Comparative study of diastereoisomer interconversion in chiral BINOL-ate and diamine platinum complexes of conformationally flexible NUPHOS diphosphines

Article abstract of DOI:10.1021/om034289f

A thorough and detailed study of diastereointerconversion in the chiral platinum complexes [(NUPHOS)Pt{(S)-BINOL}] (3a-e) has been undertaken and compared with the results of a similar study with [(BIPHEP)Pt{(S)-BINOL}]. Rate data revealed that this process obeys first-order relaxation kinetics, and rate constants for conversion of the minor to the major diastereoisomer have been obtained. Eyring analysis of the data gave ΔH^? and ΔS^? values of 22-25 kcal mol^-1 and -1 to -16 eu, respectively. In combination with computational analysis, these studies indicate that atropinversion most likely occurs via an on-metal pathway involving a planar seven-membered transition state. Substitution of (S)-BINOL for (S,S)-DPEN results in a marked reduction in the barrier to atropinversion; a ΔH^? value of 17 kcal mol^-1 has been determined for the conversion of δ-[(Ph₄-NUPHOS)Pt{(S,S)-DPEN}]-Cl₂ to λ-[(Ph₄-NUPHOS)Pt{(S,S)-DPEN}]Cl₂, which could indicate that an alternative mechanism operates.

Full text of DOI:10.1021/om034289f

Organometallics 2004, 23, 1055-1064
1055
Com p a r a tive Stu d y of Dia ster eoisom er In ter con ver sion
in Ch ir a l BINOL-a te a n d Dia m in e P la tin u m Com p lexes
of Con for m a tion a lly F lexible NUP HOS Dip h osp h in es
Simon Doherty,*,† Colin R. Newman,† Rakesh K. Rath,†
J an-Albert van den Berg,† Christopher Hardacre,† Mark Nieuwenhuyzen,† and
J ulian G. Knight*,‡
Centre for the Theory and Applications of Catalysis, School of Chemistry,
The Queen’s University of Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG,
U.K., and School of Natural Sciences, Chemistry, Bedson Building, The University of
Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K.
Received November 11, 2003
A thorough and detailed study of diastereointerconversion in the chiral platinum complexes
[(NUPHOS)Pt{(S)-BINOL}] (3a -e) has been undertaken and compared with the results of
a similar study with [(BIPHEP)Pt{(S)-BINOL}]. Rate data revealed that this process obeys
first-order relaxation kinetics, and rate constants for conversion of the minor to the major
diastereoisomer have been obtained. Eyring analysis of the data gave ∆Hq and ∆Sq values
of 22-25 kcal mol-1 and -1 to -16 eu, respectively. In combination with computational
analysis, these studies indicate that atropinversion most likely occurs via an on-metal
pathway involving a planar seven-membered transition state. Substitution of (S)-BINOL
for (S,S)-DPEN results in a marked reduction in the barrier to atropinversion; a ∆Hq value
of 17 kcal mol-1 has been determined for the conversion of δ-[(Ph4-NUPHOS)Pt{(S,S)-DPEN}]-
Cl2 to λ-[(Ph4-NUPHOS)Pt{(S,S)-DPEN}]Cl2, which could indicate that an alternative
mechanism operates.
In tr od u ction
erism to a 3:1 thermodynamic mixture at 80 °C in
2-propanol/chloroform. The relative population of the
two BIPHEP conformations has a dramatic influence
on catalyst performance, the 1:1 and 3:1 S/S,S:R/S,S
mixtures giving (R)-1-(1-naphthyl)ethanol in 63% and
84% ee, respectively. A further increase in enantiose-
lectivity was obtained at low temperature, the 3:1
diastereoenriched mixture giving 92% ee; this is a
significant improvement on the performance of the
corresponding racemic BINAP derivative. The popula-
tion of BIPHEP conformations and hence catalyst
performance can be influenced by introducing substit-
uents at the 3,5-positions of BIPHEP and/or the nitro-
gen atoms of DPEN.3 For example, the R/S,S diastereo-
isomer of a 1:1 mixture of [RuCl2(BIPHEP){(S,S)-di-
methyl-DPEN}] epimerizes completely to give diaste-
reoisomerically pure [RuCl2(S-BIPHEP){(S,S)-dimethyl-
DPEN}] at 50 °C in propan-2-ol.
Traditionally, the design of new chiral catalysts has
been guided by the concept that a conformationally
restricted enantiopure ligand is necessary to achieve
high enantioselectivities in asymmetric transforma-
tions.1 Recently, though, Mikami and Noyori have
demonstrated that the conformationally flexible tropos
diphosphine 2,2′-bis((3,5-dimethylphenyl)phosphino)bi-
phenyl (DM-BIPHEP) can be used for the enantioselec-
tive ruthenium-catalyzed reduction of 1-acetonaphtho-
ne, a strategy now termed asymmetric activation.2 In
this experiment addition of (S,S)-1,2-diphenylethylene-
diamine (S,S-DPEN) to [RuCl2(DM-BIPHEP)(dmf)n]
gave [RuCl2(DM-BIPHEP){(S,S)-DPEN}] as a 1:1 mix-
ture of diastereoisomers, which underwent atropisom-
* Corresponding author.
† The Queen’s University of Belfast.
‡ The University of Newcastle upon Tyne.
In an attempt to investigate the mechanism of atro-
pinversion, Gagne´ has studied diastereoisomer inter-
conversion in the chiral complexes [(BIPHEP)Pt{(1R,2S)-
OCHPhCHPhNTf}] and [(BIPHEP)Pt{(S)-BINOL}] (eq
1) and shown that coordination of BIPHEP to a substi-
tutionally inert metal center, such as platinum(II),
significantly slows atropinversion compared with the
(1) (a) Seyden-Penne, J . Chiral Auxiliaries and Ligands in Asym-
metric Catalysis; Wiley: New York, 1995. (b) Noyori, R. Asymmetric
Catalysis in Organic Synthesis; Wiley: New York, 1994. (c) Ojima, I.,
Ed. Catalytic Asymmetric Synthesis, 2nd ed.; Wiley-VCH: New York,
2000.
(2) (a) Mikami, K.; Korenaga, T.; Terada, M.; Ohkuma, T.; Pham,
T.; Noyori, R. Angew. Chem., Int. Ed. 1999, 38, 495. For recent
informative reviews see: (b) Walsh, P. J .; Lurain, A. E.; Balsells, J .
Chem. Rev. 2003, 103, 3297. (c) Mikami, K.; Aikawa, K.; Yusa, Y.;
J odry, J . J .; Yamanaka, M. Synlett 2002, 1561. (d) Mikami, K.; Terada,
M.; Korenaga, T.; Matsumoto, Y.; Matsukawa, S. Acc. Chem. Res. 2000,
33, 391. (e) Mikami, K.; Terada, M.; Korenaga, T.; Matsumoto, Y.; Ueki,
M.; Angeland, R. Angew. Chem., Int. Ed. 2000, 39, 3532.
(3) Korenaga, T.; Aikawa, K.; Terada, M.; Kawauchi, S.; Mikami,
K. Adv. Synth. Catal. 2001, 343, 284.
10.1021/om034289f CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/30/2004

1056 Organometallics, Vol. 23, No. 5, 2004
Doherty et al.
octahedral [(BIPHEP)RuCl2{(S,S)-DPEN}] catalyst.4
Ch a r t 1
using conformationally flexible NUPHOS-type diphos-
phines are promising alternative approaches to the use
of their enantiopure counterparts, since a wide range
of alkynes and diynes either are commercially available
or can be prepared via relatively straightforward inex-
pensive procedures. Since we ultimately intend to
investigate the use of NUPHOS-type diphosphines in
platinum-group-catalyzed asymmetric transformations,9
via either asymmetric activation or chiral environment
amplification, we have examined their coordination
chemistry and determined the rate of atropinversion
with the aim of understanding the mechanism of this
process. Herein we report the results of our studies in
this area, including a kinetic study of diastereoisomer
interconversion in the chiral platinum complexes
[(NUPHOS)Pt{(S)-BINOL}] and [(Ph4-NUPHOS)Pt-
{(S,S)-DPEN}]Cl2.
This increase in barrier to atropisomerism was used to
resolve enantiopure λ-(BIPHEP)PtCl2 and δ-(BIPHEP)-
PtCl2, by treatment of the corresponding λ(S) and 95:5
δ(S) diastereopure [(BIPHEP)Pt{(S)-BINOL}] with HCl.5
Treatment of diastereopure [(BIPHEP)Pt{(S)-BINOL}]
with trifluoromethanesulfonic acid or homochiral [(BI-
PHEP)PtCl2] with silver triflate was used to generate
the corresponding enantiopure Lewis acid complexes
λ-(BIPHEP)Pt2+ and δ-(BIPHEP)Pt2+, which catalyze
the Diels-Alder and glyoxylate-ene reactions. The high
ee’s obtained in both reactions clearly demonstrate that
conformationally flexible diphosphines can be used as
effective chiral auxiliaries to achieve high enantiose-
lectivities in asymmetric catalysis. Mikami adopted a
similar strategy to isolate enantio- and diastereomeri-
cally pure palladium complexes of BIPHEP, relying on
coordination of (R)-diaminobinaphthyl ((R)-DABN)) to
generate a diastereoisomeric mixture of [Pd{BIPHEP}-
{(R)-DABN}][SbF6]2.6 Atropinversion at 80 °C for 12 h
gave the thermodynamically favored [Pd{(R)-BIPHEP}-
{(R)-DABN}][SbF6]2, which acts as an activated catalyst
for the hetero Diels-Alder reaction between 1,3-cyclo-
hexadiene and ethyl glyoxylate to give ee values as high
as 94%. A similar strategy has been successfully applied
to control the axial chirality of bis(diphenylphosphino)-
ferrocene to effect the nickel-catalyzed asymmetric
glyoxylate-ene reaction.7
Asymmetric activation using a conformationally flex-
ible diphosphine to achieve high enantioselectivities is
an attractive strategy, since it offers immense scope for
structural variation and catalyst tuning. In this regard,
we have recently reported the synthesis of an entirely
new class of conformationally flexible diphosphine,
NUPHOS (Chart 1), based on two diphenylphosphino
groups linked by a four-carbon sp2-hybridized tether,
which clearly fulfills the criteria of a conformationally
flexible diphosphine capable of undergoing atropisom-
erism.8 Asymmetric activation and on-metal resolution
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of [(NUP HOS)-
P tCl2] (2a -e) a n d [(NUP HOS)P t{(S)-BINOL}] (3a -
e). Slow addition of a dichloromethane solution of 1a -e
into a dichloromethane solution of [(cycloocta-1,5-diene)-
PtCl2] resulted in the rapid appearance of a yellow
coloration with the formation of [(NUPHOS)PtCl2] (2a -
e) in yields of up to 88%. Compounds 2a -e have been
characterized using standard spectroscopic techniques
and, in the case of 2a , by a single-crystal X-ray study.
For each compound, the 31P NMR spectrum contains a
single sharp signal flanked by platinum satellites, with
J Pt-P between 3607 and 3814 Hz, in the region expected
for a phosphine trans to chloride.10 The 1H NMR spectra
of compounds 2a -e each contain at least one distinctive
low-field-shifted exchange-broadened signal, as a result
of restricted rotation of the diphenylphosphino phenyl
rings,8b together with numerous well-resolved multiplets
associated with the remaining aromatic protons. Com-
pounds 3a -e (Chart 2) were prepared as near 1:1
mixtures of enantiopure diastereoisomers by following
a procedure similar to that recently reported by Gagne´
and co-workers.10 Dropwise addition of a THF solution
of Na2-(S)-BINOL, generated in situ by deprotonation
of (S)-BINOL with sodium tert-butoxide, to a suspension
(4) Tudor, M. D.; Becker, J . J .; White, P. S.; Gagne´, M. R. Organo-
metallics 2000, 19, 4376.
(5) (a) Becker, J . J .; White, P. S.; Gagne´, M. R. J . Am. Chem. Soc.
2001, 123, 9478. (b) Brunkan, N. M.; Gagne´, M. R. Organometallics
2002, 21, 1576. (c) Brunkan, N. M.; White, P. S.; Gagne´, M. R.
Organometallics 2002, 21, 1565.
(6) (a) Mikami, K.; Aikawa, K.; Yusa, Y.; Hatano, M. Org. Lett. 2002,
4, 91. (b) Mikami, K.; Aikawa, K.; Yusa, Y. Org. Lett. 2002, 4, 95.
(7) Mikami, K.; Aikawa, K. Org. Lett. 2002, 4, 99.
(8) (a) Doherty, S.; Knight. J . G.; Robins, E. G.; Scanlan, T. H.;
Champkin, P. A.; Clegg, W. J . Am. Chem. Soc. 2001, 123, 5110. (b)
Doherty, S.; Robins, E. G.; Nieuwenhuyzen, M.; Knight, J . G.; Champ-
kin, P. A.; Clegg, W. Organometallics 2002, 21, 1383. (c) Doherty, S.;
Newman, C. R.; Hardacre, C.; Nieuwenhuyzen, M.; Knight, J . G.
Organometallics 2003, 22, 1452.
(9) (a) Doherty, S.; Newman, C. R.; Rath, R. K.; Luo, H.-K.;
Nieuwenhuyzen, M.; Knight, J . G. Org. Lett. 2003, 3863. (b) Ghosh,
A. K.; Matsuda, H. Org. Lett. 1999, 1, 2157. (c) Hao, J .; Hatano, M.;
Mikami, K. Org. Lett. 2000, 2, 4059. (d) Hori, K.; Kodana, H.; Ohta,
T.; Furukawa, I. J . Org. Chem. 1999, 64, 5017. (e) Hori, K.; Ohta, T.;
Furukawa, I. Tetrahedron 1998, 64, 12737. (f) Sodeoka, M.; Ryosuke,
T.; Miyazaki, F.; Hagiwara, E.; Shibasaki, M. Synlett 1997, 463. (g)
Oi, S.; Terada, E.; Ohuchi, K.; Kato, T.; Tachibana, Y.; Inoue, Y. J .
Org. Chem. 1999, 64, 8660. (h) Hamashima, Y.; Hotta, D.; Sodeoka,
M. J . Am. Chem. Soc. 2002, 124, 11240.
(10) (a) Mather, G. G.; Pidcock, A.; Rapsey, G. J . N. J . Chem. Soc.,
Dalton Trans. 1973, 2095. (b) Pidcock, A. Adv. Chem. Ser. 1982, 196,
1.

Pt Complexes of NUPHOS Diphosphines
Organometallics, Vol. 23, No. 5, 2004 1057
Ch a r t 2
Sch em e 1
of 2a -e in toluene resulted in the immediate appear-
ance of an intense orange or yellow color, characteristic
of the desired product. The reaction is typically complete
within 2-3 h, and the product was routinely isolated
in high yield. The 31P NMR spectra of 3a -e each contain
two singlets which correspond to λ(S)-3a -e and δ(S)-
correspond to the thermodynamically more stable di-
astereoisomer λ-[(Ph4-NUPHOS)Pt{(S,S)-DPEN}]Cl2
(Table 1), unequivocally confirming that δ-NUPHOS
and (S,S)-DPEN have mismatched stereochemistries.
Kin etic Stu d y of Dia ster eoisom er In ter con ver -
sion of [(NUP HOS)P t{(S)-BINOL}] (3a -e). Since the
use of conformationally flexible diphosphines such as
BIPHEP and NUPHOS in asymmetric catalysis relies
on the slow interconversion of chiral conformations,
several experimental and theoretical studies have been
undertaken in an attempt to distinguish between the
two possible pathways: (i) on-metal inversion passing
through the s-cis diene conformation of a seven-
membered metallacycle (Scheme 1a) and (ii) M-P bond
dissociation involving either an s-trans or s-cis confor-
mation (Scheme 1b). On the basis of the relative rates
of diastereointerconversion in ruthenium and platinum
complexes of BIPEHP, Gagne´ recently presented com-
pelling arguments for dissociation-induced biaryl rota-
tion but conceded that on-metal atropinterconversion
could not be dismissed.11 Unfortunately, the kinetic data
obtained in a detailed and thorough study of diastereo-
interconversion in the chiral complexes [(BIPHEP)PtX2]
(X2 ) N(Tf)CHPhCHPhO, (S)-BINOL) could not distin-
guish between these two possible mechanisms.4 Pyridine
was shown to lower the isomerization temperature of
enantiopure λ-[(BIPHEP)Pt{(S)-BINOL}] by ca. 50 °C,
which was tentatively suggested to be due to an
enhanced rate of phosphine dissociation in a five-co-
ordinate pyridine intermediate. In the case of atropoint-
erconversion of (S/S,S)- and (R/S,S)-[Ru(BIPHEP)Cl2-
{(S,S)-DPEN}], theoretical studies support a mecha-
nism involving solvent-assisted dissociation of one of the
Ru-P bonds, rotation about the biphenyl single bond,
and subsequent recoordination of phosphine.12 We have
investigated the process of atropinterconversion in 3a -
e, reasoning that the geometrical difference between the
four-carbon tethers in 3a -c and 3d ,e might manifest
itself in the activation parameters for atropintercon-
3a -e with platinum satellite coupling constants, J Pt-P
,
between 3619 and 3700 Hz, very similar to the corre-
sponding values in 2a -e. While the thermodynamically
preferred diastereoisomers of 3b-e, δ(S), were obtained
by thermolysis of a toluene solution overnight followed
by recrystallization, the thermodynamically less favored
diastereoisomer of 3a , λ(S), has been isolated pure by
fractional crystallization from a 1:1 diastereoisomeric
mixture at room temperature. A 31P NMR spectrum of
a representative sample of λ-3a confirmed that only the
thermodynamically less stable diastereoisomer crystal-
lized and that it undergoes atropinversion at elevated
temperatures to give the thermodynamically more
stable diastereoisomer.
A sample of the diastereopure δ-[(Ph4-NUPHOS)Pt-
{(S,S)-DPEN}]Cl2 (4‚Cl2), required for the kinetic study,
was prepared by reaction of enantiopure δ-[(Ph4-
NUPHOS)PtCl2]9a with (S,S)-DPEN in dichloromethane
(eq 2). Reaction was complete within 5 min, after which
time the solution was filtered, the solvent removed, and
the resulting residue triturated with hexane to give δ-
(S,S)-4‚Cl2 as a spectroscopically pure pale yellow
powder. The 31P NMR spectrum of 4‚Cl2 contains a
1
triplet at δ -6.5 with J Pt-P ) 3315 Hz, and the 1H NMR
spectrum contains two set of multiplets associated with
the amino protons and another belonging to the methine
protons of the diamine backbone. After the compound
stood in chloroform for several hours, a second set of
multiplets began to appear, which were shown to
(11) (a) Alguindigue, A. S.; Khan, M. A.; Ashby, M. T. Organome-
tallics 1999, 18, 5112. (b) Ashby, M. T.; Alguindigue, A. S.; Khan, M.
A. Organometallics 2000, 19, 547. (c) Ashby, M. T. J . Am. Chem. Soc.
1995, 117, 2000. (d) Ashby, M. T.; Govindan, G. N.; Groftan, A. K. J .
Am. Chem. Soc. 1994, 116, 4801.
(12) Yamanaka, M.; Mikami, K. Organometallics 2002, 21, 5847.

1058 Organometallics, Vol. 23, No. 5, 2004
Doherty et al.
F igu r e 1. Reversible first-order analyses for the thermal
conversion of λ(S)-3a to δ(S)-3a in toluene-chlorobenzene
(7:3). First-order analyses for the conversions of λ(S)-3b-d
to δ(S)-3b-d and of δ(S,S)-4‚Cl2 to λ(S,S)-4‚Cl2 are given
in the Supporting Information.
F igu r e 2. Eyring analysis of the rate data obtained for
the thermal conversion of λ(S)-3a -d to δ(S)-3a -d and of
δ(S,S)-4‚Cl2 to λ(S,S)-4‚Cl2.
version, since the latter necessarily involves inversion
of a cyclohexane ring, which is expected to have a higher
barrier to activation than rotation about the C(2)-C(3)
bond of an acyclic 1,3-butadiene and which cannot
undergo anti rotation in the same manner as BIPHEP
and related diphosphines. Thus, a comparative study
of diastereointerconversion of 3a -e might indicate
which of the two pathways commonly associated with
atropisomerism operates.
of which are listed in Table 1 and presented in Figure
2. During similar experiments with 3e, significant
decomposition occurred after prolonged heating, and
equilibrium values were not obtainable. An estimate of
∆Hq for 3e using the equilibrium values for 3d gave 23
kcal mol-1: i.e., within the range for 3a -d . The data
in Table 1 clearly reveal that the activation parameters
for atropinversion of 3a -c are similar to that for 3d ,e,
which strongly suggests that complexes of acyclic and
monocyclic NUPHOS diphosphines undergo atropinver-
sion via the same pathways. Since the cyclohexyl ring
in monocyclic NUPHOS diphosphines prevents anti
rotation, the only two possible pathways for atropin-
version are on-metal involving a syn transition state or
dissociation-induced rotation also via a syn transition
state. Moreover, these activation parameters are similar
to those recently reported by Gagne´ for the thermal
conversion of λ(S)-[(BIPHEP)Pt(BINOL)] to δ(S)-[(BI-
PHEP)Pt(BINOL)] (∆Hq ) 27(2) kcal mol-1, ∆Sq ) -5(5)
eu). In the case of compound 3a , a van’t Hoff analysis
of the equilibrium data obtained from the thermolysis
experiment gave ∆H° ) 0.8 kcal mol-1 and ∆S° ) 5 eu
(Figure 3), which is a clear measure of the relative
thermodynamic stability of the matched and mis-
matched diastereoisomers. In the case of 3a an equi-
librium ratio of 75:25 was observed for δ(S)-3a :λ(S)-3a ,
whereas the corresponding ratios for compounds 3b-e
are all much closer to 95:5. Solutions of diastereopure
δ(S)-3c do not show any sign of diastereointerconversion
at room temperature even after standing for 24 h, which
is consistent with the experimentally determined barrier
of 25 kcal mol-1 for atropinversion of λ(S)-3c to δ(S)-
3c.
Ta ble 1. Activa tion P a r a m eter s for th e
Atr op isom er iza tion of Com p ou n d s 3a -d a n d 4
param (error)
3a
23
-12 -16
3b
3c
25
-1 -12
3d
4‚Cl2
∆Hq, kcal mol-1 (3)
22
23
17
-23
∆Sq, eu (8)
δ/λ equilibrium ratio at 85 °C
3.8 19.1 11.1 11.1 0.34a
a
Equilibrium constant at 65 °C.
Thermolysis of a chlorobenzene/toluene solution of a
1:1 mixture of enantiopure λ(S)-3a -e and δ(S)-3a -e
within a narrow temperature range resulted in clean
conversion to a thermodynamic equilibrium ratio con-
taining a major and a minor diastereoisomer (Table 1).
The interconversion was conveniently monitored by 31
P
NMR spectroscopy, which was used to acquire kinetic
data for the atropinversion. Analysis of the data re-
vealed that the process obeys reversible first-order
relaxation kinetics. Figure 1 shows the reversible first-
order analysis for the interconversion of a 1:1 mixture
of λ(S)-3a and δ(S)-3a using the integrated rate expres-
sion for a standard reversible first-order process (A T
B).13,14 Eyring analyses of the rate data obtained for
atropinversion of 3a -d gave ∆Hq in the range 22-25
kcal mol-1 and ∆Sq between -1 and 16 eu, full details
In considering the on-metal mechanism, Gagne´ has
argued that the greater rate of atropinterconversion for
[(BIPHEP)RuCl2{(S,S)-DPEN}] compared with [(BIP-
HEP)Pt{(S)-BINOL}] could be caused by steric interac-
tions between the apical chlorides and the pseudoaxial
(13) ([B]eq/[A]0)[ln([B]eq/[B]eq - [B]t)] ) kt: Laidler, K. J . Chemical
Kinetics, 3rd ed.; Harper & Row: New York, 1987.
(14) First-order rate constants obtained for the diastereointercon-
version of compounds 3a -d and 4‚Cl2 are listed in Table S1 of the
Supporting Information.

Pt Complexes of NUPHOS Diphosphines
Organometallics, Vol. 23, No. 5, 2004 1059
Sch em e 2
and the use of conformationally flexible 2,2′-bis(di-
arylphosphino)-1,1′-biphenyl phosphines in combination
with chiral diamines for the ruthenium-catalyzed enan-
tioselective hydrogenation is one particularly relevant
example.2a
Such disparate activation parameters between atro-
pinversion in 3a -e and 4‚Cl2 clearly shows that the
ligand set trans to the tropos-diphosphine exerts a
marked influence on the barrier to this process. On this
evidence, we suggest that the difference in rates be-
tween [(BIPHEP)RuCl2{(S,S)-DPEN}] and [(BIPHEP)-
Pt{(S)-BINOL}] is more likely to be associated with the
change from DPEN to BINOL trans to BIPHEP and not
unfavorable interactions between the apical chloride
ligands and pseudoaxial P-Ph groups, as has been
suggested.4 In considering possible causes for the dif-
ference in activation parameters, we note that substitu-
tion of BINOL-ate for DPEN results in a slight length-
ening of the Pt-P bonds, as evidenced by the average
bond lengths of 2.228 and 2.239 Å in 3c and 4‚[CF3SO3]2,
respectively. Although a decrease in the strength of the
Pt-P bond could act to reduce the barrier to atropin-
version by facilitating the attenuation of strain in the
planar cyclic transition state, such a marked increase
in rate could also be taken as evidence for a change in
mechanism to the off-metal process, as either a direct
result of destablization of the ground state and/or a
conjugate-base type process in which π-donation from
an amide stabilizes the transition state to Pt-P bond
dissociation, as shown in Scheme 2. In fact, it should
also be noted that π-donation could account for attenu-
ation of strain in the planar transition state by length-
ening of the Pt-P bonds. Thus, while the kinetic data
do not allow us to distinguish between the two possible
mechanisms, the markedly disparate values of ∆Hq
between 4‚Cl2 and 3a -e could indicate that different
mechanisms operate.
F igu r e 3. van’t Hoff analyses of the equilibrium data
obtained from the thermal conversion of λ(S)-3a to δ(S)-
3a , of λ(S)-3c to δ(S)-3c, and of δ(S,S)-4‚Cl2 to λ(S,S)-4‚
Cl2.
phenyl groups, which would raise the ground-state
energy. To investigate this possibility, we have studied
diastereointerconversion in the corresponding diphos-
phine-diamine complex [(Ph4-NUPHOS)Pt{(S,S)-DP-
EN}]Cl2 (4‚Cl2). Eyring analysis of the rate data ob-
tained for interconversion of δ-[(Ph4-NUPHOS)Pt{(S,S)-
DPEN)}]Cl2 to λ-[(Ph4-NUPHOS)Pt{(S,S)-DPEN}]Cl2
revealed that atropinterconversion is significantly more
rapid in 4‚Cl2 compared to 3a -e, with activation
parameters of ∆Hq ) 17 kcal mol-1 and ∆Sq ) -23 eu
(Figure 2). A van’t Hoff analysis of the equilibrium data
obtained from the thermolysis of δ(S,S)-4‚Cl2 (Figure
3) gave a ∆H° value of 2.5 kcal mol-1 compared with
1.6 kcal mol-1 for its BINOL-ate counterpart 3c. It is
clear from the temperature dependence of Keq for 3c and
4‚Cl2 (Figure 3) that both the choice of chiral activator
(DPEN versus BINOL) and the temperature are impor-
tant for optimizing resolution using diastereoisomeric
complexes, since a large Keq value is most desirable in
the absence of kinetic resolution. The larger ∆H° for 4‚
Cl2 compared with 3c, for example, means that in order
to obtain the optimum diastereoisomeric ratio and
ultimately yield, the resolution of Ph4-NUPHOS with
(S,S)-DPEN would be most effective at elevated tem-
peratures, whereas there would be no significant ad-
vantage in performing the BINOL-ate-based resolution
at high temperature; lower temperatures would also
decrease the possibility of decomposition. Similar con-
siderations are also likely to apply to the use of
diastereoisomeric complexes in asymmetric catalysis,
Ch a r t 3
Com p u ta tion a l An a lysis. In an attempt to under-
stand the influence of coordination to a metal on the
barrier to atropinversion, an ab initio study involving
model diphosphines and their platinum complexes has
been undertaken to estimate the activation barrier to
axial torsion in the acyclic and monocyclic NUPHOS
diphosphines. Chart 3 illustrates the chemical models
that have been analyzed. In all the models studied,
chlorides were used in place of the BINOL and hydrogen

1060 Organometallics, Vol. 23, No. 5, 2004
Doherty et al.
F igu r e 4. Optimized structures for (a) the planar anti transition state of 5 and (b) the planar syn transition state of 5
using HF and (c) the ground-state structure of 7 and (d) the planar seven-membered metallacycle transition-state structure
of 7 using DFT. Color code: Pt, cyan; P, magenta; Cl, yellow; C, green; H, gray.
Ta ble 2. Ca lcu la ted En er gy Differ en ces (k ca l
m ol-1) betw een th e Gr ou n d Sta te a n d Syn
Tr a n sition Sta te of Mod el NUP HOS Dip h osp h in es
a n d P la tin u m Com p lexes, As Sh ow n in Ch a r t 3
3e can rotate about C2-C3 to achieve an anti transition
state and that the calculated activation barrier for atro-
pinversion via a syn planar transition state is signifi-
cantly higher than the experimentally determined value
both suggest that atropinversion occurs via the on-metal
pathway. Moreover, if we assume that the activation
barrier of 32.826 kcal mol-1 calculated for atropinver-
sion of 5 is a rough estimate of the barrier to atropin-
version via dissociation-induced rotation, this value does
not include a contribution from the Pt-P bond dissocia-
tion step, which would necessarily raise the barrier
quite substantially, especially given the strength of the
Pt-P bond. In this regard, a number of previous studies
have reported that solvent coordination is important for
cleavage of a strong metal-phosphorus bond to occur.12
However, in the present study a chlorobenzene/toluene
mixture has been used as the solvent, which is unlikely
to coordinate strongly and therefore cannot assist the
bond dissociation process. Therefore, if atropinversion
of 3a -e involves a Pt-P bond dissociation step, we
could reasonably expect the experimentally determined
activation barrier to be even greater than 32.826 kcal
mol-1. In considering the on-metal mechanism, the
ground and planar metallacycle transition states of mod-
el 7 were fully optimized under C2 symmetry using DFT
and the resulting structures are shown in parts c and d
Figure 4, respectively. The activation energy of 28.163
kcal mol-1 for on-metal atropinversion involving a plan-
ar syn transition state is close to the range of experi-
mentally determined values for compounds 3a -e. Thus,
each of the factors discussed above combine to indicate
that atropinversion in platinum group complexes of
NUPHOS-type diphosphines most likely occurs via an
on-metal pathway involving a syn transition state.
level of theory
5
6
7
HF
DFT
32.826
36.745
28.696
34.076
28.163
atoms were used instead of the phenyl rings attached
to the phosphorus atoms, in order to reduce computation
time while retaining the essential features of the
NUPHOS complexes and free ligands. The energy
differences between the ground state and syn transition
states of a number of the model NUPHOS diphosphines
and their platinum complexes are summarized in Table
2.
In an attempt to explore the off-metal pathway, the
transition state corresponding to syn and anti rotation
of model 5 have been optimized using HF theory. These
calculations revealed that rotation involving an anti
transition state (Figure 4a) has a significantly lower
barrier (22.154 kcal mol-1) than that of 32.826 kcal
mol-1 for the syn transition state (Figure 4b) and is in
good agreement with calculations performed on 1,1′-
binaphthyls.15 This latter value is similar to that
calculated for monocyclic NUPHOS 6 (36.745 kcal
mol-1), which can only undergo atropinversion via a syn
transition state. Therefore, if atropinversion occurs via
Pt-P bond dissociation followed by rotation about C2-
C3, it will do so via an anti transition state. Since all
the activation barriers determined experimentally for
complexes 3a -e are similar, it is reasonable to assume
that 3a -c and 3d ,e undergo diastereointerconversion
via the same mechanism. The fact that neither 3d or
X-r ay Str u ctu r es of [{1,4-bis(diph en ylph osph in o)-
1,2,3,4-t e t r a m e t h y l-1,3-b u t a d ie n e }P t {(S )-B I N -
(15) Kranz, M.; Clark, T.; Schleyer, P. v. R. J . Am. Chem. Soc. 1993,
58, 3317.

Pt Complexes of NUPHOS Diphosphines
Organometallics, Vol. 23, No. 5, 2004 1061
disparate thermodynamic stabilities of the enantiopure
diastereoisomers of compounds 3a -e prompted us to
undertake single-crystal X-ray structure determinations
of 3a ,e to obtain precise details of the relative stereo-
chemistry of the NUPHOS and BINOL fragments.
Perspective views of the molecular structures of 3a ,e
together with their atomic numbering schemes are
shown in Figures 5 and 6, and selected bond lengths
and angles for both compounds are listed in the figure
captions. The most striking feature of 3a is the relative
stereochemistry of the NUPHOS chelate, which adopts
a λ-skew conformation16 that represents mismatched
stereochemistries of NUPHOS and BINOL. The NU-
PHOS diphosphine coordinates to form a seven-mem-
bered chelate ring with a skew-boat conformation, which
is highly distorted below the square plane. Gagne´ has
also selectively crystallized the thermodynamically less
favored mismatched diastereoisomer λ(S)-[(BIPHEP)-
Pt{(S)BINOL}] and noted that the BIPHEP ligand
undergoes a similar distortion to minimize interaction
between the pseudoequatorial P-phenyl rings and the
3,3′-C-H bonds of BINOL,4 which attempt to occupy the
same quadrants. This distortion reorientates the phenyl
rings away from the classical pseudoaxial and equatorial
arrangement commonly found in complexes of BINAP,
BIPHEP, and related atropisomeric diphosphines. The
two phenyl rings attached to P(1) are evenly disposed
above and below the PtP2 plane, while those attached
to P(6) adopt pronounced axial and equatorial positions,
such that the latter lies close to the PtP2 plane. This
type of distortion has previously been quantified by the
P-Pt-P-C(ipso) dihedral angles, expressed as a rota-
tion above and below the PtP2 plane. The symmetrically
disposed phenyl rings C(11A)-C(16A) and C(11B)-
Figu r e 5. Molecular structure of λ-[(1,2,3,4-Me4-NUPHOS)-
Pt{(S)-BINOL}] (λ-(3a )), highlighting the mismatched
combination of NUPHOS and BINOL stereochemistries.
Hydrogen atoms, except for H(2BA) and H(2AA), and
dichloromethane molecules of crystallization have been
omitted for clarity. Ellipsoids are at the 40% probability
level. Selected bond lengths (Å) and angles (deg): Pt(1)-
P(1), 2.2350(11); Pt(1)-P(6), 2.2094(11); Pt(1)-O(1A),
2.037(3); Pt(1)-O(1B), 2.060(3); C(2)-C(3), 1.350(6); C(3)-
C(4), 1.489(6); C(4)-C(5), 1.330(5); P(1)-Pt(1)-P(6), 88.81(4);
O(1A)-Pt(1)-O(1B), 87.13(11); P(6)-Pt(1)-O(1A), 98.53(8);
P(6)-Pt(1)-O(1B), 169.27(9); P(1)-Pt(1)-O(1B), 86.13(8);
P(1)-Pt(1)-O(1A), 172.03(8).
OL}] (3a ) a n d [{1,2-bis(1-(d ip h en ylp h osp h in o)ben -
zylid en e)cycloh exa n e}P t {(S)-BINOL}] (3e). The
F igu r e 6. Molecular structure of δ-[(1,4-Ph2-2,3-cyclo-C6H8-NUPHOS)Pt{(S)-BINOL}] (δ-(3e)), showing the relative
stereochemistry of the NUPHOS and BINOL fragment as the matched combination. Hydrogen atoms, except for H(2A)
and H(2B), and toluene molecules of crystallization have been omitted. Ellipsoids are at the 40% probability level. Selected
bond lengths (Å) and angles (deg): Pt(1)-P(1), 2.2235(18); Pt(1)-P(6), 2.2210(19); Pt(1)-O(1A), 2.034(5); Pt(1)-O(1B),
2.037(5); C(2)-C(3), 1.311(9); C(3)-C(4), 1.491(9); C(4)-C(5), 1.389(10); P(1)-Pt(1)-P(6), 92.19(6); O(1A)-Pt(1)-O(1B),
87.79(18); P(6)-Pt(1)-O(1A), 90.59(14); P(6)-Pt(1)-O(1B), 171.82(15); P(1)-Pt(1)-O(1B), 90.56(14); P(1)-Pt(1)-O(1A),
171.50(15); C(2)-C(3)-C(4), 124.8(7); C(3)-C(4)-C(5), 125.3(6); C(2)-C(3)-C(31), 125.6(7); C(31)-C(3)-C(4), 109.2(6);
C(41)-C(4)-C(5), 119.4(8); C(3)-C(4)-C(41), 115.1(7).

1062 Organometallics, Vol. 23, No. 5, 2004
Doherty et al.
C(16B) are rotated by 41.2° below and 74.4° above the
plane.4 In contrast, the axial phenyl ring attached to
P(6) exposes its edge toward the platinum atom with
C(61A)-C(66A) rotated 109.2° above the plane, while
its pseudoequatorial counterpart C(61B)-C(66B) is
rotated 7.5° below the plane. For comparison the cor-
responding rotations in [(BIPHEP)Pt{(S)-BINOL}] are
73.8 and 44.0° for the symmetric half of the diphosphine
and 113 and 6.2° for the more distorted phenyl rings.
The distortions present in the solid-state structure of
3a clearly relate to those in λ(S)-[(BIPHEP)Pt{(S)-
BINOL)}] and arise as a result of the mismatched
stereochemistries of the BINOL and NUPHOS chelates.
The dihedral angles of 61.8° formed between the double
bonds and their substituents C(2)C(21)C(3)C(31) and
C(4)C(41)C(5)C(51) in 3a are comparable to those in
complexes of related four-carbon-bridged diphosphines.17
In the case of 3e the molecular structure confirms the
stereochemistry of the NUPHOS and BINOL as matched,
with the diphosphine adopting a δ-skew conformation.
an on-metal pathway involving a cyclic seven-membered
transition state. These studies have shown that the
conformational stability of the diphosphine stereochem-
istry in [(NUPHOS)Pt{(S)-BINOL}] is similar to that
of BIPHEP in [(BIPHEP)Pt{(S)-BINOL}]. In a com-
parative kinetic study the ligand that is trans to the
tropos diphosphine has been shown to affect the barrier
to diastereointerconversion, since substitution of (S)-
BINOL for (S,S)-DPEN results in a significant increase
in the rate of atropinversion of NUPHOS and a ∆Hq
value of 17 kcal mol-1 has been determined for
[(NUPHOS)Pt{(S,S)-DPEN}]Cl2 (4‚Cl2), which is much
closer to that reported for [(BIPHEP)RuCl2{(S,S)-
DPEN}]. On the basis of the similarity between BI-
PHEP- and NUPHOS-type diphosphines we have ini-
tiated a catalyst testing program to explore the applica-
tions of these conformationally flexible diphosphine in
platinum group asymmetric Lewis acid catalysis.
Exp er im en ta l Section
A
31P NMR spectrum of a representative sample of
Gen er a l P r oced u r es. All manipulations involving air-
sensitive materials were carried out using standard Schlenk
line techniques under an atmosphere of nitrogen or argon in
oven-dried glassware. Diethyl ether and hexane were distilled
from potassium/sodium alloy, tetrahydrofuran from potassium,
dichloromethane from calcium hydride, and methanol from
magnesium. Unless otherwise stated, commercially purchased
materials were used without further purification. The plati-
num complex [Pt(COD)Cl2]22 and the NUPHOS8 diphosphines
were prepared as previously described. Deuteriochloroform
was predried with calcium hydride, vacuum transferred, and
stored over 4 Å molecular sieves. 1H, 31P{1H}, and 13C{1H}
NMR spectra were recorded on a J EOL LAMBDA 500 or
Bruker AC 200, AMX 300, and DRX 500 machines. Optical
rotations were measured on a Perkin-Elmer 241 spectropola-
rimeter and are reported as follows: [R]D (c in units of g/100
mL, solvent).
crystals conclusively demonstrated that the X-ray study
had been conducted on the thermodynamically favored
diastereoisomer. In contrast to the structure of 3a , in
which the NUPHOS and BINOL ligands are highly
distorted above the PtP2 square plane, that of 3e has
close to C2 symmetry. The platinum atom in 3e is
distorted from an ideal square-planar geometry with a
dihedral angle of 11.4° between the PtP2 and PtO2
planes. The four rings of the diphenylphosphino groups
are arranged in the familiar alternating edge-face
manner.18 The dihedral angle of 56.1° between the least-
squares planes containing C(2)C(21)C(3)C(31) and C(4)C-
(41)C(5)C(51), which describes the atropisomerism of
NUPHOS, is similar to that reported for a number of
related conformationally restricted diphosphines.19 Sur-
prisingly, the bonding in the butadiene tether is highly
unsymmetrical, with the C(2)-C(3) bond length of
1.311(9) Å significantly shorter that that of 1.389(10) Å
associated with C(4)-C(5). The interbond angle of 109.2-
(6)° at C(3) is significantly smaller than the ideal value
of 120° for an sp2-hybridized carbon and reflects the
strong tendency of this carbon atom to adopt an angle
close to 109.5° required by a cyclohexane ring. In
contrast, the corresponding angle of 115.1(7)° at C(4) is
much larger. The natural bite angle of 92.19(6)° is close
to the ideal value of 90° and comparable to those
reported for [(BIPHEP)PdCl2] (93.03(7)°)20 and [Pt{(R)-
BINAP}(o-C6H5OMe)2] (93.1(1)°).21
Syn t h esis of Dich lor o[1,4-b is(d ip h en ylp h osp h in o)-
1,2,3,4-tetr a m eth yl-1,3-bu ta d ien e]p la tin u m (2a ). A solu-
tion of [(cyloocta-1,5-diene)PtCl2] (0.50 g, 1.3 mmol) in dichlo-
romethane (20 mL) was added to a dichloromethane solution
of 1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadi-
ene (0.635 g, 1.3 mmol), and the mixture was stirred vigorously
for 4 h. The reaction mixture was filtered and the solvent
removed to leave a pale yellow solid residue. Crystallization
by slow diffusion of n-hexane into a dichloromethane solution
at room temperature gave 2a as colorless prisms in 88% yield
(0.87 g). 31P{1H} NMR (121.5 MHz, CDCl3, δ): 6.7 (t, J PtP
)
3660 Hz, PPh2). 1H NMR (300.0 MHz, CDCl3, δ): 8.36 (br, 4H,
(16) Brunkan, N. M.; White, P. S.; Gagne´, M. R. Angew. Chem., Int.
Ed. 1998, 37, 1579.
(17) (a) Schmid, R.; Firicher, J .; Cereghetti, M.; Schonholzer, P. Helv.
Chim. Acta 1991, 74, 370. (b) Schmid, R.; Cereghetti, M.; Schonholzer,
P.; Hansen, H.-J . Helv. Chim. Acta 1988, 71, 897. (c) Schmid, R.;
Broger, E.; Cereghetti, M.; Crameri, Y.; Foricher, J .; Lalonde, M.;
Muller, R. K.; Scalone, M.; Schoettel, G.; Zutter, U. Pure Appl. Chem.
1996, 68, 131. (d) Toriumi, K.; Ito, T.; Takaya, T.; Noyori, R. Acta
Crystallogr. 1982, B38, 807.
(18) (a) Ozawa, F.; Kubo, A.; Matsumoto, Y.; Hayashi, T.; Nishioka,
E.; Yangi, K.; Moriguchi, K. Organometallics 1993, 12, 4188. (b)
Tominaga, H.; Sakai, K.; Tsubomura, T. J . Chem. Soc., Chem.
Commun. 1995, 2273.
(19) Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2000,
19, 1567.
(20) (a) Alcock, N. W.; Brown, J . M.; Perez-Torrente, J . Tetrahedron
Lett. 1992, 33, 389. (b) Brown, J . M.; Perez-Torrente, J . J .; Alcock, N.
W. Organometallics 1995, 14, 1195.
Con clu sion s
Diastereointerconversion in the NUPHOS diphos-
phine complexes [(NUPHOS)Pt{(S)-BINOL}] (3a -e)
has been investigated, and Eyring analysis of the
resulting rate data gave ∆Hq values of 22-25 kcal
mol-1. Notably, the activation barriers for atropinver-
sion of the acyclic NUPHOS diphosphine complexes
3a -c are similar to those for their monocyclic NUPHOS
counterparts 3d ,e, which suggest that the same mech-
anism operates in both cases, either via an on-metal
pathway or by Pt-P bond dissociation and syn rotation,
since the cyclohexyl ring prevents anti rotation. Com-
bined, kinetic studies and computational analysis indi-
cate that atropinversion of 3a -e most likely occurs via
(21) (a) Gugger, P.; Limmer, S. O.; Watson, A. A.; Willis, A. C.; Wild,
S. B. Inorg. Chem. 1993, 32, 5692. (b) Morton, D. A. V.; Orpen, A. G.
J . Chem. Soc., Dalton Trans. 1992, 641.
(22) (a) Drew, D.; Doyle, J . R. Inorg. Synth. 1990, 28, 246. (b) Chatt,
J .; Vallarino, L. M.; Venanzi, L. M. J . Chem. Soc. 1957, 3413.

Pt Complexes of NUPHOS Diphosphines
Organometallics, Vol. 23, No. 5, 2004 1063
C6H5), 7.58 (m, 8H, C6H5), 7.27 (m, 8H, C6H5), 1.23 (d, J PH
)
and the resulting orange solid triturated with hexane to afford
the desired product as a 1:1 mixture of diastereoisomers in
79% yield (1.01 g). Crystallization by slow diffusion of a
dichloromethane solution layered with n-hexane gave X-ray-
quality crystals of the thermodynamically less favored dias-
tereoisomer only. 31P{1H} NMR (121.5 MHz, CDCl3, δ): 3.0
(t, J PtP ) 3668 Hz, PPh2). 1H NMR (500.13 MHz, CDCl3, δ):
7.99 (t, J ) 3.2 Hz, 6H aromatic), 7.65 (d, J ) 7.95 Hz, 2H,
aromatic), 7.46-7.42 (m, 8H, aromatic), 7.27-7.22 (m, 8H,
aromatic), 7.07 (d, J ) 7.9 Hz, 2H, aromatic), 7.03 (d, J ) 8.1
Hz, 2H, aromatic), 6.98 (d, J ) 8.8 Hz, 2H, aromatic), 6.31 (d,
J ) 8.8 Hz, 2H, aromatic), 1.41 (d, J ) 5.6 Hz, 6H, CH3), 1.16
(s, 6H, CH3). Anal. Calcd for C52H44O2P2Pt: C, 65.20; H, 4.63.
Found: C, 65.09; H, 4.32. [R]D ) -0.92° (c 1.0, CHCl3).
[{1,4-b is(d ip h e n ylp h osp h in o)-1,2,3,4-t e t r a e t h yl-1,3-
bu ta d ien e}p la tin u m {(S)-BINOL}] (3b). Compound 3b was
prepared according to the procedure described above for 3a
and isolated as an orange solid in 88% yield. 31P{1H} NMR
(121.5 MHz, CDCl3, δ): 0.4 (t, J PtP ) 3700 Hz, PPh2), -2.3 (t,
J PtP ) 3630 Hz, PPh2). 1H NMR (500.13 MHz, CDCl3, δ): 8.22
(br, aromatic), 8.02 (br t, J ) 13.0 Hz, aromatic), 7.71 (t, J )
12.0 Hz, aromatic), 7.61-6.9 (m, aromatic), 6.69 (d, J ) 14.5
Hz, aromatic), 6.15 (d, J ) 14.6 Hz, aromatic), 2.01 (m 2H,
CHaHb), 1.90 (m, 4H, CHcHd + CHeHf), 1.81 (m, 2H, CHaHb),
1.48 (m, 4H, CHcHd + CHgHi), 1.32 (m, 2H, CHeHf), 1.26 (m,
2H, CHgHi), 1.01 (t, J ) 7.5 Hz, 3H, CH2CH3), 0.87 (t, J ) 7.5
Hz, 3H, CH2CH3), 0.58 (t, J ) 7.4 Hz, 3H, CH2CH3), 0.23 (t, J
) 7.4 Hz, 3H, CH2CH3). Anal. Calcd for C56H52O2P2Pt: C,
66.33; H, 5.17. Found: C, 66.67; H, 5.44. [R]D ) -8.9° (c 1.0,
CHCl3).
11.2 Hz, 6H, CH3), 1.18 (s, 6H, CH3). 13C{1H} NMR (75.4 MHz,
CDCl3, δ): 149-124 (m, C6H5 + CdC), 19.5 (t, J PC ) 3.3 Hz,
CH3), 17.5 (t, J PC ) 5.6 Hz, CH3). Anal. Calcd for C32H32Cl2P2-
Pt: C, 51.62; H, 4.33. Found: C, 51.96; H, 4.44.
Dich lor o[1,4-bis(diph en ylph osph in o)-1,2,3,4-tetr aeth yl-
1,3-bu ta d ien e]p la tin u m (2b). Compound 2b was prepared
according to the procedure described above for 2a and isolated
as yellow crystals in 82% yield by slow diffusion of a chloroform
solution layered with hexane. 31P{1H} NMR (121.5 MHz,
CDCl3, δ): 4.6 (t, J PtP ) 3814 Hz, PPh2). 1H NMR (500.13 MHz,
CDCl3, δ): 8.60 (br, 2H, C6H5), 7.56-7.09 (m, 18H, C6H5), 2.23
(m, 2H, CH2), 1.75 (br m, 2H, CH2), 1.45 (br m, 4H, CH2), 1.03
(t, J ) 7.0 Hz, 6H, CH3), 0.30 (t, J ) 7.0 Hz, 6H, CH3). Anal.
Calcd for C36H40Cl2P2Pt: C, 54.01; H, 5.04. Found: C, 54.36;
H, 5.19.
Dich lor o[1,4-bis(diph en ylph osph in o)-1,2,3,4-tetr aph en -
yl-1,3-bu ta d ien e]p la tin u m (2c). Compound 2c was prepared
according to the procedure described above for 2a and isolated
as yellow crystals in 79% yield by slow diffusion of a chloroform
solution layered with methanol. 31P{1H} NMR (121.5 MHz,
CDCl3, δ): 1.9 (t, J PtP ) 3607 Hz, PPh2). 1H NMR (500.13 MHz,
CDCl3, 193 K, δ): 9.57 (br m, 2H, C6H5), 8.41 (br m, 2H, C6H5),
8.0 (br, 6H, C6H5), 7.12 (t, J ) 7.0 Hz, 2H, C6H5), 6.94 (m, 4H,
C6H5), 6.85 (m, 4H, C6H5), 6.80 (t, J ) 7.2 Hz, 2H, C6H5), 6.77
(t, J ) 7.2 Hz, 2H, C6H5), 6.65 (t, J ) 7.3 Hz, 2H, C6H5), 6.56
(t, J ) 7.7 Hz, 2H, C6H5), 6.60 (t, J ) 7.0 Hz, 2H, C6H5), 6.18
(d, J ) 7.8 Hz, 4H, C6H5), 6.06 (d, J ) 7.8 Hz, 2H, C6H5), 5.94
(d, J ) 7.3 Hz, 2H, C6H5). Anal. Calcd for C52H40Cl2P2Pt: C,
62.91; H, 4.06. Found: C, 63.11; H, 4.21.
[{1,4-bis(d ip h en ylp h osp h in o)-1,2,3,4-t et r a p h en yl-1,3-
bu ta d ien e}p la tin u m {(S)-BINOL}] (3c). Compound 3c was
prepared according to the procedure described above for 3a
and isolated as an intense yellow solid in 63% yield. Crystal-
lization by slow diffusion of n-hexane into a dichloromethane
solution at room temperature gave X-ray-quality crystals of
the thermodynamically preferred diastereoisomer. 31P{1H}
NMR (121.5 MHz, CDCl3, δ): -1.0 (t, J PtP ) 3619 Hz, PPh2).
1H NMR (500.13 MHz, CDCl3, δ): 8.98 (br, 4H, aromatic), 7.9
(br, 6H, aromatic), 7.64 (d, J ) 8.2 Hz, 2H, aromatic), 7.46 (d,
J ) 8.8 Hz, 2H, aromatic), 7.02 (t, J ) 6.7 Hz, 2H, aromatic),
6.94-6.91 (m, 6H, aromatic), 6.87 (t, J ) 6.9 Hz, aromatic),
6.76 (m, 6H, aromatic), 6.70 (br, 2H, aromatic), 6.63 (t, J )
7.3 Hz, 4H, aromatic), 6.56 (m, 6H, aromatic), 6.4 (br, 2H,
aromatic), 6.14 (br, 4H, aromatic), 6.02 (d, J ) 7.2 Hz, 4H,
aromatic). Anal. Calcd for C72H52O2P2Pt: C, 71.69; H, 4.35.
Found: C, 71.87; H, 4.48. [R]D ) -11.1° (c 1.0, CHCl3).
[{1,2-b is((d ip h en ylp h osp h in o)et h ylm et h ylen e)cyclo-
h exa n e}p la tin u m {(S)-BINOL}] (3d ). Compound 3d was
prepared according to the procedure described above for 3a
and isolated as an orange solid in 69% yield. Crystallization
from a toluene solution at room temperature gave X-ray-
quality crystals of the thermodynamically preferred diastereo-
Dich lor o[1,2-bis((diph en ylph osph in o)eth ylm eth ylen e)-
cycloh exa n e]p la t in u m (2d ). Compound 2d was prepared
according to the procedure described above for 2a and isolated
as yellow crystals in 71% yield by slow diffusion of a chloroform
solution layered with hexane. 31P{1H} NMR (121.5 MHz,
1
CDCl3, δ): 5.2 (t, J PtP ) 3648 Hz, PPh2). H NMR (300 MHz,
CDCl3, δ): 8.19 (br, 4H, C6H5), 7.56-6.98 (m, 16H, C6H5), 1.34
(m, 4H, Cy H2 + CH2CH3), 0.92 (br m, 2H, Cy H2), 0.85 (m,
2H, CH2CH3), 0.71 (br m, 4H, Cy H2), 0.13 (t, J PH ) 7.2 Hz,
CH2CH3). 13C{1H} NMR (125.65 MHz, CDCl3, δ): 156-129 (m,
C6H5), 36.2 (s, Cy CH2), 29.1 (s, Cy-CH2), 26.1 (t, J PC ) 5.2
Hz, CH2CH3), 14.8 (s, CH2CH3). Anal. Calcd for C36H38Cl2P2-
Pt: C, 54.14; H, 4.80. Found: C, 54.21; H, 4.89.
Dich lor o[1,2-b is((d ip h en ylp h osp h in o)p h en ylm et h yl-
en e)cycloh exa n e]p la tin u m (2e). Compound 2e was pre-
pared according to the procedure described above for 2a and
isolated as yellow crystals in 67% yield by slow diffusion of a
chloroform solution layered with hexane. 31P{1H} NMR (121.5
MHz, CDCl3, δ): 1.2 (t, J PtP ) 3665 Hz, PPh2). 1H NMR (500.13
MHz, CDCl3, δ): 8.57 (br, 4H, C6H5), 7.76 (br, 6H, C6H5), 7.0
(t, J ) 7.5 Hz, 2H, C6H5), 6.96 (t, J ) 7.2 Hz, 2H, C6H5), 6.89
(t, J ) 7.4 Hz, 2H, C6H5), 6.78 (t, J ) 7.1 Hz, 2H, C6H5), 6.75
(m, 2H, C6H5), 6.25 (t, J ) 7.4 Hz, 2H, C6H5), 6.21 (d, J ) 7.3
Hz, 2H, C6H5), 5.94 (t, J ) 7.3 Hz, 2H, C6H5), 1.86 (d, J ) 13.4
Hz, 2H, Cy CH2), 1.56 (br, 2H, Cy CH2), 1.47 (br, 2H, Cy CH2),
isomer. 31P{1H} NMR (121.5 MHz, CDCl3, δ): -0.4 (t, J PtP
)
3700 Hz, PPh2), -1.7 (t, J PtP ) 3648 Hz, PPh2). 1H NMR single
diastereoisomer (500.13 MHz, CDCl3, δ): 8.17 (br, 4H, aro-
matic), 7.9-6.89 (m, 26H, aromatic), 6.53 (d, J ) 8.7 Hz, 2H,
aromatic), 2.13-0.81 (m, 12H, Cy H2 + CH2CH3), 0.31 (t, J )
7.4 Hz, 6H, CH2CH3). Anal. Calcd for C56H50O2P2Pt‚2C7H8: C,
70.28; H, 5.56. Found: C, 70.67; H, 5.73. [R]D ) -4.6° (c 1.0,
CHCl3).
1.19 (br t, J ) 9.0 Hz, 2H, Cy CH2). Anal. Calcd for C44H38
-
Cl2P2Pt‚4CHCl3: C, 42.01; H, 3.09. Found: C, 41.23; H, 2.33.
Syn th esis of [{1,4-bis(d ip h en ylp h osp h in o)-1,2,3,4-tet-
r a m eth yl-1,3-bu ta d ien e}p la tin u m {(S)-BINOL}] (3a ). A
solution of (S)-BINOL (0.383 g, 1.34 mmol) in tetrahydrofuran
(30 mL) was slowly added to a rapidly stirred solution of
freshly sublimed sodium tert-butoxide (0.259, g 2.70 mmol) in
THF (30 mL). After the mixture was stirred for ca. 15 min,
the resulting solution was transferred via cannula to a
suspension of 2a (1.0 g, 1.34 mmol) in toluene (30 mL), which
resulted in the immediate appearance of an orange coloration.
The reaction mixture was stirred for 90 min, during which time
the solid dissolved to give an intense orange solution. The
reaction mixture was filtered and the solvent removed to give
a deep orange solid, which was extracted into chloroform and
filtered to remove sodium chloride. The solvent was removed
[{1,2-b is ((d ip h e n y lp h o s p h in o )p h e n y lm e t h y le n e )-
cycloh exa n e}p la tin u m {(S)-BINOL}] (3e). Compound 3e
was prepared according to the procedure described above for
3a and isolated as an intense yellow solid in 67% yield.
Crystallization from a toluene-dichloromethane solution at
room temperature gave X-ray-quality crystals of the thermo-
dynamically preferred diastereoisomer. 31P{1H} NMR (121.5
MHz, CDCl3, δ): -0.8 (t, J PtP ) 3667 Hz, PPh2). 1H NMR
(500.13 MHz, CDCl3, δ): 8.59 (br m, 4H, aromatic), 7.70 (m,
4H, aromatic), 7.51 (d, J ) 8.8 Hz, 2H, aromatic), 7.40 (d, J )

1064 Organometallics, Vol. 23, No. 5, 2004
Doherty et al.
8.8 Hz, 2H, aromatic), 7.19-7.10 (m, 4H, aromatic), 6.92 (t, J
) 7.5 Hz, 2H, aromatic), 6.86-6.76 (m, 12H, aromatic), 6.67
(t, J ) 5.3 Hz, 4H, aromatic), 6.60 (t, J ) 6.9 Hz, 2H, aromatic),
6.52 (t, J ) 8.7 Hz, 2H, aromatic), 6.07 (d, J ) 7.55 Hz, 2H,
aromatic), 6.03 (d, J ) 8.0 Hz, 2H, aromatic), 1.84 (br d, 2H,
Cy-H2), 1.46 (br m, 4H, Cy H2), 1.18 (br, 2H, Cy H2). Anal.
Calcd for C64H50O2P2Pt‚2C7H8: C, 72.49; H, 5.15. Found: C,
72.83; H, 5.54. [R]D ) -4.7° (c 1.0, CHCl3).
tive core potential basis set28 (which was also used for DFT
level calculations) available from the EMSL database.29 Ef-
fective core potentials were used throughout to facilitate
computation of systems that included the metal and to make
comparison with free ligand calculations possible. All optimi-
zations were checked, using vibrational analysis, to ensure
that the minima on the potential energy surface were true local
minima (by ensuring the second-derivative Hessian matrix was
positive definite) and that all transition state structures had
only one imaginary vibrational frequency. Furthermore, in-
trinsic reaction coordinate following was performed to link the
transition-state structures to the structural minima.
Syn th esis of [{1,4-bis(d ip h en ylp h osp h in o)-1,2,3,4-tet-
r a p h en yl-1,3-b u t a d ien e}p la t in u m {(S,S)-DP E N}]Cl2 (4‚
Cl2). A dichloromethane solution of (S,S)-1,2-diphenylethyl-
enediamine (0.053 g, 0.25 mmol in 5 mL) was added to a
stirred solution of enantiopure δ-[(Ph4-NUPHOS)PtCl2] (0.25,
0.25 mmol) in dichloromethane. After 30 min the reaction
mixture was filtered, the solvent removed, and the residue
crystallized from a dichloromethane solution layered with
hexane to give 4‚Cl2 in 77% yield (0.233 g). 31P{1H} NMR
(121.5 MHz, CDCl3, δ): -6.5 (t, J PtP ) 3315 Hz, PPh2). 1H NMR
(500.13 MHz, CDCl3, δ): 8.11 (br, 4H, aromatic), 8.04 (t, J )
6.4 Hz, 2H, aromatic), 7.44 (d, J ) 6.8 Hz, 6H, aromatic), 7.16
(m, 6H, aromatic), 7.06 (m, 10H, aromatic), 6.82 (t, J ) 7.4
Hz, 2H, aromatic), 6.71 (t, J ) 6.4 Hz, 2H, aromatic), 6.33 (m,
6H, aromatic), 6.59 (t, J ) 7.4 Hz, 2H, aromatic), 6.21 (d, J )
7.8 Hz, 2H, aromatic), 6.05 (d, J ) 7.2 Hz, 4H, aromatic), 5.72
(br m, 2H, NH2CH), 5,67 (br, 2H, NHaHb), 3.20 (br d, 2H,
NHaHb). Anal. Calcd for C66H56Cl2N2P2Pt: C, 72.49; H, 5.15.
Found: C, 72.83; H, 5.54. Crystals of 4‚[CF 3SO3]2 suitable for
X-ray analysis were obtained by reaction of 4‚Cl2 with 2 equiv
of silver trifluoromethanesulfonate in dichloromethane fol-
lowed by crystallization from a chloroform solution layered
with n-hexane at room temperature.
Cr ysta l Str u ctu r e Deter m in a tion s of 3a a n d 3e. Data
were collected on a Bruker-AXS SMART diffractometer using
the SAINT-NT software30 with graphite-monochromated Mo
KR radiation using ψ/ω scans. A crystal was mounted onto the
diffractometer at low temperature under dinitrogen at ca. 120
K. Crystal stabilities were monitored via re-collection of the
first set of frames. There were no significant variations
(<(1%). Lorentz and polarization corrections were applied.
The structures were solved using direct methods and refined
with the SHELXTL program package,31 and the non-hydrogen
atoms were refined with anisotropic thermal parameters.
Hydrogen atoms were added at idealized positions, and a
riding model was used for subsequent refinement. Although
the structure of 3e can be solved in both P31 and P3121, the
space group P31 was chosen over P3121 because the disorder
in the model is reduced to being only associated with the
BINOL-ate ligand and this resulted in a better fit for the data
(i.e. R1 ) 0.0454 for P31 and R1 ) 0.0856 for P3121). The
2
2
function minimized was ∑[w(|Fo| - |Fc| )], with reflection
2
2
weights w-1 ) [σ2|Fo| + (g1P)2 + (g2P)], where P ) [max |Fo|
Kin etic Stu d y of th e Th er m a l In ter con ver sion of λ-
(S)-3a -e to δ(S)-3a -e. In a typical procedure a 1:1 diaster-
eoisomeric mixture of 3a (10.4 mg, 10.6 µmol) was dissolved
in a mixture of chlorobenzene (0.35 mL) and d8-toluene (0.15
mL) and placed in an NMR tube. A time zero spectrum was
recorded, the sample was heated at the specified temperature
for a known amount of time and quenched in an ice bath, and
2
+ 2|Fc| ]/3. Additional material available from the Cambridge
Crystallographic Data Center comprises relevant tables of
atomic coordinates, bond lengths and angles, and thermal
parameters.
a
31P{1H} spectrum was recorded. In this manner the progress
Ack n ow led gm en t. We gratefully acknowledge the
EPSRC (R.K.R.), CenTACat, and the McClay Trust
(C.R.N.) for funding and J ohnson Matthey for loans of
palladium salts. J .A.v.d.B. thanks SASOL Research and
Development for sponsorship (Grant R8112QLL).
of the thermal diastereointerconversion of λ(S)-3a -e to δ(S)-
3a -e was monitored.
Ch em ica l Mod els a n d Com p u ta tion a l Stu d ies. Initially,
Hartree-Fock (HF) level calculations were used to optimize
the structures. Further optimization was performed first using
Møller-Plesset second-order perturbation theory (MP2) and
using density functional theory (DFT). All the HF level
calculations were performed on the PC version of GAMESS23,25
(version 6.1; build 2032), while the DFT level (B3LYP func-
tional)24 calculations were performed on the UNIX based
version25 running on a Cygwin26 client (GAMESS version
released J une 20, 2002, R1). Initially all calculations (except
DFT level) were performed using the effective core potential
basis set by Stevens et al.27 (valence double-ú), which is part
of the GAMESS package. For the determination of better
thermodynamic properties on the chemically more relevant
models, calculations were repeated using the LanL2DZ effec-
Su p p or tin g In for m a tion Ava ila ble: Text giving revers-
ible first-order analyses for the thermal isomerization of λ(S)-
3b-d to δ(S)-3b-d and δ(S,S)-4‚Cl2 to λ(S,S)-4‚Cl2, rate
constant data, k (s-1), obtained for the atropisomerization of
compounds 3a -d and 4, and tables of crystal data, structure
solution and refinement, atomic coordinates, bond distances,
bond angles, and anisotropic thermal parameters and perspec-
tive views for compounds 3a ,c,e and 4‚[CF 3SO3]2. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org. Observed and calculated structure factor tables
are available from the authors upon request.
(23) Granovsky, A. A. Available from http://classic.chem.msu.su/
gran/gamess/index.html.
(24) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (b) Stephens,
P. J .; Devlin, F. J .; Chablowski, C. F.; Frisch, M. J . J . Phys. Chem.
1994, 98, 11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997,
268, 345.
(25) Schmidt, M. W.; Baldridge, K. K.; Boatz, J . A.; Elbert, S. T.;
Gordon, M. S.; J ensen, J . J .; Koseki, S.; Matsunaga, N.; Nguyen, K.
A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J . A. J . Comput.
Chem. 1993, 14, 1346.
OM034289F
(28) (a) Dunning, T. H., J r.; Hay, P. J . In Methods of Electronic
Structure Theory; Schaefer, H. F., Ed.; Plenum Press: New York, 1997;
Vol. 2. (b) Hay, P. J .; Wadt, R. W. J . Chem. Phys. 1985, 82, 270. (c)
Hay, P. J .; Wadt, R. W. J . Chem. Phys. 1985, 82, 284. (d) Hay, P. J .;
Wadt, R. W. J . Chem. Phys. 1985, 82, 299.
(29) Available from http://www.emsl.pnl.gov:2080/forms/basisform.
html.
(26) Available from http://www.cygwin.com.
(27) (a) Stevens, W. J .; Basch, H.; Krauss, M. J . Chem. Phys. 1984,
81, 6026. (b) Stevens, W. J .; Krauss, M.; Basch, H.; J asien, P. G. Can.
J . Chem. 1992, 70, 612. (c) Cundari, T. R.; Stevens, W. J . J . Chem.
Phys. 1993, 98, 5555.
(30) SAINT-NT, Program for Data Collection and Data Reduction;
Bruker-AXS, Madison, WI, 1998.
(31) Sheldrick, G. M. SHELXTL Version 5.0, A System for Structure
Solution and Refinement; Bruker-AXS, Madison, WI, 1998.