Bis(phosphane oxide) adducts of Rh2(MTPA)4 - Kinetics and chirality discrimination

Article abstract of DOI:10.1002/ejic.200300853

The symmetrical bis(phosphane oxide) 1 and its chiral derivatives 2 and 3 form a variety of adduct species with the dirhodium complex [Rh-Rh] which can be identified by low-temperature ¹H and ³¹P NMR spectroscopy by using varying [Rh

Full text of DOI:10.1002/ejic.200300853

FULL PAPER
Bis(phosphane oxide) Adducts of Rh2(MTPA)4 — Kinetics and Chirality
Discrimination
[a]
[a]
[a]
[b]
[b]
´
´
´
´
´
Tamas Gati, Andras Simon, Gabor Toth,* Anna Szmigielska, Anna M. Maj,
K. Michał Pietrusiewicz,[b,c] Stefan Moeller,[d] Damian Magiera,[d] and Helmut Duddeck*[d]
Dedicated to the memory of Prof. Dr. Dr.h.c.(H) Günther Snatzke (1928Ϫ1992)[‡]
Keywords: Rhodium / Phosphane ligands / NMR spectroscopy / Complexation modes / Chiral discrimination
The symmetrical bis(phosphane oxide) 1 and its chiral deriv-
atives 2 and 3 form a variety of adduct species with the dirho-
dium complex [Rh−Rh] which can be identified by low-tem-
perature 1H and 31P NMR spectroscopy by using varying
[Rh−Rh]:ligand ratios. The asymmetric bis(phosphane oxide)
3 shows a distinct preference for binding through the Ph2P=
O (Pa) as compared to the tBu(Ph)P=O (Pb) functionality due
ides) 2 and 3 form complex mixtures of various adduct spe-
cies. Nevertheless, it is possible to monitor enantiomeric
compositions from various NMR signal duplications at 213 K
and even at room temperature or slightly elevated temper-
atures.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
to the bulky tert-butyl group. The chiral bis(phosphane ox- Germany, 2004)
Introduction
NMR signals even at room temperature and that phos-
phane oxides seemed to have a different ligand behaviour
from their sulfide and selenide analogues.[4c] Moreover,
when we attempted to study some chiral bis(phosphane ox-
ides) (Scheme 1) by variable-temperature NMR spec-
troscopy we found signals of complicated mixtures of free
and ligated species.
Dirhodium complexes as well as their phosphane adducts
have been the focus of interest for many years.[1] They were
first introduced as homogeneous catalysts in various reac-
tions[2] and have even found medicinal applications.[3] We
have shown that the enantiomers of chiral phosphane chal-
cogenides can easily be discriminated by adding an equimo-
lar amount of the dirhodium complex [RhII2{(R)-(ϩ)-
MTPA}4] ([Rh؊Rh], MTPA-H ϭ methoxytrifluoromethyl-
phenylacetic acid ϵ Mosher’s acid; see Scheme 1) to their
CDCl3 solution and monitoring the diastereomeric disper-
sion ∆ν of their H or 31P NMR signals at room tempera-
1
ture (‘‘dirhodium method’’ of chirality recognition).[4] How-
ever, we noticed that coalescence effects may broaden the
[a]
Institute for General and Analytical Chemistry of the Budapest
University of Technology and Economics,
´
´
Szent Gellert ter 4, 1111 Budapest, Hungary
Fax: (internat.) ϩ36-1-463-3408
E-Mail: g-toth@tki.aak.bme.hu
[b]
Maria Curie-Sklodowska University, Department of Organic
Chemistry,
Scheme 1. Structures of the dirhodium complex and the phos-
phanyl chalcogenides studied
Lublin, Poland
Fax: (internat.) ϩ48-81-524-2251
E-Mail: michal@hermes.umcs.lublin.pl
Polish Academy of Sciences, Institute of Organic Chemistry,
ul. Kasprzaka 44/52, 01-224 Warsaw, Poland
Fax: (internat.) ϩ48-22-632 6681
[c]
[d]
[‡]
Recently, Deubel has communicated a theoretical study
based on density-functional theory (DFT) calculations of
donor-acceptor interactions contributing to the binding en-
ergy in the adducts of dirhodium() tetraformate and a rep-
resentative choice of strong and weak ligands (e.g., methyl-
ene carbene vs. benzene).[5] As a result, attractive contri-
E-Mail: kmp@icho.edu.pl
Hannover University, Institute of Organic Chemistry,
Schneiderberg 1B, 30167 Hannover, Germany
Fax: (internat.) ϩ49-511-762-4616
E-mail: duddeck@mbox.oci.uni-hannover.de
On the occasion of what would have been his 75th birthday
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200300853
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Bis(phosphane oxide) Adducts of Rh2(MTPA)4
FULL PAPER
butions were shown to be a blend of electrostatic and donor); no free ligand molecules can be detected in the
HOMOϪLUMO interactions, the former always dominat- presence of an excess of rhodium sites in the latter case, for
ing.[5] In a comparative study of the ligand properties of the example in an equimolar mixture of [Rh؊Rh] and
triphenylphosphane chalcogenides Ph3PϭX (X ϭ Se, S, and Ph2(CH3)PϭO.[6] Since bis(phosphane oxides) are con-
O), and of methyldiphenylphosphane oxide, Ph2(CH3)Pϭ sidered to be useful ligands in homogeneous catalysis, we
O, we have recently found that, in contrast to phosphane extended this work to the bis(phosphane oxides) 1Ϫ3 in
selenides and sulfides, phosphane oxides bind primarily by order to explore how bifunctional phosphane oxide ligands
an electrostatic attraction, and orbital interactions can be behave under ‘‘dirhodium method’’ experimental con-
neglected.[6] Moreover, this binding energy is strongly sensi- ditions and whether or not chirality recognition is still pos-
tive to steric crowding around the PϭO group.[6] For Ph3Pϭ sible. In addition, we wanted to see whether there is a bind-
O, the binding constant K is only about 1 (weak donor). In ing competition between the two PϭO groups in each of
contrast, K is larger than 100 for Ph2(CH3)PϭO (strong the asymmetric compounds 2 and 3.
Table 1. 1H, 13C and 31P NMR chemical shifts as well as 31P,1H and 31P,13C coupling constants (J in Hz) of the free ligands 1Ϫ3, recorded
at room temperature[a]
1H
13C
31P
nJ(31P,1H)
nJ(31P,13C)
1
CH2
ipso
ortho
meta
para
CH3
2.52
Ϫ
7.70
7.45
7.51
1.20
21.6
33.8
n.d.[b]
Ϫ
131.9
130.7
128.7
132.0
13.6
n.d.
n.d.
2[c]
Pa: 32.5
Pb: 39.0
3J ϭ 16.9
4J ഠ 0
n.d.
2J ϭ 3.2
3J ϭ 0.9
1J ϭ 69.2
2J ϭ 1.7
ϾCH2: Ha
2.48
28.8
ϾCH2: Hb
CH
2.56
2.99
n.d.
n.d.
26.9
1J ϭ 70.5
2J ϭ 4.0
Pa
Pb
ipso
Ϫ
131.1/131.0
131.1/131.0
129.3/129.2
131.9/131.8
133.4/132.6
131.5/131.5
129.1/129.1
131.9/131.9
132.9
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Ϫ
1J ഠ 97/96
2J ϭ 9.6/9.6
3J ϭ 11.3/11.7
4J ϭ 2.8/2.8
1J ϭ 98.3/99.5
2J ϭ 8.6/8.9
3J ϭ 11.9/11.4
4J ϭ 3.1/3.1
1J ϭ 80.6
ortho
meta
para
ipso
ortho
meta
para
C-2
7.68Ϫ7.76
7.37Ϫ7.47
7.47Ϫ7.58
Ϫ
7.68Ϫ7.76
7.37Ϫ7.47
7.47Ϫ7.58
Ϫ
3[c]
Pa: 32.7
Pb: 42.9
2J ϭ 7.8
C(tBu)
Ϫ
3.40
34.0
31.0
Ϫ
1J ϭ 69.2
ϾCH2: Ha
2J ϭ 15.5
3J ϭ 8.5
1J ϭ 66.7
2J ϭ 7.5
ϾCH2: Hb
3.57
6.80
2J ϭ 12.5
3J ϭ 8.5
3J ϭ 39.8
4J ϭ 1.6
ϭCH2:[d] Ha
132.3
2J ഠ 7.5
3J ഠ 7.5
ϭCH2:[d] Hb
6.04
3J ϭ 17.2
4J ഠ 0
3J ϭ 14.9
Ϫ
3J ϭ 11.7/ 11.6
n.d./n.d.
n.d./n.d.
Ϫ
3J ϭ 9.5
n.d.
CH3
ipso
ortho
meta
para
ipso
ortho
meta
para
1.16
Ϫ
7.72/7.83
7.29/7.46
7.35/7.50
Ϫ
7.62
7.37
7.46
25.2
2J ഠ 0
Pa
Pb
132.6/131.8
131.0/130.9
128.3/128.6
131.5/131.8
129.2
131.9
128.1
1J ϭ 100.3/101.6
2J ϭ 8.3/8.0
3J ϭ 12.1/11.9
4J ϭ 2.7/2.9
1J ϭ 89.1
2J ϭ 8.2
3J ϭ 10.9
131.6
n.d.
4J ϭ 2.9
[a]
For both P atoms of 2 and for Pa of 3 two entries exist for each aromatic 1H and 13C nucleus because the phenyl groups are
diastereotopic. The first entry belongs to one phenyl group and the second entry to the other; a stereochemical differentiation is not
[b]
[c]
3
3
[d]
possible.
n.d.: not detectable. Compound 2: J(31Pa,31Pb) ϭ 49.4 Hz; compound 3: J(31Pa,31Pb) ϭ 18.2 Hz.
The protons of the
methylene group (ϭCH2) can be differentiated because of their J(31P,1H)-values to Pb: Ha is trans to Pb (3J ϭ 39.8 Hz), and Hb is cis
(3J ϭ 17.2 Hz).
3
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FULL PAPER
Results and Discussion
NMR Signal Assignment
Identification of the Various Adduct Species
1,2-Bis(diphenylphosphinoyl)ethane (1) contains two
chemically equivalent PϭO residues; each is connected to
two phenyl rings and to the ethylene chain. Thus, one can
expect that its complexing properties are similar to those of
triphenylphosphane oxide and methyldiphenylphosphane
oxide.[6]
The assignment of the NMR signals of the free ligands
1Ϫ3 (cf. Scheme 1) was straightforward in most cases when
routine NMR methods such as DEPT, HMQC, HMBC and
NOE techniques were applied. Thereby, all 1H, 13C and 31
P
The low-temperature (213 K) 31P NMR spectra of 1 at
various molar ratios (Figure 1) allow the identification of
the adduct species involved (Scheme 2). Spectrum a (0.5:1)
was recorded for a solution with an excess of rhodium sites,
spectrum b (1:1) for a solution with equimolar amounts of
both [Rh؊Rh] and 1, and spectra c and d with an excess of
PϭO sites. The ratios were chosen in such a way that the
molar amount of 1 relative to that of [Rh؊Rh] is doubled
when going from one spectrum to the next (a to b, b to c,
and c to d) so that the PϭO content increases strongly from
bottom to top.
NMR signals could be identified; the unequivocal assign-
ment of the 31P signals of the two heterotopic phosphorus
1
sites in 2 and 3 was achieved by {31P} H-detected HMBC
experiments correlating Pb with the methyl or the tert-butyl
protons, respectively. In the case of [Rh؊Rh] adducts the
identification of the ligand signals was occasionally ham-
pered by overlapping Mosher acid signals, particularly
when aromatic signals were involved. All NMR signals of
the free ligands 1Ϫ3, as well their complexation shifts ∆δ
(in ppm), for 1:1 adducts at room temperature are collected
in Table 1 and 2.
Table 2. Complexation shifts in 1Ϫ3 in 1:1 adducts (in ppm); re-
corded at room temperature[a]
∆δ(1H)
∆δ(13C)
∆δ(31P)
ϩ3.9
[b]
1
2
CH2
ipso
ortho
meta
para
CH3
ϩ0.29
Ϫ
ϩ0.13
Ϫ0.20
Ϫ0.10
ϩ0.26
ϩ0.29
ϩ0.32
ϩ0.45
Ϫ0.5/Ϫ0.6[c]
ϩ0.2/ϩ0.1[c]
ϾCH2: Ha
ϾCH2: Hb
CH
ϩ0.1/Ϫ0.1[c]
n.d./n.d.
Ϫ0.1/ϩ0.1
Ϫ0.8/Ϫ0.7
n.d./Ϫ0.3
n.d./n.d.
Ϫ0.5/Ϫ0.5
Ϫ0.6/Ϫ0.6
n.d./Ϫ0.2
0.0
Pa
Pb
ipso
ortho
meta
para
ipso
ortho
meta
para
Ϫ/Ϫ
ϩ5.6
ϩ4.6
ϩ0.17/ϩ0.25
Ϫ0.28/Ϫ0.24
n.d./Ϫ0.14
Ϫ/Ϫ
Ϫ0.04 /ϩ0.08
Ϫ0.26/Ϫ0.31
n.d./Ϫ0.17
Ϫ
Ϫ
ϩ0.24
ϩ0.46
ϩ0.40
Ϫ0.10
ϩ0.03
Ϫ/Ϫ
ϩ0.16/ϩ0.08
Ϫ0.30/Ϫ0.32
Ϫ0.17/Ϫ0.20
Ϫ
Figure 1. 31P NMR spectra of 1 and [Rh؊Rh], recorded at 213 K;
molar ratios are (a) 0.5:1, (b) 1:1, (c) 2:1 and (d) 4:1
3
C-2
C(tBu)
ϾCH2: Ha
ϾCH2: Hb
ϭCH2: Ha
ϭCH2: Hb
CH3
ipso
ortho
meta
para
ϩ0.4/ϩ0.3[c]
Ϫ0.5
ϩ2.1
0.0/Ϫ0.1[c]
n.d./n.d.
ϩ0.5/ϩ0.4
Ϫ0.2/Ϫ0.7
Ϫ0.4/Ϫ0.2
n.d.
Pa
Pb
ϩ4.4
ϩ3.1
Scheme 2. Equilibria between free components and the adducts[a]
ipso
[a]
‘‘PϭO’’ symbolizes phosphane oxide groups
ortho
ϩ0.11
ϩ0.4
meta
para
Ϫ0.18
Ϫ0.09
ϩ0.3
Ϫ0.2
By comparing the 31P NMR signals and their relative
intensities in Figure 1, the following assignments can be de-
duced. There are two different signal groupings: one at δ ϭ
39.5Ϫ41.5 ppm for ligated PϭO sites (Rh···OϭP) and an-
other at δ ϭ 36Ϫ37 ppm for free PϭO sites. The free ligand
of 1 corresponds to the signal at δ ϭ 36.1 ppm (IV;
Scheme 3) as proven by its strong intensity increase upon
further addition of 1 (spectra b, c and d). The signal at δ ϭ
[a]
For both P atoms of 2 and for Pa of 3 two entries exist for
each aromatic H and 13C nucleus because the phenyl groups are
diastereotopic. The first entry belongs to one phenyl group and the
second entry to the other; a stereochemical differentiation is not
possible. [b] No 13C NMR spectroscopic data were recorded for the
[Rh؊Rh]-1 mixture because precipitation of solid material from the
solution occurred after a few hours.
dispersion effects (see text).
1
[c]
Signal duplication due to
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Bis(phosphane oxide) Adducts of Rh2(MTPA)4
FULL PAPER
41.2 ppm (singlet) belongs to a doubly complexed ligand
molecule which is in a symmetrical environment and there-
fore does not show the typical doublet splitting due to the
31P,31P coupling in unsymmetric bis(phosphane oxides) or
analogous adducts. Thus, this species is of type
I
(Scheme 3). The intensity of this peak is rather large in
spectrum a (Rh site excess), low in spectrum b (1:1), and
the signal vanishes totally in spectra c and d with their in-
creasing PϭO site excess.
1
Figure 2. Sections of the H NMR spectra of 1 and [Rh؊Rh], re-
corded at 213 K; molar ratios (a) 0.5:1, (b) 1:1, (c) 2:1 and (d) 4:1
Scheme 3. Adduct species of 1 and [Rh؊Rh] identified by low-
temperature 31P NMR spectroscopy (see text and Figure 1); the
Mosher acid residues in [Rh؊Rh] and the phenyls at the phos-
phoryl groups have been omitted for clarity
at room temperature (δ ϭ 2.77 ppm) but splits into two
(δ ϭ 2.93 and 2.52 ppm) at 213 K. The former part (δ ϭ
The two doublets at δ ϭ 40.7 and 36.5 ppm correspond 2.93 ppm) belongs to Rh···OϭPϪCH2 situations and the lat-
to an unsymmetrically ligated molecule of type II ter one (δ ϭ 2.52 ppm) to free OϭPϪCH2 groups (signal D);
(Scheme 3). The former doublet (signal IIa) is from the the relative proportion of signal D is strongly increased
complexed end of 1 (PaϭO) and the latter (signal IIb) from upon addition of larger amounts of 1 when going from
the free one (PbϭO). The signal splitting is the vicinal spectrum a to spectrum d in Figure 2. The exchange barrier
3J31P,31P coupling constant (53.5 Hz). Finally, there is a broad of the adduct formation can be estimated easily from the
hump at δ ϭ 39.7Ϫ40.5 ppm corresponding to Rh···OϭP temperature dependence of the CH2 and the methoxy pro-
situations of type III; apparently, these are the 31P NMR tons (∆G‡c ϭ 51.3 Ϯ 1 kJ/mol). This value is somewhat
resonances of doubly complexed ligand molecules incorpor- higher than those of the monophosphane chalcogenides
ated in oligomeric structures (Scheme 3).
This interpretation is supported by the H NMR signals
(∆G‡c ϭ 46.4Ϫ48.0 Ϯ 1 kJ/mol).[6]
All these spectra lead to the conclusion that the phos-
1
of the methoxy group of the Mosher acid residues of phane oxide group of 1 is only a weak donor; even under
[Rh؊Rh] at low-temperature, as displayed in Figure 2, Rh site excess there is a substantial proportion of free Pϭ
taken from the same samples as used for Figure 1. Signal A O sites (PbϭO in II and IV; Scheme 3). Thus, the steric
(δ ϭ 3.14 ppm) representing the free [Rh؊Rh] complexes congestion in 1 must be higher than in Me(Ph2)PϭO and
without any ligating 1 has an intensity of about 40% in the more similar to that of Ph3PϭO.[6] The majority of the mol-
solution at 0.5:1 molar ratio, falling to about 25% at PϭO ecules of 1 form adducts II and, eventually, oligomers III
site excess. The majority of dirhodium moieties (ca. 50%), with their composition depending on the PϭO site excess:
however, are in the 1:1 adduct situation represented by sig- oligomers become more and more dominant the higher the
nal B (δ ϭ 3.03 ppm). There is hardly any change in the PϭO content becomes.
proportion of this adduct with increasing amounts of 1. Fi-
In contrast to 1, 1,2-bis(diphenylphosphinoyl)propane (2)
nally, signal C (δ ϭ 2.93 ppm) can be attributed primarily is a C-chiral compound derived from 1 by formal meth-
to a Mosher methoxy signal in the 2:1 adduct situation; its ylation of one methylene carbon (cf. Scheme 1). Here, the
proportion varies from about 10% at a 0.5:1 ratio to about two Ph2PϭO groups are heterotopic, and separate NMR
20% at a 4:1 ratio. This proportion of the 2:1 adducts is signals are observable for each of them. In order to dis-
surprisingly low, especially with high PϭO site excess, and tinguish the two phosphoryl groups we use the superscript
is a consequence of the low donor properties of 1 and the ‘‘a’’ for that Ph2PϭO group which is attached to CH2 (C-
severe steric congestion around the PϭO group, similar to 1) and thereby structurally closest to 1. The same procedure
Ph3PϭO.[6] Additionally, this small amount of 2:1 adducts was applied to differentiate the two PϭO groups in 3 (see
may be associated with the observation that some insoluble below). The method of assigning the two phosphorus atoms
material precipitated from the CDCl3 solution after several to the two 31P signals has already been described above.
hours. We suspect that this material is a copolymer of type
III (Scheme 3).
The room-temperature 31P NMR signals of 2 in the pres-
ence of [Rh؊Rh] are broad humps showing averaged peaks.
It should be noted that the temperature dependence of As can be seen in Figure 3, however, the low-temperature
1
the H NMR spectrum proves that a small part of signal C spectra (213 K) display a number of signals indicating the
in spectrum a belongs to the CH2 signal, which is averaged existence of several species, the concentrations of which de-
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G. Toth, H. Duddeck et al.
FULL PAPER
pend on the molar ratio of the components 2 and [Rh؊Rh]. as PbϭO, of bis(phosphane oxide) 2 are weak donors, and
The ratio producing the lower spectrum was 2:1 — it was that there is no significant selection between the two com-
recorded under PϭO site excess — whereas that in the top peting PϭO groups.
spectrum was 0.5:1 — Rh site excess. It should be noted
that further spectra were taken from samples with 1:1 and (213 K) H NMR spectrum in the ligand methyl group re-
This interpretation is supported by the low-temperature
1
4:1 molar ratios. These are not displayed in Figure 3 but gion recorded under 2:[Rh؊Rh] ϭ 0.5:1 molar ratio con-
support the following conclusions.
ditions (Rh site excess; Figure 4). It shows signals of the
various species whereas at room temperature the averaged
3
signal is a doublet of doublets due to the J1H,1H (7.0 Hz)
3
31P,1H
and J
(16.8 Hz) couplings (not depicted). The assign-
ment shown was proven unequivocally by a {31P} H-de-
tected HMBC spectrum (not depicted) correlating the
methyl protons with the Pb atoms in each species. The inte-
gration shows that only about 30% of all ligand molecules
are complexed to rhodium on both sides (᭛); the content
of free ligand molecules (∆) cannot be derived safely due to
signal overlap but there is no doubt that a considerable
amount of it exists. Moreover, the amount of the species
with only one complexed phosphoryl (᭺ for Rh···OPaϳPbO
and ᮀ for OPaϳPbO···Rh) is roughly similar so that the
complexation constants of both phosphoryl groups must be
similar as well; the methyl group has, indeed, no significant
directing effect for an adduct formation preference of Pa vs.
Pb (or vice versa).
1
Figure 3. 31P NMR spectra of 2 in the presence of various amounts
of [Rh؊Rh], recorded at 213 K; top, molar ratio 2:[Rh؊Rh] 0.5:1,
bottom 2:1; see Scheme 4 for the meaning of the symbols
The different ratio conditions enabled us to assign the
peaks. Moreover, we recorded a 31P,31P COSY spectrum
(not depicted) correlating the two heterotopic 31P nuclei
within each species. For sake of clarity we used the follow-
ing icons for the 31P signal assignment (Scheme 4): ᭡/∆ for
the free ligand OPaϳPbO, ᭹/᭺ for the adduct
Rh···OPaϳPbO, ᭿/ᮀ for the adduct OPaϳPbO···Rh and
᭜/᭛ for the adduct Rh···OPaϳPbO···Rh; filled icons refer
to Pa and open ones to Pb (see also Scheme 4; ‘‘ϳ’’ depicts
the carbon chain between the phosphorus atoms).
Figure 4. Methyl 1H NMR signal of 2 in the presence of two molar
equivalents of [Rh؊Rh], recorded at 213 K
Scheme 4. Assignment of the symbols to the phosphorus atoms in
the Rh*-ligand species; filled icons refer to Pa and open ones to Pb,
‘‘ϳ’’ represents the carbon chain between the phosphorus atoms
Three factors make the 1H NMR signals appear as broad
multiplets instead of doublets of doublets: (a) the existence
It is obvious that the two doublet signals of the free li- of different species regarding the 1:1 and 2:1 adduct situ-
gand 2 (᭡/∆) are the largest ones in the bottom spectrum ations of the RhϪRh fragments, (b) high-order spin sys-
due to an excess of 2. On the other hand, four small signals tems (ABX) because the methylene protons are dia-
appear in the top spectrum that are not visible in the bot- stereotopic, and (c) diastereomeric dispersion effects (com-
tom spectrum; they were assigned to ЈЈsymmetricalЈЈ adduct pare the two signals marked by ‘‘ᮀ’’).
species (᭜/᭛). The fact that each of them appears twice can
Whereas the bis(phosphane oxide) 2 has a chiral carbon
be attributed to the existence of two diastereomeric adducts center, 2-(tert-butylphenylphosphinoyl)-3-(diphenylphos-
(R)-[Rh؊Rh]···[(R)-OPaϳPbO]···(R)-[Rh؊Rh] and (R)- phinoyl)propene (3) is P-chiral (Pb). In analogy to 2, the
[Rh؊Rh]···[(S)-OPaϳPbO]···(R)-[Rh؊Rh], an example of two 31P NMR signals of 3 were unambiguously assigned by
1
chirality recognition (2 exists as a racemate). The signals of a {31P} H-detected HMBC experiment at room tempera-
the ‘‘unsymmetrical’’ species (᭹/᭺ and ᭿/ᮀ) are visible in ture. Figure 5 shows the 31P NMR spectra of 3 in the pres-
both spectra. Taking the relative intensities (᭹/᭺ vs. ᭿/ᮀ) ence of three different molar equivalents of [Rh؊Rh], re-
into account and the fact that the signals of the free ligand corded at 213 K. The assignment of the various species was
2 (᭡/∆) still exist under excess Rh site conditions, one can performed in analogy to those of 2 and is given in Figure 5
state that, like in 1, both phosphoryl groups, PaϭO as well by icons similar to those used for 2 in Scheme 4. The signals
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Bis(phosphane oxide) Adducts of Rh2(MTPA)4
FULL PAPER
of Pa appear at δ ϭ 35Ϫ39 ppm (filled icons) and those of
Pb at δ ϭ 44Ϫ49.5 ppm (open icons).
Figure 5. 31P NMR spectrum of 3 in the presence of various
amounts of [Rh؊Rh], recorded at 213 K; top: 0.5:1 molar ratio
(3:[Rh؊Rh]), middle: 1:1, bottom: 2:1; the icons used for the as-
signment of the various species of 3 have the same meaning as for
2 (see Scheme 4)
1
Figure 6. H NMR signals of the tert-butyl group of 3, recorded
at 213 K; top: 1:2 molar ratio (3:[Rh؊Rh]), middle: 1:1, bottom:
2:1; the icons used for the assignment of the various species of 3
have the same meaning as for 2 (see Scheme 4)
The basic difference in the complexing behaviour of the
bis(phosphane oxides) 2 and 3 lies in the relative concen-
trations of the monoligated species Rh···OPaϳPbO (᭹/᭺)
and OPaϳPbO···Rh (᭿/ᮀ). 31P NMR signal integrations
show that the ratio of the former relative to the latter is
about 1.4 (3:[Rh؊Rh] ϭ 2:1, PϭO site excess; Figure 5, bot-
tom), about 1.7 (1:1; middle) and about 2.5 (0.5:1, Rh site
excess; Figure 5, top). This clearly reflects the steric effects
during complexation and reveals that the tert-butyl group
at Pb creates a larger steric congestion during adduct forma-
tion and thereby repels approaching [Rh؊Rh] molecules
much more than the second phenyl group at Pa. Figure 6
shows the 1H NMR signals of the tert-butyl group attached
to Pb; some of these signals are multiplets for the same re-
asons as those described for the methyl protons of 2 (see
above). The signal group at δ ഠ 1 ppm appears to be two
overlapping doublets of the Rh···OPaϳPbO species, appar-
ently another case of chirality recognition (see below).
The difference in adduct formation energies of the two
PϭO groups is also evident when comparing the intensities
Moreover, the carbon signals of the tert-butyl group (re-
corded at 300 K) are duplicated as well: ∆ν(CH3) ϭ 2.5 Hz
and ∆ν[C(tBu)]
ϭ 6.5 Hz. Interestingly, these dia-
stereomeric dispersions occur preferably at the phosphane
oxide group of 3 (Pb) that is clearly the weaker donor and,
time-averaged, is further away from the Rh sites. Appar-
ently, the branched structure of 3 guarantees that free Pbϭ
O moieties cannot easily escape such interactions so that
they come into closer spatial contact with the Mosher
acid residues.
1
of the tert-butyl H NMR signal (᭺ vs. ᮀ; Figure 6).
Chirality Recognition
Dispersion effects, ∆ν, due to the existence of dia-
stereomeric adducts have been identified in some low-tem-
1
perature H NMR spectra (see previous section). They are
1
Figure 7. H NMR signal of the tert-butyl group of 3 in the pres-
difficult to detect at room temperature because most NMR
signals are severely broadened by coalescence effects. How-
ever, some atoms in the aliphatic part of the two chiral bis-
(phosphane oxides) 2 and 3 offer the possibility of enanti-
omeric discrimination. In the case of the racemic 2, the 13C
NMR signals are split: ∆ν(CH3) ϭ 11.1 Hz, ∆ν(CH2) ϭ
18.2 Hz and ∆ν(CH) ϭ 9.9 Hz. The time-averaged tert-bu-
ence of an equimolar amount of [Rh؊Rh], recorded at 330 K, reso-
lution enhanced; ∆ν ϭ 4.9 Hz and J
3
31
1
ϭ 15.1 Hz
P, H)
Conclusion
The bidentate 1,2-bis(diphenylphosphinoyl)ethane de-
tyl signal in the 1H NMR spectrum of 3 (ratio 3:[Rh؊Rh] ϭ rivatives 1Ϫ3 are weak donors in adducts to the dirhodium
2:1) recorded at elevated temperature (330 K; Figure 7) dis- complex [Rh؊Rh]. Consequently, all conceivable species ex-
plays a signal dispersion of 4.9 Hz (recorded at 11.74 T). ist: free ligands, 1:1 adducts, 2:1 adducts and various iso-
Eur. J. Inorg. Chem. 2004, 2160Ϫ2166
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2165

´
G. Toth, H. Duddeck et al.
FULL PAPER
point in the H, 0.24 Hz/point in the 13C, and 0.22 Hz/point in the
31P NMR spectra. One drop of [D6]acetone was added to each
NMR sample before measurement in order to increase the solu-
bility of [Rh؊Rh].[10]
1
mers and oligomers thereof. Their relative concentrations
strongly depend on the molar ratio of [Rh؊Rh] and the
ligands so that an identification by low-temperature 31P
NMR spectroscopy is possible. Enantiomers can easily be
resolved in the case of chiral bidentate ligands 2 and 3.
Variable-temperature 1H (500.1 MHz) and 31P{1H} NMR
(202.4 MHz) spectra were recorded in the presence of [Rh؊Rh] on
a Bruker Avance DRX-500 spectrometer (11.74 T). Temperatures
varied from 213 to 330 K (333 K in the case of 1) and were read
from the instrument panel; no further measurements for more pre-
cise temperature determinations were taken.
Experimental Section
Substances: The synthesis of [Rh؊Rh] has been communicated by
us previously.[7] 1,2-Bis(diphenylphosphinoyl)ethane (1)[8] was pre-
pared from the respective commercially available bisphosphane by
oxidation with H2O2 in water.
{31P} 1H-detected HMBC experiments were recorded at 213 K;
relaxation delay 1.5 s, number of scans 32, delay optimized for
long-range couplings (8 Hz); spectral width δ ϭ 30Ϫ50 (31P) and
δ ϭ 0Ϫ9 (1H). Gradient-edited 31P,31P COSY spectra were re-
corded at 213 K; relaxation delay 4.0 s, number of scans 32, spec-
tral width δ ϭ 31Ϫ52 (31P).
1,2-Bis(diphenylphosphinoyl)propane (2): Diphenylphosphane oxide
(1.19 g, 5.9 mmol) and diphenyl(propadienyl)phosphane oxide[9]
(1.43 g, 5.9 mmol) were dissolved in toluene (25 mL), and the re-
sulting solution was heated to reflux for 3 days. Evaporation of the
solvent and recrystallization of the solid residue from toluene/di-
ethyl ether (10:1) gave 1.84 g of 2,3-bis(diphenylphosphinoyl)prop-
ene as white crystals. Yield 70%; m.p. 132Ϫ133 °C. IR (KBr): ν˜ ϭ
Acknowledgments
This work was performed within the project ‘‘Biologically Active
Natural Products: Synthetic Diversity’’ (Department of Chemistry,
Hannover University) and was supported by the Deutsche For-
schungsgemeinschaft (Projects Du 98/22 436 UNG 113/148 and
436 POL 113/83), by the Hungarian Academy of Sciences (Project
MTA/DFG 2002Ϫ2004) and the Hungarian National Research
1482, 1437, 1397, 1261, 1199 cmϪ1 1H NMR (CDCl3): δ ϭ 3.39
.
(br. d, J ϭ 12.7 Hz, 1 H), 3.46 (br. d, J ϭ 12.2 Hz, 1 H), 5.49 (dd,
J ϭ 20.5, 2.5 Hz, 1 H), 6.64 (dd, J ϭ 41.6, 1.7 Hz), 7.21Ϫ7.89 (m,
20 H) ppm. 31P NMR (CDCl3): δ ϭ 30.5 (d, J ϭ 24 Hz), 32.6 (d,
J ϭ 24 Hz) ppm. C27H24O2P2 (442.4) calcd. C 72.96, H 5.84; found
C 72.51, H 5.96.
´ ´
Foundation (OTKA No. T032180). A. S. thanks for Bekesy and
Varga/Rohr fellowships.
Pd/C (20%; 0.096 g) was added to a solution of 2,3-bis(diphenyl-
phosphinoyl)propene (0.2 g, 0.45 mmol) in 10 mL of methanol, and
the resulting mixture was stirred at room temperature under a hy-
drogen atmosphere (balloon) for 2 days. The catalyst was then re-
moved by filtering the reaction mixture through a thin bed of celite,
and the methanol was evaporated to dryness. Recrystallization of
the solid residue from toluene/hexane gave 0.175 g of 2. Yield 87%;
m.p. 128Ϫ131 °C. EI-HRMS: C27H26O2P2 [Mϩ] calcd. 444.1408;
found 444.1399. See Table 2 for 1H, 13C and 31P NMR spectro-
scopic data.
[1] [1a]
E. B. Boyar, S. D. Robinson, Coord. Chem. Rev. 1983, 50,
[1b]
109Ϫ208.
Multiple Bonds between Metal Atoms, 2nd ed.
(Eds.: F. A. Cotton, R. A. Walton), 1993, Clarendon, Oxford.
[2] [2a]
C. Mertis, M. Kravaritoy, M. Chorianopoulou, S. Koinis,
N. Psaroudakis, Top. Mol. Org. Engineer. 1994, 11, 321Ϫ329.
[2b] Modern Catalytic Methods for Organic Synthesis with Diazo
Compounds: From Cyclopropanes to Ylides (Eds.: M. P. Doyle,
M. A. McKervey, T. Ye), 1998, John Wiley & Sons, New York.
[2c]A. Endres, G. Maas, Tetrahedron 2002, 58, 3999Ϫ4005; and
references cited therein.
2-(tert-Butylphenylphosphinoyl)-3-(diphenylphosphinoyl)propene (3):
tert-Butylphenylphosphane oxide (0.38 g, 2.1 mmol) was added to
a solution of diphenyl(propadienyl)phosphane oxide[9] (0.51 g,
2.1 mmol) in 15 mL of toluene and the resulting solution was
heated to 110 °C for 2 days. Evaporation of the solvent and recrys-
tallization of the solid residue from diethyl ether/hexane (20:1) gave
[3]
M. J. Clarke, F. Zhu, D. R. Frasca, Chem. Rev. 1999, 99,
2511Ϫ2533.
[4] [4a]
S. Rockitt, H. Duddeck, J. Omelanczuk, Chirality 2001, 13,
[4b]
214Ϫ223.
S. Malik, H. Duddeck, J. Omelanczuk, M. I.
[4c]
Choudhary, Chirality 2002, 14, 407Ϫ411.
D. Magiera, S.
Moeller, Z. Drzazga, Z. Pakulski, K. M. Pietrusiewicz, H.
Duddeck, Chirality 2003, 15, 391Ϫ399.
˜
0.51 g of 3. Yield 57%; m.p. 84Ϫ86 °C. IR (KBr): ν ϭ 1484, 1474,
[5]
[6]
1462, 1439, 1277, 1258, 1214, 1184 cmϪ1. C25H28O2P2 (422.4)
D. V. Deubel, Organometallics 2002, 21, 4303Ϫ4305.
´
´
T. Gati, A. Simon, G. Toth, D. Magiera, S. Moeller, H. Dud-
deck, Magn. Reson. Chem., in print.
K. Wypchlo, H. Duddeck, Tetrahedron: Asymmetry 1994, 5,
27Ϫ30.
1
calcd. C 71.08, H 6.68; found C 70.71, H 6.80. See Table 2 for H,
13C and 31P NMR spectroscopic data.
[7]
[8]
1
NMR Spectroscopy: Room-temperature H (400.1 MHz), 13C{1H}
M. Quenard, V. Bonmarin, G. Gelbard, L. Krumenacker, New
J. Chem. 1989, 13, 183Ϫ91.
A. P. Boiselle, N. A. Meinhardt, J. Org. Chem. 1962, 27,
1828Ϫ1833.
(100.6 MHz), and 31P{1H} (161.9 MHz), NMR measurements
were performed on a Bruker Avance DPX-400 spectrometer
(9.4 T). Chemical shift standards were internal tetramethylsilane
[9]
1
(δ ϭ 0 ppm) for H and 13C, and external aqueous H3PO4 for 31P
[10]
C. Meyer, H. Duddeck, Magn. Reson. Chem. 2000, 38, 29Ϫ32.
(δ ϭ 0 ppm). Signal assignments were assisted by NOE-difference,
COSY, HMQC and HMBC (standard Bruker software) as well as
by inspecting couplings to 31P. Digital resolutions were 0.14 Hz/
Received November 19, 2003
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Eur. J. Inorg. Chem. 2004, 2160Ϫ2166