMe were prepared, and their structures, in solution and solid
states, are compared. Abstraction of the chloride anion by the
addition of silver salt of either toluenesulfonate or chiral BINOL-
phosphates generates active catalysts for the intramolecular cyclization of 6-methyl-1,1-diphenylhepta-4,5-dien-1-ol, where up to
47% ee can be obtained. Match and mismatch effects between chiral ligands and counteranions are highlighted.
INTRODUCTION
■
Chiral ferrocenylphosphine ligands were first described by
Kumada and Hayashi in 1974.1,2 Boosted by their commercial
availability,3 they have been extensively studied in asymmetric
reactions4−8 and many industrial processes,9 resulting in the
production of the herbicide (S)-metolachlor, the largest
commercial application of asymmetric catalysis.10 On the
other hand, the development of gold catalysis for stereo-
selective organic synthesis is a highly topical area of research in
recent years, particularly for the activation of unsaturated
carbon−carbon bonds toward C−C and C−X bond
formations.11,12 Early work by Hayashi and Togni included
the application of ferrocenylphosphine−gold(I) complexes to
aldol reactions of isocyanoalkanoates with aldehydes to afford
oxazolines in high enantioselectivities.13−15 Since then, they
have also been used in C−C bond-forming processes, such as
the addition of isocyanoacetates to N-sulfonylimines,16
alkoxycyclization of enynes,17 and cyclodimerization of
dialkynes.18 However, more recent reports of enantioselective
gold catalysis are dominated by the use of chiral diphosphine
ligands, while the use of nonchelating ligands for asymmetric
catalysis is restricted to sterically bulky NHCs19 and
phosphoramidites.20−22 In this paper, we will report the
coordination chemistry of three structurally similar chiral
ferrocenylphosphines, L1, L2, and L3, to gold (Figure 1).
The resultant complexes will be evaluated in the intramolecular
cyclization of a γ-allenol (6-methyl-1,1-diphenylhepta-4,5-dien-
1-ol, 6).
Figure 1. Chiral mono- and diphosphine ligands used in this work.
synthetic and recrystallization purposes were distilled under Ar (THF
from Na/benzophenone; CH2Cl2, MeOH, EtOH, and acetonitrile
from CaH2; pentane, hexane, toluene, and Et2O from Na/K) at ETH
Zurich, or by passing through solvent purification towers under N
̈
2
over molecular sieves (Imperial College London).
31P, 13C, and 1H NMR spectra were measured in the stated solvent
using the following instruments: ETH Zurich: Bruker Avance 250
̈
1
[frequency in MHz: 31P, 101.26; 13C, 62.90; H, 250.14] or Bruker
1
Avance 300 [frequency in MHz: 31P, 121.49; 13C, 75.47; H, 300.13]
or Bruker Avance 400 [frequency in MHz: 31P, 161.98; 13C, 100.61;
1H, 400.13] or Bruker Avance DPX500 [frequency in MHz: 31P,
202.46; 13C, 125.75; 1H, 500.23] at room temperature. Imperial
College London: Bruker AVANCE 400 [frequency in MHz: 31P,
1
161.9; 13C, 100.6; H, 400.3]. Chemical shifts are reported in δ (parts
per million), referenced to TMS for 1H/13C, and 85% H3PO4 for 31P. J
values are given in hertz. Multiplicity is abbreviated to s (singlet), d
(doublet), dd (double doublet), t (triplet), q (quartet), dq (double
quartet), tq (triple quartet), and m (multiplet). Melting points were
recorded using an Electrothermal Gallenhamp apparatus and were
uncorrected. Chiral HPLC was performed on Gilson and Hewlett-
Packard HPLC systems, each equipped with a variable-wavelength UV
detector set at 210 nm and autoinjectors with 10 μL loops, using a
EXPERIMENTAL SECTION
■
General Procedures. All reactions with air- or moisture-sensitive
materials were carried out under Ar or N2 using standard Schlenk
techniques or in a glovebox (under N2). The solvents used for
Received: March 19, 2012
Published: May 2, 2012
© 2012 American Chemical Society
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Organometallics
Article
Daicel Chiralcel OJ-H column (250 × 4.6 mm). Mass spectra (MS)
were recorded on either a Micromass Autospec Premier or a VG
Platform II spectrometer using EI or FAB+ techniques. Elemental
analyses were carried out by the Science Technical Support Unit at
London Metropolitan University. Crystal structure determination of
L2-BH3 and L3 was performed at ETH, while those of the gold
complexes were carried out at Imperial College London.
Unless otherwise stated, all chemical reagents and precursors were
procured from commercial sources and used without purification. (S)-
(R)-PPF-OMe (L1) was obtained as a gift to the Hii research group by
Sigma-Aldrich, while (R)-(+)-[1-(dimethylamino)ethyl]ferrocene (4)
was provided by SOLVIAS AG to the Togni group and was
recrystallized with tartaric acid according to the reported literature
procedure.23 6-Methyl-1,1-diphenylhepta-4,5-dien-1-ol (6) was syn-
thesized by the literature procedure.24
toluene-d8): 94.5 (d, JCP = 5.0, CCp), 89.8 (d, JCP = 6.9, CCp), 84.1 (s,
CCp), 82.5 (s, CCp), 69.1 (s, Cp), 69.1 (s, CHCp), 68.8 (s, Cp), 67.7 (s,
CHCp), 65.9 (s, CHCp), 65.5 (s, CHCp), 65.0 (s, CHCp), 64.7 (d, JCP
=
2.5, CHCp), 34.6 (d, JCP = 22.2, CHCy), 32.8 (d, JCP = 22.2, CH2Cy),
29.4 (d, JCP = 5.2, CH2(Cy)), 29.4 (s, CH2), 28.8 (d, JCP = 23.2,
CHMe), 27.3 (s, CH2(Cy)), 27.2 (d, JCP = 17.1, CH2(Cy)), 26.4 (s,
CH2(Cy)), 26.1 (d, JCP = 19.4, CHMe), 20.4 (d, JCP = 23.8, CHMe),
19.3 (d, JCP = 29.7, CHMe). δP (101.26 MHz, toluene-d8): 39.1. m/z
(HR-MALDI): 552.132 ([M+], 100%). Anal. Calcd. for C31H37PFe2:
C, 67.42; H, 6.75; P, 5.61. Found: C, 67.33; H, 6.56; P, 5.63%.
(2R,3R,7R,8R)-2,8-Dimethyl-1-cyclohexyl-diferroceno-[c,f ]-phos-
phocin-κP-borane, L2-BH3. To a solution of 100 mg (0.18 mmol) of
L2 in 5 mL of THF, BH3·SMe2 was added dropwise (51 μL, 41 mg,
0.47 mmol). After stirring overnight at r.t., the solvent was evaporated,
yielding a crude product that was purified by recrystallization from
hexane (90 mg, 88%). Orange powder. δH (250 MHz, toluene-d8):
4.33 (1H, m, CHCp), 4.12 (1H, m, CHCp), 4.05 (5H, s, Cp), 4.04 (1H,
t, J = 2.3, CHCp), 3.99 (5H, s, Cp), 3.97 (1H, m, CHCp), 3.93 (1H, m,
CHCp), 3.69 (1H, t, J = 2.4, CHCp), 3.42 (2H, AB spin system, CH2),
3.92 (1H, dq, JHP = 14.3, 7.2, CHMe), 2.51 (1H, dq, JHP = 9.5, 7.7,
CHMe), 2.12 (1H, m, Cy), 2.00−0.80 (13H, m, Cy and BH3), 1.77
(3H, dd, JHP = 14.7, 7.2, CHMe,), 1.33 (3H, dd, JHP = 15.0, 7.7,
CHMe). δC (62.90 MHz, CDCl3): 69.5 (s, Cp), 69.1 (s, Cp), 69.0 (s,
CHCp), 68.2 (s, CHCp), 66.9 (s, CHCp), 66.5 (s, CHCp), 66.4 (s, CHCp),
66.2 (s, CHCp), 32.2 (d, JCP = 24.3, CαHCy), 29.3 (s, CH2(Cy)), 28.6 (s,
CH2(Cy)), 28.0 (d, JCP = 3.1, CH2(Cy)), 27.9 (d, JCP = 27.0, CHMe),
27.3 (d, JCP = 4.6, CH2(Cy)), 27.1 (d, JCP = 1.9, CH2(Cy)), 26.0 (s,
CH2(Cy)), 24.1 (d, JCP = 26.6, CHMe), 16.8 (d, JCP = 4.2, CHMe), 16.1
(d, JCP = 3.8, CHMe). δP (101 MHz, CDCl3): 47.1 (br s, w1/2 = 125
Hz). Anal. Calcd. for C31H40BPFe2: C, 65.77; H, 7.12; P, 5.47. Found:
C, 65.49; H, 7.28; P, 5.38%.
Preparation of L2 and L3. 1,1″-Carbonyl-bis[(2S)-2-[(1R)-1-
(dimethylamino)ethyl]ferrocene], 2. A solution of n-BuLi (1.53 M in
hexane, 9.7 mL, 14.9 mmol) was added at −78 °C to a solution of 1-
bromo-(S)-2-[(R)-1-(dimethylamino)ethyl]ferrocene 1 (5 g, 14.9
mmol) in THF (200 mL). After stirring for 2 h, diethyl carbonate
(0.91 mL, 7.4 mmol) was added. After warming to r.t. overnight, the
solution was treated with TBME (200 mL). The organic layer was
washed with saturated NaHCO3 (2 × 200 mL) and brine (2 × 200
mL) and dried (MgSO4). Evaporation of the solvent yielded a crude
product that was purified by flash chromatography (hexane/EtOAc, 5/
1 to 1/1 containing 2% NEt3) and recrystallization from CH2Cl2/
hexane: 2.87 g (71%). Red crystals. δH (250 MHz, CDCl3): 4.74 (2H,
q, J = 7.0, CHMe), 4.56 (2H, dd, J = 2.6, 1.5, CHCp), 4.42 (2H, dd, J =
2.6, 1.3, CHCp), 4.28 (2H, t, J = 2.6, CHCp), 4.12 (10H, s, Cp), 2.13
(12H, s, NMe2), 1.50 (6H, d, J = 6.8, CHMe). δC (62.9 MHz,
CD2Cl2): 203.6 (CO), 93.7 (CCp), 80.4 (CCp), 70.7 (Cp), 70.5
(CCp), 70.1(CCp), 67.7(CCp), 54.5 (CHMe), 40.4 (NMe2), 13.9
(CHMe). m/z (HR-MALDI): 453.1 ([MH+ − 2NMe2], 100%).
Anal. Calcd. for C29H38N2OFe2: C, 64.47; H, 6.72; N, 5.18. Found: C,
64.42; H, 6.83; N, 5.12%.
2,2″-Methylenebis[(1R)-1-[(1R)-1-(dimethylamino)ethyl]-
ferrocene], 3. A solution of 2 (2.5 g, 4.63 mmol) in THF (80 mL) was
slowly added to a suspension of LiAlH4 (0.70 g, 18.5 mmol) in THF
(200 mL). After stirring for 1 h at r.t., AlCl3 (2.46 g, 18.5 mmol) was
added and the mixture was stirred for an additional 2 h. The solution
was treated with sat. aq NaHCO3 (200 mL), and the aqueous layer
was extracted with Et2O (3 × 150 mL). The combined organic layers
were washed with saturated NaHCO3 (2 × 200 mL) and brine (2 ×
200 mL) and dried (MgSO4). Evaporation of the solvent yielded a
crude product that was purified by recrystallization from CH2Cl2/
hexane: 1.92 g (79%). Orange crystals. δH (250 MHz, CDCl3): 4.38
(2H, m, CHCp), 4.01 (10H, s, Cp), 4.01 (2H, m, CHCp), 4.00 (2H, q, J
= 7.0, CHMe), 3.93 (2H, t, J = 2.2, CHCp), 3.57 (2H, m, CH2), 2.24
(12H, s, NMe2), 1.34 (6H, d, J = 7.0, CHMe). δC (62.9 MHz,
CD2Cl2): 89.4 (CCp), 89.3 (CCp), 69.2 (Cp), 69.2 (CHCp), 65.6
(CHCp), 65.3 (CHCp), 56.3 (CHMe), 39.5 (NMe2), 25.5 (CH2), 8.3
(CHMe). m/z (HR-MALDI): 437.1 ([M+ − NMe2 − HNMe2],
100%). Anal. Calcd.. for C29H38N2Fe2: C, 66.18; H, 7.28; N, 5.32.
Found: C, 66.02; H, 7.36; N, 5.15%.
(2R,3R,7R,8R)-2,8-Dimethyl-1-cyclohexyl-diferroceno-[c,f ]-phos-
phocin, L2. The aminophosphine 3 (1.82 g, 3.46 mmol) was dissolved
in degassed glacial acetic acid (30 mL), to which a solution of
cyclohexylphosphine (0.471 M in acetic acid, 7.7 mL, 3.63 mmol) was
added, and the mixture was stirred at 75 °C until no starting material
was detected (31P NMR; 3 h). Evaporation of the solvent yielded a
crude product that was purified by flash chromatography (hexane/
Et2O, 1:1) and recrystallization from hexane: 1.16 g (61%). Orange
powder. δH (250 MHz, toluene-d8): δ 4.16 (1H, m, CHCp), 3.99 (5H,
s, Cp), 3.99 (1H, m, CHCp), 3.97 (5H, s, Cp), 3.97 (1H, m, CHCp),
3.94 (1H, m, CHCp), 3.90 (1H, t, J = 2.4, CHCp), 3.67 (1H, m, CHCp),
3.46 (2H, AB spin system, CH2), 2.72 (1H, quintet, J = 7.0, CHMe),
2.23 (1H, q, J = 7.7, CHMe), 1.85 (1H, m, Cy), 1.68 (2H, m, Cy),
1.60 (1H, m, Cy), 1.56 (3H, dd, JHP = 17.6, J = 7.0, CHMe), 1.45 (1H,
m, Cy), 1.33 (3H, dd, JHP = 18.4, J = 7.7, CHMe), 1.28 (1H, m, Cy),
1.20 (1H, m, Cy), 1.16 (1H, m, Cy), 1.16 (3H, m, Cy). δC (62.9 MHz,
Crystal Data for L2-BH3: C31H40BFe2P, M = 566.11, orthorhombic,
P212121 (No. 19), a = 12.9833(2) Å, b = 18.9236(3) Å, c = 22.7786(2)
Å, V = 5596.49(13) Å3, Z = 8, Dc = 1.344 g cm−3, μ(Mo Kα) = 1.112
mm−1, T = 293 K, red plates, Siemens SMART Platform; 13 297
independent measured reflections (Rint = 0.0374), F2 refinement,
R1(obs) = 0.0401, wR2(all) = 0.0779, 10 548 independent observed
absorption-corrected reflections [|Fo| > 3σ(|Fo|), 2θmax = 56°], 637
parameters. The absolute structure of L2-BH3 was determined by use
of the Flack parameter [x+ = 0.038(10)]. CCDC 875937.
1,1″-(Phenylphosphinidene)bis[(2S)-2-[(1R)-1-(dimethylamino)-
ethyl]]ferrocene, 5. A solution of t-BuLi (1.55 M in pentane, 5.46 mL,
8.46 mmol) was added at −78 °C to a solution of R-(+)-[1-
(dimethylamino)ethyl]ferrocene 4 (2.16 g, 8.38 mmol) in Et2O (20
mL). After stirring for 30 min, the reaction mixture was warmed to r.t.
and stirred for a further hour. The mixture was then cooled to −78 °C,
and the dichlorophenylphosphine (0.57 mL, 0.75 g, 4.2 mmol) was
added. After warming to r.t. overnight, the solution was treated with
H2O. The organic layer was washed with H2O (3×) and brine (3×)
and dried (MgSO4). Evaporation of the solvent afforded an orange oil,
which was purified by flash chromatography on silica (Et2O/hexane,
2/1, containing 1% Et3N): 1.63 g (63%). δH (250 MHz, CDCl3): δ
7.70−7.66 (m, 2H, Ph), 7.29−7.21 (3H, m, Ph), 4.54 (1H, m, CHCp),
4.46 (1H, m, CHCp), 4.37 (1H, m, CHCp), 4.36 (1H, m, CHCp), 4.22
(1H, m, CHCp), 4.21 (1H, m, CHCp), 4.29 (1H, dq, J = 4.2, 6.8,
CHMe), 3.98 (5H, s, Cp), 3.98 (1H, m, CHMe), 3.58 (5H, s, Cp),
2.31 (6H, s, NMe2), 1.66 (6H, s, NMe2), 1.48 (3H, d, J = 6.8, CHMe),
1.26 (3H, d, J = 6.8, CHMe). δP (101.26 MHz, CDCl3): −45.6 (s).
(2S,4R,6R,8S)-4,6-Dimethyl-5-cyclohexyl-1-phenyl-diferroceno-
[b,g][1,5]diphosphocin, L3. The aminophosphine 4 (0.82 g, 1.32
mmol) was dissolved in degassed glacial acetic acid (12 mL).
Cyclohexylphosphine was added as a solution (0.365 M in glacial
acetic acid, 3.6 mL, 1.32 mmol), and the reaction mixture was stirred at
70 °C, until no starting material was detectable in the reaction aliquot
(31P NMR). Evaporation of the solvent yielded a crude product that
was purified by flash chromatography (hexane/Et2O, 1/1) and
recrystallization from MeOH/hexane/CH2Cl2: 567 mg (66%). δH
(500 MHz, toluene-d8): 7.85 (2H, m, Ph), 7.20 (2H, m, Ph), 7.08 (1H,
m, Ph), 4.62 (1H, m, CHCp), 4.24 (1H, m, CHCp), 4.21 (1H, m,
CHCp), 4.09 (1H, m, CHCp), 4.08 (5H, s, Cp), 4.05 (1H, m, CHCp),
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4.01 (5H, s, Cp), 3.87 (1H, m, CHCp), 2.84 (1H, dq, JHP = 6.9, J = 6.8,
CHMe), 2.06 (1H, q, JHP = 7.8, CHMe), 1.84 (1H, m, Cy), 1.75 (2H,
m, Cy), 1.67 (1H, m, Cy), 1.45 (1H, m, Cy), 1.45 (3H, dd, JHP = 18.4,
J = 7.8, CHMe), 1.34 (1H, m, Cy), 1.33 (1H, m, Cy), 1.28 (3H, dd, JHP
= 17.1, J = 6.8, CHMe), 1.23 (3H, m, Cy), 1.19 (1H, m, Cy). δC
J
CP = 28.8, Cy), 29.5 (d, JCP = 27.0, Cy), 27.1 (d, JCP = 13.1, CHMe),
26.9 (d, JCP = 7.9, CHMe), 26.6 (CH2), 25.8 (Cy), 20.8 (d, JCP = 9.2,
CHMe), 20.1 (d, JCP = 8.3, CHMe). δP (CDCl3): 70.9. m/z (FAB):
784 (M+, 75%), 749 ([M − Cl]+, 40), 154 (100). Anal. Calcd. for
C31H37AuPFe2Cl: C, 47.42; H, 4.72. Found: C, 47.30; H, 4.71%.
Crystal Data for (L2)AuCl: C31H37AuClFe2P, M = 784.69,
orthorhombic, P212121 (No. 19), a = 7.21077(17) Å, b =
18.2833(4) Å, c = 20.8090(5) Å, V = 2743.39(11) Å3, Z = 4, Dc =
1.900 g cm−3, μ(Mo Kα) = 6.554 mm−1, T = 173 K, yellow prisms,
Oxford Diffraction Xcalibur 3 diffractometer; 6252 independent
measured reflections (Rint = 0.0365), F2 refinement, R1(obs) =
0.0231, wR2(all) = 0.0332, 5755 independent observed absorption-
corrected reflections [|Fo| > 4σ(|Fo|), 2θmax = 58°], 325 parameters.
The absolute structure of (L2)AuCl was determined by a combination
of R-factor tests [R1+ = 0.0231, R1− = 0.0705] and by use of the Flack
parameter [x+ = 0.000(3)]. CCDC 875940.
(L3)Au2Cl2 was obtained from L3, as orange crystals (92%). mp
215 °C (dec.). δH (CDCl3): 7.77−7.59 (2H, m, Ph), 7.57−7.39 (3H,
m, Ph), 5.26−5.09 (1H, m, CHCp), 4.83 (1H, dd, J = 4.6, 2.3 Hz,
CHCp), 4.68−4.66 (1H, m, CHCp), 4.66−4.64 (1H, m, CHCp), 4.64−
4.62 (1H, m, CHCp), 4.49−4.46 (1H, m, CHCp), 4.34 (5H, s, Cp), 4.32
(5H, s, Cp), 2.69 (1H, dq, JHP = 13.7, J = 6.9, CHMe), 2.13 (1H, dq,
JHP = 15.5, J = 7.8, CHMe), 1.92−185 (1H, m, Cy), 1.85−1.75 (2H,
m, Cy), 1.71 (3H, dd, JHP = 19.0, J = 7.8, CHMe), 1.60−1.44 (2H, m,
Cy), 1.34 (3H, dd, JHP = 19.4, J = 6.9, CHMe), 1.27−1.20 (3H, m,
(125.75 MHz, toluene-d8): 135.9 (d, JCP = 25.9, Ph), 134.2 (d, JCP
=
20.6, Ph), 128.1 (s, Ph), 127.6 (d, JCP = 6.4, Ph), 99.9 (dd, JCP = 8.4,
6.4, C2b), 94.1 (dd, JCP = 7.5, 5.5, C2a), 77.3 (d, JCP = 53.0, C5b), 75.0
(d, JCP = 30.4, C5a), 74.0 (dd, JCP = 21.0, JCP = 0.9, C1b), 72.5 (d, JCP
4.1, C1a), 70.7 (s, Cp), 69.6 (s, Cp), 69.4 (d, JCP = 12.1, C4b), 69.1 (d,
JCP = 7.8, C4a), 68.5 (m, C3a), 67.7 (d, JCP = 4.6, C3b), 34.4 (d, JCP
=
=
22.6, Cy), 33.4 (d, JCP = 22.6, Cy), 29.6 (d, JCP = 24.0, CHMe), 29.5
(d, JCP = 4.6, Cy), 27.9 (d, JCP = 4.8, Cy), 27.6 (d, JCP = 13.3, Cy), 26.8
(d, JCP = 0.9, Cy), 25.0 (d, JCP = 18.3, CHMe), 21.5 (d, JCP = 20.8,
CHMe), 21.1 (d, JCP = 23.1, CHMe). δP (202.46 MHz, toluene-d8):
39.5, −19.1. Anal. Calcd. for C36H40P2Fe2: C, 66.89; H, 6.23. Found:
C, 66.64; H, 6.42%.
Crystal Data for L3: C36H40Fe2P, M = 646.32, monoclinic, P21 (No.
4), a = 8.8418(2) Å, b = 10.4886(3) Å, c = 16.5551(4) Å, V =
1529.73(7) Å3, Z = 2, Dc = 1.403 g cm−3, μ(Mo Kα) = 1.077 mm−1, T
= 293 K, red cubes, Siemens SMART Platform; 5501 independent
measured reflections (Rint = 0.0316), F2 refinement, R1(obs) = 0.0365,
wR2(all) = 0.0931, 4582 independent observed absorption-corrected
reflections [|Fo| > 3σ(|Fo|), 2θmax = 53°], 363 parameters. The absolute
structure of L3 was determined by use of the Flack parameter [x+ =
−0.030(15)]. CCDC 875938.
General Procedure for the Synthesis of Gold(I)−Chloride
Complexes. Thiodiglycol (3 equiv) was added dropwise to an ice-
cold solution of NaAuCl4·H2O (1 equiv) in water. When the orange
solution turned to transparent, a solution of the appropriate phosphine
(1 equiv for L1 and L2, 0.5 equiv for L3) in CHCl3 was added and the
mixture was allowed to stir for 3 h. The organic layer was separated,
dried over MgSO4, and filtered. Ethanol was added, and the mixture
was put it into the freezer (−20 °C), whereupon the requisite gold
complex crystallizes.
(L1)AuCl was obtained from (S)-(R)-PPF-OMe, as orange crystals
(95%). mp 123−125 °C. δH (CDCl3): 7.77−7.35 (10H, m, Ph), 5.27
(1H, q, J = 6.3, CHMe), 4.69−4.66 (1H, m, CHCp), 4.40 (1H, t, J =
2.4, CHCp), 4.24 (5H, s, Cp), 3.80 (1H, dd, J = 3.8, 2.4, CHCp), 2.91
(3H, s, OMe), 1.54 (3H, d, J = 6.3, CHMe). δC (CDCl3): 135.1 (d, JCP
= 14.0, Ph), 133.5(d, JCP = 14.0, Ph), 132.1 (Ph), 131.7 (Ph), 131.4 (d,
Cy), 1.20−1.09 (3H, m, Cy). δC (CDCl3): 133.8 (Ph), 133.4 (d, JCP
13.9, Ph), 131.7 (Ph), 128.4 (d, JCP = 11.9, Ph), 94.1 (d, JCP = 4.1,
CCp), 90.8 (CCp), 78.1 (d, JCP = 27.6, CCp), 77.2 (CCp), 75.3 (d, JCP
21.3, CCp), 72.0 (Cp), 71.9 (CCp), 71.6 (d, JCP = 12.0, CCp), 70.7 (Cp),
70.6 (d, JCP = 6.1, CCp), 68.8 (CCp), 68.1 (CCp), 35.0 (d, JCP = 26.0,
=
=
Cy), 32.8 (Cy), 31.0 (d, JCP = 26.6, CHMe), 28.3 (Cy), 26.6 (d, JCP
=
10.7, Cy), 26.35 (d, JCP = 17.2, CHMe), 26.15 (d, JCP = 3.6, Cy), 25.23
(Cy), 23.10 (d, JCP = 10.0, CHMe), 22.08 (d, JCP = 9.2, CHMe). δP
(CDCl3): 70.1, 24.4. m/z (FAB): 1110 ([M]+, 50%), 1075 ([M −
Cl]+, 60), 154 (100). Anal. Calcd. for C36H40Au2P2Fe2Cl2: C, 38.91;
H, 3.63. Found: C, 38.81; H, 3.64%.
Crystal Data for (L3)Au2Cl2: C36H40Au2Cl2Fe2P2·0.5(CHCl3), M =
1170.84, monoclinic, P21 (No. 4), a = 11.9744(4) Å, b = 8.9423(3) Å,
c = 18.3431(5) Å, β = 96.613(3)°, V = 1951.09(11) Å3, Z = 2, Dc =
1.993 g cm−3, μ(Mo Kα) = 8.571 mm−1, T = 173 K, orange blocky
needles, Oxford Diffraction Xcalibur 3 diffractometer; 10 484
independent measured reflections (Rint = 0.0486), F2 refinement,
R1(obs) = 0.0434, wR2(all) = 0.1101, 8310 independent observed
absorption-corrected reflections [|Fo| > 4σ(|Fo|), 2θmax = 65°], 433
parameters. The absolute structure of (L3)Au2Cl2 was determined by a
combination of R-factor tests [R1+ = 0.0434, R1− = 0.0694] and by use
of the Flack parameter [x+ = 0.000(8), x− = 1.014(8)]. CCDC 875941.
Synthesis of AgOTs. Ag2CO3 (0.5 equiv) was added in one
portion to a solution of p-toluenesulfonic acid (1 equiv) in EtOH (5
mL). The resulting mixture was protected from light and stirred
vigorously overnight. The mixture was filtered and concentrated under
vacuum, to give AgOTs was as a white solid (98%). mp 220 °C (dec.).
δH (DMSO-d6): 7.48 (2H, d, J = 8.0, tol), 7.12 (2H, d, J = 8.0, tol),
2.28 (3H, s, CH3). δC (DMSO-d6): 145.4, 138.0, 128.3, 125.6, 20.9. m/
z (FAB) 279 ([M]+, 2%), 260 (21), 154 (100), 136 (72), 107 (43).
Anal. Calcd. for C7H7AgO3S: C, 30.13; H, 2.53. Found: C, 30.05; H,
2.55%.
(S)-P*Ag was similarly prepared from Ag2CO3 and (S)-(+)-1,1′-
binaphthyl-2,2′-diyl hydrogen phosphate, as a white solid (87%). mp >
261 °C (dec.). δH (DMSO-d6): 8.07 (d, J = 8.8, 2H), 8.04 (d, J = 8.1,
2H), 7.45 (t, J = 7.9, 4H), 7.36−727 (m, 2H), 7.22 (d, J = 8.5, 2H). δC
(DMSO-d6): 149.9 (d, JCP = 8.9), 132.0, 130.5, 130.0, 128.5, 126.2,
126.1, 124.6, 122.6, 121.7. δP (DMSO-d6): +5.1. m/z (FAB): 1019
([M + Ag2]+, 12%), 911 ([M2]+, 16), 563 ([M + Ag]+, 65), 455 ([M]+,
100), 349 ([MH − Ag]+, 46), 268 (50). Anal. Calcd. for
C20H12AgO4P: C, 52.74; H, 2.62. Found: C, 52.59; H, 2.61%.
(R)-P*Ag was similarly prepared from Ag2CO3 and (R)-(-)-1,1′-
binaphthyl-2,2′-diyl hydrogen phosphate, as a white solid (90%). mp >
261 °C (dec.). δH (DMSO-d6): 8.08 (d, J = 8.8, 2H), 8.04 (d, J = 8.1,
2H), 7.46 (t, J = 7.5, 4H), 7.38−7.27 (m, 2H), 7.22 (d, J = 8.5, 2H). δC
JCP = 2.3, Ph), 131.0 (Ph), 129.0 (d, JCP = 11.8, Ph), 128.6 (d, JCP
=
12.1, Ph), 93.8 (d, JCP = 8.1, CH), 74.3 (d, JCP = 6.0, CCp), 74.0 (d, JCP
= 3.1, CCp), 71.3 (d, JCP = 7.1, CCp), 71.1 (Cp), 70.2 (d, JCP = 7.5, CCp),
69.4 (Cp), 55.7 (OMe), 17.0 (CHMe). δP (CDCl3): +24.6. m/z
(FAB): 661 (M+, 62%), 473 (57), 154 (100). Anal. Calcd. for
C25H25AuOPFeCl: C, 45.36; H, 3.78. Found: C, 45.32; H, 3.81%.
Crystal Data for (L1)AuCl: C25H25AuClFeOP, M = 660.69,
orthorhombic, P212121 (No. 19), a = 11.18245(8) Å, b =
12.26925(8) Å, c = 17.17066(11) Å, V = 2355.82(3) Å3, Z = 4, Dc
= 1.863 g cm−3, μ(Cu Kα) = 18.268 mm−1, T = 173 K, yellow plates,
Oxford Diffraction Xcalibur PX Ultra diffractometer; 4610 independ-
ent measured reflections (Rint = 0.0324), F2 refinement, R1(obs) =
0.0183, wR2(all) = 0.0433, 4445 independent observed absorption-
corrected reflections [|Fo| > 4σ(|Fo|), 2θmax = 145°], 271 parameters.
The absolute structure of (L1)AuCl was determined by a combination
of R-factor tests [R1+ = 0.0183, R1− = 0.0476] and by use of the Flack
parameter [x+ = 0.000(6), x− = 1.013(6)]. CCDC 875939.
(L2)AuCl was obtained from L2, as orange crystals (89%). mp 160
°C (dec.). δH (CDCl3): 4.41−4.39 (1H, m, CHCp), 4.29 (1H, t, J = 2.5,
CHCp), 4.25−4.22 (1H, m, CHCp), 4.15 (5H, s, Cp), 4.12−4.11 (1H,
m, CHCp), 4.08 (1H, t, J = 2.5, CHCp), 4.04 (5H, s, Cp), 3.94−3.90
(1H, m, CHCp), 3.78 (1H, d, J = 16.6, CH2), 3.59 (1H, d, J = 16.6,
CH2), 3.01 (1H, dq, JHP = 13.9, J = 6.9, CHMe), 2.48 (1H, dq, JHP
15.1, J = 7.6, CHMe), 2.02−1.95 (1H, m, Cy), 1.69 (3H, dd, JHP
=
=
19.1, J = 6.9, CHMe), 1.54 (3H, dd, JHP = 18.9, J = 7.6, CHMe), 1.54−
1.53 (4H, m, Cy), 1.30−1.11 (6H, m, Cy). δC (CDCl3): 89.9 (CCp),
85.5 (CCp), 85.0 (CCp), 82.4 (CCp), 69.9 (CCp), 69.8 (Cp), 69.4 (Cp),
68.9 (CCp), 67.3 (CCp), 67.0 (CCp), 66.4 (CCp), 61.3 (d, JCP = 8.8,
CCp), 35.3 (Cy), 34.5 (d, JCP = 25.7, Cy), 33.5 (d, J = 2.4, Cy), 30.9 (d,
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Scheme 1. Preparation of Enantiopure Chiral Ferrocenyl Phosphines L2 and L3
Figure 2. 31P NMR spectra of L3 to gold (collected in CDCl3). (A) M/L ratio = 0; (B) M/L ratio = 2; (C) M/L ratio = 1, after 10 min; (D) M/L
ratio = 1, after 1 h.
(DMSO-d6): 149.8 (d, JCP = 8.4), 132.3, 131.0, 130.6, 128.9, 126.7,
126.5, 125.2, 122.7, 122.0. δP (DMSO-d6): +4.5. m/z (FAB): 1019
([M + Ag2]+, 19%), 911 ([M2]+, 18), 563 ([M + Ag]+, 98), 455 ([M]+,
90), 349 ([MH − Ag]+, 78), 154 (100). Anal. Calcd. for
C20H12AgO4P: C, 52.74; H, 2.64. Found: C, 52.93; H, 2.66%.
Halide Abstraction. One equivalent of the requisite gold(I)
chloride complex (L1)AuCl or (L2)AuCl was added to a solution of
the silver salt AgOTs, (S)-P*Ag or (R)-P*Ag (1 equivalent) in CHCl3,
and stirred for 12 h. The resultant suspension was centrifuged, and the
decanted liquid was evaporated under vacuum to yield the required
complex. Cationic complexes of (L3)Au2Cl2 were not isolated, but
generated in situ, by mixing the gold complex with the corresponding
silver complex, prior to catalytic reactions.
(L1)AuOTs was obtained from AgOTs and (L1)AuCl, as an
orange powder (98%). mp 70−72 °C. δH (CDCl3): 7.93 (2H, d, J =
8.2, Tol), 7.73−7.35 (10H, m, Ph), 7.25 (2H, d, J = 8.2, Tol), 5.31
(1H, q, J = 6.2, CH), 4.71−4.68 (1H, m, CHCp), 4.41−4.38 (1H, m,
CHCp), 4.27 (5H, s, Cp), 3.78 (1H, dd, J = 3.7, 2.5, CHCp), 2.88 (3H,
s, OMe), 2.39 (3H, s, Me), 1.55 (3H, d, J = 6.2, CHMe). δP (CDCl3):
19.4. m/z (FAB): 796 ([M]+, 10%), 746 (65), 473 (100). Anal. Calcd.
for C32H32AuPFeO4S: C, 48.30; H, 4.03. Found: C, 48.28; H, 4.05%.
(L1)Au[(S)-P*] was obtained from (S)-P*Ag and (L1)AuCl, as an
orange powder (99%). mp 123−125 °C. δH (CDCl3): 8.02−7.33
(22H, m, Ph), 5.24 (1H, q, J = 6.3, CH), 4.61−4.59 (1H, m, CHCp),
4.32−4.28 (1H, m, CHCp), 4.02 (5H, s, Cp), 3.65−3.61 (1H, m,
CHCp), 2.90 (3H, s, OMe), 1.45 (3H, d, J = 6.3, Me). δP (CDCl3):
18.4, 8.8. m/z (FAB): 973 ([M]+, 8%), 746 (22), 593 (33), 473 (100).
Anal. Calcd. for C45H37AuP2FeO5: C, 55.50; H, 3.80. Found: C, 55.69;
H, 3.82%.
(L1)Au[(R)-P*] was obtained from (R)-P*Ag and (L1)AuCl, as
an orange powder (95%). mp 125−127 °C. δH (CDCl3): 7.88−7.35
(22H, m, Ph), 5.08 (1H, q, J = 6.2, CHMe), 4.62−4.58 (1H, m,
CHCp), 4.37−4.32 (1H, m, CHCp), 4.21 (5H, s, Cp), 3.74−3.69 (1H,
m, CHCp), 2.63 (3H, s, OMe), 1.33 (3H, d, J = 6.2, CHMe). δP
(CDCl3): 18.5, 7.5. m/z (FAB): 746 (20), 625 (5) [M −
C20H12PO4]+, 473 (60), 136 (68), 73 (100). Anal. Calcd. for
C45H37AuP2FeO5: C, 55.50; H, 3.80%. Found: C, 55.45; H, 3.79%.
(L2)AuOTs was obtained from AgOTs and (L2)AuCl, as a red
solid (79%). mp 195 °C (dec.). δH (CDCl3): 7.78 (2H, d, J = 7.7,
Tol), 7.19 (2H, d, J = 7.7, Tol), 4.70−4.66 (1H, m, CHCp), 4.44−4.39
(1H, m, CHCp), 4.34−4.30 (1H, m, CHCp), 4.22−4.20 (1H, m,
CHCp), 4.20−4.18 (1H, m, CHCp), 4.17 (5H, s, Cp), 4.06 (5H, s, Cp),
3.96−3.91 (1H, m, CHCp), 3.75−3.69 (1H, m, CH2), 3.49−3.40 (1H,
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m, CH2), 3.05−2.87 (1H, m, CHMe), 2.62−2.42 (1H, m, CHMe),
2.37 (3H, s, Me), 1.78−1.72 (1H, m, Cy), 1.61−1.56 (3H, m, CHMe),
1.51 (3H, dd, JHP = 19.5, J = 7.7, CHMe), 1.30−1.06 (10H, m, Cy). δP
(CDCl3): 68.6. m/z (FAB): 920 ([M]+, 3%), 749 ([M − C7H7SO3]+,
62), 627 (23), 315 (25), 154 (47), 55 ([Fe]+, 100). Anal. Calcd. for
C38H44AuPFe2O3S: C, 49.59; H, 4.82. Found: C, 49.75; H, 4.80%.
(L2)Au[(S)-P*] was obtained from (S)-P*Ag and (L2)AuCl, as a
red solid (89%). mp 205 °C (dec.). δH (CDCl3): 8.11−7.39 (12H, m,
Ph), 4.83 (1H, d, J = 20.2, CHCp), 4.70−4.66 (1H, m, CHCp), 4.43−
4.39 (1H, m, CHCp), 4.33−4.29 (1H, m, CHCp), 4.23−4.18 (5H, m,
Cp), 4.09−4.06 (1H, m, CHCp), 4.03−3.97 (5H, m, Cp), 3.93−3.87
(1H, m, CHCp), 3.78−3.70 (1H, m, CH2), 3.51−3.38 (1H, m, CH2),
2.96−2.84 (1H, m, CHMe), 2.54−2.40 (1H, m, CHMe), 1.67−1.61
(1H, m, Cy), 1.60 (3H, dd, JHP = 13.4, J = 5.8, CHMe), 1.54−1.48
(3H, m, CHMe), 1.30−1.13 (10H, m, Cy). δP (CDCl3): 67.0, 8.1. m/z
(FAB): 1096 ([M]+, 19%), 793 (85), 749 ([M − C20H12PO4]+, 27),
627(32), 55 ([Fe]+, 100). Anal. Calcd. for C51H49AuP2Fe2O4: C,
55.86; H, 4.50. Found: C, 55.69; H, 4.52%.
Scheme 2. Coordination Behavior of Gold(I) to L3
(L2)Au[(R)-P*] was obtained from (R)-P*Ag and (L2)AuCl, as a
red solid (92%). mp 207 °C (dec.). δH (CDCl3): 7.97−7.39 (12H, m,
Ph), 4.84−4.80 (1H, m, CHCp), 4.46−4.42 (1H, m, CHCp), 4.28 (2H,
t, J = 5.4, CHCp), 4.20 (5H, s, CHCp), 4.17−4.15 (1H, m, CHCp), 4.01
(5H, s, Cp), 3.95−3.90 (1H, m, CHCp), 3.79−3.72 (1H, m, CH2),
3.47−3.41 (1H, m, CH2), 2.85−2.76 (1H, m, CHMe), 2.31−2.19 (1H,
m, CHMe), 1.70−1.65 (1H, m, Cy), 1.63−1.52 (3H, m, CHMe), 1.40
(3H, dd, JHP = 20.8, J = 7.0, CHMe), 1.30−1.07 (10H, m, Cy). δP
(CDCl3): 66.6 (s), 8.6 (s). m/z (FAB): 1096 ([M]+, 3%), 793 (33),
749 ([M − C20H12PO4]+, 13), 627(48), 419 (73), 149 (95), 57 ([Fe]+,
100). Anal. Calcd. for C51H49AuP2Fe2O4: C, 55.86; H, 4.50. Found: C,
55.75; H, 4.53%.
Typical Procedure for Catalytic Reactions. A screw-cap vial was
charged with a magnetic stir bar and 6-methyl-1,1-diphenylhepta-4,5-
dien-1-ol (6), and toluene or DCE was added. The catalyst was added
as a solid or in solution, and the vial was then left to stir in the dark.
Conversions were monitored by TLC and/or NMR integration. Upon
completion, the solvent was evaporated and the residue purified by
column chromatography (hexanes/EtOAc, 9:1).
2-(2-Methylprop-1-en-1-yl)-4,4-diphenyltetrahydrofuran, 7:
Obtained as a white solid. mp 51−52 °C. Rf = 0.83 (hexanes/EtOAc,
3:1). δH (CDCl3): 7.57−7.44 (4H, m), 7.32 (4H, m), 7.27−7.15 (2 H,
m), 5.41 (1H, dq, J = 8.4, 1.2), 4.85 (1H, dd, J = 14.9, 8.4), 2.81−2.55
(2H, m), 2.17−2.01 (1H, m), 1.79 (3H, d, J = 0.8), 1.75 (3H, s),
1.65−1.76 (1H, m). δC (CDCl3) 147.3, 146.9, 135.5, 128.2, 128.0,
126.6, 126.5, 126.4, 126.0, 126.0, 87.9, 75.9, 39.4, 32.7, 25.9, 18.3. m/z
(EI): 278 ([M]+, 15%), 222 (41), 180 (100). HPLC conditions:
Chirapak OJ-H column, 0.5% IPA in n-hexane, 0.3 mL/min, tR (major)
= 46.0 min, tR (minor) = 55.4 min. [α]2D5 = +27.0° (47% ee, c = 1.0,
CHCl3).
Figure 3. Crystal structure of (L1)AuCl. H atoms are omitted for
clarity.
BuLi, followed by the addition of diethyl carbonate, coupled
two ferrocenyl fragments via a carbonyl linkage, which was
reduced to a methylene group by using lithium aluminum
hydride. Finally, the dimethylamino groups were sequentially
substituted by cyclohexylphosphine, to form the eight-
membered phosphocin L2, which was isolated and purified as
its borane adduct (L2-BH3). For the synthesis of the 1,5-
diphosphocin L3, the chiral ferrocenyl units were coupled via a
phosphine moiety, by the reaction of lithiated 4 with
dichlorophenylphosphine, to afford 5 in 63% yield, followed
by cyclization under acidic conditions with cyclohexylphos-
phine, to furnish L3 in 61% yield.
Formation of Au(I) Complexes. The reaction of L1 and
L2 with (thiodiglycol)AuCl, in a metal-to-ligand (M/L) ratio of
1:1, furnished the corresponding gold(I) chloride complexes
(L1)AuCl and (L2)AuCl, respectively, while the reaction of the
gold precursor with L3 in a 2:1 ratio afforded the bimetallic
complex (L3)Au2Cl2. The different electronic environments of
the two donor atoms in L3 correspond well with their
equivalents in L1 and L2, reflected by their distinctive 31P
NMR chemical shifts, observed at −20.5 and +39.4 ppm for P-
Ph and P-Cy moieties, respectively (Figure 2, spectrum A). As
RESULTS AND DISCUSSION
■
Three enantiopure mono- and diphosphocins containing
planar-chiral ferrocenyl units and stereogenic carbons were
chosen for this work (Figure 1). (S)-(R)-PPF-OMe (L1) was
first reported as a highly enantioselective ligand for nickel-
catalyzed asymmetric cross-coupling by Hayashi et al., where up
to 95% ee can be attained, a significant achievement that is
rarely superseded.25,26 Since then, it has been utilized with
moderate success in asymmetric hydrosilylation of 1,3-dienes,27
intramolecular cyclization of 1,5-dienes,28 alkylation of allenes
by malonates,29 Rh-catalyzed C−H/olefin coupling,30,31 and
Pd-catalyzed α-arylation of amides.32 Rather surprisingly, the
coordination chemistry of this ligand to gold has never been
reported.
Synthesis of Diferrocenyl(di)phosphocins L2 and L3.
In an earlier project,33 mono- and diphosphines L2 and L3 have
been prepared from lithiated derivatives of N,N-dimethyl-(S)-1-
ferrocenylethylamine (Scheme 1). Halide exchange of 1 with n-
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Figure 4. Crystal structures for L2-BH3, L3, (L2)AuCl, and (L3)Au2Cl2. H atoms are omitted for clarity.
expected, their coordination to gold caused a downfield shift to
+24.4 and +70.1 ppm (Figure 2, spectrum B). To investigate
preferential binding of the donor atoms to gold, the precursor
was mixed with L3 in a 1:1 M/L ratio. Within 10 min of mixing,
the binding of the cyclohexylphosphine moiety to gold was
clearly indicated by the appearance of the slightly broadened
31P resonance signal at +70.1 ppm. However, only a broad
resonance signal was observed at +24.4 ppm (Figure 2,
spectrum C), which sharpened up slowly over an hour (Figure
2, spectrum D). During this time, two minor 31P resonance
signals were also observed at +29.0 and +68.2 ppm. Given the
absence of resonances corresponding to uncoordinated P(III)
species, this suggests that the binding of the gold to the L3
occurs initially with the formation of the oligomeric or
polymeric (L3AuCl)n, in a homoleptic “head-to-head” coordi-
nation mode.34 The assembled structure is unstable, under-
going fast, reversible dissociation at the Au−PCy bond,
followed by a slower Au−PPh cleavage (Scheme 2). The
chelation of the diphosphocin or the formation of a “head-to-
tail” polymer (where Au is bound to P1 and P2 simultaneously)
will be expected to give rise to AB coupling patterns, but these
were not observed in the 31P NMR spectrum.
Molecular Structures. Single crystals of suitable quality for
X-ray crystallography were obtained for L2-BH3, L3, and also
all three isolated gold(I) complexes, (L1)AuCl, (L2)AuCl, and
(L3)Au2Cl2, allowing direct comparisons of their coordination
environments. In all cases, the complexes consist of gold nuclei
with essentially linear geometries, with a maximum distortion of
up to 6.6°. None of these complexes display noticeable
intermolecular Au···Au interactions, and the absolute stereo-
chemistry of all five structures was unambiguously determined
from the X-ray data, using a combination of R-factor tests and
the Flack parameter.
Interaction between oxygen and gold is not evident in the
gold(I) complex of (S)-(R)-PPF-OMe, (L1)AuCl (Figure 3),
the measured Au···O distance [3.705(3) Å] is longer than the
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Table 1. Selected Bond Lengths, Bond Angles, and Observed 31P NMR Resonances
a
compound
bond lengths (Å)
bond angles (deg)
θ (deg)
314.9
(L1)AuCl
Au(1)−Cl(1) = 2.2867(9)
Au(1)−P(1) = 2.2357(9)
P(1)−Au(1)−Cl(1) = 174.18(4)
P(1)−C(1)−C(2) = 125.5(3)
C(1)−C(2)−C(23) = 126.1(3)
C(1)−C(2)−C(6) = 124.3(3), 124.6(2)
C(2)−C(1)−C(15) = 124.4(2), 124.6(2)
C(10)−C(14)−C(15) = 130.1(3), 130.7(2)
C(8)−C(10)−C(14) = 128.4(2), 129.2(2)
P(1)−Au(1)−Cl(1) = 177.93(3)
C(1)−C(2)−C(6) = 124.7(3)
b
L2-BH3
317.0
315.5
(L2)AuCl
Au(1)−Cl(1) = 2.2863(8)
Au(1)−P(1) = 2.2379(8)
C(2)−C(1)−C(15) = 124.5(3)
C(10)−C(14)−C(15) = 130.7(3)
C(8)−C(10)−C(14) = 129.1(3)
P(1)−C(1)−C(2) = 132.5(3)
L3
P(1) = 310.3
P(2) = 307.7
C(1)−C(2)−C(6) = 125.9(3)
C(14)−C(15)−C(19) = 129.0(3)
P(1)−C(14)−C(15) = 134.5(3)
P(1)−Au(1)−Cl(1) = 173.41(13)
P(2)−Au(2)−Cl(2) = 178.61(9)
P(1)−C(1)−C(2) = 128.0(7)
(L3)Au2Cl2
Au(1)−Cl(1) = 2.273(3)
Au(2)−Cl(2) = 2.285(3)
Au(1)−P(1) = 2.235(2)
Au(2)−P(2) = 2.229(2)
P(1) = 319.1
P(2) = 324.6
C(1)−C(2)−C(6) = 126.0(8)
C(14)−C(15)−C(19) = 129.7(8)
P(1)−C(14)−C(15) = 131.6(6)
a
b
Sum of three C−P−C angles around each P donor. There are two unique structures in the unit cell for L2-BH3.
a
Table 2. 31P NMR Data of Cationic Gold Complexes
ment did not reveal any other transannular interactions, nor any
close allylic (A1,3) contact between the methyl substituents and
the Cp ring. Hence, it is assumed that ring strain is mostly
contained within distortions in bond lengths and angles in the
eight-membered ring. In L2-BH3 (which crystallized with two
independent complexes), this manifested itself in wider bond
angles in one of the Cp rings (averaged C(8)−C(10)−C(14)
and C(10)−C(14)−C(15) bond angles of 128.8° and 130.4°,
respectively), while the corresponding angles in the other Cp
ring are all very similar, ranging between 124.3(3) and 124.6(2)
° across both independent complexes. In comparison, for both
of the Cp rings contained in L3, the equivalent bond angles are
notably larger than 125°, being 125.9(3), 129.0(3), 132.5(3),
and 134.5(3)° (Table 1). The θ values in L3 were found to be
310.3° and 307.7° for the PPh and PCy moieties, respectively.
The latter increases to 317.0° in the BH3 adduct, in keeping
with coordination of the phosphorus’ lone pair to boron.
The boat−boat conformations are retained upon the
coordination of gold(I) chloride to L2 and L3. The structural
parameters of L2-BH3 and (L2)AuCl are very similar. The
pyramidalization at P (θ = 315.5°) is only slightly smaller than
that found in the BH3 adduct. As a result, there is little
difference in the bond angles observed in L2-BH3 and
(L2)AuCl. The coordination of AuCl to the highly strained
L3, on the other hand, causes an increase in θ from 310.3° to
319.1° at P1, and from 307.7° to 324.6° at P2. This inevitably
causes changes to the equivalent bond angles within the
medium-sized ring; in this case, significant decreases in P(1)−
C(1)−C(2) from 132.5(3)° to 128.0(7)°, and P(1)−C(14)−
C(15) from 134.4(3)° to 131.6(6)°, were observed.
entry
AuCl
P*Ag
AgOTs
product
δP (ppm)
+19.4
1
2
(L1)AuCl
(L1)AuCl
(L1)Au(OTs)
(L1)Au(R-P*)
(R)-P*Ag
+18.5, +7.5
(br)
3
(L1)AuCl
(S)-P*Ag
(L1)Au(S-P*)
+18.4, +8.8
(br)
4
5
(L2)AuCl
(L2)AuCl
AgOTs
(L2)Au(OTs)
(L2)Au(R-P*)
+68.6
(R)-P*Ag
+66.6, +8.6
(br)
6
(L2)AuCl
(S)-P*Ag
(L2)Au(S-P*)
+67.0, +8.1
(br)
7
8
(L3)Au2Cl2
(L3)Au2Cl2
AgOTs
(L3)Au(OTs)
+66.9, +18.2
2 × (R)-P*Ag
(L3)Au2(R-P*)2
+66.2, +19.9,
+8.5 (br)
9
(L3)Au2Cl2
2 × (S)-P*Ag
(L3)Au2(S-P*)2
+65.9, +17.3,
+8.2 (br)
a
NMR samples prepared in CDCl3, using 85% H3PO4 as internal
standard.
sum of their van der Waals radii. The crystal structure provides
measurements of P(1)−C(1)−C(2) and C(1)−C(2)−C(23)
angles of 125.5(3) and 126.1(3)°, respectively, and the sum of
three C−P−C angles (θ) provided a value of 314.87° (which is
within the expected range for similar R3P−Au−Cl com-
plexes35), which set a useful benchmark (vide infra).
The X-ray crystal structures of L2-BH3 and L3 revealed a
(somewhat flattened) boat−boat conformation in both
compounds, with the exocyclic P−C bonds in pseudoaxial
positions (Figure 4). These conformations are retained in
solution, as indicated by the observation of distinct chemical
shifts and coupling patterns in their H NMR spectra. A H-
NOESY experiment performed with L3 confirmed the boat−
boat endo−endo conformation of the eight-membered
diphosphocin ring, by the observation of NOE cross-peaks
between the P-Ph and P-Cy substituents (Supporting
Information). Barring these close contacts, the NOE experi-
1
1
Halide Abstraction. Metathesis reactions of (L1)AuCl and
(L2)AuCl with the achiral and chiral silver salts AgOTs, (R)- or
(S)-P*Ag, cause upfield shifts and a certain amount of
broadening of the 31P resonance signals (Table 2). More
significantly, observed chemical shifts are different between
diastereomeric pairs (entries 2 vs 3, 5 vs 6, and 8 vs 9),
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Figure 5. 31P NMR spectra of addition of (S)-P*Ag to (L3)Au2Cl2 (in CDCl3). (A) (S)-P*Ag; (B) (L3)Au2Cl2; (C) (L3)Au2Cl2 + (S)-P*Ag; (D)
(L3)Au2Cl2 + 2 (S)-P*Ag.
Characterization of these compounds was achieved by 1H
Scheme 3. Intramolecular Cyclization of γ-Allenol
NMR spectroscopy, FAB-MS, and elemental analysis. Attempts
to isolate the corresponding complexes of L3 were
unsuccessful.
Asymmetric Catalysis. Given ongoing interests in our
research laboratories on the development of ferrocenylphos-
Table 3. Catalytic Performance of the Cationic Gold(I)
Complexes
a
phines38−40 and group 11 catalysts41−44 for stereoselective
addition of X−H to CC bonds, the potential application of
these gold(I) complexes as enantioselective catalysts was
evaluated. To date, only axially chiral diphosphines have been
reported to be highly selective ligands for the gold-catalyzed
intramolecular addition of O−H or N−H to allenes.36,45−51 In
our recent work on the development of silver catalysts for such
reactions,52 2,2-diphenyl-substituted γ-allenol 6 was found to be
one of the more challenging substrates and was thus chosen as
the acyclic precursor in this study (Scheme 3, Table 3). With
the three ferrocenyl-based ligands in hand, we will examine the
effect of the various chiral components on the enantioselectivity
of the reaction; the diphosphocin L3 can be regarded as a
composite of L1 and L2, containing a relatively electron-
deficient and sterically congested P1 substituted by three
aromatic groups, and a sterically less congested, but more Lewis
basic P2, substituted by three alkyl groups.
c
T
(°C)
time
(h)
%
% ee
d
b
entry
catalyst (mol %)
conversion
( )
1
2
3
4
5
6
7
8
9
10
(L1)Au(OTs) (2)
(L1)Au(R-P*) (2)
(L1)Au(S-P*) (2)
(L2)Au(OTs) (2)
(L2)Au(R-P*) (2)
(L2)Au(S-P*) (2)
−40
−40
−40
−40
−40
−40
−40
−40
−40
−40
20
48
50
48
48
72
48
48
48
48
100
100
100
100
62
10.5 (+)
4 (+)
47 (+)
11.5 (+)
9 (−)
25 (+)
18.5 (+)
7 (+)
32
e
(L3)Au2(OTs)2
100
100
100
100
e
(L3)Au2(R-P*)2
(L3)Au2(S-P*)2
e
31.5 (+)
22.5 (+)
f
(L3)Au2Cl(S-P*)
a
See the Experimental Section for general reaction conditions.
b
c
Determined by 1H NMR spectroscopy. Determined by chiral
d
e
HPLC. Determined by polarimetry. Generated by mixing 1 mol %
(L3)Au2Cl2 with 2 mol % Ag salt. Generated by mixing 1 mol %
f
As expected, the gold(I) sulfonate and phosphonate salts
showed good to moderate catalytic activity in the cyclization of
6 under (sub)ambient conditions (≤room temperature), while
the equivalent reactions with chloride salts did not afford any
turnover. Preliminary studies were conducted with gold
complexes in toluene at −40 °C using 2 mol % of the catalyst.
Overall, the catalytic activity of comparative (L)AuX complexes
decreases in the order: L1 > L3 > L2, which can be rationalized
on the grounds of sterics (L1 vs L3) and electronic factors (L1
and L3 vs L2).
(L3)Au2Cl2 with 1 mol % Ag salt.
suggestive of weak interactions between the counteranions and
gold, by either a weak dative coordination or the formation of
an ion pair.36,37 Addition of 1 equiv of the silver salt (S)-P*Ag
(where P* = 1,1′-binaphthyl-2,2′-diyl phosphate) to (L3)-
Au2Cl2 led to an equal mixture of starting materials and the
dicationic complex (Figure 5); that is, the extraction of the
halide from the metal center occurs spontaneously and
unselectively. The broadened peaks suggest that the interaction
between the phosphate anion and gold is likely to be highly
dynamic. (L1)AuX and (L2)AuX (where X = OTs, (R)-P*,
(S)-P*) can be isolated as orange and red solids, respectively.
Using the tosylate complex, complete conversions can be
obtained with all three complexes, to afford the product with
low enantioselectivities of between 10.5 and 18.5% (entries 1, 4,
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Article
and 7). In comparison, complexes derived from the phosphate
anions were less active. However, dramatic match−mismatch
effects can be observed; the S enantiomer of the gold phosphate
complex of L1 enhanced the ee of the product to 47%, while its
R congener afforded only 4% (entries 2 and 3). Very
interestingly, the use of the S enantiomer of the phosphate
anion only slightly enhanced the stereoselectivity of the gold
complexes of L2 (entries 4 and 6), while the incorporation of
the mismatched R-phosphate resulted in lower and opposite
stereoinduction (entries 5 and 6).
In the case of L3, each of the gold metal centers was
expected to behave independently; their reactivity tuned by the
electronically and sterically different PR3 moieties that they are
ligated to. In this regard, the incorporation of the additional
PPh group in the eight-membered ring appeared to only
slightly enhance the enantioselectivity achieved in (L3)Au2X2,
compared to L2 (entries 4 vs 7, 5 vs 8, and 6 vs 9). Using a Au/
P* ratio of 2:1, the selectivity reduced from 31.5 to 22.5%
(entries 9 and 10).
Foundation are also gratefully acknowledged (Ph.D. fellowships
to D.F.D.B. and R.S.).
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CONCLUSIONS
■
The preparation and characterization of two conformationally
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ASSOCIATED CONTENT
■
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* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: togni@inorg.chem.ethz.ch (A.T.), mimi.hii@imperial.
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Notes
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ACKNOWLEDGMENTS
■
We are grateful to Fundacion
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Barrie
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de la Maza and Xunta
Galicia (Angeles Alvarino program) for the award of
̃
postdoctoral fellowships to E.M.B. and L.A.A., respectively.
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