Synthesis of 2,5-difunctional phosphaferrocenes

Article abstract of DOI:10.1021/om9010164

The reaction of FeCl₂ with a 1:1 mixture of 2, 5diacylphospholides and Cp*Li in the presence of ZnCl₂ affords 2,5-bis(RCO)phosphaferrocenes when R = OEt, Ph and the tetrafunctional 1,1'-diphosphaferrocene when R = ^tBu. We explain these results by a combination of two effects: the push-pull stabilization of the ferrocene structure by Cp* (donor) and phospholide http://phospholi.de (acceptor) and the classical steric protection of the η⁵ complexes by bulky α-substituents.

Full text of DOI:10.1021/om9010164

Organometallics 2010, 29, 1053–1056 1053
DOI: 10.1021/om9010164
Synthesis of 2,5-Difunctional Phosphaferrocenes
Aholibama Escobar and Franc-ois Mathey*
Nanyang Technological University, Division of Chemistry & Biological Chemistry, 21 Nanyang Link,
Singapore 637371
Received November 24, 2009
Summary: The reaction of FeCl2 with a 1:1 mixture of 2,5-
diacylphospholides and Cp*Li in the presence of ZnCl2
affords 2,5-bis(RCO)phosphaferrocenes when R = OEt,
Ph and the tetrafunctional 1,10-diphosphaferrocene when
R=tBu. We explain these results by a combination of two
effects: the push-pull stabilization of the ferrocene struc-
ture by Cp* (donor) and phospholide (acceptor) and the
classical steric protection of the η5 complexes by bulky
R-substituents.
ring.4 This contradiction is suppressed if we admit that a
sizable electronic transfer from cyclopentadienyl to phos-
pholyl takes place through iron in phosphaferrocenes. This
transfer must induce a stabilization of the phosphaferro-
cene by a classical push-pull effect. In order to evaluate
this effect, we decided to compute the energies of the
two members of the formal equation (1) by DFT at the
6-31G(d)-Lanl2dz (Fe) level.5
Together with phosphinines, phosphaferrocenes are one
of the two fundamental cIasses of aromatic species featur-
ing an sp2 phosphorus center. They are readily functiona-
lizable, and they have already found a certain number of
catalytic applications.1 At the present time, however, es-
sentially no 2,5-difunctional phosphaferrocene is known
aside from the silyl derivatives,2 thus preventing the easy
incorporation of the phosphaferrocene unit into rings or
polymers. This point has been raised by Ganter,2 who
unsuccessfully tried to prepare 2,5-diacylphosphaferro-
cenes. We show hereafter how it is possible to circumvent
this limitation.
The results are ambiguous. The two molecules of phos-
phaferrocene appear to be more stable than the 1 þ 1
mixture of ferrocene and 1,10-diphosphaferrocene by only
0.9 kcal mol-1 (ZPE included). Even though this effect is
limited, we could expect to enhance it on replacing Cp by
Cp* and the nonfunctional phospholyl by a phospholyl
substituted with an electron-withdrawing group. Two
practical consequences can be expected: (1) if we react a
1:1 mixture of cyclopentadienide and phospholide with
FeCl2, we must get more phosphaferrocene than the sta-
tistical mixture; (2) we can envisage compensating the very
poor π-complexing ability of 2,5-diacylphospholyl by
using the strong Cp* donor as the coligand of iron. This
led us to investigate the reaction of a 1:1 mixture of the
pentamethylcyclopentadienide 1 and 2,5-diethoxycarbo-
nyl-3,4-dimethylphospholide (2)6 with FeCl2 in the pre-
sence of ZnCl2 as acidic coreagent.7 We were delighted to
Results and Discussion
Our initial reasoning was that the failure reported by
Ganter was due to the very poor π-donating ability of 2,5-
diacylphospholides. Indeed, it is well established, on the
basis of comparative IR studies of cymantrene and phos-
phacymantrene, that the nonfunctional η5-phospholyl is
already a poorer π-donor than the η5-cyclopentadienyl
ligand.3 From another standpoint, this fact seems to con-
tradict the observation that the Friedel-Crafts acylation of
phosphaferrocenes selectively takes place at the phospholyl
(5) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez,
C.; Pople, J. A. Gaussian 03, Revision B.05; Gaussian, Inc., Pittsburgh,
PA, 2003.
*To whom correspondence should be addressed. E-mail: fmathey@
ntu.edu.sg.
(1) Melaimi, M.; Mathey, F.; Le Floch, P. J. Organomet. Chem. 2001,
640, 197. Qiao, S.; Fu, G. C. J. Org. Chem. 1998, 63, 4168. Tanaka, K.; Qiao,
S.; Tobisu, M.; Lo, M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 9870.
Ganter, C.; Kaulen, C.; Englert, U. Organometallics 1999, 18, 5444.
Shintani, R.; Lo, M. M.-C.; Fu, G. C. Org. Lett. 2000, 2, 3695. Ogasawara,
M.; Yoshida, K.; Hayashi, T. Organometallics 2001, 20, 3913. Shintani, R.;
Fu, G. C. Org. Lett. 2002, 4, 3699. Shintani, R.; Fu, G. C. J. Am. Chem. Soc.
2003, 125, 10778. Fu, G. C. Acc. Chem. Res. 2006, 39, 853. Ogasawara, M.;
Ito, A.; Yoshida, K.; Hayashi, T. Organometallics 2006, 25, 2715. Willms,
H.; Frank, W.; Ganter, C. Organometallics 2009, 28, 3049.
ꢀ
(2) Sava, X.; Mezailles, N.; Maigrot, N.; Nief, F.; Ricard, L.; Mathey,
F.; Le Floch, P. Organometallics 1999, 18, 4205. Loschen, R.; Loschen, C.;
Frank, W.; Ganter, C. Eur. J. Inorg. Chem. 2007, 553.
(3) Poizat, O.; Sourisseau, C. J. Organomet. Chem. 1981, 213, 461.
(4) de Lauzon, G.; Deschamps, B.; Fischer, J.; Mathey, F.; Mitschler,
A. J. Am. Chem. Soc. 1980, 102, 994.
(6) Holand, S.; Jeanjean, M; Mathey, F. Angew. Chem., Int. Ed. 1997,
36, 98.
(7) Escobar, A.; Donnadieu, B.; Mathey, F. Organometallics 2008,
27, 1887.
r
2010 American Chemical Society
Published on Web 01/28/2010
pubs.acs.org/Organometallics

1054 Organometallics, Vol. 29, No. 4, 2010
Escobar and Mathey
Figure 1. X-ray crystal structure of phosphaferrocene 3. Main
˚
distances (A) and angles (deg): Fe1-P1=2.2780(3), Fe1-C1=
2.0643(11), Fe1-C13=2.0773(12), P1-C1=1.7868(12), P1-
C4 = 1.7832(12), C1-C2 = 1.4384(16), C2-C3 = 1.4254(16),
C3-C4 = 1.4386(16); C1-P1-C4 = 87.88(5).
Figure 3. X-ray crystal structure of 1,10-diphosphaferrocene 6.
˚
Main distances (A) and angles (deg): Fe1-P1 = 2.2746(9),
Fe1-C1=2.092(3), Fe1-C2=2.086(3), P1-C1=1.782(3), C1-
C2=1.430(5), C2-C3=1.426(5), C3-C4=1.423(5), P1-C4=
1.795(3); C1-P1-C4 = 88.72(15).
phosphaferrocene 3, both rings are in a staggered conforma-
tion. The phosphole ring is quasi planar, the bending around
C1 C4 being only 1.67°. The Cp* and phospholyl planes
3 3 3
are almost parallel (angle 1.33°). The phospholyl plane is
closer to iron than the Cp* plane (1.637 vs 1.670 A). As
˚
expected, the phospholyl ring shows practically no alterna-
tion between the C-C bonds. In phosphole 4, the steric bulk
of the Cp* substituent induces a lengthening of the P-C
˚
exocyclic bond at 1.8967(16) A but the phosphorus pyramid
P
is not flattened ( (angles at P) = 302.6°, the same as in
1-benzylphosphole8). The intracyclic P-C bonds are long,
and the alternation between C-C and CdC bonds is high.
The bending of the ring around C4-C9 is significant at 9.65°.
Using the same experimental protocol, but replacing the 2,5-
bis(ethoxycarbonyl) by 2,5-dibenzoylphospholide,9 we ob-
tained the 2,5-dibenzoylphosphaferrocene 5 in 33% yield.
The surprise came when we used the 2,5-bis(pivaloyl)phos-
pholide.
Figure 2. X-ray crystal structure of phosphole 4. Main dis-
˚
tances (A) and angles (deg): P1-C13 = 1.8967(16), P1-C9 =
1.8033(17), P1-C4=1.8011(16), C4-C5=1.362(2), C5-C7=
1.464(2), C7-C9 = 1.367(2), C4-C3 = 1.474(2), C9-C10 =
1.479(2); C4-P1-C9=89.86(8), C4-P1-C13=103.57(7), C9-
P1-C13 = 109.16(7).
find that this reaction provides a very simple access to the
2,5-difunctional phosphaferrocene 3 (eq 2).
In the presence of a stoichiometric amount of Cp*Li, only
traces of the expected phosphaferrocene 7 are obtained. By
far the main product of the reaction is the tetrafunctional
1,10-diphosphaferrocene 6. In the absence of Cp*Li, we got
an impressive 80% yield of this product. The identity of 6 was
unambiguously established by X-ray crystal structure ana-
lysis (Figure 3). The conformation of the molecule lies half-
way between staggered and eclipsed. One ring is rotated with
respect to the other by 23.24°. The two phospholyl planes are
almost parallel (interplane angle 1.26°). The two rings are
This result is remarkable because we have shown pre-
viously that the reaction of 2 with FeCl2 leads exclusively to
the tetrafunctional P-P coupling product.7 In line with this
previous result, we just get a small quantity of the Cp*-
substituted phosphole 4. Both products were characterized
by X-ray crystal structure analysis (Figures 1 and 2). In
(8) Coggon, P.; McPhail, A. T. J. Chem. Soc., Dalton Trans. 1973,
1888.
(9) Toullec, P.; Mathey, F. Synlett 2001, 1977.

Note
Organometallics, Vol. 29, No. 4, 2010 1055
slightly folded around their CR-CR0 axis, both by 1.78°. The
two planes are equidistant from iron at 1.655 A. The
preferential formation of 6 vs 7 is somewhat mysterious. It
might have something to do with the relative stabilities of the
intermediate σ-complexes.
to react with an excess of lithium wire for 5 h at room
temperature. After excess lithium was removed, the solution
was treated with tert-butyl chloride (0.6 mL, 5.32 mmol) and
heated to 60 °C for 1 h. Benzoyl chloride (0.7 mL, 5.32 mmol)
was added dropwise at -78 °C. The solution was warmed to
room temperature and stirred for 20 min. tBuOK was added
(0.597 g, 5.32 mmol) at 0 °C. The resulting mixture was heated at
60 °C for 2 h. Another 1 equiv of benzoyl chloride (0.7 mL, 5.32
mmol) was added at -78 °C, immediately followed by addition
of another 1 equiv of tBuOK (0.597 g, 5.32 mmol) at 0 °C. The
resulting mixture was stirred at 60 °C for 2.5 h. The solution was
cooled to 0 °C, ZnCl2 (1.450 g, 10.63 mmol) was added, and the
mixture was stirred at room temperature for 30 min. The
resulting 2,5-bifunctional phospholide solution was treated in
the same manner as for 3. Purification was performed via cold
column chromatography on silica using 1:1 dichloromethane-
hexanes. A red band was collected and, on concentration, 900
mg (1.76 mmol) of a red oil (5) was obtained in 33% yield.
Data for 5 are as follows. 1H NMR (CD2Cl2): δ 1.61 (s, 15H,
Me), 2.19 (s, 6H, Me), 7.38 (t, 4H meta), 7.48 (t, 2H para), 7.74 (d,
4H ortho). 13C NMR (CD2Cl2): δ 9.24 (s, Me Cp*), 12.83 (s, Me
˚
Experimental Section
2,5-Bis(ethoxycarbonyl)phosphaferrocene 3 and 1-pentamethyl-
cyclopentadienylphosphole 4. 1-Phenyl-3,4-dimethylphosphole10
(1 g, 5.32 mmol) in dry THF (15 mL) was allowed to react with
an excess of lithium wire for 5 h at room temperature. After
excess lithium was removed, the solution was treated with tert-
butyl chloride (0.6 mL, 5.32 mmol) and heated to 60 °C for 1 h.
Ethyl chloroformate (0.55 mL, 5.85 mmol) was added dropwise
at -78 °C. The solution was heated at 65 °C for 2 h. tBuOK was
added (0.597 g, 5.32 mmol) at 0 °C. The resulting mixture was
heated at 60 °C for 2 h. Another 1 equiv of ethyl chloroformate
(0.6 mL, 5.85 mmol) was added at -78 °C, immediately followed
by addition of another 1 equiv of tBuOK (0.597 g, 5.32 mmol) at
0 °C. The resulting mixture was stirred at 40 °C for 18 h. The
solution was cooled to 0 °C, ZnCl2 (1.450 g, 10.63 mmol) was
added, and the mixture was stirred at room temperature for 30
min. In a separate reaction flask, Cp*Li was prepared by adding
n-BuLi (2.85 mL, 4.5 mmol) to 1,2,3,4,5-pentamethylcyclopen-
tadiene (0.84 mL, 5.32 mmol) in dry THF (15 mL) at -78 °C.
The solution was stirred at -78 °C for 30 min and warmed to
room temperature for 1 h. The 2,5-bifunctionalized phospholide
mixture was then added to the Cp*Li solution at -78 °C and
stirred for 20 min at -78 °C. In a separate flask, FeCl2 (0.725 g,
5.32 mmol) was stirred in dry THF at room temperature for 30
min. After the FeCl2 solution was cooled to -78 °C, the 2,5-
bifunctionalized phospholide/Cp*Li solution was added drop-
wise by cannula. The resulting mixture was stirred at -78 °C for
1 h and at room temperature for 18 h. The crude solution
mixture was concentrated, dissolved in methylene chloride,
filtered through silica, and concentrated. Purification was per-
formed via cold column chromatography on silica using 1:1
dichloromethane-hexanes. An orange band was collected and,
once concentrated, 570 mg (1.28 mmol) of an orange solid (3)
was obtained in 30% yield. The product was crystallized by slow
evaporation of methylene chloride. When the polarity of the
eluent was increased to 2:1 dichloromethane-hexanes, a yellow
band was recovered from the same column. On concentration,
100 mg (0.26 mmol) of a yellow solid (4) was obtained in 4.8%
yield. The solid was crystallized by slow evaporation of methy-
lene chloride.
1
phosphole), 85.27 (s, C Cp*), 92.45 (d, JPC = 61.2 Hz, C-P),
=
98.60 (d, 2JPC =5.4Hz, Cβ), 127.74 (s, CH meta), 128.81 (d, JPC
6.5 Hz, CH ortho), 131.47 (s, CH para), 141.88 (s, C ipso), 200.33
(d, 2JPC = 20.4 Hz, CO). 31P NMR (CD2Cl2) δ-17.5. Exact mass:
m/z calcd for C30H32FeO2P 511.1489, found 511.1475.
2,20,5,50-Tetrapivaloyl-1,10-diphosphaferrocene 6. 1-Phenyl-
3,4-dimethylphosphole (1 g, 5.32 mmol) in dry THF (15 mL)
was allowed to react with an excess of lithium wire for 5 h at
room temperature. After excess lithium was removed, the solu-
tion was treated with tert-butyl chloride (0.6 mL, 5.32 mmol)
and heated to 60 °C for 1 h. Pivaloyl chloride (0.7 mL, 5.32
mmol) was added dropwise at -78 °C. The solution was warmed
to room temperature and stirred for 20 min. tBuOK was added
(0.597 g, 5.32 mmol) at 0 °C. The resulting mixture was heated at
60 °C for 2 h. Another 1 equiv of pivaloyl chloride (0.7 mL, 5.32
mmol) was added at -78 °C, immediately followed by addition
of another 1 equiv of tBuOK (0.597 g, 5.32 mmol) at 0 °C. The
resulting mixture was stirred at 60 °C for 2.5 h. The solution was
cooled to 0 °C, ZnCl2 (1.450 g, 10.63 mmol) was added, and the
mixture was stirred at room temperature for 30 min. The
resulting 2,5-bifunctional phospholide solution was treated in
the same manner as for 3. Purification was performed via cold
column chromatography on silica using 3:1 dichloromethane-
hexanes. An orange band was first collected. After concentra-
tion, 50 mg (0.11 mmol) of an orange solid (7) was obtained in
2% yield. A second red band corresponding to the diphospha-
ferrocene 6 was collected and, on concentration, 320 mg
(0.52 mmol) of a red solid was obtained in 20% yield. The
product was crystallized by slow evaporation of hexanes.
For the optimized synthesis of 6, the initial preparation of the
2,5-diacylphospholide was identical. After the addition of
ZnCl2, FeCl2 (0.335 g, 2.66 mmol) was added at room tempera-
ture. The solution was stirred for 18 h at room temperature. The
crude solution mixture was filtered through silica and concen-
trated. Purification was performed via column chromatography
on silica using 3:1 dichloromethane-hexanes. The red band was
collected and, on concentration, 1.311 g (2.136 mmol) of a red
solid (6) was obtained in 80% yield.
Data for 3 are as follows. 1H NMR (CD2Cl2): δ 1.31 (t, 6H,
Me), 1.64 (s, 15H, Me), 2.24 (s, 6H, Me), 4.12 (q, 4H, OCH2). 13
C
NMR (CD2Cl2): δ 8.99 (s, Me Cp*), 11.90 (s, Me), 14.15 (s, Me),
1
60.00 (s, OCH2), 82.78 (d, JPC = 56.8 Hz, C-P), 84.74 (s, C
Cp*), 97.83 (d, 2JPC = 6.4 Hz, Cβ), 171.58 (d, 2JPC = 18.5 Hz,
CO). 31P NMR (CD2Cl2) δ -29.7. Exact mass: m/z calcd for
C22H32FeO4P 447.1388, found 447.1386. Anal. Calcd for
C22H31FeO4P: C, 59.21; H, 7.00. Found: C, 60.08; H, 7.34.
Data for 4 are as follows. 1H NMR (CD2Cl2): δ 1.23 (d, 3JHP
= 15.6 Hz, Me Cp*), 1.31 (t, Me OEt), 1.60 (s, Me Cp*), 1.67 (s,
Me Cp*), 2.21 (s, Me phosphole), 2.22 (s, Me phosphole), 4.08
and 4.21 (2 m, OCH2). 13C NMR (CD2Cl2): δ 10.63 (s, Me Cp*),
10.77 (d, JCP = 5.8 Hz, Me Cp*), 14.09 (s, Me OEt), 15.30 (s, Me
Data for 6 are as follows. 1H NMR (CD2Cl2): δ 1.16 (s, 36H,
Me tBu), 1.99 (s, 12H, Me phosphole). 13C NMR (CD2Cl2): δ
12.99 (s, Me phosphole), 27.15 (s, Me tBu), 45.39 (s, C tBu),
98.79 (d, 1JPC = 72.5 Hz, C-P), 102.19 (d, 2JPC = 3.8 Hz, Cβ),
210.41 (d, 2JPC = 16.3 Hz, CO). 31P NMR (CD2Cl2) δ -61.3.
Exact mass: m/z calcd for C32H49FeO4P2: 615.2456, found
615.2466. Anal. Calcd for C32H48FeO4P2: C, 62.54; H, 7.87;
Found: C, 62.85: H, 8.14.
2
phosphole), 18.84 (d, JCP = 12.5 Hz, Me-C-P), 60.57 (s,
1
OCH2), 133.52 (s, =C Cp*), 135.98 (d, JCP = 9.6 Hz, C-P),
2
139.74 (s, dC Cp*), 153.38 (d, JCP = 8.6 Hz, Cβ), 166.07 (d,
2JCP = 19.2 Hz, CO). 31P NMR (CD2Cl2) δ 29.7. Exact mass: m/
z calcd for C22H32O4P 391.2038, found 391.2043.
2,5-Dibenzoylphosphaferrocene 5. 1-Phenyl-3,4-dimethyl-
phosphole (1 g, 5.32 mmol) in dry THF (15 mL) was allowed
Data for 7 are as follows. 1H NMR (CD2Cl2): δ 1.21 (s, Me
tBu), 1.62 (s, Me Cp*), 2.04 (s, Me phosphole). 13C NMR
(CD2Cl2): δ 9.52 (s, Me Cp*), 12.05 (s, Me phosphole), 27.63
ꢁ
(10) Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983.

1056 Organometallics, Vol. 29, No. 4, 2010
Escobar and Mathey
and 27.71 (2s Me tBu), 45.69 (s, C tBu), 84.72 9 (s, C Cp*), 91.22
work and to Dr. Li Yong Xin for the X-ray crystal structure
analyses.
(d, 1JPC = 66.7 Hz, C-P), 99.50 (d, 2JPC = 4.8 Hz, Cβ), 211.78
2
(d, JPC = 16.8 Hz, CO). 31P NMR (CD2Cl2) δ -66.3. Exact
mass: m/z calcd for C26H40FeO2P 471.2115, found 471.2109.
Supporting Information Available: CIF files giving X-ray
crystal structure analyses of compounds 3, 4, and 6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. Thanks are due to Nanyang Techno-
logical University in Singapore for financial support of this