Enantioselective synthesis of platinum group metal complexes with the chiral PCP pincer ligand R,R-{C6H4-2,6-(CH2P*PhBu t)2} · The crystal structure of R,R-PdCl{C6H3-2,6-(CH2P*PhBu t)2}

Article abstract of DOI:10.1016/S0022-328X(02)01371-2

The novel PCP chiral ligand R,R-{C₆H₄-2,6-(CH₂P*PhBu ^t)₂} and its platinum metal complexes have been easily synthesized in enantiomeric pure form. The palladium compound R,R-PdCl{C₆H₃-2,6-(CH₂P*PhBu ^t)₂} has been characterized by X-ray crystal structure analysis thus confirming the absolute configuration of the complexated ligand. Preliminary catalytic experiments with the iridium and palladium complexes R,R-IrH₄{C₆H₃-2,6-(CH₂P*PhB u^t)₂} and R,R-PdCl{C₆H₃-2,6-(CH₂P*PhBu ^t)₂} in the dehydrogenation of cycloalkanes and in the hydrosilylation of styrene, allylic alkylation and Heck coupling reactions, respectively have been carried out.

Full text of DOI:10.1016/S0022-328X(02)01371-2

Journal of Organometallic Chemistry 654 (2002) 44Á50
/
www.elsevier.com/locate/jorganchem
Enantioselective synthesis of platinum group metal complexes with
the chiral PCP pincer ligand R,R-{C6H4-2,6-(CH2P*PhBut)2}. The
crystal structure of R,R-PdCl{C6H3-2,6-(CH2P*PhBut)2}
David Morales-Morales a,*, Roger E. Cramer b, Craig M. Jensen b
a Instituto de Qu´ımica, Universidad Nacional Auto´noma de Me´xico, Cd. Universitaria, Circuito Exterior, Coyoaca´n, D.F. 04510, Mexico
b Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA
Received 9 January 2002; received in revised form 28 February 2002; accepted 28 February 2002
Abstract
The novel PCP chiral ligand R,R-{C6H4-2,6-(CH2P*PhBut)2} and its platinum metal complexes have been easily synthesized in
enantiomeric pure form. The palladium compound R,R-PdCl{C6H3-2,6-(CH2P*PhBut)2} has been characterized by X-ray crystal
structure analysis thus confirming the absolute configuration of the complexated ligand. Preliminary catalytic experiments with the
iridium and palladium complexes R,R-IrH4{C6H3-2,6-(CH2P*PhBut )2} and R,R-PdCl{C6H3-2,6-(CH2P*PhBut)2} in the
dehydrogenation of cycloalkanes and in the hydrosilylation of styrene, allylic alkylation and Heck coupling reactions, respectively
have been carried out. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Chiral phosphines; Asymmetric catalysis; PCP pincer complexes
1. Introduction
for the introduction of chiral motifs in similar ligands
have been carried out before by Venanzi and coworkers
[5] for the synthesis of the chiral ligand {C6H4-2,6-
(CH3C*HPPh2)2}, however the synthetic method
probed to be very difficult and tedious. Further efforts
by the same group [6] led them to the synthesis of the
Among the most popular ligands used today in
asymmetric synthesis, chiral phosphines occupy an
important place [1]. These compounds have probed to
be very efficient ligands in different catalytic processes
and outstanding enantiomeric yields have continued the
increasing interest for the synthesis of these ligands and
its complexes [2]. The iridium PCP pincer complexes
ꢀ
/
chiral ligand 1S,2S-{C6H4-2,6-({2,3-O-(CH3)2C-2,3-O
C*HCH2O}-1-C*/H(PPh2)2}; the platinum complex of
ꢂ
this specie was tested in the enantioselective aldol
reaction of methyl isocyanoacetate and aldehydes with
good enantiomeric yields (65%). A more convenient
approach by Zhang and coworkers [7] resulted in the
efficient synthesis of the chiral compound 1R,1R-
{C6H4-2,6-(CH3C*HPPh2)2}. The palladium and plati-
num complexes were synthesized and the X-ray crystal
structure analysis of both species obtained, the palla-
dium complex was examined in the allylic alkylation [7a]
couplings and aldol reactions [7b] of methyl isocyanoa-
cetate with enantiomeric yields similar to those obtained
by Venanzi. Also by Zhang and coworkers [8], the
compounds R,R-{C6H4-2,6-[CH2CH2P*Ph(o-An)]2}
and R,R-{C5H3N-2,6-[CH2CH2P*Ph(o-An)]2} were
synthesized and applied in the asymmetric allylic
alkylation [8a] and hydrosilylation [8b] reactions with
IrH2fC6H3-2; 6-(CH2PBut2)2g [3] and PdClfC6H3-2; 6-
/
/
(OPPri2)2g [4] have been found to be highly efficient
and robust catalysts for the dehydrogenation of alkanes
[3a,3b,3c,3d,3e,3f], ethers and alkyl arenes [3c], amines
[3g], alcohols [3h] and in the Heck reaction [4],
respectively. Following our continuous interest in the
design and screening of highly reactive and thermally
stable organometallic species for organic transforma-
tions, we have turned our efforts to the design and
synthesis of the chiral counterparts of the PCP pincer
ligand in IrH2fC6H3-2; 6-(CH2PBut2)2g: First attempts
* Corresponding author. Tel.: ꢀ
E-mail address: damor@servidor.unam.mx (D. Morales-Morales).
/
52-5-6224514; fax: ꢀ52-5-6162217.
/
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 3 7 1 - 2

D. Morales-Morales et al. / Journal of Organometallic Chemistry 654 (2002) 44Á
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45
modest enantiomeric yields. In this case the active
species were assumed to be chelated complexes with
the PCP or PNP compounds behaving as bidentated or
tridentated ligands, respectively. As noted above by
exception of Zhang’s compounds the chiral information
has been located on the a-benzylic positions. It has been
suggested that in order to achieve high enantiomeric
yields the chiral information must be located in a closer
proximity to the metal center [9]. Based on these
observations we have synthesized the R,R-{C6H4-2,6-
(CH2P*PhBut)2} ligand and its platinum group metal
complexes in enantio-pure form; preliminary catalytic
results are also reported herein.
were placed 5.0 g (3.9 ml, 27.93 mmol) of dichlorophe-
nylphosphine and 10 ml of dry THF under Ar. The flask
was cooled to ꢁ78 8C, and a solution of tert-butyl-
/
magnesium chloride (20.54 ml of 1.36 M THF solution)
was added with stirring by syringe during 1 h. After
addition, the cooling bath was removed and stirring was
continued at room temperature (r.t.) for 1 h. The flask
was then immersed in an ice bath and a THF solution of
LiAlH4 (27.94 ml of 1 M THF solution) and neat BH3ꢃ
/
SMe2 (2.84 ml, 30.0 mmol) were added consecutively by
syringe and the solution stirred for and additional 1 h.
The reaction mixture was carefully poured into a
mixture of concd. HCl (15 ml), ice (ca. 30 g) and
CH2Cl2 (20 ml). The organic layer was separated and
the aq. phase was extracted with CH2Cl2 (2ꢃ5 ml). The
/
2. Experimental
combined extracts were dried over MgSO4 and concen-
trated in vacuum. NMR spectroscopy showed the
product to be better than 98% pure and it was used in
2.1. General
further steps without further purification. Yield: 3.5 g,
1
All manipulations were carried out using standard
Schlenk and glovebox techniques under purified argon.
Solvents were degassed and dried using standard
procedures. The following were purchased and used
without further purification, Cl2PPh, a,a-1,3-dibromox-
70%. H-NMR (400 MHz, CDCl3, d ppm): 0.1Á
/
1.0 (q,
14.8 Hz, 9H, PC(CH3)3), 4.64
363.6 Hz, 1H, PH), 7.4Á7.7 (m, 5H,
Ar); 31P-NMR (161.90 MHz, CDCl3, d ppm): 31.00 (q,
Jꢂ
91.65 Hz, 1P); 13C-NMR (100.6 MHz, CDCl3, d
ppm): 26.41 (d, Jꢂ2.82 Hz, PC(CH3)3), 28.33 (d, Jꢂ
32.69 Hz, PC(CH3)3), 124.51Á133.92 (m, Ar). Anal.
br, 3H, PBH3), 1.2 (d, Jꢂ
/
and 5.55 (dq, Jꢂ
/
/
/
ylene, (ꢁ
BuLi (Aldrich Chemical Co.). The complexes [Ir(m-
Cl)(COE)2]2 [10] (COEꢂcyclooctene), [PdCl2(COD)]
[11] (CODꢂ1,5-cyclooctadiene) were synthesized by
/
)-sparteine, BH3×
/
SMe2, HBF4×
/
Et2O and n-
/
/
/
/
Calc. for C10H18BP (180): C, 66.71; H, 10.08. Found: C,
66.67; H, 9.97%.
/
the literature methods. The 1H-NMR spectra were
recorded on a Varian Unity Inova 400 spectrometer.
Chemical shifts are reported in ppm down field of TMS
using the solvent as internal standard (CDCl3, d 7.26 or
C6D6 d 7.16). 13C- and 31P-NMR spectra were recorded
with complete proton decoupling and are reported in
ppm downfield of TMS with solvent as internal stan-
dard (CDCl3, d 77.0 or C6D6 d 128.4) and external 85%
H3PO4, respectively. GC analyses were carried out in a
HP 5890A flame ionization detector (FID) and HP 5890
SERIES II with an 5971A mass selective detector gas
chromatographs, and an HP-1 capillary column (25.0
m) from Hewlett Packard or a SPBTM-1 capillary
column (30.0 m) from SUPELCO†. Optical rotations
were measured at 25 8C on a JASCO-DIP-370 Digital
2.3. Synthesis of R,R-{C6H4-2,6-(CH2P*PhButBH3)2}
(2)
A Schlenk flask equipped with a magnetic stirring bar
was charged with ether (100 ml), (ꢁ)-sparteine [13] (5.0
/
ml, 21.67 mmol), and tert-butylphenylphosphine-bor-
ane (1) (3.0 g, 16.67 mmol). The solution was cooled to
ꢁ78 8C and n-butyllithium (10.42 ml, 1.6 M in C6H14)
/
was added. The solution was allowed to warm to r.t.,
whereupon a thick suspension of white precipitate was
formed. After the reaction mixture had stirred for 1 h at
r.t., it was again cooled to ꢁ78 8C and a,a-1,3-
/
dibromoxylene (2.2 g, 8.33 mmol) was added as a
solution in THF (15 ml). The solution was allowed to
slowly warm to r.t. (overnight, ca. 20 h). The reaction
mixture was washed with 5% aq. H2SO4 (35 ml) and the
Polarimeter with a sodium lamp and reported as
follows: [a]Tl
ꢀC
(c g/100 ml of solvent). The ee values
were determined by analytical high performance liquid
chromatography (HPLC) on a IBM LC/9533 ternary
liquid chromatograph with a variable wavelength de-
tector, using a Daicel CHIRALPAK† AD column
aq. phase was extracted with ether of CH2Cl2 (3ꢃ15
/
ml). The combined organic phases were washed with
water (15 ml), brine (15 ml), dried (MgSO4) and filtered
through a short plug of celite. The solution was
concentrated under vacuum to yield a white solid. Yield:
(250ꢃ4.6 mm).
/
3.66 g, 95%. [a]2D5 ꢂꢀ115:4ꢀ (cꢂ
(400 MHz, CDCl3, d ppm): 0.1Á
1.11 (d, Jꢂ13.5 Hz, 18H, PC(CH3)3), 3.24Á
4H, PCH2Ar), 6.90Á
7.90 (m, 14H, Ar); 31P-NMR
(161.90 MHz, CDCl3, d ppm): 33.28 (d, Jꢂ55.47 Hz,
2P); 13C-NMR (100.6 MHz, CDCl3, d ppm): 25.67 (s,
/
1.0, CH2Cl2); 1H-NMR
1.0 (q, br, 6H, PBH3),
3.44 (m,
2.2. Synthesis of HPPhButBH3 (1)
/
/
/
Phosphine 1 was synthesized using a modification of
the procedure reported in the literature [12]. In a 100 ml
Schlenk flask equipped with a magnetic stirring bar,
/
/

46
D. Morales-Morales et al. / Journal of Organometallic Chemistry 654 (2002) 44Á50
/
PC(CH3)3), 26.85 (d, Jꢂ
/
29.68 Hz, PCH2Ar, d ppm):
2.6. Synthesis of R,R-IrHCl{C6H3-2,6-
(CH2P*PhBut)2} (5)
29.88 (d, Jꢂ29.68 Hz, PC(CH3)3), 126.00Á
/
/
134.00 (m,
Ar). Anal. Calc. for C28H42B2P2 (462.2): C, 72.76; H,
9.16. Found: C, 72.66; H, 9.19%.
A mixture of [Ir(COE)2(m-Cl)]2 (0.575 g, 0.575 mmol),
R,R-[C6H4-2,6-(CH2P*PhBut)2] (3) (0.5 g, 1.15 mmol)
and C6H5CH3 (50 ml), was heated under reflux for 24 h.
After this time the solvent is evaporated under vacuum
2.4. Synthesis of R,R-{(C6H4-2,6-(CH2P*PhBut)2}
(3)
and the solid residue taken off in C5H12 (4ꢃ50 ml), the
/
solution filtered through a short plug of celite and the
solvent removed under vacuum to yield a microcrystal-
line dark-brown powder. Yield: 632 mg, 83%. [a]2D5 ꢂ
A Schlenk flask equipped with a magnetic stirring bar
was charged with 2 (1.0 g, 2.16 mmol) and CH2Cl2 (20
1
ml). The solution was cooled to ꢁ5 8C and tetrafluor-
/
ꢀ78:0ꢀ (C6H6, 1 M); H-NMR (400.00 MHz, C6D6, d
oboric acid diethyl ether complex (3.75 ml, 21.63 mmol)
was added dropwise by syringe. The solution was
allowed to slowly warm to r.t. (overnight, ca. 20 h).
The reaction mixture is then diluted with Et2O (40 ml)
and added to a degassed, saturated aq. solution of
NaHCO3 (110 ml). The resulting biphasic mixture was
stirred vigorously under Ar for 10 min, after this time
the organic layer was separated and the aq. phase
ppm): ꢁ
/
25.4 (t, Jꢂ
/
11.2 Hz, 1H, Ir ꢃ
/
H), 1.15 (vt, Jꢂ
/
6.4 Hz, 18H, C(CH3)3), 3.57 (m, 4H, PCH2Ar), 6.9Á
/
7.7
(m, 13H, Ar); 31P-NMR (161.90 MHz, CDCl3, d ppm):
45.77 (s, 2P); 13C-NMR (100.6 MHz, CDCl3, d ppm):
24.67 (s, PC(CH3)3), 32.90 (vt, Jꢂ
35.29 (vt, Jꢂ12.02 Hz, PC(CH3)3), 121.7Á
Ar). Anal. Calc. for C28H36ClIrP2 (662.2): C, 50.78; H,
5.48. Found: C, 50.74; H, 5.46%.
/11.21 Hz, PCH2Ar),
/
/152.6 (m,
extracted with ether (2ꢃ
phases were washed with water (2ꢃ
/
20 ml). The combined organic
/20 ml), brine (20
2.7. Synthesis of R,R-IrH4{C6H3-2,6-(CH2P*PhBut)2}
(6)
ml), dried (MgSO4) and filtered through a short plug of
celite. The solution was concentrated under vacuum to
yield a viscous colorless oil. Yield: 921 mg, 98%. [a]2D5 ꢂ
The title complex was synthesized using the method
reported by Jensen and coworkers [3] by reacting the
hydrochloride complex 5 (0.6 g, 0.906 mmol) with
superhydride [LiB(C2H5)3H] (0.91 ml, 1 M solution in
THF) under a hydrogen atmosphere in C5H12 (100 ml).
Filtration through a short plug of celite and evaporation
of the solvent under vacuum affords complex 6 as pale-
orange powder. Yield: 0.548 g, 96%. [a]2D5 ꢂꢁ132:31ꢀ
ꢀ196:0ꢀ (cꢂ
d ppm): 0.91 (d, Jꢂ
Jꢂ14, 2 Hz, 2H, PCH(H)Ar), 3.07 (dd, Jꢂ
2H, PCH(H)Ar), 6.91Á
7.52 (m, 14H, Ar); 31P-NMR
(161.90 MHz, CDCl3, d ppm): 9.44 (s, 2P); 13C-NMR
(100.6 MHz, CDCl3, d ppm): 27.58 (d, Jꢂ13.38 Hz,
PC(CH3)3), 29.11 (d, Jꢂ20.12 Hz, PCH2Ar), 29.46 (d,
Jꢂ15.39 Hz, PC(CH3)3), 127.00Á139.40 (m, Ar). Anal.
/
1.0, CH2Cl2); 1H-NMR (400 MHz, CDCl3,
/11.6 Hz, 18H, PC(CH3)3), 2.92 (dd,
/
/
14, 2.8 Hz,
/
/
/
/
/
(C6H6, 1 M); 1H-NMR (400.00 MHz, C6D6, d ppm): ꢁ
8.27 (t, Jꢂ10, 4 Hz, 4H, Ir ꢃH), 1.10Á1.50 (vt, Jꢂ7.2
Hz, 18H, C(CH3)3), 3.67 (dt, Jꢂ16.4, 4.4 Hz, 2H,
PCH(H)Ar), 3.87 (dt, Jꢂ16.8, 3.8 Hz, 2H,
PCH(H)Ar), 6.86Á
7.78 (m, 13H, Ar); 31P-NMR
(161.90 MHz, CDCl3, d ppm): 50.09 (s, 2P); 13C-
NMR (100.6 MHz, CDCl3, ppm): 26.72 (s,
PC(CH3)3), 32.96 (vt, Jꢂ11.32 Hz, PCH2Ar), 35.43
(vt, Jꢂ12.12 Hz, PC(CH3)3), 127.00Á160.00 (m, Ar).
/
Calc. for C28H36P2 (434.5): C, 77.39; H, 8.35. Found: C,
77.32; H, 8.29%.
/
/
/
/
/
/
/
2.5. Synthesis of R,R-PdCl{C6H3-2,6-
(CH2P*PhBut)2} (4)
d
/
To a C6H5CH3 solution of the ligand R,R-[C6H4-2,6-
(CH2P*PhBut)2] (3) (0.5 g, 1.15 mmol) was added under
stirring a suspension of [PdCl2(COD)] (0.33 mg, 1.15
mmol) in C6H5CH3 (50 ml). The resulting mixture was
set to reflux for 5 h, after this period the solvent was
evaporated under vacuum to yield the title complex as
white powder in good yield (650 mg, 98%). [a]2D5 ꢂ
/
/
Anal. Calc. for C28H39IrP2 (629.8): C, 53.4; H, 6.24.
Found: C, 53.37; H, 6.24%.
2.8. General procedure for the hydrosilylation of styrene
Complex 4 (3 mg, 5.2ꢃ
10ꢁ3 mmol) and styrene (1
/
ꢁ96:44ꢀ (cꢂ
CDCl3, d ppm): 1.24 (vt, Jꢂ
3.27 (m, 4H, PCH2Ar), 6.90Á
8.24 (m, 13H, arom.); 31P-
NMR (161.90 MHz, CDCl3, d ppm): 52.27 (s, 2P); 13C-
NMR (100.6 MHz, CDCl3, ppm): 26.72 (s,
PC(CH3)3), 32.96 (vt, Jꢂ11.32 Hz, PCH2Ar), 35.43
(vt, Jꢂ12.12 Hz, PC(CH3)3), 124.60Á160.00 (m, Ar).
/
1.0, CH2Cl2); 1H-NMR (400.00 MHz,
ml, 8.7 mmol) were mixed in 4 ml of C6H6. The addition
of trichlorosilane (1.1 ml, 10.9 mmol) started the
reaction. The mixture was stirred for 24 h. The crude
product was carefully poured into a suspension of KF
(10 g, 0.17 mol) in 80 ml of MeOH, and stirred for 30
min. The solvent was removed in vacuo. The resulting
solid was suspended in 100 ml of DMF, H2O2 (30%
water solution, 10 ml) was added and the mixture heated
/7.6 Hz, 18H, PC(CH3)3),
/
d
/
/
/
Anal. Calc. for C28H35ClP2Pd (575.4): C, 58.45; H, 6.13.
Found: C, 58.42; H, 6.09%.
for 1 h at 60Á70 8C. The residue was isolated by aq.
/

D. Morales-Morales et al. / Journal of Organometallic Chemistry 654 (2002) 44Á
/50
47
workup and extraction and analyzed by GCÁ
HPLC. Yield: 97% of PhCH(OH)CH3.
/
MS and
correction was applied based upon c-scans of six
reflections. Transmission coefficients ranged from 0.85
to 0.94. The diffractometer auto indexing routine
indicated a hexagonal unit cell. The only systematic
2.9. General procedure for the allylic alkylation
absence was OOOL, Lꢂ
/
2nꢀ1. Since the crystal was
/
To a solution of complex 4 (3 mg, 5.2ꢃ
in CH2Cl2 (2 ml) silver triflate (2 mg, 7.7ꢃ
/
10ꢁ3 mmol)
10ꢁ3) was
known to be chiral, the space group was uniquely
determined to be P63. Using SHELX-97, both Patterson
and direct methods yielded the position of the Pd atom.
The remaining atoms were located via a few cycles of
least-square refinements and difference Fourier maps.
Hydrogen atoms were input at calculated positions, and
allowed to ride on the atoms to which they are attached.
Three group thermal parameters were refined for
hydrogen atoms, one each for phenyl, methylene and
methyl protons. The final cycle of refinement was
carried out on all non-zero data using SHELXL-97 [15]
and anisotropic thermal parameters for all non-hydro-
gen atoms. The absolute configuration was determined
/
added, the solution was stirred for 30 min. After this
time the solution was filtered through a short plug of
celite. Then (E)-3-acetoxi-1,3-diphenyl-1-propene (50.4
mg, 0.2 mmol), dimethylmalonate (68 ml, 0.6 mmol),
BSA (148 ml, 0.6 mmol), and KOAc (1 mg) were added,
and the resulting reaction mixture stirred at r.t. for 4 h.
After this time the resulting reaction mixture was diluted
with ether, washed with water and brine, and then dried
over MgSO4. The solvent was evaporated under vacuum
to yield an oily residue, which slowly crystallizes. Yield:
100% of rac-PhCH{CH(CO2Me)2}CHꢄ
/CHPh.
via the Flack parameter [16], which refined to ꢁ0.03(6).
/
2.10. General procedure for the Heck reaction
The absolute configuration at the two P atoms is R,R.
To a solution of 4 (3 mg, 5.2ꢃ
/
10ꢁ3 mmol) in C6H6 (3
10ꢁ3) was added, the
ml) silver triflate (2 mg, 7.7ꢃ
/
3. Results and discussion
solution was stirred for 30 min. After this time the
solution was filtered through a short plug of celite. Then
phenyl triflate (81 ml, 0.5 mmol), N,N-diisopropylethy-
lamine (261 ml, 1.5 mmol), and 2,3-dihydrofuran (189 ml,
2.5 mmol) were added. The solution was stirred at
60 8C for 4 days under Ar. The reaction mixture was
cooled, diluted with C5H12 (200 ml) and filtered to
remove solid materials. The solution was then washed
with 0.1 N HCl and saturated with NaHCO3 and dried
over MgSO4. The solvent was removed under vacuum
3.1. Synthesis of the ligand
The ligand R,R-{C6H4-2,6-(CH2P*PhBut)2} (3) has
been synthesized in two easy steps according to Scheme
1. In the first step, phenyl tert-butylphenylphosphine-
borane (1) is reacted with n-butyl lithium in the presence
of (ꢁ
/
)-sparteine to afford the lithium salt of the
phosphine. This compound is further reacted at ꢁ
78 8C with a,a-1,3-dibromoxylene to afford the borane
/
and the residue analyzed by GCÁMS and HPLC. Yield:
/
100% of 2-phenyl-3,4-dihydrofuran.
2.11. General procedure for the catalytic
dehydrogenation experiments [3a,3b,3c,3d,3e,3f]
A solution of cyclooctane (4.0 ml, 37.0 mmol) and the
(tert-butylethylene) (2.0 ml, 1.6 mmol) were charged
with 6 (5 mg, 0.008 mmol) in a sealed tube under Ar, the
tube was fully immersed in an oil-bath for 2 h at 150 8C.
The production of cyclooctene was quantified by GC.
2.12. Single crystal X-ray structure determination R,R-
PdCl{C6H3-2,6-(CH2P*PhBut)2} (4)
A crystalline colorless prism of R,R-PdCl{C6H3-2,6-
(CH2P*PhBut)2}, grown from a CH2Cl2Á
MeOH sol-
vent system was glued to a glass fiber. A Siemens P3
diffractometer was used for unit cell measurements and
/
for data collection using MoÁKa radiation. Three check
/
reflections, monitored every 100 reflections, showed no
significant decay. The data were processed using the
SHELXTL [14] program package, and an absorption
Scheme 1.

48
D. Morales-Morales et al. / Journal of Organometallic Chemistry 654 (2002) 44Á50
/
complex R,R-{C6H4-2,6-(CH2P*PhButBH3)2} (2) in
good yield (95%). The bisborane intermediate 2 was
isolated as a white powder, this compound is air stable
allowing the purification to be carried out easily in
Reaction of [Ir(m-Cl)(COE)2]2 with two equivalents of
the PCP pincer ligand 3 under reflux in toluene for 24 h
yields the hydrochlorideÁiridium complex R,R-
/
IrHCl{C6H3-2,6-(CH2P*PhBut)2} (5) as a deep red
powder in good yield. The complex was characterized
by multinuclear NMR. The 1H-NMR spectra shows
absorptions corresponding to the phenyl, tert-butyl and
methylene groups in the molecule, additionally a triplet
at high field due to the presence of the hydride is
observed. The multiplicity is attributed to the coupling
of the hydride with the two equivalent phosphorus
nuclei. The 31P-NMR shows a single sharp signal at
45.76 ppm accounting for equivalent mutually trans
phosphorus centers. Treatment of a solution (pentane)
of the hydrochloride complex 5 with an excess of
superhydride [LiB(C2H5)3H] in a hydrogen atmosphere
(1 atm) at r.t. affords the tetrahydride complex R,R-
IrH4{C6H3-2,6-(CH2P*PhBut)2} (6) as light orange
powder (Scheme 3). Characteristic signals for the
hydrocarbon backbone and phosphorus substituents
1
aerobic conditions. The H-NMR spectra of 2 exhibits
the signals corresponding to the phenyl, tert-butyl and
methylene groups in the ligand, in addition a set
(quartet) of broad signals at 0.9 ppm corresponding to
the presence of the borane protecting group is observed.
The 31P-NMR shows a single absorption at 33.28 ppm
as a broad doublet, the broadness and multiplicity of
1
both the quartet at high field in the H and the doublet
in the 31P-NMR are due to quadrupolar effects and spin
value (sꢂ/3/2) of boron [17]. In the second step,
deprotection of the pincer-borane compound 2 with
Et2O [18] in dichloromethane affords the free
HBF4×
/
ligand R,R-{C6H4-2,6-(CH2P*PhBut)2} (3) in pure
enantiomeric form as colorless oil in almost quantitative
1
yield (98%). The H-NMR spectra of this compound is
similar to that of the borane complex 2, signals
corresponding to the phenyl, tert-butyl and methylene
groups were observed, absence of the broad quartet at
high field accounts for complete deprotection of the
borane adduct.
1
are observed in the H-NMR spectra, the presence of
hydrides was inferred by a triplet at ꢁ8.27 ppm with the
/
proper integration. A single sharp signal was observed
in the 31P-NMR spectrum (50.09 ppm) which is in
agreement with equivalent phosphorus nuclei.
3.2. Synthesis of R,R-PdCl{C6H3-2,6-
(CH2P*PhBut)2}, R,R-IrHCl{C6H3-
2,6(CH2PPhBut)2} and R,R-IrH4{C6H3-2,6-
(CH2P*PhBut)2}
The palladium complex R,R-PdCl{C6H3-2,6-
(CH2P*PhBut)2} (4) was synthesized (Scheme 2) by
refluxing stoichiometric amounts of ligand R,R-{C6H3-
2,6-(CH2P*PhBut)2} (3) and [PdCl2(COD)] in toluene
for 5 h. Evaporation of the solvent under vacuum
affords compound 4 as a white powder in good yield
(98%). Complex 4 is air and moisture stable, convenient
characteristics that make this complex an excellent
candidate for its screening in asymmetric catalysis.
Compound 4 was characterized by multinuclear NMR.
The 31P-NMR of this complex exhibits a single absorp-
tion (d 52.27 ppm) as expected for equivalent phospho-
rus nuclei.
Scheme 2.
Scheme 3.

D. Morales-Morales et al. / Journal of Organometallic Chemistry 654 (2002) 44Á
/50
49
Table 2
3.3. Catalysis
˚
Selected bond distances (A) and angles (8) for R,R-PdCl{C6H3-
2,6(CH2P*PhBut )2} (4)
For completeness, we have used 4 as a catalyst in the
hydrosilylation of styrene, allylic alkylation (with
Bond distances
CH(CO2Me)ꢁ2
as
nucleophile
to
afford
Pdꢃ
Pdꢃ
Pdꢃ
Pdꢃ
/
C(1)
P(1)
P(2)
Cl
2.020(14)
2.299(4)
2.302(4)
2.385(4)
/
PhCH{CH(CO2Me)2}CHꢄ/CHPh) and Heck coupling
/
reactions with phenyl triflate and 2,3-dihydrofuran.
However, neither of these reactions gave substantial
ee’s (6.5, 4.3 and 3.0%, respectively). This might be due
to the similarity in size of both substituents at the
phosphorus centers. Current efforts are focus in the
synthesis of new series of PCP P*-homochiral ligands in
order to prove this theory.
/
Bond angles
C(1)ꢃ
C(1)ꢃ
P(1)ꢃPdꢃ
C(1)ꢃPdꢃ
P(1)ꢃPdꢃCl
P(2)ꢃPdꢃCl
/
Pdꢃ
/
P(1)
82.2(4)
82.1(4)
/
Pdꢃ
/P(2)
/
/P(2)
163.60(13)
178.7(4)
96.50(14)
99.28(13)
/
/
Cl
/
/
/
/
As expected complex 6 is an efficient catalyst for the
transfer dehydrogenation of cyclooctane to cyclooctene
in the presence of tert-butylethylene at 200 8C, with
turnover numbers similar to those observed for
IrH2fC6H3-2; 6-(CH2PBut2)2g [3a,3b,3c,3d,3e,3f].
3.4. X-ray crystal structure of R,R-PdCl{C6H3-2,6-
(CH2P*PhBut)2} (4)
Crystals suitable for X-ray structure analysis were
obtained form a CH2Cl2ÁMeOH double layer solvent
/
system as colorless prisms. The details of the structure
determination are given in Table 1 and selected bond
lengths and angles are listed in Table 2. A thermal
ellipsoid drawing of 4 with atomic number scheme of the
obtained structure is presented in Fig. 1. The palladium
center is found in a distorted square planar geometry
Table 1
Crystal data and structure refinement for R,R-PdCl{C6H3-2,6-
(CH2P*PhBut )2} (4)
Empirical formula
Formula weight
Temperature (K)
C28H35ClP2Pd
575.35
293(2)
with angles of 163.60 and 178.78 for P(1)ꢃ
C(1)ꢃPdꢃCl, respectively. Two chelated five-membered
rings formed by PdꢃP(2)ꢃC(8)ꢃC(6)ꢃC(1) and Pdꢃ
P(1)ꢃC(7)ꢃC(2)ꢃC(1) are present in the complex. These
/
Pdꢃ/P(2) and
/
/
˚
Wavelength (A)
0.71073
Hexagonal
P63
/
/
/
/
/
Crystal system
Space group
Unit cell dimensions
/
/
/
rings are quite strained due to constrains imposed by the
atoms forming the two adjacent five-membered chelated
rings. Part of this strain is released by formation of
˚
a (A)
20.575(12)
20.575(12)
11.709(7)
90
˚
b (A)
˚
c (A)
unequal P(1)ꢃ
/
Pdꢃ
/
Cl (96.58) and P(2)ꢃ
/
PdꢃCl (99.288)
/
a (8)
b (8)
g (8)
angles. In addition the methylene groups on C(7) and
90
120
3
˚
V (A )
Z
4293(4)
6
Dcalc (g cmꢁ3
Absorption coefficient (mmꢁ1
F(000)
)
1.335
0.867
1776
)
Crystal size (mm)
u range for data collection (8)
Index ranges
0.15ꢃ
1.98Á20.02
175h 5
115l 511
5951
2684 [Rint
/
0.15ꢃ
/
0.50
/
ꢁ
ꢁ/
/
/
/
17, ꢁ
/175
/
k 517,
/
/
/
Reflections collected
Independent reflections
Refinement method
ꢂ/0.1235]
Full-matrix least-squares on F2
Data/restraints/parameters
2536/3/297
b
Final R indices [I ꢀ
/
2s(I)]
R1ꢂ
R1ꢂ
0.03(6)
/0.0499, wR2ꢂ
/
0.0807
b
R indices (all data)
Absolute structure parameter
/0.0938, wR2ꢂ
/0.0938
ꢁ/
Goodness-of-fit on F2
Largest difference peak and hole
0.927
0.701 and ꢁ0.528
a
/
ꢁ3
˚
(e A
)
a
Fig. 1. Thermal ellipsoid (50% probability) drawing of R,R-
PdCl{C6H3-2,6-(CH2P*PhBut )2} (4). Hydrogen atoms have been
omitted for clarity.
Sꢂ
/
[w(Fo)2ꢁ
p, total number of parameters.
/
(Fc)2)2/(nꢁp)]1/2, where n, number of reflections and
/
b
R1ꢂ
/
jFoꢁ
/
Fcj/jFoj, wR2ꢂ
/
[w((Fo)2ꢁ(Fc)2)2/w(Fo)2]1/2
.
/

50
D. Morales-Morales et al. / Journal of Organometallic Chemistry 654 (2002) 44Á50
/
C(8) located in the axial position of the puckered five-
membered chelated rings result in a C2-symmetry metal
complex with the phenyl groups and tert-butyl groups
on the phosphines being in pseudoaxial and -equatorial
positions, respectively. Furthermore, while the pseu-
doaxial substituent on one side of the molecule is above
the coordination plane, the corresponding substituent
on the other side is below the coordination plane. This
imposed asymmetric environment is expected to lead to
high enantioselectivity in asymmetric catalysis. The
absolute configuration of both stereocenters is deter-
mined to be R (based on refinement of the Flack
parameter). In other respects, bond lengths and angles
are similar to those observed in other PCP-complexes
[3b,3h,4a].
References
[1] R. Noyori, Asymmetric Catalysis in Organic Synthesis, John
Wiley & Sons, New York, 1994.
[2] (a) K.M. Pitrusiewicz, M. Zabloka, Chem. Rev. 94 (1994) 1375;
(b) I. Ojima (Ed.),Catalytic Asymmetric Synthesis, John Wiley &
Sons, New York, 2000.
[3] (a) M. Gupta, C. Hagen, W.C. Kaska, R. Flesher, C.M. Jensen, J.
Chem. Soc. Chem. Commun. (1996) 2083;
(b) M. Gupta, W.C. Kaska, R.E. Cramer, C.M. Jensen, J. Am.
Chem. Soc. 119 (1997) 840;
(c) M. Gupta, W.C. Kaska, C.M. Jensen, J Chem. Soc. Chem.
Commun. (1997) 2083;
(d) W.-W. Xu, G. Rossini, M. Gupta, C.M. Jensen, W.C. Kaska,
K. Krough-Jespersen, A.S. Goldman, J. Chem. Soc. Chem.
Commun. (1997) 2273;
(e) F. Liu, E.B. Pak, B. Singh, C.M. Jensen, A.S. Goldman, J.
Am. Chem. Soc. 121 (1999) 4086;
(f) C.M. Jensen, J. Chem. Soc. Chem. Commun. (1999) 2443;
(g) D. Morales-Morales, W. Chen, C.M. Jensen, J. Mol. Catal.
Sect. A (2002) submitted;
4. Conclusions
(h) R. Redo´n, D. Morales-Morales, D.W. Lee, Z. Wang, C. Yung,
C.M. Jensen, Can. J. Chem. 79 (2001) 823.
[4] (a) D. Morales-Morales, C. Grause, K. Kasaoka, R. Redo´n, R.E.
Although the present system did not achieved the goal
of being able to provide high enantioselective yields in
the catalytic reactions studied, we believe that by careful
tuning of both the steric and electronic properties of the
R groups in the phosphorus donor centers we will have
better possibilities to provide not only an efficient
catalytic system but also a better understanding of the
factors that govern the reactivity of these ligands.
Cramer, C.M. Jensen, Inorg. Chim. Acta 300Á302 (2000) 958;
/
(b) D. Morales-Morales, R. Redo´n, C. Yung, C.M. Jensen, J.
Chem. Soc. Chem. Commun. (2000) 1619.
[5] F. Gorla, L.M. Venanzi, A. Albinati, Organometallics 13 (1994)
43.
[6] F. Gorla, A. Togni, L.M. Venanzi, A. Albinati, F. Lianza,
Organometallics 13 (1994) 1607.
[7] (a) J.M. Longmire, X. Zhang, Tetrahedron Lett. 38 (1997) 1725;
(b) J.M. Longmire, X. Zhang, M. Shang, Organometallics 17
(1998) 4374.
[8] (a) G. Zhu, M. Terry, X. Zhang, Tetrahedron Lett. 37 (1996)
4475;
5. Supplementary material
(b) G. Zhu, M. Terry, X. Zhang, J. Organomet. Chem. 547 (1997)
97.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 177277 for complex 4. Copies
of this information may be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2
[9] (a) J.G. Donkervoort, J.L. Vicario, E. Rijnberg, J.T.B.H.
Jastrzebski, H. Kooijman, A.L. Spek, G. van Koten, J. Organo-
met. Chem. 550 (1998) 463;
(b) J.G. Donkervoort, J.L. Vicario, J.T.B.H. Jastrzebski, W.J.J.
Smeets, A.L. Spek, G. van Koten, J. Organomet. Chem. 551
(1998) 1.
1EZ, UK (Fax: ꢀ44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
/
[10] A. van der Ent, A.L. Onderdelinden, Inorg. Synth. 28 (1990) 91.
[11] D. Drew, J.R. Doyle, Inorg. Synth. 28 (1990) 348.
[12] T. Imamoto, T. Oshiki, T. Onozawa, T. Kusumoto, K. Sato, J.
Am. Chem. Soc. 112 (1990) 5244.
[13] B. Wolfe, T. Livinghouse, J. Am. Chem. Soc. 120 (1998) 5116.
[14] SHELXTL V.5, Siemens Industrial Automation, Inc., 1994.
[15] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure
Acknowledgements
Refinement, University of Gottingen, Germany, 1997.
¨
The support of this research by CONACYT (37288-
E), DGAPA-UNAM (IN116001) and the US Depart-
ment of Energy Hydrogen Program is gratefully ac-
knowledged.
[16] H.D. Flack, Acta Crystallogr. Sect. A 39 (1983) 876.
[17] H. Friebolin, Basic One- and Two-dimensional NMR Spectro-
scopy, John Wiley & Sons, Weinheim, Germany, 1998.
[18] L. McKinstry, T. Livinghouse, Tetrahedron 51 (1995) 7655.