A versatile approach to planar chiral monophosphane ligands

Article abstract of DOI:10.1055/s-2006-950291

The enantioselective reaction of a chiral bisamide and an electrophile with benzyl ether complexes of arene tricarbonyl chromium(0) was used to introduce the asymmetry into a synthesis of novel enantiopure planar chiral ligands. Elaboration of the resulting ether complexes [η⁶-4-t-BuC ₆H₄CHE¹(OR)]Cr(CO)₃ via a diastereo-selective ortholithiation/chlorophosphane quench protocol led to the synthesis of planar chiral monophosphane complexes with high enantiomeric purity (typically 96-98% ee) for assessment in the palladium-catalyzed asymmetric hydrosilylation of styrene. The identity of benzylic substituent E¹ (installed using the chiral base reaction) was found to have a significant effect on the enantioselectivity of resulting catalysts. Removal of the tricarbonylchromium(0) unit from the structurally optimized phosphane ligand was accomplished using a high-yielding borane protection-aerial oxidation-deprotection strategy. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-950291

PAPER
3631
A Versatile Approach to Planar Chiral Monophosphane Ligands
P
lana
r
Chiral
M
u
onophosph
s
ane
L
igan
a
ds n E. Gibson,* Jacob T. Rendell, Matthew Rudd
Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
Fax +44(207)5945804; E-mail: s.gibson@imperial.ac.uk
Received 21 August 2006; revised 29 August 2006
Dedicated to Professor Marty Semmelhack on the occasion of his 65th birthday
E1
HR
i) (+)-3 (1 equiv),
BuLi (2 equiv)
Abstract: The enantioselective reaction of a chiral bisamide and an
electrophile with benzyl ether complexes of arene tricarbonyl chro-
mium(0) was used to introduce the asymmetry into a synthesis of
novel enantiopure planar chiral ligands. Elaboration of the resulting
ether complexes [h6-4-t-BuC6H4CHE1(OR)]Cr(CO)3 via a diastereo-
selective ortholithiation/chlorophosphane quench protocol led to
the synthesis of planar chiral monophosphane complexes with high
enantiomeric purity (typically 96–98% ee) for assessment in the
palladium-catalyzed asymmetric hydrosilylation of styrene. The
identity of benzylic substituent E1 (installed using the chiral base re-
action) was found to have a significant effect on the enantioselectiv-
ity of resulting catalysts. Removal of the tricarbonylchromium(0)
unit from the structurally optimized phosphane ligand was accom-
plished using a high-yielding borane protection–aerial oxidation–
deprotection strategy.
HS
OR
90–99% yield
96–99% ee
HS
OR
Cr(CO)3
ii) E1X
Cr(CO)3
1
2
Ph
NH HN
(+)-3
Ph
Ph
Ph
Scheme 1
thesis of a range of bisphosphane complexes,8 which were
examined in an asymmetric hydrogenation and asymmet-
ric Heck reaction.9
Key words: arene complexes, asymmetric synthesis, chromium,
hydrosilylation, phosphane
E1
E1
OR
i) LiTMP
ii) E2X
tBu
tBu
55–98% yield
96–99% ee
99% de
OR
E2
The ability to modulate the steric and electronic properties
of chiral ligands is a highly desirable feature in asymmet-
ric catalysis. Many of the ‘privileged ligands’,1 such as the
Josiphos family of planar chiral ferrocene ligands, have
undergone structural modifications in order to give in-
creased activity and/or selectivity across a range of cata-
lytic reactions and substrates.2 As a result of the striking
success of the Josiphos ligands,3 there has been increasing
interest in planar chiral arene tricarbonyl chromium(0)
complexes with similar stereochemistry.4
Cr(CO)3
Cr(CO)3
5
4
Scheme 2
In light of the ongoing interest in the use of monophos-
phanes in asymmetric catalysis,10 we envisaged that
monophosphane complexes derived from the route devel-
oped within our group would merit an investigation in the
asymmetric hydrosilylation (AHS) reaction, one of the
benchmark reactions of monophosphane ligated cataly-
sis.11 To date there has been just one report of the use of
arene tricarbonyl chromium based ligands in this reac-
tion.12
Some time ago we showed that tricarbonylchromium(0)
complexes of dibenzyl or alkyl benzyl ethers, such as 1,
give chiral ether complexes 2 in high yield and enantio-
meric excess upon treatment with n-butyllithium and a
chiral diamine 3, followed by quenching with an electro-
phile (Scheme 1).5 Use of the (+)-R,S,S,R-enantiomer of
the base, leads to preferential replacement of the pro-R
benzylic proton. (Recently we reported the use of (+)-3 to
synthesize enantiopure C2- and C3-symmetric chromium
complexes.6)
The aims of the study described herein were to test wheth-
er or not monophosphanes derived via a route based on the
two reactions depicted in Schemes 1 and 2 would sustain
the AHS reaction, and to fully utilize the flexible nature of
the ligand synthesis in order to optimize promising ligand
structures. We were particularly interested in answering
the following questions:
Developing this methodology further we showed that tert-
butyl complex 4 undergoes a diastereoselective ortho-
lithiation/electrophilic quench sequence to give access to
planar chiral complexes 5 with high stereochemical purity
(Scheme 2).7 The use of phosphane electrophiles in both
the chiral base and ortholithiation steps allowed the syn-
What is the optimum position for the primary phosphorus
donor atom (E1 or E2 in Scheme 2)?
Does placing secondary donor groups in the ligand frame-
work affect the performance of the resulting catalysts?
Does variation of the ether group OR alter the potency/se-
lectivity of the ligands?
SYNTHESIS 2006, No. 21, pp 3631–3638
x
x.
x
x
.
2
0
0
6
How important is the presence of planar chirality in the
ligand to the enantioselectivity of resulting catalysts?
Advanced online publication: 09.10.2006
DOI: 10.1055/s-2006-950291; Art ID: Z17106SS
© Georg Thieme Verlag Stuttgart · New York

3632
S. E. Gibson et al.
PAPER
To start to address the first two questions, six ligand can-
didates 6–11 (Figure 1) were targeted and their perfor-
mance in AHS chemistry was probed.
Table 2 Introduction of Phosphanes and the Synthesis of Ligands 7–
11
E1
E1
i) LiTMP, –78 °C, 1 h
tBu
tBu
OMe
OMe
ii) ClPR2,
PR2
PPh2
OMe
PPh2
OMe
tBu
tBu
tBu
–78 °C (2 h) to r.t. (1 h)
Cr(CO)3
Cr(CO)3
OMe
PPh2
PPh2
Entry
S.M.
E1
PR2
Product Yield (%)ee (%)
Cr(CO)3
Cr(CO)3
Cr(CO)3
8
6
7
1
2
3
4
5
(+)-13 Me
PPh2
PPh2
PPh2
(–)-7
(–)-8
(–)-9
88
75
86
75
82
96
96
95
95
96
SPh
Ph
tBu
tBu
tBu
(+)-6
PPh2
SPh
OMe
OMe
PPh2
OMe
PiPr2
PPh2
(–)-14
Cr(CO)3
Cr(CO)3
Cr(CO)3
9
10
11
(+)-13 Me
(+)-15 Bn
P(i-Pr)2 (–)-10
PPh2 (–)-11
Figure 1 Target ligands for assessment in an AHS assay
Phosphanes 6–11 were synthesized using the chiral base
reaction depicted in Scheme 1, followed by the diastereo-
selective deprotonation/electrophilic quench shown in
Scheme 2.8 Chiral amine (+)-3 is accessible over two
steps from commercially available (R)-(+)-a-methylben-
zylamine.13 Treatment of the readily available complex
128 with the chiral diamide derived from (+)-3 and
quenching with chlorodiphenylphosphane, iodomethane,
diphenyl disulfide or benzyl bromide gave complexes (+)-
6, (+)-13, (–)-14 and (+)-15 in good yields and enantiopu-
rities (Table 1).14
lution resulted indicating an interaction between the
ligand and palladium. At this point trichlorosilane was
added and the reaction incubated under an inert atmo-
sphere and in the absence of light, for two days. Where
formed, the desired benzylic silane 16 was isolated by
Kugelrohr distillation and then oxidized using the Tamao
method16 to give 1-phenylethanol 17 (Table 3). The opti-
cal purity of 17 was then determined by HPLC analysis.
All assays were carried out twice, and gave results which
typically varied by 1–2% for the hydrosilylation yield,
2–3% for the oxidation step and 0–1% for the ee of the al-
cohol. The average values from the two runs are reported
throughout.
Treatment of complexes (+)-6, (+)-13, (–)-14 and (+)-15
with lithium tetramethylpiperidide at –78 °C and quench-
ing with chlorodiphenylphosphane or chlorodiisopropyl-
phosphane gave the five ligand candidates 7–11 as single
diastereomers in good yield and excellent enantiopurity
(Table 2).
Table 3 AHS/Oxidation Using Ligands 6–11
Pd(COD)Cl2 (0.25 mol%)
Ligand (0.5 mol%)
HSiCl3 (1.5 equiv)
CH2Cl2, r.t., 48 h
The six ligand candidates (+)-6, (–)-7, (–)-8, (–)-9 (–)-10
and (–)-11 were subjected to a catalytic AHS assay. After
some experimentation, a protocol that was similar to the
one used by the group of Johannsen was found to be very
reliable.15 Catalysts were formed in situ by mixing 0.25
mol% of Pd(COD)Cl2, 0.50 mol% of the ligand and 10.0
mmol of styrene in a 2 M solution of dichloromethane for
15 minutes (Table 3). In all cases a red homogeneous so-
KF, KHCO3,
H2O2, r.t., 16 h
SiCl3
OH
16
17
Entry
Ligand
none
Yield 16 (%) Yield 17 (%)a ee (%)a
1
2
3
4
5
6
7
6
83
98
0
n/a
96
n/a
1
(+)-6
(–)-7
Table 1 Synthesis of Ligand 6 and Precursors to 7–11
97
46
n/a
n/a
0
i) (+)-3 (1 equiv), BuLi (2 equiv),
LiCl (1 equiv), –78 °C, 30 min
E1
(–)-8
n/a
n/a
96
tBu
tBu
OMe
OMe
ii) E1X, –78 °C, 30–40 min
(–)-9
0
Cr(CO)3
Cr(CO)3
(–)-10
(–)-11
33
98
12
95
55
Entry
E1
Time (min) Product
Yield (%) ee (%)
a n/a = not applicable.
1
2
3
4
PPh2
Me
SPh
Bn
30
30
40
30
(+)-6
94
90
97
87
96
96
95
96
(+)-13
(–)-14
(+)-15
The results for ligand candidates 6–11 are summarized in
Table 3. A control experiment in the absence of ligand
was performed (entry 1) and led to a 6% conversion of sty-
rene, underlining the need for ligand additives in order to
obtain an active palladium catalyst. Bisphosphane (–)-8
Synthesis 2006, No. 21, 3631–3638 © Thieme Stuttgart · New York

PAPER
Planar Chiral Monophosphane Ligands
3633
gave a palladium complex that was completely inactive
(entry 4). This result is consistent with the findings of Ha-
yashi, who noted that bisphosphanes such as BINAP or
dppe gave palladium complexes that are inactive at tem-
peratures as high as 80 °C.17 In a similar fashion novel
P–S ligand (–)-9 gave an inactive palladium complex (en-
try 5). In contrast, the three monophosphanes (+)-6, (–)-7
and (–)-10 gave palladium complexes which did display
significant activity in the AHS of styrene. Benzylic phos-
phane (+)-6, a precursor to bisphosphane (–)-8 gave a cat-
alyst that produced 16 in 83% yield (entry 2). Upon
Tamao oxidation, however, the resulting alcohol 17 was
found to be essentially racemic.
Table 4 Synthesis of Benzyl Iodides for Electrophilic Quench
Yn
Yn
NaI, CeCl3·7H2O,
MeCN, 80 °C, 16 h
OH
I
Entry
Yn
Product
Yield (%)
74
1
2
3
2,3,5,6-Me4
4-t-Bu
4-MeO
19
20
22
68
mixture
Encouragingly, the catalyst formed from planar chiral
monophosphane (–)-7 gave 16 in 98% yield after 48 hours
(entry 3). Subsequent Tamao oxidation to 17 revealed an
enantioselectivity of 46% ee. Ligand (–)-10, with a similar
structure, but containing a di(isopropyl)phosphane donor
on the complexed arene ring, was much less active giving
only 33% of the silane, which was found to be optically
inactive upon Tamao oxidation (entry 6). Benzylated
complex (–)-11 was tested in the AHS assay and was
found to give a significant improvement over the meth-
ylated complex (–)-7. Silane 17 was obtained in 98% yield
and after Tamao oxidation, the resulting alcohol was
found to have an enantiomeric excess 9% higher than was
found for (–)-7 (55% vs. 46% ee, entries 7 and 3).
enantiomeric excesses between 90 and 97% (Table 5, en-
tries 2, 3 and 5). Complexes (+)-23 and (+)-26 were ob-
tained satisfactorily by employing bromides 18 and 21 in
the chiral base reaction (Table 5, entries 1 and 4).
Table 5 Asymmetric Introduction of a Range of Benzyl Groups
Yn
i) (+)-3 (1 equiv), BuLi (2 equiv)
LiCl (1 equiv), –78 °C, 30 min
tBu
tBu
OMe
ii) E1X, –78 °C, 30 min
OMe
Cr(CO)3
Cr(CO)3
12
Entry
Yn
E1X
Product
(+)-23
(+)-24
(+)-25
(+)-26
(+)-27
Yield (%) ee (%)
In view of the catalytic activity and selectivity exhibited
by ligand 11, the effect of substituting its benzyl group
was examined next. For the synthesis of ligands with in-
creased steric demand around the benzyl ring, benzyl ha-
lides 18, 19 and 20 were required, and electrophilic
reagents 21 and 22 were needed to probe the role of elec-
tronic effects (Figure 2). The two substituted benzyl bro-
mides (18 and 21) are commercially available whereas
benzyl iodides 19, 20 and 22 needed to be synthesized.
1
2
3
4
5
3,5-Me2
18
99
73
88
75
80
96
97
90
95
95
2,3,5,6-Me4 19
4-t-Bu
4-F
20
21
22
4-MeO
Complexes 23–27 were subjected to the established
ortholithiation protocol in order to provide monophos-
phane ligand candidates with differing properties around
the benzyl group (Table 6). Good yields were obtained
(81–90%) and the enantiomeric excesses of the products
28–32 were the same as those of their respective starting
materials.
tBu
F
OMe
I
Br
Br
I
I
19
20
21
22
18
Figure 2 Benzyl halide electrophiles with a range of steric and elec-
tronic properties
Monophosphane ligand candidates 28–32 were subse-
quently screened in the AHS of styrene. From the results
(Table 7, entries 1–5) it appeared that the steric properties
of the substituents situated on the benzyl ring do have an
effect on the observed enantioselectivity of resulting cat-
alysts. 3,5-Dimethylbenzyl ligand (–)-28 was the most se-
lective ligand giving the product in 73% ee (entry 1), a
significant improvement over the 55% ee observed for
(–)-11, which possesses an unsubstituted benzyl group.
However, 2,3,5,6-tetramethyl substituted ligand (–)-29
was less selective, giving the product benzyl alcohol 17 in
only 42% ee (entry 2). Ligand (–)-30 with a tert-butyl
group in the para-position, gave selectivity at a similar
level to (–)-28 (69% ee, entry 3). Overall these results sug-
Iodides 19, 20 and 22 were obtained via the Lewis acid as-
sisted iodination of the corresponding benzyl alcohols.18
Iodides 19 and 20 were obtained in good yields (Table 4),
but the synthesis of 4-methoxybenzyl iodide 22 proved
more problematic, due to its marked instability, and it was
not possible to isolate an analytically pure sample.
Although benzyl iodides 19, 20 and 22 were unstable, it
proved possible to use them in the chiral base reaction by
adding them to the reaction immediately after purifica-
tion. This gave the required ortholithiation substrates (+)-
24, (+)-25 and (+)-27 in yields between 73 and 88% and
Synthesis 2006, No. 21, 3631–3638 © Thieme Stuttgart · New York

3634
S. E. Gibson et al.
PAPER
Table 6 Introduction of Phosphane onto 23–27
Table 8 Introduction of New Ether Substituents
Yn
i) NaH, 0 °C, 30 min
Yn
tBu
tBu
OR
Cr(CO)3
OH
ii) RBr, r.t., 3 h
i) LiTMP, –78 °C, 1 h
tBu
tBu
Cr(CO)3
OMe
OMe
ii) ClPPh2,
–78 °C (2 h) to r.t. (1 h)
PPh2
12
Cr(CO)3
Cr(CO)3
Entry
1
R
Product
Yield (%)
Entry
Yn
S.M.
Product
(–)-28
(–)-29
(–)-30
(–)-31
(–)-32
Yield (%) ee (%)
Bn
33
34
97
94
1
2
3
4
5
3,5-Me2
(+)-23
90
83
86
87
81
96
97
90
95
95
2a
ArCH2
2,3,5,6-Me4 (+)-24
a Ar = 3,5-dimethylphenyl.
4-t-Bu
4-F
(+)-25
(+)-26
(+)-27
Given that variation of E1 had been shown to have a sig-
nificant effect on the selectivity of the AHS, attention then
turned to a study of the effect of varying the adjacent ether
substituent OR (Scheme 2, structure 5). The ether groups
chosen for initial study were tert-butyl, phenyl and ben-
zyl, but only the benzyl ether proved to be a good sub-
strate for the chiral base reaction. As increasing steric
hindrance around E1 had enhanced selectivity, a 3,5-di-
substituted benzyl ether ligand was also targeted. To com-
mence the synthesis of the two assay candidates, benzyl
ethers 33 and 34 were generated from complex 12
(Table 8).
4-MeO
gest that steric bulk around the 3, 4 and 5 positions, but not
the 2 and 6 positions of the benzyl ring improved the se-
lectivity.
Ligand (–)-31 with an electron-poor benzyl ring due to a
fluorine atom in the 4-position delivered silane 16 in 97%
yield, which upon oxidation to 17 was found to have 62%
ee (entry 4). Monophosphane (–)-32, with an electron-rich
benzyl ring due to a 4-methoxy group gave 60% ee (entry
5) under identical conditions. Comparing the enantiose-
lectivity of the palladium catalysts derived from (–)-31
and (–)-32 with that derived from the parent benzyl ligand
(–)-11 (55% ee, Table 3, entry 7), suggests that the elec-
tronic properties of the benzyl ring are not decisive in the
selectivity of resulting catalysts.
Benzyl ethers 33 and 34 both reacted smoothly with the
chiral base and a corresponding benzyl electrophile at
–78 °C, to give (+)-35 and (+)-36 in good yield and excel-
lent enantiomeric purity (Table 9).
Table 9 Elaboration of Ether Complexes where R = Bn and ArCH2
i) (+)-3 (1 equiv), BuLi (2 equiv),
LiCl (1 equiv), –78 °C, 30 min
E1
tBu
Table 7 AHS Oxidation Using Ligands 28–32, 37, 38 and 40
tBu
OR
OR
ii) E1X, –78 °C, 30 min
Pd(COD)Cl2 (0.25 mol%)
Ligand (0.5 mol%)
HSiCl3 (1.5 equiv)
Cr(CO)3
Cr(CO)3
KF, KHCO3,
H2O2, r.t., 16 h
SiCl3
OH
Entry
1
R
E1
S.M.
Product Yield (%) Ee (%)
CH2Cl2, r.t., 48 h
Bn
Bn
33
(+)-35
(+)-36
88
88
97
98
16
17
2a
ArCH2 ArCH2 34
Entry
Ligand
Yield 16 (%) Yield 17 (%) ee (%)
a Ar = 3,5-dimethylphenyl.
1
2
3
4
5
6
7
8
(–)-28
(–)-29
(–)-30
(–)-31
(–)-32
(–)-37a
(–)-38a
(+)-40b
99
91
93
96
94
95
94
96
93
73
42
69
62
60
65
70
28
98
99
97
99
96
98
98
Subjecting ether complexes (+)-35 and (+)-36 to the
ortholithiation step resulted in the isolation of two novel
monophosphane complexes (–)-37 and (–)-38 in modest
yield and high enantiopurity (Table 10). The yields were
lower than those obtained for the corresponding methyl
ether complexes, presumably as a result of the increased
steric bulk in these systems.
Ligand candidates (–)-37 and (–)-38 were progressed to
the AHS assay. In the case of the dibenzyl monophos-
phane (–)-37 an improvement in the selectivity of the re-
sulting catalyst was observed (65% ee, Table 7, entry 6)
over its analogous methyl ether (–)-11 (55% ee, Table 3,
entry 7). However, for monophosphane (–)-38, containing
a For synthesis of 37 and 38, see Table 10.
b For synthesis of 40, see Scheme 3.
Synthesis 2006, No. 21, 3631–3638 © Thieme Stuttgart · New York

PAPER
Planar Chiral Monophosphane Ligands
3635
Table 10 Synthesis of Monophosphanes 37 and 38
4 steps
tBu
82% yield
E1
E1
i) LiTMP, –78 °C, 1 h
tBu
tBu
tBu
OR
OH
OMe
PPh2
OR
ii) ClPPh2,
PPh2
Cr(CO)3
–78 °C (2 h) to r.t. (1 h)
Cr(CO)3
Cr(CO)3
(–)-28
Entry
1
R
E1
S.M.
Product Yield (%) ee (%)
i) BH3·THF, r.t., 1.5 h
ii) air and light
Bn
Bn
(+)-35 (–)-37
48
53
97
98
94% yield
2a
ArCH2 ArCH2 (+)-36 (–)-38
DABCO
a Ar = 3,5-dimethylphenyl.
tBu
tBu
OMe
3 h, 50 °C
OMe
PPh2
PPh2
99% yield
H3B
a 3,5-dimethylbenzyl group at both positions R and E1 the
enantioselectivity of 70% ee observed in product 17
(Table 7, entry 7) represented no improvement over the
analogous methyl ether (–)-28, which gave 73% ee
(Table 7, entry 1).
(+)-40
(+)-39
Scheme 3
was envisaged that lowering the reaction temperature
would have a beneficial effect on the enantioselectivity.
Monophosphane (–)-28 was used under the standard con-
ditions, except a reaction temperature of 0 °C was main-
tained over the 48 hours incubation period. After this time
To investigate the role of planar chirality further, the chro-
mium-free analogue of optimal ligand (–)-28 was targeted
(Scheme 3). Removal of the chromium moiety was
achieved using a protection–oxidation–deprotection strat-
egy in order to shield the air-sensitive phosphane donor of
(–)-28 during the oxidation of the tricarbonylchromium
moiety. The phosphane donor of (–)-28 was converted to
its borane, a strategy commonly employed to protect air-
sensitive phosphanes.19 (Borane protected phosphanes
can also be deprotected in situ for use in a catalytic reac-
tion.20) Stirring (–)-28 with a slight excess of BH3·THF in
the absence of air and light led to complete consumption
of starting material. The crude phosphane-borane com-
plex was then exposed to both air and light in an open ves-
sel, which resulted in complete oxidation of the
tricarbonylchromium moiety after 16 h to give the air- and
light-stable borane (+)-39 as a fluffy white solid in 94%
yield (Scheme 3). Cleavage of the phosphorus–boron
bond was achieved via an aminolysis protocol using
DABCO (1,4-diazabicyclooctane).21 Borane removal pro-
ceeded smoothly at 50 °C giving, after careful flash col-
umn chromatography, the desired phosphane (+)-40 as an
air-sensitive oil in almost quantitative yield (99%).
a
1H NMR spectrum of the crude product recorded
showed only styrene, indicating that the catalyst was com-
pletely inactive at this lower temperature (Table 11, entry
3). It is postulated that interaction between palladium and
a second donor atom (either the ether group of (–)-28 or a
second equivalent of phosphane) are too strong at this
temperature to allow the coordination of the styrene sub-
strate and therefore catalysis does not proceed. This is
reminiscent of the behavior of bisphosphanes,17 which are
inactive up to 80 °C, and where a stronger chelating inter-
action is present. It is of note that successful AHS reac-
tions were characterized by a color change of solution
from red to yellow during the reaction. In this unsuccess-
ful run the bright red color observed on mixing the metal
and ligand was maintained throughout the 48 hours incu-
bation period.
Table 11 Effect of Solvent and Temperature on AHS/Oxidation
Using Ligand (–)-28
Chromium-free monophosphane (+)-40 was assayed in
the AHS of styrene under the usual conditions, and led to
complete conversion of styrene after 48 h. Upon Tamao
oxidation to 17 the product was found to have an ee of
28% (Table 7, entry 8), revealing that the planar chirality
plays an important role in the selectivity of the catalyst.
Pd(COD)Cl2 (0.25 mol%)
(–)-28 (0.5 mol%)
HSiCl3 (1.5 equiv)
Solvent, Temp, 48 h
KF, KHCO3,
H2O2, r.t., 16 h
SiCl3
OH
16
17
At this point, the effects of temperature and solvent varia-
tion on reactions based on the optimal ligand (–)-28 were
investigated. Firstly, ligand (–)-28 was utilized in the
AHS at different temperatures to examine the effect on
enantioselectivity. At an elevated reaction temperature of
40 °C, the catalyst retained its activity producing silane 16
in 93% yield after the 48 h incubation (Table 11, entry 2).
Tamao oxidation of 16 gave alcohol 17 with 51% ee. As
this was a lower selectivity than that observed at room
temperature with (–)-28 (73% ee, Table 11, entry 1), it
Entry
Solvent
Temp (°C) Yield 16 Yield 17 ee
(%)
99
93
0
(%)
(%)
1
2
3
4
CH2Cl2
CH2Cl2
CH2Cl2
benzene
r.t.
40
0
91
73
92
51
n/a
95
n/a
77
r.t.
97
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cooled solution of complex 12 (0.628 g, 2.00 mmol) in THF (6 mL).
After 30 min, 3,5-dimethylbenzyl bromide (0.797 g, 4.00 mmol) in
THF (4 mL) was added via a cannula and stirring continued for a
further 30 min before the addition of MeOH (2 mL) to quench the
reaction. The crude residue was purified by flash column chroma-
tography (silica gel; hexane–Et2O, 98:2 → 80:20) to afford (+)-23
as a yellow solid (0.830 g, 99%, 96% ee as determined by HPLC);
mp 75–76 °C; [a]D20 +39 (c = 0.030, CHCl3); Rf 0.35 (hexane–Et2O,
9:1).
The role of the reaction solvent was also probed. The use
of benzene as a solvent has led to dramatic improvements
in enantioselectivity in some systems22 and small im-
provements for others.23 Ligand (–)-28 was applied to the
AHS of styrene using the standard conditions, except ben-
zene rather than CH2Cl2 was used as the solvent. The high
catalytic activity was maintained and in addition the enan-
tioselectivity rose to 77% ee (entry 4) for the oxidation
product 17.
HPLC: Chiralcel OD-H; n-hexane–propan-2-ol (99:1); flow rate,
0.3 mL/min; detection at 330 nm; S-enantiomer tR 25.5 min (minor);
R-enantiomer tR 26.7 min (major).
IR (CH2Cl2): 1883 (C≡O), 1762 cm–1 (C≡O).
1H NMR (500 MHz, CDCl3): d = 1.29 [s, 9 H, C(CH3)3], 2.26 (s, 6
H, CArCH3), 2.83 (dd, J = 5.0, 13.5 Hz, 1 H, CHCHH), 2.98 (dd,
J = 6.2, 13.5 Hz, 1 H, CHCHH), 4.06 (virt t, J = 5.5 Hz, 1 H,
CHCH2), 4.87 (d, J = 6.9 Hz, 1 H, CCrH), 5.35 (d, J = 6.9 Hz, 1 H,
CCrH), 5.39 (d, J = 6.8 Hz, 1 H, CCrH), 5.47 (d, J = 6.8 Hz, 1 H,
CCrH), 6.69 (s, 2 H, CArH), 6.86 (s, 1 H, CArH).
13C NMR (125 MHz, CDCl3): d = 21.2 (CArCH3), 31.1 [C(CH3)3],
33.9 [C(CH3)3], 44.0 (CHCH2), 58.5 (OCH3), 82.0 (CHCH2), 89.5,
90.0, 90.6, 91.0 (CCrH), 112.4 (CCrCH), 123.1 [CCrC(CH3)3], 127.6,
128.2 (CArH), 136.8, 137.7 (CAr), 233.6 (C≡O).
This study has shown that members of a novel class of
enantiopure monophosphanes, synthesized by a simple
and versatile route, are active in the palladium-catalyzed
AHS reaction. The flexibility of the ligand synthesis, and
in particular the way that the chiral base reaction allows
variation at the benzylic position, has facilitated the en-
hancement of the initially observed enantioselectivity.
The reduced enantioselectivities obtained with complex 6
and chromium-free phosphane 40 indicate that planar
chirality plays an important role in the outcome of the re-
action.
MS (EI, 70 eV): m/z (%) = 432 (9, [M]+), 348 (100, [M – 3 CO] +)
All reactions involving the use of organometallic compounds were
performed under dry N2 using standard Schlenk techniques.24 Reac-
tions involving the use of arene tricarbonylchromium(0) complexes
were protected from sunlight. The concentrations of alkyllithiums
were determined by titration against diphenylacetic acid in THF.25
THF was distilled from sodium benzophenone ketyl and used im-
mediately. CH2Cl2 was distilled from CaH2 and also used immedi-
ately. MeOH was distilled from CaH2 and stored under an inert
atmosphere over 3 Å molecular sieves. Chlorodiphenylphosphane
was distilled prior to use and then stored under nitrogen. Tetra-
methylpiperidine was stored over KOH pellets under N2. All other
reagents were used as purchased from commercial sources. Flash
column chromatography was performed using BDH silica gel (70–
330 mm particle size). TLC was performed on Merck TLC plates
coated with Merck silica gel 60.
316 (24, [M – 3 CO – CH3OH]+), 177 (22, [M – Cr(CO)3
CH2C6H4C(CH3)3]+).
–
Anal. Calcd for C24H28CrO4: C, 66.65; H, 6.53. Found: C, 66.71; H,
6.63.
Diastereoselective Ortholithiation; (–)-(1pR,1¢¢R)-Tricarbon-
yl{1-diphenylphosphane-2-[1-methoxy-2-(3,5-dimethylphe-
nyl)ethyl]-5-tert-butylbenzene}chromium(0) [(–)-28]; Typical
Procedure
MeLi (1.48 mL, 1.60 M in Et2O, 2.40 mmol) was added dropwise
to a stirred solution of tetramethylpiperidine (0.40 mL, 2.4 mmol)
in anhyd THF (30 mL) at –78 °C. Complex (+)-23 (0.830 g, 1.98
mmol) in THF (10 mL) was added via a cannula and the reaction
stirred for 1 h before chlorodiphenylphosphane (1.09 mL, 5.90
mmol) was added in one portion. The reaction was then stirred for
2 h at –78 °C followed by 1 h at r.t. before MeOH (2 mL) was added
and the solvent removed. Purification of the residue by flash column
chromatography (hexane–Et2O, 99:1 → 90:10) gave (–)-28 as a yel-
low foam (1.099 g, 90%, 96% ee as determined by HPLC); mp 59–
61 °C; [a]D20 –293 (c = 0.011, CHCl3); Rf 0.15 (hexane–Et2O, 9:1).
Melting points were recorded on a Büchi 510 melting point appara-
tus in open capillaries and are uncorrected. Optical rotations were
recorded on a PerkinElmer 241 Polarimeter using a 1 dm path
length and with concentrations given as g/mL. IR spectra were re-
corded on a PerkinElmer 1600 FT-IR spectrometer using NaCl or
KBr plates. Analytical HPLC was performed using a Unicam Crys-
tal 200 pump, a Unicam 100 UV/Vis Detector and Daicel Chiralcel
OD-H and Daicel Chiralpak AD and AS columns (25 × 0.46 cm).
NMR spectra were recorded at r.t. on Bruker AC 300F, AM 360,
DRX 400 and AM 500 instruments. J values are reported in Hz and
chemical shifts in ppm. Apparent multiplets which result from coa-
lescent coupling constants are marked virtual (virt). Mass spectra
were recorded on Micromass Platform II and Micromass AutoSpec-
Q spectrometers at Imperial College London or a JEOL AX 505W
spectrometer at Kings College London. Elemental analysis was per-
formed by the London Metropolitan University microanalytical ser-
vice.
HPLC: Chiralcel OD-H; n-hexane–propan-2-ol (99:1); flow rate,
0.5 mL/min; detection at 330 nm; pR,R-enantiomer tR 11.1 (major);
pS,S-enantiomer tR 12.5 min (minor).
IR (CH2Cl2): 1889 (C≡O), 1763 cm–1 (C≡O).
1H NMR (500 MHz, CDCl3): d = 1.10 [s, 9 H, C(CH3)3], 2.30 (s, 6
H, CArCH3), 2.64 (s, 3 H, OCH3), 2.93 (dd, J = 9.5, 14.0 Hz, 1 H,
CHCHH), 3.27 (dd, J = 3.2, 14.0 Hz, 1 H, CHCHH), 4.92 (virt t,
J = 1.9 Hz, 1 H, CCrH), 5.03 (ddd, J = 3.2, 5.8, 9.5 Hz, 1 H,
CHCH2), 5.48 (dd, J = 3.0, 6.8 Hz, 1 H, CCrH), 5.55 (d, J = 6.8 Hz,
1 H, CCrH), 6.86 (s, 1 H, CArH), 6.98 (s, 2 H, CArH), 7.34–7.43 (m,
10 H, CArH).
Asymmetric Benzylic Functionalization; (+)-(R)-Tricarbon-
yl{1-[1-methoxy-2-(3,5-dimethylphenyl)ethyl]-4-tert-butylben-
zene}chromium(0) [(+)-23]; Typical Procedure
n-BuLi (1.60 mL, 2.50 M in hexane, 4.00 mmol) was added drop-
wise to diamine (+)-3 (0.840 g, 2.00 mmol) in anhyd THF (20 mL)
at –78 °C, and the resulting solution allowed to warm to r.t. over a
period of 30 min. The resulting deep red solution was then recooled
to –78 °C before a solution of heat-gun-dried LiCl (0.084 g, 2.00
mmol) in THF (10 mL) was added via a cannula followed by a pre-
13C NMR (125 MHz, CDCl3): d = 21.3 (CArCH3), 30.8 [C(CH3)3],
33.9 [C(CH3)3], 40.4 (CH2), 56.8 (OCH3), 79.8 (d, J = 16.2 Hz,
CHCH2), 89.8, 91.2, 95.7 (CCrH), 102.7 (d, J = 24.0 Hz, CCrP),
117.1 (d, J = 20.5 Hz, CCrCH), 122.3 [CCrC(CH3)3], 127.2, 128.1
(CArH), 128.5 (d, J = 6.9 Hz, CArH), 128.6 (d, J = 6.3 Hz, CArH),
128.9, 129.6 (CArH), 131.9 (CAr), 133.4 (d, J = 17.7 Hz, CArH),
134.4 (CAr), 134.5 (d, J = 17.2 Hz, CArH), 134.8 (CArH), 136.1 (d,
J = 9.7 Hz, CAr), 137.8, 138.4 (CAr), 232.7 (C& equiv;O).
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PAPER
Planar Chiral Monophosphane Ligands
3637
31P NMR (202 MHz, CDCl3): d = –13.3 (PPh2).
MS (EI, 70 eV): m/z (%) = 616 (46, [M]+), 588 (23, [M – CO]+), 532
(100, [M – 3 CO]+), 465 (34, [M – Cr(CO)3 – CH3]+), 449 (22,
[M – Cr(CO)3 – OCH3]+).
(+)-(R)-1-Diphenylphosphane-2-[1-methoxy-2-(3,5-dimethyl-
phenyl)ethyl]-5-tert-butylbenzene [(+)-40]
Phosphane-borane complex (+)-39 (0.342 g, 0.69 mmol) and
DABCO (0.155 g, 1.38 mmol) were dissolved in anhyd toluene (20
mL) and stirred under an inert atmosphere at 50 °C for 3 h before
being allowed to cool back to r.t. The solvent was removed in vacuo
and the resulting residue subjected to flash column chromatography
(hexane–EtOAc, 9:1) to give monophosphane (+)-40 as an oil
(0.330 g, 99%); [a]D20 +55 (c = 0.048, CHCl3); Rf 0.77 (hexane–
EtOAc, 9:1).
Anal. Calcd for C36H37CrO4P: C, 70.12; H, 6.05. Found: C, 69.99;
H, 5.98.
Benzyl Iodide Formation; 1-(Iodomethyl)-2,3,5,6-tetramethyl-
benzene (19); General Procedure
A 250 mL flask containing a magnetic stirrer bar was charged with
2,3,5,6-tetramethylbenzyl alcohol (1.642 g, 10.00 mmol), NaI
(1.798 g, 12.00 mmol), CeCl3·7H2O (5.580 g, 15.00 mmol) and
MeCN (100 mL). A reflux condenser was fitted and the mixture was
stirred vigorously and heated at 85 °C for 24 h before being allowed
to cool to r.t. The mixture was diluted with Et2O (500 mL), to pre-
cipitate any CeCl3, filtered through Celite and then concentrated in
vacuo. The resulting residue was purified by flash column chroma-
tography (hexane–EtOAc, 95:5) to give 19 as a white crystalline
solid (2.031 g, 74%); mp 84–85 °C; Rf 0.79 (hexane–EtOAc, 95:5).
1H NMR (360 MHz, CDCl3): d = 2.24 (s, 6 H, CArCH3), 2.25 (s, 6
H, CArCH3), 4.52 (s, 2 H, CH2I), 6.94 (s, 1 H, CArH).
13C NMR (75 MHz, CDCl3): d = 5.6 (CH2I), 15.1, 20.4 (CArCH3),
131.9 (CArH), 132.9, 134.2 (CArCH3), 134.7 (CArCH2I).
1H NMR (500 MHz, CDCl3): d = 1.15 [s, 9 H, C(CH3)3], 2.25 (s, 6
H, CArCH3), 2.68 (dd, J = 3.2, 16.5 Hz, 1 H, CHCHH), 2.61 (dd,
J = 9.5, 16.5 Hz, 1 H, CHCHH), 3.05 (s, 3 H, OCH3), 5.17 (ddd,
J = 3.2, 6.3, 9.5 Hz, 1 H, CHCH2), 6.62 (s, 2 H, CArH), 6.80 (s, 1 H,
CArH), 6.92 (dd, J = 2.0, 4.9 Hz, 1 H, CArH), 7.29–7.54 (m, 12 H,
CArH).
13C NMR (125 MHz, CDCl3): d = 21.2 (CArCH3), 31.1 [C(CH3)3],
34.6 [C(CH3)3], 44.3 (CH2), 56.8 (OCH3), 81.2 (d, J = 24.0 Hz,
CHCH2), 125.6 (d, J = 4.0 Hz, CArH), 126.2, 127.2, 127.6 (CArH),
128.4 (d, J = 6.1 Hz, CArH), 128.5 (d, J = 7.0 Hz, CArH), 128.6 (d,
J = 7.0 Hz, CArH), 128.8, 130.8 (CArH), 133.9 (d, J = 17.6 Hz,
CArH), 134.2 (d, J = 17.6 Hz, CArH), 134.5 (d, J = 14.7 Hz, CArH),
136.5 (d, J = 10.7 Hz, CArH), 137.2, 139.2 (CAr), 144.1 (d, J = 22.0
Hz, CArP), 149.9 (CAr).
MS (EI, 70 eV): m/z (%) = 274 (57, [M]+), 147 (100, [M – I]+).
HRMS: m/z [M]+ calcd for C11H15I: 274.0217; found: 274.0226.
31P NMR (202 MHz, CDCl3): d = –15.7 (PPh2).
MS (EI, 70 eV): m/z (%) = 480 (20, [M]+), 465 (100, [M – CH3]+),
449 (25, [M – OCH3]+), 433 (6, [M – OCH3 – CH3 – H]+), 377 (46,
[M – C(CH3)3 – OCH3 – CH3]+), 361 (21, [M – C(CH3)3 – OCH3 –
2 CH3 – H]+), 331 (5, [M – (CH3)2C6H3CH2 – 2 CH3]+), 317 (4,
[M – (CH3)2C6H3CH2CHOCH3]+).
(+)-(R)-1-Boronatodiphenylphosphane-2-[1-methoxy-2-(3,5-
dimethylphenyl)ethyl]-5-tert-butylbenzene [(+)-39]
Phosphane complex (–)-28 (0.486 g, 0.79 mmol) in anhyd THF (30
mL) was treated with borane-THF complex (0.87 mL, 1.00 M in
THF, 0.90 mmol) under an inert atmosphere. The mixture was
stirred at r.t. for 1.5 h before the solvent was removed in vacuo. The
residue was then dissolved in CH2Cl2 (50 mL) and stirred in an open
vessel in the presence of air and light for 16 h. After this time, the
mixture was concentrated and the residue was chromatographed
(hexane–EtOAc, 9:1) to give (+)-50 as a white solid (0.365 g, 94%);
mp 178–179 °C; [a]D20 +58 (c = 0.016, CHCl3); Rf 0.51 (hexane–
EtOAc, 4:1).
1H NMR (500 MHz, CDCl3): d = 1.08 [s, 9 H, C(CH3)3], 2.29 (s, 6
H, CArCH3), 2.56 (dd, J = 9.3, 9.5 Hz, 1 H, CHCHH), 2.61 (dd,
J = 2.9, 9.3 Hz, 1 H, CHCHH), 2.89 (s, 3 H, OCH3), 5.11 (dd,
J = 2.9, 9.5 Hz, 1 H, CHCH2), 6.53 (s, 2 H, CArH), 6.78 (s, 1 H,
CArH), 6.92 (dd, J = 2.0, 12.6 Hz, 1 H, CArH), 7.44–7.72 (m, 12 H,
CArH).
Anal. Calcd for C33H37OP: C, 82.47; H, 7.76. Found: C, 82.37; H,
7.79.
Hydrosilylation; 1-Trichlorosilyl-1-phenylethane (16); Typical
Procedure
An oven-dried Schlenk tube was placed under an inert atmosphere
and charged with dichloro(1,5-cyclooctadiene)palladium(II)
(0.0075 g, 0.025 mmol), ligand (0.050 mmol) and anhyd benzene (5
mL). The contents were stirred for 1 min leading to a color change
of solution to red before styrene (1.14 mL, 10.0 mmol) was added
via a syringe. Stirring was then continued for a further 15 min be-
fore trichlorosilane (1.51 mL, 15.0 mmol) was added in one portion.
The reaction was stirred for a total of 48 h under a slight pressure of
N2 (ca. 50 mbar). After this time the solvent was removed and the
product 16 isolated by Kugelrohr distillation.
13C NMR (125 MHz, CDCl3): d = 21.2 (CArCH3), 30.9 [C(CH3)3],
34.6 [C(CH3)3], 44.3 (CH2), 56.8 (OCH3), 79.2 (d, J = 6.0 Hz,
CHCH2), 126.6, 127.0 (CArC), 127.2, 127.6 (CArH), 128.1 (d,
J = 9.8 Hz, CArH), 128.7 (d, J = 8.3 Hz, CArH), 128.9 (d, J = 9.9 Hz,
CArH), 129.1 (d, J = 24.9 Hz, CArC), 129.6 (d, J = 7.5 Hz, CArC),
130.1 (CArC), 131.2 (d, J = 6.7 Hz, CArH), 133.4 (d, J = 8.9 Hz,
CArH), 133.7 (d, J = 8.4 Hz, CArH), 137.1, 138.7 (CArC), 145.3 (d,
J = 12.4 Hz, CArP), 150.0 (d, J = 8.0 Hz, CArP).
1H NMR (360 MHz, CDCl3): d = 1.55 (d, J = 7.6 Hz, 3 H, CH3),
2.84 (q, J = 7.6, 1 H, CHCH3), 7.15–7.28 (m, 5 H, CArH).
13C NMR (90 MHz, CDCl3): d = 14.8 (CH3), 36.5 (CSiCl3), 127.4,
128.7, 129.0 (CArH), 138.3 (CArC).
MS (EI, 70 eV): m/z (%) = 244 (7, [PhCH(Si37Cl3)(CH3)]+), 242 (7,
[PhCH(Si37Cl235Cl)(CH3)]+), 240 (13, [PhCH(Si37Cl35Cl2)(CH3)]+),
238 (13, [PhCH(Si35Cl3)(CH3)]+), 105 (100, [M – SiCl3]+).
31P NMR (202 MHz, CDCl3): d = 17.9 (PPh2).
11B NMR (160 MHz, CDCl3): d = –35 (BH3).
MS (FAB, +ve): m/z (%) = 494 (4, [M]+), 480 (8, [M – BH3]+), 465
(35, [M – BH3 – CH3]+), 449 (100, [M – BH3 – OCH3]+), 433 (10,
[M – BH3 – CH3OH – CH3]+), 375 (15, [M – BH3 – C6H3(CH3)2]+),
331 (15, [M – BH3 – CH2C6H3(CH3)2 – OCH3 + H]+).
Tamao Oxidation; 1-Phenylethanol (17); Typical Procedure
A 100 mL flask containing a magnetic stirrer bar was charged with
silane 16 (0.60 g, 2.5 mmol), KF (0.87 g, 15.0 mmol), KHCO3 (1.50
g, 15.0 mmol), MeOH (20 mL) and THF (20 mL). After stirring for
15 min at r.t., H2O2 (3 mL) was added in one portion and stirring
was continued for 16 h. At this point, aq sat. Na2S2O3 (12 mL) was
added and stirring continued for 1 h before the mixture was extract-
ed with CH2Cl2 (3 × 100 mL). The combined organic fractions were
washed with aq sat. NaHCO3 (50 mL) followed by brine (50 mL)
and then dried (MgSO4) and concentrated. Flash column chroma-
Anal. Calcd for C33H40BOP: C, 80.16; H, 8.15. Found: C, 80.04; H,
8.14.
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3638
S. E. Gibson et al.
PAPER
(10) For example: Hayashi, T. Acc. Chem. Res. 2000, 33, 354.
tography of the residue (hexane–EtOAc, 85:15) gave 17 as a trans-
parent oil.
(11) For reviews on the AHS, see: (a) Hiyama, T.; Kusumoto, T.
In Comprehensive Organic Synthesis, Vol. 8; Trost, B. M.;
Fleming, I., Eds.; Pergamon: Oxford, 1991, 120–131.
(b) Hayashi, T. In Comprehensive Asymmetric Catalysis,
Vol. 1; Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.;
Springer-Verlag: Berlin, 1999, 317–333. (c) Nishiyama, H.;
Ito, K. Catalytic Asymmetric Synthesis, 2nd ed.; Ojima, I.,
Ed.; Wiley-VCH: New York, 2000, 111–132. (d) Tang, J.;
Hayashi, T. In Catalytic Heterofunctionalisation; Togni, A.;
Grutzmacher, H., Eds.; Wiley-VCH: New York, 2001, 73–
90. (e) Tietze, L.; Hiriyakkanavar, I.; Bell, H. P. Chem. Rev.
2004, 104, 3453.
HPLC: Chiralcel OD-H; n-hexane–propan-2-ol (90:10); flow rate
0.5 mL/min; detection at 254 nm; R-enantiomer tR 12.9 min (ma-
jor); S-enantiomer tR 14.1 min (minor); Rf 0.35 (hexane–EtOAc,
85:15).
IR (KBr): 3367 cm–1 (OH).
1H NMR (300 MHz, CDCl3): d = 1.52 (d, J = 6.5 Hz, 3 H, CHCH3),
2.01 (s, 1 H, OH), 4.90 (q, J = 6.5 Hz, 1 H, CHCH3), 7.28–7.39 (m,
5 H, CArH).
13C NMR (90 MHz, CDCl3): d = 25.6 (CH3), 70.8 (CHCH3), 125.8,
127.9, 128.9 (CArH), 146.2 (CArCOH).
(12) Weber, I.; Jones, G. B. Tetrahedron Lett. 2001, 42, 6983.
(13) Bambridge, K.; Begley, M. J.; Simpkins, N. S. Tetrahedron
Lett. 1994, 35, 3391.
MS (EI, 70 eV): m/z (%) = 122 (41, [M]+), 107 (100, [M – CH3]+),
105 (24, [M – OH]+), 79 (71, [M – H2O – CH3]+).
(14) All HPLC analyses were carried out with racemic samples
created using t-BuLi instead of the diamide of (+)-3.
(15) Jensen, J. F.; Svendsen, B. Y.; la Cour, T. V.; Pedersen, H.
L.; Johannsen, M. J. Am. Chem. Soc. 2002, 124, 4558.
(16) For a review and early reports on this area, see: (a) Jones,
G. R.; Landais, Y. Tetrahedron 1996, 52, 7599. (b) Tamao,
T.; Kakui, K.; Kumada, M. J. Am. Chem. Soc. 1978, 100,
2268. (c) Tamao, K.; Ishida, N.; Tanaka, T.; Kumada, M.
Organometallics 1983, 2, 1694.
(17) Uozumi, Y.; Hayashi, T. J. Am. Chem. Soc. 1991, 113, 9887.
(18) Bartoli, G.; Belluci, M.; Bosco, M.; Di Deo, M.; Marcantoni,
E.; Sambri, L.; Torregiani, E. J. Org. Chem. 2000, 65, 2830.
(19) For example, see: Gourdel, Y.; Ghanimi, A.; Pellon, P.; Le
Corre, M. Tetrahedron Lett. 1993, 34, 1011.
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(b) Schmaltz, H.-G. Transition Metals for Organic
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(20) Brisset, H.; Gourdel, Y.; Pellon, P.; Le Corre, M.
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(21) For a review, see: Carboni, B.; Monnier, L. Tetrahedron
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