Ag+-assisted hydrosilylation: Complementary behavior of Rh and Ir catalysts (reversal of enantioselectivity)

Article abstract of DOI:10.1021/ol0702730

Figure presented The presence of a suitably situated hydroxy function in a PHOX ligand leads to an enhancement of the enantioselectivity in Rh-catalyzed hydrosilylations of prochiral ketones in the presence of AgBF₄ (95% ee for acetophenone as compared to 75% using i-Pr-phosphinooxazoline (PHOX)). Exchanging Rh for Ir affords the product with the opposite absolute configuration (78% ee).

Full text of DOI:10.1021/ol0702730

ORGANIC
LETTERS
+
2007
Vol. 9, No. 7
371-1374
Ag -Assisted Hydrosilylation:
1
Complementary Behavior of Rh and Ir
Catalysts (Reversal of Enantioselectivity)
Anders Fr o1 lander and Christina Moberg*
KTH School of Chemical Science and Engineering, Organic Chemistry,
SE 100 44 Stockholm, Sweden
kimo@kth.se
Received February 2, 2007
ABSTRACT
The presence of a suitably situated hydroxy function in a PHOX ligand leads to an enhancement of the enantioselectivity in Rh-catalyzed
hydrosilylations of prochiral ketones in the presence of AgBF (95% ee for acetophenone as compared to 75% using i-Pr phosphinooxazoline
PHOX)). Exchanging Rh for Ir affords the product with the opposite absolute configuration (78% ee).
4
−
(
The introduction of functional groups capable of taking part
in secondary interactions with the metal ion in transition-
metal catalysts may occasionally lead to enhanced selectivity
bonding to electron-rich heteroatoms and, in some cases, to
low valent metal ions. We have been able to show that
4
Pd(0) serves as a hydrogen-bond acceptor in π-olefin
complexes with ligands containing properly situated hydroxy
groups, thereby affecting the conformation and, consequently,
the stereochemistry in palladium-catalyzed allylic alkyla-
1
in catalytic reactions. The presence of internal hydroxy
groups has, for example, been shown to provide significant
effects on the selectivity in catalytic reactions. An early
example was provided by Knowles and co-workers, who
demonstrated that substitution of methoxy for hydroxy groups
in DIPAMP led to increased enantioselectivity in Rh-
catalyzed hydrogenations, probably as a result of coordination
5
tions. Major differences were also found between hydroxy-
6
and methoxy-containing phosphinooxazoline (PHOX) ligands
1-4 (Figure 1) in palladium- and iridium-catalyzed allyla-
7
tions. These differences may also be explained by a
2
of oxygen to Rh. Several examples have subsequently been
hydrogen bond with the metal ion acting as the proton
acceptor. PHOX ligands containing properly situated hydroxy
described, whereby the oxygen atom in a hydroxy group
takes part in coordination to the metal center in a catalytically
active complex. This may result in conformational restrictions
and, as a consequence, enhanced enantioselectivity, although
occasionally at the expense of the reactivity.3
(
4) (a) Epstein, L. M.; Shubina, E. S. Coord. Chem. ReV. 2002, 231,
1
65-181. (b) Brammer, L.; Zhao, D.; Lapido, F. T; Braddock-Wilking, J.
Acta Crystallogr., Sect. B 1995, 51, 632-640. (c) Braga, D.; Grepioni, F.;
Tedesco, E.; Biradha, K.; Desiraju, G. R. Organometallics 1997, 16, 1846-
1
856.
In addition to serving as hemilabile oxygen ligands to
metal ions, hydroxy groups can be engaged in hydrogen
(
5) Hallman, K.; Wondimagegn, T.; Fr o¨ lander, A.; Svensson, M.;
Moberg, C. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5400-5404.
6) (a) von Matt, P.; Pfaltz, A. Angew. Chem., Int. Ed. Engl. 1993, 32,
(
5
66-568. (b) Sprinz, J.; Helmchen, G. Tetrahedron Lett. 1993, 34, 1769-
(
(
(
1) Sawamura, M.; Ito, Y. Chem. ReV. 1992, 92, 857-871.
2) Knowles, W. S. Acc. Chem. Res. 1983, 16, 106-112.
3) B o¨ rner, A. Eur. J. Inorg. Chem. 2001, 327-337 and references
1772. (c) Dawson, G. J.; Frost, C. G.; Williams, J. M. J.; Coote, S. J.
Tetrahedron Lett. 1993, 34, 3149-3150.
(7) Fr o¨ lander, A.; Lutsenko, S.; Privalov, P.; Moberg, C. J. Org. Chem.
2005, 70, 9882-9891.
therein.
1
0.1021/ol0702730 CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/27/2007

oxygen to the metal is a more likely secondary interaction
which might be of importance for the enantioselectivity.
First, Rh-catalyzed hydrosilylation of acetophenone was
performed employing diphenylsilane together with [Rh(cod)-
Cl]
2
and ligands 1-4. Moderate enantioselectivities were
observed (entries 1, 3, 5, and 8, Table 1), indeed lower than
Table 1. Hydrosilylations of Acetophenone Using [Rh(cod)Cl]
2
Figure 1. Phosphinooxazoline ligands.
groups can therefore be expected to interact with both low
valent metal ions (via hydrogen bonding) and high valent
metal ions (via oxygen coordination). Both types of interac-
tions may be expected to have an influence on the stereo-
chemistry of catalytic reactions.
entrya
conv (%)b
ee (%)c
abs conf d
ligand
1
2
3
4
5
6
7
8
9
1
1
100
100
100
100
100
97
100
100
100
100
91
12
63
70
60
66
91
95
56
87
73
69
S
S
S
S
R
R
R
R
R
R
R
e
2
2e
Having access to these functionalized PHOX ligands, we
decided to investigate their behavior in Rh- and Ir-catalyzed
3
8
e
hydrosilylations of prochiral ketones. Metal-catalyzed hy-
3
e,f
3
drosilylations of ketones have emerged as highly versatile
reactions, affording enantioenriched products under mild
conditions, using complexes based on Rh, Ir, Ti, Zn, and
4
4
e
1
1
0
1
5
5
9
Cu.
e
PHOX ligands were early employed in hydrosilylations
of ketones and resulted in moderate to high enantioselec-
a
Reaction conditions unless otherwise noted: [Rh(cod)Cl]2 (0.5 mol %),
ligand (2 mol %), acetophenone (1.0 mmol), diphenylsilane (1.2 mmol),
THF (1 mL), 16 h reaction time, rt. Determined by NMR. Yield of silyl
1
0
b
tivities. Reduction of acetophenone with diphenylsilane,
often used as a benchmark reaction, resulted in 73-82% ee
using a Rh catalyst containing i-Pr-PHOX (5). Improved
selectivity was achieved using an aminoindanol-derived
c
d
enol ether: 2-10%. Determined by chiral GC. e Assigned by comparing
the sign of optical rotation with the literature data. AgBF4 (2 mol %) added.
f
Reaction performed at 0 °C. Yield of silyl enol ether: <4%.
11
PHOX (94% ee) and an oxazolylferrocene-phosphine
ligand (Rh, 91%; Ir, 96% ee).9 The particular aim of our
study was to examine whether an interaction of the hydroxy
group in 1 and 3 with the metal ion would influence the
stereochemistry of the reaction.
b,12
those reported using 5 and other standard PHOX ligands.10
To favor coordination of oxygen to the metal ion, cationic
rhodium complexes, obtained by abstraction of the chloride
ion by AgBF , were then employed for the reaction. We were
4
Rh-catalyzed hydrosilylations are known to proceed by
oxidative addition of the silane to Rh(I) followed by
coordination of the substrate and insertion of the carbonyl
bond into the rhodium-silicon bond and subsequent reduc-
tive elimination.13 Because the enantiodetermining step
involves Rh(III) complexes only, it is highly unlikely that
hydrogen bonding between the hydroxy group and the metal
affects the stereochemistry of the product. Coordination of
pleased to find that under these conditions, the product was
obtained with considerably higher enantioselectivity, the best
result (91% ee, entry 6, Table 1) being obtained with 3. An
even higher ee was obtained at 0 °C under otherwise identical
conditions (95% ee, entry 7). The counterion evidently had
an effect because addition of other silver salts led to inferior
results (AgOTf, 79% ee; AgPF , 80% ee). Other experimental
6
details were also found to be important: employing only 1
equiv of the ligand gave 83% ee at room temperature, and
(
8) (a) Nishiyama, H.; Itoh, K. In Catalytic Asymmetric Synthesis; Ojima,
using the alternative Rh precursor Rh(cod)
only 58% ee.
2 4
BF resulted in
I., Ed.; Wiley: New York, 2000. (b) Riant, O.; Mostefa ¨ı , N.; Courmarcel,
J. Synthesis 2004, 2943-2958.
(
To determine whether these intriguing effects accompany-
ing the addition of AgBF were connected to our PHOX
9) For representative examples, see: (a) Tao, B.; Fu, G. C. Angew.
Chem., Int. Ed. 2002, 41, 3892-3894. (b) Nishibayashi, Y.; Segawa, K.;
Takada, H.; Ohe, K.; Uemura, S. Chem. Commun. 1996, 847-848. (c) Yun,
J.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 5640-5644. (d) Bette,
V.; Mortreux, A.; Savoia, D.; Carpentier, J-.F. Tetrahedron 2004, 60, 2837-
4
ligands or were the result of some more general phenomenon,
we subjected the standard i-Pr-PHOX ligand 5 to the
hydrosilylation of acetophenone. With the neutral rhodium
complex, the result achieved was in agreement with those
2
2
842. (e) Lipshutz, B. H.; Noson, K.; Chrisman, W. J. Am. Chem. Soc.
001, 123, 12917-12918.
(10) (a) Newman, L. M.; Williams, J. M. J.; McCague, R.; Potter, G. A.
Tetrahedron: Asymmetry 1996, 7, 1597-1598. (b) Langer, T.; Janssen, J.;
10
published (73% ee, entry 10, Table 1; 75% ee at 0 °C).
Helmchen, G. Tetrahedron: Asymmetry 1996, 7, 1599-1602.
4
Together with AgBF , this ligand afforded a slightly lower
(11) Sudo, A.; Yoshida, H.; Saigo, K. Tetrahedron: Asymmetry 1997,
1
4
8
, 3205-3208.
12) Nishibayashi, Y.; Segawa, K.; Ohe, K.; Uemura, S. Organometallics
995, 14, 5486-5487.
13) Imamoto, T.; Itoh, T.; Yamanoi, Y.; Narui, R.; Yoshida, K.
Tetrahedron: Asymmetry 2006, 17, 560-565 and references therein.
ee (69%, entry 11), indicating that the ee enhancement for
(
ligands 1 and 3 as well as 4 is somehow linked to the oxygen-
1
(
(14) This is consistent with previous observations; see ref 10a.
1372
Org. Lett., Vol. 9, No. 7, 2007

containing substituents. A probable explanation for the
observed results is that the oxygen atom coordinates to
rhodium(III), thereby increasing the rigidity of the catalyst
and therefore the enantiodiscriminating power.
Use of our ligands together with [Ir(cod)Cl]
hydrosilylation of acetophenone led to products with moder-
ate ee’s (entries 1, 3, 5, and 8, Table 2). Addition of AgBF
2
in the
4
Table 2. Hydrosilylations of Acetophenone Using [Ir(cod)Cl]
2
Figure 2. Complex 6.
the proposed interaction of the hydroxy group with the
cationic metal. A signal for the hydroxy proton (at 3.66 ppm
at -40 °C) shows that the ligand is not deprotonated.
Encouraged by the results for acetophenone, we decided
to subject a number of prochiral ketones to the hydrosilyla-
entrya
ligand
conv (%)b
ee (%)c
abs conf d
1
2
3
4
5
6
7
8
9
1
1
99
99
100
98
97
98
97
100
98
98
99
2
22
8
26
33
77
78
16
53
8
S
R
S
R
S
S
S
R
S
R
S
e
2
e
2 2
tion using ligand 3 and either [Rh(cod)Cl] or [Ir(cod)Cl]
(Table 3). We found that for all ketones the two catalytic
2
3
3
3
4
4
5
5
e
e,f
e
e
Table 3. Hydrosilylations of Various Ketones Using
[Rh(cod)Cl] or [Ir(cod)Cl] and 3
2
1
1
0
1
32
a
Reaction conditions unless otherwise noted: [Ir(cod)Cl]2 (0.5 mol %),
ligand (2 mol %), acetophenone (1.0 mmol), diphenylsilane (1.2 mmol),
b
THF (1 mL), 16 h reaction time, rt. Determined by NMR. Yield of silyl
c
d
enol ether: 2-15%. Determined by chiral GC. Assigned by comparing
e
the sign of optical rotation with the literature data. AgBF4 (2 mol %) added.
Reaction performed at 0 °C. Yield of silyl enol ether: <5%.
f
also in this case substantially improved the enantioselectivi-
ties. The best result at room temperature was again obtained
with ligand 3 (77% ee, entry 6). To our surprise, this product
had the opposite absolute configuration to those of the
product obtained in the Rh-catalyzed process. The Rh- and
Ir-catalyzed processes do thus exhibit complementary be-
havior. Such behavior has previously been observed for an
9b,12
a Reaction conditions unless otherwise noted: [M(cod)Cl] (0.5 mol %),
oxazolylferrocene-phosphine
as well as for a few other
2
ligand 3 (2 mol %), AgBF4 (2 mol %), ketone (1.0 mmol), diphenylsilane
less selective ligands.15 In analogy to what was observed in
the Rh-catalyzed reactions, a less pronounced selectivity
1.2 mmol), THF (1 mL), 16 h reaction time, rt. b Determined by NMR.
(
Yield of silyl enol ether, entries 1-4: <6%, entries 5-8: <3%.
c
Determined by chiral GC or HPLC. d Assigned by comparing the sign of
improvement was observed when AgOTf or AgPF
added in place of AgBF . The enantioselectivities in Ir-
catalyzed reactions with ligand 5 were low in both the
presence and absence of AgBF (entries 10 and 11).
6
was
optical rotation with the literature data.
4
4
systems gave rise to products with different absolute
configurations. For 4-bromoacetophenone and 4-methoxy-
acetophenone, the ee’s were good but somewhat lower than
that for acetophenone (entries 1-4, Table 3). For benzyl-
acetone and 2-octanone, good ee’s were observed in the
rhodium-catalyzed reactions, but low ee’s were observed in
the iridium system.
To find evidence for the assumption that oxygen coordi-
nates to the metal center, the Ir(I) complex 6 (Figure 2) was
prepared by stirring 3 with [Ir(cod)Cl]
exchange with AgBF . Although the complex has not yet
been fully characterized, its H and C NMR spectra have
been assigned. Large downfield shifts were observed for H
from 4.11 to 5.91 ppm) and C (from 77.4 to 84.0 ppm) in
the complex compared to the free ligand, consistent with
2
followed by anion
4
1
13
a
(
a
In conclusion, we have found that, in the presence of
AgBF , PHOX ligand 3, easily available in two steps from
4
commercial starting materials, provides an excellent enan-
tioselectivity in the Rh-catalyzed hydrosilylation of acetophe-
none, due to involvement of the oxygen function. The
(
15) (a) Faller, J. W.; Chase, K. J. Organometallics 1994, 13, 989-992.
b) Kinting, A.; Kreuzfeld, H.-J.; Abicht, H.-P. J. Organomet. Chem. 1989,
70, 343-349.
(
3
Org. Lett., Vol. 9, No. 7, 2007
1373

analogous catalyst based on Ir was shown to exhibit
complementary behavior, giving the opposite enantiomer of
the product also with good enantioselectivity.
Supporting Information Available: Experimental pro-
cedures for the catalytic reactions. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was financed by the
Swedish Research Council.
OL0702730
1374
Org. Lett., Vol. 9, No. 7, 2007