Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric induction as a function of the substrate and the backbone flexibility of C 1-symmetric diphosphines in rhodium-catalysed hydrogenations

Article abstract of DOI:10.1039/b607479k

The new unsymmetrical, optically active ligands 1,2-C₂H ₄(PPh₂)(2′R,5′R-2′,5′- dimethylphospholanyl) (L_a) and 1,3-C₃H ₆(PPh₂)(2′R,5′R-2′,5′- dimethylphospholanyl) (L_b) form complexes of the type [Rh(L)(cyclooctadiene)][BF₄] where L = L_a (1a) or L _b (1b), [PtCl₂(L)] where L = L_a (2a) or L _b (2b) and [PdCl₂(L)] where L = L_a (3a) or L_b (3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt δ-twist and flattened chair conformations respectively. Asymmetric hydrogenation of enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned diphosphine L_a gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue L_b. The greatest enantioselectivities were obtained with 1a and enamide substrates with ees up to 91%. Substrate-induced conformational changes in the Rh-diphosphine chelates are proposed to explain some of the ees observed in the hydrogenation of enamides. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b607479k

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Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric
induction as a function of the substrate and the backbone flexibility of
C1-symmetric diphosphines in rhodium-catalysed hydrogenations
Angharad Baber,a Johannes G. de Vries,b A. Guy Orpen,a Paul G. Pringle*a and Karl von der Luehea
Received 25th May 2006, Accepted 6th July 2006
First published as an Advance Article on the web 9th August 2006
DOI: 10.1039/b607479k
The new unsymmetrical, optically active ligands 1,2-C2H4(PPh2)(2ꢀR,5ꢀR-2ꢀ,5ꢀ-dimethylphospholanyl)
(La) and 1,3-C3H6(PPh2)(2ꢀR,5ꢀR-2ꢀ,5ꢀ-dimethylphospholanyl) (Lb) form complexes of the type
[Rh(L)(cyclooctadiene)][BF4] where L = La (1a) or Lb (1b), [PtCl2(L)] where L = La (2a) or Lb (2b) and
[PdCl2(L)] where L = La (3a) or Lb (3b). The crystal structures of 2a and 2b show the chelate ligand
backbones adopt d-twist and flattened chair conformations respectively. Asymmetric hydrogenation of
enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned
diphosphine La gives a more efficient catalyst in terms of asymmetric induction than the
propylene-backboned analogue Lb. The greatest enantioselectivities were obtained with 1a and enamide
substrates with ees up to 91%. Substrate-induced conformational changes in the Rh–diphosphine
chelates are proposed to explain some of the ees observed in the hydrogenation of enamides.
with enforced d-conformation chelates leads to enhanced enan-
tioselectivities, i.e. the chiral elements are matched.
Introduction
Previous results5,6 with the optically active, phenylene diphos-
phine Lc containing PPh2 and 2,5-dimethylphospholane groups
prompted us to investigate ligands La and Lb which contain
the same substituents but on more flexible backbones. Here we
report the coordination chemistry of La and Lb with rhodium(I),
platinum(II), and palladium(II) and compare the performance
of their rhodium complexes in asymmetric hydrogenation with
previously reported7,8 catalysts derived from Ld, Le, bpe and bpp
(see Scheme 1 for the ligand structures).
The quadrant diagram model is the simplest and most effective
guide for the design of diphosphines for the asymmetric hydro-
genation of enamides.1 Gridnev and Imamoto2 have elegantly
shown how quadrant diagrams can be used to predict accurately
the sense of enantioselectivity. The simple rule with C2-symmetric
diphosphines is blocked upper left/lower right quadrants leads to
R-selectivity and this holds for the variety of diphos ligands shown
in Fig. 1.1–3
Fig. 1 Quadrant diagram for Rh complexes of the ligands shown (drawn
in the conformation they adopt upon coordination).1–3
Another general rule3 for five membered chelates is that the
d-conformation (as represented in Fig. 1 for S,S-dipamp and S,S-
chiraphos) favours R-selection and conversely k-conformation
favours S. The explanation for this rule is that the d-conformation
puts the more sterically demanding pseudo-axial substituents
in the upper left and lower right quadrants. By using 1,2-
diphosphinocyclopentanes, we4 and particularly Dahlenburg
et al.3 have shown that combining upper left quadrant blocking
aSchool of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS
bCatalysis & Development, DSM Pharma Chemicals-Advanced Synthesis,
P. O. Box 18, 6160 MD Geleen, Netherlands
Scheme 1
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Scheme 2
We postulate that substrate-induced ligand conformational
preferences (an allosteric effect) may explain some of the catalytic
results with La.
Results and discussion
Ligands La and Lb were made according to the route shown in
Scheme 2 and have been fully characterised (see Experimental); the
primary phosphine precursors Ph2P(CH2)nPH2 have been reported
previously.9,10
The coordination chemistry of La and Lb is summarised in
Scheme 3. For each of the complexes the 31P NMR spectra
were especially diagnostic, showing AX patterns with 103Rh or
195Pt coupling. The chelates formed by La showed large, positive
coordination chemical shifts (Dd) typical of 5-membered chelates
(see Table 1 for the data). The assignments of PA and PB in 1a
1
and 1b are based on the similarity of d(PB) and J(RhPB) to the
data reported8 for [Rh(bpe)(cod)][SbF6] (d(P) 76.7 ppm, 1J(RhP)
146 Hz) and [Rh(bpp)(cod)][SbF6] (d(P) 27.7 ppm, J(RhP) 140
1
Scheme 3
Hz). The assignments of PA and PB in 2a,b and 3a,b are based
1
on the similarity of d(PA) and J(PtPA) to the data reported11,12
with platinum centres square planar and bite angles in the
normal ranges for C2 and C3 backbone ligands (87.05(6) and
95.51(13)◦) respectively. The chelate ring in 2a has the expected
twist conformation and is in the d form. In 2b the phospholane
ring is disordered and occupies two sites in ca. 3 : 1 ratio in
which the ring is rotated by ca. 18◦. The chelate ring is in a
flattened chair conformation albeit the methylene carbon atoms
show displacement ellipsoids that indicate unresolved disorder.
The implication is that an alternative conformation of the chelate
ring may be present in the crystal.
for [MCl2(dppe)] (M = Pt, d(P) 41.9 ppm, 1J(PtP) 3631 Hz; M =
Pd, d(P) 64.2 ppm) and [MCl2(dppp)] (M = Pt, d(P) −5.6 ppm,
1J(PtP) 3412 Hz; M = Pd, d(P) 11.9). Other characterising data for
the rhodium(I) (1a,b), platinum(II) (2a,b) and palladium(II) (3a,b)
complexes are given in the Experimental section.
The crystal structures of 2a and 2b were determined (see
Tables 2 and 3, and Fig. 2 and 3). These structures confirm the
R configuration at the stereogenic carbons in the phospholane
rings. The coordination geometry in both cases is as expected
Table 1 31P NMR dataa
Compound
d(PA)
1J(MPA)
d(PB)
1J(MPB)
J(PAPB)
Dd
1a
1b
2a
2b
3a
3b
57.8
11.6
45.1
−4.3
68.7
13.5
152
141
3729
3511
76.1
27.0
70.5
13.9
99.2
33.9
142
135
3416
3282
23
46
0
21
11
12
68.8, 79.3
27.9, 30.2
56.1, 64.5
12.0, 17.2
79.7, 102.4
29.8, 37.1
a All spectra measured in CDCl3, at +23 ◦C at 121 or 162 MHz with chemical shifts to high frequency of 85% H3PO4. PA is in the PPh2 group and PB is
in the phospholane group. The coordination chemical shift, Dd = d(complex) − d(ligand).
4822 | Dalton Trans., 2006, 4821–4828
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Fig. 3 Molecular geometry of [PtCl2(Lb)] (2b) showing 50% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Only the major
orientation of the disordered phospholane ring is shown.
Fig. 2 Molecular geometry of [PtCl2(La)] (2a) showing 50% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity.
Table 4 Hydrogenations of substrates A–Ca
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for [PtCl2(La)] (2a)
Entry
Ligand
Substrate
eeb
Ref.c
Pt1–P1
Pt1–P2
Pt1–Cl2
Pt1–Cl1
P1–C7
2.2217(13)
2.2256(15)
2.3703(14)
2.3790(14)
1.818(6)
P1–C1
P1–C13
C7–P1–C1
C7–P1–C13
C1–P1–C13
1.831(6)
1.845(6)
105.3(3)
105.3(3)
106.9(3)
1
2
3
4
5
6
7
8
La
A
A
A
A
A
A
A
B
B
B
B
B
C
C
C
C
C
+9
Lb
−48
+81
+60
−38
+85
+60
+80
−5
Lc
5
7
7
8
8
Ld
Le
◦
bpe
bpp
La
˚
Table 3 Selected bond length (A) and angles ( ) for [PtCl2(Lb)] (2b)
Pt1–P1
Pt1–P2
Pt1–Cl1
Pt1–Cl2
P1–C7
P1–C4A
P1–C4B
P1–C1B
P1–C1A
P2–C9
2.225(4)
2.241(3)
2.360(3)
2.364(3)
1.815(13)
1.826(14)
1.828(17)
1.837(17)
1.852(14)
1.805(12)
1.810(12)
1.838(13)
C7–P1–C4A
C7–P1–C4B
C7–P1–C1B
C4B–P1–C1B
C7–P1–C1A
C4A–P1–C1A
C9–P2–C10
C9–P2–C16
C10–P2–C16
107.1(10)
100(2)
100(2)
9
Lb
Lc
bpe
bpp
La
Lb
Lc
bpe
bpp
10
11
12
13
14
15
16
17
+94
+90
+18
+91
+16
+95
+90
+33
5
97(2)
104.1(11)
93.6(9)
103.4(6)
101.7(6)
104.6(6)
5
P2–C10
P2–C16
a Conversions were all 100% under the condition used (see Experimental).
b The ee is reported as %R and a negative ee means the S enantiomer
predominates. c Data not given a reference are from this work.
Asymmetric hydrogenations
case under the mild conditions used (see Experimental for details).
The rates were similar to those we measured for the catalysts
derived from bpe and bpp under the same conditions which means
that there is not a synergic effect on the kinetics (as a result of the
mixed donor) that we previously reported5 with the unsymmetrical
phenylene ligand Lc.
The rhodium complexes of La and Lb (1a and 1b) were screened
for asymmetric hydrogenation of the acetamidocinnamic ester A
and the enamides B and C; the results are collected in Table 4 and
for comparison, some reported5,7,8 data with Lc, Ld, Le, bpe and
bpp (see Scheme 1) obtained under similar conditions are given.
Substrate A has been widely used for asymmetric hydrogenation
and is a useful benchmark of catalyst enantioselectivity. We had
hoped that, in the asymmetric hydrogenation of A, La would give
greater selectivity than Lc because, if La adopted a d-conformation
in 1a (as it does in the crystal structure of model complex 2a),
this would be expected to increase the R-selectivity.3 The rationale
for this is that lower right quadrant blocking should be greater
with the pseudo-axial aryl group in 1a than the isoclinal array
of aryl groups observed in the structure of [PtCl2(Lc)].5 However,
the fragility of making such predictions is cruelly illustrated by the
much lower ee obtained with ligand La (entry 1) than with the more
rigid analogue Lc (entry 3). Indeed we note that in the structure
Scheme 4
The catalysts derived from La and Lb were efficient with 100%
conversions observed (by monitoring H2 uptake) in ca. 1 h in each
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Fig. 4 Chelate conformations for Ld and Le
of 2a the phenyl rings adopt inverted conformations from those
in [PtCl2(Lc)]5 but in neither case are they clearly edge or face. It
may be that the conformations of the chelate rings formed by La
in solution are a mixture of d and k forms, as discussed below.
The optical induction with La (entry 1) is almost exactly the
average of those reported by Brown et al.7 with ligands Ld and
Le (entries 4 and 5) which feature a stereogenic P atom. The
quadrant model can be used to rationalise these results using the
suggestions that Imamoto and Gridnev have made2 to explain the
‘anomalous’ enantioselectivity of diphosphines containing PArArꢀ
(Arꢀ = ortho-substituted aryl) groups. They propose that the
more bulky Arꢀ adopts a face-on, pseudo-equatorial orientation
to minimise steric congestion, thereby fixing the other Ar group
in an edge-on psuedo-axial position; it is therefore the less bulky
aryl group that is more effective at quadrant blocking. In addition
the Arꢀ group fixes (or at least shifts the balance in favour of) the
chelate ring conformation—upper right Arꢀ favours d and lower
right Arꢀ favours k conformation. Therefore Brown’s ligand7 Ld
would be predicted to form a d–chelate with blocked upper left
and bottom right quadrants (Fig. 4(a)) and thus be the matched
diastereoisomer for R-selectivity in the product, and it is (entry 4).
The mis-matched isomer Le might be predicted to have upper-left
and upper-right quadrants blocked (Fig. 4(b)) and therefore give a
low ee, but in fact the ee is significant and S (entry 5). This could be
rationalised by the bulky anisyl group promoting the adoption of
a k-conformation of the Le chelate as shown in Fig. 4(c) which now
has a more blocked upper-right quadrant (containing a pseudo-
axial phenyl) and a less blocked upper-left quadrant.2 A similar
situation pertains with chelates of R,R-dipamp and S,S-BisP*
which also adopt a k-conformation.2,3
Returning to our results with La, a comparison of entries 1 and 6
in Table 2 indicates that the effect of the PPh2 group is to reduce an
ee of 85% (R) with bpe to 9% (R) with La. It can be appreciated that
the optical induction with La (entry 1), being approximately the
average of the values for Ld and Le (entries 4 and 5), is consistent
with a mixture of d and k chelates for La being present in solution.
There is commercial interest in the asymmetric hydrogenation
of the enamides B and C13 and the results with these substrates
are given in Table 4, entries 8–17. These data illustrate how critical
the substrate is when considering the efficiency of a ligand for
asymmetric catalysis. The catalyst with ligand La which gave 9%
ee with substrate A gives 80% and 91% ee with substrates B (entry
8) and C (entry 13) and thus, in sharp contrast to the results
with substrate A, the effect of the PPh2 group is to make the C1-
symmetric ligand performance comparable to the C2-symmetric
bpe (entries 11 and 16).
k/dconformer preference.3 It seems to us very likely that these two
variables are not independent, i.e. substrate binding will influence
k/dpopulations of Rh chelates of ligands such as dipamp, bpe and
La. In the putative Rh(La)(substrate) intermediates, the La chelate
may favour a dconformation in order to better accommodate more
bulky a-substituents of the substrate in the lower left quadrant (see
Fig. 5); this allosteric effect could explain the greater ee observed
with substrates B and C (which have a-aryl substituents) than A
(with a smaller a-CO2Me substituent).
Fig. 5 Chelate conformations for La
Comparing the results with substrate A for the C3 ligands Lb
(entry 2) and bpp (entry 7) shows that the effect of the PPh2 is
to invert the ee of 60% (R) with bpp to 48% (S) with Lb. The
twist-boat conformation of the chelate formed by Lb detected in
the crystal structure of the model compound 2b puts the phenyl
group occupying the upper right quadrant in a more axial position
and it may be that this type of blocking is particularly effective
and is responsible for the bias for S-product. The catalyst with
ligand Lb which gave an ee of 48% (S) with substrate A gives very
low ee with substrates B and C (5% (R) and 16% (S), entries 9
and 14 respectively); bpp gives low R-selectivity (entries 12 and
17) with substrates B and C. Clearly with ligand Lb, the ee is
sensitive to the substrate and this may similarly be associated with
substrate-induced conformational changes in the Rh(Lb) chelate.
The conformational diversity (chair, boat etc.) available to Lb
would make the interpretation more complicated than with La.
Conclusion
In this article we have shown, from a comparison of the new ligands
La and Lb with bpe and bpp, that the backbone is more important
than C2-symmetry in their performance as ligands for asymmetric
hydrogenation of enamides B and C. The structure of the substrate
is crucial to the catalyst’s performance, as dramatically illustrated
for La by the <10% ee with substrate A being transformed into
>90% ee with substrate B. This has led us to suggest that this may
be partly due to substrate-induced conformational changes in the
metal–diphosphine chelates. The design of ligands for asymmetric
catalysis is indeed a subtle science.
The enantioselectivity of a hydrogenation catalyst featuring a
flexible Rh(PCCP) chelate is a function of the substrate14 and the
4824 | Dalton Trans., 2006, 4821–4828
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3
Ar). dC (100 MHz; CDCl3) 16.4 (d, Me, J(PC) 5.4 Hz), 20.3
Experimental
1
2
(dd, CH2P(O)(OEt)2, J(PC) 14.6 Hz, J(PC) 6.2 Hz), 22.0 (dd,
1
2
General experimental details
CH2PPh2, J(PC) 40.3 Hz, J(PC) 16.5 Hz), 61.6 (d, OCH2CH3,
2J(PC) 6.2 Hz), 128.3 (d, ipso-C, J(PC) 13.1 Hz), 128.5 (d, Ar,
1
Unless otherwise stated, all reactions were carried out under
a nitrogen atmosphere using standard Schlenk line techniques.
Solvents were dried and deoxygenated using the Grubbs’ solvent
system.15 After purification, all phosphines were stored under
nitrogen at room temperature. Most complexes were stable to air in
the solid state and were stored in air at room temperature. Starting
materials prepared by literature methods were [PtCl2(cod)],16
[RhCl(cod)]2,17 [Rh(cod)2]BF4,18 [PdCl2(NCMe)2]19 and the N-
2J(PC) 6.9 Hz), 132.6 (d, Ar, 3J(PC) 18.5 Hz), 137.4 (d, Ar, 4J(PC)
13.1 Hz). EI mass spectrum: m/z 350 (M+).
Preparation of 1-diethylphosphonato-3-(diphenylphosphino)-
propane. To a solution of diphenylphosphine (9.8 cm3, 56.8
mmol) in THF (100 cm3) was added n-BuLi (1.6 M in hexanes,
35.5 cm3, 56.8 mmol) dropwise at 0 ◦C. The resulting solution
was stirred at 0 ◦C for 30 min. The solution was cooled to −78 ◦C
and a solution of 1-diethylphosphonato-3-bromopropane (10 cm3,
51.7 mmol) in THF (100 cm3) added dropwise. The mixture
was stirred for 20 min at −78 ◦C and then for 2 h at room
temperature. Degassed water (50 cm3) was added and the organic
layer extracted, dried over MgSO4 and filtered. The volatiles were
removed in vacuo to leave the product as a pale yellow oil (17.7 g,
94%). dP (162 MHz; CDCl3) −18.4 (d, PPh2, 4J(PP) 3.0 Hz), 30.4
(d, P(O)(OEt)2, 4J(PP) 3.0 Hz).
acetyl a-arylenamide substrates.20 The 31P{ H}, 13C{ H} and H
NMR spectra were recorded using Jeol D300 (300 MHz) or Jeol
GX400 (400 MHz) spectrometers. The microanalytical laboratory
of the School of Chemistry, University of Bristol, carried out
elemental analyses. The mass spectrometry service, University of
Bristol, recorded electron impact and fast atom bombardment
mass spectra on a MD800 and an Autospec.
1
1
1
Preparation of 1-bromo-2-(diethylphosphonato)ethane. Tri-
ethylphosphite (19.4 g, 116.6 mmol) was added to 1,2-
dibromoethane (87.6 g, 466.5 mmol) at room temperature and
the mixture heated to 160 ◦C for 4 h. The apparatus allowed
continuous distillation of bromoethane from the reaction mixture.
The mixture was allowed to cool to room temperature and the
volatiles removed in vacuo. The colourless liquid product was
collected at 91–92 ◦C/0.9 mm (18.2 g, 74.4 mmol, 64%). dP
(121 MHz; CDCl3) 26.2 (s). dH (400 MHz; CDCl3) 1.07–1.14 (m,
6H, Me), 2.09–2.21 (m, 2H, CH2Br), 3.25–3.34 (m, 2H, PCH2),
3.82–3.95 (m, 4H, OCH2CH3). dC (100 MHz; CDCl3) 16.0 (d, Me,
3J(PC) 6.2 Hz), 23.4 (s, CBr), 30.3 (d, PC, 1J(PC) 134.5 Hz), 61.5
(d, OCH2CH3, 2J(PC) 6.9 Hz).
Preparation of 1-diphenylphosphino-2-phosphinoethane.
A
solution of 1-diethylphosphonato-2-diphenylphosphinoethane
(4.70 g, 13.4 mmol) in diethyl ether (200 cm3) was added dropwise
to a suspension of LiAlH4 (0.64 g, 16.8 mmol) in diethyl ether
◦
(100 cm3) at 0 C. The mixture was stirred at room temperature
for 20 h. The resultant mixture was cooled to 0 ◦C and HCl (2M,
60 cm3) added dropwise. The organic layer was extracted, dried
over MgSO4 and filtered. The diethyl ether was distilled off to leave
an oily residue. The product was collected by vacuum distillation
◦
(140–142 C, 0.6 mm) as a colourless liquid (1.88 g, 7.65 mmol,
3
57%). dP (121 MHz; CDCl3) −127.4 (d, PH2, J(PP) 16.8 Hz),
−13.7 (d, PPh2, 3J(PP) 16.8 Hz). dH (400 MHz; CDCl3) 2.16 (dm,
2H, CH2PH2, 2J(PH) 1.0 Hz), 2.22–2.29 (m, 2H, CH2PPh2), 2.84
Preparation of 1-bromo-3-(diethylphosphonato)propane. Tri-
ethylphosphite (19.7 cm3, 115 mmol) was added to 1,3-
dibromopropane (46.7 cm3, 460 mmol) at room temperature and
the mixture heated to 160 ◦C for 4 h. The apparatus allowed
continuous distillation of bromoethane from the reaction mixture.
The mixture was allowed to cool to room temperature and the
volatiles removed in vacuo. The colourless liquid product was
collected at 95–97 ◦C/0.5 mm (22.85 g, 77%). dP (162 MHz;
CDCl3) 31.0 (s). dH (400 MHz; CDCl3) 1.17–1.24 (m, 6H, Me),
1.73–1.84 (m, 2H, PCH2CH2), 2.00–2.06 (m, 2H, PCH2), 3.32–
3.73 (m, 2H, CH2Br), 3.93–4.04 (m, 4H, OCH2CH3).
1
(d, 2H, PH2, J(PH) 195.0 Hz), 7.31–7.35 (m, 6H, Ar), 7.38–
1
7.43 (m, 4H, Ar). dC (100 MHz; CDCl3) 10.4 (dd, CPH2, J(PC)
16.1 Hz, 2J(PC) 9.2 Hz), 31.5 (dd, CPPh2, 1J(PC) 13.8 Hz, 2J(PC)
2
1
3.1 Hz), 128.5 (d, Ar, J(PC) 6.9 Hz), 128.6 (d, Ar, J(PC) 20.0
Hz), 132.7 (d, Ar, 3J(PC) 18.5 Hz), 137.9 (d, Ar, 4J(PC) 12.3 Hz).
Preparation of 1-diphenylphosphino-3-phosphinopropane.
A
solution of 1-diethylphosphonato-2-diphenylphosphinopropane
(17.7 g, 48.6 mmol) in diethyl ether (200 cm3) was added dropwise
to a suspension of LiAlH4 (2.3 g, 60.7 mmol) in diethyl ether
◦
(100 cm3) at 0 C. The mixture was stirred at room temperature
for 20 h. The resultant mixture was cooled to 0 ◦C and HCl (2M,
80 cm3) added dropwise. The organic layer was extracted, dried
over MgSO4 and filtered. The diethyl ether was distilled off to leave
an oily residue. The product was collected by vacuum distillation
(128–130 ◦C, 0.5 mm) as a colourless liquid (8.32 g, 66%). dP
(162 MHz; CDCl3) −137.3 (s, PH2), −16.3 (s, PPh2). dH (400 MHz;
CDCl3) 1.53–1.65 (m, 4H, H2PCH2CH2), 2.07 (t, 2H, Ph2PCH2,
3J(HH) 7.2 Hz), 2.61 (dt, 2H, PH2, 1J(PH) 194.8 Hz, 3J(HH) 7.2
Hz), 7.25–7.28 (m, 6H, Ar), 7.35–7.40 (m, 4H, Ar).
Preparation of 1-diethylphosphonato-2-(diphenylphosphino)-
ethane. To a solution of diphenylphosphine (3.35 g, 18 mmol)
in THF (50 cm3) was added n-BuLi (1.6 M in hexanes, 11.3 cm3,
18 mmol) dropwise at −10 ◦C. The resulting solution was stirred
at 0 ◦C for 30 min. The solution was cooled to −78 ◦C and
a solution of 1-diethylphosphonato-2-bromoethane (4.41 g, 18
mmol) in THF (30 cm3) added. The mixture was stirred for 20 min
at −78 ◦C and then for 2 h at room temperature. Degassed water
(50 cm3) was added and the organic layer extracted, dried over
MgSO4 and filtered. The volatiles were removed in vacuo to leave
the product as a white oil (4.71 g, 13.5 mmol, 75%). dP (121 MHz;
Preparation of 1-[(2ꢀR,5ꢀR)-2ꢀ,5ꢀ-dimethylphospholano]-2-(di-
phenylphosphino)ethane
(La). 1-Diphenylphosphino-2-phos-
3
CDCl3) −12.1 (d, PPh2, J(PP) 62.3 Hz), 31.8 (d, P(O)(OEt)2,
phinoethane (1.88 g, 7.6 mmol) was dissolved in THF (100 cm3)
and n-BuLi (1.6M in hexanes, 4.8 cm3, 7.6 mmol) was added
dropwise at −78 ◦C. The solution was allowed to warm to room
temperature and stirred for 2 h. The resultant orange solution
3
3J(PP) 62.3 Hz). dH (400 MHz; CDCl3) 1.29 (t, 6H, Me, J(HH)
7.1 Hz), 1.71–1.82 (m, 2H, CH2P(O)(OEt)2), 2.23–2.31 (m, 2H,
CH2PPh2), 4.01–4.11 (m, 4H, OCH2CH3), 7.29–7.44 (m, 10H,
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The Royal Society of Chemistry 2006
Dalton Trans., 2006, 4821–4828 | 4825
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was cooled to 0 ◦C and (2S,5S)-2,5-hexanediol cyclic sulfate
(1.36 g, 7.6 mmol) added. The mixture was allowed to warm
to room temperature and stirred for 2 h. The yellow solution
was cooled to −78 ◦C and another portion of n-BuLi (1.6M
in hexanes, 6.1 cm3, 9.9 mmol) added dropwise. The mixture
was allowed to warm to room temperature and stirred for 2 h.
Degassed water (100 cm3) was added followed by diethyl ether
(40 cm3). The organic layer was extracted, dried over MgSO4
and filtered. Volatiles were removed in vacuo to leave a white low
melting point solid (2.11 g, 6.44 mmol, 85%). dP (162 MHz; C7D8)
−11.0 (d, PPh2, 3J(PP) 27.9 Hz), 5.0 (d, phospholane, 3J(PP)
27.9 Hz). dH (400 MHz; C7D8) 0.90 (dd, 3H, Me, 3J(PH) 9.5 Hz,
3J(HH) 7.3 Hz), 0.94–1.04 (m, 1H, CH), 1.06–1.15 (m, 1H, CH),
Preparation of [Rh(cod)(La)]BF4. To
a
solution of
[Rh(cod)2]BF4 (0.50 g, 1.23 mmol) in CH2Cl2 (10 cm3) was
added a solution of La (0.40 g, 1.23 mmol) in CH2Cl2 (10 cm3).
The solution was stirred at room temperature for 20 min. The
solution was concentrated to ca 10 cm3 under reduced pressure
before the addition of diethylether (50 cm3). The resultant solid
was filtered and dried in vacuo to give the product as an orange
powder (0.34 g, 0.54 mmol, 44%). dP (121 MHz; CDCl3) 57.8 (dd,
PPh2, 1J(RhP) 151.7 Hz, 3J(PP) 23.3 Hz), 76.1 (dd, phospholane,
3
1J(RhP) 142.4 Hz, J(PP) 23.3 Hz). dH (400 MHz; CD2Cl2) 1.12
3
3
(dd, 3H, Me, J(PH) 14.8 Hz, J(HH) 6.7 Hz), 1.41 (dd, 3H,
3
3
Me, J(PH) 18.1 Hz, J(HH) 6.8 Hz), 1.63–1.87 (m, 1H, CH),
2.01–2.54 (br m, 17H, CH2 of C8H12, backbone and ring, ring
CH), 4.55 (br s, 1H, CH of C8H12), 4.86 (br s, 1H, CH of C8H12),
5.15 (br s, 1H, CH of C8H12), 5.40 (br s, 1H, CH of C8H12),
7.31–7.40 (m, 2H, Ar), 7.41–7.49 (m, 5H, Ar), 7.50–7.56 (m, 1H,
Ar), 7.66–7.73 (m, 2H, Ar). dC (100 MHz; CD2Cl2) 14.0 (s, Me),
3
3
1.20 (dd, 3H, Me, J(PH) 17.6 Hz, J(HH) 7.3 Hz), 1.26–1.36
(m, 2H, ring CH2), 1.61–1.72 (m, 2H, C6H12PCH2), 1.79–1.89
(m, 2H, Ph2PCH2), 2.06–2.13 (m, 1H, ring CH2), 2.13–2.25 (m,
1H, ring CH2), 7.03–7.10 (m, 7H, Ar), 7.38–7.44 (m, 3H, Ar). dC
(100 MHz; C7D8) 14.4 (s, Me), 19.8 (d, Me, 2J(PC) 15.4 Hz), 21.5
(d, C6H12PC, 1J(PC) 30.8 Hz), 25.5 (dd, Ph2PC, 1J(PC) 19.2 Hz,
2
20.2 (d, Me, J(PC) 8.5 Hz), 21.5–21.8 (m, C6H12PC), 27.2–27.4
(m, Ph2PC), 30.1 (m, CH2 of C8H12), 30.2 (m, CH2 of C8H12),
30.5 (m, CH2 of C8H12), 31.1 (m, CH2 of C8H12), 34.2 (m, PCH),
1
2J(PC) 14.6 Hz), 34.5 (d, PCH, J(PC) 12.3 Hz), 37.0 (d, ring
CH2, 2J(PC) 4.6 Hz), 37.4 (d, ring CH2, 2J(PC) 1.5 Hz), 38.2 (d,
34.5 (d, ring CH2, J(PC) 1.5 Hz), 35.7 (s, ring CH2), 37.1 (m,
2
1
4
PCH, J(PC) 11.5 Hz), 128.6 (d, Ar, J(PC) 2.3 Hz), 128.8 (d,
PCH), 95.8–96.1 (br m, CH of C8H12), 97.6–97.9 (br m, CH of
C8H12), 102.7–103.0 (br m, CH of C8H12), 103.0–103.2 (br m,
CH of C8H12), 129.9 (dd, Ar, J(RhC) 5.4 Hz, J(PC) 10.0 Hz),
131.7 (d, Ar, J(PC) 12.3 Hz), 132.6 (d, Ar, J(PC) 2.3 Hz), 134.5
(d, Ar, J(PC) 10.8 Hz). ESI mass spectrum: m/z 539 (M+–BF4).
3
2
5
Ar, J(PC) 6.2 Hz), 133.2 (dd, Ar, J(PC) 18.4 Hz, J(PC) 6.2
Hz), 139.7 (dd, ipso-C, 1J(PC) 20.0 Hz, 4J(PC) 15.4 Hz). EI mass
spectrum: m/z 328 (M+).
1
Elemental analysis (presence of solvent confirmed by H NMR)
Preparation of 1-[(2ꢀR,5ꢀR)-2ꢀ,5ꢀ-dimethylphospholano]-3-(di-
phenylphosphino)propane (Lb). 1-Diphenylphosphino-3-phos-
phinopropane (0.95 g, 3.66 mmol) was dissolved in THF (100 cm3)
and n-BuLi (1.6M in hexanes, 2.3 cm3, 3.66 mmol) was added
dropwise at −78 ◦C. The solution was allowed to warm to room
temperature and stirred for 2 h. The resultant orange solution
was cooled to 0 ◦C and (2S,5S)-2,5-hexanediol cyclic sulfate
(0.66 g, 3.66 mmol) added. The mixture was allowed to warm
to room temperature and stirred for 2 h. The yellow solution
was cooled to −78 ◦C and another portion of n-BuLi (1.6M
in hexanes, 3.0 cm3, 4.76 mmol) added dropwise. The mixture
was allowed to warm to room temperature and stirred for 2 h.
Degassed water (70 cm3) was added followed by diethyl ether
(40 cm3). The organic layer was extracted, dried over MgSO4
and filtered. Volatiles were removed in vacuo to leave a white oil
(1.12 g, 3.26 mmol, 89%). dP (121 MHz; C7D8) −16.3 (s, PPh2),
−3.2 (s, phospholane). dH (400 MHz; C7D8) 0.90 (dd, 3H, Me,
(calc·2CH2Cl2) C: 45.85 (45.26), H: 5.10 (5.32).
Preparation of [Rh(cod)(Lb)]BF4. To
a
solution of
[Rh(cod)2]BF4 (0.40 g, 0.99 mmol) in CH2Cl2 (10 cm3) was
added a solution of Lb (0.34 g, 0.99 mmol) in CH2Cl2 (10 cm3).
The solution was stirred at room temperature for 30 min. The
solution was concentrated to ca 10 cm3 under reduced pressure
before the addition of diethylether (50 cm3). The resultant solid
was filtered and dried in vacuo to give the product as a yellow
powder (0.30 g, 0.46 mmol, 48%). dP (121 MHz; CDCl3) 11.6 (dd,
PPh2, 1J(RhP) 141.4 Hz, 3J(PP) 45.6 Hz), 27.0 (dd, phospholane,
3
1J(RhP) 134.9 Hz, J(PP) 45.6 Hz). dH (400 MHz; C7D8) 1.03
3
3
(dd, 3H, Me, J(PH) 15.0 Hz, J(HH) 7.2 Hz), 1.46 (dd, 3H,
Me, 3J(PH) 7.2 Hz, 3J(HH) 4.0 Hz), 1.59–2.97 (br m, 20H, CH2
of C8H12, backbone and ring, ring CH), 4.49 (br s, 1H, CH of
C8H12), 4.87 (br s, 1H, CH of C8H12), 5.37 (br s, 1H, CH of C8H12),
5.43 (br s, 1H, CH of C8H12), 7.46–7.54 (m, 1H, Ar), 7.59–7.77
(m, 5H, Ar), 7.78–7.90 (m, 2H, Ar), 7.91–8.07 (m, 2H, Ar). dC
3
3J(PH) 17.6 Hz, J(HH) 7.3 Hz), 0.95–1.03 (m, 2H, ring CH),
1.10 (dd, 3H, Me, 3J(PH) 9.8 Hz, 3J(HH) 7.1 Hz), 1.14–1.24 (m,
2H, ring CH2), 1.32–1.42 (m, 2H, C6H12PCH2), 1.67–1.79 (m, 2H,
Ph2PCH2CH2), 1.80–1.90 (m, 2H, Ph2PCH2), 1.94–2.03 (m, 1H,
ring CH2), 2.05–2.13 (m, 1H, ring CH2), 6.97–7.05(m, 6H, Ar),
7.26–7.38 (m, 4H, Ar). dC (100 MHz; C7D8) 14.4 (s, Me), 21.0 (d,
Me, 2J(PC) 12.3 Hz), 21.3 (d, C6H12PC, 1J(PC) 30.8Hz), 25.3 (dd,
(100 MHz; CD2Cl2) 14.3 (s, Me), 19.3 (dd, C6H12PC, J(RhC)
2
1
2
7.3 Hz, J(PC) 21.1 Hz), 19.9 (d, Me, J(PC) 6.9 Hz), 24.9 (dd,
1
2
Ph2PC, J(PC) 28.8 Hz, J(RhC) 6.5 Hz), 28.3 (dd, Ph2PCH2C,
2J(RhC) 1.5 Hz, 2J(PC) 11.5 Hz), 30.0 (br s, CH2 of C8H12), 30.6
(br s, CH2 of C8H12), 30.8 (br s, CH2 of C8H12), 31.1 (br s, CH2 of
C8H12), 32.2 (dd, PCH, 2J(RhC) 7.7 Hz, 1J(PC) 7.7 Hz), 33.5 (d,
ring CH2, 2J(PC) 2.3 Hz), 36.5 (d, ring CH2, 2J(PC) 5.4 Hz), 36.7
(d, PCH, 1J(PC) 1.5 Hz), 96.8 (dd, CH of C8H12, 2J(RhC) 8.8 Hz,
1
3
Ph2PC, J(PC) 22.3 Hz, J(PC) 11.5 Hz), 30.2 (dd, Ph2PCH2C,
2J(PC) 13.1 Hz, 2J(PC) 11.5 Hz), 33.9 (d, PCH, 1J(PC) 11.5 Hz),
2
2
2
3
37.0 (d, ring CH2, J(PC) 4.6 Hz), 37.5 (d, ring CH2, J(PC) 1.5
Hz), 38.3 (d, PCH, 1J(PC) 11.5 Hz), 128.9 (dd, Ar, 3J(PC) 5.4 Hz,
3J(PC) 7.3 Hz), 98.2 (dd, CH of C8H12, J(RhC) 8.1 Hz, J(PC)
8.1 Hz), 99.3 (dd, CH of C8H12, 2J(RhC) 9.2 Hz, 3J(PC) 6.9 Hz),
7J(PC) 1.5 Hz), 129.0 (dd, Ar, J(PC) 4.6 Hz, J(PC) 2.3 Hz),
101.7 (dd, CH of C8H12, J(RhC) 8.8 Hz, J(PC) 7.3 Hz), 129.3
(d, Ar, J(PC) 10.0 Hz), 129.8 (d, Ar, J(PC) 10.0 Hz), 132.0 (d, Ar,
J(PC) 10.0 Hz), 134.3 (d, Ar, J(PC) 11.5 Hz). ESI mass spectrum:
m/z 639 (M+), 553 (M+–BF4). Elemental analysis (presence of
4
8
2
3
133.5 (dd, Ar, 2J(PC) 18.5 Hz, 6J(PC) 13.8 Hz), 140.2 (dd, ipso-C,
1J(PC) 23.1 Hz, J(PC) 14.6 Hz). EI mass spectrum: m/z 342
5
(M+).
4826 | Dalton Trans., 2006, 4821–4828
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©

View Article Online
3
6
solvent confirmed by 1H NMR) (calc·2CH2Cl2) C: 45.01 (45.96),
Ar, J(PC) 11.1 Hz, J(PC) 2.7 Hz). EI mass spectrum: m/z 472
(M+–HCl). Elemental analysis (calc.) C: 46.58 (47.50), H: 3.96
(5.18).
H: 5.82 (5.47).
Preparation of [PtCl2(La)]. To a suspension of [PtCl2(cod)]
(0.20 g, 0.54 mmol) in toluene (20 cm3) was added a solution
of La (0.175 g, 0.54 mmol) in toluene (6 cm3). The mixture was
stirred at room temperature for 2 h. The volatiles were removed
in vacuo and the resultant residue was washed with diethylether
(20 cm3). The precipitate was filtered and dried in vacuo to give
the product as a colourless powder (0.27 g, 0.45 mmol, 85%).
Preparation of [PdCl2(Lb)]. To a solution of [PdCl2(NCMe)2]
(0.08 g, 0.29 mmol) in CH2Cl2 (10 cm3) was added a solution of
Lb (0.10 g, 0.29 mmol) in toluene (5 cm3). The mixture was stirred
at room temperature for 1.5 h. The volatiles were removed in
vacuo and the resultant residue dissolved in a minimum of CH2Cl2.
Hexane (60 cm3) was added and the mixture stirred for 15 min. The
precipitate was filtered and dried in vacuo to give the product as
a yellow powder (0.08 g, 0.15 mmol, 52%). dP (121 MHz; CDCl3)
13.5 (d, PPh2, 3J(PP) 11.5 Hz), 33.9 (d, phospholane, 3J(PP) 11.5
Hz). dH (400 MHz; CDCl3) 0.85–2.52 (br m, 12H, backbone and
1
dP (162 MHz; CDCl3) 45.1 (d, PPh2, J(PtP) 3729 Hz), 70.5 (d,
1
phospholane, J(PtP) 3416 Hz). dH (400 MHz; CDCl3) 1.09 (dd,
3
3
3H, Me, J(PH) 16.2 Hz, J(HH) 7.2 Hz), 1.36 (dd, 3H, Me,
3J(PH) 18.8 Hz, 3J(HH) 7.1 Hz), 1.48–2.63 (br m, 12H, backbone
and ring CH2, ring CH), 7.33–7.47 (m, 6H, Ar), 7.70–7.81 (m, 4H,
3
3
ring CH2, ring CH), 1.20 (dd, 3H, Me, J(PH) 15.9 Hz, J(HH)
7.1 Hz), 1.49 (dd, 3H, Me, 3J(PH) 19.8 Hz, 3J(HH) 6.1 Hz), 7.40–
7.58 (m, 6H, Ar), 7.68–7.75 (m, 2H, Ar), 7.97–8.04 (m, 2H, Ar).
EI mass spectrum: m/z 485 (M+–Cl).
2
Ar). dC (100 MHz; CDCl3) 13.5 (d, Me, J(PC) 1.5 Hz), 18.2 (d,
2
1
2
Me, J(PC) 3.8 Hz), 22.9 (dd, C6H12PC, J(PC) 33.1 Hz, J(PC)
1
2
9.2 Hz), 30.3 (dd, Ph2PC, J(PC) 43.4 Hz, J(PC) 7.3 Hz), 33.6
1
2
(d, PCH, J(PC) 36.9 Hz), 35.2 (d, ring CH2, J(PC) 2.3 Hz),
35.3 (d, ring CH2, 2J(PC) 4.6 Hz), 37.7 (d, PCH, 1J(PC) 37.7 Hz),
127.3 (dd, ipso-C, 1J(PC) 62.7 Hz, 4J(PC) 2.7 Hz), 128.8 (dd, Ar,
Standard procedure for asymmetric hydrogenation of substrates
A–C
3J(PC) 11.5 Hz, J(PC) 8.5 Hz), 131.9 (dd, Ar, J(PC) 10.4 Hz,
7J(PC) 2.7 Hz), 133.4 (dd, Ar, 2J(PC) 15.4 Hz, 5J(PC) 10.8 Hz). EI
mass spectrum: m/z 593 (M+). Elemental analysis (calc.) C: 40.93
(40.42), H: 4.05 (4.41).
6
4
Catalyst 3a or 3b (0.010 mmol) and the substrates (1.0 mmol)
were mixed in glass vessels. Up to eight vessels were placed in the
EndeavorTM parallel screening apparatus and flushed with nitro-
gen. Nitrogen-saturated methanol (5.0 cm3) was added and the
apparatus sealed. The vessels were purged 10 times with nitrogen
and once with hydrogen. The autoclave was then pressurised with
5 bar H2 and the reaction mixture stirred for 3 h. The resulting
mixture was filtered through a 1.5 cm pad of silica to remove the
metal complex catalyst. The conversion was measured by 1H NMR
spectroscopy and in each case was 100%; the ee determination was
by chiral GC.
Preparation of [PtCl2(Lb)]. To a solution of [PtCl2(cod)]
(0.11 g, 0.29 mmol) in CH2Cl2 (10 cm3) was added a solution
of Lb (0.10 g, 0.29 mmol) in toluene (5 cm3). The mixture was
stirred at room temperature for 2 h. The volatiles were removed
in vacuo and the resultant residue was dissolved in a minimum
of CH2Cl2. Diethylether (60 cm3) was added and the precipitate
filtered and dried in vacuo to give the product as a colourless
powder (0.08 g, 0.13 mmol, 45%). dP (162 MHz; CDCl3) −4.3 (dd,
Crystal structure analysis
1
3
PPh2, J(PtP) 3511 Hz, J(PP) 20.5 Hz), 13.9 (dd, phospholane,
1J(PtP) 3284 Hz, J(PP) 20.5 Hz). dH (400 MHz; CDCl3) 1.17
3
X-Ray diffraction experiments on 2a and 2b were carried out
at −100 ◦C on Bruker CCD diffractometers using Mo Ka X-
(dd, 3H, Me, 3J(PH) 15.5 Hz, 3J(HH) 7.2 Hz), 1.35 (dd, 3H, Me,
3J(PH) 18.6 Hz, 3J(HH) 6.6 Hz), 1.65–2.62 (br m, 12H, backbone
and ring CH2, ring CH), 7.38–7.55 (m, 6H, Ar), 7.66–7.74 (m, 2H,
Ar), 7.93–8.01 (m, 2H, Ar). EI mass spectrum: m/z 572 (M+–HCl),
535 (M+–2HCl).
˚
radiation, k = 0.71073 A. Crystal data and refinement data
are given in Table 5. The intensity data were integrated using
the SAINT21 program. Absorption corrections were based on
Table 5 Crystallographic data for 2a and 2b
Preparation of [PdCl2(La)]. To a solution of [PdCl2(NCMe)2]
(0.15 g, 0.45 mmol) in CH2Cl2 (10 cm3) was added a solution of
La (0.12 g, 0.45 mmol) in toluene (5 cm3). The mixture was stirred
at room temperature for 1 h. The volatiles were removed in vacuo
and the resultant solid was washed with diethylether (10 cm3) and
dried under reduced pressure to give the product as an orange
powder (0.14 g, 0.28 mmol, 62%). dP (121 MHz; CDCl3) 68.7 (d,
PPh2, 3J(PP) 11.1 Hz), 99.2 (d, phospholane, 3J(PP) 11.1 Hz). dH
(400 MHz; CDCl3) 1.19 (dd, 3H, Me, 3J(PH) 16.4 Hz, 3J(HH) 7.2
Hz), 1.54 (dd, 3H, Me, 3J(PH) 19.7 Hz, 3J(HH) 6.9 Hz), 1.73–2.65
(br m, 12H, backbone and ring CH2, ring CH), 7.42–7.54 (m, 6H,
Ar), 7.81–7.88 (m, 4H, Ar). dC (100 MHz; CDCl3)13.8 (s, Me),
18.4 (d, Me, 2J(PC) 4.6 Hz), 23.6 (dd, C6H12PC, 1J(PC) 25.4 Hz,
Compound
2a
2b
Colour, habit
Size/mm
Empirical Formula
M
Colourless block
Colourless block
0.25 × 0.25 × 0.20 0.30 × 0.30 × 0.25
C20H26Cl2P2Pt
594.34
C21H28Cl2P2Pt
608.36
Monoclinic
P21
8.9411(11)
11.7365(14)
11.1592(14)
106.754(2)
1121.3(2)
2
Crystal system
Space group
Monoclinic
P21
˚
a/A
8.8106(11)
13.7761(17)
9.1967(11)
108.001(2)
1061.6(2)
2
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
l/mm−1
7.012
6.641
5015
0.0894
0.0559
1
2
2J(PC) 14.6 Hz), 30.2 (dd, Ph2PC, J(PC) 35.4 Hz, J(PC) 12.3
Hz), 35.3 (d, ring CH2, 2J(PC) 3.8 Hz), 35.5 (d, PCH, 1J(PC) 30.0
Hz), 35.6 (s, ring CH2), 39.5 (d, PCH, 1J(PC) 30.8 Hz), 128.1 (d,
Independent reflections
Rint
4836
0.0323
0.0282
1.565, −1.515
R1
−3
˚
Largest peak, hole/eA
1.251, −1.532
1
2
5
ipso-C, J(PC) 53.0 Hz), 129.0 (dd, Ar, J(PC) 11.5 Hz, J(PC)
Absolute structure parameter −0.021(7)
−0.015(14)
3.9 Hz), 132.0 (dd, Ar, 4J(PC) 5.4 Hz, 7J(PC) 3.1 Hz), 133.4 (dd,
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 4821–4828 | 4827
©

View Article Online
equivalent reflections. The structures were solved by direct meth-
ods and refined using full-matrix least-squares against F2 using
SHELXTL.21 Hydrogen atoms were included in idealisedpositions
with isotropic displacement parameters
4 E. Fernandez, A. Gillon, K. Heslop, E. Horwood, D. J. Hyett, A. G.
Orpen and P. G. Pringle, Chem. Commun., 2000, 1663.
5 S. Basra, J. G. de Vries, D. J. Hyett, G. Harrison, K. M. Heslop,
A. G. Orpen, P. G. Pringle and K. von der Luehe, Dalton Trans., 2004,
1901.
6 K. Matsumura, H. Shimizu, T. Saito and H. Kumobayashi, Adv. Synth.
CCDC reference numbers 609102 and 609103 for compounds
2a and 2b. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b607479k
Catal., 2003, 345, 180.
7 D. Carmichael, H. Doucet and J. M. Brown, Chem. Commun., 1999,
261.
8 M. J. Burk, J. M. Feaster and R. L. Harlow, Tetrahedron: Asymmetry,
1991, 2, 569.
Acknowledgements
9 (a) R. B. King and P. N. Kapoor, Angew. Chem., Int. Ed. Engl., 1971,
10, 734; (b) R. B. King and J. C. Cloyd, J. Am. Chem. Soc., 1975, 97,
46; (c) D. M. Schubert, P. F. Brandt and A. D. Norman, Inorg. Chem.,
1996, 35, 6204.
We would like to thank Dr A.H.M. de Vries and Dr J.A.F. Boogers
from DSM for useful discussions and Mairi Haddow for assistance
in preparing data for publication. The Leverhulme Trust for a
Research Fellowship (to P.G.P.), DSM Research and EPSRC for
funding and Johnson-Matthey for the loan of precious metal
compounds.
10 (a) U. W. Meier, F. Hollmann, U. Thewalt, M. Klinga, M. Leskelae
and B. Reiger, Organometallics, 2003, 22, 3905; (b) C. E. Lloyd-Jones,
Ph.D. Thesis, University of Bristol, 2001.
11 E. Lindner, R. Fawzi, H. A. Mayer, K. Elchele and W. Hiller,
Organometallics, 1992, 11, 1033.
12 C. H. Lindsay, L. S. Benner and A. L. Balch, Inorg. Chem., 1980, 19,
3503.
13 J. S. Ma, Chim. Oggi, 2003, 21, 65.
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