Synthesis, Characterization, and Application of Platinum(II) Complexes Incorporating Racemic and Optically Active 4-Chloro-5-Methyl-1-Phenyl-1,2,3,6-Tetrahydrophosphinine Ligand

Article abstract of DOI:10.1002/hc.21305

An efficient resolution method was elaborated for the preparation of (+)-4-chloro-5-methyl-1-phenyl-1,2,3,6-tetrahydrophosphinine oxide using the acidic Ca²⁺ salt of (-)-O,O-di-p-toluoyl-(2R,3R)-tartaric acid. Crystal structure of the diastereo

Full text of DOI:10.1002/hc.21305

Heteroatom Chemistry
Volume 27, Number 2, 2016
Synthesis, Characterization, and Application
of Platinum(II) Complexes Incorporating
Racemic and Optically Active 4-Chloro-5-
Methyl-1-Phenyl-1,2,3,6-Tetra-
hydrophosphinine Ligand
Pe´ter Bagi,1 Konstantin Karaghiosoff,2 Ma´tya´s Czugler,1
Do´ra Hessz,3,4 Miha´ly Ka´llay,5,3 Miklo´s Kubinyi,3,4 Tibor Szilva´si,6
Pe´ter Pongra´cz,7,8 La´szlo´ Kolla´r,7,8 Istva´n Tima´ri,9 Katalin E. Ko¨ve´r,9
La´szlo´ Drahos,4 Eleme´r Fogassy,1 and Gyo¨rgy Keglevich1
1Department of Organic Chemistry and Technology, Budapest University of Technology and
Economics, Budapest, 1521, Hungary
2Department Chemie und Biochemie, Ludwig-Maximilians-Universita¨t Mu¨ nchen, Mu¨ nchen,
81377, Germany
3Department of Physical Chemistry and Material Science, Budapest University of Technology and
Economics, Budapest, 1521, Hungary
4Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, 1525, Hungary
5MTA-BME Lendu¨ let Quantum Chemistry Research Group
6Department of Inorganic and Analytical Chemistry, Budapest University of Technology and
Economics, Budapest, 1521, Hungary
7Department of Inorganic Chemistry, University of Pe´cs and Szenta´gothai Research Center, Pe´cs,
7624, Hungary
8MTA-PTE Research Group for Selective Chemical Syntheses, 7624 Pe´cs, Hungary
9Department of Inorganic and Analytical Chemistry, University of Debrecen, 4032 Debrecen,
Hungary
Received 23 July 2015; revised 27 October 2015; accepted 10 November 2015
P-configuration was also determined by a circular
ABSTRACT: An efficient resolution method was
dichroism (CD) spectroscopic study including
elaborated for the preparation of (+)-4-chloro-5-
theoretical calculations. The tetrahydrophosphinine
methyl-1-phenyl-1,2,3,6-tetrahydrophosphinine oxide
oxide was then converted to the corresponding
using the acidic Ca2+ salt of (–)-O,O-di-p-toluoyl-
platinum complex whose stereostructure was investi-
(2R,3R)-tartaric acid. Crystal structure of the
gated by high-level quantum chemical calculations.
diastereomeric complex was evaluated by single
The Pt complex was tested as
in the hydroformylation of styrene.
a
ꢀC
catalyst
2016 Wi-
crystal X-ray analysis. Beside this, the absolute
ley Periodicals, Inc. Heteroatom Chem. 27:91–101,
2016; View this article online at wileyonlineli-
brary.com. DOI 10.1002/hc.21305
Correspondence to: Gyo¨rgy Keglevich, e-mail: gkeglevich@mail.
bme.hu.
2016 Wiley Periodicals, Inc.
ꢀC
91

92 Bagi et al.
INTRODUCTION
higher than those with five-membered P-rings
[17,25,27,28].
The phosphines are important, as their transition
metal complexes are widely applied catalysts in ho-
mogeneous catalytic reactions (e.g., hydrogenation
and hydroformylation) [1, 2]. Among the P(III)
Inspired by the results obtained with the opti-
cally active 3-phospholene platinum complexes, we
were interested in if the ring size influences the
catalytic activity and selectivity of the platinum
complexes. For this, we synthesized the optically
active platinum complex of 4-chloro-1-phenyl-5-
methyl-1,2,3,6-tetrahydrophosphinine to be tested
as catalyst in the hydroformylation of styrene.
ligands, the derivatives incorporating
a
P-
heterocyclic moiety are of great importance [3, 4].
Chiral phosphines also form an important class, as
they are potential ligands in asymmetric syntheses.
Most of such ligands contain C-chirality center(s),
and only a few of them possess a P-stereogenic
center [5,6].
RESULTS AND DISCUSSION
Enantioselective hydroformylation that can be
applied in the synthesis of anti-inflammatory drugs
(e.g., ibuprofen and naproxen) is a direct method
to obtain optically active derivatives [7, 8]. Among
the ligands used in asymmetric hydroformylations,
several P-chiral heterocyclic phosphines were
described that were employed mainly in rhodium
complexes [4, 9–14]. Optically active P-heterocyclic
ligands were rarely used in platinum-catalyzed
hydroformylations [13]. Gouygou and co-workers
synthesized a few platinum complexes incorporating
optically active bisphosphole ligands that were
tested in enantioselective hydroformylations [15].
Our research group synthesized a series of
optically active 1-aryl- and 1-alkyl-3-methyl-3-
phospholenes that were then converted to the
corresponding platinum complexes, which were
tested as catalysts in the hydroformylation of
styrene. Enantiomeric excess (Ee-s) up to 29%
were obtained [16–20]. The key step of the ligand
synthesis is the optical resolution of the correspond-
ing racemic phospholene oxides [21–24]. These
methods developed were then extended to the
preparation of optically active six-membered
Synthesis of Optically Active 4-Chloro-1-phenyl-
5-methyl-1,2,3,6-tetrahydrophosphinine
1-Oxide (1)
The racemic 4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine 1-oxide (1) was prepared
as described earlier [26]. The resolution of the
phenyl-tetrahydrophosphinine oxide (1) was carried
out applying the acidic Ca2+ salt of (−)-O,O’-di-
p-toluoyl-(2R,3R)-tartaric acid [Ca(H-DPTTA)2] in
a mixture of ethanol and water (Scheme 1) [26].
However, a better purification of the corresponding
diastereomeric
associates
was
elaborated
performing the digestion in ethanol. This
modification led to a more efficient resolution,
as the (+)-(R)-4-chloro-5-methyl-1-phenyl-1,2,3,6-
tetrahydrophosphinine 1-oxide [(R)-1] was obtained
in a yield of 60% and with an enantiomeric excess
(ee) of 98%, whereas in the previous study [26], the
yield and the ee were 28% and 99%, respectively.
Moreover, it became possible to characterize the
diastereomeric intermediate of the resolution and
identify the P-stereogenic center.
Single Crystal X-Ray Analysis of Diastereomeric
Complex Ca(1)2(H-DPTTA)2
phosphine oxides, including
a
3-phenyl-3-
1-phenyl-
phosphabicyclo[3.1.0]hexane oxide,
a
1,2-dihydrophosphinine oxide, and a 1-phenyl-
1,2,3,6-tetrahydrophosphinine oxide [25, 26].
A few platinum complexes incorporating racemic
P-heterocyclic ligands, such as 1,2,3,6-tetrahydro-
phosphinine and a 1,4-dihydrophosphinine deriva-
tive, were also prepared by us [16].
Our research group also synthesized platinum
complexes of bidentate P-ligands incorporating five-
and six-membered P-heterocycles with an exocyclic
P-function, such as 3-diphenylphosphino-1-phenyl-
As
turned
out
from
the
experiment,
Ca(1)2(H-DPTTA)2 crystallized as an ethanol
inclusion species with two partially occupied (one-
third of approximate populations) and disordered
alcohol molecules in the asymmetric unit of the
crystal. Figure 1 shows the catemer subunit from
phospholane
(LuPhos),
3-diphenylphosphino-
1-phenyl-1,2,3,6-tetrahydrophosphinine, and 3-
diphenylphosphino-1-phenyl-1,2,3,4,5,6-hexahydro-
phosphinine. These platinum complexes were
also tested as catalysts in the hydroformylation of
styrene. The catalytic activity of the complexes incor-
porating six-membered P-ligand was significantly
SCHEME 1
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Application of Platinum(II) Complexes 93
FIGURE 1 Fifty percent atomic displacements plot of Ca(1)2(H-DPTTA)2 showing carboxyl/carboxylate H-bridge contacts in
broken lines. Disordered ethanol molecules are omitted for clarity from this drawing.
the polymer chain that makes up the crystal. Most
basic feature of this coordination salt assembly is
its perfect twofold symmetry that matches a crys-
tallographic C2 symmetry axis going through the
Ca2+ ion. The coordination sphere of the Ca2+ ion
is made up from six O atoms and can be described
as a slightly distorted tetragonal bipyramid. The
Ca–O1=P coordination distance is the shortest one
CD Spectroscopic Investigation of the Optically
Active 4-Chloro-1-phenyl-5-methyl-1,2,3,6-tetra-
hydrophosphinine 1-Oxide (1)
The theoretical calculations were performed on the
randomly selected (R)-isomer of 4-chloro-1-phenyl-
5-methyl-1,2,3,6-tetrahydrophosphinine 1-oxide (1).
It was started with a molecular mechanics (MM)
conformation analysis, which resulted in 14 stable
conformers lying at most at 10 kJ/mol above the
lowest energy one. Geometry optimizations were
performed at the density functional theory (DFT)
level for the low-energy conformers, and accurate
conformational energies were calculated with the di-
rect random-phase approximation (dRPA) approach
at the DFT-optimized structures. Three low-energy
conformers were identified differing in the orien-
tation of the phenyl group with the two higher
energy ones lying at around 1.1 and 4.4 kJ/mol
above the most stable conformer. Other conformers
were higher by at least 15 kJ/mol in energy and were
not considered in the further calculations. The op-
timized geometry for the most stable conformer
is presented in Fig. 2; the geometrical parameters
are given in Table 1. These calculated data are
compared with the parameters obtained from the
single crystal X-ray analysis of the optically active
P-heterocyclic moiety in the diastereomer complex
Ca(1)2(H-DPTTA)2. As can be seen, the theoreti-
cally calculated data generally agree within the usual
3*sigma (3σ) criteria. The only notable exceptions
concern the bonds around the P atom.
(2.291(2)A) around the Ca2+ ion. This distance value
˚
falls on the slightly shorter side than the mean of 13
˚
(out of 15) such Ca—O distances (2.306 A, sample
˚
s.d. 0.028 A) distribution from the Cambridge Struc-
tural Database (CSD) [29]. It is not only significantly
shorter than the other two independent distances
˚
˚
˚
(2.342(2)A and 2.354(2)A, ꢀ = 0.051 A and 0.063
˚
A, i.e., 25σ and 31σ differences), but those longer
contacts are maintained by partial negative charged
carboxyl/carboxylate pairs. In other words, it seems
from these distances that the O1 = P oxygen atom
is more negatively polarized than those in the
COOH/COO– pair. Otherwise, the covalent bonds
largely conform to the database mean values as
attested by a MOGUL analysis [30] of corresponding
bonds and angles.
The H-principal O—H···O H-bridges may ideally
form a symmetric COOH/COO– H-bridge, apart from
which there are a few C—H···O contacts in the crys-
tal. The Cl atom also maintains numerous C—H···Cl
short contacts.
The chirality of the P-center was established as R
with high confidence marked by a Flack parameter
of 0.01(5).
Heteroatom Chemistry DOI 10.1002/hc

94 Bagi et al.
The theoretical absorption and CD spectra for
the selected three conformers were evaluated at the
time-dependent DFT (TD-DFT) level. The theoreti-
cal spectra of the mixture of conformers obtained
as the Boltzmann-weighted sum of their spectra, to-
gether with the experimental spectra of the tetra-
hydrophosphinine oxide (1), are displayed in Fig. 3.
The agreement of the measured and calculated UV
and CD spectra of the compound is satisfactory. The
measured and calculated CD spectra are rather
similar, and the sign of the dominant features in
the two spectra are identical, and their positions are
very close to each other that indicates that the ab-
solute P-configuration of the compound synthesized
is identical to that of the molecule considered in the
calculations, that is the (R) P-configuration. More-
over, this absolute P-configuration is also confirmed
by the result of the single crystal X-ray analysis.
Synthesis of 4-Chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine Borane and Platinum
Complexes
FIGURE 2 Optimized geometry for the lowest energy
conformer of (R)-4-chloro-1-phenyl-5-methyl-1,2,3,6-tetra-
hydrophosphinine 1-oxide [(R)-1].
The deoxygenation of racemic and optically active
4-chloro-1-phenyl-5-methyl-1,2,3,6-tetrahydrophos-
phinine 1-oxide [1 and (R)-1] was carried out
with trichlorosilane at 0°C. In case of optically
active phosphine oxides, the deoxygenation with
trichlorosilane takes place with retention of the con-
figuration of the P-stereogenic center [33]. Earlier,
the deoxygenation of phenyl-tetrahydrophosphinine
oxide was carried out with a mixture of trichlorosi-
lane and pyridine at 110°C [16].
The racemic and optically active phenyl-
tetrahydrophosphinines [2 and (S)-2] obtained
were then reacted with dichlorodibenzonitrile
platinum to give the corresponding platinum com-
plexes [3 and (R,R)-3] (Schemes 2 and 3). Starting
from racemic phenyl-tetrahydrophosphinine (2),
the corresponding platinum complexes were
obtained as a ca. 2:1 mixture of the racemic [(R,R)-3
and (S,S)-3] and meso [(R,S)-3] forms.
The (S)-phenyl-tetrahydrophosphinine [(S)-2]
was reacted with dimethylsulfide-borane to furnish
the corresponding optically active borane complex
[(S)-4] (Scheme 3). The importance of the optically
active phosphine boranes lies in the fact that they
can be regarded as protected phosphines, since the
P(III)-compounds may be liberated from them using
secondary amines (e.g., diethylamine) [31].
The structure of the platinum and borane
complexes [3, (R,R)-3 and (S)-4] was characterized
by 1H, 13C, and 31P NMR, as well as high
resolution MS (HRMS). In case of the phenyl-
tetrahydrophosphinine platinum complexes [3 and
TABLE 1 Comparison of bonds and angles of tetra-
hydrophosphinine 1, obtained by theoretical calculations (dT)
against the X-ray data (dX) from [32] and the data obtained
from the Ca(1)2(H-DPTTA)2 crystal structure at 173K (dXn
with the standard uncertainties in parentheses (σ)
)
˚
˚
˚
dT (A/°)
dX (A/°) [32]
dXn (A/°)
|ꢀ/σ|a
P1-O1
P1-C6
P1-C1
C5-C6
C4-C5
C2-C4
C1-C2
P1-C7
1.508
1.811
1.818
1.526
1.501
1.338
1.512
1.811
1.497
1.761
1.398
114
101
112
114
107
107
123
118
122
1.484
1.798
1.801
1.536
1.488
1.323
1.513
1.489(3)
1.792(4)
1.798(4)
1.535(6)
1.499(5)
1.320(5)
1.501(6)
1.784(4)
1.511(6)
1.761(4)
1.407(6)
113.6(2)
102.4(2)
112.0(2)
113.2(2)
107.3(2)
107.7(2)
121.3
6.3
4.8
5
1.5
0.4
3.6
1.8
6.8
2.3
0
1.2
2
7
2.5
4
1.5
3
C2-C3
C4-Cl1
C7-C8
O1-P1-C6
C1-P1-C6
O1-P1-C7
O1-P1-C1
C6-P1-C7
C1-P1-C7
P1-C7-C8
P1-C1-C2
C1-C2-C4
C2-C4-Cl1
C1-C2-C3
114.8
100.1
112.5
114.1
106.5
107.9
6.3
3.7
2.5
1.3
0.8
116.9(3)
123.0(4)
119.6(3)
112.7(4)
120
113
aꢀ is defined as ꢀ = |dT – dXn|, while dXn values from the
Ca(1)2(H-DPTTA)2 crystal are averaged for the two o-position bonds
and angles. Values of |ꢀ/σ| that are larger than the 3*σ level are
printed in italics.
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Application of Platinum(II) Complexes 95
60000
40000
20000
0
120
80
measured
calculated
measured
calculated
40
0
-40
200
250
300
350
200
250
300
350
wavelength [nm]
(a)
wavelength [nm]
(b)
FIGURE 3 Calculated (dashed line) UV absorption (a) and CD (b) spectra of (R)-4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine 1-oxide [(R)-1] together with the measured (solid line) spectra of (+)-4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine 1-oxide [(+)-1]. The solvent was acetonitrile.
SCHEME 2
SCHEME 3
(R,R)-3], 2D 1H-1H COSY, 1H-13C HSQC, and HMBC
spectra were also recorded to verify the structures.
The relative position of the tetrahydrophosphinine
ligands in the platinum complexes [3 and (R,R)-3]
including the 195Pt–13C and 31P–13C couplings of the
optically active platinum complex [(R,R)-3], were
determined by first-order analysis.
1
was confirmed by the stereospecific JPt-P couplings
Stereostructure of the (R,R)-4-Chloro-1-phenyl-
5-methyl-1,2,3,6-tetrahydrophosphinine
Platinum Complex [(R,R)-3]
that were 3567 and 3568 Hz for 3 and (R,R)-3,
respectively. It is known that the coupling
constants of ca. 3500 Hz indicate that the
corresponding coordinated phosphorus donor atom
and chloro ligand are in the trans disposition.
All relevant heteronuclear coupling constants,
Stereostructure of the 4-chloro-1-phenyl-5-methyl-
1,2,3,6-tetrahydrophosphinine platinum complex
(3) was calculated at the ωB97X-D/cc-pVTZ//
Heteroatom Chemistry DOI 10.1002/hc

96 Bagi et al.
FIGURE 4 Perspective view of cis-bis(4-chloro-1-phenyl-5-methyl-1,2,3,6-tetrahydrophosphinino)-dichloro-platinum(II) [(R,R)-
3] calculated by the ωB97X-D/cc-pVTZ//RI-B97-D/6-31G(d) method. For Pt atoms, cc-pVTZ-PP pseudopotential was applied
in all cases. Gray, white gray, orange, green, and white colors are referred to carbon, hydrogen, phosphorus, chlorine, and
˚
platinum atoms, respectively. Selected bond lengths (A), bond angles (°), and torsion angles (°) are as follows: Pt-Cl(1) 2.394,
Pt-P(1) 2.252, P(1)-C2 1.858, C2-C3 1.542, C3-C4 1.515, C4-C5 1.351, C5-C6 1.523, P(1)-C6 1.861, P(1)-C1’ 1.832, C1’-C2’
1.411, C2’-C3’ 1.403, C3’-C4’ 1.404; Cl(1)-Pt-Cl(2) 88.7, Cl(1)-Pt-P(1) 83.8, Pt-P(1)-C2 121.7, Pt-P(1)-C6 110.8, Pt-P(1)-C1’
113.0, P(1)-C2-C3 112.7, C2-C3-C4 114.7, C3-C4-C5 127.5, C4-C5-C6 123.0, C5-C6-P1 119.4, C2-P(1)-C6 98.8, C2-P(1)-
C1’ 104.0, C6-P(1)-C1’ 106.8; P(1)ꢀꢀꢀCl(1)ꢀꢀꢀCl(2)ꢀꢀꢀP(2) –4.6, Cl(1)-Pt-P(1)-C2 170.8, Pt-P(1)-C2-C3 –172.4, P(1)-C2-C3-C4
52.5, C2-C3-C4-Cl 158.5, C2-C3-C4-C5 –24.5, C3-C4-C5-C6 –0.5.
˚
RI-B97-D/6-31G(d) level of theory with cc-pVTZ-PP
pseudopotential on the Pt atom. Accurate ωB97X-D
single point energy calculations suggested that
the relative energies of the corresponding
homochiral [(R,R) and (S,S)] and heterochiral
the ortho-HꢀꢀꢀPt distance of 2.95 A (where ortho-H
is the corresponding H of the phenyl group, see
Fig. 4, blue dotted line).
Catalytic Activity of the cis-PtCl2(L)2 (where L
Stands for 2 or (S)-2) Complex in the
Hydroformylation of Styrene
(R,S)-1,2,3,6-tetrahydrophosphinine
platinum
complexes (3) are close to each other as the values
are within 0.9 kcal/mol. The most stable structure
of the (R,R)-4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine platinum complex [(R,R)-3]
synthesized is displayed in Fig. 4. The geometry
around the Pt atom is considered square planar,
as the P(1)ꢀꢀꢀCl(1)ꢀꢀꢀCl(2)ꢀꢀꢀP(2) pseudo-torsion
angle is –3.9°. It was found that the conformer with
rotational symmetry (C2 symmetry group) is the
most favorable structure, which is stabilized by
intramolecular nonbonding interactions between
the phosphinine rings.
The “preformed” PtCl2L2-type complexes (3 or
(R,R)-3 containing ligands 2 or (S)-2, respectively)
were tested as a catalyst precursors in the hydro-
formylation of styrene. The catalysts formed in situ
from 3 (or (R,R)-3) and two equivalents of tin(II)
chloride were used at 80 bar [syngas; p(CO) =
p(H2) = 40 bar]. In addition to the branched and
linear formyl regioisomers (2-phenylpropanal
(A) and 3-phenylpropanal (B), respectively),
ethylbenzene (C) as hydrogenation by-product was
also formed (Eq. 1).
CHꢀꢀꢀHC interactions were identified between
the C(2)H2 moieties of the two phosphinine rings
CO/H2
PhCH = CH2 −→ PhCH(CHO)CH3
˚
(the shortest HꢀꢀꢀH distance is 2.21 A, see Fig. 4,
A
red dashed line). These secondary interactions
also define the conformation of the phenyl groups,
and thus the environment of the platinum. In
case of (R,R)-3, due to the C2 symmetry, one
ortho-H of each phenyl rings is situated close to
the platinum center significantly influencing the
environment of the platinum that is shown by the
HꢀꢀꢀPtꢀꢀꢀCl(2) ꢀꢀꢀCl(1) dihedral angle of 116.9° and
+ PhCH2CH2CHO + PhCH2CH3 (1)
B
C
As expected, both catalytic systems formed
either from the racemic or enantiomerically pure
tetrahydrophosphinine ligand (2 or (S)-2, respec-
tively) provided similar catalytic activity (compare,
for instance, entries 2 and 6, as well as 4 and 9,
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Application of Platinum(II) Complexes 97
TABLE 2 Hydroformylation of Styrene in the Presence of In Situ Formed Catalysts from PtCl2(L)2 Complex (L = 2 or (S)-2)
and Tin(II) Chloridea
b
c
Entry
L
Temperature (°C)
Reaction time (h)
Conversion (%)
Rc (%)
Rbr (%)
ee (%)
1
2
3
4
5
6
7
8
9
2
40
60
100
100
40
20
20
5
10
20
20
2
11
91
91
98
20
90
54
96
99
83
82
87
84
84
81
87
88
85
62
58
55
54
64
61
59
57
58
–
–
–
2
2
2
(S)-2
(S)-2
(S)-2
(S)-2
(S)-2
–
29 (R)
17 (R)
10 (R)
9 (R)
8 (R)
60
100
100
100
4
10
aReaction conditions: Pt/styrene = 1/100, Pt/SnCl2 = 1/2; p(CO) = p(H2) = 40 bar, 1 mmol of styrene, solvent: 10 mL of toluene.
bChemoselectivity toward aldehydes (A, B). [(moles of A + moles of B)/(moles of A + moles of B + moles of C) × 100], where A is
2-phenylpropanal, B is 3-phenylpropanal, and C is ethylbenzene.
cRegioselectivity toward branched aldehyde (A). [moles of A/(moles of A + moles of B) × 100].
Table 2). Reasonable activity was observed even at
40°C (entries 1 and 5).
hydroformylation of styrene, and its stereostructure
was evaluated by theoretical calculations.
The formation of the aldehydes (A and B) was
highly preferred in all cases, i.e., chemoselectivities
above 81% were obtained in all cases. A slight in-
crease in chemoselectivity (up to 88%) was obtained
at higher temperature (entry 8).
EXPERIMENTAL
General (Instruments)
The formation of branched aldehyde (A) is pre-
dominated in all cases; however, slightly higher
branched selectivities were obtained at lower
temperatures (entries 1, 2 and 5, 6) than at 100°C
(entries 3, 4 and 8, 9). It is worth noting that the
major trends regarding chemo- and regioselectivities
keep in line with those observed for Pt catalysts with
5-membered P-heterocycles as P-ligands in hydro-
formylation [17–19].
The application of (R,R)-3 complex with the op-
tically active P-ligand, (S)-2 resulted in low enan-
tioselectivities by the predominance of the R enan-
tiomer of the branched aldehyde. The ee-s obtained
at low temperature (40–60°C) are definitely higher,
than those detected at 100°C (compare entries 5, 6
and 7–9).
The 31P NMR spectra were recorded on a Bruker
AV-300 (Wien, Austria) spectrometer operating at
1
121.5 MHz for 31P, whereas the 13C and H NMR
spectra were obtained on a Bruker DRX-500 (Wien,
Austria) spectrometer operating at 125.7 for 13C and
500 MHz for 1H, respectively. 2D 1H-1H COSY,
1H-13C HSQC, and HMBC spectra of platinum com-
plexes [3 and (R,R)-3] were acquired on a Bruker
Avance II (Wien, Austria) spectrometer operating at
500 MHz 1H frequency equipped with a BBI or a TXI
z-gradient probe. The couplings are given in hertz.
The exact mass measurements were performed
using a Q-TOF Premier mass spectrometer in
positive electrospray mode. The ee of the 4-chloro-
1-phenyl-5-methyl-1,2,3,6-tetrahydrophosphinine 1-
oxide (1) was determined by chiral HPLC using
KromasilꢀR 5-Amycoat 250 × 4.6 mm ID, hexane/
ethanol 85/15 as an eluent with a flow rate of
0.8 mL/min, T = 20°C, UV detector α = 254 nm.
Retention times: 15.8 min for (+)-1 and 23.2 min
for (−)-1. The determination of the ee-s of
2-phenylpropanal (A) was carried out on a Thermo
Scientific FOCUS gas-chromatograph equipped
with a Cyclodex column (20 m × 0.25 mm,
0.25 μm film, FID detector, helium as carrier
gas, injector 250°C, detector 280°C, head pressure:
14.5 psi). (S)-2-Phenylpropanal was eluted before the
(R)-enantiomer. Optical rotations were determined
on a Perkin-Elmer 241 polarimeter (Boston, MA).
CONCLUSIONS
Absolute configuration of (+)-4-chloro-5-methyl-
1-phenyl-1,2,3,6-tetrahydrophosphinine oxide was
determined by single crystal X-ray investigation
of the diastereomeric associate obtained by
resolution with the acidic Ca2+ salt of (–)-O,O-di-
p-toluoyl-(2R,3R)-tartaric acid. An independent
CD spectroscopic study also suggested “R”
configuration of the P-stereogenic center of
the tetrahydrophosphinine oxide that was then
converted to the corresponding platinum complex
after deoxygenation. The Pt complex showed
moderate enantioselectivity as catalyst in the
The
4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine 1-oxide (1) was synthesized
Heteroatom Chemistry DOI 10.1002/hc

98 Bagi et al.
as described earlier [32]. The (–)-O,Oꢁ-di-p-toluoyl-
(2R,3R)-tartaric acid was purchased from Aldrich
(Budapest, Hungary).
ral form), 1.86–2.04 (m, 4H) and 2.40–2.60 (m, 4H)
C3-H2 of the homo- and heterochiral forms, 2.09–
2.20 (m, 2H) and 2.84–2.94 (m, 2H) C6-H2 of the
homochiral form, 2.21–2.29 (m, 2H) and 2.76–2.85
(m, 2H) C2-H2 of the homochiral form, 2.27–2.35 (m,
2H) and 3.01–3.14 (m, 2H) C6-H2 of the heterochi-
ral form, 2.31–2.38 (m, 2H) and 2.62–2.72 (m, 2H)
C2-H2 of the heterochiral form, 7.25–7.34 (m, 8H,
C2’-H of the homo- and heterochiral forms), 7.27–
7.36 (m, 8H, C3’-H of the homo- and heterochiral
forms), 7.39–7.47 (m, 4H, C4’-H of the homo- and
heterochiral forms); 13C NMR (CDCl3) δ 22.0–22.4
(m, C2), 22.5–22.6 (m, CH3), 27.5–27.9 (m, C6
of the homochiral form), 28.0–28.4 (m, C6 of the
heterochiral form), 30.3–30.6 (m, C3), 123.9–124.2
(m, C5 of the homochiral form), 124.4–124.5 (m,
C5 of the heterochiral form), 127.7–127.8 (m, C4),
128.0–128.7 (m, C1’), 129.2 (m, C3’), 131.5–131.7
(m, C2’), 131.8 (C4’ of the heterochiral form), 131.9
(C4’ of the homochiral form); 31P NMR (CDCl3) δ:
–14.3 (JPt–P = 3568, 65%), –14.7 (JPt–P = 3568, 35%),
Preparation of (R)-4-Chloro-1-Phenyl-5-Methyl-
1,2,3,6-Tetrahydrophosphinine 1-Oxide
[(R)-1]
To 0.55 g (1.4 mmol) of (−)-(R,R)-di-p-toluoyl-
tartaric acid monohydrate and 0.039 g (0.68 mmol)
of CaO in a mixture of 1.8 mL of ethanol and 0.35 mL
of water, 0.65 g (2.7 mmol) of racemic 4-chloro-5-
methyl-1-phenyl-1,2,3,6-tetrahydrophosphinine 1-
oxide (1) was added in 1.8 mL of ethanol.
After standing at 26°C for 24 h, the crystals
were filtered off to give 0.67 g (76%) of complex
Ca[(R)-1]2(H-DPTTA)2 with
a
diastereomeric
excess (de) of 86%. The complex was purified
by two digestions, the crystals were taken up
in 3.6 mL of ethanol and the suspension was
stirred for 24 h at 26°C to afford 0.60 g (69%)
(δP [16] –14.7); HRMS [M–Cl]+
= 677.0370,
of complex Ca[(R)-1]2(H-DPTTA)2 with
a de
found
C24H28P2Cl3Pt requires 677.0359 for the 35Cl and
of 98%. The (R)-4-chloro-5-methyl-1-phenyl-
1,2,3,6-tetrahydrophosphinine 1-oxide [(R)-1] was
recovered by treating the dichloromethane solution
(4 mL) of the complex with 2 mL of 10% aqueous
ammonia. The organic phase was washed with 1
mL of water, dried (Na2SO4), and concentrated to
give 0.20 g (60%) of (R)-1 with an ee of 98%. 31P
NMR (CDCl3) δ: 27.8, (δP [26] 27.9); [α]2D5 = +19.5
(c 0.7; CHCl3). (In this experiment, the yields were
based on the half of the racemate that is regarded to
be 100% for each antipode.)
195Pt isotopes.
The optically active cis-[bis-(R)-4-chloro-
1-phenyl-5-methyl-1,2,3,6-tetrahydrophosphinino]-
dichloro-platinum(II) [(R,R)-3] was prepared in
a similar manner from (R)-4-chloro-5-methyl-1-
phenyl-1,2,3,6-tetrahydrophosphinine-1-oxide [(R)-1]
with an ee of 98%. Yield of (R,R)-3: 80%.
1H NMR (CDCl3) δ: 1.58 (bs, 6H, CH3), 1.86–
1.99 (m, 2H) and 2.44–2.60 (m, 2H) C3-H2, 2.10–2.22
(m, 2H) and 2.84–2.94 (m, 2H) C6-H2, 2.22–2.33
(m, 2H) and 2.75–2.85 (m, 2H) C2-H2, 7.26–7.33
(m, 4H, C2’-H), 7.28–7.35 (m, 4H, C3’-H), 7.41–7.46
Preparation of
cis-[Bis(4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinino)-dichloro-platinum(II)]
(3)
(m, 2H, C4’-H); 13C NMR (CDCl3) δ 22.3 (1JP–C
=
3
5
48.7, JP–C = 6.9, C2), 22.5 (3JP–C = 4.0, JP–C = 4.0,
1
3
CH3), 27.7 (2JPt–C = ꢁ 36, JP–C = 45.6, JP–C = 6.6,
2
4
C6), 30.4 (3JPt–C = ꢁ 24, JP–C = 3.4, JP–C = 3.4,
The solution of 0.070 g (0.29 mmol) of racemic
4-chloro-5-methyl-1-phenyl-1,2,3,6-tetrahydrophos-
phinine 1-oxide (1) in 2 mL of benzene was de-
gassed and cooled to 0°C, then 0.18 mL (1.7 mmol)
of trichlorosilane was added. The mixture was
stirred at 0°C for 3 h, and then at 26°C for 3 h
under nitrogen to afford the corresponding phenyl-
tetrahydrophosphinine (2). Then, 0.068 g (0.15
mmol) of dichlorodibenzonitrile platinum(II) was
added to the reaction mixture under nitrogen. The
mixture was stirred at 26°C for 1 day, whereupon
the complex precipitated. Separation by filtration
led to 0.081 g (78%) of complex 3 as a 2:1 mixture of
the homo- [(R,R) and (S,S)] and heterochiral (R,S)
forms.
2
4
C3), 124.0 (3JPt–C = ꢁ 29, JP–C = 4.0, JP–C = 4.0,
3
5
C5), 127.7 (4JPt–C = ꢁ 39, JP–C = 5.6, JP–C = 5.6,
C4),128.2 (m, C1’), 129.2 (4JPt–C = ꢁ 39, JP–C = 5.4,
3
5JP–C = 5.4, C3’), 131.6 (3JPt–C = ꢁ 35, JP–C = 5.0,
2
4JP–C = 5.0, C2’), 131.9 (C4’); 31P NMR (CDCl3) δ:
–14.3 (JPt–P = 3567); HRMS [M–Cl]+found = 677.0381,
C24H28P2Cl3Pt requires 677.0359 for the 35Cl and
195Pt isotopes; [α]2D5 = +33.6 (c 1; CHCl3).
Preparation of (S)-4-Chloro-1-phenyl-5-methyl-
1,2,3,6-tetrahydrophosphinine-borane
[(S)-4]
The deoxygenation of 0.064 g (0.27 mmol) of
(R)-4-chloro-5-methyl-1-phenyl-1,2,3,6-tetrahydro-
phosphinine-1-oxide [(R)-1] was carried out in
1H NMR (CDCl3) δ: 1.58 (bs, 6H, CH3 of the ho-
mochiral form), 1.70 (bs, 6H, CH3 of the heterochi-
Heteroatom Chemistry DOI 10.1002/hc

Synthesis, Characterization, and Application of Platinum(II) Complexes 99
benzene using 0.16 mL (1.6 mmol) of trichlorosi-
lane according to the procedure described above.
Then, 0.13 mL of 2 M dimethyl sulfide borane in
tetrahydrofuran (0.27 mmol) was added and the
solution was stirred at 26°C for 3 h under nitrogen.
Then, the mixture was treated with 3 mL of water
and stirred for 15 min. The precipitated boric acid
was removed by filtration and the organic phase
dried (Na2SO4). Volatile components were removed
under reduced pressure and the residue so obtained
was purified by column chromatography (silica gel,
3% methanol in dichloromethane) to give 0.050
g (78%) of (S)-4-chloro-1-phenyl-5-methyl-1,2,3,6-
tetrahydrophosphinine-borane [(S)-4].
data were collected at 173 K using monochroma-
tized Mo Kα radiation. Completeness were equal or
greater to 2θ = 98.7%. Semiempirical absorption
correction from equivalents was applied to the data
with minimum and maximum transmission factors
of 0.958 and 0.983. Initial structure models were
obtained by direct methods [34, 35] and subsequent
difference syntheses. Anisotropic full-matrix least-
squares refinement on F2 were applied [34, 35] for
all nonhydrogen atoms except the disordered solvent
atomic sites and the model was refined to conver-
gence. Final R indices [I > 2σ(I)] are R1 = 0.0534,
wR2 = 0.1288; absolute structure parameter value
of 0.01(5) shows assignments of the correct hand
and of the absolute configuration. Hydrogen atomic
positions were calculated from assumed geometries
where appropriate; O–H hydrogen atoms were lo-
cated from difference electron density maps. Hydro-
1H NMR (CDCl3) δ: 0.28-1.33 (m, 3H, BH3), 1.99
(bs, 3H, CH3), 2.07–2.20 (m, 2H, C3–H2), 2.44–2.85
(m, 4H, C2–H2 and C6–H2), 7.44–7.54 (m, 3H, Ar–H),
7.61–7.68 (m, 2H, Ar–H); 13C NMR (CDCl3) δ: 21.2
(1JP–C = 35, C2), 23.0 (3JP–C = 7, CH3), 27.8 (1JP–C
36, C6), 30.0 (2JP–C = 6, C3), 124.6 (2JP–C = 7,
C5), 127.8 (2JP–C = 11, C4), 128.4 (1JP–C = 52, C1ʹ),
129.1 (3JP–C = 10, C3ʹ), 131.4 (3JP–C = 9, C2ʹ), 131.7
(4JP–C = 3, C4ʹ); 31P NMR (CDCl3) δ: –0.3 (bs);
=
gen atoms were included in structure factor calcula-
tions and only the nontrivial hydrogen positions and
their isotropic displacements were refined. Other
H atoms were kept riding on their anchor atoms,
with isotropic displacement parameters of these
hydrogen atoms were approximated from the U(eq)
value of the atom they were bonded. The maximum
[M + Na]+
= 261.0752, C12H17PBClNa requires
found
261.0747 for the 35Cl 11B isotope; [α]2D5 = +46.7 (c 1;
CHCl3).
and minimum residual electron densities in the
–3
˚
final difference maps are 0.87 and –0.25 e.A
and are acceptable due to solvent disorder. All
further calculations and drawings were done by
using PLATON [36] and SCHAKAL [37]. Crystal
structure data are deposited with the Cambridge
Crystallographic Data Centre under CCDC 1409703
and can be obtained free of charge upon application.
Hydroformylation Experiments
In a typical experiment, a solution of cis-PtCl2(L)2
(where L stands for 2 or (S)-2), 0.01 mmol) and
tin(II) chloride (3.8 mg; 0.02 mmol) in toluene
(10 mL) containing styrene (0.115 mL, 1.0 mmol)
was transferred under argon into a 100-mL stainless
steel autoclave. The reaction vessel was pressurized
to 80 bar total pressure (CO/H2 = 1:1) and placed
in an oil bath of constant temperature. The mixture
was stirred with a magnetic stirrer for the given reac-
tion time. The pressure was monitored throughout
the reaction. After cooling and venting of the
autoclave, the pale yellow solution was removed and
immediately analyzed by gas chromatography.
CD Measurements
The UV and CD spectra were measured in
acetonitrile solutions at 25°C. The UV spectra were
recorded on an Agilent 8453 diode array spectrome-
ter. The CD spectra were obtained on a Jasco J-810
spectropolarimeter.
Theoretical Calculations
X-Ray Measurements
The MM, DFT, and dRPA calculations were car-
ried out using the Marvin [38], Gaussian 09 [39],
and MRCC [40] programs, respectively. The
Merck Molecular Force Field (MMFF) [41] was
employed at the MM conformational analysis. DFT
geometry optimizations were performed with the
PBE0 [42, 43] functional and the 6–311++G** [44]
basis set. The absorption and CD spectra were com-
puted using the TD-DFT [45] method and the same
functional and basis set. Rotator strengths were cal-
culated in the velocity gauge. The DFT calculations
X-ray quality crystals of the diastereomeric
complex Ca(1)2(H-DPTTA)2 were grown from a
saturated ethanol solution of 5.2 mg (0.022 mmol)
of (R)-1 and 4.4 mg (0.0054 mmol) of Ca(H-DPTTA)2
prepared in situ as described above.
A crystal of the complex Ca(1)2(H-DPTTA)2 was
mounted on a glass fiber. Cell parameters were de-
termined by least-squares from the respective setting
angles of reflections. Space group was determined
as chiral orthorhombic P21212 (No. 18). Diffraction
Heteroatom Chemistry DOI 10.1002/hc

100 Bagi et al.
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model [46] with acetonitrile as the solvent since the
latter was employed in the experiments. The dRPA
energies were evaluated with the aug-cc-pVTZ basis
set using PBE0 Kohn-Sham orbitals. Temperature
corrections, entropy contributions, and Gibbs
energies of solvation evaluated at the DFT level with
the above functional and basis set were added to
the gas-phase dRPA energies to obtain 298 K Gibbs
energies of the conformers in the solvent. For all the
three conformers, the absorption (CD) spectra were
simulated as superpositions of Gaussian functions
placed at the wavelengths of the computed
transitions with heights proportional to the
corresponding computed oscillator (rotator)
strengths. To obtain the final theoretical spectra, the
spectra of the individual conformers were weighted
using the Boltzmann factors calculated from the
conformational energies. The averaged curves were
normalized so that the height of the dominant
band will be identical to that of the experimental
spectra, and the spectra were shifted by –2 nm so
that the most intense band in the theoretical and
experimental absorption spectra will appear at the
same wavelength (191 nm).
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ACKNOWLEDGMENTS
The authors are thankful to Hungarian Research
Fund for the financial support (Grant No. PD116096,
K83118, K115421, K108752, K104769, K113177,
and K105459 (K.E.K.)). Financial support from
Richter Gedeon Talentum Alap´ıtva´ny (Ph.D. schol-
arship to I.T.) is gratefully acknowledged. T.S. is
grateful for the support of The New Sze´chenyi Plan
TAMOP-4.2.2/B-10/1-2010-0009.
[26] Bagi, P.; Laki, A.; Keglevich, G. Heteroatom Chem
2013, 24, 179–186.
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Heteroatom Chemistry DOI 10.1002/hc