Bidentate planar chiral η6-arene tricarbonyl chromium(0) complexes: Ligands for catalytic asymmetric alkene hydrosilylation

Article abstract of DOI:10.1016/S0040-4039(01)01471-X

S_EAr-type reactions of various heterocycles with ortho-substituted (1′S,P)-tricarbonyl[(1-chloroethyl)-η⁶- benzene]chromium(0) derivatives lead to a family of novel planar chiral bidentate and pseudo-bidentate ligands. Two members of this family (1′S,P)-2-[1′-tricarbonyl-η⁶-((2″-P, P-diphenylphosphino)phenyl)-chromium(0))ethyl] furan and thiophene show high activity and enantioselectivity in the Pd-catalyzed hydrosilylation of styrenes.

Full text of DOI:10.1016/S0040-4039(01)01471-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6983–6986
Bidentate planar chiral h6-arene tricarbonyl chromium(0)
complexes: ligands for catalytic asymmetric alkene
hydrosilylation
Immo Weber and Graham B. Jones*
Bioorganic and Medicinal Chemistry Laboratories, Department of Chemistry, Northeastern University, Boston, MA 02115, USA
Received 17 July 2001; accepted 8 August 2001
Abstract—SEAr-type reactions of various heterocycles with ortho-substituted (1%S,P)-tricarbonyl[(1-chloroethyl)-h6-ben-
zene]chromium(0) derivatives lead to a family of novel planar chiral bidentate and pseudo-bidentate ligands. Two members of this
family (1%S,P)-2-[1%-tricarbonyl-h6-((2¦-P,P-diphenylphosphino)phenyl)-chromium(0))ethyl] furan and thiophene show high activ-
ity and enantioselectivity in the Pd-catalyzed hydrosilylation of styrenes. © 2001 Published by Elsevier Science Ltd.
The catalytic hydrosilylation of alkenes with trichlorosi-
lane provides a powerful method for the in situ conver-
sion of olefins to alcohols via oxidation of the
alkyltrichlorosilane, complementing hydroboration pro-
tocols.1 Of significance, Pd-catalyzed hydrosilylation
reactions require much lower catalyst loadings (ca.
0.01–0.1 mol%) than conventional Rh-catalyzed
hydroborations (ca. 1.0 mol%). The first enantioselec-
tive Pd-catalyzed hydrosilylation of olefins was
reported by Hayashi using chiral ferrocenyl phosphine
ligand 1.2 Further improvements were obtained with
the less basic analogue 2,3 and the substituted heterocy-
cle 3.4 The use of ferrocenyl catalysts in asymmetric
synthesis is now a maturing field,5 and has prompted
the search for additional planar chiral templates includ-
dination site for the substrate olefin. Viable templates
include 4 and 5, where the coordinative ability of
heteroatoms (X, Y) are modulated by the tricarbonyl
chromium tripod. A number of such ligands have been
synthesized, though application in asymmetric hydrosil-
ylation has not been reported.7 Although numerous
ligand candidates could be envisioned, the principle
drawback lies in a lack of suitable chiral pool tem-
plates, which thus requires either resolution or
diastereoselective synthesis. An attractive possibility for
the latter lies in the use of the complex 6, derived from
(S)-phenethylamine, which is known to undergo highly
selective O-metallation.8 Given our ongoing interest in
the development of related h6-arene chromium(0)-based
catalysts,9 we therefore elected to develop a series of
bidentate planar chiral complexes containing hetero-
cyclic pendant groups. Given the flexibility for hetero-
cycle functionalization, such systems would be expected
to offer considerable advantages for subsequent and
systematic ligand optimization.
ing analogous h6-arene chromium(0) derivatives.6
A
prerequisite for such templates in Pd-catalyzed hydrosil-
ylations is the presence of one strongly coordinating
and one weakly coordinating functional group, dissoci-
ation of the labile group providing the necessary coor-
CH3
CH3
CH3
N(CH3)2
OAc
N
N
Fe
PPh2
Fe
Fe
PPh2
PPh2
3
2
1
mesityl
X
X
Y
N
Y
H
Cr
Cr
Cr
OC
CO
5
6
OC
CO
4
OC
CO
CO
CO
CO
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)01471-X

6984
I. Weber, G. B. Jones / Tetrahedron Letters 42 (2001) 6983–6986
Synthesis commenced from 6, which was subjected to
diastereoselective metallation, quenched with the appro-
priate electrophile, followed by chlorination, to give 7 or
12, respectively (Scheme 1).10 Silver mediated coupling of
7 with furan, thiophene, pyrrole, or N-methyl pyrrole
gave products 8 through 11, respectively. Alternatively,
thiophenyl substrate 12 gave adducts 13 and 14 in good
yield. Of significance, the heterocycle adducts were
formed as single diastereomers, retention of stereochem-
istry (precedented on the basis of exo attack on the
intermediate benzylidene 1511) confirmed in the case of
8 by X-ray crystallography.12 While many applications
of these ligands could be envisioned, we elected to study
the Pd-catalyzed hydrosilylation of olefins as a test case
sincethisreactionhasyettobesuccessfullyaddressedwith
a planar chiral h6-arene chromium-based catalyst.
note that the major product enantiomer was S with all
substrates examined, in contrast to results obtained with
related catalysts.4 In the absence of solvent, catalysts
derived from 8 and 9 also proved to be extremely reactive.
At ambient temperature and with a catalyst loading of
only 0.04 mol% the hydrosilylation of styrene is complete
within 2 min, equivalent to a turnover rate of 60,000 h−1
yet still returning a product e.e. of >50% (entry 19).
Results were identical either using in situ formation of
the allyl–Pd complex, or with a pre-formed admix, e.g.
16, which proved more practical on a smaller scale. For
comparison ligand 17 was prepared from 7 (MeOH,
THF), and its poor selectivity (entry 20) supports our use
of the heterocyclic template. Based on the ready availabil-
ity of 8 and 9, coupled with their promising activity (Table
1), we believe that this new class of ligand will be of broad
interest. It is well precedented that modifications on the
phosphine groups of related catalysts results in strong
improvements in enantioselectivity.4 This, coupled with
the potential for introduction of modulating substituents
on the heterocycle ring itself may give rise to highly
discriminating catalysts for use in numerous asymmetric
transformations.
Hydrosilylation was conducted using a variety of substi-
tuted styrene substrates (Scheme 2). Reaction end points
were determined by NMR (typically involving incubation
for up to 48 h), then followed by in situ addition of fluoride
and immediate oxidation, which in each case gave good
yields of the corresponding (S) product alcohols (85–90%
isolated yields on a 10 mmol scale), with moderate to high
enantioselectivity (Table 1).
1. HSiCl3 / CH2Cl2 /
0.05 - 0.15 mol% ligand
Pd(µ-Hal)(η3-C3H3)2 or PdCl2
OH
As can be seen, a temperature dependence for enantiose-
lectivity exists, providing high levels of control at −50°C
(entry 7) though poor solubility precludes reaction below
this temperature. Nodiscernable electronic trend emerged
from the substrates examined, though it is interesting to
R
R
2. KF/ MeOH then H2O2
S
>85%
Scheme 2. Catalytic hydrosilylation–oxidation of alkenes.
1. t BuLi,
Ph2PCl
1. t BuLi,
PhSSPh
Cl
H
Cl
6
H3C
H3C
H
PhS
OC
Ph2P
OC
2. MeCO2Cl
85%
2. MeCO2Cl
Cr
Cr
CO
12
82%
CO
CO
CO
7
X
AgBF4
AgBF4
exo
X
X
75-90%
30-70%
H3C
H
Y
*
Cr(CO)3
X
15
X
H3C
H3C
H
8 X=O
9 X=S
10 X=NH
11 X=NCH3
H
Ph2P
OC
13 X=S
14 X=NH
PhS
Cr
Cr
CO
CO
OC
CO
CO
Scheme 1. Preparation of bidentate planar chiral h6-arene tricarbonyl chromium(0) ligands.
OCH3
O
Br
Ph
P
Pd
Ph
P
Ph
Cr
Ph
OC
CO
Cr
CO
OC
CO
16
17
CO

I. Weber, G. B. Jones / Tetrahedron Letters 42 (2001) 6983–6986
Table 1. Catalytic enantioselective hydrosilylation using planar chiral ligandsa
6985
Entry
Ligand
Pd species
Equiv.
R
Temp. (°C)
% e.e.b
1
2
3
4
5
6
7
8
8
8
8
9
9
9
9
11
13
14
8
8
8
9
9
8
9
8
8
PdCl(h3-C3H5)
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdCl2
PdCl2
PdCl2
0.05
0.05
0.25
0.05
0.25
0.25
0.25
0.05
0.05
0.05
0.15
0.10
0.05
0.10
0.05
0.05
0.05
0.25
0.04
0.05
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
25
−25
−40
−25
−40
−50
−50
−40
25
47
86
87
82
87
87
92
65
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdCl2
9
C6H5
C6H5
No reaction
No reaction
10
11
12
13
14
15
16
17
18
19c
20
25
p-CH3(C6H4)
p-CH3O(C6H4)
p-F(C6H4)
p-CH3(C6H4)
p-F(C6H4)
p-Cl(C6H4)
p-Cl(C6H4)
p-CF3(C6H4)
C6H5
−25
−25
−25
−25
−25
−25
−25
−25
25
83
77
82
77
76
83
73
86
52
17
PdCl2
PdBr(h3-C3H5)
PdCl2
PdBr(h3-C3H5)
PdBr(h3-C3H5)
PdCl(h3-C3H5)
17
C6H5
25
a Reactions conducted in CH2Cl2 or CDCl3 for 48 h. Product configuration (S) determined by optical rotation.
b Determined by GC [Supelco b-DEX].
c Reacted in the absence of solvent for 2 min.
In summary a new class of heterocyclic planar chiral
catalysts has been developed, and applied to the first
example of highly enantioselective alkene hydrosilyla-
tion using a h6-arene chromium catalyst. The scope and
versatility of this family of catalysts will be reported in
due course.
5. (a) Hayashi, T. In Ferrocenes; Togni, A.; Hayashi, T.,
Eds.; VCH: New York, 1995; Chapter 2, p. 105 and
references cited therein; (b) Togni, A. In Metallocenes.
Synthesis, Reactivity, Applications; Togni, A.; Halter-
mann, R. L., Eds.; Wiley-VCH: Weinheim, 1998; pp.
687–721.
6. Bolm, C.; Muniz, K. Chem. Soc. Rev. 1999, 28, 51.
7. For representative publications, see: (a) Englert, U.;
Haerter, R.; Vasen, D.; Salzer, A.; Eggeling, E. B.; Vogt,
D. Organometallics 1999, 18, 4390; (b) Englert, U.;
Salzer, A.; Vasen, D. Tetrahedron: Asymmetry 1998, 9,
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Chung, Y. K. Tetrahedron: Asymmetry 1999, 10, 347; (d)
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1998, 9, 193; (h) Taniguchi, N.; Uemura, M. Tetrahedron
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Acknowledgements
We thank the PRF (Administered by the American
Chemical Society) for financial support of this work
(33920-AC1).
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