Efficient asymmetric hydrogenation with rhodium complexes of C 1-symmetric 2,5-dimethylphospholane-diphenylphosphines

Article abstract of DOI:10.1039/b404827j

The unsymmetrical, optically active ligands 1,2-C₆H ₄(pph₂)((R,R)-2,5-dimethylphospholanyl) (1a) and the new 1,1′-Fe(C₅H₄)₂(PPh ₂)((R,R)-2,5-dimethylphospholanyl) (1b) form compl

Full text of DOI:10.1039/b404827j

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F U L L P A P E R
Efficient asymmetric hydrogenation with rhodium complexes of
C1-symmetric 2,5-dimethylphospholane-diphenylphosphines
Sandeep Basra,a Johannes G. de Vries,b David J. Hyett,b Gayle Harrison,a Katie M. Heslop,a
A. Guy Orpen,a Paul G. Pringle*a and Karl von der Luehea
a
School of Chemistry, University of Bristol, Cantocks Close, Bristol UK BS8 1TS
DSM Pharma Chemicals-Advanced Synthesis, Catalysis & Development, P.O. Box 18, 6160 MD,
b
Geleen, Netherlands
Received 31st March 2004, Accepted 5th May 2004
First published as an Advance Article on the web 20th May 2004
The unsymmetrical, optically active ligands 1,2-C6H4(PPh2)((R,R)-2,5-dimethylphospholanyl) (1a) and the new
1,1′-Fe(C5H4)2(PPh2)((R,R)-2,5-dimethylphospholanyl) (1b) form complexes of the type [PtCl2(diphos)] (2a,b) and
[Rh(diphos)(diene)][BF4] (3a,b). The crystal structure of 2a reveals that only one quadrant is blocked. Asymmetric
hydrogenation of acrylic esters and enamides using 3a and 3b as catalysts show that the phenylene-backboned diphosphine
gives a more efficient catalyst in terms of asymmetric induction than the more flexible ferrocene-backboned diphosphine.
The best results, which were obtained with 3a and enamide substrates, exceeded those obtained with Duphos catalysts.
The rate of hydrogenation of the enamides with 3a was 10 times faster than with [Rh(Duphos)(diene)][BF4]. A quadrant
diagram can be used to predict the configuration of the major product, provided it is assumed to be derived from the less
sterically congested intermediate.
Table 1 31P NMR dataa
Introduction
Asymmetric hydrogenations of CC, CO and CN function-
alities have found important applications in organic synthesis and
in the fine chemicals business.1 Over the last 30 years, hundreds
of chiral diphosphine ligands have been screened2 for asymmetric
hydrogenation and from this empirical endeavour, some guiding
principles for the design of diphosphines for efficient asymmetric
hydrogenation have emerged. These include, the presence of a rigid
backbone, PPh2 groups or 2,5-dimethylphospholanes; two of the
most successful chiral diphosphines, binap3 and Duphos4 embody
these features. In parallel with the ligand work, detailed mechanistic
studies5 have illuminated the elementary steps in asymmetric
hydrogenation and given rise to a useful model, based on quadrant
diagrams,6 which has been used to rationalise the efficiency of
ligands such as binap and Duphos.5
(PA)
(PB)
1J(PAPB)
1J(MPA), 1J(MPB)/Hz
1a
1b
2a
2b
3a
3b
−12.1
−16.7
42.0
10.3
60.0
−0.1
−1.1
66.4
36.8
75.7
47.3
159
0
3
12
27
28
3468, 3666
3807, 3664
158, 160
25.5
162, 158
aAll spectra measured in CDCl3 at +23 °C at 121 MHz with chemical shifts
 to high frequency of 85% H3PO4.
terised. The intermediates in the synthesis of the phenylene diphos-
phine 1a have been previously reported by us10 and others,11 while
the ferrocenyl diphosphine 1b is new and all intermediates have
been fully characterised (see Experimental section).
Theligands1aand1bweredesignedtoinvestigatetheefficiencyof
a chiral diphosphine containing PPh2 and 2,5-dimethylphospholane
groups. Here we report the coordination chemistry of 1a and 1b
with platinum(II) and rhodium(I) and the applications of 1a and 1b
in asymmetric hydrogenation. While this work was in progress,7 the
synthesis of ligand8 1a and some of its applications in asymmetric
hydrogenation have been reported.9
Scheme 1 Ligand synthesis.
Results and discussion
The coordination chemistry of 1a,b is summarised in Scheme 2
and the characterising data for the platinum(II) complexes (2a,b)
and rhodium(I) complexes (3a,b) are given in Table 1 (31P NMR)
Ligands 1a,b were made according to the route shown in Scheme 1
(which is similar to the literature route9) and have been fully charac-
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 9 0 1 – 1 9 0 5
1 9 0 1

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Table 2 Selected bond distances (Å) and angles (°) for 2a
Table 4 Hydrogenations of substrates D–H
Pt(11)–P(11)
Pt(11)–P(12)
Pt(11)–Cl(11)
Pt(11)–Cl(12)
Pt(21)–P(21)
Pt(21)–P(22)
Pt(21)–Cl(21)
Pt(21)–Cl(22)
2.214(7)
2.228(8)
2.370(8)
2.343(8)
2.184(8)
2.191(6)
2.362(7)
2.374(7)
P(11)–Pt(11)–P(12)
Cl(11)–Pt(11)–P(11)
Cl(11)–Pt(11)–P(12)
Cl(12)–Pt(11)–P(11)
Cl(12)–Pt(11)–P(12)
Cl(12)–Pt(11)–Cl(11)
P(21)–Pt(21)–P(22)
Cl(21)–Pt(21)–P(21)
Cl(21)–Pt(21)–P(22)
Cl(22)–Pt(21)–P(21)
Cl(22)–Pt(21)–P(22)
88.4(3)
91.4(3)
177.7(4)
177.6(3)
89.9(3)
90.4(3)
87.4(2)
90.6(2)
177.9(3)
176.9(3)
89.9(3)
Entry
Substrate
Solvent
Ee
1
2
3
4
5
6
7
8
9
10
11
12
13
D
D
D
E
E
E
F
MeOH
CH2Cl2
i-PrOH
MeOH
CH2Cl2
i-PrOH
MeOH
i-PrOH
MeOH
CH2Cl2
i-PrOH
EtOAc
MeOH
12 (S)
38 (S)
33 (S)
42 (R)
42 (R)
39 (R)
94 (R)
86 (R)
95 (R)
70 (R)
86 (R)
78 (R)
96 (R)
F
G
G
G
G
H
Table 3 Hydrogenations of substrates A–C
Entry
Catalyst
Substrate
Ee
1
2
3
4
5
6
3a
3b
3a
3b
3a
3b
A
A
B
B
C
C
81 (R)
51 (R)
46 (R)
43 (R)
82 (S)
55 (S)
and in the Experimental section. The crystal structure of (R,R)-2a
was determined (see Fig. 1 and Table 2 which lists important bond
lengths and angles) and shows square-planar platinum with near
equal Pt–P distances. The methyl group at C(223) blocks the upper
left quadrant adjacent to the site occupied by Cl(22). Indeed the
shortest methyl hydrogen to Cl(22) contact (2.85 Å) is shorter than
the methine H(220)Cl(22) contact (3.05 Å).
Fig. 1 Molecular structure of (R,R)-2a. Hydrogen atoms (other than those
on tertiary carbon atoms) have been omitted for clarity.
There is commercial potential for the products of the asym-
metric hydrogenation of -amino esters D and E or enamides
F–H.12 The results for the hydrogenation of D–H with 3a as
catalyst are collected in Table 4. The enantioselectivities for D
and E are disappointingly low, especially when compared to the
ee’s we obtained with Duphos under similar conditions in MeOH
(79 and 69% for D and E, respectively). The best results obtained
with 3a were for the hydrogenation of the enamides F–H. The
enantioselectivities were superior to the <90% ee we obtained
with Duphos under similar conditions in MeOH. Saito et al.9 also
obtained excellent results with enamides. Our results suggest that
the enantioselectivities are sometimes sensitive to the solvent used;
e.g. see entries 9 and 10 where the ee drops from 95% in MeOH to
70% in CH2Cl2.
Scheme 2
Asymmetric hydrogenations
The rhodium complexes 3a,b were screened for asymmetric
hydrogenation of the dehydroamino acid derivatives A and B
and itaconic acid C and the results are collected in Table 3. The
catalysts were efficient in terms of conversions under mild condi-
tions (see Experimental section for details). For all three substrates,
the enantioselectivities were greater for the phenylene diphos com-
plex 3a than for the ferrocene diphos complex 3b. The absolute
configuration of the product from A was the same as that obtained
with the parent Duphos catalyst but the ee’s with 3a,b were signifi-
cantly inferior to the 98% ee obtained with Duphos under similar
conditions.4
1 9 0 2
D a l t o n T r a n s . , 2 0 0 4 , 1 9 0 1 – 1 9 0 5

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With the enamide substrates F–H, it was noted that the hydro-
genations with 3a as catalyst were completed much more rapidly
than with [Rh(nbd)(Duphos)][BF4] and therefore, we monitored
the reaction progress more closely. Fig. 2 shows plots of the up-
take of H2 with time for substrate F; the rate with 3a was over 10
times faster than with [Rh(nbd)(Duphos)][BF4]. The source of the
higher activity of 3a is presumably the presence of the PPh2 group.
For comparison, the complex [Rh(cod){o-C6H4(PPh2)2}][BF4] was
screened and shown to be more than five times slower than 3a. Thus
it appears that the unsymmetrical catalyst 3a is significantly more
efficient than either of its symmetrical analogues.
spectra were recorded on a JEOL 300 or JEOL GX400 spectrom-
eters; coupling constant values are given in Hz. 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.
Preparation of 1-diphenylphosphino-2-phosphinobenzene
Chlorotrimethylsilane (1.9 cm3, 15 mmol) was added slowly
via syringe to a suspension of LiAlH4 (0.58 g, 15 mmol) in THF
(15 cm3) at −78 °C. The mixture was stirred for 20 min, allowed to
warm to room temperature and then stirred for a further 2 h. After
cooling the mixture to −30 °C, a solution of 1-diethylphosphonato-
2-diphenylphosphinobenzene (2.0 g, 5.0 mmol) in THF (15 cm3)
was added dropwise over 30 min and the resulting mixture was
stirred at room temperature for 18 h, by which time 31P{1H} NMR
spectroscopy indicated the reaction had gone to completion. The
mixture was cooled with an ice–water bath and then water (7.4 cm3)
and 1 M aqueous NaOH (7.4 cm3) added. Diethyl ether (20 cm3) was
added, the organic layer separated and the aqueous layer washed
with diethyl ether (2 × 10 cm3). The combined organic component
was dried over MgSO4 and then the solvent was removed under
reduced pressure to give the product as a cream solid (1.43 g, 97%).
Elemental analysis (%): found (calc.): C 73.4 (73.5), H 5.7 (5.5). EI
mass spectrum: m/z 294 (M+). 31P (CDCl3):  −11.3 (d), −124.7 (td),
1J(PH) 206, 3J(PP) 98. 13C{1H} (CDCl3): three groups of complex
multiplets at  136.9–135.7, 134.0–133.2, 128.8–128.4. 1H (CDCl3):
 3.89 (dd, 2H, PH2, 1J(PH) 205.9, 4J(HH) 12.0); 6.81–6.83 (m, 2H),
7.11–7.29 (m, 10H), 7.48–7.50 (m, 2H).
Fig. 2 Uptake of H2 as a function of time for the hydrogenation of F
catalysed by (i) [Rh(cod)(1a)][BF4]; (ii) [Rh(cod){o-C6H4(PPh2)2}][BF4];
(iii) [Rh(nbd)(Duphos)][BF4].
Quadrant diagrams for Duphos catalysts have been the subject
of debate recently.5,13 They can be used to predict the absolute
configuration of the hydrogenated product provided the substrate is
presumed to bind to the metal in such a way that steric congestion is
minimised (i.e. contrary to Burke’s original proposal4). The crystal
structure of (R,R)-2a discussed above is consistent with the quadrant
diagram proposed by Saito and co-workers.9 and shown in Fig. 3.
Preparation of 1-diphenylphosphino-1′-phosphinoferrocene
Chlorotrimethylsilane (1.8 cm3, 14.2 mmol) was added slowly via sy-
ringe to a suspension of LiAlH4 (0.54 g, 14.2 mmol) in THF (15 cm3)
at −78 °C. The mixture was stirred for 20 min, allowed to warm to
room temperature and then stirred for a further 2 h. After cooling
the mixture to −30 °C, a solution of 1-diethylphosphonato-1′-diphe-
nylphosphinoferrocene (2.4 g, 4.7 mmol) in THF (15 cm3) was added
dropwise over 30 min and the resulting mixture was stirred at room
temperature for 18 h, by which time 31P{1H} NMR spectroscopy in-
dicated the reaction had gone to completion. The mixture was cooled
with an ice–water bath and then water (7.4 cm3) and 1 M aqueous
NaOH (7.4 cm3) added. Diethyl ether (20 cm3) was added, the organic
layer separated and the aqueous layer washed with diethyl ether (2 ×
10 cm3). The combined organic component was dried over MgSO4,
concentrated to ca. 5 cm3 under reduced pressure and then methanol
(8 cm3) was added. The mixture was cooled in a freezer overnight to
give the product as a yellow–orange solid (1.24 g, 86%). Elemental
analysis (%): found (calc.): C 64.8 (65.6), H 5.0 (5.0). EI mass spec-
trum: m/z 402 (M+). 31P (CDCl3):  −17.1 (s), −144.6 (t), 1J(PH) 204.
13C{1H} (CDCl3): three groups of complex multiplets at  133.3–
133.8, 128.6–128.1, 76.6–68.6 (Cp). 1H (CDCl3):  3.56 (d, 2H, PH2,
1J(PH) 203.7); 3.87–4.32 (m, 8H, Cp), 7.19–7.37 (m, 10H).
Fig. 3 Quadrant diagrams for complexes of Duphos and 1a.9
From this, the configurations of the products from A–H can be
rationalised if A–H bind to the metal via the re-face of the alkene.
While this model is predictive, it is at odds with the observation that
a Halpern-type mechanism (i.e. that product formation occurs from
the less stable diastereoisomer of the substrate adduct) is known to
operate for Duphos catalysts.14
Preparation of 1-[(2R,5R)-2,5-dimethylphospholano]-2-
diphenylphosphinobenzene (1a)
Experimental
1-Diphenylphosphino-2-phosphinobenzene (1.10 g, 3.74 mmol)
was dissolved in THF (45 cm3) and n-BuLi (2.3 cm3, 3.74 mmol,
1.6 M solution in hexane) was added dropwise at room temperature.
The deep red solution was stirred for 90 min, after which time a so-
lution of (2S,5S)-2,5-hexanediol cyclic sulfate (0.67 g, 3.74 mmol)
in THF (6 cm3) was added dropwise giving an orange solution. The
mixture was then stirred for 2 h, and then another portion of n-BuLi
(2.55 cm3, 5.05 mmol, 1.6 M solution in hexane) was added drop-
wise, again giving a deep red solution. After stirring for a further
2 h, any excess n-BuLi was quenched with methanol (2 cm3) and
the yellow–orange mixture was filtered through Celite to remove the
gelatinous precipitate. The yellow filtrate was concentrated to 5 cm3
and pentane (25 cm3) was added. The white precipitate was filtered
off and the solvent removed to give the product as a viscous white oil
Unless otherwise stated, all reactions were carried out under a
nitrogen atmosphere using standard Schlenk-line techniques. Dried,
nitrogen-saturatedsolventswerecollectedfromaGrubbssystem15 or
obtained by refluxing under a nitrogen atmosphere over appropriate
drying agents: calcium hydride (for CH2Cl2), sodium/benzophenone
(for diethyl ether and THF). After purification, all phosphines were
stored under nitrogen at room temperature. The complexes were
stable to air in the solid state and were stored in air at room tempera-
ture. The starting materials prepared by literature methods were:
[PtCl2(cod)],16 [Rh(nbd)2][BF4],17 [Rh(cod)2][BF4],18 and the N-
acetyl--arylenamide substrates.19 1-Bromo-2-diphenylphosphino-
benzene was prepared using the palladium-catalysed cross-coupling
reaction reported by Stille et al.20 31P{1H}, 13C{1H} and H NMR
1
D a l t o n T r a n s . , 2 0 0 4 , 1 9 0 1 – 1 9 0 5
1 9 0 3

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(0.9 g, 64%). EI mass spectrum: m/z 376 (M+). 31P (CDCl3):  −12.1
(d), −0.1 (d), 3J(PP) 159.2. The optical purity of 1a was determined
to be >99.5% by a 31P NMR method: reaction with the R-enantiomer
of the orthometallated complex21 [Pd2Cl2(Me2NCHMeC6H4)2] gave
a product for which a single pair of two doublets at  72.0 and 41.7,
2J(PP) 22, were observed; a different set of doublets ( 73.5 and
collected to afford the product as an orange–brown solid (385 mg,
75%). FAB mass spectrum: m/z 571 [(M+) − BF4]. Elemental analysis
(%): found (calc): C 53.9 (54.0), H 5.0 (5.0). 31P (CDCl3):  75.7 (dd,
1J(RhP) 160), 60.0 (dd, 1J(RhP) 158), 2J(PP) 27. 1H (CDCl3):  1.0
(dd, 3H, CH3, J(HH) 7.2, J(PH) 15.4), 1.4 (dd, 3H, CH3, J(HH)
7, 3J(PH) 19.1), 1.51 (m, 1H, CH), 1.83 (d, 1H, nbd), 1.96 (d, 1H,
nbd), 2.36 (m, 1H, CH), 2.38–2.46 (m, 2H, CH2), 2.69–2.78 (m, 2H,
CH2), 4.25 (s, 1H, nbd), 4.29 (s, 1H, nbd), 5.07 (s, 1H, nbd), 5.33 (s,
1H, nbd), 5.96 (m, 2H, nbd), 7.35–7.78 (m, 14H, Ph). The complex
[Rh(cod)(1a)][BF4] was made similarly from [Rh(cod)2][BF4].
3
3
3
2
41.0, J(PP) 22) were observed for the diastereoisomeric product
formed when the S-enantiomer was used.
Preparation of 1-[(2R,5R)-2,5-dimethylphospholano]-1′-
diphenylphosphinoferrocene (1b)
Preparation of [Rh(nbd)(1b)][BF4] (3b)
To a solution of 1-diphenylphosphino-1′-phosphinoferrocene
(0.50 g, 1.24 mmol) in THF (20 cm3) was added n-BuLi (0.78 cm3,
1.24 mmol, 1.6 M in hexane) dropwise via syringe and the orange
solution became deep red. After stirring for 90 min, a solution of
(2S,5S)-2,5-hexanediol cyclic sulfate (0.22 g, 1.24 mmol) in THF
(4 cm3) was added dropwise and the mixture was stirred for 2 h. A
second portion of n-BuLi (0.85 cm3, 1.37 mmol, 1.6 M solution in
hexane) was then added, and again the solution became deep red.
After stirring for a further 2 h, the mixture was quenched with meth-
anol (2 cm3), and filtered through Celite. The filtrate was concen-
trated to 5 cm3, pentane (20 cm3) added and the mixture was placed
in the freezer overnight. The precipitate was removed by filtration
and the solvent removed in vacuo to give the product as an orange
oil (0.4 g, 67%). EI mass spectrum: m/z 484 (M+). 31P (CDCl3):
To a solution of 1b (630 mg, 1.3 mmol) in dichloromethane (50 cm3)
was added [Rh(nbd)2][BF4] (486 mg, 1.3 mmol) and the solution was
stirred overnight. The solution was concentrated to 10 cm3, diethyl
ether (30 cm3) and pentane (10 cm3) were added and, after stirring for
1 h, the precipitate was collected to afford the product as an orange–
brown solid (735 mg, 74%). FAB mass spectrum: m/z 679 [(M+) −
BF4]. Elemental analysis (%): found (calc): C 52.7 (52.7), H 5.2 (4.9).
31P (CDCl3):  47.3 (dd, 1J(RhP) 158), 25.5 (dd, 1J(RhP) 162), 2J(PP)
28. 1H (CDCl3):  0.89 (dd, 3H, CH3, 3J(HH) 7.3, 3J(PH) 14.8), 1.26
(m, 1H, CH), 1.38 (d, 1H, nbd), 1.6 (d, 1H, nbd), 1.81 (m, 2H, CH2),
1.88 (dd, 3H, CH3, 3J(HH) 7.1, 3J(PH) 19.6), 1.93 (m, 1H, CH), 2.30
(m, 1H, CH2), 2.96 (m, 1H, CH2), 11 broad singlets at  3.72, 3.92,
4.03, 4.21, 4.32, 4.39, 4.45, 4.63, 4.69, 4.77, 4.98 (14H, Cp, nbd),
7.29–7.34 (m, 2H, Ph), 7.43 (s, 3H, Ph), 8.05–8.08 (m, 2H, Ph).
1
 −16.7 (s), −1.1 (s). H (CDCl3):  0.60 (dd, 3H, CH3), 0.80 (m,
1H, CH), 1.18 (m, 1H, CH), 1.25 (dd, 3H, CH3), 1.70–1.86 (m, 2H,
CH2), 2.01–2.14 (m, 1H, CH2), 2.28–2.41 (m, 1H, CH2), 3.75 (s, 1H,
Cp), 3.97 (s, 1H, Cp), 4.01 (m, 1H, Cp), 4.07–4.13 (m, 3H, Cp), 4.27
(s, 1H, Cp), 4.33 (s, 1H, Cp), 7.19–7.32 (m, 10H, Ph).
Standard experimental procedure for asymmetric
hydrogenation of substrates A–C
A50 cm3 glass vessel was placed in the steel autoclave and the reac-
tor was sealed. The reactor was deoxygenated three times with nitro-
gen, evacuated and then sealed under vacuum and the temperature of
the reactor was set to the required value. A methanolic (10 cm3) so-
lution containing [Rh(nbd)(1a)][BF4] or [Rh(nbd)(1b)][BF4] (0.02
mmol), the substrate (2 mmol) and triethylamine (0.28 ml, 2 mmol)
was syringed into the glass vessel. Apressure of 1 bar hydrogen was
applied, the reactor sealed and the solution stirred for 20 h. The pres-
sure was then released, the reactor opened and the reaction mixture
worked-up as follows. The solvent was removed from the solution
taken from the autoclave by rotary evaporation and the residue was
cooled in an ice-bath and dissolved in NaOH (5 cm3, 0.5 M solution).
The insoluble material was filtered off and the filtrate was acidified
with 2 M HCl. The solution was extracted with diethyl ether (3 ×
5 cm3) and the combined organic fractions dried over MgSO4. The
solvent was removed by rotary evaporation to give the product as a
white solid. For substrate A, the ee of the product was determined by
chiral GC (L-Chiracel-Val from Chrompack, 300 °C). For substrates
B and C the ee of the products were calculated from polarimetry by
comparison of the specific rotation of the product with literature
values of an authentic sample (for N-acetyl-(R)-phenylalanine B,
[]D26 = −51.8 (c = 1, solvent = EtOH); for (R)-(−)-methyl succinic
acid C, []D20 = +16.88 (c = 2.16, solvent = EtOH)).
Preparation of [PtCl2(1a)] (2a)
A solution of 1a (90 mg, 0.24 mmol) and [PtCl2(cod)] (90 mg, 0.24
mmol) in dichloromethane (5 cm3) was stirred overnight. Diethyl
ether (10 cm3) was added to the yellow solution and the precipitate
collected to give the product as a cream solid (120 mg, 78%). FAB
mass spectrum: m/z 607 [(M+) − Cl]. Elemental analysis (%): found
1
(calc): C 41.5 (41.3), H 4.1 (3.9). 31P (CDCl3):  66.4 (d, J(PtP)
1
2
3666), 42.0 (d, PPh2, J(PtP) 3468), J(PP) 3). 13C{1H} (CDCl3):
1
 134.1–131.8 (m, Ph), 129.0–131.8 (m, Ph), 40.9 (d, CH, J(CP)
38.4), 36.8 (d, CH, 1J(CP) 37.9), 35.8 (d, CH2, 2J(CP) 4.6), 31.0 (s,
CH3), 17.0 (d, CH2, 2J(CP) 4.4), 13.9 (s, CH3). 1H (CDCl3):  0.95
(dd, 3H, CH3, 3J(HH) 7.1, 3J(PH) 19.4), 1.48 (dd, 3H, CH3, 3J(HH)
6.8), 3J(PH) 19.4), 1.67–1.80 (m, 1H, CH2), 2.18–2.79 (m, 4H, CH2/
CH), 3.59–3.63 (m, 1H, CH), 7.42–7.80 (m, 14H, Ph).
Preparation of [PtCl2(1b)] (2b)
A solution of 1b (95 mg, 0.2 mmol) and [PtCl2(cod)] (73 mg, 0.2
mmol) in dichloromethane (10 cm3) was stirred overnight. The so-
lution was then concentrated to ca. 2 cm3 and diethyl ether (6 cm3)
was added. The precipitate was collected to give the product as an
orange solid (120 mg, 82%). Elemental analysis (%): found (calc):
1
C 45.1 (44.8), H 4.0 (4.1). 31P (CDCl3):  36.8 (d, J(PtP) 3664),
Standard experimental procedure for asymmetric
hydrogenation of substrates D–H
10.3 (d, 1J(PtP) 3807), 2J(PP) 12. 13C{1H} (CDCl3):  136.4–127.6
(m, Ph), 75.8–70.2 (m, Cp), 35.0 (d, CH, 1J(CP) 38.9), 33.8 (d, CH,
1J(CP) 39.1), 33.4 (s, CH2), 22.1 (s, CH2), 15.3 (s, CH3), 13.9 (s,
Catalyst [Rh(cod)(1a)][BF4] (0.010 mmol) and the substrates (1.0
mmol) were mixed in glass vessels. Up to eight vessels were placed
in the Endeavor™ parallel screening apparatus and flushed with ni-
trogen. The nitrogen-saturated solvent was added (5.0 cm3) and the
apparatus sealed. The vessels were purged 10 times with nitrogen
and once with hydrogen. The autoclave was 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
1
3
3
CH3). H (CDCl3):  0.98 (dd, 3H, CH3, J(HH) 7, J(PH) 17.1),
1.37 (dd, 3H, CH3, 3J(PH) 20.1), 1.45 (m, 2H, CH2), 1.88–1.97 (m,
2H, CH2), 2.29 (m, 1H, CH), 2.70 (m, 1H, CH), 3.90 (s, 1H, Cp),
4.28–4.38 (m, 4H, Cp), 4.53 (s, 1H, Cp), 4.63 (s, 1H, Cp), 4.90 (s,
1H, Cp), 7.15–7.2 (m, 1H, Ph), 7.39–7.47 (m, 4H, Ph), 7.6–7.64 (m,
1H, Ph), 7.78–7.84 (m, 2H, Ph), 8.03–8.1 (m, 2H, Ph).
1
catalyst. The conversion was measured by H NMR spectroscopy
Preparation of [Rh(norbornadiene)(1a)][BF4] (3a)
and ee determination by chiral HPLC (for H) using a Chiralpak AD
250 × 4.6 mm column; n-hexane–i-PrOH (92:8, v/v) or chiral GC
(for D–G) using a CP Chiracel-Dex CB column from Chrompack
(25 m × 0.25 mm × 0.25 m), operated at 150 °C or 160 °C. Race-
mates were used to check the technique. Absolute configurations
To a solution of 1a (220 mg, 0.58 mmol) in CH2Cl2 (8 cm3) was added
[Rh(nbd)2][BF4] (219 mg, 0.58 mmol) and the red solution was al-
lowed to stir overnight. Addition of diethyl ether (15 cm3) resulted in
the formation of a precipitate and after stirring for 1 h, the solid was
1 9 0 4
D a l t o n T r a n s . , 2 0 0 4 , 1 9 0 1 – 1 9 0 5

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were determined by comparison with reference compounds and
literature values.22 Retention times for the products from substrate
D were 11.08 and 11.27 min, from substrate E were 37.50 and 38.10
min, from substrate F were 5.1 and 5.5 min, from substrate G were
6.3 and 6.6 min and from substrate H were 22 and 27 min.
References
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X-Ray crystal structure of (R,R)-2a. X-Ray diffraction
experiments on (R,R)-2a as its dichloromethane solvate were
carried out at −100 °C on a Bruker SMART diffractometer using
Mo-K X-radiation,  = 0.71073 Å. Crystal and refinement data
for (R,R)-[PtCl2{(MeC4H6Me)PC6H4PPh2}]·CH2Cl2, 2a·CH2Cl2:
C25H28Cl4P2Pt, M = 727.3, triclinic, space group P1 (no. 1),
a = 10.057(3), b = 10.276(3), c = 14.575(4) Å,  = 92.175(5),
 = 102.839(5),  = 114.204(4)°, V = 1325.4(6) Å3, Z = 2,
 = 5.83 mm−1, T = 173(2) K, 8686 unique data, Rint = 0.034, R1
(for all 7884 data with F2 > 2(F2)) = 0.059.Absorption corrections
were based on equivalent reflections and structures refined against
2
all Fo data with hydrogen atoms riding in calculated positions.
The analysis is hampered by the presence of a local near-centre of
symmetry between the two independent molecules in the P1 unit
cell. The PtCl2PC6H4PPh2 units of the two molecules are essentially
exactly related by this pseudo-inversion centre. The resultant refine-
ment is consequently rather unstable and has large difference elec-
tron features close to the platinum atoms presumably resulting from
poor absorption correction that is not in this case concealed by, e.g.
the anisotropic displacement parameters of those atoms.Alternative
refinement models with S,S-configuration of the phospholane or P1
symmetry gave significantly worse residuals.
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See http://www.rsc.org/suppdata/dt/b4/b404827j/ for crystallo-
graphic data in CIF or other electronic format.
Acknowledgements
We would like to thank DrA. H. M. de Vries and Dr J. A. F. Boogers
for useful discussions, 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.
D a l t o n T r a n s . , 2 0 0 4 , 1 9 0 1 – 1 9 0 5
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