Synthesis, resolution, and crystallographic characterization of a C2-symmetric diphosphaferrocene

Article abstract of DOI:10.1021/om970845s

Treatment of FeCl₂ with potassium 2-phenyl-3,4-dimethylphospholide provides a new C₂-symmetric diphosphaferrocene, (±)-3,3′,4,4′-tetramethyl-2,2′-diphenyl-1,1′- diphosphaferrocene (1). Separation by chiral HPLC furnishes both antipodes of complex 1, the first diphosphaferrocene to be prepared in enantiopure form. The absolute configuration of (-)-1 has been determined by X-ray crystallography.

Full text of DOI:10.1021/om970845s

Organometallics 1998, 17, 773-774
773
Syn th esis, Resolu tion , a n d Cr ysta llogr a p h ic
Ch a r a cter iza tion of a C2-Sym m etr ic Dip h osp h a fer r ocen e
Shuang Qiao, Diego A. Hoic, and Gregory C. Fu*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received September 29, 1997
Summary: Treatment of FeCl2 with potassium 2-phenyl-
3,4-dimethylphospholide provides a new C2-symmetric
diphosphaferrocene, (()-3,3′,4,4′-tetramethyl-2,2′-diphen-
yl-1,1′-diphosphaferrocene (1). Separation by chiral
HPLC furnishes both antipodes of complex 1, the first
diphosphaferrocene to be prepared in enantiopure form.
The absolute configuration of (-)-1 has been determined
by X-ray crystallography.
We have initiated a program directed at the develop-
ment of applications of planar-chiral heterocycles in
asymmetric catalysis. Our early work focused on the
chemistry of π-bound nitrogen heterocycles, which we
established serve as effective chiral nucleophilic cata-
lysts1 and as useful chiral ligands.2 More recently, we
have begun to explore the corresponding chemistry of
π-bound phosphorus heterocycles.3 At the time that we
started this investigation, there were no reports of
resolutions of this family of planar-chiral complexes.
However, very recently Ganter has described the prepa-
ration of an enantiopure phosphaferrocene derivative
through the use of a chiral auxiliary.4 In this note, we
establish, in the context of obtaining enantiopure diphos-
phaferrocene complex 1, that chiral HPLC represents
a viable alternate method for the resolution of planar-
chiral phosphorus heterocycles. We have determined
the absolute configuration of (-)-1 by X-ray crystal-
lography.
F igu r e 1. HPLC trace for (()-1 on a DAICEL CHIRACEL
OD column (R ) 1.70).
found that treatment of 3 with FeCl2 provides the
desired diphosphaferrocene 1 and its meso isomer in
73% yield (eq 1). The diastereomers can be separated
by HPLC on silica, and the enantiomers of 1 can then
be resolved by HPLC on a chiral stationary phase
(DAICEL CHIRALCEL OD column; R ) 1.70; Figure
1).8
In light of Cowley’s report that 1,1′-diphosphafer-
rocenes can serve as bidentate ligands for transition
metals,5 we decided to prepare an enantiopure C2-
symmetric diphosphaferrocene.6 Complex 1 was an
appealing target since phospholide 3 is readily available
by a two-step route developed by Mathey.7 We have
(6) (a) For the synthesis of 1, 1′-diphosphaferrocene, see: De Lauzon,
G.; Mathey, F.; Simalty, M. J . Organomet. Chem. 1978, 156, C33-
C36. (b) For the synthesis of a racemic C2-symmetric 1,1′-diphospha-
ferrocene derivative, see: de Lauzon, G.; Deschamps, B.; Fischer, J .;
Mathey, F.; Mitschler, A. J . Am. Chem. Soc. 1980, 102, 994-1000.
(7) Holand, S.; J eanjean, M.; Mathey, F. Angew. Chem., Int. Ed.
Engl. 1997, 36, 98-100. Phosphole 2 can be synthesized in one step
from commercially available PhPCl2 and 2,3-dimethyl-1,3-butadiene.
See: Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983-985.
(8) For precedent for the use of chromatography to resolve planar-
chiral transition metal complexes, see: (a) Falk, H.; Schlo¨gl, K.
Tetrahedron 1966, 22, 3047-3053. (b) Falk, H.; Schlo¨gl, K.; Steyrer,
W. Monatsh. Chem. 1966, 97, 1029-1044.
(1) (a) Ruble, J . C.; Latham, H. A.; Fu, G. C. J . Am. Chem. Soc.
1997, 119, 1492-1493. (b) Ruble, J . C.; Fu, G. C. J . Org. Chem. 1996,
61, 7230-7231.
(2) Dosa, P. I.; Ruble, J . C.; Fu, G. C. J . Org. Chem. 1997, 62, 444-
445.
(3) Garrett, C. E.; Fu, G. C. J . Org. Chem. 1997, 62, 4534-4535.
(4) Ganter, C.; Brassat, L.; Ganter, B. Tetrahedron: Asymmetry
1997, 8, 2607-2611.
(5) Atwood, D. A.; Cowley, A. H.; Dennis, S. M. Inorg. Chem. 1993,
32, 1527-1528.
S0276-7333(97)00845-5 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/22/1998

774 Organometallics, Vol. 17, No. 4, 1998
Notes
F igu r e 3. View of 1 from above.
F igu r e 2. ORTEP illustration, with thermal ellipsoids
drawn at the 35% probability level, of 1.
diphosphaferrocene (1), the first diphosphaferrocene to
be prepared in enantiopure form. HPLC on a chiral
stationary phase provided an effective means for sepa-
rating the enantiomers of 1. We anticipate that complex
1 will find application in asymmetric catalysis.
Ta ble 1. Cr ysta llogr a p h ic Da ta for 1
empirical formula
formula weight
cryst color, habit
cryst dimens (mm3)
cryst syst
C24H24FeP2
430.22
red block
0.15 × 0.27 × 0.45
Exp er im en ta l Section
monoclinic
space group
a (Å)
b (Å)
C2
22.9303(8)
11.3262(4)
16.6256(8)
105.5510(10)
4159.8(3)
Syn th esis of 3,3′,4,4′-Tetr a m eth yl-2,2′-d ip h en yl-1,1′-
d ip h osp h a fer r ocen e (1). In a closed Schlenk tube, a mixture
of 3,4-dimethyl-1-phenylphosphole (1.88 g, 10.0 mmol) and
t-BuOK (1.35 g, 12.0 mmol) in THF (10 mL) was heated (140
°C) with stirring under N2 for 12 h.7 The resulting yellow
solution was transferred by cannula to a stirred slurry of FeCl2
(Strem; 0.89 g, 7.0 mmol) in THF (20 mL) at room tempera-
ture, immediately forming a dark red mixture. The reaction
was stirred at room temperature for 12 h, and then the solvent
was removed in vacuo. The resulting dark brown residue was
purified by column chromatography (20% benzene/hexane),
which afforded 1.58 g (73%) of complex 1 and its meso isomer
(0.95:1) as a red solid. The diastereomers were separated by
HPLC (Alltech Econosphere 10 µ silica; 250 mm × 22 mm;
CH2Cl2:hexane 8:92; 20 mL/min; the racemate eluted from 11.5
to 12.6 min).
c (Å)
â (deg)
V (Å3)
Z
Fcalc
8
1.374 Mg/m3
radiation, monochromator
temp (K)
Mo KR (λ ) 0.710 69 Å), graphite
183(2)
µ(Mo KR)
diffractometer
0.885 mm-1
Siemens SMART/CCD
(3-circle, ø fixed at 54.78°
ω; -19 e h e 25,
-11 e k e 12,
-17 e l e 18
scan type; limiting indices
θ range for collection
total no. of reflns
no. of unique reflns
corrections
1.27° to 23.23°
8372
(()-1: 1H NMR (300 MHz, CD2Cl2) δ 7.30 (m, 4H), 7.14 (m,
6H), 3.44 (m, 2H), 2.15 (s, 6H), 2.09 (s, 6H); 13C NMR (75 MHz,
CD2Cl2) δ 139.6 (apparent t, J C-P ) 8.6 Hz), 131.0 (apparent
t, J C-P ) 4.8 Hz), 128.1, 126.5, 100.8, 94.5, 83.4 (d, J C-P ) 6.1
Hz), 82.6 (d, J C-P ) 6.2 Hz), 15.5, 13.9; 31P NMR (CD2Cl2, 122
MHz): - 63.7; IR (KBr) 3448, 3025, 2922, 1595, 1492, 1441,
1374, 1028, 839, 749; HRMS calcd for C24H24P2Fe 430.0703,
found 430.0704. Anal. Calcd for C24H24P2Fe: C, 67.00; H,
5.62. Found: C, 67.30; H: 5.89.
5422 (Rint ) 0.0342)
Lorentz-polarization; absorption
(semiempirical from ψ-scans);
max and min trans.,
0.6199 and 0.5557
direct methods, full-matrix
least-squares on F2
5415/1/488
structure solution,
refinement
data/restraints/params
R1; wR2 (data with I > 2σI)
R1; wR2 (all data)
goodness of fit
0.0396; 0.1020
0.0432; 0.1143
1.057
The enantiomers of complex 1 were separated by chiral
HPLC (14 mg per injection; DAICEL CHIRALCEL OD; 25 cm
× 1 cm; chloroform:hexane 14:86; 2.5 mL/min). Enantiomer
ext coeff.; largest peak, hole 0.0007(2); 0.348 e Å-3
,
-0.559 e Å-3
-0.01(2)
(+)-1 ([R]20 ) +441°, c ) 0.10, THF) was collected from 5.8
absolute structure
(Flack) param
programs used
D
to 6.8 min, and (-)-1 was collected from 7.6 to 10.0 min.
Siemens software package:
SMART, SAINT, XPREP,
SHELXTL 5.0
Ack n ow led gm en t. Support has been provided by
the Alfred P. Sloan Foundation, the American Cancer
Society, the Camille and Henry Dreyfus Foundation, Eli
Lilly, Firmenich, Glaxo Wellcome, the National Science
Foundation (Young Investigator Award to G.C.F., with
funding from Procter & Gamble, Merck, Bristol-Myers
Squibb, DuPont, Pfizer, Rohm & Haas, Pharmacia &
Upjohn, and Bayer), and the Research Corporation.
Acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the ACS, for partial
support of this research.
Crystals of (-)-1 grown from CH2Cl2/pentane proved
to be suitable for an X-ray diffraction study, which
established the absolute configuration of this enanti-
omer (Figure 2; Table 1). The structure resembles that
of 3,3′,4,4′-tetramethyl-1,1′-diphosphaferrocene9 sthe
two phospholyl rings are nearly parallel (6° deviation
from coplanarity) and adopt an eclipsed geometry rela-
tive to one another (Figure 3).10 The Fe-centroid
distance is 1.67 Å.
Su p p or tin g In for m a tion Ava ila ble: Text giving the
experimental details of the X-ray crystal structure determi-
nation and tables of crystal data, atomic coordinates, bond
lengths and angles, anisotropic displacement parameters, and
hydrogen coordinates (13 pages). Ordering information is
given on any current masthead page.
Thus, we have developed a straightforward method
for the synthesis and resolution of a new C2-symmetric
(9) de Lauzon, G.; Deschamps, B.; Fischer, J .; Mathey, F.; Mitschler,
A. J . Am. Chem. Soc. 1980, 102, 994-1000.
(10) For the structural characterization of 2,2′,5,5′-tetraphenyl-1,1′-
diphosphaferrocene, see: Hitchcock, P. B.; Lawless, G. A.; Marziano,
I. J . Organomet. Chem. 1997, 527, 305-308.
OM970845S