Synthesis and characterization of benzo[ b ]phosphaferrocene derivatives

Article abstract of DOI:10.1021/om400496b

The first examples of (η⁵-phosphindolyl)iron(II) species, which are planar-chiral, have been prepared and characterized by NMR spectroscopy and X-ray crystallography, and their partial optical resolution has been examined.

Full text of DOI:10.1021/om400496b

Note
pubs.acs.org/Organometallics
Synthesis and Characterization of Benzo[b]phosphaferrocene
Derivatives
Masamichi Ogasawara,*,† Sachie Arae,† Susumu Watanabe,† Velusamy Subbarayan,† Hiroyasu Sato,‡
and Tamotsu Takahashi*,†
†Catalysis Research Center and Graduate School of Life Science, Hokkaido University, Kita-ku, Sapporo 001-0021, Japan
‡Application Laboratory, Rigaku Corporation, Akishima, Tokyo 196-8666, Japan
S
* Supporting Information
ABSTRACT: The first examples of (η5-phosphindolyl)iron(II) spe-
cies, which are planar-chiral, have been prepared and characterized by
NMR spectroscopy and X-ray crystallography, and their partial optical
resolution has been examined.
he phospholyl anion (phospholide) 2 is isoelectronic with
cyclopentadienide 1 and is capable of coordinating to
transition-metal complexes 6 are limited to those with Mn(I)7
and Sm(II).8 The X-ray crystal structure of a phosphindo-
lyllithium also shows an η5 coordination of the phosphindolide
to a lithium cation.9 In addition to the species above, a couple
of examples of (η5-isophosphindolyl)metal complexes were
described recently.7b,10 The lack of “benzannelated” phospha-
metallocene chemistry can be partially attributed to the lack of
convenient synthetic procedures of supplying various benzo-
phospholes,7,8,11,12 which are the logical precursors to the
phosphindolyl anions. In addition, benzannelation on the
phospholide anion dramatically decreases its ability for π
coordination. For example, benzophospholides 5 and dibenzo-
phospholides 9 tend to act as σ donors, occasionally bridging
two metal centers (e.g., 7 and 10),7,13,14 and the η5
coordination of dibenzophospholides 9 has never been
reported to date.14 The latter is a clear contrast to the η5
coordination capability of fluorenyl anions 8.
T
various transition-metal cations as a six-electron donor to form
the corresponding phosphametallocenes 3 (Figure 1).1 The
Figure 1. Comparison among cyclopentadienide, phospholide, and
their benzannelated derivatives.
Among the numerous phosphametallocene complexes
reported so far, those with a central iron(II) cation, i.e.
phosphaferrocenes, have been arguably the most extensively
investigated.1 Considering these facts, it is quite surprising that
the synthesis of (η5-phosphindolyl)iron(II) derivatives has not
been achieved yet. In the paper published in 1982,7a Mathey
and co-workers examined a reaction of a phosphindolyl anion
with FeCl2, but the expected bis(η5-phosphindolyl)iron(II)
species could not be obtained, probably because the
benzannelation lowered the stability of the phosphaferrocenes.
Here we wish to report the synthesis and characterization of a
series of substituted benzo[b]phospholes (phosphindoles) and
their application in the preparation of the first (η5-
phosphindolyl)iron(II) complexes. It should be noted that
phosphindolyl anions are unsymmetrical, either substituted or
unsubstituted, and their η5 coordination to a transition-metal
cation always induces planar chirality in the corresponding
phosphorus atom in an η5-phospholide possesses a lone pair on
it; phosphametallocenes are Lewis basic. Thus, they have been
utilized as nucleophilic catalysts2 or a new class of P-donor
ligands.1,3−5 Due to an sp2-hybridized phosphorus atom,
phosphametallocenes show unique electronic properties: in
general, they are good π acceptors and poor σ donors.1
Meanwhile, various chirally modified phosphaferrocenes, which
include characteristic planar-chiral derivatives, have been
prepared and applied to a wide range of metal-catalyzed
asymmetric reactions as a new class of chiral ligands showing
excellent performances.4,5
The indenyl anion 4 and its derivatives also serve as η5 six-
electron donors. Whereas (η5-indenyl)metal complexes show a
clear contrast to (η5-cyclopentadienyl)metal species in their
reactivity as well as in their steric characteristics, they share
important positions in organotransition-metal chemistry.6 On
the other hand, metal complexes of phosphaindenyl (phos-
phindolyl) anions 5 have attracted surprisingly less attention.
To the best of our knowledge, examples of η5-phosphindolyl
Received: May 31, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400496b | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Note
complexes.15 Thus, the benzo[b]phosphaferrocenes obtained
here are also planar-chiral.
propyne provided a mixture of 14c and 14′c,19g which are
difficult to separate, in 71% yield with the ratio 14c/14′c = 72/
28.
The benzophospholes are thermally stable but slightly air-
sensitive. Diethyl derivative 14a was distilled under high
vacuum and readily converted into the corresponding phosp-
hole oxide 15a and phosphole sulfide 16a quantitatively by
H2O2 or S8 treatment, respectively.
Preparation and Characterization of Benzo[b]-
phosphaferrocenes. The preparation of the as yet unknown
(η5-benzophospholyl)iron(II) species was examined. Treat-
ment of an THF solution of 14a with excess lithium metal
resulted in a dark red color in several minutes. A 31P NMR
spectrum of the crude reaction mixture revealed a single sharp
resonance at δ 23.2, indicating quantitative conversion to the
corresponding lithium phosphindolide 17a. As an initial trial,
the reactions of 17a with FeCl2 or Fe(acac)2 were examined.
However, similar to the previous report,7a the reactions did not
afford any tractable iron-containing products. Then, we turned
our attention to the preparation of benzo-fused mono-
phosphaferrocene species. The THF solution of 17a, which
contained equimolar PhLi, was treated with [(η6-mesitylene)-
FeCp]PF6 (2 equiv to 17a) and the mixture was refluxed for 12
h.25 After the usual workup, the crude material was purified by
silica gel chromatography under nitrogen to afford benzo-
monophosphaferrocene 18a in 60% yield as a dark red viscous
liquid (Scheme 2). The phosphaferrocene is thermally stable,
Preparation of Benzo[b]phospholes (Phosphindoles).
One of the most versatile synthetic methods of phospholes
known to date is the metallacycle transfer reaction from
zirconium to phosphorus reported by Fagan and Nugent in
1988.16 Whereas various regio- and chemoselective methods of
preparing substituted zirconacyclopentadienes have been
developed,17 the metallacycle transfer process allows access to
diverse phosphole derivatives in a highly selective manner. A
typical method of generating the zirconacyclopentadiene
species is the reductive coupling of two alkynes with a divalent
zirconium reagent. Here the reaction of a zirconocene benzyne
species with an alkyne reagent gives the corresponding
zirconaindene derivative;18,19 the metallacycle transfer from
the zirconaindene to an appropriate phosphorus compound
should afford the benzophosphole derivative. Indeed, this
possibility was demonstrated in a few previous reports.8,20 We
also utilized the zirconaindene-mediated process for the
preparation of a series of novel benzo[b]phosphole derivatives
with slight modifications (Scheme 1).
Scheme 1. Zirconium Benzyne Mediated Synthesis of
Benzo[b]phospholes 14
Scheme 2. Synthesis of Benzo[b]phosphaferrocenes 18
An equimolar mixture of zirconocene diphenyl 11, which was
generated in situ from zirconocene dichloride and PhLi (2
equiv to Zr), and 3-hexyne in THF was heated to reflux for 8 h
to give a deep red solution of zirconaindene 13a.19b−f
Subsequently, the solution of 13 was treated with CuCl (2
equiv to Zr) and PhPCl2, followed by the addition of
diethylamine to give 2,3-diethyl-1-phenylbenzophosphole
(14a) in 67% yield as a colorless liquid. It was found that
CuCl facilitated the metallacycle transfer process via the Zr to
Cu transmetalation: the reaction took place faster and at lower
temperature in the presence of CuCl.21,22 The metallacycle
transfer reaction is sensitive to steric factors and becomes very
sluggish, especially with sterically demanding substituents in
THF, which is a conventional solvent for the zirconacycle
formation. The solvent exchange from THF to dichloro-
methane accelerates the metallacycle transfer process in same
cases,23,24 but the addition of CuCl is as effective as the use of
the dichloromethane solvent and could avoid the bothersome
solvent exchange.
and the compound stored in an argon glovebox at room
temperature for several months shows no sign of decom-
position. On the other hand, 18a is very sensitive to air:
exposure of its hexane solution to air bleaches the dark red
color in a few minutes and brown anonymous solids precipitate.
In the 13C NMR spectrum of 18a, the two α-phosphindolyl
carbons are detected as doublets at δ 95.4 (JPC = 54.1 Hz) and
103.9 (JPC = 60.4 Hz) with relatively large coupling constants,
while the β-phosphindolyl carbons show smaller JPC values at δ
93.7 (JPC = 6.7 Hz) and 98.2 (JPC = 4.0 Hz). The 31P NMR
spectrum recorded in C6D6 shows a sharp singlet at high field at
δ −85.3.
In the same way, 14b could be converted to benzo[b]-
phosphaferrocene 18b in 58% yield. Compound 18b is
crystallin, and deep red needles suitable for X-ray analysis can
be grown from the hexane solution. The crystal structure of
18b is shown in Figure 2 with the selected bond lengths and
angles, which confirms the η5 coordination of the phosphin-
dolyl ligand to the Fe(II) center (see the Supporting
Information for details). The phosphindolyl ligand in 18b is
planar: all of the atoms of the phosphindolyl core are located
on a plane within 0.014 Å deviation. The phosphindolyl ligand
and the Cp ligand are almost parallel with a dihedral angle of
With an unsymmetrical alkyne component, the benzophosp-
hole could be in two regioisomeric forms. The reaction using 1-
(trimethylsilyl)propyne, however, afforded α-silylbenzophosp-
hole 14b in 75% yield exclusively and the β-silyl isomer 14′b
was not detected, since the formation of 13b was highly
regioselective, giving the α-silylzirconaindene exclusively.19c,d
On the other hand, the analogous reaction using 1-phenyl-
B
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Note
Figure 2. ORTEP drawing of 18b with 50% thermal ellipsoids.
Selected bond lengths (Å) and angles (deg): Fe(1)−P(1) = 2.284(1),
Fe(1)−C(1) = 2.127(3), Fe(1)−C(2) = 2.088(3), Fe(1)−C(3) =
2.056(3), Fe(1)−C(4) = 2.094(2), P(1)−C(1) = 1.792(3), P(1)−
C(4) = 1.797(3), Fe(1)−phosphindolyl =1.656, Fe(1)−Cp = 1.659;
C(1)−P(1)−C(4) = 89.7(1).
3.73° in the nearly eclipsed conformation. The Fe−Cp (1.659
Å) and Fe−phosphindolyl (1.656 Å) distances are nearly the
same.
Analytical Resolution and Diastereomeric Complex-
ation of Planar-Chiral Benzo[b]phosphaferrocenes. Due
to their unsymmetrical structure, benzo[b]phosphaferrocenes
18 are planar-chiral,15 and optical resolution of rac-18a was
examined. Nearly complete separation of the two enantiomers
was achieved by HPLC on a chiral stationary phase (Daicel
Chiralcel OD-H column) on an analytical scale (see the
Supporting Information for details);26 however, the HPLC
method is not practical for the macroscale resolution of 18a
due to the air-sensitive nature of the compound. An attempt at
resolving rac-18a on a 5 mg scale resulted in decomposition of
the benzo[b]phosphaferrocene and resolved 18a could not be
obtained.
Figure 3. Reactions of rac-18a with chiral palladacycle (S)-Pd* and
31P{1H} NMR spectra at 162 MHz in CH2Cl2 at −90 °C: (a) free rac-
18a; (b) rac-18a/[(S)-Pd*]2 = 1.0/0.55; (c) rac-18a/[(S)-Pd*]2 =
1.0/0.11; (d) rac-18a/[(S)-Pd*]2 = 1.0/0.43.
tially resolvable by the chiral HPLC or the selective
complexation with the chiral palladacycle species. However,
complete and macroscale optical resolution of the phospha-
ferrocenes cannot be achieved due to the air sensitivity of the
complexes.
Then, we turned our attention to the enantioselective
complexation of rac-18a with the chiral palladacycle (S)-
Pd*.27,28 Treatment of rac-18a with a slight excess of [(S)-
Pd*]2 in dichloromethane afforded the pair of diastereomeric
complexes [(S)-Pd*][(S)-18a] and [(S)-Pd*][(R)-18a] at the
time of mixing with complete consumption of free 18a (Figure
3b). The 31P{1H} NMR spectrum of the mixture showed two
clearly resolved resonances of equal intensities at δ 2.4 and δ
3.2. The use of less than stoichiometric [(S)-Pd*]2 realized an
uneven complexation between (S)- and (R)-18a. For example,
a solution of rac-18a with 0.11 equiv of [(S)-Pd*]2 in
dichloromethane showed three signals in the 31P{1H} NMR
spectrum at δ −89.1, 2.4, and 3.2 which are integrated with
79.1:12.7:8.2 relative intensities (Figure 3c). The enantiomeric
purity of the unligated portion of 18a in this mixture, detected
at δP −89.1, was estimated to be ca. 6% ee. Although the
selectivity of this asymmetric complexation is rather poor, the
enantiomeric purity of the unligated 18a could be amplified to
27% ee in the mixture of rac-18a and [(S)-Pd*]2 with a 1.0/
0.43 molar ratio, which shows the three 31P resonances at δ
−89.1, 2.4, and 3.2 with a 14.9/44.6/40.5 ratio (Figure 3d).
In summary, two novel benzo[b]phosphaferrocenes, which
are the first examples of Fe(II) species with an η5-
phosphindolyl ligand, have been prepared and fully charac-
terized by NMR spectroscopy and X-ray crystallography. The
benzo[b]phosphaferrocenes are planar-chiral and are poten-
ASSOCIATED CONTENT
* Supporting Information
Text giving experimental procedures and compound character-
ization data, figures giving NMR spectra (1H, 13C, and 31P) for
all the new compounds, and a CIF file giving crystallographic
data for 18b. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: ogasawar@cat.hokudai.ac.jp (M.O.); tamotsu@cat.
hokudai.ac.jp (T.T.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Challenging
Exploratory Research (24655039) from the MEXT of Japan (to
M.O.). S.W. and S.A. are recipients of JSPS Research
Fellowships for Young Scientists.
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dx.doi.org/10.1021/om400496b | Organometallics XXXX, XXX, XXX−XXX