(R)- and (S)-1,3-Bis(diphenylphosphino)-2-((diphenylphosphino)methyl)-1-phenylpropane ((R)- and (S)-heliphos): Solid-state and solution conformations of a chiral, tripodal tris(phosphine) ligand that is restrained to one helical conformation upon coordination to a rhodium(I) metal center

Article abstract of DOI:10.1021/om961073n

(R)- and (S)-1,3-bis(diphenylphosphino)-2-((diphenylphosphino)methyl)-1-phenylpropane ((R)- and (S)-heliphos) were synthesized via an acid-catalyzed, diastereoselective Michael addition of diphenylphosphine to bis(1R,2S,5R)-(-)-menthyl) benzylidenemalonate. The crystal structure of [Rh((R)-heliphos)(NBD)](ClO₄) was determined by X-ray diffraction. As predicted by studies of molecular models, the framework of coordinated (R)-heliphos adopted a left-handed (Λ) helical conformation in the solid state, and NMR experiments indicated that it strongly favored this conformation in solution as well. The sense of helicity was controlled by the absolute configuration of the stereogenic carbon atom in the ligand.

Full text of DOI:10.1021/om961073n

1890
Organometallics 1997, 16, 1890-1896
(R)- a n d (S)-1,3-Bis(d ip h en ylp h osp h in o)-2-
((d ip h en ylp h osp h in o)m eth yl)-1-p h en ylp r op a n e ((R)- a n d
(S)-h elip h os): Solid -Sta te a n d Solu tion Con for m a tion s of
a Ch ir a l, Tr ip od a l Tr is(p h osp h in e) Liga n d Th a t Is
Restr a in ed to On e Helica l Con for m a tion u p on
Coor d in a tion to a Rh od iu m (I) Meta l Cen ter
Yiming Yao, Christopher J . A. Daley, Robert McDonald,† and
Steven H. Bergens*
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada
Received December 18, 1996X
(R)- and (S)-1,3-bis(diphenylphosphino)-2-((diphenylphosphino)methyl)-1-phenylpropane
((R)- and (S)-heliphos) were synthesized via an acid-catalyzed, diastereoselective Michael
addition of diphenylphosphine to bis(1R,2S,5R)-(-)-menthyl) benzylidenemalonate. The
crystal structure of [Rh((R)-heliphos)(NBD)](ClO4) was determined by X-ray diffraction. As
predicted by studies of molecular models, the framework of coordinated (R)-heliphos adopted
a left-handed (Λ) helical conformation in the solid state, and NMR experiments indicated
that it strongly favored this conformation in solution as well. The sense of helicity was
controlled by the absolute configuration of the stereogenic carbon atom in the ligand.
In tr od u ction
of the ligand framework.4d,e,f There is an element of
chirality in triphos and structurally equivalent tris-
(phosphines) that has not been directly controlledsthe
helical conformations they adopt upon coordination to
metal centers.4,5
Figure 1 shows a model of the left (Λ)- and right-
handed (∆) helical conformations of coordinated triphos.
The Λ and ∆ conformations are enantiomers, and it is
expected that interconversion between them is facile.
Figure 1 also shows a model of the left (Λ)- and right-
handed (∆) helical conformations of coordinated (R)-1,3-
bis(diphenylphosphino)-2-((diphenylphosphino)methyl)-
1-phenylpropane ((R)-heliphos). The Λ and ∆ conforma-
An essential component of asymmetric induction for
many enantioselective catalytic systems is the confor-
mation of the chiral ligand’s framework. In many cases,
the conformations of the framework are in themselves
chiral and dictate the absolute spatial configuration of
the pendant groups near the active sites on the catalyst.
Control over framework conformations therefore greatly
facilitates understanding the origins of enantioselection
in these systems. Further, conformationally rigid or
restrained ligands are common components of highly
enantioselective catalysts. Approaches used to control
conformational fluxionality have included use of rigid
ligand frameworks (type I)1 and use of derivatives of
fluxional parent frameworks that are biased toward one
conformer (type II).2 Experimental studies of the solu-
tion framework conformation(s) of type II chiral ligands
are limited in number3 and have rarely been applied to
chiral tripodal tris(phosphines).
(2) For representative examples, see the following. Bis(phos-
phines): (a) Kagan, H. B.; Dang, T.-P. J . Am. Chem. Soc. 1972, 94,
6429. (b) Vineyard, B. D.; Knowles, W. S.; Sabacky, M. J .; Bachman,
G. L.; Weinkauft, D. J . J . Am. Chem. Soc. 1977, 99, 5946. (c) Fryzuk,
M. D.; Bosnich, B. J . Am. Chem. Soc. 1977, 99, 6262. (d) Burns, C. J .;
Martin, C. A.; Sharpless, K. B. J . Org. Chem. 1989, 54, 2826. Oxygen
donor atoms: (e) Seebach, D.; Behrendt, L.; Felix, D. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 1321. See also reviews in refs 1g,h.
(3) For examples see the following. (a) Brown, J . M.; Chaloner, P.
A. J . Am. Chem. Soc. 1978, 100, 4307. (b) Brunner, H.; Beier, P.; Riepl,
G. Organometallics 1985, 4, 1732. (c) Alcock, N. W.; Brown, J . M.;
Derome, A. E.; Lucy, A. R. J . Chem. Soc., Chem. Commun. 1985, 575.
(d) Brown, M. B.; Maddox, P. J . J . Chem. Soc., Chem. Commun. 1987,
1276. (e) Giovannetti, J . S.; Kelly, C. M.; Landis, C. R. J . Am. Chem.
Soc. 1993, 115, 4040. (f) Brown, M. J .; Guiry, P. J .; Price, D. W.
Tetrahedron: Asymmetry 1994, 5, 561. (g) Beyreuther, S.; Hunger, J .;
Zsolnai, L. Chem. Ber. 1996, 129, 745.
(4) (a) Ward, T. R.; Venanzi, L. M.; Albinati, A.; Lianza, F.; Gerfin,
T.; Gramlich, V.; Tombo, G. M. R. Helv. Chim. Acta 1991, 74, 983. (b)
Burk, M. J .; Harlow, R. L. Angew. Chem., Int. Ed. Engl. 1990, 29, 1462.
(c) Walter, O.; Klein, T.; Huttner, G.; Zsolnai, L. J . Organomet. Chem.
1993, 458, 63. (d) Heidel, H.; Huttner, G.; Helmchen, G. Z. Naturforsch.
1993, 48B, 1681. (e) Seitz, T.; Muth, A.; Huttner, G. Z. Naturforsch.
1994, 50B, 1045. (f) Heidel, H.; Huttner, G.; Zsolnai, L. Z. Naturforsch.
1995, 50B, 729.
There have been few chiral analogs of 1,1,1-tris-
((diphenylphosphino)methyl)ethane (triphos) reported in
the literature.4 These ligands contain stereogenic cen-
ters either at phosphorus,4a at the ancillary groups
attached to phosphorus,4b,c or at the central carbon atom
* To whom correspondence should be addressed. Telephone: (403)
492-9703 (voice); (403) 492-8231 (fax). E-mail: Steve.Bergens@
UAlberta.ca.
† Faculty Service Officer, Structure Determination Laboratory.
X Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) For representative examples see the following. Bis(phos-
phines): (a) Fu, T. Y.; Liu, Z.; Scheffer, J . R.; Trotter, J . Tetrahedron
Lett. 1994, 35, 7593. (b) Brunner, H. J . Organomet. Chem. 1986, 300,
39. (c) Miyashita, A.; Takaya, H.; Souchi, T.; Noyori, R. Tetrahedron
1984, 40, 1245. Nitrogen donor atoms: (d) Fritschi, H.; Leutenegger,
U.; Siegmann, K.; Pfaltz, A.; Keller, W.; Kratky, Ch. Helv. Chim. Acta
1988, 71, 1541. (e) Helmchen, G.; Krotz, A.; Ganz, K.-T.; Hannen, D.
Synlett 1991, 257. Oxygen donor atoms: (f) Maruoka, K.; Itoh, T.;
Araki, Y.; Shirasaka, T.; Yamamoto, H. Bull. Chem. Soc. J pn. 1988,
61, 2975. For recent extensive reviews on enantioselective catalysis
see the following. (g) Noyori, R. Asymmetric Catalysis In Organic
Synthesis; Wiley-Interscience: New York, 1994. (h) Catalytic Asym-
metric Synthesis; Ojima, I., Ed.; VCH: New York, 1993.
(5) (a) Heidel, H.; Scherer, J .; Asam, A.; Huttner, G.; Walter, O.;
Zsolnai, L. Chem. Ber. 1995, 128, 293. (b) J anssen, B. C.; Asam, A.;
Huttner, G.; Sernau, V.; Zsolnai, L. Chem. Ber. 1994, 127, 501. (c) Seitz,
Th.; Muth, A.; Huttner, G.; Klein, Th.; Walter, O.; Fritz, M.; Zsolnai,
L. J . Organomet. Chem. 1994, 469, 155. (d) Muth, A.; Walter, O.;
Huttner, G.; Asam, A.; Zsolnai, L.; Emmerich, Ch. J . Organomet. Chem.
1994, 468, 149. (e) J anssen, B. C.; Sernau, V.; Huttner, G.; Asam, A.;
Walter, O.; Buchner, M.; Zsolnai, L. Chem. Ber. 1995, 128, 63 and
references cited therein.
S0276-7333(96)01073-4 CCC: $14.00 © 1997 American Chemical Society

A Chiral, Tripodal Tris(phosphine) Ligand
Organometallics, Vol. 16, No. 9, 1997 1891
heliphos, locking them into the less strained ∆-(S) and
Λ-(R) conformers, respectively. Similar framework con-
formational arguments were recently published for the
tetradentate tetraamine ligands (1-(2-pyridyl)-1-ethyl)-
bis(2-(pyridylmethyl)amine and bis(2-quinolylmethyl)
(1-(2-pyridyl)-1-ethyl)amine6 while this work was in
progress.7
Our interests are to develop several structural ana-
logs of heliphos with various terminal phosphines, with
differing substituents at the stereogenic center, and
with further substitution on the backbone. Our ulti-
mate objective is to investigate the uses of these chiral
ligands in asymmetric catalysis. In this report, we
describe the syntheses of the chiral tris(phosphine)
ligands (R)- and (S)-heliphos as well as their helical
conformations in the solid state and in solution upon
coordination to rhodium in [Rh((R)-heliphos)(NBD)]-
(ClO4).
Exp er im en ta l Section
Ma t er ia ls a n d Met h od s. The solvents n-hexane (K,
Ph2CO), methylene chloride (CaH2), tetrahydrofuran (K, Ph2-
CO), diethyl ether (K, Ph2CO), toluene (K, Ph2CO), trieth-
ylamine (CaH2), tributylamine (CaH2), and xylenes (CaH2)
were distilled from drying agents under argon gas, except
methanol, which was deoxygenated by bubbling with argon
gas or dinitrogen gas for 1 h. The argon and dinitrogen gases
were passed through a bed of Drierite drying agent. Unless
stated otherwise, commercial reagents were used without
further purification and all operations were performed under
an inert atmosphere of argon or dinitrogen gas.
All 1H, 13C, and 31P NMR spectra were measured with a
Bruker AM-400 NMR spectrometer operating at 400.13,
100.61, and 161.97 MHz, respectively. 1H and 13C NMR
chemical shifts are reported in parts per million (δ) relative
to tetramethylsilane using the solvent as an internal reference.
31P NMR chemical shifts are reported in parts per million (δ)
relative to an 85% H3PO4 external reference. All 13C and 31P
NMR spectra are proton-decoupled unless stated otherwise.
Mass spectra were measured using Kratos ms50 or AEI ms9
mass spectrometers. Microanalyses were performed at the
University of Alberta Microanalysis Laboratory. Optical rota-
tions were measured with a Perkin-Elmer 241 polarimeter at
589 nm (sodium D line) using 1.0 dm cells. Specific rotations,
[R]D, are reported in degrees per decimeter at 25 °C, and the
concentration (c) is given in grams per 100 mL. Melting points
were measured with a Gallenkamp capillary melting point
apparatus and are uncorrected.
C6H 5CHdC(C(O)OMen t h )2 (2; Men t h ) (1R,2S,5R)-
(-)-m en th ol). In air, dimenthyl malonate (1; 33.102 g, 87.1
mmol),8 dissolved in benzene (120 mL), was sequentially
treated with acetic acid (1.0 mL, 17.4 mmol), piperidine (0.35
mL, 3.5 mmol), and benzaldehyde (10.0 mL, 95.8 mmol) in
an azeotropic distillation apparatus. The mixture was re-
fluxed at 120 °C for 1.5 h and then at 130 °C for an additional
24 h. When the mixture was cooled to room temperature,
benzene (100 mL) was added, and the resultant yellow-red
solution was washed with 1 N HCl (2 × 100 mL), saturated
NaHCO3 (100 mL), and saturated NaCl (150 mL). The organic
layer was dried over Na2SO4, filtered, and concentrated
under reduced pressure, affording a yellow-white solid. The
solid was recrystallized from methanol to afford 2 as pure
white needle-shaped crystals in 89.5% yield (36.53 g): mp
F igu r e 1. Schematic representations of Λ- and ∆-coordi-
nated triphos and of (R)-heliphos showing the overlap of
phenyl rings in the ∆-(R)-heliphos conformer. The chiral
spatial arrangements of equatorial (e) and axial (a) phenyl
rings as viewed from the perspective of the active sites on
the metal center for Λ- and ∆-coordinated (R)-heliphos are
also depicted at the bottom of the figure.
tions would be enantiomers in the absence of the phenyl
substituent on the framework (as in triphos). The
phenyl rings of the phosphine groups are held in chiral
spatial arrangements of opposite absolute configuration
by Λ- and ∆-(R)-heliphos. For Λ- and ∆-heliphos, one
phenyl ring of each phosphine group is disposed roughly
in the plane defined by the phosphorus atoms (equato-
rial), and the other projects toward an active site on the
metal center (axial). Interconversion between the Λ and
∆ conformations causes the equatorial and axial phenyl
rings to exchange orientations, thereby inverting the
asymmetric environment of the active sites on the
catalyst.
Liga n d Design
A ball and stick molecular model (HGS) of coordinated
(R)-heliphos shows that severe steric repulsions exist
between the phenyl ring on the framework and the
equatorial phenyl ring of the syn-phosphine group in
∆-(R)-heliphos (Figure 1). Although we have not cal-
culated the magnitude of this interaction, molecular
models show that these phenyl rings physically overlap
in the ∆ configuration. This steric repulsion is absent
in the Λ configuration. Conversely, models show that
this severe steric repulsion occurs in Λ-(S)-heliphos but
not in ∆-(S)-heliphos. We propose that these steric
repulsions dictate the conformations of (R)- and (S)-
(6) Canary, J . W.; Allen, C. S.; Castagnetto, J . M.; Wang, Y. J . Am.
Chem. Soc. 1995, 117, 8484.
(7) This work was first presented in preliminary form at the 78th
CSC Conference of Chemistry, Guelph, Waterloo, Canada, May 28-
J une 1, 1995.
(8) Rule, G. R.; Harrower, J . J . Chem. Soc. 1930, 2319.

1892 Organometallics, Vol. 16, No. 9, 1997
Yao et al.
103-104 °C; 1H NMR (CDCl3) δ 0.70-0.95 (m, 21 H), 1.05
(m, 3 H), 1.39 (m, 2 H), 1.50 (m, 2 H), 1.67 (br, 4 H), 1.81 (quint-
d, J ) 6.9, 2.6 Hz, 1 H), 1.92 (quint-d, J ) 6.9, 2.6 Hz, 1 H),
2.07 (br d, J ) 12.0 Hz, 1 H), 2.13 (br d, J ) 12.0 Hz, 1 H),
4.85 (ddd, J ) 24.4, 10.9, 4.4 Hz, 2 H), 7.25-7.40 (br, 3 H),
7.48 (m, 2 H), 7.68 (s, 1 H); 13C NMR (CDCl3) δ 15.87, 16.13,
20.81, 20.90, 22.00, 22.03, 22.91, 23.19, 25.35, 25.97, 31.32,
31.39, 34.07, 34.30, 40.22, 40.86, 46.70, 47.25, 75.39, 75.53,
127.31, 128.56, 129.40, 130.21, 133.02, 141.02, 163.59, 166.32;
HRMS (EI) calcd for C30H44O4 468.3240, found 468.3224. Anal.
Calcd for C30H44O4: C, 76.94; H, 9.47. Found: C, 76.79; H,
9.62.
was rinsed with ether (50 mL); the reaction mixture was
stirred for 20 min at 0 °C and then for 1 h at room tempera-
ture. The LiAlH4 was neutralized by the dropwise addition
of 1.1 mL of deoxygenated distilled water, 1.1 mL of deoxy-
genated 10% NaOH solution at 0 °C, and 3.3 mL of deoxygen-
ated distilled water at room temperature. The mixture was
stirred for 30 min, filtered, washed with CH2Cl2 (3 × 100 mL),
and concentrated under reduced pressure, yielding a white
solid. The solid was recrystallized from toluene (100 mL),
affording white crystals of the phosphine diol (4). The phos-
phine diol was dissolved in CH2Cl2 (400 mL, undistilled) and
oxidized by adding a solution of 10% H2O2 (35 mL). The
reaction mixture was stirred for 5 min and then washed with
a solution of saturated Na2S2O3 (50 mL) until the excess
H2O2 was neutralized (negative result with saturated solution
of KI). The organic layer was dried over Na2SO4, filtered, and
concentrated under reduced pressure, affording (R)-5 in
77.3% yield (2.68 g), as a white powder. Further purification
was unnecessary: 1H NMR (CDCl3) δ 2.48 (m, 1 H), 3.25 (m,
2 H), 3.50 (dd, J ) 4.9, 10.7 Hz, 1 H), 3.60 (dd, J ) 11.8, 8.1
Hz, 1 H), 4.29 (dd, J P-H ) 9.9 Hz, J H-H ) 3.5 Hz, 1 H), 7.10-
7.35 (m, 6 H), 7.40-7.60 (m, 7 H), 7.92 (m, 2 H); 13C NMR
(CDCl3) δ 44.06, 44.75, 60.52, 61.33 (J P-C ) 11.8 Hz), 127.35,
128.14, 128.25, 128.41, 128.82, 128.94, 130.92, 131.04, 131.23,
131.31, 131.36, 131.92, 132.11, 132.91 (J P-C ) 3.1 Hz); 31P
NMR (CDCl3) δ 37.48; CIMS (carrier gas NH3) calcd for
C22H23O3P 366.3972, found 367.4 (M + H), 368.3 (M + 2H),
369.4 (M + 3H). Anal. Calcd for C22H23O3P: C, 72.12; H, 6.33.
Found: C, 72.17; H, 6.11. (S)-5 was obtained by the same
method.
C6H5CH(P P h 2)CH(C(O)OMen th )2 ((R)-3 a n d (S)-3). A
solution of 2 (10 g, 21.3 mmol), p-toluenesulfonic acid hydrate
(0.1 g, 0.53 mmol), and diphenylphosphine (4.05 g, 21.76 mmol)
in CH2Cl2 (100 mL) was stirred at room temperature for 72 h.
The solution was stirred over NaHCO3 (1 g) for 2 h, filtered,
washed with toluene (3 × 10 mL), and concentrated under
reduced pressure to afford a white solid in 100% yield (13.97
g). The solid was determined to be a mixture of both (R)-3
and (S)-3 in a 70:30 ratio. The solid was dissolved in toluene
(20 mL), and methanol was added (300 mL) under reflux.
White needlelike crystals of (R)-3 crystallized overnight and
were isolated by filtration and washed with cold methanol (20
mL, 0 °C). The filtrate was concentrated under reduced
pressure, affording a white solid which was dissolved in boiling
methanol (300 mL) and concentrated to 200 mL for crystal-
lization. Solid cubelike crystals of (S)-3 were isolated by
filtration and washed with cold methanol (20 mL, 0 °C). The
1
filtrate was concentrated (∼ /2 volume), and fractional crystal-
lization afforded more (R)-3 and (S)-3. The combined portions
of (R)-3 were recrystallized from toluene and methanol (1:20),
affording white needlelike crystals of (R)-3 in 80.1% yield (7.83
C6H5CH[P (O)P h 2]CH(CH2OMs)2 ((R)-6 a n d (S)-6). To
a -35 °C CH2Cl2 (250 mL) solution of (R)-5 (2.50 g, 6.82 mmol)
and dry triethylamine (2.85 mL, 20.46 mmol) was added a
solution of methanesulfonyl chloride (1.31 mL, 16.95 mmol)
in dry CH2Cl2 (15 mL) over 10 min. The mixture was warmed
to room temperature over 1 h and then stirred for an additional
45 min. In air, the reaction flask was placed in an ice bath
and the solution was quenched by the addition of crushed ice
(25 g), followed by cold water (25 g), with vigorous stirring.
The ice bath was removed, and the solution was stirred until
the ice melted. The organic layer was separated and the
aqueous layer back-extracted with CH2Cl2 (50 mL). The
combined organic layers were sequentially washed with cold
water (2 × 60 mL), cold saturated NaHCO3 (2 × 100 mL), and
cold brine (2 × 60 mL). The organic layer was dried over Na2-
SO4, filtered, and concentrated under reduced pressure to
afford a light yellow solid. The solid was recrystallized from
methanol (10 mL) at -10 °C overnight, affording white crystals
of (R)-6 in 86.1% yield (3.07 g): mp 142-143 °C; 1H NMR
(CDCl3) δ 2.80 (s, 3 H), 2.80 (s, 3 H), 2.95 (m, 1 H), 3.87 (m, 2
H), 4.15 (d, J ) 6.1 Hz, 2 H), 4.85 (dd, J P-H ) 10.7 Hz, J H-H
) 2.9 Hz, 1 H), 7.00-7.30 (m, 6 H), 7.30-7.60 (m, 7 H), 7.95
(m, 2 H); 13C NMR (CDCl3) δ 36.98, 37.07, 39.19, 43.38 (d, J P-C
) 67.6 Hz), 67.56 (m), 128.07, 128.15, 128.26, 128.96, 129.13,
129.25, 130.37, 130.43, 130.65, 130.74, 130.95, 131.04, 131.48,
131.95, 132.27, 132.35 (br); 31P NMR (CDCl3) δ 31.72. Anal.
Calcd for C24H27O7S2P: C, 55.16; H, 5.21. Found: C, 54.93;
H, 5.27. (S)-6 was obtained by the same method.
1
g): mp 96 °C; H NMR (CDCl3) δ 0.41 (d, J ) 6.9 Hz, 3 H),
0.62 (q, 1 H), 0.70-0.80 (m, 10 H), 0.83-1.10 (m, 10 H), 1.18-
1.35 (m, 2 H), 1.42-1.63 (m, 6 H), 1.69 (m, 2 H), 1.91 (m, 2
H), 3.94 (dd, J H-H ) 12.1 Hz, J P-H ) 6.3 Hz, 1 H), 4.39 (d, J
) 12.1 Hz, 1 H), 4.45 (td, J ) 4.4, 10.9 Hz, 1 H), 4.80 (td, J )
4.4, 10.9 Hz, 1 H), 6.68 (d, J ) 7.2 Hz, 2 H), 6.91-7.03 (m, 3
H), 7.04-7.09 (m, 2 H), 7.13-7.18 (m, 2 H), 7.25-7.36 (m, 4
H), 7.55 (m, 2 H); 13C NMR (CDCl3) δ 15.69, 15.98, 20.84, 20.94,
21.98, 22.06, 22.93, 23.09, 25.50, 25.67, 31.20, 31.45, 34.12,
34.27, 40.05, 40.51, 44.93 (d, J P-C ) 21.6 Hz), 46.60, 47.06,
56.73 (d, J P-C ) 31.8 Hz), 75.38, 75.91, 126.20, 127.54, 127.82,
127.90, 128.21, 128.26, 128.36, 128.73, 129.42, 132.65, 132.82,
134.04 (d, J P-C ) 14.8 Hz), 135.52, 135.75, 135.88 (d, J P-C
)
14.8 Hz), 137.59, 166.93 (d, J P-C ) 19.2 Hz), 167.78; 31P NMR
(CDCl3) δ 4.36 (s); HRMS (EI) calcd for C42H55O4P 654.3838,
found 654.3838. Anal. Calcd for C42H55O4P: C, 77.03; H, 8.47.
Found: C, 77.04; H, 8.73.
The combined portions of (S)-3 were recrystallized from pure
methanol affording solid white cubes of (S)-3 in 63.9% yield
1
(2.68 g): mp 95 °C; H NMR (CDCl3) δ 0.45 (d, J ) 5.9 Hz, 3
H), 0.65-1.00 (m, 20 H), 1.08 (m, 1 H), 1.25 (m, 2 H), 1.35
-1.65 (m, 6 H), 1.70 (m, 2 H), 2.08 (m, 2 H), 3.92 (dd, J H-H
)
11.8 Hz, J P-H ) 8.7 Hz, 1 H), 4.39 (d, J ) 11.8 Hz, 1 H), 4.51-
(td, J ) 4.4, 10.9 Hz, 1 H), 4.80 (td, J ) 4.4, 10.9 Hz, 1 H),
6.64 (d, J ) 7.3 Hz, 2 H), 6.95-7.10 (m, 5 H), 7.15 (m, 2 H),
7.25-7.36 (m, 4 H), 7.55 (m, 2 H); 13C NMR (CDCl3) δ 15.74,
16.49, 20.92, 21.88, 22.06, 22.92, 23.43, 25.33, 26.19, 31.20,
31.38, 34.10, 34.30, 40.26, 40.42, 45.13 (d, J P-C ) 23.3 Hz),
46.64, 46.97, 56.43 (d, J P-C ) 32.1 Hz), 75.29, 75.97, 126.23,
127.58, 127.81, 127.89, 128.12, 128.22, 128.27, 128.60, 128.62,
129.61, 132.38, 132.55, 133.64 (d, J P-C ) 16.3 Hz), 135.79,
C6H5CH[P (O)P h 2]CH(CH2P P h 2)2 ((R)-7 a n d (S)-7). Po-
tassium hydride (3.25 g, 28.48 mmol, 35% mineral oil disper-
sion) was washed with THF (3 × 30 mL) and then suspended
in THF (100 mL). Diphenylphosphine (6.23 g, 33.49 mmol)
was added dropwise, resulting in a red solution. (R)-6 (2.5 g,
4.78 mmol) was added at -10 °C in one portion as a powder.
Stirring was continued at -10 °C for 20 min and then at room
temperature for an additional 40 min. The reaction was
quenched by the addition of deoxygenated saturated NH4Cl
(10 mL). The mixture was stirred until the organic layer
became clear (20 min) and a white precipitate remained. The
organic layer was filtered from the white precipitate, and the
precipitate was washed with THF (2 × 20 mL). The combined
organic fractions were then dried over deoxygenated MgSO4,
136.02, 136.52 (d, J P-C ) 15.4 Hz), 137.51, 167.21 (d, J P-C
)
19.5 Hz), 167.81; 31P NMR (CDCl3) δ 7.92; HRMS (EI) calcd
for C42H55O4P 654.3838, found 654.3878. Anal. Calcd for
C
42H55O4P: C, 77.09; H, 8.47. Found: C, 76.99; H, 8.47.
C6H5CH[P (O)P h 2]CH(CH2OH)2 ((R)-5 a n d (S)-5). To a
stirred mixture of LiAlH4 (1.08 g, 28.46 mmol) and ether (100
mL) was added dropwise a solution of (R)-3 (6.5 g, 9.47 mmol)
in ether (100 mL) at 0 °C over 10 min. The dropping funnel

A Chiral, Tripodal Tris(phosphine) Ligand
Organometallics, Vol. 16, No. 9, 1997 1893
filtered, and concentrated under reduced pressure in a warm
water bath. The resulting oil was sonicated in the presence
of hexanes (30 mL), leaving a white powder and clear solution.
The mixture was filtered and the solid washed with hexanes
(2 × 30 mL) and dried under reduced pressure. The solid was
recrystallized from methanol, affording white crystals of (R)-7
in 81.7% yield (2.57 g): mp 211-212 °C; [R]D ) -91.74°
(toluene, c 0.5047); 1H NMR (CDCl3) δ 1.70 (m, 2 H), 2.21 (br,
1 H), 3.10 (br, 1 H), 3.80 (m, 1 H), 4.68 (m, 1 H), 7.00-7.40
(m, 29 H), 7.56 (m, 4 H), 7.71 (br, 2 H); 13C NMR (CDCl3) δ
32.45 (m), 32.85 (m, two carbons overlapped), 46.95 (ddd, J P-C
) 70.2, 10.9, 7.3 Hz), 125.38, 127.29, 128.03, 128.14, 128.23,
128.31, 128.39, 128.47, 128.54, 128.88, 129.11, 130.76, 130.84,
130.97, 131.09, 131.17, 131.56, 131.99, 132.07, 132.13, 132.20,
132.31, 132.38, 132.69 (d, J P-C ) 29.8 Hz), 133.49, 133.68,
solution was concentrated under reduced pressure, and the
resulting orange solid was washed with hexanes (3 × 5 mL).
The solid was crystallized from methanol, affording red-orange
crystals of [Rh((R)-heliphos)(NBD)](ClO4) in 88.2% yield (216
mg): [R]D ) +127.18° (toluene, c 0.5032); 1H NMR (CD2Cl2) δ
1.41 (br, 2 H), 2.37 (dd, J ) 5.3, 16.2 Hz, 1 H), 2.67 (dd, J )
4.9, 15.2 Hz, 1 H), 3.35 (m, 1 H), 3.55 (m, 5 H), 3.81 (d, J )
9.0 Hz, 1 H), 3.87 (br, 3 H), 6.27 (m, 2 H), 6.70 (m, 8 H), 7.00-
7.35 (m, 15 H), 7.50 (br, 5 H), 7.12 (m, 3 H), 7.85 (m, 2 H);
1H{31P} NMR (CD2Cl2:CDCl3 ) 1:1) δ 1.44 (br, 2 H), 2.42 (d,
2
2J H-H ) 16.2 Hz, 1 Hâpro-S(C13)), 2.68 (d, J H-H ) 15.6 Hz , 1
2
3
Hâpro-S(C12)), 3.40 (dd, J H-H ) 16.2 Hz, J H-H ) 7.0 Hz, 1
Hâpro-R(C13)), 3.49 (br, 2 H), 3.53 (br, 2 H), 3.63 (dd, J H-H
)
2
15.6 Hz, J H-H ) 7.6 Hz, 1 Hâpro-R(C12)), 3.82 (s, 1 Hâpro-S(C11)),
3
3.89 (br, 2 H), 3.98 (br, 1H), 6.28 (d, 2 H), 6.70 (m, 8 H), 7.00-
133.73 (d, J P-C ) 10.7 Hz), 134.24, 134.45, 136.28 (d, J P-C
)
7.35 (m, 15 H), 7.50 (br, 5 H), 7.12 (m, 3 H), 7.85 (d, 2 H); 13
C
11.1 Hz), 136.73 (d, J P-C ) 13.6 Hz), 138.71 (d, J P-C ) 9.1 Hz),
139.25 (d, J P-C ) 10.3 Hz); 31P NMR (CDCl3) δ 32.62, -20.99,
-23.97; HRMS (EI) calcd for C46H41OP3 702.2371, found
702.2372. Anal. Calcd for C46H41OP3: C, 78.68; H, 5.88.
Found: C, 78.37; H, 5.93. (S)-7 was obtained by the same
method: [R]D ) +91.93° (toluene, c 0.5058).
NMR (CD2Cl2) δ 25.26 (t, J P-C ) 17.2 Hz), 27.12 (dd, J P-C
)
12.6, 20.4 Hz), 37.83 (br), 44.27 (t, J P-C ) 13.47 Hz), 46.67,
46.91 (br), 47.42 (m), 61.82, 127.75, 127.86, 128.16, 128.61,
128.71, 129.05, 129.14, 129.53, 129.55, 129.64, 130.01, 130.10,
130.46, 130.70, 130.77, 130.88, 130.99, 131.48, 131.52, 131.59,
131.64, 132.25, 132.32, 132.43, 134.45 (d, J P-C ) 5.4 Hz),
134.78 (d, J P-C ) 5.4 Hz), 136.27, 136.59, 136.70, 136.83,
137.03 (d, J P-C ) 10.8 Hz), 139.95 (d, J P-C ) 8.9 Hz); 31P NMR
(CD2Cl2) δ -7.23 (ddd, J P-Rh ) 111.6, J P-P ) 19.3, 35.9 Hz),
-0.765 (ddd, J P-Rh ) 113.9, J P-P ) 19.3, 36.7 Hz), 32.25
(pseudo-dt, J P-Rh ) 116.7, J P-P ) 36.7, 35.9 Hz). Anal. Calcd
for C53H49P3RhClO4: C, 64.89; H, 5.04. Found: C, 64.89; H,
5.08. [Rh ((S)-h elip h os)(NBD)](ClO4) was obtained by the
same method: [R]D ) -128.64° (toluene, c 0.4975).
C6H5CH(P P h 2)CH(CH2P P h 2)2 ((R)-h elip h os a n d (S)-
h elip h os). A high-pressure reactor was charged with a
solution of (R)-7 (2.5 g, 3.56 mmol) in distilled p-xylene (20
mL). The reactor was placed in an ice-water bath. Distilled
tri-n-butylamine (5.04 mL, 21.2 mmol) and trichlorosilane
(1.80 mL, 17.8 mmol) were added, and the reactor was sealed.
The reaction mixture was warmed to room temperature and
then heated to 140 °C for 96 h. The reactor was cooled to room
temperature and the reaction mixture treated with 30%
deoxygenated aqueous NaOH (50 mL). The reaction mixture
was stirred at 60 °C for 1.5 h until both layers became
transparent. The layers were separated, and the aqueous
layer was back-extracted with toluene (2 × 25 mL). The
combined organic layers were washed sequentially with 30%
deoxygenated aqueous NaOH (50 mL), deoxygenated distilled
water (50 mL), 0.1 N HCl (75 mL), and deoxygenated distilled
water (50 mL). The organic layer was dried over Na2SO4,
filtered, and concentrated under reduced pressure. The
remaining white solid was dried under reduced pressure for
an additional 1 h and then recrystallized from methanol,
affording white cube-shaped crystals of (R)-heliphos in 82.0%
yield (2.00 g): mp 140-141 °C; [R]D ) -71.74° (toluene, c
1.0174); 1H NMR (CDCl3) δ 1.80 (br, 3 H), 2.99 (br, 1 H), 3.26
(br, 1 H), 4.59 (br, 1 H), 6.90-7.60 (m, 35 H); 13C NMR (CDCl3)
δ 31.48 (m), 32.11 (m), 33.51 (m), 47.20 (dd, J P-C ) 8.5, 9.4
Hz), 126.64, 127.82, 127.89, 128.14, 128.24, 128.38, 128.44 (br),
128.83, 131.29, 131.38, 132.06, 132.15, 132.23, 132.33, 133.53,
133.72, 133.85, 134.05, 134.22, 136.44 (d, J P-C ) 14.3 Hz),
137.01 (d, J P-C ) 4.9 Hz), 137.14 (d, J P-C ) 3.2 Hz), 137.39 (d,
[Rh (tr ip h os)(NBD)](ClO4). A stirred solution of [Rh-
(NBD)2](ClO4) (48.4 mg, 0.125 mmol)10 in CH2Cl2 (2.0 mL) at
0 °C was slowly treated with a solution of triphos (78.2 mg,
0.125 mmol, Aldrich) in CH2Cl2 (1.0 mL). The yellow-red
reaction mixture was stirred at room temperature for 1 h. The
solution was concentrated under reduced pressure, and the
resulting orange solid was washed with hexanes (3 × 10 mL).
The solid was used without further purification: 1H NMR
(CD2Cl2) δ 1.46 (s, 2 H), 1.59 (q, 3 H), 2.42 (dd, 6 H,
(CH2)3CCH3), 3.61 (br, 4 H), 3.88 (br, 2 H), 6.89-7.25 (m, 24
H), 7.28-7.41 (t, 6 H); 31P NMR (CD2Cl2) δ -9.59 (d, J P-Rh
)
113.7 Hz); 1H NMR (CD2Cl2, -80 °C) δ 1.38 (s, 2 H), 1.50 (br,
3 H), 2.29 (br, 6 H, (CH2)3CCH3), 3.46 (br, 4 H), 3.90 (br, 2 H),
6.92-7.02 (br, 12 H), 7.09 (t, 12 H), 7.28 (t, 6 H); 31P NMR
(CD2Cl2, -80 °C) δ 8.99 (d, J P-Rh ) 113.7 Hz).
Cr ysta llogr a p h ic An a lysis of [Rh ((R)-h elip h os)(NBD)]-
(ClO4)‚MeOH. Crystals suitable for an X-ray diffraction study
were obtained by slow evaporation of methanol solutions of
[Rh((R)-heliphos)(NBD)](ClO4) under a stream of argon gas.
The crystal system was monoclinic, and the space group P21
(No. 4) was consistent with an enantiomerically pure com-
pound. Crystal data, details of data collection, and structure
solution and refinement details are outlined in Table 1.
J P-C ) 14.1 Hz), 137.96 (d, J P-C ) 9.2 Hz), 139.54 (d, J P-C
)
10.9 Hz); 31P NMR (CDCl3) δ -12.23, -21.72, -24.08; HRMS
(EI) calcd for C46H41P3 686.2421, found 686.2437. Anal. Calcd
for C46H41P3: C, 80.51; H, 6.02. Found: C, 80.68; H, 6.09. (S)-
heliphos was obtained by the same method: [R]D ) +72.50°
(toluene, c 0.5310).
Resu lts a n d Discu ssion
Syn th esis of (R)- a n d (S)-h elip h os. The ligands
(R)- and (S)-heliphos were prepared according to Schemes
1 and 2. Knovenagel condensation of 1 with benzal-
dehyde in methylene chloride solution yielded bis-
(1R,2S,5R)-(-)-menthyl) benzylidenemalonate (2). The
most notable feature in this synthetic pathway was the
acid-catalyzed, diastereoselective Michael addition of
diphenylphosphine to 2 (Scheme 1). The diastereomers
(R)-3 and (S)-3 were easily separated by fractional
recrystallization from methanol/toluene solution. The
separated diastereomers were then reduced with lithium
Op tica l P u r ity of (R)-h elip h os. Crystallized (R)-heliphos
was oxidized to the corresponding tris(phosphine oxide) by
following the same experimental procedure used to oxidize
(R)-3 to (R)-4. A mixture of (R)-heliphos tris(phosphine oxide)
6 equiv of (R)-(+)-1,1′-bi-2-naphthol were dissolved in CDCl3.9
31P NMR (C6D6) δ 35.51, -21.45, -23.88.
[Rh ((R)-h elip h os)(NBD)](ClO4). A stirred solution of
[Rh(NBD)2](ClO4) (98.5 mg, 0.25 mmol)10 in CH2Cl2 (1.0 mL)
at 0 °C was slowly treated with a solution of (R)-heliphos
(0.1752 g, 0.25 mmol) in CH2Cl2 (1.0 mL). The yellow-red
reaction mixture was stirred at room temperature for 1 h. The
(10) Schenck, T. G.; Downes, J . M.; Milne, C. R. C.; Mackenzie, P.
B.; Boucher, H.; Whelan, J .; Bosnich, B. Inorg. Chem. 1985, 24,
2334. Bachechi, F.; Ott, J .; Venanzi, L. M. Acta Crystallogr. 1989, C45,
724.
(9) Mikhalev, O. V.; Malyshev, O. R.; Vinogradov, M. G.; Chel’tsova-
Bebutova, G. V.; Ignatenko, A. V. Izv. Akad. Nauk, Ser. Khim. 1995,
5, 900.

1894 Organometallics, Vol. 16, No. 9, 1997
Yao et al.
Ta ble 1. Cr ysta l Da ta a n d Refin em en t Deta ils for
Sch em e 1
[((R)-h elip h os)Rh (NBD)](ClO4)‚MeOH
formula
fw
C54H53ClO5P3Rh
1013.23
cryst dimens (mm)
cryst syst
space group
unit cell paramsa
a (Å)
0.32 × 0.25 × 0.24
monoclinic
P21 (No. 4)
9.5295(9)
b (Å)
18.307(2)
c (Å)
13.564(2)
â (deg)
99.362(13)
2334.9(5)
V (Å3)
Z
2
Fcalcd (g cm-3
)
1.441
µ (mm-1
diffractometer
)
0.575
Enraf-Nonius CAD4b
radiation (λ (Å))
monochromator
temp (°C)
Mo KR (0.710 73)
incident beam, graphite cryst
-50
scan type
θ-2θ
data collection 2θ limit (deg)
total no. of data collected
index rangesc
50.0
8826
-11 e h e 11, -21 e k e 21,
-16 e l e 16
no. of indep rflns
no. of observns (NO)
structure soln method
8169
2
7116 (Fo g 2σ(Fo2))
direct methods/fragment
search (DIRDIF-94d)
full-matrix least squares
on F2 (SHELXL-93e)
DIFABSf
refinement method
abs cor method
range of abs cor factors
no. of data/restraints/params
Flack absolute structure
param
1.258-0.840
2
8169 (Fo g - 3σ(Fo2)]/0/579
-0.06 (4)g
goodness of fit (S)h
1.054 (Fo g - 3σ(Fo2)]
2
Sch em e 2a
final R indicesi
2
2
Fo > 2σ(Fo
)
R1 ) 0.0579, wR2 ) 0.1567
R1 ) 0.0706, wR2 ) 0.1715
+1.488 and -0.987
all data
largest diff peak and
hole (e Å-3
)
a
Obtained from least-squares refinement of 24 reflections with
b
20.2° < 2θ < 25.4°. Programs for diffractometer operation and
data collection were those supplied by Enraf-Nonius. c Data in the
d
quadrants (h,+k,+l and (h,-k,-l were collected. Beurskens,
P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de Gelder, R.;
Israel, R.; Smits, J . M. M. The DIRDIF-94 Program System;
Crystallography Laboratory, University of Nijmegen, The Neth-
erlands, 1994. e Sheldrick, G. M. SHELXL-93. Program for
Crystal Structure Determination; University of Go¨ttingen, Go¨t-
2
tingen, Germany, 1993. Refinement was carried out on Fo for
2
all reflections (all having Fo > -3σ(Fo2)). Weighted R factors
wR2 and all goodness of fit values S are based on Fo2; conven-
tional R factors R1 are based on Fo, with Fo set to zero for nega-
2
2
tive Fo
.
The observed criterion of Fo > 2σ(Fo2) is used only for
calculating R1, and is not relevant to the choice of reflections
for refinement. R factors based on Fo are statistically about
2
twice as large as those based on Fo, and R factors based on all
data will be even larger. f Walker, N.; Stuart, D. Acta Crystallogr.
g
1983, A39, 158-166. Flack, H. D. Acta Crystallogr. 1983, A39,
876-881. The Flack parameter will be refined to a value near 0
if the structure is in the correct configuration and will be refined
h
2
to a value near 1 for the inverted configuration. S ) [∑w(Fo
-
Fc2)2/(n - p)]1/2 (n ) number of data; p ) number of parameters
varied; w ) [σ2(Fo2) + (0.1185P)2 + 0.4729P]-1, where P
)
i
2
[Max(Fo2,0) + 2Fc2]/3). R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo
- Fc2)2/∑w(Fo4)]1/2
.
a
aluminum hydride to produce the phosphine diol 4
(Scheme 2). The phosphine diol 4 was oxidized with
peroxide to produce 5 in 89% yield. Oxidation of 4 to
the phosphine oxide 5 was necessary to prevent in-
tramolecular displacement of a mesyl group by the
phosphine in (R)-6 and (S)-6. Reaction of 5 with 2 equiv
of mesyl chloride in the presence of triethylamine
produced the dimesylate 6. Displacement of the mesyl
(S)-heliphos is prepared by starting with (S)-2.
groups with diphenylphosphine was performed by the
reaction of 6 with KPPh2 to produce 7. We originally
used a 10% excess of potassium hydride to ensure
complete formation of KPPh2 from diphenylphosphine.
The resultant bis(phosphine) 7 was partially racemized
(∼20%) in the process, as determined by the 31P NMR

A Chiral, Tripodal Tris(phosphine) Ligand
Organometallics, Vol. 16, No. 9, 1997 1895
F igu r e 2. The crystal structure of Λ-[Rh((R)-heliphos)(NBD)](ClO4), as determined by X-ray diffraction. Also shown is
the arrangement of phenyl groups as viewed by the active sites on the metal (NBD is omitted for clarity). Atoms are
represented at the 20% probability level. Selected bond lengths (Å) and torsional angles (deg) are as follows: Rh-P(1),
2.352(2); Rh-P(2), 2.376(2); Rh-P(3), 2.308(2); Rh-C(1), 2.216(7); Rh-C(2), 2.254(10); Rh-C(4), 2.177(7); Rh-C(5), 2.260-
(13); Rh-P(1)-C(11)-C(10), -30.6(6); Rh-P(2)-C(12)-C(10), -35.1(7); Rh-P(3)-C(13)-C(10), -27.9(6).
spectrum recorded in methylene chloride solution con-
taining (R)-(+)-1,1′-bi-2-naphthol. The racemization of
7 was averted upon using an excess of diphenylphos-
phine to potassium hydride, ensuring that no free
hydride was available to deprotonate 6 or 7. Finally,
both enantiomers of optically pure heliphos were ob-
tained by reducing 7 with trichlorosilane in the presence
of tributylamine in a sealed high-pressure reactor. The
optical purity of (R)-heliphos was confirmed upon oxida-
tion to (R)-heliphos tris(phosphine oxide). Addition of
6 equiv (R)-(+)-1,1′-bi-2-naphthol in CDCl3 resulted in
three peaks in the 31P NMR spectrum. Addition of (S)-
heliphos tris(phosphine oxide) resulted in three new
peaks, indicating that (S)-heliphos was absent from the
purified (R)-heliphos.
tances for the equatorial phenyl rings are -0.21(8)
(C(41)), -0.168(9) (C(61)), and -0.107(8) (C(71)) (incli-
nations toward the ligand framework are assigned a
negative value). Figure 2 also shows the propeller-like
asymmetric array of pendant phenyl rings around the
rhodium center, with alternating axial and equatorial
orientations.
We believe the framework of [Rh((R)-heliphos)(NBD)]-
(ClO4) also strongly favored the Λ conformer in solution
for two reasons. First, the pro-S (assuming replacement
by a phenyl ring) protons at C(12) and C(13) of Λ-(R)-
heliphos are oriented toward an adjacent arm’s equato-
rial phenyl ring. This orientation is similar to that of
the framework phenyl ring in the unfavored ∆-(R)-
heliphos (Figure 1). For both C(12) and C(13),11 the
1H{31P} NMR signal for the pro-S proton was shifted
upfield from that for the pro-R proton by approximately
0.9 ppm. We believe this difference in chemical shift
was caused by ring current effects of the equatorial
phenyl group and indicated that interconversion with
appreciable amounts of ∆-(R)-8 did not occur. Second,
the coupling constants between the apical proton (at
C(10)) and the two pro-R methylene protons (at C(12)
and C(13)) were approximately 7 Hz, while those with
the three pro-S methylene protons (at C(11), C(12), and
C(13)) were approximately 0 Hz. According to the
Karplus relationship,12 a coupling constant of 0 Hz cor-
responds to HR(C(10)) - Hâ(pro-S) dihedral angles of ap-
proximately 90°. These angles are supported by those
obtained from molecular models (90°), and the cal-
culated13 hydrogen atom positions in the crystal struc-
R h od iu m (I) Nor bor n a d ien e Com p lexes of Tr i-
p h os a n d Helip h os. The Rh(I) complex [Rh(triphos)-
(NBD)](ClO4) was prepared by reaction of triphos with
[Rh(NBD)2](ClO4). The methylene protons of Λ- and
∆-triphos (CH3C(CH2PPh2)3) are diastereotopic, and
exchange magnetic environments upon interconversion
between the Λ and ∆ conformations. To determine if
interconversion between coordinated Λ- and ∆-triphos
1
was rapid, we recorded the H NMR spectrum of [Rh-
(triphos)(NBD)](ClO4) at -80 °C in methylene chloride
solution. We were unable to separate the signals for
the diastereotopic methylene protons, indicating that
interconversion between Λ- and ∆-triphos was rapid on
the NMR time scale.
The Rh(I) complex [Rh((R)-heliphos)(NBD)](ClO4) ((R)-
8) was prepared by the slow addition of (R)-heliphos to
[Rh(NBD)2](ClO4) in methylene chloride solution. The
solid-state structure of [Rh((R)-heliphos)(NBD)](ClO4)
was determined by X-ray diffraction. Figure 2 shows
the molecular structure of (R)-8. The framework of
(R)-8 adopted the predicted Λ configuration in the solid
state, with the Rh-P-C(methylene)-C(10) torsional
angles ranging from 35.1(7) to 27.9(6)°. These values
are comparable to those observed for other triphos-type
ligands.5 The distances (in Å) between the plane
containing the phosphorus atoms and the ipso carbons
of the axial phenyl rings are 0.474(8) (C(31)), 0.372(9)
(C(61)), and 0.415(9) (C(81)). The corresponding dis-
ture of Λ-[Rh((R)-heliphos)(NBD)](ClO4) (HR(C(10))
-
Hâ(pro-S)(C(11)) ) 87°, HR(C(10)) - Hâ(pro-S)(C(12)) ) 82°,
HR(C(10)) - Hâ(pro-S)(C(13)) ) 75°). For comparison, we note
that the signals for the methylene protons in the
published 1H NMR spectrum of [Fe(CH(CH2PPh2)3)-
(NCCH3)3](BPh4)2 coincided at room temperature, and
the corresponding HR(apical) - Hâ(methylene) coupling con-
stants were approximately 4 Hz.5b We believe the
framework of [Fe(CH(CH2PPh2)3)(NCCH3)3](BPh4)2 was
conformationally fluxional and that 4 Hz is the averaged
coupling constant of the two helical environments.
(12) Karplus, M. J . J . Am. Chem. Soc 1963, 85, 2870.
(13) Positions were calculated assuming idealized sp3 geometries
about the carbon atoms of the framework.
(11) The signals for all framework protons could be unambiguously
assigned on the bases of chemical shifts, COSY, and NOE experiments.

1896 Organometallics, Vol. 16, No. 9, 1997
Yao et al.
Our NMR studies could not exclude a rapid equilib-
rium with small amounts of the ∆ or some intermediate
conformer in solution. We doubt, however, that the
severe steric repulsions predicted by models of ∆-[Rh-
((R)-heliphos)(NBD)](ClO4) would allow its formation.
Ack n ow led gm en t. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada and by the University of Alberta. C.J .A.D. and
Y.Y. thank the University of Alberta for support pro-
vided by University of Alberta Ph.D. fellowships.
Su m m a r y
Su p p or tin g In for m a tion Ava ila ble: A full report on the
X-ray structure of Λ-[Rh((R)-heliphos)(NBD)](ClO4), including
tables of experimental details, atomic coordinates, interatomic
distances and angles, torsional angles, and anisotropic thermal
parameters and figures giving additional views of the structure
(22 pages). Ordering information is given on any current
masthead page.
The chiral tris(phosphines) (R)- and (S)-heliphos were
easily prepared in enantiomerically pure form. It
appears that the predicted locked helical conformations
of (R)- and (S)-heliphos in [Rh(heliphos)(NBD)](ClO4)
were adopted in both the solution and solid state.
Further, the asymmetric array of pendant phenyl rings
around the metal center contains axial phenyl groups
that project toward the spatial domains of the active
sites. We are now determining if this projection is
enantiogenic14 in enantioselective catalytic reactions.
OM961073N
(14) We define enantiogenic as the components of chirality respon-
sible for the enantioselection during an enantioselective reaction.