Resolution of BIPHEP on Rh with a chiral diene auxiliary

Article abstract of DOI:10.1016/j.jorganchem.2005.09.050

Resolution of the atropisomeric chiral BIPHEP ligand on Rh has been achieved with the aid of 2-(4-tert-butyl-phenyl)-8-methoxy-1,8-dimethyl-bicyclo[2.2.2]octa-2,5-diene, a chiral chelating diene ligand. The diene complex 3 containing an (S)-BIPHEP ligand was found to be configurationally stable in CDCl₃ solution at RT. Conversion of the diene complex 3 to a dicarbonyl Rh complex 4 that had a barrier of 25.0 kcal/mol for atropisomerization of the BIPHEP ligand. Preliminary studies of the use of the resolved complex 3 for catalysis are presented.

Full text of DOI:10.1016/j.jorganchem.2005.09.050

Journal of Organometallic Chemistry 691 (2006) 2207–2212
www.elsevier.com/locate/jorganchem
Resolution of BIPHEP on Rh with a chiral diene auxiliary
*
J.W. Faller , Jeremy C. Wilt
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107, USA
Received 13 July 2005; received in revised form 28 September 2005; accepted 28 September 2005
Available online 15 November 2005
Abstract
Resolution of the atropisomeric chiral BIPHEP ligand on Rh has been achieved with the aid of 2-(4-tert-butyl-phenyl)-8-methoxy-1,8-
dimethyl-bicyclo[2.2.2]octa-2,5-diene, a chiral chelating diene ligand. The diene complex 3 containing an (S)-BIPHEP ligand was found
to be configurationally stable in CDCl3 solution at RT. Conversion of the diene complex 3 to a dicarbonyl Rh complex 4 that had a
barrier of 25.0 kcal/mol for atropisomerization of the BIPHEP ligand. Preliminary studies of the use of the resolved complex 3 for catal-
ysis are presented.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Asymmetric catalysis; Resolution; Chiral induction
1. Introduction
lyst, followed by the addition of a substoichiometric
amount of a ‘‘chiral poison’’. The chiral poison effectively
serves to deactivate one enantiomer of the racemic catalyst,
usually by strong, diastereoselective binding to a vacant
site. This allows the non-poisoned enantiomer of the cata-
lyst to carry out the asymmetric reaction. Excellent enanti-
oselectivities have been obtained via this strategy in a
number of cases [14,21,22].
Another strategy involves the selective activation of one
enantiomer of a chiral, racemic metal complex, known as
‘‘asymmetric activation’’ [14,23–25]. This approach entails
the addition of another enantiomerically pure chiral ligand
to the racemic metal complex to create two diastereomeric
species which catalyze the reaction in question at different
rates. If this difference in rates is great, then the reaction
will proceed selectively through only one of the diastereo-
meric catalysts, giving potentially high enantioselectivities.
An interesting variation of these strategies makes use of
racemic atropisomeric ligands such as 2,20-bis(diph-
enylphosphino)-1,10-biphenyl (BIPHEP) [26]. The BIPHEP
ligand exhibits hindered rotation about its biaryl axis, but
the barrier to atropisomerization has been determined to
be only 22 kcal/mol (Fig. 1) [27]. As such, the BIPHEP
ligand racemizes slowly at room temperature, making it
unresolvable by traditional methods. However, upon
Axially chiral C2 symmetric ligands such as Bis(diph-
enylphosphino)-1,10-binaphthyl (BINAP) and 2,20-Dihy-
droxy-1,10-binaphthyl (BINOL) have received much
attention owing to their ability to induce high levels of
enantioselectivity for a variety of catalytic reactions. These
ligands, and others similar to them, maintain their chirality
due to the large barrier to atropisomerization involving
hindered rotation about the bond connecting the naphtha-
lene moieties. The syntheses of enantiomerically pure
binaphthyl derivatives usually entail a classical resolution
of the two enantiomers [1–6]. Although methods exist for
the asymmetric coupling of biaryl fragments, such method-
ology is far from being general in scope, or (in most cases)
insufficiently enantioselective to produce the desired non-
racemic biaryl in adequate enantiomeric purity for use as
a chiral ligand [7–12].
Alternatively, several strategies exist for the use of race-
mic ligands in asymmetric catalysis [13,14]. One such
approach is termed ‘‘chiral poisoning’’ [14–20]. This
method involves the preparation of a chiral, racemic cata-
*
Corresponding author. Tel.: +1 203 432 3954; fax: +1 203 432 6144.
E-mail address: jack.faller@yale.edu (J.W. Faller).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.09.050

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J.W. Faller, J.C. Wilt / Journal of Organometallic Chemistry 691 (2006) 2207–2212
MeO
Me
Me
ΔG = 22 kcal / mol
PPh2
PPh2
PPh2
PPh2
Fig. 1. Racemization of BIPHEP.
t-Bu
1
complexation to a transition metal, this barrier is raised,
allowing the possibility of resolution. By using a second
enantiopure chiral ligand as a chirality-controlling element,
the BIPHEP ligand may be dynamically resolved on a par-
ticular transition metal. This chirality control stems from
steric interactions between the racemic BIPHEP ligand
and the added enantiopure ligand.
Several examples of the dynamic resolution of the BIP-
HEP ligand have been reported in the last decade. Mikami
has performed dynamic resolutions of BIPHEP on Ru
[28,29], Pd [30,31], and most recently Rh [32] with the use
of enantiopure chiral diamine ligands. As the barriers to
atropisomerization are generally higher when BIPHEP is
complexed to a transition metal, these complexes can some-
times be used for asymmetric catalysis without significant
erosion of the enantiopurity of the catalyst. For example,
the epimerization of 3,30-dimethyl-BIPHEP on Rh with
(R)-2,20-diamino-1,10-binaphthyl (BINAM) as the chirality
inducer took place at 80 ꢁC within 5 h to give the diastereo-
merically pure (R,R) complex. The amine could then be
removed by protonolysis at 5 ꢁC, and the Rh complex
was used as a catalyst for the asymmetric cycloisomeriza-
tion of some 1,6-enynes with excellent enantioselectivity
[32].
Fig. 2. A chiral diene developed by Carreira.
of these ligands are that their chirality is derived from com-
mercially available, inexpensive (ꢀ)-carvone, and that the
syntheses of these ligands are facile. Carreira has also
recently extended the utility of derivatives of these dienes
to the Rh-catalyzed 1,4-addition of arylboronic acids to a
wide variety a,b-unsaturated ketones with excellent ee [41].
2. Results and discussion
The diene, 2-(4-tert-butyl-phenyl)-8-methoxy-1,8-dimethyl-
bicyclo[2.2.2]octa-2,5-diene 1 was synthesized in good
overall yield according to the method of Careirra [40]. The
strategy involved in our approach was to take advantage
of the protruding t-BuC6H4 group on this particular chiral
diene to exert control over the axial chirality of the BIPHEP
ligand in a square-planar arrangement. As the t-BuC6H4
group is relatively large, and all other diene positions of 1
are unsubstituted, it would appear that the diphenylphosph-
ino group of BIPHEP underneath this large group may pref-
erentially orient itself away from it due to unfavorable steric
interactions.
´
Gagne [33] has also studied the resolution of BIPHEP–
Rhodium dimer 2 was obtained in 97% yield from [(cyc-
looctene)2RhCl]2 (Fig. 3). Preparation of BIPHEP complex
3 was equally straightforward; abstraction of chloride ion
from 2 in THF followed by slow addition of a BIPHEP
solution produced the desired product in 95% yield. Ini-
tially, product 3 exists as a 3:1 diastereomeric ratio in
CDCl3 solution, where BIPHEP has adopted either an
(R) or (S) configuration. Allowing this solution to stand
for a period of 1 week at RT produced no change in the
diastereomeric ratio.
All attempts at heating this mixture (CHCl3, THF, tolu-
ene) resulted in extensive decomposition of the Rh complex;
therefore, the two diastereomers of 3 could not be dynami-
cally resolved utilizing this strategy. Fractional crystalliza-
tion by slow evaporation of a rigorously degassed EtOH
solution provided the major diastereomer, (S)-3, in 50%
yield. An X-ray crystallographic study of this complex
proved the structure of (S)-3 unambiguously (Fig. 4). The
remaining supernatant consisted of an approximately 1:1
ratio of the two diastereomers of 3, along with a small
amount of decomposition product. Unfortunately, further
purification by crystallization of this mixture was not suc-
cessful in our hands. Complex (S)-3 was configurationally
stable in CDCl3 under N2 for a period of 1 week, showing
no trace of the other diastereomer. Thus, the chelation
Pt complexes with BINOL as the chiral controller. After
performing a similar dynamic resolution as the above
method, protonolysis of the BINOL ligand gave the (BIP-
HEP)PtII dication, which was used as a highly enantioselec-
tive Lewis acid catalyst in the asymmetric Diels–Alder
reaction [34].
Herein, we present the resolution of the BIPHEP ligand
on Rh with the aid of a chelating chiral diene auxiliary
ligand [35]. The first example of a chelating chiral diene
ligand used in asymmetric catalysis was HayashiÕs C2-sym-
metric diene based on the norbornadiene skeleton [36]. This
ligand, though rather difficult to synthesize, proved to be
very effective in the Rh-catalyzed 1,4-addition of arylbo-
ronic acids to a,b-unsaturated ketones. Hayashi has also
developed C2-symmetric chiral dienes based on the
[2.2.2]-bicyclooctadiene skeleton, which are useful for the
Rh-catalyzed asymmetric arylation of N-tosylarylimines
[37] and the cyclization of alkynals [38], as well as the
1,4-addition reaction mentioned above [39].
Carreira has also prepared a chiral diene family based
on the [2.2.2]-bicyclooctadiene skeleton for use in the
Ir-catalyzed kinetic resolution of aryl-substituted allylic
carbonates during the course of an asymmetric allylic
etherification reaction (Fig. 2) [40]. Two important features

J.W. Faller, J.C. Wilt / Journal of Organometallic Chemistry 691 (2006) 2207–2212
2209
t-Bu
MeO
Me
Me
[(COE)2RhCl]2
CH2Cl2, RT
Me
Rh
MeO
Cl
2
t-Bu
1
2
SbF6
t-Bu
Ph
1.) AgSbF6
Ph
P
Me
2.) BIPHEP
thf, RT
Rh
MeO
P
Ph
Ph
3
Fig. 3. Synthesis of 2 and 3.
SbF6
Ph
P
Ph
Rh
Ph
OC
OC
CO
3
P
CHCl3
Ph
4
Fig. 5. Removal of diene 1.
P2
plex in optically active form. Though 4 may have racemized
to some extent during its preparation (< 5 min), it retained
sufficient optical activity to permit a study of its racemiza-
tion using polarimetry. The rate constant for loss of optical
rotation of 4 was determined to be 5.3 · 10ꢀ6 sꢀ1; therefore,
the rate constant for atropisomerization of the BIPHEP
ligand on Rh is 0.5krac, or 2.6 · 10ꢀ6 sꢀ1. This rate constant
for atropisomerization of the BIPHEP ligand in 4 corre-
sponds to a DGà value of 25.0 kcal/mol, which is ꢁ 3 kcal/
mol higher than that of the free ligand.
Rh1
P1
O1
C4
C5
The relatively high barrier to atropisomerization of BIP-
HEP on Rh, in addition to the recent literature example
[32], suggested that complex (S)-3 may be used in asymmet-
ric catalysis without substantial enantiomeric degradation
of the catalyst. Initially, hydrogenation of prochiral olefins
such as dimethyl itaconate was attempted with (S)-3. Inter-
estingly, complex (S)-3 showed no activity towards H2 at
1 atm. Presumably, this lack of activity is derived from
the difficulty of the hydrogenation of a trisubstituted olefin.
An attempted hydrosilylation of dimethyl itaconate with
diphenylsilane unfortunately led to the hydrogenated prod-
uct in racemic form (Fig. 6). Although this type of reaction
Fig. 4. An ORTEP diagram of (S)-3 showing 30% probability ellipsoids.
and the diene ligand appear to be preventing the atropiso-
merization of the BIPHEP ligand.
To examine the atropisomerization of BIPHEP on Rh, it
was necessary to remove the chirality inducer 1. When CO
was bubbled through a solution of (S)-3, the diene ligand
was quickly replaced by carbonyls, giving complex 4 as a
yellow solid (Fig. 5). Quick evaporation of the solvent fol-
lowed by trituration of 4 with Et2O provided the pure com-

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J.W. Faller, J.C. Wilt / Journal of Organometallic Chemistry 691 (2006) 2207–2212
Chiral diene 1 was prepared as previously described [40].
1% (S)-3
O
O
OMe
OMe
[(cyclooctene)2RhCl]2 was prepared via a published proce-
dure [45]. Optical rotations were measured on a Perkin–
Elmer model 341 polarimeter at 589 nm and 25 ꢁC, using
a 1 dm path length. Enantiomeric excesses of sec-phenethyl
alcohol were measured by the 1H NMR integration of
appropriate resonances of the Mosher ester.
*
MeO
MeO
PhSiH2
thf, RT, 12h
O
O
0% ee
Fig. 6. Hydrosilylation of dimethyl itaconate with (S)-3.
OH
3.2. Preparation of 2
1% (S)-3
*
To a stirred solution of [(cyclooctene)2RhCl]2 (204 mg,
0.28 mmol) in CH2Cl2 (8 mL) was added diene 1 (196 mg,
0.66 mmol). The resulting orange-red solution was stirred
at RT for 16 h. The mixture was filtered through a pad
of Celite and the solvent was removed under reduced pres-
sure to leave a crude solid. This was subjected to column
chromatography over silica gel, eluting first with pentane
to remove displaced cyclooctene, then with Et2O to elute
an orange-yellow band. Removal of the solvent gave pure
catecholborane
thf, RT, 5h
57% Yield
12 % ee
Fig. 7. Hydroboration of styrene with (S)-3.
with diphenylsilane is unusual, it has been previously doc-
umented [42–44]. This complex was also tested in the hyd-
roboration of styrene with catecholborane. Although
complex 3 was catalytically active, the product alcohol
was produced in only 12% ee at ambient temperature
(Fig. 7).
Upon lowering the temperature to ꢀ78 ꢁC, complex (S)-
3 showed no catalytic activity. When adding catecholbo-
rane to complex (S)-3 at RT, followed by cooling to
ꢀ78 ꢁC and then adding styrene, the reaction proceeded
only to <5% conversion.
In conclusion, we have developed a novel method for
the resolution of the atropisomerically chiral BIPHEP
ligand on Rh with the aid of CarreiraÕs chiral diene ligand
1. Complex (S)-3 is configurationally stable for over one
week in CDCl3 solution at RT. The barrier to atropisomer-
ization of BIPHEP on Rh was found to be 25.0 kcal/mol
after removal of 1 with CO. Thus, the barrier is dependent
upon the other ligands, as well as chelation. Other non-
racemic Rh(BIPHEP) catalysts should be available by dis-
placement of the diene. Though a successful catalytic asym-
metric reaction was not realized with this particular
complex in our hands, the modularity of these chiral diene
ligands should allow for sufficient tuning to make this a
possibility in future studies.
1
2 as an orange-yellow powder (240 mg, 97%). H NMR
(400 MHz, CDCl3): d 7.8 (4H, br s); 7.35 (4H, br s); 4.33
(2H, br t, J = 6.0 Hz); 3.87 (2H, br t, J = 6.0 Hz); 3.62
(2H, br s); 3.48 (2H, br s); 3.20 (6H, s, –OMe); 1.79 (6H,
s, Me); 1.34 (18 H, s, t-Bu); 1.21 (6H, s, Me); 1.12 (2H,
d, J = 14.0 Hz); 0.92 (2H, d, J = 14.0 Hz). 13C NMR
(125 MHz, CDCl3): d 149.9, 134.3, 130.2, 124.0, 80.3,
71.4, 56.5, 53.1, 53.0, 49.8, 49.5, 49.0, 48.7, 34.5, 31.3,
21.8, 21.7. Anal. Calc. for C42H56Cl2O2Rh2: C, 58.01; H,
6.49. Found: C, 57.63; H, 6.70.
3.3. Preparation of 3
To compound 2 (25 mg, 28.8 lmol) and AgSbF6 (18 mg,
52.4 lmol) in a Schlenk flask equipped with an addition
funnel was added 5 mL THF. The resulting cloudy, light
orange solution was stirred at RT for 0.5 h. A solution of
BIPHEP (27.4 mg, 52.4 lmol) in 10 mL THF was trans-
ferred to the addition funnel via cannula, and was added
dropwise to the reaction mixture. The solution quickly
turned deep orange in color. After addition was complete,
the reaction mixture was stirred at RT for an additional
0.5 h, then filtered through a pad of Celite and concen-
trated. Trituration of the resulting orange solid with Et2O
followed by filtration gave crude 3 as a 3:1 diastereomeric
mixture (56 mg, 93%). The major diastereomer (S)-3 could
be isolated as red crystals by slow evaporation of a solution
of crude 3 in carefully degassed EtOH under argon in 50%
yield.
Future work will focus on expanding this strategy to the
resolution of other atropisomerically flexible chiral ligands
analogous to BIPHEP.
3. Experimental
3.1. General
Major diastereomer (S)-3. 1H NMR (400 MHz,
CDCl3): d 7.82–6.87 (28H, BIPHEP aryl region); 6.86
(2H, J = 8.2 Hz); 6.74 (2H, J = 8.2 Hz); 5.04 (1H, t,
J = 5.9 Hz); 4.85 (1H, br s); 4.75 (1H, br s); 3.48 (1H, br
s); 3.16 (3H, OMe, s); 1.62 (3H, Me, s); 1.14 (3H, Me, s);
1.12 (9H, t-Bu, s); 0.97 (1H, d, J = 13.6 Hz); 0.89 (1H, d,
J = 13.6 Hz); 31P NMR (162 MHz, CDCl3); d 30.6 (dd,
All synthetic manipulations were carried out under inert
gas using standard Schlenk techniques. Dichloromethane
was distilled from CaH2 prior to use. THF was distilled
over sodium metal before use. Ethanol was dried over
˚
4 A molecular sieves and subjected to three freeze–pump–
thaw cycles prior to use. BIPHEP was purchased from
Strem Chemicals and used without further purification.
JRh–P = 162.7 Hz, JPP = 43.7 Hz); 25.6 (dd, JRh–P
=
162.7 Hz, JPP = 43.7 Hz). 13C NMR (125 MHz, CDCl3):

J.W. Faller, J.C. Wilt / Journal of Organometallic Chemistry 691 (2006) 2207–2212
2211
d 151.6–124.9 (aryl region, 42C); 108.2, 93.0, 90.5, 87.1,
83.7, 52.9, 51.5, 51.3, 50.6, 34.7, 31.4, 22.3, 22.2. [a]25
(589 nm) = ꢀ34.0 (c = 0.0025, CHCl3). Anal. Calc. for
C57H56F6OP2RhSb: C, 59.14; H, 4.88. Found: C, 59.21;
H, 4.96.
by reference to the diene and by inverting the coordinates
which yielded: R = 0.0569 and Rw = 0.0511. Detailed
information is provided in the supporting information.
Appendix A. Supplementary data
3.4. Preparation of 4
Crystallographic data for the structural analysis has been
deposited with the Cambridge Crystallographic Data Cen-
tre, CCDC No. 278397 for compound 3. Copies of this
information may be obtained free of charge from: The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ
UK; fax (int code): +44 (1223) 336 033; or e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk. The
metrical parameters are also available in the supporting
Information. Supplementary data associated with this arti-
cle can be found, in the online version, at doi:10.1016/
j.jorganchem.2005.09.050.
Carbon monoxide was vigorously passed through an
orange solution of 3 (15 mg, 13 lmol) in chloroform
(1 mL). Within 5 min, the solution became bright yellow,
and the solvent was removed in vacuo. The resulting light
yellow powder was triturated with Et2O to remove dis-
placed 1, filtered, and dried under vacuum to leave the
1
desired compound (11.2 mg, 95%). H NMR (400 MHz,
CDCl3): d 7.72–7.35 (20H, BIPHEP aryl region); 7.14–
7.00 (6H, BIPHEP aryl region); 6.58 (2H, BIPHEP aryl
region). 31P NMR (162 MHz, CDCl3): d 22.1 (d, JRh–P
=
123 Hz). 13C NMR (125 MHz, CDCl3, carbonyl only): d
143.3 (dd, J = 7.2, 13.7 Hz). IR (thin film solid): 2098
(mCO), 2054(mCO). [a]25 (589 nm) = +65.6 (c = 0.0032,
CHCl3). Anal. Calc. for C38H28F6O2P2RhSb: C, 49.76;
H, 3.08. Found: C, 49.44; H, 3.52.
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Orange crystals were obtained by slow evaporation of
an EtOH solution of complex 3. Data were collected on a
0.1 · 0.1 · 0.2 crystal at ꢀ100 ꢁC on a Nonius KappaCCD
(Mo Ka radiation) diffractometer and were not specifically
corrected for absorption other than the inherent correc-
tions provided by SCALEPACK [46]. The structure was solved
by direct methods (SIR 92) [47] and refined on F for all
reflections [48]. Non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms
were included at calculated positions. An asymmetric unit
containing two independent [C57H56OP2Rh][SbF6] moieties
was located in a primitive monoclinic cell with Z = 4 and
˚
˚
lattice parameters of: a = 12.322(2) A; b = 21.0771(4) A;
3
˚
˚
c = 19.9387(3) A; b = 99.004(2)ꢁ; V = 5114.6(5) A . The
space group was P21 (#4). The total data collected was
21717 reflections of which 13036 were unique
(Rint = 0.055). A rotational disorder in the orientation of
the t-butyl groups was modeled as a 67:33 occupancy.
The refinement converged with residuals: R = 0.0529 and
Rw = 0.0456. The absolute configuration was determined

2212
J.W. Faller, J.C. Wilt / Journal of Organometallic Chemistry 691 (2006) 2207–2212
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