A study on the optical resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide and its use in the synthesis of borane and platinum complexes

Article abstract of DOI:10.1016/j.jorganchem.2015.08.013

A 1-isopropyl-3-phospholene 1-oxide was prepared and the two enantiomers were isolated from the racemate by resolution using optically active TADDOL derivatives or the acidic Ca²⁺ salts of (-)-O,O-diaroyl-(2R,3R)-tartaric acids. The single crystal X-ray structure of the 1-isopropyl-3-phospholene oxide - spiro-TADDOL 1:2 associate revealed the mode of binding between the host and guest molecules. The role of the interactions between the three molecules was supported not only by the contact data, but also force field and semiempirical calculations. Beside X-ray analysis, the absolute configuration of the P-stereogenic center was also determined on the basis of CD spectroscopy and high level quantum chemical calculations. The racemic and optically active 1-isopropyl-3-phospholenes obtained after deoxygenation were converted to the corresponding borane complexes and Pt(II) complexes. Stereostructure of the latter species was evaluated by high level quantum chemical calculations and the Pt complexes were tested as catalysts in the hydroformylation of styrene.

Full text of DOI:10.1016/j.jorganchem.2015.08.013

Journal of Organometallic Chemistry 797 (2015) 140e152
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A study on the optical resolution of 1-isopropyl-3-methyl-3-
phospholene 1-oxide and its use in the synthesis of borane and
platinum complexes
a
a
b
b
c, d
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
€ ꢀ
ꢀ
ꢀ
Peter Bagi , Kinga Juhasz , Istvan Timari , Katalin E. Kover , David Mester
,
c
,
d
d,
e
f
g
g
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Mihaly Kallay , Miklos Kubinyi , Tibor Szilvasi , Peter Pongracz , Laszlo Kollar ,
Konstantin Karaghiosoff , Matyas Czugler , Laszlo Drahos , Elemer Fogassy ,
h
a
e
a
ꢀ ꢀ
ꢀ
ꢀ
ꢀ
a, *
€
Gyorgy Keglevich
a Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, 1521 Budapest, Hungary
b Department of Inorganic and Analytical Chemistry, University of Debrecen, 4032 Debrecen, Hungary
c MTA-BME Lendület Quantum Chemistry Research Group, Budapest University of Technology and Economics, 1521 Budapest, Hungary
d Department of Physical Chemistry and Material Science, Budapest University of Technology and Economics, 1521 Budapest, Hungary
e Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1525 Budapest, Hungary
f Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, 1521 Budapest, Hungary
g
ꢀ
ꢀ
ꢀ
Department of Inorganic Chemistry, University of Pecs and Szentagothai Research Center, 7624 Pecs, Hungary
Department Chemie und Biochemie, Ludwig-Maximilians-Universitat München, 81377 München, Germany
h
€
a r t i c l e i n f o
a b s t r a c t
Article history:
A 1-isopropyl-3-phospholene 1-oxide was prepared and the two enantiomers were isolated from the
racemate by resolution using optically active TADDOL derivatives or the acidic Ca2þ salts of (ꢀ)-O,O-
diaroyl-(2R,3R)-tartaric acids. The single crystal X-ray structure of the 1-isopropyl-3-phospholene oxide
e spiro-TADDOL 1:2 associate revealed the mode of binding between the host and guest molecules. The
role of the interactions between the three molecules was supported not only by the contact data, but also
force field and semiempirical calculations. Beside X-ray analysis, the absolute configuration of the P-
stereogenic center was also determined on the basis of CD spectroscopy and high level quantum
chemical calculations. The racemic and optically active 1-isopropyl-3-phospholenes obtained after
deoxygenation were converted to the corresponding borane complexes and Pt(II) complexes. Stereo-
structure of the latter species was evaluated by high level quantum chemical calculations and the Pt
complexes were tested as catalysts in the hydroformylation of styrene.
Received 10 June 2015
Received in revised form
23 July 2015
Accepted 18 August 2015
Available online 28 August 2015
Keywords:
3-Phospholene oxide
Optical resolution
X-ray structure
Theoretical calculations
Pt-complexes
© 2015 Elsevier B.V. All rights reserved.
Hydroformylation
1. Introduction
Although an increasing number of examples can be found in the
literature for the asymmetric syntheses of P-stereogenic compounds
The transition metal phosphine complexes are widely used
catalysts in homogenous catalytic reactions, such as hydrogenation
and hydroformylation that underlines the importance of phosphines
among organophosphorus compounds [1,2]. Among the phosphine
ligands, the P-stereogenic P(III)-compounds are of great importance.
The P-heterocyclic derivatives form a special group [3]. However,
there are only a few examples in the literature for P-heterocyclic
phosphines bearing an asymmetric phosphorus atom [4e6].
[7e9], in most cases the resolution of the corresponding racemic
compounds is the method of choice for the preparation of optically
active organophosphorus compounds including P-heterocyclic
derivatives with a P-stereogenic center [6,10]. Many examples can
be found in the literature for the resolution of cyclic P-chiral
phosphines, phosphine oxides and phosphonium salts via the for-
mation of covalent diasteromers [11e13], diastereomeric coordina-
tion [14e19], molecular complexes [10,20,21] or diastereomeric
salts [22e27].
In the literature, the synthesis of a few optically active P-chiral
heterocyclic ligands was described and the transition metal
complexes of these P(III)-compounds were used as catalysts mainly
* Corresponding author.
E-mail address: gkeglevich@mail.bme.hu (G. Keglevich).
http://dx.doi.org/10.1016/j.jorganchem.2015.08.013
0022-328X/© 2015 Elsevier B.V. All rights reserved.

P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
141
in enantioselective hydrogenation reactions [11,13,16,18,21,28e32].
There are only a few examples for P-heterocyclic ligands that were
used in hydroformylation [5]. Among the transition metal phos-
phine complexes that can be used as catalyst in hydroformylation,
the platinum complexes of P-heterocycles form a class that needs
further investigations. Prigle et al. reported the synthesis of the
platinum complexes of several 5-, 6- and 7-membered P-hetero-
cycles [33,34]. Gouygou et al. synthesized a few bisphosphole
platinum-complexes [35]. Recently, our research group contributed
to the research of optically active P-stereogenic phosphine oxides,
as well as to the synthesis of platinum-complexes incorporating P-
chiral heterocyclic ligands. TADDOL-derivatives and the Ca2þ salts
of dibenzoyl- and di-p-toluoyl-tartaric acid were applied as the
resolving agents to prepare the enantiomers of the aryl- and alkyl-
3-phospholene oxides [36e42], as well as a few six-membered P-
Scheme 1.
diastereomer appeared by gradually cooling down the reaction
mixture to 26 ꢁC (Scheme 2).
In all instances, the corresponding diastereomers were sepa-
rated from the mother liquor by filtration after 3 h of crystallization,
and they were purified further by two recrystallizations. The
composition of the corresponding diastereomers was established
by 1H NMR spectroscopy. The enantiomeric mixture of isopropyl-
phospholene oxide (3) was recovered from the given diaste-
reomer by column chromatography using silica gel and a 97:3
mixture of dichloromethane and methanol as the eluent. The
enantiomeric excess of the isopropyl-phospholene oxide (3) was
analysed by chiral GC.
The results leading to enantiomeric separation of the target
molecule 3 were summarized in Table 1. Beside the yield and the
enantiomeric excess, the resolving capability (S) was also deter-
mined. The resolving capability (S) is used to describe the overall
efficiency of a given resolution and this value can be calculated as
the product of the yield and the enantiomeric excess.
heterocyclic
phosphine
oxides,
embracing
a
1,2-
dihydrophosphinine oxide and a 1,2,3,6-tetrahydrophosphinine
oxide [43,44]. We have also reported the synthesis of a few
racemic and optically active borane and platinum-complexes
bearing aryl- and alkyl-3-phospholene ligands. The 3-
phospholene - platinum complexes were applied as catalysts
in the asymmetric hydroformylation of styrene and
enantioselectivities up to 29% were obtained [45e48].
As a continuation of this ongoing research, we wished to pro-
ceed with our systematic investigation of the resolution of the 3-
phospholene oxides. Another aim of ours was to gain a deeper
understanding how the substituents of the 3-phospholene moiety
influence the catalytic activity of the platinum-complexes incor-
porating 3-phospholene ligands. In this paper, we report the
synthesis and the resolution of the 1-isopropyl-3-methyl-3-
phospholene oxide. The 1-isopropyl-3-methyl-3-phospholene-
borane and platinum complexes were also prepared in racemic and
optically active forms, and the platinum-complexes were tested as
catalysts in the hydroformylation of styrene.
Applying TADDOL [(ꢀ)-4] as the resolving agent for the enan-
tiomeric separation of 3-phospholene oxide 3, the crystalline
diastereomers were obtained in the cases, when ethyl acetate/
hexane or isopropyl alcohol was used (Table 1, Entries 1 and 2).
However, the enantiomeric discrimination between the two anti-
podes of 1-isopropyl-3-phospholene oxide (3) was poor in these
instances, as the highest enantiomeric excess and resolving capa-
bility values were 6% and 0.05, respectively (Table 1, Entry 1). It is
noteworthy that no diastereomeric complex was obtained when
methanol or ethanol was used as the solvent, and the TADDOL
[(ꢀ)-4] precipitated exclusively in these instances.
2. Results and discussion
2.1. Synthesis and resolution of 1-isopropyl-3-methyl-3-
phospholene 1-oxide (3)
Applying spiro-TADDOL [(ꢀ)-5] as the resolving agent, the
enantiomeric excess and the resolving capability values were sol-
vent dependent, and the ee was between 57 and 95%, and the
resolving capability fell in the range of 0.34e0.47 (Table 1, Entries
3e6). It is noteworthy that the solvent also influenced which
isopropyl-phospholene oxide (3) enantiomer was incorporated into
1-Isopropyl-3-methyl-3-phospholene 1-oxide (3) was prepared
by extending the synthetic method elaborated for other alkyl- and
aryl-3-methyl-3-phospholene oxides [37,47,49]. 1-Hydroxy-3-
methyl-3-phospholene 1-oxide (1) was reacted with thionyl chlo-
ride to form the corresponding phosphinic chloride (2) which was
immediately reacted with isopropylmagnesium bromide to afford
1-isopropyl-3-methyl-3-phospholene 1-oxide (3) in a yield of 49%
(Scheme 1). As a new compound, the isopropyl-phospholene oxide
(3) was characterized by 1H, 13C and 31P NMR spectroscopy, as well
as HRMS.
To prepare the enantiomers of isopropyl-phospholene oxide (3),
the resolution of racemic phosphine oxide 3 was first attempted
with TADDOL-derivatives [(ꢀ)-4 and (ꢀ)-5] that were found to be
efficient resolving agents for other aryl- and alkyl-3-phospholene
oxides [36,37,40].
A few resolution experiments of isopropyl-phospholene oxide
(3) with TADDOL-derivatives [(ꢀ)-4 and (ꢀ)-5] were carried out in
a mixture of ethyl acetate and hexane. In these cases, the mixture of
the racemic compound (3) and the given resolving agent [(ꢀ)-4 or
(ꢀ)-5] was dissolved in hot ethyl acetate, and the corresponding
diastereomer precipitated on adding hexane to the reaction
mixture.
Applying alcohols, such as methanol, ethanol or isopropanol as
the solvent, the racemic compound (3) and the resolving agent
[(ꢀ)-4 or (ꢀ)-5] were dissolved in hot alcohols, and the
the
corresponding
diastereomer.
(þ)-(R)-1-Isopropyl-3-
phospholene oxide [(þ)-3] could be prepared with spiro-TADDOL
[(ꢀ)-5] in ethyl acetate/hexane or methanol (Table 1, Entries 3
and 4). Carrying out the resolution in ethanol or in isopropyl
alcohol led to the other (ꢀ)-(S)-1-isopropyl-3-phospholene oxide
antipode [(ꢀ)-3] (Table 1, Entries 5 and 6). Moreover, the solvent
also affected the composition of the diastereomers.
A
diastereomeric complex incorporating isopropyl-phospholene
oxide (3) and spiro-TADDOL [(ꢀ)-5] in a 1:1 ratio was obtained in
ethyl acetate/hexane and isopropyl alcohol (Table 1, Entries 3 and
6), and the diastereomer having the composition of (3)∙(spiro-
TADDOL)2 was separated when methanol or ethanol was used as
the solvent (Table 1, Entries 4 and 5).
These results indicated that the TADDOL analogue (ꢀ)-5 could
be used more efficiently for the enantiomeric separation of phos-
phine oxide 3, than the TADDOL [(ꢀ)-4] itself (compare Table 1,
Entries 1e2 and 3e6).
The observation that both isopropyl-phospholene oxide anti-
podes [(þ)- and (ꢀ)-3] could be prepared with the same resolving
agent, (spiro-TADDOL [(ꢀ)-5]) in different solvents allowed us to
develop
a
procedure for the preparation of the (þ)- and

142
P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
Scheme 2.
(ꢀ)-isopropyl-phospholene oxide enantiomers [(þ)- and (ꢀ)-3]
from racemic (Scheme 3). The resolution of isopropyl-
at the de of 64%, which hinders the purification of this diaste-
reomer. As a consequence, methanol was chosen for the prepara-
tion of (þ)-(R)-1-isopropyl-3-phospholene oxide [(þ)-3] by
resolution, as it was detailed above.
Single crystal X-ray structure determination from a proper
sample obtained from recrystallizations (Table 1, Entry 5), revealed
as the most conspicuous feature, also the 1:2 composition of
phospholene oxide (ꢀ)-3 and spiro-TADDOL (ꢀ)-5 in the asym-
metric unit of the crystal (Fig. 1).
The appearance of two independent hosts in the crystallographic
asymmetric units is a frequently observed stoichiometry feature in
this line of experiments. Two examples from our recent studies with
2:2 resolving host e target guest ratio are the 1-butyl-3-methyl-3-
phospholene oxide [40] and the 1-propoxy-3-methyl-3-
phospholene oxide [41] guest containing crystal structures. This
directs attention to the altered stoichiometry from the frequently
observed 2:2 host/guest ratio. Lately, we expounded the idea of
competitive resolution [42], where the 2:2 ratio is violated, such that
the inclusion of the solvent i-PrOH instead of a second phospholene
3
phospholene oxide 3 utilizing spiro-TADDOL [(ꢀ)-5] in ethanol
afforded diastereomer [(ꢀ)-3 ∙ (ꢀ)-5] that was purified by three
recrystallizations to give the (ꢀ)-(S)-1-isopropyl-3-phospholene
oxide [(ꢀ)-3] with an ee of 97% and in a yield of 45% after a re-
covery from the diastereomer by column chromatography.
To prepare (þ)-(R)-1-isopropyl-3-phospholene oxide [(þ)-3],
the mother liquors of the crystallization and recrystallizations were
combined, and the volatiles were removed to afford a mixture of
(þ)-3 (ee: 23%) and spiro-TADDOL [(ꢀ)-5]. To this mixture, a suf-
ficient amount of spiro-TADDOL [(ꢀ)-5] was added and the
resolution was carried out in methanol to obtain the (þ)-isopropyl-
phospholene oxide [(þ)-3] with an ee of 87% and in a yield of 40%
after recrystallizations and decomplexation of the corresponding
diastereomer [(þ)-3 ∙ (ꢀ)-5].
Besides the methanol, the mixture of ethyl acetate and hexane
was also a suitable solvent combination to prepare the (þ)-(R)-1-
isopropyl-3-phospholene oxide [(þ)-3] by resolution with spiro-
TADDOL [(ꢀ)-5], as it was indicated by the preliminary experi-
ments (Table 1, Entries 3 and 4). Moreover, considering the
resolving capability values, the application of ethyl acetate/hexane
seemed to be more advantageous than methanol (Compare Table 1,
Entries 3 and 4). However, the purity of the corresponding diaste-
reomer [(þ)-3 ∙ (ꢀ)-5] could not be improved by recrystallizations
from ethyl acetate/hexane with the maximum diastereomeric
excess (de) was 64%. This phenomenon may be explained by the
fact that the (þ)-3 ∙ (ꢀ)-5 diastereomer has a eutectic composition
occurred yielding
a 1:1:2 guest/solvent/hosts stoichiometry.
Here we have another case as there is only one (ꢀ)-3 guest
embedded between two (ꢀ)-5 hosts. It is apparent that the guest is
primarily anchored by the H-bridges donated from each (ꢀ)-5
molecules. Classical H-bonding dimensions (cf. Table 2) are all well
defined and comply with expectations. The H-bonding pattern re-
tains the well-known internal H-bridge structure of the resolving
tool, providing a single-donor function only for H-bridging from
each spiro-TADDOL [(ꢀ)-5] host molecules. The double acceptor role
Table 1
Resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3) with TADDOL derivatives [(ꢀ)-4 and (ꢀ)-5].
Entry
Resolving agent
Eq.
Solventsa,b
Diastereomeric complexc
Yield (%)d,g
ee (%)e,g
S (ꢀ)f,g
Abs. config.h
1
2
3
4
5
6
TADDOL
TADDOL
spiro-TADDOL
spiro-TADDOL
spiro-TADDOL
spiro-TADDOL
0.5
0.5
0.5
1
1
0.5
2 ꢂ EtOAc/10 ꢂ hexane
6 ꢂ iPrOH
(3) (TADDOL)
(3) (TADDOL)
(3) (spiro-TADDOL)
(3) (spiro-TADDOL)2
(3) (spiro-TADDOL)2
(3) (spiro-TADDOL)
(98) 81
(81) 2*
(90) 60
(94) 60
(88) 50
(84) 48
(2) 6
(2) 5*
(35) 64
(15) 57
(29) 95
(38) 89
(0.02) 0.05
(0.02) 0.00*
(0.31) 0.39
(0.14) 0.34
(0.26) 0.47
(0.32) 0.43
(R)
(R)
(R)
(R)
(S)
(S)
4 ꢂ EtOAc/10 ꢂ hexane
6 ꢂ MeOH
6 ꢂ EtOH
6 ꢂ iPrOH
*Diastereomeric complex was purified with one recrystallization.
a
Mixture of solvents for the crystallization [mL of solvent/g of resolving agent].
Mixture of solvents for the recrystallization(s) [mL of solvent/g of resolving agent].
The ratio of 3 and (ꢀ)-4 or (ꢀ)-5 was determined by 1H NMR.
b
c
d
e
f
Based on the half of the racemate 3 that is regarded to be 100% for each antipode.
Determined by chiral GC.
Resolving capability, also known as the Fogassy parameter (S ¼ Yield ꢂ ee) [50].
g
h
Results obtained after the first crystallization are shown in “( )”, while results obtained after two recrystallizations are shown in boldface.
The absolute configuration of 3 was determined by X-Ray analysis and CD spectroscopy.

P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
143
Scheme 3.
Fig. 1. Atomic displacement representation (50% probability) of the diastereomeric complex incorporating (ꢀ)-3 and (ꢀ)-5 in a ratio of 1:2 in the asymmetric unit with atomic
numbering and indication of the “classical” OeH…O type H-bridges (in broken lines).

144
P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
Table 2
are shown in Fig. 2. The “hot spots” can be seen well, where in-
teractions of the given kind are the most concentrated.
It is clear that the dominant hydrophilic interactions roughly
match the observed “hot spots” in Fig. 2. A further support for the
tight encapsulation of (ꢀ)-3 comes from the UNI intermolecular
potential force filed (FF) type calculation [52,53].
Hydrogen Bonds (Å, ꢁ) for (ꢀ)-1-isopropyl-3-methyl-3-phospholene oxide [(ꢀ)-3].
The rows beneath each contact (marked “Normalized”) indicate geometry after
alteration the DeH bond lengths to generally accepted standard distances as shown.
2A) Classical OeH…O H-bridges; 2B) CeH…contact distances.
DdH…A
DdH (Å) H…A (Å)
D…A (Å)
DdH…A (ꢁ) Ser. No.
2A)
The graphical view (Fig. 3) of the results underlines that the two
strongest interactions are between (ꢀ)-3 and each of the inde-
pendent (ꢀ)-5 molecules in the asymmetric unit, with ꢀ50.7
and ꢀ48.4 kJ/mol. Inter-fragment energies are [(ꢀ)-3 and
(ꢀ)-5I]: ꢀ50.7 kJ/mol, [(ꢀ)-3 and (ꢀ)-5II]: ꢀ48.4 kJ/mol, [(ꢀ)-5I and
(ꢀ)-5II]: ꢀ36.4 kJ/mol. A considerable portion (ꢀ135.5 kJ/mol) of
the total of ꢀ484 kJ/mol packing energy is thus in the interactions
between these three molecules of the asymmetric unit.
Subsequently, semi-empirical series of calculations were also used
to assess feasibility of the FF results. These were done by using
modified PM6 [54] Hamiltonians suggested by Korth (PM6-DHþ)
[55] and as implemented in MOPAC012 [56,57]. The aim was to
derive and estimate independently the binding energy of (ꢀ)-3 to
the two molecules of (ꢀ)-5 as in the lattice of the associate crystal.
The proposed strategy for this goal is to use the
PM6-DHþ procedure to optimize the geometry of each component
separately and also to obtain heats of formations for each of them.
As FF calculations indicated essential attraction between the two
independent (ꢀ)-5 molecules a change was made such that the
geometries were optimized and heat of formation was calculated
for this assembly of the two spiro-TADDOL molecules. Then the
content of the asymmetric unit is optimized and the heats of for-
mation are obtained. The binding (or intermolecular interaction)
O2 – H200 .. O1
Normalized
O3 – H300 .. O2
Normalized
O6 – H600 .. O7
Normalized
O7 – H700 .. O1
Normalized
2B)
C5 – H5C .. O6i
Normalized
C17 – H17 .. O2
e
0.89(3)
0.98
0.86(3)
0.98
0.85(3)
0.98
0.77(3)
0.98
1.85(3)
1.75
1.85(3)
1.73
1.82(3)
1.68
1.90(3)
1.69
2.727(2)
2.727(2)
2.705(3)
2.705(3)
2.666(3)
2.666(3)
2.671(2)
2.671(2)
172(3)
172
171(2)
170
175(3)
175
176(4)
175
1
2
3
4
0.99
1.08
0.99
e
2.50
2.44
2.43
e
3.307(3)
3.307(3)
2.791(3)
e
138
136
100
e
1
2
C21 – H21 .. O4
Normalized
C26 – H26 .. O5
Normalized
C30 – H30 .. O3
e
0.96
1.08
0.99
1.08
0.99
e
2.31
2.23
2.55
2.52
2.42
e
2.967(3)
2.967(3)
3.026(3)
3.026(3)
2.792(3)
e
126
123
109
107
101
e
3
4
5
C32 – H32 .. O5
Normalized
C49 – H49 .. O8
Normalized
C51 – H51 .. O7
e
0.96
1.08
0.96
1.08
0.96
e
2.24
2.18
2.51
2.46
2.38
e
2.872(3)
2.872(3)
3.031(3)
3.031(3)
2.729(3)
e
122
120
114
111
101
e
6
7
8
C64 – H64 .. O9
Normalized
C66 – H66 .. O6
e
1.03
1.08
0.95
e
2.33
2.31
2.41
e
2.935(3)
2.935(3)
2.768(3)
e
116
115
102
e
9
10
11
energy is then given by
D
HB
¼
D
HF[(ꢀ)-3 - (ꢀ)-5I - (ꢀ)-5II] -
D
HF[(ꢀ)-3] e HF[(ꢀ)-5I/(ꢀ)-5II]. Heats of formations components
D
C70 – H70 .. O9
Normalized
1.00
1.08
2.58
2.57
2.948(3)
2.948(3)
101
100
were ꢀ1596, ꢀ409, ꢀ1062 kJ/mol, for [(ꢀ)-3 - (ꢀ)-5I - (ꢀ)-5II],
[(ꢀ)-3], and for the [(ꢀ)-5I/(ꢀ)-5II] assembly. The resulting binding
Transformation applied: i ¼ 1 þ x,y,z.
of the 3-phospholene oxide [(ꢀ)-3] ideally uses both lone pairs of
the O atom. Nevertheless, it is not the classical H-bridge scheme that
delivers surprise. The (ꢀ)-3 guest molecule boasts with an unusually
large number (11) of short CeH…O contacts to the two [(ꢀ)-5]
molecules of the stoichiometric entity (the crystallographic asym-
metric unit). In other words, all but one of these contacts are of
intramolecular (correctly intra-associate) type, 10 out of 11. A closer
scrutiny is thus warranted. The first step to go ahead is to
“normalize” the CeH bond lengths to the “industry - standard”
1.08 Å internuclear distance. For comparison Table 2A lists these
for the classical H-bridges, too, while Table 2B for the less trivial
CeH…O ones as well. The emerging pattern is also interesting, as
almost half of the non-trivial contacts are then eliminated, 5 or 6 out
of 11. These fail to qualify as H-bridge as their DdH…A angles got
under 100ꢁ. This is not so surprising, as the classical H-bridges are
nearly linear, so the CeH bond increasing will only improve on their
geometry. In contrast, the CeH…O contacts are more bent, and in
cases where the control angle gets under 100ꢁ the analysing pro-
gram drops them from the list.
Nevertheless, this does not mean that there cannot be other,
non-H-bridge interactions between such entities. There are, for
example, indications that some CeH…Caryl type of short contacts
exist. We could just stop here and conclude that the (ꢀ)-3 guest is
so tightly bound to its two (ꢀ)-5 molecules in their lap that the host
conformations may not permit inclusion of a second (ꢀ)-3 mole-
cule. A full interaction mapping was attempted by the aid of
program Mercury [51]. The chemical functions defined for the
mapping were the OH, aryleCH, and methyl groups. These were
used as target interactions in the mapping program, and the results
Fig. 2. A full interaction mapping [51] of the asymmetric unit containing (ꢀ)-3 and
(ꢀ)-5 in a ratio of 1:2 in the crystal with (ꢀ)-3 in space filling representation. Light
contours correspond to at least two, while darker more than six contributors. Numbers
represent “hot spots” i.e. the maximum interaction directions.

P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
145
Fig. 3. UNI intermolecular potential calculation results [52,53] as implemented in Mercury [51] show the two largest lattice contribution in red broken lines to molecule (ꢀ)-3 from
each independent (ꢀ)-5 molecule with ꢀ50.7 and ꢀ48.4 kJ/mol, respectively.
energy of ꢀ125 kJ/mol compares well with those obtained from the
FF computations. During this procedure basic geometry features,
such as conservation of H-bridges, etc., were checked and all major
features were retained in the resulting models. All models
converged using default gradients for optimization. A modest in-
[(þ)-3] fell in the range of 36e48% after one recrystallization, and
the corresponding diastereomers were obtained in low yields
meaning resolving capability values of 0.02e0.03 (Table 3, Entries
1e3).
When Ca(H-DPTTA)2 [(ꢀ)-7] was applied as the resolving
agent, the enantiomeric excess of phospholene oxide 3 was in the
range of 85e92% (Table 3, Entries 4e6). Despite the promising ee
values, the yield of the corresponding diastereomers remained
low in all instances, when Ca(H-DPTTA)2 [(ꢀ)-7] was applied and
the corresponding resolving capability values fell in the range of
0.01e0.09 (Table 3, Entries 4e6). It is noteworthy that the solvent
influenced which isopropyl-phospholene oxide antipode (3) was
incorporated into the corresponding diastereomer. Applying
Ca(H-DPTTA)2 [(ꢀ)-7], the (ꢀ)-(S)-1-isopropyl-3-phospholene
oxide [(ꢀ)-3] could be prepared in a mixture of ethanol and
water or in a mixture of ethanol, ethyl acetate and water (Table 3,
Entries 4 and 5), while the (þ)-(R)-1-isopropyl-3-phospholene
ternal calibration was also provided as
D
HF[(ꢀ)-5I] and HF[(ꢀ)-5II]
D
may be expected to be identical. These were ꢀ480 and ꢀ488 kJ/mol
[(ꢀ)-5I] and [(ꢀ)-5II], respectively. The less than 2% difference
between the
DHF values would probably be a too optimistic
estimate of the uncertainty. Nevertheless, the important message is
that both approaches confirm and similarly quantify the energy
background of the geometry conditions of binding in the crystal
structure, in spite of the more or less differing starting geometries.
Besides the TADDOL-derivatives [(ꢀ)-4 and (ꢀ)-5], the resolution
of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3) was also
attempted with the acidic Ca2þ salts of (ꢀ)-O,O0-dibenzoyl- or
(ꢀ)-O,O0-di-p-toluoyl-(2R,3R)-tartaric acid [Ca(H-DBTA)2 (ꢀ)-6 or
Ca(H-DPTTA)2 (ꢀ)-7]. The Ca(H-DBTA)2 [(ꢀ)-6] was prepared in
advance as described earlier [38], whereas the Ca(H-DPTTA)2 [(ꢀ)-7]
was always formed in situ in the reaction of (ꢀ)-O,O0-di-p-toluoyl-
(2R,3R)-tartaric acid and CaO in the mixture of ethanol and water.
During the resolution experiments, the given solution of the
isopropyl-phospholene oxide (3) was added to the hot ethanolic
solution of Ca(H-DBTA)2 (ꢀ)-6 or Ca(H-DPTTA)2 (ꢀ)-7. The
diastereomers appeared after cooling down the reaction mixture
gradually to 26 ꢁC (Scheme 4). The crystals were filtered off after
24 h of crystallization and were purified further by digestion (i.e. by
stirring the diastereomers in the corresponding solvent or solvent
mixture at 26 ꢁC). The composition of the diastereomers was
determined by 1H NMR spectroscopy. To obtain the enantiomeric
mixture of isopropyl-phospholene oxide (3), the crystalline
diastereomer was treated with 10% aqueous ammonia and the
solution was extracted with dichloromethane. The enantiomeric
excess of phospholene oxide 3 was determined by chiral GC. The
results are summarized in Table 3.
oxide [(þ)-3] could be obtained in
a mixture of ethanol,
acetonitrile and water (Table 3, Entry 6).
The results shown in Table 3 indicated that the overall efficiency
of resolutions with Ca(H-DBTA)2 (ꢀ)-6 or Ca(H-DPTTA)2 (ꢀ)-7 was
considerably lower than those obtained with spiro-TADDOL (ꢀ)-5
which agent was proved to be the best for the enantiomeric
separation of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3) in
our study.
2.2. CD spectra of the optically active 1-isopropyl-3-methyl-3-
phospholene 1-oxide (3)
The absolute configuration of the title compound (3) was
determined by combined spectroscopic and theoretical in-
vestigations. To compute the spectra of the title compound, fore-
most, a molecular mechanical (MM) conformation analysis was
performed for the molecule, and 12 stable conformers were found,
which lie at most at 10 kJ/mol above the lowest energy one. For
each of these conformers, the geometry was further optimized at
the density functional theory (DFT) level, and accurate
Using Ca(H-DBTA)2 [(ꢀ)-6] as the resolving agent, the enantio-
meric excess of the (þ)-(R)-1-isopropyl-3-phospholene oxide

146
P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
Scheme 4.
Table 3
Resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3) with 0.25 equivalent of Ca(H-DBTA)2 [(ꢀ)-6] or Ca(H-DPTTA)2 [(ꢀ)-7].
Entry Resolving agent Eq.
Solvents
Diastereomeric complexc Yield (%)d,g ee (%)e,g S (ꢀ)f,g
Abs. config.h
1
2
3
4
5
6
Ca(H-DBTA)2
Ca(H-DBTA)2
Ca(H-DBTA)2
Ca(H-DPTTA)2
Ca(H-DPTTA)2
Ca(H-DPTTA)2
0.25 6 ꢂ EtOHa,b
Ca(3)2(H-DBTA)2
Ca(3)2(H-DBTA)2
Ca(3)2(H-DBTA)2
Ca(3)2(H-DPTTA)2
(33) 8*
(15) 4*
(49) 5*
(50) 1
(35) 43* (0.11) 0.03* (R)
(24) 36* (0.04) 0.02* (R)
(44) 48* (0.22) 0.02* (R)
0.25 3 ꢂ EtOH/3 ꢂ EtOAca1 ꢂ EtOH/1 ꢂ EtOAcb
0.25 3 ꢂ EtOH/3 ꢂ MeCNa,b
0.25 6 ꢂ EtOH/5% H2Oa,b
(4) 90
(0.02) 0.01
(0.21) 0.09
(0.18) 0.02
(S)
(S)
(R)
0.25 3 ꢂ EtOH/3 ꢂ EtOAc/10% H2Oa1 ꢂ EtOH/1 ꢂ EtOAcb Ca(3)2(H-DPTTA)2
(45) 10
(44) 2
(46) 92
(41) 85
0.25 3 ꢂ EtOH/3 ꢂ MeCN/10% H2Oa
Ca(3)2(H-DPTTA)2
1 ꢂ EtOH/1 ꢂ MeCNb
*Diastereomeric complex was purified with one recrystallization.
a
Mixture of solvents for the crystallization [mL of solvent/g of resolving agent].
b
Mixture of solvents for the recrystallization(s) [mL of solvent/g of resolving agent].
c
The ratio of 3 and (ꢀ)-4 or (ꢀ)-5 was determined by 1H NMR.
d
Based on the half of the racemate 3 that is regarded to be 100% for each antipode.
e
Determined by chiral GC.
f
Resolving capability, also known as the Fogassy parameter (S ¼ Yield ꢂ ee) [50].
g
Results obtained after the first crystallization are shown in “( )”, while results obtained after two recrystallizations are shown in boldface.
The absolute configuration of 3 was determined by X-Ray analysis and CD spectroscopy.
h
conformational energies were computed using the direct random
oxides [3 and (S)-3] were deoxygenated with trichlorosilane in
separate reactions to obtain the corresponding racemic and
optically active isopropyl-phospholenes [8 and (R)-8] (Schemes 5
and 6). Deoxygenations with trichlorosilane are known to proceed
with retention of configuration of the P-stereogenic center in case
of optically active phosphine oxides [58].
The racemic and optically active 3-phospholenes [8 and (R)-8]
so obtained were immediately reacted with dimethyl sulfide-
borane to furnish the corresponding isopropyl-phospholene-
boranes [9 and (R)-9] (Schemes 5 and 6). Beside the synthetic in-
terest, the importance of phosphine boranes is demonstrated by
phase approximation (dRPA) approach. Three low-energy con-
formers were found differing in the orientation of the isopropyl
group, two of them are practically isoenergetic, whereas the third
one lies at around 1.3 kJ/mol below the two others. The optimized
structure and geometrical parameters for the most stable
conformer are presented in Fig. 4.
The excitation energies, as well as oscillator and rotator
strengths for all the three conformers were computed at the time-
dependent DFT (TD-DFT) level. The theoretical UV absorption and
CD curves for each conformer were obtained as superpositions of
Gaussian functions placed at the wavelengths of the computed
transitions with heights proportional to the corresponding calcu-
lated oscillator (rotator) strengths. The spectra of the conformers
were Boltzmann-weighted. The averaged spectra were normalized
so that the height of the highest peak was identical to that of the
experimental spectra and were not shifted. The theoretical and
experimental spectra are presented in Fig. 5.
Considering the similarity of the measured and calculated ab-
sorption spectra of compound 3, we can conclude that the selected
theoretical methods are adequate. The structure of the measured
and calculated CD spectra is rather similar. The signs of the domi-
nant peaks in the two spectra are identical, and their positions are
close to each other. Consequently, the absolute configuration of the
enantiomer obtained by the resolution (Table 1, Entry 5) is also (S),
like the configuration of the enantiomer considered in the
calculations.
Fig. 4. DFT-optimized geometry for the most stable conformer of phospholene oxide 3
with the randomly selected (S) configuration. Characteristic bond lengths (in Å) and
angles (in degree) are as follows: P1eO1 1.492, P1eC1 1.838, C1eC2 1.503, C2eC3
1.490, C2eC4 1.335, C4eC5 1.496, C5eP1 1.839, P1eC6 1.835, C6eC7 1.525, C6eC8
1.525; O1eP1eC6 113, C1eP1eC5 95, C1eP1eC6 106, O1eP1eC1 117, O1eP1eC5 117,
C5eP1eC6 106, C7eC6eC8 111, C1eC2eC4 116, C4eC5eP1 104; O1eP1eC6eC7 61,
O1eP1eC6eC8 -61, P1eC1eC2eC4 -9, C3eC2eC4eC5 179.
2.3. Synthesis of 1-isopropyl-3-methyl-3-phospholene borane and
platinum complexes (9 and 10)
First, the racemic and optically active isopropyl-phospholene

P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
147
Fig. 5. The calculated (dashed line) UV absorption (a) and CD (b) spectra of (S)-3 with the corresponding measured spectra (solid line).
the fact that they may be regarded as protected phosphines, from
which the corresponding P(III)-compounds can be liberated by
treatment with secondary amines (e.g. diethylamine) [59].
(10) was calculated at the
level of theory with cc-pVTZ-PP pseudopotential on the Pt atom.
Accurate B97X-D single point energy calculations suggested that
uB97X-D/cc-pVTZ//RI-B97-D/6-31G(d)
u
The racemic and optically active isopropyl-phospholenes [8 and
(R)-8] were also converted to the corresponding platinum complexes
[cis-10 and cis-(S,S)-10] by reaction with dichlorodibenzonitrile
platinum. Using racemic isopropyl-phospholene (8) as the starting
material, the given platinum complex was obtained as a 1:2 mixture
of the homo- [cis-(R,R)- and cis-(S,S)-10] and the heterochiral isomers
[cis-(R,S)-10]. At the same time, application of the (R)-1-isopropyl-3-
phospholene [(R)-8] afforded the platinum complex exclusively in
the corresponding homochiral form [cis-(S,S)-10] (Schemes 5 and 6).
The borane and platinum complexes [9 and 10] of i-propyl-3-
phospholene were characterized by 1H, 13C and 31P NMR
spectroscopy, as well as with HRMS. Due to the highly congested
spectra of platinum complexes [10 and (S,S)-10], 2D 1He1H COSY,
1He13C HSQC and HMBC experiments were also performed to
verify the structure. According to expectations, the racemic and
optically active platinum complexes [10 and (S,S)-10] contained the
3-phospholene [8 and (R)-8] ligands in cis position that was
confirmed by the stereospecific 1JPt-P coupling constants of 3489 Hz
measured for both [10] and [(S,S)-10]. It is worth noting that the
coupling constants of ca. 3500 Hz are of diagnostic value, and are
characteristic for those coordinated phosphorus donor atoms
possessing the chloro ligand in the trans position. The 13C NMR
spectra of the platinum complexes [10 and (S,S)-10] were rather
complicated because many signals were split due to 195Pte13C and
31Pe13C couplings, respectively. The corresponding coupling
constants were determined by a first order analysis.
the relative energies of the corresponding homochiral [(R,R) and
(S,S)] and heterochiral (R,S) isopropyl-phospholene - platinum
complexes (10) are close to each other within 0.6 kcal/mol. The most
stable structure of the synthesized cis-bis((S)-1-isopropyl-3-methyl-
3-phospholeno)-dichloro-platinum(II) [(S,S)-10] is displayed in Fig. 6.
We found that the conformer with rotational symmetry (C2
symmetry group) is the most favourable structure which is stabilized
by intramolecular nonbonding interactions between the phospho-
lene ring and the corresponding alkyl group. The geometry around
the Pt atom is considered square planar, as the P(1)…Cl(1)…Cl(2)…
P(2) pseudo torsion angle is ꢀ3.9ꢁ.
The structure of the complexes is determined by nonbonding
interaction between the P-ligands. CH…HC interactions were
identified between the C(2)H2 moieties of the two phospholene
rings (the shortest H…H distance is 2.29 Å) and between the C(2)H2
unit of the phospholene ring and the methyl group of the isopropyl
substituent of the neighbouring phospholene moiety (the shortest
H…H distance is 2.22 Å, see Fig. 6). These secondary interactions
also define the environment of the platinum influencing the
properties of the catalyst. In case of (S,S)-10, one methyl group of
each isopropyl substituent is situated, due to the C2 symmetry,
close to the platinum atom, and it significantly influences the
environment of the platinum that is shown by the H…Pt…Cl(2)…
Cl(1) dihedral angle of 96.1ꢁ and the H2CH…Pt distance of 3.04 Å
(where H is the corresponding proton of one methyl group of the
isopropyl substituent, See Fig. 6.)
Stereostructure of the isopropyl-phospholene -platinum complex
Scheme 5.

148
P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
Scheme 6.
2.4. Catalytic activity of the 1-isopropyl-3-methyl-3-phospholene -
platinum complex in the hydroformylation of styrene
catalytic activity was rather low.
The formation of the aldehydes (A and B) was preferred in all
cases and chemoselectivities up to 93% were obtained. Slight in-
crease in aldehyde selectivity was observed by increasing the re-
action temperature (Table 4, Entries 1e3).
As regards the regioselectivity, the influence of the structure of
the catalyst is even more pronounced. The branched aldehyde (A)
predominated, when the ligand was used in racemic form, i.e., the
catalyst precursor was a mixture of homo- and heterochiral forms
((R,R)-10, (S,S)-10, (R,S)-10). A lower regioselectivity was observed,
when the ligand was used in enantiomerically pure form, i.e., the
The PtCl2L2-type complexes [cis-10; cis-(S,S)-10] were tested as
catalyst precursor in the hydroformylation of styrene. The catalysts
formed in situ from cis-10 (or cis-(S,S)-10) and two equivalents of
tin(II) chloride were used under standard ‘oxo-conditions’
[p(CO) ¼ p(H2) ¼ 40 bar, 100 ꢁC]. As expected in the hydro-
formylation of styrene, in addition to the two types of formyl
regioisomers, 2-arylpropanal (A) and 3-arylpropanal (B), ethyl-
benzene (C) as hydrogenation by-product was also formed (Eq. (1)).
catalyst precursor was
a homochiral complex ((S,S)-10). For
instance, 73% and 49% regioselectivities were obtained at 100 ꢁC,
when racemic and enantiomerically pure ligands were used,
respectively (Compare for instance Table 4 entries 3 and 6). A
CO=H2
PhCH ¼ CH2
PhCHðCHOÞCH þ PhCH CH CHO
ƒƒƒƒƒƒ!
3
2
2
B
A
(1)
þ PhCH2CH3
similar change in regioselectivity was observed at 40 ꢁC and 60 ꢁ
(Table 4, entries 1, 4 and 2, 5, respectively).
C
C
The catalytic system is active at a temperature as low as 40 ꢁC.
Considering the conversions obtained with similar catalysts but
containing P-alkyl-phospholenes and phospholanes [46], the
The explanation for the above phenomenon might be based on
the different structure and reactivity of the diastereomeric transition
states coordinating the substrate. The transition state complexes,
coordinating the prochiral alkene (styrene) either to (S,S)-10 or (S,R)-
10, reveal different reactivity towards alkene migratory insertion.
Consequently, the ratio of the platinum(II)-branched alkyl interme-
diate to the corresponding linear alkyl intermediate might be
different in case of homochiral and heterochiral complexes. If we
suppose that the stereochemical outcome of the reaction is decided
in this step (and not in the following CO-insertion step), the unex-
pectedly large difference in regioselectivities can be rationalized.
Enantioselective hydroformylation only with low ee-s could be
performed when the complex (S,S)-10 was used. The temperature-
dependence of the ee-s is not pronounced.
Table 4
Hydroformylation of styrene in the presence of in situ formed catalysts from cis-
PtCl2(L)2 complex [cis-10; cis-(S,S)-10] and tin(II) chloride.a
Entry
L
Temp. (ꢁC) R. Time (h) Conv. (%) Rcb (%) Rbrc (%) e.e.d (%)
1
2
3
4
5
6
8
8
8
40
60
100
40
96
72
24
96
72
24
28
30
65
24
27
85
73
87
91
84
90
93
89
77
73
69
52
49
e
e
e
Fig. 6. Perspective view of cis-bis((S)-1-isopropyl-3-methyl-3-phospholeno)-dichloro-
(R)-8
(R)-8
(R)-8 100
7 (S)
10 (S)
10 (S)
platinum(II) [(S,S)-10] calculated by
uB97X-D/cc-pVTZ//RI-B97-D/6-31G(d) method.
60
For Pt atoms, cc-pVTZ-PP pseudopotential was applied in all cases. Grey, white grey,
orange, green and white colours are referred to carbon, hydrogen, phosphorus, chlo-
rine and platinum atoms, respectively. Selected bond lengths (Å) and angles (ꢁ) are as
follows: PteCl(1) 2.402, PteP(1) 2.233, P(1)-C2 1.875, C2eC3 1.520, C3eC4 1.348,
C4eC5 1.510, P(1)-C5 1.871, P(1)-C10 1.866, C10-C20 1.540; Cl(1)-Pt-Cl(2) 90.7, Cl(1)-Pt-
P(1) 84.6, PteP(1)-C2 121.7, PteP(1)-C5 115.6, PteP(1)-C10 112.0, P(1)-C2-C3 105.3,
C2eC3eC4 116.3, C3eC4eC5 118.7, C2eP(1)-C5 94.2, C2eP(1)-C10 105.3, C5eP(1)-C10
105.8, C20-C10-C30 112.4; P(1)…Cl(1)…Cl(2)…P(2) ꢀ3.9, Cl(1)-Pt-P(1)-C2 161.1, PteP(1)-
C10-C30 ꢀ55.6, PteP(1)-C2-C3 e132.4, P(1)-C2-C3-C4 6.9, P(1)-C2-C3-CH3 e174.4,
C2eC3eC4eC5 e0.5.
A: 2-phenylpropanal, B: 3-phenylpropanal, C: ethylbenzene.
a
Reaction conditions: Pt/styrene ¼ 1/100, Pt/SnCl2 ¼ 1/2; p(CO) ¼ p(H2) ¼ 40 bar,
1 mmol of styrene, solvent: 10 mL of toluene.
b
Chemoselectivity towards aldehydes (A, B). [(moles of A þ moles of B)/(moles of
A þ moles of B þ moles of C) ꢂ 100].
c
Regioselectivity towards branched aldehyde (A). [moles of A/(moles of A þ moles
of B) ꢂ 100].
d
Determined by chiral GC (See Experimental).

P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
149
3. Conclusions
(159.7 mmol) of isopropyl bromide) at 0 ꢁC, and the mixture was
stirred overnight. The mixture was then quenched with a 3 M HCl at
The family of optically active 3-methyl-3-phospholene oxides
was expanded by the enantiomers of 1-isopropyl derivative. The
optical resolution of the racemate was performed in different ways
applying either TADDOL derivatives, or the acidic Ca2þ salts of
(ꢀ)-O,O0-diaroyl-(2R,3R)-tartaric acids. In one case, the diaste-
reomer associate could be analysed by X-ray chrystallography. The
stochiometry of the associate and the stabilizing intermolecular
contacts between the guest and the two host molecules were
evaluated. The absolute configuration of the P-stereogenic center
was also confirmed by CD spectroscopy and quantum chemical
calculations. The racemic and optically active P-ligands obtained
after deoxygenation were converted to the corresponding borane
and platinum(II) complexes. Stereostructure of the latter PtL2Cl2
0
ꢁC. The two phases were separated, and the organic layer was
washed with NaHCO3, brine and then dried (Na2SO4). After
evaporating the solvent, the residue so obtained was purified by
column chromatography (silica gel, 3% methanol in dichloro-
methane) to give 8.2
phospholene 1-oxide (3) as a dense oil.
1H NMR (CDCl3)
g (49%) of 1-isopropyl-3-methyl-3-
d
1.20 (dd, J ¼ 2.2, 7.2, 3H) and 1.23 (dd, J ¼ 2.2,
7.1, 3H) CH(CH3)2, 1.79 (bs, 3H, C3eCH3), 1.95e2.05 (m, 1H,
CH(CH3)2), 2.38e2.52 (m, 4H, CH2PCH2), 5.49 (m, 1H, CH ¼ ); 13C
NMR (CDCl3)
d
15.5 and 15.6 (CH(CH3)2), 20.3 (3JP-C ¼ 10, C3eCH3),
28.4 (1JP-C ¼ 64, C1 ), 30.4 ( JP-C ¼ 61, C5), 33.5 (1JP-C ¼ 64, C2), 121.2
1
0
(2JP-C ¼ 7, C4), 137.1 (2JP-C ¼ 12, C3); 31P NMR (CDCl3)
d 76.0; HRMS
½M þ Hꢃþfound ¼ 159.0934, C8H16OP requires 158.0861.
species (where
L
¼
isopropyl-3-methyl-3-phospholene) was
evaluated by high level calculations and they were tested as catalyst
in the hydroformylation of styrene.
4.3. Resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3)
with TADDOL [(ꢀ)-4] (Representative Procedure I.)
0.15
g (0.95 mmol) of racemic 1-isopropyl-3-methyl-3-
4. Experimental
phospholene 1-oxide (3) and 0.22 g (0.48 mmol) of TADDOL
[(ꢀ)-4] was dissolved in 0.44 mL of hot ethyl acetate, and then
2.2 mL of hexane was added. Colourless crystalline diastereomeric
complex (R)-3∙(TADDOL) appeared immediately. After standing at
26 ꢁC for 3 h, the crystals were separated by filtration to give 0.29 g
(98%) of (R)-3∙(TADDOL) with a de of 2%. The diastereomeric
complex (R)-3∙(TADDOL) was purified further by two re-
crystallizations from a mixture of 0.44 mL of ethyl acetate and
2.2 mL of hexane to afford 0.25 g (84%) of the complex (R)-
3∙(TADDOL) with a de of 6%. The (R)-1-isopropyl-3-methyl-3-
phospholene 1-oxide [(R)-3] was recovered from the diaste-
reomer by column chromatography (silica gel, dichloromethane-
methanol 97:3) to give 0.061 g (81%) of phospholene oxide (R)-3
with an ee of 6% (Table 1, Entry 1). Resolution of 1-isopropyl-3-
methyl-3-phospholene 1-oxide (3) with TADDOL [(ꢀ)-4] was also
performed using isopropyl alcohol. In this instance, the racemic
isopropyl-phospholene oxide 3 and the TADDOL [(ꢀ)-4] were dis-
solved in hot isopropyl alcohol, and the corresponding diastereo-
meric complex [(R)-3∙(TADDOL)] precipitated by cooling the
mixture to 26 ꢁC (Table 1, Entry 2).
4.1. General (instruments)
The 31P NMR spectra were recorded on a Bruker AV-300 spec-
trometer operating at 121.5 MHz, while the 13C and 1H NMR spectra
were obtained on a Bruker DRX-500 spectrometer operating at
125.7 and 500 MHz, respectively. 2D 1He1H COSY, 1He13C HSQC
and HMBC spectra of platinum complexes [10 and (S,S)-10] were
acquired on a Bruker Avance II spectrometer operating at 500 MHz
1H frequency equipped with a BBI or a TXI z-gradient probe. The
couplings are given in Hz. The exact mass measurements were
performed using a Q-TOF Premier mass spectrometer in positive
electrospray mode. The enantiomeric excess (ee) values of the
phospholene oxide 3 were determined by chiral GC on Agilent
4890D instrument equipped with a GAMMA DEX™ 120 column
(30 m ꢂ 0.25 mm, 0.25
mm film, FID detector, nitrogen as carrier gas,
injector 240 ꢁC, detector 300 ꢁC, head pressure: 10 psi, at 1:100 split
ratio). Retention times of 3 by chiral GC (program: 40 min at
150 ꢁC): 30.2 min for (R)-3 and 30.9 min for (S)-3. The determi-
nation of the ee-s of 2-phenylpropanal (A) was carried out on
Thermo Scientific FOCUS gas-chromatograph equipped with a
Cyclodex column (20 m ꢂ 0.25 mm, 0.25
mm film, FID detector,
4.4. Resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3)
with spiro-TADDOL [(ꢀ)-5]
helium as carrier gas, injector 250 ꢁC, detector 280 ꢁC, head pres-
sure: 14.5 psi). (S)-2-Phenylpropanal was eluted before the (R)-
enantiomer. Optical rotations were determined on a PerkineElmer
241 polarimeter.
The resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide
(3) with spiro-TADDOL [(ꢀ)-5] was carried out according to
Representative Procedure I. The conditions and the results are shown
in Table 1, Entries 3e6.
The
(ꢀ)-(4R,5R)-4,5-bis(diphenylhydroxymethyl)-2,2-
dimethyldioxolane [(ꢀ)-4], the (ꢀ)-(2R,3R)-
a,a, ,a
a0 0-tetraphenyl-
1,4-dioxaspiro[4.5]decan-2,3-dimethanol [(ꢀ)-5] and the calcium
hydrogen (ꢀ)-O,O0-dibezoyl-(2R,3R)-tartrate [(ꢀ)-6] were synthe-
sized as described earlier. (ꢀ)-O,O0-Dibezoyl- and (ꢀ)-O,O0-di-p-
toluoyl-(2R,3R)-tartaric acid were purchased from Aldrich Chemi-
cal Co.
4.5. Preparation of both enantiomers of 1-isopropyl-3-methyl-3-
phospholene 1-oxide [(S)- and (R)-3] with spiro-TADDOL [(ꢀ)-5]
The (S)-1-isopropyl-3-methyl-3-phospholene 1-oxide [(S)-3]
was obtained by the resolution of 0.71 g (4.5 mmol) of racemic 3
with 2.3 g (4.5 mmol) of spiro-TADDOL [(ꢀ)-5] in 13.6 mL of ethanol
according to Representative Procedure I. The diastereomeric
complex [(S)-3∙(spiro-TADDOL)2] was purified by three re-
crystallizations in 13.6 mL of ethanol. The (S)-1-isopropyl-3-
methyl-3-phospholene 1-oxide [(S)-3] was recovered by column
chromatography (silica gel, dichloromethane-methanol 97:3) to
give 0.16 g (45%) of (S)-3 in an ee of 97% [½aꢃ2D5 ¼ ꢀ12.8 (c 0.8,
CHCl3)]. The mother liquors of the crystallization and re-
crystallizations were combined, and the solvent was evaporated to
afford 1.71 g (150%) of a white powder as a 1:0.69 mixture of (R)-3
4.2. Preparation of 1-isopropyl-3-methyl-3-phospholene 1-oxide
(3)
To 14.1 g (106.4 mmol) of 1-hydroxy-3-phospholene 1-oxide (1)
in 60 mL of chloroform was added 10.1 mL (138.4 mmol) of thionyl
chloride and the solution was stirred overnight. The volatile com-
ponents were removed in vacuo, and the residue was dissolved in
70 mL of THF. To the solution so obtained was added dropwise
159.7 mmol of isopropylmagnesium bromide in 90 mL of THF
(prepared from 4.1 g (167.7 mmol) of magnesium and 15 mL

150
P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
with an ee of 23% and spiro-TADDOL [(ꢀ)-5]. 0.91 g (1.8 mmol) of
spiro-TADDOL [(ꢀ)-5] was added to this mixture and the resolution
was performed in 11.5 mL of methanol according to Representative
Procedure I. The [(R)-3∙(spiro-TADDOL)2] complex was purified by
two recrystallizations in 11.5 mL of methanol and it was decom-
posed by column chromatography (silica gel, dichloromethane-
methanol 97:3) to give 0.14 g (40%) of (R)-1-isopropyl-3-methyl-
3-phospholene 1-oxide [(R)-3] in an ee of 87%.
4.8. Preparation of 1-isopropyl-3-methyl-3-phospholene-borane
(9)
The solution of 0.11 g (0.68 mmol) of racemic 1-isopropyl-3-
methyl-3-phospholene 1-oxide (3) in
2 mL of toluene was
degassed and cooled to 0 ꢁC, then 0.56 mL (4.1 mmol) of tri-
chlorosilane 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
phospholene (8) that was immediately reacted further. 0.34 mL of
2 M dimethyl sulfide borane in tetrahydrofuran (0.68 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 0.041 g (38%) of 1-isopropyl-3-methyl-3-
4.6. Resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3)
with calcium hydrogen O,O0-dibenzoyl-(2R,3R)-tartrate [(ꢀ)-6]
(Representative Procedure II.)
To 0.14 g (0.17 mmol) of Ca(H-DBTA)2∙(H2O)2 [(ꢀ)-6∙(H2O)2] in
0.40 mL of hot ethanol was added 0.11 g (0.69 mmol) of racemic
1-isopropyl-3-methyl-3-phospholene 1-oxide (3) in 0.40 mL of
ethanol. After the addition, the solution was allowed to cool to
26 ꢁC, whereupon colourless crystals appeared. After standing
at 26 ꢁC for 24 h, the crystals were filtered off to give 0.60 g (33%) of
phospholene-borane (9). 1H NMR (CDCl3)
1.12 (d, J ¼ 7.1, 3H) and 1.18 (d, J ¼ 7.1, 3H) CH(CH3)2, 1.80 (bs, 3H,
d 0.35e1.04 (m, 3H, BH3),
d
C3eCH3), 1.84e1.97 (m, 1H, CH(CH3)2), 2.34e2.56 (m, 4H, CH2PCH2),
Ca[((R)-3)2(H-DBTA)2] with
a de of 35%. The diastereomeric
5.43 (m, 1H, CH ¼ ); 13C NMR (CDCl3)
d
16.6 and 16.7 CH(CH3)2, 19.1
0
complex was purified further by one digestion, by stirring the
suspension of the diastereomeric complex at 26 ꢁC in 0.80 mL of
ethanol for 24 h to give 0.022 g (12%) of Ca[((R)-3)2(H-DBTA)2]
with a de of 43%. The (R)-1-isopropyl-3-methyl-3-phospholene
1-oxide [(R)-3] was recovered from the diastereomeric complex
by treatment of the 2 mL dichloromethane suspension of Ca
[((R)-3)2(H-DBTA)2] with 2 mL of a 10% aqueous ammonia. The
organic layer was washed with 0.5 mL of water, dried (Na2SO4), and
concentrated to give 0.004 g (8%) of (R)-1-isopropyl-3-methyl-3-
phospholene 1-oxide [(R)-3] with an ee of 43% (Table 3, Entry 1).
Resolution of isopropyl-3-methyl-3-phospholene 1-oxide (3)
with Ca(H-DBTA)2 [(ꢀ)-6] was also performed in a mixture of
ethanol and ethyl acetate and that of ethanol and acetonitrile ac-
cording to Representative Procedure II. The conditions and the re-
sults are shown in Table 3, Entries 2 and 3.
1
(3JP-C ¼ 7, C3eCH3), 24.1 (1JP-C ¼ 31, C1 ), 28.4 ( JP-C ¼ 33, C5), 32.2
(1JP-C ¼ 35, C2), 122.2 (C4), 138.2 (2JP-C ¼ 3, C3); 31P NMR (CDCl3)
d
42.9 (broad); ½M þ Naꢃþfound ¼ 179.1137, C8H18PBNa requires
179.1137 for the 11B isotope.
The optically active (R)-1-isopropyl-3-methyl-3-phospholene-
borane [(R)-9] was prepared in a similar manner from (S)-1-
isopropyl-3-methyl-3-phospholene 1-oxide [(S)-3] with an ee of
97%. Yield of (R)-9: 40%; 1H NMR (CDCl3)
d
0.40e1.05 (m, 3H, BH3),
d
1.12 (d, J ¼ 7.0, 3H) and 1.18 (d, J ¼ 7.0, 3H) CH(CH3)2, 1.80 (bs, 3H,
C3eCH3),1.86e1.97 (m, 1H, CH(CH3)2), 2.32e2.55 (m, 4H, CH2PCH2),
5.43 (m, 1H, CH ¼ ); 13C NMR (CDCl3)
d
16.6 and 16.7 CH(CH3)2, 19.2
0
(3JP-C ¼ 7, C3eCH3), 24.0 (1JP-C ¼ 31, C1 ), 28.4 ( JP-C ¼ 33, C5), 32.1
1
(1JP-C ¼ 35, C2), 122.2 (C4), 138.2 (2JP-C ¼ 3, C3); 31P NMR (CDCl3)
d
43.2 (broad); [½aꢃ2D5 ¼ þ3.8 (c 1, CHCl3)].
4.9. Preparation of cis-[bis(1-isopropyl-3-methyl-3-phospholeno)-
dichloro-platinum(II)] (10)
4.7. Resolution of 1-isopropyl-3-methyl-3-phospholene 1-oxide (3)
with calcium hydrogen O,O0-di-p-toluoyl-(2R,3R)-tartrate [(ꢀ)-7]
(Representative Procedure III.)
The deoxygenation of 0.067 g (0.43 mmol) of racemic 1-
isopropyl-3-methyl-3-phospholene 1-oxide (3) was carried out in
benzene using 0.26 mL (2.6 mmol) of trichlorosilane according to
the procedure described in Section 4.8. Then, 0.10 g (0.21 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
0.10 g (87%) of 10 as a 2:1 mixture of the hetero- (R,S) and homo-
To 0.19 g (0.48 mmol) of DPTTA∙H2O in a mixture of 0.60 mL of
ethanol and 0.06 mL of water was added 0.014 g (0.24 mmol) of
CaO, and the mixture was heated at the boiling point until it
became clear. 0.15 g (0.97 mmol) of racemic 1-isopropyl-3-methyl-
3-phospholene 1-oxide (3) in 0.60 mL of ethyl acetate was
then added to the solution of the in situ formed resolving agent
Ca(H-DPTTA)2 [(ꢀ)-7]. After the addition, the solution was allowed
to cool down to 26 ꢁC, whereupon colourless crystals appeared.
After standing at 26 ꢁC for 24 h, the crystals were filtered off to give
0.12 g (45%) of Ca[((S)-3)2(H-DPTTA)2] with a de of 46%. The
diastereomeric complex was purified further by two digestions, by
stirring the suspension of the diastereomeric complex at 26 ꢁC for
24 h in a mixture of 0.60 mL of ethanol and 0.60 mL of ethyl acetate
to give 0.038 g (14%) Ca[((S)-3)2(H-DPTTA)2] with a de of 92%.
The (S)-1-isopropyl-3-methyl-3-phospholene 1-oxide [(S)-3]
was recovered from the corresponding diastereomeric complex
similarly to the procedure described in Section 4.6 to afford 0.008 g
(10%) of (S)-1-isopropyl-3-methyl-3-phospholene 1-oxide [(S)-3]
with an ee of 92% (Table 3, Entry 5).
chiral [(R,R) and (S,S)] forms. 1H NMR (CDCl3)
d 1.13e1.24 (m, 12H,
CH(CH3)2), 1.81 (bs, 6H, C3eCH3), 2.51e2.63 (m, 2H, CH(CH3)2),
2.74e3.12 (m, 8H, CH2PCH2), 5.47 (m, 2H, CH ¼ ); 13C NMR (CDCl3)
d
17.8e17.9 (m) and 18.1e18.3 (m) CH(CH3)2 of the homochiral
form, 17.9e18.1 (m, CH(CH3)2 of the heterochiral form), 18.8e18.9
0
(m, C3eCH3), 27.7e28.1 (m, C1 ), 30.5e31.2 (m, C5), 34.4e35.0 (m,
C2), 122.7 (3JPt-C ¼ cannot be observed, C4 of the homochiral form),
122.9 (3JPt-C ¼ 29, C4 of the heterochiral form),138.9e139.0 (m, C3 of
the heterochiral form), 139.2e139.3 (m, C3 of the homochiral form);
31P NþMR (CDCl3)
and 195Pt isotopes.
d
29.7 (1JPt-P
¼
3489); HRMS
½M ꢀ Clꢃfound ¼ 513.1146, C16H30P2ClPt requires 513.1138 for the 35Cl
The optically active cis-[bis-(S)- 1-isopropyl-3-methyl-3-
phospholeno)-dichloro-platinum(II)] [(S,S)-10] was prepared in a
similar manner from (S)-1-isopropyl-3-methyl-3-phospholene 1-
Resolution of isopropyl-3-methyl-3-phospholene 1-oxide (3)
with Ca(H-DPTTA)2 [(ꢀ)-7] was also performed in a mixture of
ethanol and water and that of ethanol, water and acetonitrile ac-
cording to Representative Procedure III. The conditions and the re-
sults are shown in Table 3, Entries 4 and 6.
oxide [(S)-3] with an ee of 97%. Yield of (R)-9: 75%. ½aꢃ2D5 ¼ þ18.8
(c 1, CHCl3); 1H NMR (CDCl3)
d
1.15 (dd, 3JH-H ¼ 7.1, 3JP-H ¼ 16.7, 6H,

P. Bagi et al. / Journal of Organometallic Chemistry 797 (2015) 140e152
151
CH(CH3)2), 1.18 (dd, 3JH-H ¼ 7.2, 3JP-H ¼ 17.2, 6H, CH(CH3)2), 1.79 (bs,
6H, C3eCH3), 2.48e2.58 (m, 2H, CH(CH3)2), 2.77e2.89 (m, 6H,
CH2PCH2), 2.95e3.10 (m, 2H, CH2PCH2), 5.44 (m, 2H, CH ¼ ); 13C
calculations of the absorption and CD spectra, a time-dependent
DFT (TD-DFT) [70] method was used with the same functional and
basis set. Rotator strengths were calculated in the velocity gauge. To
mimic the experimental conditions all the DFT calculations were
performed using the polarized continuum model (PCM) [71] with
acetonitrile as the solvent. The dRPA energies were computed with
the aug-cc-pVTZ basis set and PBE0 orbitals. To calculate the 298 K
Gibbs energies of the conformers in the solvent, the gas-phase dRPA
energies were adjusted by temperature corrections, entropy con-
tributions, and Gibbs energies of solvation evaluated at the DFT
level.
NMR (CDCl3)
d
17.8 (3J PteC ¼ 22) and 18.2 (3J PteC ¼ 20) CH(CH3)2,
3
5
18.8 (4JPt-C ¼ 33, JP-C ¼ 4, JP-C ¼ 4, C3eCH3), 27.9 (2JPt-C ¼ 31,
JP-C ¼ 40, 3JP-C ¼ 6, C1 ), 30.7 ( JPt-C ¼ 54,1JP-C ¼ 42, 3JP-C ¼ 4, C5), 34.8
1
2
0
(2JPt-C ¼ 46, 1JP-C ¼ 46, JP-C ¼ 4, C2), 122.7 (3JPt-C ¼ 26, C4), 139.2
3
(3JPt-C ¼ 28, C3); 31P NMR (CDCl3)
d
29.7 (1JPt-P ¼ 3489); HRMS
½M ꢀ Clꢃþfound ¼ 513.1129, C16H30P2ClPt requires 513.1138 for the 35Cl
and 195Pt isotopes.
4.10. Hydroformylation experiments
4.13. Theoretical calculations
In a typical experiment, a solution of PtCl2(ligand)2 (10 or (S,S)-
10, 5.5 mg, 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 ¼ 1:1) and placed in an oil bath of constant temperature. The
mixture was stirred with a magnetic stirrer for the given reaction
time. The pressure was monitored throughout the reaction. After
cooling and venting of the autoclave, the pale yellow solution was
removed and immediately analysed by GC, chiral GC (on a capillary
Cyclodex column, (R)-2-phenylpropanal was eluted before the (S)
enantiomer) and GCeMS.
Geometries were computed at the RI-B97-D/6-31G(d) level of
theory [72e75] then single point energy calculations were per-
formed at the optima using
uB97X-D/cc-pVTZ level [76,77]. For Pt
atoms, the cc-pVTZ-PP basis set and the corresponding pseudo-
potential [78] was applied for both geometry optimization and
single point energy calculations. Minima on the potential energy
surface (PES) were characterized by harmonic vibrational fre-
quency calculations. Calculations were carried out using Gaussian
09 [64] program. Avogadro was utilized for visualization [79].
Acknowledgements
The authors are thankful to Hungarian Research Fund for
financial support (Grant No. PD116096, K83118, K108752, K104769
and K113177). This work was financially supported by the BME
4.11. X-ray measurements
A crystal of the diastereomeric complex incorporating (ꢀ)-3 and
(ꢀ)-5 in a ratio of 1:2 was mounted on a glass fibre. Cell parameters
were determined by least-squares from the respective setting an-
ꢀ
RþDþI Project supported by the grant (TAMOP-4.2.1/B-09/1/KMR-
ꢀ
2010-0002). TS is grateful for the support of The New Szechenyi
Plan TAMOP-4.2.2/B-10/1-2010-0009. Financial support from OTKA
gles as quoted. Completeness were equal or greater to 2
q
¼ 99.5%.
ꢀ
K 105459 (to K.E.K.) and from Richter Gedeon Talentum Alapítvany
Multi-scan absorption correction was applied to the data with
minimum and maximum transmission factors of 0.965 and 0.992.
Initial structure models were obtained by direct methods [60] and
subsequent difference syntheses. Anisotropic full-matrix least-
squares refinement on F2 were applied [60] for all non-hydrogen
atoms and the model was refined to convergence. Hydrogen
atomic positions were calculated from assumed geometries where
appropriate. Four OeH hydrogen atoms were located from differ-
ence electron density maps. Hydrogen atoms were included in
structure factor calculations and the non-trivial hydrogen positions
and their isotropic displacements were refined. Other H atoms
were kept riding on their anchor atoms, with isotropic displace-
ment parameters of these hydrogen atoms were approximated
from the U(eq) value of the atom they were bonded. The maximum
and minimum residual electron densities in the final difference
maps are 0.19 and ꢀ0.23 e.Åꢀ3 and are normal. All further
calculations and drawings were done by using PLATON [61], Mer-
cury [51] and SCHAKAL [62]. Crystal structure data are deposited
with the Cambridge Crystallographic Data Centre under CCDC
1405185 and can be obtained free of charge upon application.
(Ph.D. scholarship to I.T.) is gratefully acknowledged.
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