Bulky phosphinines: From a molecular design to an application in homogeneous catalysis

Article abstract of DOI:10.1002/chem.201300557

The design and preparation of an asymmetrically substituted and bulky phosphinine was achieved by introducing sterically demanding substituents into specific positions of a rigid phosphorus-heterocyclic framework. Compound 5 shows, at the same time, axial chirality and a sufficiently high energy barrier for internal rotation to prevent enantiomerization. Both enantiomers of 5 were isolated by means of chiral analytical HPLC, and their absolute configurations could be assigned by combining experimental data and DFT calculations. Despite its substitution pattern, 5 can still coordinate to transition-metal centers through the lone pair of electrons on the phosphorus atom. Rapid C-H activation on the adjacent aryl substituent at the 2-position of the phosphorus heterocycle was achieved by using [{CpIrCl₂}₂] (Cp=1,2,3,4,5- pentamethylcyclopentadienyl) as a metal precursor. A racemic mixture of 5 was applied as a π-accepting low-coordinate phosphorus ligand in the Rh-catalyzed hydroformylation of trans-2-octene, which showed a clear preference for the formation of 2-methyloctanal. Bulky phosphinines: An asymmetrically substituted, bulky, and atropisomeric phosphinine has been prepared and characterized (see figure). Rapid C-H activation on the 2-aryl substituent of the phosphorus heterocycle was achieved with [{CpIrCl₂}₂] (Cp=1,2,3,4,5-pentamethylcyclopentadienyl) as a metal precursor. A racemic mixture of the phosphinine acted as a π-accepting low-coordinate phosphorus ligand in the Rh-catalyzed hydroformylation of 2-octene. Copyright

Full text of DOI:10.1002/chem.201300557

FULL PAPER
DOI: 10.1002/chem.201300557
Bulky Phosphinines: From a Molecular Design to an Application in
Homogeneous Catalysis
Jarno J. M. Weemers,[b] Willem N. P. van der Graaff,[b] Evgeny A. Pidko,[b]
Martin Lutz,[c] and Christian Mꢀller*[a, b]
ꢀ
Abstract: The design and preparation
of an asymmetrically substituted and
bulky phosphinine was achieved by in-
troducing sterically demanding sub-
stituents into specific positions of a
rigid phosphorus-heterocyclic frame-
work. Compound 5 shows, at the same
time, axial chirality and a sufficiently
high energy barrier for internal rota-
tion to prevent enantiomerization.
Both enantiomers of 5 were isolated by
means of chiral analytical HPLC, and
their absolute configurations could be
assigned by combining experimental
data and DFT calculations. Despite its
substitution pattern, 5 can still coordi-
the phosphorus atom. Rapid C H acti-
vation on the adjacent aryl substituent
at the 2-position of the phosphorus het-
erocycle was achieved by using
[{Cp*IrCl2}2] (Cp*=1,2,3,4,5-pentame-
thylcyclopentadienyl) as a metal pre-
cursor. A racemic mixture of 5 was ap-
plied as a p-accepting low-coordinate
phosphorus ligand in the Rh-catalyzed
hydroformylation of trans-2-octene,
which showed a clear preference for
the formation of 2-methyloctanal.
nate
to
transition-metal
centers
through the lone pair of electrons on
ꢀ
Keywords: atropisomerism · C H
activation · coordination chemistry ·
DFT calculations · homogeneous
catalysis · phosphinines
Introduction
We recently started to investigate in detail the synthesis
and coordination chemistry of (donor-)functionalized 2,4,6-
triaryl-substituted phosphinines,[5] which are generally acces-
sible by a modular route that used pyrylium salts reported
by Mꢀrkl.[1] Nevertheless, this route is mostly restricted to
simple aryl substituents. The introduction of more bulky res-
idues within the heterocyclic framework is problematic be-
cause of the facile formation of byproducts. Retro-aldol con-
densation at higher temperature and in the presence of
HBF4·Et2O with subsequent (random) recondensation usual-
ly occurs before the formation of the pyrylium salt, thus
making the rational design of bulky phosphinines rather
challenging. On the other hand, access to this type of phos-
phorus heterocycle would provide the possibility of prepar-
ing atropisomeric and asymmetrically substituted phosphi-
nines, which could be interesting p-accepting ligands for ho-
mogeneous catalysis.
l3-Phosphinines (phosphabenzenes), the higher homologues
of pyridines, have been known for almost 50 years due to
the pioneering work of Mꢀrkl and Ashe III.[1,2] These het-
erocycles are planar aromatic systems in which one of the
ꢀ
C H groups in benzene is substituted for an isoelectronic,
low-coordinate, and trivalent phosphorus atom. Phosphi-
nines were long regarded as laboratory curiosities until Zen-
neck and co-workers[3] and Breit et al.[4] reported the first
applications of phosphinines as p-accepting phosphorus li-
gands in homogeneous catalysis. For the complete imple-
mentation of phosphinine-based ligands in applied research
fields, the facile modification of steric and electronic ligand
properties and the incorporation of chirality is, however, an
essential feature.
Breit et al.[6] have demonstrated the synthesis of the 2-
naphthyl-substituted phosphinine B (Figure 1) and the relat-
ed phosphinine C, which was formed by a domino-reaction
sequence that started from a 2-naphthyl-substituted chal-
cone. Although somewhat bulky according to Figure 1, this
substitution pattern does not provide the possibility of pre-
paring atropisomeric (axially chiral) phosphinines. In fact,
due to the formal sp2-hybridization of the phosphorus atom,
the introduction of P stereogenic centers within the resulting
planar heterocycle is unfortunately impossible and opposed
to classical phosphines and phosphites based on a trivalent
phosphorus atom. The incorporation of chirality was, until
recently, only limited to the introduction of chiral auxillaries
within the phosphinine framework, such as oxazoline,[7] dicy-
clohexyl-1,2-ethanediol,[8] binaphthyl,[9] or (+)-camphor[10]
[a] Prof. Dr. C. Mꢁller
Institut fꢁr Chemie und Biochemie—Anorganische Chemie
Freie Universitꢀt Berlin
Fabeckstr. 34/36, 14195 Berlin (Germany)
Fax : (+49)30 838 52440
E-mail: c.mueller@fu-berlin.de
Homepage: www.bcp.fu-berlin.de/ak-mueller
[b] J. J. M. Weemers, W. N. P. van der Graaff, Dr. E. A. Pidko,
Prof. Dr. C. Mꢁller
Chemical Engineering and Chemistry
Eindhoven University of Technology
Den Dolech 2, 5600 MB Eindhoven (The Netherlands)
[c] Dr. M. Lutz
Bijvoet Center for Biomolecular Research
Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
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such as A (Figure 1), might provide the possibility for an
impact on regioselectivity because they could influence the
alignment of the substrates at the catalytically active transi-
tion-metal center.
We report herein the design of the first enantiopure bulky
and rigid atropisomeric phosphinine with a sufficiently high
rotational energy barrier to prevent enantiomerization at
elevated temperatures. Moreover, the first results on the co-
ordination chemistry of such a ligand and the application of
the racemic mixture in the Rh-catalyzed hydroformylation
of 2-octene are presented.
Figure 1. Bulky phosphinines.
Results and Discussion
groups. Recently, the first C2-symmetrical chiral phosphinine
To increase the barrier for internal rotation and to prevent
enantiomerization, sterically demanding substituents have to
be introduced preferably at the 5- and 6-positions of the
phosphorus heterocycle. As a further development of our
was synthesized from the corresponding bis(camphorpyryli-
um) salt and PACHTUNGTRENNUNG
(SiMe3)3 by Garner and co-workers.[11]
At the same time, our group started to investigate syn-
thetic access to atropisomeric phosphinines by locating spe-
cific substituents on the heterocyclic framework and by
making use of the above-mentioned modular synthetic ap-
proach through the pyrylium salt precursor. Racemic phos-
phinine D could be prepared by introducing an ortho-xylyl
group at the 6-position and an additional CH3 group at the
5-position of the phosphorus heterocycle by starting from
the corresponding functionalized propiophenone derivative
(Figure 1).[12] Both enantiomers could be isolated by em-
ploying analytical chiral HPLC and a rotational barrier of
DG° =109.5 kJmolꢀ1 (298 K) was determined experimental-
ly by means of racemization experiments. Although phos-
phinine D undergoes slow enantiomerization at room tem-
perature, chiral phosphinines could in general be very inter-
esting p-accepting phosphorus ligands for asymmetric homo-
geneous catalysis. Despite the fact that the application of
phosphinines in homogeneous catalysis is still limited, some
very important contributions have been described, especially
by Breit and co-workers.[4] In fact, the application of phos-
phinines in the Rh-catalyzed hydroformylation of various al-
kenes is probably one of the most prominent examples for
the use of phosphinines as ligands in homogeneous catalytic
reactions.[13] The observed high activity in the hydroformyla-
tion of 1-octene and trans-2-octene and the isomerization
and hydroformylation activities for less-reactive alkenes can
be attributed to the presence of a monoligated Rh species in
combination with the strong p-acceptor capabilities of the
ligand.
previously reported first axially chiral phosphinine
(Figure 1), we came up with the design of phosphinine E
(Scheme 1).
D
Scheme 1. Design of the novel phosphinine E.
The key compound for the preparation of E is (2-methyl-
naphthalen-1-yl)propan-1-one (1). This propiophenone de-
rivative can easily be prepared in high yield in a two-step
synthesis[14] by starting from commercially available 1-
bromo-2-methylnaphthalene (Scheme 2). In this way, the
methyl groups are located at the 2-position of the naphthyl
substituent and in the 5-position of the corresponding het-
erocycle (Scheme 1 and below), which is a prerequisite to
preserve the axial chirality in the target ligand.
In general, triaryl-substituted pyrylium salts can be syn-
thesized either through the chalcone route (i.e., a,b-unsatu-
Inspired by these recent investigations, we started to ex-
plore the development of bulky phosphinines in more detail,
but especially for two distinct reasons: 1) Is it possible to
prepare atropisomeric phosphinines with a sufficiently high
energy barrier for internal rotation to prevent enantiomeri-
zation? Such compounds could find application as new p-ac-
cepting phosphorus ligands in asymmetric homogeneous cat-
alysis. 2) Is it possible to prepare asymmetrically substituted,
bulky phosphinines that would show a preferred regioselec-
tivity in homogeneous catalytic reactions? Phosphinines,
Scheme 2. Synthesis of the key compound 1.
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Synthesis and Application of a Bulky Phosphinine Ligand
FULL PAPER
rated aryl ketones) or by cyclization of 1,5-diketones in the
presence of HBF4·Et2O or BF3·Et2O. However, both syn-
thetic protocols usually have their limitations and the right
reaction conditions have to be selected individually.
Unfortunately, the racemic atropisomeric naphthyl-func-
tionalized pyrylium salt F cannot be obtained directly by re-
action of 1 with two equivalents of trans-benzylideneaceto-
phenone in the presence of either HBF4·Et2O or BF3·Et2O
(in ClCH2CH2Cl or neat) at 708C. Instead, substantial retro-
aldol condensation was observed under these reaction con-
ditions, thus leading to a product mixture that made purifi-
cation impossible. Among these byproducts, 2,4,6-triphenyl-
pyrylium tetrafluoroborate could be identified by means of
single-crystal X-ray diffraction studies. Moreover, the pres-
ence of 3-benzyl-2,4,6-triphenylpyrylium tetrafluoroborate
could be proven by 1H NMR spectroscopic analysis. This
compound is the condensation product of two trans-benzyli-
deneacetophenones that react with one another (Scheme 3).
Figure 2. Rotational energy barrier of phosphinine G as a function of the
torsion angle.
AHCTUNGTRENNUNG
(naphthyl)-based backbones,[15] which find numerous appli-
cations in the preparation of axially chiral ligands for asym-
metric homogeneous catalysis.
Thus, the tetralone-based chalcone 2 was chosen as a pre-
cursor and prepared by following a modified reported pro-
cedure.[16] Unfortunately, the chromenium salt 3 could not
be obtained directly either under similar reaction conditions
to those described for the pyrylium salt of the corresponding
phosphinine D (Scheme 5).
Scheme 3. Attempted direct synthesis of pyrylium salt F.
To prevent the formation of the observed side products
and to introduce more rigidity into the framework, we con-
sidered a redesign of the original phosphinine E (Scheme 4).
Scheme 5. Direct synthesis of chromenium salt 3 from 1 and 2.
We therefore considered the preparation and isolation of
the 1,5-diketone intermediate 4 before treating with the cor-
responding chromenium salt 3. This procedure can be the
preferred route, especially for highly asymmetrically substi-
tuted pyrylium and chromenium salts. The preparation of
1,5-diketones has been extensively studied. However, incor-
porating functionalized ketone 1 into the framework turned
out to be far from straightforward. Mixing all the starting
materials with a mortar and pestle in the presence of NaOH
or KOH as described by Cave and Raston[17] did not yield
the desired 1,5-diketone 4. This procedure has been shown
by us to work efficiently for the introduction of pyridyl or
thienyl substituents into pyrylium salts and the correspond-
ing phosphorus heterocycles.[5b] By making use of stronger
bases, such as sodium hydride or lithium diisopropylamide
(LDA),[18] or alternatively by performing the reaction in the
liquid phase did not result in any product formation either.
1,5-Diketone 4 could only be obtained in two specific
ways:[19,20] The reaction of functionalized ketone 1 with chal-
cone 2 in dry DMF in an inert atmosphere and in the pres-
Scheme 4. Design of the novel phosphinine G.
We first decided to calculate the rotational barrier for the
enantiomerization of compound G. An activation Gibbs free
energy for the internal rotation of DG° =165 kJmolꢀ1
(298 K) was predicted by using DFT calculations (Figure 2).
This rotational barrier is significantly higher relative to the
experimentally determined value found for D and is expect-
ed to be high enough for configurational stability, even
under elevated temperatures. As a matter of fact, this value
is in the range of the rotational barrier reported for bis-
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C. Mꢁller et al.
ence of BaACHTUNGTRENNUNG(iPr)2. This procedure gave the desired product in
only 10% yield of the isolated product after stirring for
3 weeks. The much faster reaction in a mixture of DMSO
and 10% aqueous KOH solution and in the presence of tet-
rabutylammonium iodide as a phase-transfer agent afforded
4 as white crystals in an acceptable yield of the isolated
product after 4.5 h. Subsequent reaction with one equivalent
of benzylidene-2-acetophenone in the presence of BF3·Et2O
at 708C afforded the racemic chromenium salt 3 as a yellow
solid in 95% yield (Scheme 5). Although racemic 3 gave
bright-yellow cube-shaped crystals after slow evaporation
from a concentrated solution in methanol, the crystal quality
was unfortunately not sufficient for X-ray diffraction stud-
ies.
Chromenium salt 3 was further converted into the corre-
sponding racemic phosphinine 5 by reaction with a slight
excess of PACHTUNGTRENNUNG(SiMe3)3 in DME at 858C overnight (Scheme 6).
Figure 3. 1H NMR (400 MHz, CD2Cl2) spectrum of rac-5. Inset:
31P{1H} NMR (162 MHz, CD2Cl2) spectrum of rac-5.
After flash-column chromatography and further workup, 5
could be isolated as an off-white solid in 13% yield
(Scheme 7).
the 5-position of the phosphorus heterocycle and the CH3
group at the 2-position of the naphthyl moiety, respectively.
Racemate 5 was dissolved in a hexane/CH2Cl2 mixture
(2:1) and analyzed by means of chiral HPLC. Baseline sepa-
ration of 5-E1 and 5-E2 (first and second eluted enantiomers,
respectively) was only achieved at very low concentrations
by using preparative HPLC on a chiral bulk stationary
phase with n-hexane as the eluent (chiralpak IA, t1 =15.88,
t2 =20.23 min, T=258C, flow rate=4.7 mLminꢀ1; Figure 4).
1
Figure 3 shows the H and 31P{1H} NMR spectra of rac-5
in CD2Cl2. The 31P{1H} NMR spectrum clearly illustrates the
characteristic downfield shift of the phosphorus signal at d=
1
187.1 ppm. The H NMR spectrum shows a doublet at d=
1.65 ppm with a coupling constant of 3JH,P =2.1 Hz and a
4
doublet at d=2.32 ppm with a coupling constant of JH,P
=
1.0 Hz. These signals can be assigned to the CH3 group at
Figure 4. Front: HPLC analysis of baseline separation of both enantiom-
ers of 5 by using preparative HPLC on a chiral bulk stationary phase
with n-hexane as the eluent (Chiralpak IA, t1 =15.88, t2 =20.23 min, T=
258C, flow rate=4.7 mLminꢀ1). Back: first eluted enantiomer 5-E1 after
separation.
Scheme 6. Synthesis of 1,5-diketone 4 and chromenium salt 3. Reaction
conditions: 1) BaACHTUNGTRENNUNG(iPr)2, DMF; 2) tetrabutylammonium iodide, KOH
(10% aq. solution), DMSO; 3) trans-benzylideneacetophenone (1 equiv),
BF3·Et2O, ClCH2CH2Cl, 708C, overnight. DMSO=dimethyl sulfoxide.
Unfortunately, rac-5 tends to precipitate more easily from
solution at higher concentrations, and both peaks in the
HPLC chromatogram start to overlap significantly, thus
making separation on a multi-milligram scale very tedious
and almost impossible. We therefore abandoned the idea to
separate the enantiomers of 5 on a larger scale.
Analysis of rac-5 with the same chiral column by means
of analytical HPLC revealed, however, that the first enan-
tiomer 5-E1 could be obtained with an enantiopurity of 99%
(Figure 4). This result essentially enabled us to determine
Scheme 7. Synthesis of phosphinine 5. 1) P
night. DME=1,2-dimethoxyethane.
ACHTUNGTREN(NGNU SiMe3)3, DME, 858C, over-
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Synthesis and Application of a Bulky Phosphinine Ligand
FULL PAPER
the energy barrier for internal rotation experimentally or to
give at least an indication for a temperature range in which
the axial chirality is still retained. For the enantiomerization
experiments, we first chose a temperature of 808C, and the
thermal enantiomerization of 5-E1 in n-heptane at 808C was
followed over regular time intervals (Figure 5).
no racemization of the sample had occurred, nor after
2 days; however, after 7 days at 1408C (in n-decane), slow
decomposition of the ligand could be observed. We there-
fore abandoned increasing the temperature further and our
attempt to determine the rotational barrier experimentally.
Nevertheless, these experiments impressively show that our
novel phosphinine has indeed a very high barrier for inter-
nal rotation, as predicted, and that the configurational sta-
bility of the enantiomers is guaranteed at least up to 1408C.
Moreover, phosphinine 5 can also be considered to be
fairly stable toward air and moisture because the enrichment
procedure was conducted without the protection of an inert
atmosphere over several days, whereas essentially no deteri-
oration of the sample could be observed in solution. In fact,
phosphinine 5 can be kept in the liquid phase for over
2 weeks without any loss of quality. This finding proves once
again that 2,4,6-triaryl-substituted phosphinines show con-
siderable kinetic stability and are fairly inert toward water
and oxygen in contrast to less-substituted phosphinines.
To assign the absolute configurations of 5-E1 and 5-E2, the
optical properties of the separated enantiomers were investi-
gated. The experimentally recorded circular dichroism (CD)
and UV spectra of 5-E1 and 5-E2 in ClCH2CH2Cl are shown
in Figure 6a. The UV spectra of both enantiomers (Fig-
ure 6a, right) show the same characteristic profile, with a
shoulder at approximately l=360 nm, followed by two ab-
Figure 5. HPLC chromatograms of the thermal racemization of 5-E1 in n-
heptane at 808C.
Interestingly, and in contrast to phosphinine D, it is evi-
dent from Figure 5 that the enantiomeric excess of 5-E1 re-
mained 99% even after 11 days at 808C, and no racemiza-
tion of the sample could be observed. Performing the enan-
tiomerization process at 1208C (in n-decane) revealed that
Figure 6. Experimental CD (top left) and UV (top right) spectra of 5-E1 (b) and 5-E2 (c) in ClCH2CH2Cl. The experimental CD spectra (gray
lines) have been smoothed (black lines) to decrease the high noise level. Theoretical CD (bottom left) and UV (bottom right) spectra of enantiomers 5-
Sa (b) and 5-Ra (c).
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C. Mꢁller et al.
sorption maxima at approximately lmax =300 and 270 nm.
The high-energy part of these spectra is typical for 2,4,6-sub-
stituted triarylphosphinines (2,4,6-triphenylphosphinine:
To deduce the absolute configuration of both enantiom-
ers, the geometries for 5-Ra and 5-Sa were optimized at the
B3LYP/6-311+G
CD and UV spectra were calculated by means of a time-de-
pendent DFT method at the TD-B3LYP/6-311+G(d,p)
ACHTUNGTRENUN(NG d,p) level (Figure 8) and their theoretical
lmax =278, l=314 nm (sh); 2,4,6-triphenylpyridine: lmax
=
254, l=312 nm (sh); 2,4,6-triphenylbenzene: lmax =254 nm,
in methanol).
ACHTUNGTRENNUNG
level (Figure 6b). This procedure has been shown to provide
a reasonably accurate description of the photophysical prop-
erties of various compounds, which includes quite accurate
predictions of the CD spectra.[23]
In earlier reports, these bands were assigned as an n!p*
transition (long-wave absorption shoulder) and a p!p*
transition 1L(a) transition (short-wave absorption).[21,22]
However, the HOMO of the parent phosphinine has no con-
tribution to the lone-pair located on the phosphorus atom
and is consequently relatively low in energy.[21] As a matter
of fact, the lone pair of electrons on the phosphorus atom in
5
is essentially represented in the molecular orbitals
HOMOꢀ4, HOMOꢀ5, and HOMOꢀ6, with the latter having
the largest contribution (Figure 7). The multisignate (ꢀ/+
/ꢀ) CD curve of 5-E1 shows the presence of two exciton
Figure 8. Optimized structures of 5-Ra and 5-Sa.
Most strikingly, a high degree of similarity is obvious by
comparing the experimental UV spectra of 5-E1 and 5-E2
with the theoretical UV spectra of both enantiomers. The
experimental and theoretical spectra show the same qualita-
tive features, and the overall shape of the bands is very simi-
lar for both cases. However, the experimental spectra show
some broadening and a small shift toward lower energy.
This difference in peak positioning along the x axis can
most likely be attributed to solvent effects. Analysis of the
calculated electronic absorption bands reveals that the
shoulder at approximately l=360 nm can in fact be assigned
to the p!p* transition with a major contribution from the
HOMO!LUMO excitation. In the same way, the absorp-
tion maxima at about lmax =300 and 270 nm were also attrib-
uted to p!p* transitions with major contributions from the
HOMOꢀ1!LUMO excitation and the HOMOꢀ2!LUMO
excitation, respectively. Furthermore, in the lower-energy
region, all the other electronic absorption bands could be as-
signed to n!p* transitions (Figure 6, bottom right). The re-
spective frontier molecular orbitals (FMOs) are depicted in
Figure 7.
Interestingly, the theoretical CD spectra of both enan-
tiomers also display the same characteristic features as the
experimentally obtained spectra. The calculated spectra
clearly show the multisignate shape and the sequence and
positions of the negative/positive Cotton effects. Both the
(ꢀ/+/ꢀ) pattern and the (+/ꢀ/+) pattern are clearly visi-
ble; however, the positioning of peaks is more compact and
slightly offset, whereas the intensity maxima and minima of
the theoretical spectra are, in general, larger than those in
the experimental spectra. For the theoretical calculations,
both conformers have been structurally optimized. Although
the rotation of the phenyl group at the 4-position of the het-
erocycle and the fluxional half-chaired -C2H4- bridge have
an influence on the shape of the theoretical CD spectrum,
the overall multisignate character, as seen in the theoretical
Figure 7. Frontier molecular orbitals (FMOs) of phosphinine 5.
split couplets with relatively small amplitudes of approxi-
mately SDe=13 and 18, whereas the multisignate (+/ꢀ/+)
CD curve of 5-E2 has two couplets both with amplitudes of
approximately SDe=12. The CD spectra also display three
Cotton effects for each enantiomer at about l=325, 295,
and 260 nm. The deviation in SDe values between the two
enantiomers can be attributed to a difference in concentra-
tion as a result of the enrichment and the dilution proce-
dure. This difference can also be observed in the UV spectra
in which 5-E1 has a slightly lower absorbance than 5-E2.
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Synthesis and Application of a Bulky Phosphinine Ligand
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and the experimental spectra, remains unchanged and is in-
dependent of the conformations of these side groups.
By applying additional conventions to the Cahn–Ingold–
Prelog priority rules,[24] the absolute configuration of both
enantiomers could be determined. Because the calculated
data have such a close resemblance to the experimental re-
sults, we are confident to assign the 5-Sa and 5-Ra configura-
tions unambiguously to the 5-E1 and 5-E2 enantiomers, re-
spectively (Figure 9).
Scheme 8. Synthesis of the AuI complex 6 and the RhI complex 7.
Figure 9. Left: enantiomer E1 (first eluted compound) with the Sa confor-
mation. Right: enantiomer E2 (second eluted compound) with the Ra
conformation.
Coordination chemistry: Several reported examples of X-
ray crystal structures of 2,4,6-triarylphosphinines or their
corresponding transition-metal complexes show that the aryl
substituents at the 2- and 6-positions of the phosphinine
core are rotated out of the plane of the heterocycle.[5,25] Tor-
sion angles between 30 and 908 can be found depending on
the substitution pattern of the aryl groups. Because the
steric demand of the methylnaphthyl group at the 6-position
of 5 is rather high and the aryl group at the 2-position is
almost in plane with the phosphorus heterocycle due to the
rigid -C2H4- bridge in the backbone, we wondered whether
this situation might hamper coordination through the lone
pair of electrons on the phosphorus atom to a metal center.
As a matter of fact, phosphinines containing SiMe3 substitu-
ents in the 2- and 6-positions of the heterocycle show a pref-
erence for h6-coordination through the aromatic ring rather
than for h1-coordination through the lone pair of electrons
on the phosphorus atom.[26] We therefore investigated the
coordination chemistry of rac-5 to obtain more insight into
the behaviour toward selected transition metals.
Figure 10. 31P{1H} NMR spectra of the AuI complex 6 (left) and the RhI
complex 7 (right) in CD2Cl2 (1JRh P =221.9 Hz).
ꢀ
1
coupling constant of JRh P =221.9 Hz. This value is in the
ꢀ
range observed for complexes of the type [Rh
ACHTUNGTRENN(UNG acac)CO-
AHCTUNGTRENNUNG
ligand). Unfortunately, all attempts to obtain crystals of 6 or
7 suitable for X-ray diffraction studies have so far been un-
successful.
Most importantly, however, we can conclude from these
experiments that despite the presence of a sterically de-
manding group at the 6-position of the phosphorus hetero-
cycle and the fact that the aryl group at the 2-position is lo-
cated almost in the same plane as the phosphinine moiety,
coordination to transition-metal centers through the lone
pair of electrons on the phosphorus atom can still occur.
Cyclometalation of 5: We recently demonstrated, for the
first time, that the base-assisted ortho-metalation of 2,4,6-tri-
phenylphosphinine with [{Cp*IrCl2}2] and [{Cp*RhCl2}2]
(Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) as metal pre-
cursors in the presence of NaOAc leads to new phosphi-
nine-based metallacycles,[25a,28] which might open up new
perspectives for the application of phosphinines in molecu-
lar materials and homogeneous catalysis. We therefore con-
sidered rac-5 as a suitable phosphinine ligand to test the cy-
clometalation with [{Cp*IrCl2}2] (Scheme 9).
The reaction of rac-5 with one equivalent of [AuCl-
ACHTUNGTRENNUNG(SMe2)] proceeded quantitatively in CD2Cl2 and only a
single resonance at d=156.26 ppm was observed in the
31P{1H} NMR spectrum (Scheme 8 and Figure 10). The up-
field chemical shift of approximately Dd=30 ppm is charac-
teristic for phosphinines coordinated to an AuI metal center
and is in the typical region for phosphinine/transition-metal
complexes.[5,25,27] Upon the addition of rac-5 to one equiva-
lent of [Rh
ACHTUNGTRENNUNG
dium complex [Rh
G
E
As the aryl group at the 2-position is located almost in
the same plane as the phosphinine moiety due to the rigid
though the reaction did not proceed quantitatively and the
free ligand was still observed. The 31P{1H} NMR spectrum of
7 in CH2Cl2 revealed a doublet at d=180.71 ppm, with a
ꢀ
-C2H4- spacer in the backbone, we anticipated that the C H
activation on that aryl ring might consequently proceed
Chem. Eur. J. 2013, 19, 8991 – 9004
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8997

C. Mꢁller et al.
cyclometalation reaction is much faster relative to 2,4,6-tri-
phenylphosphinine, as already anticipated due to the fact
that this reaction proceeds at room temperature. We further
assume that the reaction follows the mechanism described
by Jones and Feher[29] for 2-phenylpyridine, which suggests
that the ortho-metalation follows a concerted base-assisted
metalation deprotonation mechanism. Red crystals were ob-
tained by slow diffusion of Et2O into a solution of crude
MC1 and MC2 in CH2Cl2. As shown by 31P{1H} NMR spec-
troscopy, the red crystals consist exclusively of the major
product MC1 and were suitable for single-crystal X-ray dif-
fraction studies. The compound forms racemic, but diaster-
eomerically pure, crystals in the monoclinic space group
P21/n (no. 14), and the molecular structure along with select-
ed bond lengths and angles is shown in Figure 12.
The X-ray crystal structure analysis of MC1 unambigu-
ously confirms the cyclometalation of rac-5 by IrIII with the
elimination of HCl and formation of 9. The molecular struc-
ture of 9 in the crystal shows the characteristic three-legged
Scheme 9. Cyclometalation of rac-5 with a IrIII center.
much faster than originally observed for 2,4,6-triphenylphos-
phinine. The reaction of rac-5 with [{Cp*IrCl2}2] and NaOAc
in a ratio of 2:1:2.2 in CD2Cl2 at 208C in a sealed NMR
tube leads indeed initially to a new species in a quantitative
yield, with
a broad signal at d=125.4 ppm in the
31P{1H} NMR spectrum (Scheme 9 and Figure 11 at t=0).
Figure 11. Distribution of species plot derived from the 31P{1H} NMR
spectroscopic data for the ortho-metalation of rac-5 with [{Cp*IrCl2}2] in
the presence of NaOAc at 208C.
By analogy to the reaction of 2,4,6-triphenylphosphinine
with [{Cp*IrCl2}2], this species can be considered as the
simple monomeric coordination compound [Cp*IrCl2ACTHNUTRGNE(NUG rac-
5)] (8) after opening of the Ir dimer. From Figure 11, it is
obvious that the mono-coordinated species 8 is rapidly in
equilibrium with free rac-5 because immediately after the
start of the reaction almost 35% of the free ligand rac-5 is
present in the reaction mixture. A conversion of 80% based
on cyclometalated products is achieved after 2 days at 208C
(Figure 11). A mixture of diastereomeric cyclometalated
complexes MC1 and MC2 is formed in a ratio of 6:1 and
with resonances in the 31P{1H} NMR spectrum of d=162.2
and d=161.3 ppm, respectively, because rac-5 was used. The
Figure 12. Molecular structure of 9 in the crystal. Only one diastereomer
of the racemic crystal structure is shown. Bottom representation shows
ꢀ
the view along the C(1) C(15) bond. Displacement ellipsoids are shown
at the 50% probability level. Compound 9 cocrystallized with one mole-
cule of CH2Cl2, which is omitted for clarity. Selected bond lengths [ꢃ]:
ꢀ
ꢀ
ꢀ
ꢀ
Ir(1) P(1): 2.2366(7), Ir(1) C(11): 2.090(3), Ir(1) Cl(1): 2.4042(7), Ir(1)
ꢀ
ꢀ
ꢀ
Cp*ACTHNUGRTNEUNG(cent.): 1.862, P(1) C(1): 1.711(3), P(1) C(13): 1.708(3), C(1) C(2):
ꢀ
ꢀ
ꢀ
1.418(4), C(2) C(3): 1.413(4), C(3) C(4): 1.409(4), C(4) C(13): 1.391(4);
and torsion angles [8]: C(1)-P(1)-C(13): 105.96(13), C(11)-Ir(1)-P(1):
79.22(7); P(1)-C(1)-C(15)-C(24): ꢀ83.3(3), P(1)-C(13)-C(12)-C(11):
2.0(3).
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Chem. Eur. J. 2013, 19, 8991 – 9004

Synthesis and Application of a Bulky Phosphinine Ligand
FULL PAPER
“piano-stool” arrangement around the iridium atom and
represents the first crystallographic characterization of a
phosphinine/MIII complex with axial chirality reported so
far. It is clear that the naphthyl group at the 6-position is
almost perpendicular to the phosphorus heterocycle with a
P(1)-C(1)-C(15)-C(24)
(Figure 12).
torsion
angle
of
ꢀ83.3(3)8
The two coordinated aromatic rings are essentially copla-
nar with respect to one another with a P(1)-C(13)-C(12)-
C(11) torsion angle of only 2.0(3)8. The P(1)–C(1) and P(1)–
C(13) bond lengths are 1.711(3) and 1.708(3) ꢃ, respectively,
and thus somewhat shorter than in free 2,4,6-triarylphosphi-
nines (i.e., 1.74–1.76 ꢃ), whereas the carbon–carbon bond
lengths in the aromatic phosphinine subunit are on the usual
order of 1.391(4)–1.418(4) ꢃ observed for both free and co-
ordinated phosphinine ligands.[5a] Figure 12 (top) nicely
shows that the metal center is not located in the ideal axis
of the lone pair of electrons on the phosphorus atom but
clearly shifted toward the carbon atom linked to the metal
atom. Obviously, this effect is necessary for an efficient
complexation of the metal atom by the chelating P^C ligand
and facilitated by the diffuse and less-directional lone pair
on the low-coordinate phosphorus atom.
Scheme 10. Rh-catalyzed hydroformylation of trans-2-octene and possible
reaction products.
nine ligand 5 might have an influence on the product distri-
bution in the hydroformylation reaction.
Reported examples of the regioselective hydroformlyation
of internal alkenes are rare and have only been observed
with supramolecular systems, not with classical phosphorus-
based ligands.[30] Although not yet separated on a larger
scale, we set out to test the racemic mixture of 5 in the hy-
droformylation reaction and compared it with Rh catalysts
based on standard phosphorus ligands, such as the s-donat-
ing PPh3 and bulky p-accepting tris(2,4-di-tert-butylphenyl)
phosphite ligands.[31] The catalytic reactions were carried out
in a 75 mL stainless steel autoclave equipped with a drop-
The large opening of the aC–P–C angle (105.9 vs approx-
imately 1008 in free phosphinines) reflects significant disrup-
tion of the aromaticity; consequently, a high reactivity of
the P=C double bond is expected. Nevertheless, the IrIII
complex 9 can easily be isolated, handled, and character-
ized.[25] We assume that the nonmetalated naphthyl substitu-
ent in the 6-position of the heterocyclic framework might
contribute to a steric protection of the P=C double bond as
we have observed before for the neutral phosphinine/RhIII
ping funnel and sample unit. A solution of [RhACHTUNTRGENUG(N acac)(CO)2]
and 20 equivalents of the ligand in toluene were loaded into
the autoclave, and the reaction was performed at 808C with
a syngas pressure of 20 bar (CO/H2 =1:1) for 2 hours. After-
ward, a mixture of trans-2-octene and n-decane as an inter-
nal standard (molar ratio=3:1) was added through the
dropping funnel at 808C and 20 bar, which initiated the start
of the reaction. In the case of the [Rh]/PPh3 system, the re-
action reached a conversion of >99% after 70 h. The reac-
tion is first order with respect to 2-octene up to the point at
which almost 90% of all the substrate has been consumed
(Figure 13).
and phosphinine/IrIII complexes [Cp*M
ACTHNUTRGNE(NUG P^C)Cl] that con-
tain a cyclometalated 2,4,6-triphenylphosphinine ligand.
Hydroformylation of trans-2-octene: We were further inter-
ested in the application of the novel bulky phosphinine
ligand 5 in homogeneous catalytic reactions. The sterically
demanding 2,6-bis(2,4-dimethylphenyl)-4-phenylphosphinine
has shown superior performance as a ligand in the Rh-cata-
lyzed hydroformylation of 1-octene over classical PPh3-
based catalysts, with reported turnover frequencies of up to
45370 hꢀ1 (1308C, 40 bar CO/H2, Rh/L=1:20).[6] Most strik-
ingly, selective hydroformylation of a-pinene and tetrame-
thylethylene, which undergo a tandem isomerization–hydro-
formylation reaction, can be achieved with this bulky triar-
yl-substituted phosphinine, although with low rates. These
substrates are among the least reactive alkenes and cannot
be hydroformylated with the [Rh]/PPh3 system. A drawback
in the hydroformylation of internal alkenes is the presence
of the two possible reaction products 2-methyloctanal and 2-
ethylheptanal as well as substantial isomerization that leads
to the byproducts nonanal and 2-propylhexanal
(Scheme 10). Nevertheless, trans-2-octene is a very interest-
ing substrate because the expected main-products 2-methyl-
octanal and 2-ethylheptanal are chiral aldehydes, and we
wondered whether the asymmetrically substituted phosphi-
Figure 13. Product-distribution plot for the hydroformylation of 2-octene
with the [Rh]/PPh3 system. Preformation conditions: T=808C, p=20 bar
(CO/H2 =1:1), t=2 h. Reaction conditions: T=808C, p=20 bar (CO/
H2 =1:1), Vtot =12.0 mL, Rh/L/S=1:20:1500, cRh =1.4ꢄ10ꢀ4 molLꢀ1
.
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8999

C. Mꢁller et al.
Analysis of the product distribution revealed that the
main products 2-methyloctanal and 2-ethylheptanal (57 and
41%, respectively) were formed in a ratio of approximately
1.4. Moreover, almost no isomerization products were ob-
served: 2-propylhexanal was formed in a yield of approxi-
mately 2%, whereas nonanal was present in only 0.4%
yield.
On the other hand, the catalyst based on the bulky p-ac-
cepting tris(2,4-di-tert-butylphenyl) phosphite was much
more active and the substrate was completely consumed
within 3.5 hours (Figure 14). The two internal aldehydes 2-
Figure 15. Product-distribution plot for the hydroformylation of 2-octene
with the [Rh]/rac-5 system. Preformation conditions: T=808C, p=20 bar
(CO/H2 =1:1), t=2 h. Reaction conditions: T=808C, p=20 bar (CO/
H2 =1:1), Vtot =12.0 mL, Rh/L/S=1:20:1500, cRh =1.4ꢄ10ꢀ4 molLꢀ1
.
These initial results show that there is obviously a clear
preference for the formation of one of the internal (chiral)
aldehydes, that is, 2-methyloctanal. At this stage, we assume
that the distinct substitution pattern of the phosphinine
ligand might indeed have a pronounced influence on the
outcome of the hydroformylation reaction. Other substitut-
ed phosphinines do not show this distinct behavior in the hy-
droformylation of trans-2-octene.[6,13] Future investigations
will thus focus on the influence of the substitution pattern
on the regioselectivity in the hydroformylation of internal
alkenes in more detail. Interestingly, and in contrast to clas-
sical phosphorus ligands, the modular synthetic route
toward 2,4,6-triarylphosphninines allows access to a large
variety of asymmetrically substituted p-accepting phospho-
rus heterocycles with different electronic and steric proper-
ties.
Figure 14. Product-distribution plot for the hydroformylation of 2-octene
with the [Rh]/P[O(2,4-di-tBuPh)]3 system. Preformation conditions: T=
808C, p=20 bar (CO/H2 =1:1), t=2 h. Reaction conditions: T=808C,
p=20 bar (CO/H2 =1:1),
Vtot =12.0 mL, Rh/L/S=1:20:1500, cRh =1.4ꢄ
10ꢀ4 molLꢀ1
.
methyloctanal and 2-ethylheptanal (51 and 32%, respective-
ly) were formed in a ratio of approximately 1.6:1 (ratios are
given relative to 1), which is in the same range as observed
for the s-donating PPh3. However, 2-propylhexanal and
nonanal, which are formed by isomerization prior to hydro-
formylation, were formed in yields as high as 10 and 5%, re-
spectively. Nevertheless, the final product distributions were
essentially very similar, especially with respect to the ratio
of the main products 2-methyloctanal and 2-ethylheptanal
(1.4:1 vs 1.6:1).
Finally, we wanted to compare these results with the p-ac-
cepting rac-5 as the ligand. When rac-5 was applied to the
reaction, full conversion was reached after approximately
90 hours (Figure 15). It is clear that the reaction had a
longer initiation period relative to both benchmark reac-
tions. During the reaction, a maximum amount of 20% of
octene isomers was present, which consequently resulted in
a somewhat higher degree of isomerization products; 2-pro-
pylhexanal and nonanal were formed in yields of 15 and
13%, respectively. Most interestingly however, and in con-
trast to the s-donating phosphine and p-accepting bulky
phosphite ligands, both internal aldehydes 2-methyloctanal
and 2-ethylheptanal were formed in a much higher ratio
(ca. 2.5:1), whereas 2-methyloctanal was the predominant
reaction product among the internal aldehydes.
Conclusion
We have demonstrated for the first time the rational design
and preparation of an asymmetrically substituted and bulky
phosphinine derivative by introducing sterically demanding
substituents into specific positions of a rigid phosphorus-het-
erocyclic framework. By choosing the right substituents, we
could achieve, at the same time, axial chirality and a suffi-
ciently high energy barrier for internal rotation to prevent
enantiomerization. Both enantiomers could be isolated by
means of chiral analytical HPLC. The absolute configura-
tions of both enantiomers were assigned by combining ex-
perimental data and DFT calculations. Despite its substitu-
tion pattern, the bulky low-coordinate phosphorus ligand
can still coordinate to selected transition-metal centers, such
as AuI and RhI, through the lone pair of electrons on the
phosphorus atom rather than through the aromatic ring.
ꢀ
Rapid C H activation at room temperature on the adjacent
aryl substituent in the 2-position of the phosphorus hetero-
cycle was achieved by using [{Cp*IrCl2}2] as a metal precur-
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Chem. Eur. J. 2013, 19, 8991 – 9004

Synthesis and Application of a Bulky Phosphinine Ligand
FULL PAPER
19.40, 38.68, 123.97, 125.45, 126.83, 128.27, 128.55, 128.66, 129.28, 130.09,
131.74, 138.80, 211.20 ppm; elemental analysis calcd (%) for C14H14O
(198.26): C 84.81, H 7.12; found: C 82.55, H 6.91.
sor in the presence of NaOAc, and the product could be
characterized by single-crystal X-ray diffraction studies. A
racemic mixture of 5 was applied as a p-accepting low-coor-
dinate phosphorus ligand in the Rh-catalyzed hydroformyla-
tion of trans-2-octene, thus showing a clear preference for
the formation of 2-methyloctanal. Future investigations will
focus on the application of enantiopure phosphinine ligands
for the asymmetric version of the hydroformylation reac-
tion, especially for the preparation of optically active inter-
nal aldehydes. Work in this direction is currently being per-
formed in our laboratories.
(E)-2-Benzylidene-3,4-dihydronaphthalen-1-one (2): A mixture of KOH
pellets (8.11 g, 144.47 mmol, 0.88 equiv) in EtOH (820 mL) was stirred
until all of the base had dissolved. Next, a-tetralone (24.00 g,
164.17 mmol, 1.0 equiv) was added in one portion to result in a dark-blue
reaction mixture. Subsequently, benzaldehyde (47.04 g, 443.26 mmol,
2.7 equiv) was added to the reaction mixture, which was stirred for 24 h
and resulted in a green suspension. The reaction mixture was poured into
water (820 mL) and enough diethyl ether was added to completely dis-
solve the precipitate. After removal of the diethyl ether and a large part
of the EtOH by using a rotary evaporation, the product crystallized. The
solid was removed by filtration, washed with cold EtOH, and dried in
vacuo to obtain the product as pale-yellow crystals (36.99 g, 157.89 mmol,
96%). M.p. 1068C; 1H NMR (400.16 MHz, CDCl3, 258C): d=2.95 (t, J=
6.6 Hz, 2H), 3.14 (dt, J=6.6, J=1.8 Hz, 2H), 7.26 (d, J=7.6 Hz, 1H),
7.33–7.47 (m, 6H), 7.50 (dt, J=7.4, J=1.4 Hz, 1H), 7.88 (s, 1H),
8.14 ppm (dd, J=7.8, J=1.2 Hz, 1H); 13C NMR (100.63 MHz, CDCl3,
258C): d=27.19, 28.88, 127.02, 128.20 (shoulder), 128.22, 128.44, 128.54,
129.87, 133.27, 133.48, 135.46, 135.85, 136.64, 143.23, 187.89 ppm; elemen-
tal analysis calcd (%) for C17H14O (234.29): C 87.15, H 6.02; found: C
86.94, H 6.00.
Experimental Section
General: All manipulations were carried out in an argon atmosphere by
using modified Schlenk techniques, unless stated otherwise. All the glass-
ware was dried prior to use by heating under vacuum. All the common
chemicals were commercially available and purchased from Aldrich
Chemical Co. or Acros and were used as received. The solvents were
2-[2-Methyl-3-(2-methylnaphthalen-1-yl)-3-oxo-1-phenylpropyl]-3,4-dihy-
dronaphthalen-1-one (4): Method 1: A suspension of anhydrous BaACHTUNGTRENNUNG(iPr)2
dried and deoxygenated by using custom-made solvent-purification col-
[32]
umns filled with Al2O3. PACTHNUTRGNEUGN(SiMe3)3 was prepared according to a report-
(268.5 mg, 1.05 mmol, 0.10 equiv) in dry and degassed DMF (35 mL) was
stirred at room temperature in an argon atmosphere for 10 min. The ad-
dition of 1 (2.50 g, 12.61 mmol, 1.2 equiv) resulted in a black reaction
mixture. Compound 2 (2.46 g, 10.51 mmol, 1.0 equiv) was added to the
reaction mixture, which was stirred for 3 weeks. The reaction mixture
was quenched by the addition of a saturated ammonium chloride solution
(60 mL), and the crude mixture was extracted with CH2Cl2 (3ꢄ225 mL),
dried over MgSO4, filtered, and the filtrate concentrated to obtain a
dark-red solution. The crude product was further purified by means of
column chromatography (silica gel, lꢄd=35ꢄ10 cm, n-hexane/EtOAc=
5:1) to obtain the crude product as a pale-yellow solid. After double re-
crystallization from EtOH, only one of the diastereomers was obtained
as white starlike crystals (574.7 mg, 1.33 mmol, 11%). 1H NMR
(400.16 MHz, CDCl3, 258C): d=1.04 (d, J=7.6 Hz, 3H; -CH-CH3), 1.70–
1.84 (m, 1H), 2.15–2.31 (m, 1H; partly overlapping with naphthyl-CH3
singlet), 2.26 (s, 3H; naphthyl-CH3), 2.70–2.83 (m, 1H), 2.97–3.10 (m,
1H), 3.14–3.25 (m, 1H), 3.60 (quin, J=7.2 Hz, 1H), 4.43 (t, J=7.3 Hz,
1H), 7.15–7.42 (m, 12H), 7.46 (dt, J=7.4, J=1.4 Hz, 1H), 7.72 (d, J=
8.4 Hz, 1H), 7.77 (d, J=8.4 Hz, 1H), 8.04 ppm (dd, J=7.8, J=1.4 Hz,
1H); 13C NMR (100.63 MHz, CDCl3, 258C): d=14.04, 19.82, 26.06, 27.18,
43.04, 49.30, 49.44, 124.27, 125.39, 126.66, 126.73, 126.80, 127.75, 128.14,
128.26, 128.71, 128.81, 128.96, 129.98, 130.64, 131.24, 131.75, 132.52,
133.37, 137.55, 138.99, 143.43, 199.35, 211.94 ppm.
ed procedure and 2-benzylidene-3,4-dihydronaphthalen-1-one (2) was
prepared according to a modified reported procedure.[16] Elemental anal-
yses were obtained from H. Kolbe, Mikroanalytisches Laboratorium,
Mꢁlheim a.d. Ruhr (Germany). The 1H, 13C{1H}, and 31P{1H} NMR spec-
tra were recorded on a Varian Mercury 400 MHz spectrometer, and all
the chemical shifts are reported relative to the residual proton resonance
in the deuterated solvents or referred to an 85% aqueous solution of
H3PO4, respectively.
1-(2-Methylnaphthalen-1-yl)propan-1-ol (1a): 1-Bromo-2-methylnaphtha-
lene (40.0 g, 180.9 mmol, 1.0 equiv) in dry THF (20 mL) was added drop-
wise to a suspension of Mg turnings (5.28 g, 217.1 mmol, 1.2 equiv) in dry
THF (180 mL) at room temperature in an argon atmosphere. The addi-
tion was complete after 1 h, and the mixture was stirred for 1.5 h, heated
to reflux for 1 h, and then allowed to cool to room temperature. Under
ice cooling, propanal (15.76 g, 271.4 mmol, 1.5 equiv) was added dropwise
and stirring was continued for 1 h with ice cooling followed by 1 h at
room temperature. The mixture was quenched with saturated aqueous
NH4Cl and extracted with Et2O (3ꢄ300 mL). The combined organic
phases were back-extracted with water (3ꢄ200 mL) and dried over
MgSO4. After filtration, all the volatiles were removed in vacuo to obtain
the crude product. For further purification, a double filtration over silica
gel was performed (eluting first with n-pentane followed by CH2Cl2),
thus obtaining a pale-yellow liquid (29.56 g, 147.6 mmol, 82%). 1H NMR
(400.16 MHz, CDCl3, 258C): d=1.03 (t, J=7.4 Hz, 3H; -CH2-CH3), 2.02–
2.09 (m, 1H; -CH2-), 2.22–2.30 (m, 1H; -CH2-), 2.17 (s, 1H; OH), 2.56 (s,
3H; -CH3), 5.48 (t, J=7.2 Hz, 1H), 7.26 (d, J=8.4 Hz, 1H), 7.40–7.48
(m, 2H), 7.67 (d, J=8.0 Hz, 1H), 7.80 (d, J=8.0 Hz, 1H), 8.66 ppm (d,
J=8.4 Hz, 1H); 13C NMR (100.63 MHz, CDCl3, 258C): d=11.13, 21.05,
29.87, 73.31, 124.68, 125.42, 125.94, 127.90, 128.72, 129.55, 131.42, 133.15,
133.47, 136.18 ppm; elemental analysis calcd (%) for C14H16O (200.28): C
83.96, H 8.05; found: C 82.00, H 7.98.
Method 2: DMSO (200 mL) followed by tetrabutylammonium iodide
(1.14 g, 3.09 mmol, 0.10 equiv) was added to
1 (6.12 g, 30.86 mmol,
1.0 equiv). The addition of an aqueous KOH solution (10%, 24.0 mL) to
the reaction mixture resulted in a color change from clear orange to
black. The reaction mixture was stirred for 45 min, and
2 (7.23 g,
30.68 mmol, 1.0 equiv) was added. Stirring was continued for 4.5 h, and
the reaction mixture was quenched by the addition of water (200 mL),
thus forming a brownish precipitate. The crude product was removed by
filtration and washed with water. The crude product was taken up in
CH2Cl2 (150 mL), extracted with water (3ꢄ100 mL), dried over MgSO4,
filtered, and the filtrate was concentrated in vacuo to obtain a brownish
fluffy solid. After slow crystallization from EtOH, a mixture of diaster-
eomers was obtained as either white crystals or white solid (2.25 g,
5.20 mmol, 17%). 1H NMR (400.16 MHz, CDCl3, 258C): d=1.33 (d, J=
7.2 Hz, 3H; -CH-CH3), 1.79 (dd, J=13.7, J=3.9 Hz, 1H), 2.06–2.17 (m,
1H; partly overlapping with naphthyl-CH3 singlet), 2.20 (s, 3H; naph-
thyl-CH3), 2.83–2.94 (m, 1H), 2.97–3.11 (m, 2H), 3.72 (dd, J=10.6, J=
4.1 Hz, 1H), 7.02–7.53 (m, 13H), 7.72 (dd, J=18.2, J=8.0 Hz, 2H),
8.01 ppm (d, J=8.2 Hz, 1H); 13C NMR (100.63 MHz, CDCl3, 258C): d=
16.53, 20.13, 28.81, 29.94, 49.45, 49.69, 51.87, 124.93, 125.14, 126.40,
(2-Methylnaphthalen-1-yl)propan-1-one (1): A solution of CrO3 (15.20 g,
152.0 mmol, 1.05 equiv) in
a mixture of H2SO4 (67 mL) and H2O
(135 mL) was added dropwise to a solution of 1a (29.0 g, 144.8 mmol,
1.0 equiv) in acetone (320 mL) under ice cooling. Stirring was continued
for 1.5 h. The mixture was extracted with CH2Cl2 (3ꢄ300 mL), and the
combined organic phases were back-extracted with water (3ꢄ150 mL)
and dried over MgSO4. After filtration, all the volatiles of the filtrate
were removed in vacuo to obtain the ketone product (26.89 g, 82%).
1H NMR (400.16 MHz, CDCl3, 258C): d=1.29 (t, J=7.4 Hz, 3H; -CH2-
CH3), 2.40 (s, 3H; -CH3), 2.88 (q, J=7.2 Hz, 2H; -CH2-), 7.28 (d, J=
8.6 Hz, 1H), 7.40–7.58 (m, 3H), 7.75 (d, J=8.0 Hz, 1H), 7.82 ppm (dd,
J=6.8, J=2.1 Hz, 1H); 13C NMR (100.63 MHz, CDCl3, 258C): d=7.94,
Chem. Eur. J. 2013, 19, 8991 – 9004
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9001

C. Mꢁller et al.
126.61, 126.76, 127.33, 128.01, 128.58, 128.69, 129.07, 130.29, 130.66,
131.69, 132.32, 133.29, 133.51, 137.33, 141.90, 143.77, 199.68, 211.43 ppm;
elemental analysis calcd (%) for C31H28O2 (432.55): C 86.08, H 6.52;
found: C 85.65, H 6.58.
9.13 (C
tyl-CH3), 29.29 (d, JC,P =1.8 Hz, -CH2-CH2-), 29.36 (d, JC,P =6.3 Hz, -CH2-
CH2-), 96.02 (d, JC,P =3.3 Hz, C5A(CH3)5), 122.19, 125.37, 125.73, 126.96,
127.30, 128.45 (d, JC,P =2.6 Hz), 128.53, 128.75 (d, JC,P =1.4 Hz), 129.07 (d,
(CH3)5), 21.03 (d, 3JC,P =6.3 Hz, heterocyclic CH3), 21.77 (naph-
5ACHTUNGTRENNUNG
CTHUNGTRENNUNG
J
J
C,P =7.7 Hz), 129.33 (d, JC,P =4.4 Hz), 129.72, 129.77, 129.79, 131.80 (d,
C,P =17.3 Hz), 132.86, 133.57 (d, JC,P =7.0 Hz), 134.82 (d, JC,P =8.8 Hz),
rac-3-Methyl-2-(2-methylnaphthalen-1-yl)-4-phenyl-5,6-dihydrobenzo-
chromenium tetrafluoroborate (3): Benzylidene-2-acetophenone (0.76 g,
135.97 (d, JC,P =8.1 Hz), 136.64, 140.87 (d, JC,P =26.5 Hz), 142.06 (d, JC,P
10.0 Hz), 142.18 (d, JC,P =4.1 Hz), 142.97 (d, JC,P =11.4 Hz), 143.12 (d,
=
3.68 mmol, 1.0 equiv) and dichloroethane (15 mL) were added to
4
(1.59 g, 3.68 mmol, 1.0 equiv). BF3·Et2O (4.17 g, 29.4 mmol, 8.0 equiv)
was added to the reaction mixture, and the clear red solution was heated
to 708C and stirred overnight. The dark reaction mixture was allowed to
cool and was added slowly to diethyl ether (300 mL) with a pipette, thus
resulting in a yellow precipitate. The precipitate was collected by filtra-
tion, washed with diethyl ether (100 mL), and dried in vacuo to obtain
the product as a bright-yellow solid (1.75 g, 3.50 mmol, 95%). Bright-
yellow crystals were obtained by slow diffusion of diethyl ether into a sol-
ution of the product in CH2Cl2. M.p. 208–2098C; 1H NMR (400.16 MHz,
CD2Cl2, 258C): d=1.95 (s, 3H), 2.48 (s, 3H), 3.00–3.07 (m, 2H; -CH2-),
3.11–3.18 (m, -CH2-, 2H), 7.34–7.43 (m, 2H), 7.47–7.52 (m, 1H), 7.52–
7.70 (m, 9H), 7.94 (dd, J=7.8, J=0.8 Hz, 1H), 7.97–8.01 (m, 1H),
8.10 ppm (d, J=8.4 Hz, 1H); 13C NMR (100.63 MHz, CD2Cl2, 258C): d=
16.27, 20.34, 25.11, 26.43, 124.29, 125.41, 125.94, 126.77, 126.82, 127.23,
127.41, 128.64, 128.94, 129.61, 129.75, 129.81, 130.83, 131.62, 132.15,
132.58, 132.84, 133.63, 133.72, 136.72, 137.90, 143.23, 168.26, 169.86,
170.47 ppm; 19F NMR (188.1 MHz, CD2Cl2, 258C): d=ꢀ152.35 ppm; ele-
mental analysis calcd (%) for C31H25BF4O·CH2Cl2 (585.27): C 65.67, H
4.65; found: C 66.09, H 4.77.
1
J
C,P =27.6 Hz), 146.52 (d, JC,P =26.5 Hz), 150.91, 160.19 ppm (d, JC,P
=
42.4 Hz); 31P NMR (161.98 MHz, CD2Cl2, 258C): d=164.25 ppm.
HPLC analysis: HPLC-fraction collection was performed by using HPLC
equipment that consisted of a Shimadzu LC-8A pump, a Shimadzu SPD-
20A prominence UV/Vis detector, a Shimadzu SIL-10AP autosampler,
and a Shimadzu FRC-10A fraction collector. Column and analysis speci-
fications: Chiralpak IA (250ꢄ10 mm, particle size: 5 mm, purchased from
Daicel), eluent=n-hexane, column temperature=258C, flow rate=
4.7 mLminꢀ1, p=34 bar, l=254 nm (UV detector), injection volume=
100 mL. HPLC analysis was performed by using HPLC equipment that
consisted of a Shimadzu LC-20AD pump, a Shimadzu SPD-20A promi-
nence UV/Vis detector, a Shimadzu SIL-20A HT prominence autosam-
pler, and a CTO-20AC prominence column oven. Column and analysis
specifications: Chiralpak IA (250ꢄ4.6 mm, particle size: 5 mm, purchased
from Daicel), eluent=n-hexane, column temperature=258C, flow rate=
1.0 mLminꢀ1, p=28 bar, l=254 nm (UV detector), injection volume=
1 mL (808C sampling) or 5 mL (1208C and 1408C sampling).
Enrichment and determination of the maximum temperature for configu-
rational stability of 5-E1: Enantiomerization at 808C: Racemate
5
rac-3-Methyl-2-(2-methylnaphthalen-1-yl)-4-phenyl-5,6-dihydrobenzo-
(50.0 mg, 1.2ꢄ10ꢀ4 mol) was dissolved in n-hexane/CH2Cl2 (2:1, 6 mL)
prior to HPLC enrichment. Enantiomer 5-E1 was eluted and collected
without an argon atmosphere, and the solvents were evaporated. This
procedure was performed sixteen times, and the combined residues of 5-
E1 were redissolved in degassed n-heptane (6.0 mL) to give a final con-
centration of approximately 10ꢀ4 m. This solution was stirred at 808C and
aliquots (0.7 mL) were analyzed by using HPLC at time intervals of t=0,
phosphinoline (5): PACHTUNGTRENNUNG(SiMe3)3 (863.9 mg, 3.45 mmol, 1.2 equiv) was added
to a suspension of 3 (1.44 g, 2.87 mmol, 1.0 equiv) in 1,2-dimethoxyethane
(24 mL) under argon. The mixture was heated at 908C for 17 h. After-
ward, the reaction mixture was allowed to cool to room temperature and
taken up in diethyl ether. Flash column chromatography with diethyl
ether as the eluent over neutral alumina resuled in a pale-yellow fluffy
solid. The crude produced was precipitated twice more with EtOH and
3, 6, and 11 d. Enantiomerization at 120 and 1408C: Racemate
5
once with MeCN to give the product as
(156.0 mg, 13%). M.p. 159–1608C;
a
white/pale-yellow solid
1H NMR
(30.0 mg, 7.0ꢄ10ꢀ5 mol) was dissolved in n-hexane/CH2Cl2 (2:1, 4.5 mL)
prior to HPLC enrichment. Enantiomer 5-E1 was eluted and collected
without an argon atmosphere, and the solvents were evaporated. This
procedure was performed five times, and the combined residues of 5-E1
were redissolved in degassed n-decane (5.0 mL) to give a final concentra-
tion of approximately 10ꢀ4 m. This solution was stirred at 1208C and ali-
quots (0.5 mL) were analyzed by using HPLC at time intervals of t=0, 2,
and 7 d. The temperature was adjusted to 1408C after 7 d, and the sam-
ples were analyzed by using HPLC at time intervals of t=0, 2, and 7 d.
3.6ꢄ10ꢀ1 mmol,
(400.16 MHz, CDCl3, 258C): d=1.65 (d, J=2.1 Hz, 3H), 2.31 (d, J=
1.0 Hz, 3H), 2.55–2.62 (m, 2H), 2.73–2.81 (m, 2H), 7.20–7.31 (m, 5H),
7.47–7.54 (m, 4H), 7.83 (d, J=8.2 Hz, 1H), 7.86 (d, J=8.0 Hz, 1H), 8.13–
8.20 ppm (m, 1H); 13C NMR (100.63 MHz, CDCl3, 258C): d=20.44,
21.41, 29.10, 29.42, 125.23, 125.80 (d, JC,P =28.7 Hz), 126.33 (d, JC,P
13.3 Hz), 127.34, 127.38, 127.73, 127.75, 128.13, 128.23, 128.52 (d, JC,P
=
=
10.3 Hz), 129.15, 129.16, 129.20, 129.23, 132.59, 133.38 (d, JC,P =5.2 Hz),
133.76 (d, C,P =6.6 Hz), 136.35 (d, C,P =8.5 Hz), 137.73 (d, JC,P
22.1 Hz), 138.83 (d, JC,P =22.9 Hz), 140.70 (d, JC,P =11.1 Hz), 141.78 (d,
J
J
=
UV/Vis and CD analysis: UV/Vis and CD spectroscopic measurements
were performed at 258C by using a Jasco J-815 spectropolarimeter. Ap-
propriate settings were chosen for the sensitivity, time constant, and scan
1
J
C,P =13.3 Hz), 142.87, 142.89, 145.36 (d, JC,P =15.5 Hz), 162.67 (d, JC,P =
51.6 Hz), 165.75 ppm (d, 1JC,P =42.8 Hz); 31P NMR (161.98 MHz, CDCl3,
258C): d=187.11 ppm; elemental analysis calcd (%) for C31H25P·1/
3CH2Cl2 (456.81): C 82.38, H 5.66; found: C 81.26, H 5.46.
rate. Cuvettes of 10.00 mm were used. Racemate
5 (30.0 mg, 7.0ꢄ
10ꢀ5 mol) was dissolved in n-hexane/CH2Cl2 (2:1, 4.5 mL) prior to HPLC
separation. Enantiomers 5-E1 and 5-E2 were eluted and collected without
an argon atmosphere, and the solvent was evaporated. This procedure
was performed five times, and the combined residues of 5-E1 and 5-E2
were each redissolved in ClCH2CH2Cl (1.5 mL) and diluted to give final
concentrations of approximately 10ꢀ6 m. The UV and CD spectra were re-
corded immediately.
Chloro(h5-pentamethylcyclopentadienyl)
thalen-1-yl)-4-phenyl-k-C10-5,6-dihydrobenzophosphinoline-k-P]}iridium-
(III) (9): A mixture of [{Cp*IrCl2}2] (27.9 mg, 0.035 mmol, 0.5 equiv), 5
ACHTUNGTRNE{NUNG rac-[3-methyl-2-(2-methylnaph-
ACHTUNGTRENNUNG
(30.0 mg, 0.070 mmol, 1.0 equiv), and NaOAc (6.5 mg, 0.079 mmol,
1.1 equiv) was suspended in CH2Cl2 (1.1 mL) and transferred to a Young
NMR tube under argon. The suspension was allowed to stand for 2 d at
room temperature (198C), shaken from time to time, and monitored by
means of 31P{1H} NMR spectroscopic analysis. The orange solution was
filtered over celite, and the complex was obtained by recrystallization
(diffusion of diethyl ether into the filtrate) to give red crystals (23.7 mg,
0.030 mmol, 42.8%). Compound 9 crystallized as a racemic, but diaster-
eomerically pure, mixture in the monoclinic space group P21/n (no. 14).
X-ray crystal-structure determination of 9: Crystals suitable for X-ray dif-
fraction studies were obtained by slow diffusion of Et2O into a solution
of the crude reaction mixture in CH2Cl2. Crystallographic data:
C41H39ClIrP·CH2Cl2, Mr =875.27, orange block, 0.43ꢄ0.35ꢄ0.18 mm3,
monoclinic; P21/n (no. 14), a=11.6873(3), b=13.1547(4), c=
23.6233(9) ꢃ, b=90.031(1)8, V=3631.89(19) ꢃ3, Z=4, Dx =1.601 gcmꢀ3
,
31P NMR spectroscopy of the isolated crystals displayed a diastereomeric
m=3.97 mmꢀ1; 51158 reflections were measured on a Bruker Kappa
ApexII diffractometer with sealed tube and Triumph monochromator
(l=0.71073 ꢃ) at a temperature of 150(2) K up to a resolution of (sinq/
l)max =0.65 ꢃꢀ1; intensity data were integrated with the Eval15 soft-
ware;[33] Absorption correction and scaling was performed based on mul-
tiple measured reflections with SADABS[34] (correction range 0.30–0.43);
8335 reflections were unique (Rint =0.020), of which 7871 were observed
ratio of 97:3. 1H NMR (400.16 MHz, CD2Cl2, 258C): d=1.35 (d, JH,P
=
4
2.9 Hz, 15H; C5ACHTUNGTRENNUNG
(CH3)5), 1.64 (d, 3JH,P =2.7 Hz, 3H; heterocyclic-CH3),
2.41 (d, JH,P =1.6 Hz, 3H; naphthyl-CH3), 2.54–2.75 (m, 2H), 2.81–3.00
(m, 2H), 6.75 (d, JH,P =7.0 Hz, 1H), 6.94 (dt, JH,P =7.4, JH,P =1.0 Hz, 1H),
7.18–7.24 (m, 1H), 7.28–7.33 (m, 1H), 7.34–7.56 (m, 7H), 7.67–7.75 (m,
1H), 7.82–7.93 ppm (m, 2H); 13C NMR (100.62 MHz, CD2Cl2, 258C): d=
9002
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8991 – 9004

Synthesis and Application of a Bulky Phosphinine Ligand
FULL PAPER
[I>2s(I)]; the structure was solved with direct methods by using the pro-
gram SHELXS-97;[37] least-squares refinement was performed with
SHELXL-97[35] against F2 of all reflections; non-hydrogen atoms were re-
fined freely with anisotropic displacement parameters and hydrogen
atoms were introduced in calculated positions and refined with a riding
model; 431 parameters were refined without restraints; R1/wR2 [I>
2s(I)]: 0.0230/0.0490; R1/wR2 (all reflections): 0.0247/0.0495; S=1.097; re-
sidual electron density between 1.18 and 1.38 eꢃ3. Geometry calculations
and checking for higher symmetry was performed with the PLATON
program.[36]
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CCDC-919083 (9) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_re-
quest/cif.
Computational details: Quantum-chemical calculations were carried out
with DFT by using the Gaussian 09 program[37] at the B3LYP/6-311G+
(d,p) level. Full-geometry optimizations were performed for compounds
5-R and 5-S. The nature of the stationary points was tested by analyzing
the analytically calculated harmonic normal modes. The local-minimum
structures did not show imaginary frequencies, whereas the transition-
state structure showed a single imaginary frequency that corresponded to
an eigenvector along the reaction path, that is, internal rotation about
ꢀ
the Caryl Caryl’ bond. The theoretical UV/Vis spectra were calculated at
the same level as the geometry optimizations by using the time-depend-
ent DFT (TD-DFT) method that is implemented in the Gaussian 09 pro-
gram package.[37] The number of states included in the TD-DFT calcula-
tions was 15. To simulate the absorption spectra, the full width at half-
maximum of the gaussian curves used to generate the spectra was set to
10 m. The CD and vibrational CD (VCD) spectra were simulated with
the assumption that the bandwidth at half of the height of the gaussian
curves used to generate the spectrum was equal to 20 nm and 15 cmꢀ1, re-
spectively.
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General procedures for the hydroformylation of trans-2-octene: [Rh-
ACHTUNGTRENNUNG
(acac)(CO)2] (3.1 mg, 1.2ꢄ10ꢀ4 mol) and the ligand (20 equiv, 2.4ꢄ
10ꢀ3 mol) were dissolved in toluene (8.0 mL) and stirred for 15 min at
room temperature. The mixture was transferred into an autoclave. A
dropping funnel was loaded with the substrate (2.02 g, 1500 equiv,
0.18 mmol) and n-decane (0.85 g, 0.060 mmol) as an internal standard.
The autoclave was pressurized to p=20 bar (CO/H2 =1:1) and heated
within 10 min to 808C, and the mixture was stirred (500 minꢀ1). The reac-
tion mixture preformed for 2 h followed by olefin addition, which initiat-
ed the start of the reaction. After the indicated reaction time, the auto-
clave was cooled to room temperature and depressurized, and the reac-
tion mixture was analyzed by GC analysis.
Hydroformylation of trans-2-octene: a) [Rh]/PPh3 catalyst: 99.3% con-
version after 70 h; product distribution: 2-methyloctanal, 2-ethylheptanal,
and 2-propylhexanal (56.5, 40.6, and 1.8%, respecitvely); and n-nonanal
(0.4%). b) [Rh]/P[O(2,4-di-tBuPh)]3 catalyst: full conversion after 3.5 h;
product distribution: 2-methyloctanal, 2-ethylheptanal, and 2-propylhexa-
nal (51.2, 32.3, and 9.8%, respectively), and n-nonanal (5.1%). c) [Rh]/
rac-5 catalyst: full conversion after 90 h; product distribution: 2-methyl-
octanal, 2-ethylheptanal, and 2-propylhexanal (51.5, 20.8, and 14.5%, re-
spectively), and n-nonanal (13.2%).
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Acknowledgements
The authors would like to thank Ing. Ton Staring (TU/e; Laboratory for
Catalysis and Organometallic Chemistry) for technical assistance with
the HPLC equipment. C.M. thanks The Netherlands Organization for
Scientific Research (NWO-CW) for a personal grant (NWO Vidi and
NWO Echo). The COST action PhoSciNet (CM0802) is gratefully ac-
knowledged.
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Received: February 12, 2013
Revised: March 27, 2013
Published online: May 13, 2013
9004
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