Synthesis, characterization, and catalytic application of a new chiral P,N-indene ligand derived from (R)-BINOL

Article abstract of DOI:10.1016/j.jorganchem.2009.02.015

Lithiation of 2-dimethylaminoindene followed by quenching with [(R)-(1,1′-binaphthalene-2,2′-diyl)]chlorophosphite and treatment with triethylamine afforded the crystallographically characterized enantiopure P,N-indene 3 in 71% isolated yield. In the cour

Full text of DOI:10.1016/j.jorganchem.2009.02.015

Journal of Organometallic Chemistry 694 (2009) 1943–1947
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis, characterization, and catalytic application of a new chiral
P,N-indene ligand derived from (R)-BINOL
Dominik Wechsler a, Gabriele Schatte b, Mark Stradiotto a,
*
a Department of Chemistry, Dalhousie University, Halifax, NS, Canada B3H 4J3
b Saskatchewan Structural Sciences Centre, University of Saskatchewan, Saskatoon, SK, Canada S7N 5C9
a r t i c l e i n f o
a b s t r a c t
Lithiation of 2-dimethylaminoindene followed by quenching with [(R)-(1,10-binaphthalene-2,20-
diyl)]chlorophosphite and treatment with triethylamine afforded the crystallographically characterized
enantiopure P,N-indene 3 in 71% isolated yield. In the course of rhodium coordination chemistry studies
Article history:
Received 4 February 2009
Received in revised form 17 February 2009
Accepted 18 February 2009
Available online 28 February 2009
involving 3, the formation of the isolable complex [(j j
2-P,N-3)( 1-P,N-3)RhCl] (7) (81%) was observed,
thereby confirming the propensity of this new ligand to form LnRh(3)2 complexes. Such coordination
chemistry behavior may contribute in part to the generally poor catalytic performance exhibited by mix-
tures of 3 and rhodium precursor complexes in the asymmetric hydrogenation and hydrosilylation stud-
ies described herein.
Keywords:
Asymmetric catalysis
Chiral
Ó 2009 Elsevier B.V. All rights reserved.
Indene
P,N-ligand
Rhodium
1. Introduction
ral P,N-substituted indenes prepared from (R)-BINOL (i.e. 2, or
alternatively the vinylic isomer 3) as attractive targets of inquiry
Chiral platinum-group metal complexes are widely employed
as catalysts in the synthesis of chiral, non-racemic molecules on
both bench-top and industrial scales [1]. Notwithstanding the util-
ity of enantiopure (often C2-symmetric) P,P- and N,N-ligands in
promoting such metal-mediated asymmetric transformations, re-
lated heterobidentate C1-symmetric P,N-ligands that combine soft
(P) and hard (N) donor fragments have in some cases been shown
offer inroads to reactivity that cannot be accessed by use of homo-
bidentate ligand systems [2]. Indeed, the successful application of
heterobidentate chelates including the phosphinooxazoline
(PHOX) family of ligands [3] has prompted the development of
alternative classes of chiral P,N-ligands for use in promoting new
and/or challenging metal-mediated asymmetric substrate transfor-
mations [2].
[6]. Herein we report on the synthesis and characterization of the
new chiral indene ligand 3, our efforts to develop the rhodium
coordination chemistry of this ligand, and the application of 3 in
rhodium-mediated asymmetric alkene hydrogenation and ketone
hydrosilylation.
2. Experimental
2.1. General considerations
All manipulations were conducted in the absence of oxygen and
water under an atmosphere of dinitrogen, either by use of standard
Schlenk methods or within an mBraun glovebox apparatus, utiliz-
ing glassware that was oven-dried (130 °C) and evacuated while
hot prior to use. 2-Dimethylaminoindene 4 [7] and [(R)-(1,10-
binaphthalene-2,20-diyl)]chlorophosphite [8] were prepared by
use of literature procedures and were dried in vacuo for 24 h prior
to use. Otherwise, the purification and handling of reagents, as well
as the rhodium-catalyzed alkene hydrogenation and ketone hydro-
silylation experiments, were carried out by using published proto-
cols [9]. 1H, 13C, and 31P NMR characterization data were collected
at 300 K on a Bruker AV-500 spectrometer operating at 500.1,
125.8, and 202.5 MHz (respectively) with chemical shifts reported
in parts per million downfield of SiMe4 (for 1H and 13C) or 85%
H3PO4 in D2O (for 31P). 1H and 13C NMR chemical shift assignments
are made on the basis of data obtained from 13C DEPT, 1H–1H COSY,
In this context we have demonstrated that P,N-substituted ind-
enes including 1-PiPr2-2-NMe2-indene (1; Scheme 1) can be em-
ployed in the synthesis of neutral, cationic, and formally
zwitterionic
j
2-P,N platinum-group metal complexes that are of
use as catalysts in the reduction of unsaturated substrates [4]. In
seeking to advance this research, we became interested in develop-
ing chiral variants of 1; given the well-established utility of enan-
tiopure 1,10-bi-2-naphthol (BINOL)-derived ligands including
MonoPhos [5] in platinum-group metal catalysis, we identified chi-
* Corresponding author. Tel.: +1 902 494 7190; fax: +1 902 494 1310.
E-mail address: mark.stradiotto@dal.ca (M. Stradiotto).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.02.015

1944
D. Wechsler et al. / Journal of Organometallic Chemistry 694 (2009) 1943–1947
clean conversion to 5. The THF and other volatile materials were
then removed in vacuo, and the residue was taken up in toluene.
The solution was then filtered through Celite followed by removal
of the toluene and other volatiles in vacuo and the solid was used
PiPr2
NMe2
H
O
O
without further purification. 1H NMR (C6D6):
d 7.50 (d,
O
O
P
P
H
3JHH = 7.5 Hz, 1H, C7–H), 7.27–7.20 (m, 2H, C4–H and C6–H), 7.03
(t, 3JHH = 7.5 Hz, 1H, C5–H), 5.54 (s, 1H, C3–H), 4.02 (d, 2JPH = 8.0 Hz,
1H, C1–H), 3.04–2.91 (m, 4H, 2 CH2CH3), 2.86–2.74 (m, 4H, 2
CH2CH3), 2.61 (s, 6H, NMe2), 1.03 (m, 6H, 2 CH2CH3), 0.84 (m, 6H,
NMe2
NMe2
H
1
H
H
H
3
2
2
2 CH2CH3); 13C{1H} NMR (C6D6): d 160.0 (d, JPC = 6.7 Hz, C2),
Scheme 1.
3
146.8 (C7a), 138.8 (C3a), 126.1 (C6), 123.9 (d, JPC = 2.3 Hz, C7),
1
119.8 (C5), 117.7 (C4), 101.6 (C3), 53.0 (d, JPC = 42.6 Hz, C1), 43.4
2
2
(d, JPC = 19.2 Hz, 2 CH2CH3), 42.9 (d, JPC = 18.9 Hz, 2 CH2CH3),
41.9 (s, NMe2), 14.7 (2 CH2CH3), 14.1 (2 CH2CH3); 31P{1H} NMR
(C6D6): d 113.6.
1H–13C HSQC, and 1H–13C HMBC NMR experiments. Elemental
analyses were performed by Canadian Microanalytical Service
Ltd., Delta, British Columbia, Canada.
2.4. Synthesis of 7
2.2. Synthesis of 3
A vial containing a magnetic stir bar was charged with 3
A vial containing a magnetic stir bar was charged with 4 (0.14 g,
0.88 mmol) and 4 mL of toluene. The solution was cooled to ꢀ35 °C
followed by the addition of n-BuLi (1.6 M in hexanes, pre-cooled to
ꢀ35 °C, 0.55 mL, 0.88 mmol). The mixture was stirred for 1 h at
ambient temperature. Meanwhile, a second vial was charged with
(0.044 g, 0.094 mmol) and 3 mL of THF. To a separate vial,
[(COE)2RhCl]2 (0.017 g, 0.024 mmol) and THF (2 mL) were added.
The rhodium-containing solution was added to the THF slurry of
3 and magnetically stirred for 2 h. The solvent was then removed
in vacuo, and the residue washed with pentane. Any residual sol-
vent and other volatiles were removed in vacuo, leaving behind 7
as a light brown solid (0.041 g, 0.038 mmol, 81%). Anal. Calc. for
C62H48P2N2O4RhCl: C, 68.59; H, 4.46; N, 2.58. Found: C, 68.67; H,
[(R)-(1,10-binaphthalene-2,20-diyl)]chlorophosphite
(0.31 g,
0.88 mmol) and 3 mL toluene. The solution of [(R)-(1,10-binaphtha-
lene-2,20-diyl)]chlorophosphite was added dropwise to the solu-
tion of the indenyl lithium salt, followed by 2 h of stirring.
Triethylamine (0.49 mL, 3.5 mmol) was then added to the reaction
mixture, and the mixture was stirred overnight, during which time
a significant quantity of solid 3 (including crystals suitable for X-
ray diffraction analysis) precipitated out of solution. The mixture
was then concentrated to near dryness and washed with benzene
(2 mL). The remaining solid was then dried in vacuo yielding 3 as
an analytically pure white solid (0.30 g, 0.63 mmol, 71%). Anal.
Calc. for C31H24PNO2: C, 78.63; H, 5.11; N, 2.96. Found: C, 78.40;
3
4.46; N, 2.29%. 1H NMR (C6D6): d 9.52 (d, JHH = 7.4 Hz, 1H, aryl
3
3
C–H), 8.61 (d, JHH = 8.7 Hz, 1H, aryl C–H), 7.89 (d, JHH = 8.8 Hz,
3
1H, aryl C–H), 7.80 (d, JHH = 8.1 Hz, 1H, aryl C–H), 7.63–7.69 (m,
2H, aryl C–Hs), 7.59 (t, JHH = 7.3 Hz, 1H, aryl C–H), 7.41–7.52 (m,
4H, aryl C–Hs), 7.25 (d, JHH = 8.7 Hz, 1H, aryl C–H), 7.13–7.17 (m,
3
3
1H, aryl C–H), 7.03–7.07 (m, 2H, aryl C–Hs), 6.98–7.02 (m, 2H, aryl
3
C–Hs), 6.97 (d, JHH = 8.8 Hz, 1H, aryl C–H), 6.66–6.83 (m, 5H, aryl
C–Hs), 6.51 (m, 1H, aryl C–H), 6.43 (m, 1H, aryl C–H), 6.11 (t,
3
3JHH = 7.4 Hz, 1H, aryl C–H), 6.06 (d, JHH = 6.0 Hz, 1H, aryl C–H),
3
3
3
H, 4.95; N, 2.51%. 1H NMR (CD2Cl2): d 8.07 (d, JHH = 9.0 Hz, 1H,
5.67 (d, JHH = 8.9 Hz, 1H, aryl C–H), 5.42 (d, JHH = 8.9 Hz, 1H, aryl
3
3
C–H Naph.), 8.02 (d, JHH = 8.5 Hz, 1H, C–H Naph.), 7.91 (d,
C–H), 5.23 (d, JHH = 7.8 Hz, 1H, aryl C–H), 5.21 (s, 1H, aryl C–H),
3
3JHH = 8.0 Hz, 1H, C–H Naph.), 7.73 (d, JHH = 8.5 Hz, 1H, C–H
3.10 (s, 3H, bound NMe), 2.86 (s, 3H, bound NMe), 2.48–2.34 (m,
3
Naph.), 7.59 (d, JHH = 8.5 Hz, 1H, C–H Naph.), 7.55–7.45 (m, 4H,
4H, CH2), 1.98 (s, 6H, unbound NMe2); 13C{1H} NMR (C6D6): d
C–Hs Naph.), 7.39–7.31 (m, 2H, C–Hs Naph.), 7.22 (d, 3JHH = 8.5 Hz,
176.4 (j j
2-P,N ligand C2), 156.8 ( 1-P,N ligand C2), 156.7 (quat),
3
1H, C–H Naph.), 7.11 (d, JHH = 7.5 Hz, 1H, C7–H), 6.59 (t,
151.4 (quat), 151.3 (quat), 151.1 (quat), 150.7 (quat), 150.2 (quat),
149.8 (quat), 149.7 (quat), 148.0 (quat), 147.9 (quat), 142.3 (C3a or
C7a), 137.2 (quat), 134.8 (quat), 133.6 (quat), 133.2 (quat), 132.3
(quat), 131.9 (quat), 131.8 (quat), 131.4 (quat), 131.0 (quat),
130.3 (aryl C–H), 130.0 (quat), 129.1 (aryl C–H), 128.4 (aryl C–H),
128.2 (aryl C–H), 128.0 (aryl C–H), 127.3 (aryl C–H), 127.2 (aryl
C–H), 127.1 (2 aryl C–Hs), 126.8 (aryl C–H), 126.7 (aryl C–H),
126.0 (aryl C–H), 125.8 (aryl C–H), 125.5 (aryl C–H), 125.3 (aryl
C–H), 125.1 (2 aryl C–Hs), 125.0 (aryl C–H), 124.6 (aryl C–H),
124.5 (2 aryl C–Hs), 123.9 (aryl C–H), 123.7 (aryl C–H), 123.5 (aryl
C–H), 122.7 (aryl C–H), 122.4 (aryl C–H), 121.6 (aryl C–H), 120.2
(aryl C–H), 117.2 (aryl C–H), 103.4 (aryl C–H), 55.7 (aryl C–H),
55.6 (aryl C–H), 50.7 (bound NMe), 48.5 (bound NMe), 40.8 (un-
bound NMe2), 29.7 (CH2), 29.6 (CH2); 31P{1H} NMR (C6D6): d
3
3JHH = 7.5 Hz, 1H, C5–H or C6–H), 6.10 (d, JHH = 8.0 Hz, 1H, C4–
3
H), 6.02 (t, JHH = 7.5 Hz, 1H, C6–H or C5–H), 3.63 (s, 2H, CH2),
5
3.38 (d, JPH = 3.5 Hz, 6H, NMe2); 13C{1H} NMR (CD2Cl2): d 170.9
2
(d, JPC = 27.7 Hz, C2), 152.6 (quat Naph.), 151.6 (quat Naph.),
146.3 (C3a or C7a), 133.9 (C7a or C3a), 133.1 (quat Naph.), 133.0
(quat Naph.), 131.5 (quat Naph.), 131.1 (quat Naph.), 130.6 (C–H
Naph.), 130.1 (C–H Naph.), 128.4 (C–H Naph.), 128.4 (C–H Naph.),
128.2 (C–H Naph.), 126.8 (C–H Naph.), 126.5 (C–H Naph.), 126.1
(C–H Naph.), 125.9 (C–H Naph.), 125.5 (C6 or C5), 124.7 (C–H
Naph.), 124.3 (C–H Naph.), 123.0 (quat), 122.2 (C–H Naph.),
121.8 (C7), 121.5 (C4), 120.3 (C5 or C6), 70.6 (quat), 45.5 (d,
3
4JPC = 23.0 Hz, NMe2), 41.1 (d, JPC = 4.4 Hz, CH2); 31P{1H} NMR
(CD2Cl2): d 197.4.
2
1
199.4 (d of d, JPP = 36.4 Hz, JRhP = 261.2 Hz), 170.3 (d of d,
1
2.3. Formation and characterization of 5
2JPP = 36.4 Hz, JRhP = 291.6 Hz).
A vial containing a magnetic stir bar was charged with 4 (0.40 g,
0.25 mmol) and 2 mL of THF. The mixture was cooled to ꢀ35 °C,
and magnetic stirring was initiated followed by the dropwise addi-
tion of n-BuLi (1.6 M in hexanes, pre-cooled to ꢀ35 °C, 0.16 mL,
0.25 mmol). Following the addition, the resulting mixture was stir-
red for 1 h. To the reaction mixture was then added (Et2N)2PCl by
using an Eppendorf pipette, followed by stirring for 3.5 h. 31P NMR
data obtained from an aliquot of the reaction mixture indicated the
2.5. Crystallographic solution and refinement details for 3
Crystallographic data for this compound were obtained at
173( 2) K on a Nonius KappaCCD 4-Circle Kappa FR540C diffrac-
tometer using a graphite-monochromated Mo K
a (k = 0.71073 Å)
radiation, employing a sample that was mounted in inert oil and
transferred to a cold gas stream on the diffractometer. Cell param-
eters were initially retrieved using the COLLECT software (Nonius),

D. Wechsler et al. / Journal of Organometallic Chemistry 694 (2009) 1943–1947
1945
and refined with the HKL DENZO and SCALEPACK software [10]. Data
H
H
P(NEt2)2
NMe2
H
1. n-BuLi
2. (Et2N)2PCl
reduction and absorption correction (multi-scan) were also per-
formed with the HKL DENZO and SCALEPACK software. The structure
was solved by using direct methods, and refined by use of full-ma-
NMe2
trix least-squares procedures (on F2) with R1 based on F2 ꢁ 2
r
ðF2oÞ
H
5
H
4
o
and wR2 based on F2 ꢁ ꢀ3
r
ðF2oÞ. Anisotropic displacement param-
o
(R)-BINOL
eters were employed throughout for the non-H atoms, and all H-
atoms were added at calculated positions and refined by use of a
riding model employing isotropic displacement parameters based
on the isotropic displacement parameter of the attached atom.
The near-zero final refined value of the absolute structure param-
eter supported that the correct absolute structure had been chosen
[11]. The crystal structure diagram in Fig. 1 was generated by use
of the ORTEP-3 for Windows program [12].
O
O
4
+
+
P
NEt2
HNEt2
6
Scheme 2.
3. Results and discussion
3.1. Synthetic investigations
1. n-BuLi
H
H
The conversion of (sp2-R)P(NEt2)2 precursors into (sp2-R)P(BI-
NOL-2H) and HNEt2 upon treatment with BINOL has been success-
fully exploited in the synthesis of a range of chiral phosphorus-
containing compounds [13]. Using this approach, 2-dimethylami-
noindene 4 was lithiated, followed by quenching with ClP(NEt2)2
to give 1-P(NEt2)2-2-NMe2-indene 5 (Scheme 2). While 5 was com-
pletely consumed over the course of 24 h upon reaction with (R)-
BINOL in refluxing toluene, 1H and 31P NMR analysis of the reaction
mixture revealed an equimolar mixture of 4, the known phospho-
ramidite 6 [14], and HNEt2, rather than the desired product 2 (or
the isomer 3). This product mixture can be viewed as arising from
a combination of the desired protonolysis by (R)-BINOL of a P–N
linkage in 5, accompanied by unwanted P-C(sp3) bond protonoly-
sis. Given that previously reported synthetic protocols of this type
have employed precursors that feature more robust P-C(sp2) link-
ages [13], we sought to promote the isomerization of the allylic
phosphine 5 to the vinylic phosphine 3-P(NEt2)2-2-NMe2-indene;
unfortunately, we were unable to effect the clean isomerization
2. (RO) PCl
2
O
3. NEt
3
NMe
O
2
P
H
4
NMe
2
3
H
H
Scheme 3.
of 5 by use of heating and/or treatment with triethylamine [6a].
As such, alternative synthetic inroads to 2 (or 3) were explored.
In using a protocol similar to that employed in the synthesis of 1
[15], compound 4 was lithiated, followed by quenching with [(R)-
(1,10-binaphthalene-2,20-diyl)]chlorophosphite (represented as
(RO)2PCl in Scheme 3). Whereas a mixture of phosphorus-contain-
ing products was formed initially (including 3; 31P NMR), treat-
ment of this reaction mixture with triethylamine promoted the
conversion of at least one of these products (possibly correspond-
Table 1
Crystallographic data for 3.
Empirical formula
Formula weight
Crystal dimensions (mm3)
Color, habit
Crystal system
Space group
a (Å)
C31H24N1O2P1
473.82
0.20 ꢂ 0.20 ꢂ 0.10
Colorless, plate
Monoclinic
P21
9.4950(6)
7.2100(4)
17.9760(8)
90
b (Å)
c (Å)
a
(°)
b (°)
103.444(3)
90
1196.90(11)
2
c
(°)
V (Å3)
Z
Range of transmission
2h Limit (°)
0.9857–0.9716
52.74
ꢀ11 6 h 6 11
ꢀ8 6 k 6 9
ꢀ22 6 l 6 22
4656
Independent reflections
Observed reflections
3840
Data/restraints/parameters
Goodness-of-fit
4656/1/319
1.057
Fig. 1. ORTEP diagram of 3, shown with 50% displacement ellipsoids and with the
indene atomic numbering scheme depicted; selected H-atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (°) for 3: P–C3 1.777(3); P–O1
1.671(3); P–O2 1.669(2); N–C2 1.351(4); N—CHa3 1:467ð4Þ; N—CH3b 1:455ð5Þ; C1–
C2 1.511(4); C2–C3 1.399(4); C2—N—CHa3 120:8ð3Þ; C2—N—CH3b 123:2ð3Þ; CH3a —
N—CHb3 115:7ð3Þ.
Absolute structure parameter
ꢀ0.08(14)
R1 ½F2o ꢁ 2
r
ðF2oÞꢃ
0.0561
wR2 ½F2o ꢁ ꢀ3
r
ðF2oÞꢃ
0.1331
0.257, ꢀ0.270
Largest peak, hole (e Åꢀ3
)

1946
D. Wechsler et al. / Journal of Organometallic Chemistry 694 (2009) 1943–1947
Table 2
ing to 2) into 3. The P,N-indene 3 was obtained as an analytically
pure white solid in 71% isolated yield. To complement the solution
spectroscopic characterization of 3, a single-crystal X-ray diffrac-
tion experiment was conducted; an ORTEP [12] diagram of 3 is pre-
sented in Fig. 1 and crystallographic data are collected in Table 1.
The overall geometric features in 3 are comparable to those of
the related achiral compound 3-PPh2-2-NMe2-indene [6a]. Nota-
bly, the apparent co-planarity of the non-hydrogen indene and
dimethylamino atoms, the relatively short N–C2 distance
(1.351(4) Å), and the sum of the angles at nitrogen (ca. 360°), col-
lectively indicate that the lone pair of electrons on nitrogen in 3 is
Hydrogenation of methyl-2-acetamidoacrylate.a
O
O
H2
O
O
*
NH
catalyst
NH
(1)
O
O
8
9
b
Entry
Mol% 3 added
Solvent
%
% eec
2-1
2-2
2-3
2-4
0
THF
>99
>99
26
<5
40
<5
67
6.0
6.0
12.0
THF
CH2Cl2
THF
in conjugation with the indene
p-system. Furthermore, the abso-
lute configuration observed in the crystallographically character-
ized sample of 3 served to confirm that the stereochemistry of
the (R)-BINOL synthon is retained in this new P,N-substituted
indene.
32
Conditions: 24 °C; 1 atm H2; 5.0 mol% ½ðCODÞRhðTHFÞ ꢃþBF4ꢀ prepared in situ
a
2
from 0.5 [(COD)MCl]2 and AgBF4 in THF (COD =
g
4-1,5-cyclooctadiene).
b
Percent conversion based on the consumption of methyl-2-acetamidoacrylate
Having successfully prepared the new chiral P,N-indene ligand
3, we turned our attention to the preparation of rhodium coordina-
tion complexes. However, unlike 1-PiPr2-2-NMe2-indene (1),
which served as a precursor to the cationic complexes [(COD)
(8) at 24 h.
c
Enantiomeric excess of 9 determined on the basis of chiral GC-FID data [9].
Rh(
j
2-1/3-PiPr2-2-NMe2-indene)]+Xꢀ (COD =
g
4-1,5-cyclooctadi-
2-3-PiPr2-2-
ene), as well as the related zwitterion (COD)Rh(
j
hydrogenation and hydrosilylation studies (vide infra) were con-
ducting by using primarily catalyst mixtures that were prepared
in situ.
NMe2-indenide) [4c,15], rather complex reactivity was observed
when employing 3 under similar conditions. In this regard, treat-
ment of
3 with either [(COD)RhCl]2 or in situ generated
[(COD)Rh(THF)2]+Xꢀ afforded an intractable mixture of products.
Furthermore, our efforts to generate an indenyl salt of 3 by use
of reagents such as n-BuLi or NaN(SiMe3)2 under conditions that
proved effective for the metalation of 1, invariably resulted in the
formation of complex reaction mixtures. Although we are pres-
ently uncertain as to the identity of the products formed in the lat-
ter metalation reactions involving 3, it is possible that P–O bond
cleavage occurs in the presence of the alkali metal reagents
employed.
3.2. Asymmetric hydrogenation
The rhodium-mediated asymmetric hydrogenation of methyl-
2-acetamidoacrylate 8 to afford 9 (Eq. (1)) under mild conditions
(24 °C, ca. 1 atm H2, 5.0 mol% Rh) was selected as a test reaction
to use in benchmarking the catalytic performance of 3 [16]; repre-
sentative results of these hydrogenation studies are collected in
Table 2. Control experiments confirmed the ability of catalyst mix-
tures derived from 0.5 [(COD)RhCl]2 and AgBF4 in THF (without
added 3) to efficiently mediate the reduction of 8 (entry 2-1). Un-
der analogous conditions employing 6.0 mol% 3 (1.2 equiv. relative
to rhodium) in THF, the quantitative hydrogenation of 8 was also
achieved, affording 9 in a modest 40% ee (entry 2-2); inferior per-
formance was observed in dichloromethane (entry 2-3). While
increasing the amount of 3 to 12.0 mol% provided a rise in enanti-
oselectivity in THF (67% ee; entry 2-4), such gains were made at the
expense of catalyst productivity (32% conversion). Negligible con-
version was achieved by using 6.0 mol% of the isolable Rh complex
7 in either THF or CH2Cl2 under similar experimental conditions.
In exploring the reactivity of [(COE)2RhCl]2 (COE =
g
2-cyclooc-
tene) with 3, a 2:1 ligand-to-metal complex 7 was formed invari-
ably (31P NMR), even when using a 1:1 stoichiometry of 3:Rh
(Scheme 4); complex 7 was also generated cleanly when using a
2:1 ratio of 3 and (PPh3)3RhCl. Complex 7 was obtained as an ana-
lytically pure light brown solid in 81% isolated yield, and was char-
acterized spectroscopically. Our assignment of 7 as featuring both
j
2-P,N-3 and 1-P,N-3 ligands in which the phosphorus atoms are
j
cis-disposed is in keeping with the observation of two 31P NMR res-
2
onances that exhibit relatively small JPP values (36.4 Hz). This
structural assignment is also consistent with the observation of
distinct 1H and 13C NMR resonances for each of the inequivalent
3.3. Asymmetric hydrosilylation
methyl groups of the
onances for the methyl groups of the
j
2-P,N-3 ligand, as well as a single set of res-
j
1-P,N-3 ligand which are
In an effort to evaluate further the catalytic utility of 3, we
turned our attention to the asymmetric hydrosilylation of ketones
[17] – a challenging transformation for which rhodium catalysts
rendered equivalent by rotation and inversion processes at nitro-
gen [4c] that are rapid on the NMR timescale. In light of our limited
success in preparing isolable rhodium complexes of 3, asymmetric
supported by appropriately designed chiral
j
2-P,N ligands have
proven to be highly effective [18]. Representative results of our cat-
alytic investigations examining the asymmetric hydrosilylation of
acetophenone with Ph2SiH2 (Eq. (2)) are collected in Table 3. In
preliminary experiments conducted in THF using catalyst mixtures
comprised of 2.5 mol% [(COD)RhCl]2 or [(COE)2RhCl]2 and 6.0 mol%
3, conversions of 74% and 63% were achieved (entries 3-1 and 3-2).
Altering the conditions to promote the formation of 7 as a pre-cat-
alyst (2.5 mol% [(COE)2RhCl]2 and 12.0 mol% 3) offered no reactiv-
ity advantages (55%; entry 3-3), and the catalytic performance of
pre-formed 7 proved to be identical to this in situ prepared catalyst
mixture. While related cationic rhodium catalyst systems did not
offer significantly improved performance in THF (30–66%; entries
3-4 to 3-7), in moving to dichloromethane and employing 3:Rh
in an equilmolar ratio, somewhat higher conversions were
achieved (57–93%; entries 3-8 to 3-11). However, despite the range
H
H
Me N
2
O
O
0.25 [(COE) RhCl]
P
2
2
P
O
3
or 0.5 (PPh ) RhCl
Rh
O
3 3
Cl
N
H
Me
2
H
7
Scheme 4.

D. Wechsler et al. / Journal of Organometallic Chemistry 694 (2009) 1943–1947
1947
Table 3
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
ta_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.jorganchem.
2009.02.015.
Addition of diphenylsilane to acetophenone.
O
OH
H+
Ph2SiH2
catalyst
(2)
Ph
Ph
*
10
H2O
References
b
Entry
Metal precursora (5.0 mol% Rh)
Mol% 3 added
Solvent
%
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
[(COD)RhCl]2
[(COE)2RhCl]2
[(COE)2RhCl]2
6.0
6.0
12.0
6.0
12.0
6.0
12.0
6.0
THF
THF
THF
THF
THF
THF
THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
74
63
55
66
52
50
30
84
57
93
90
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½ðCODÞRhðTHFÞ2ꢃþBF4ꢀ
½ðCOEÞ2RhðTHFÞ2ꢃþBF4ꢀ
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½ðCODÞRhðTHFÞ2ꢃþBF4ꢀ
½ðCODÞRhðTHFÞ2ꢃþBF4ꢀ
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4. Summary and conclusions
In conclusion, we have reported herein on the synthesis and
characterization of a new enantiopure heterobidentate P,N-indene
ligand 3 that is derived from (R)-BINOL. In contrast to the estab-
lished ability of 1/3-PiPr2-2-NMe2-indene to give rise to isolable
neutral, cationic, and formally zwitterionic j
2-P,N rhodium coordi-
nation complexes in high yield, more complex reactivity was ob-
served for 3. In the course of these coordination chemistry
studies, the formation of the isolable complex [(j j
2-P,N-3)( 1-P,N-
3)RhCl] (7) was observed, thereby confirming the propensity of this
new ligand to form LnRh(3)2 complexes; such ligation behavior
may contribute in part to the generally poor catalytic performance
exhibited by mixtures of 3 and rhodium precursor complexes in
the asymmetric hydrogenation and hydrosilylation studies re-
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Acknowledgment is made to Dalhousie University and the Nat-
ural Sciences and Engineering Research Council of Canada for their
generous support of this work. We also thank Drs. Michael Lums-
den and Katherine Robertson (Atlantic Region Magnetic Resonance
Center, Dalhousie) for assistance in the acquisition of NMR data.
Appendix A. Supplementary material
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CCDC 719217 contains the supplementary crystallographic data
for 3. These data can be obtained free of charge from The Cam-