A 1,1′-ferrocenyl phosphine-borane: Synthesis, structure and evaluation in Rh-catalyzed hydroformylation

Article abstract of DOI:10.1039/c0nj00117a

The new ambiphilic ligand Ph₂P-(1,1′-ferrocenyl)-BMes ₂, prepared by sequential lithiation/electrophilic trapping of 1,1′-dibromoferrocene, adopts a monomeric structure free of dative P → B and Fe → B interactions. This flexible phosphine-borane and the related o-phenylene bridged system have been evaluated in Rh-catalyzed hydroformylation.

Full text of DOI:10.1039/c0nj00117a

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LETTER
www.rsc.org/njc | New Journal of Chemistry
A 1,10-ferrocenyl phosphine-borane: synthesis, structure and evaluation
in Rh-catalyzed hydroformylationwzy
a
a
a
´
Magnus W. P. Bebbington,z Sebastien Bontemps,8 Ghenwa Bouhadir,
Martin J. Hanton,b Robert P. Tooze,b Hendrick van Rensburgb and
Didier Bourissou*a
Received (in Montpellier, France) 12th February 2010, Accepted 18th March 2010
DOI: 10.1039/c0nj00117a
The new ambiphilic ligand Ph2P–(1,10-ferrocenyl)–BMes2,
prepared by sequential lithiation/electrophilic trapping of
1,10-dibromoferrocene, adopts a monomeric structure free of
dative P - B and Fe - B interactions. This flexible phosphine-
borane and the related o-phenylene bridged system have been
evaluated in Rh-catalyzed hydroformylation.
Scheme 1
There has been growing interest in ambiphilic derivatives over
The target ligand 2 was prepared by sequential lithiation/
electrophilic trapping of 1,10-dibromoferrocene (Scheme 1), a
very similar strategy to that used to prepare PB derivatives
with conventional aromatic spacer groups. Borylation of the
1-bromo,10-diphenylphosphinoferrocene 111 afforded 2, which
was isolated as a purple powder after work up (64% yield).
The 31P NMR chemical shift for 2 (d = ꢀ17.5 ppm) is very
similar to that of 1 (d = ꢀ18.1 ppm), and the 11B NMR
chemical shift (d = 84.7 ppm) is in the typical range for a free
triarylborane, suggesting a monomeric structure without
P - B interaction.12 This was unambiguously confirmed by
an X-ray diffraction study (Fig. 1).** Despite the free rotation
of the two Cp rings, the phosphorus and boron groups do not
position themselves anti to one another, but even so they are
still shifted markedly from an eclipsed arrangement (the C1P
axis is rotated by 45.21 with respect to the C6B axis). Due to
steric repulsion of the two bulky functional substituents, the
two Cp rings are not exactly parallel (tilt angle = 7.81). The
large PB distance (4.87 A, compared with 3.75 A for the sum
of the van der Waals radii),13 and the planar environment
around boron (the sum of CBC bond angles SBa = 359.71)
rule out any significant P - B interaction. The absence of any
the last few years. The range of applications of phosphine-
boranes (PB), for example, has grown considerably: e.g.
p-conjugated systems,1 ligands for transition metals able to
adopt different coordination modes,2 metal-free systems
capable of H2 activation and transfer,3 precursors for photo-
isomerizable systems,4 and molecular probes for putative
reaction intermediates.5
These developments have stimulated the preparation of an
increasing variety of PB compounds differing in the number of
accompanying donor buttresses, in the substitution pattern at
phosphorus and boron, and in the nature of the spacer group,
which permits the P–B distance and the rigidity of the system
to be altered.6 In this respect, purely organic linkers have been
so far largely preferred, and only a few metallocene-bridged
phosphine-boranes have been reported.7–9 This includes
the recent preparation of Ph2P–(1,2-ferrocenyl)–BMes2
(Mes = 2,4,6-trimethylphenyl).9 Here, we report the synthesis
and complete characterization of the position isomer
Ph2P–(1,10-ferrocenyl)–BMes2 2. This new phosphine-borane
and the related rigid system Ph2P–(o-phenylene)–BMes2 310
have been evaluated in Rh-catalyzed hydroformylation of
1-octene.
a
´
Universite de Toulouse, UPS, LHFA, and CNRS, LHFA UMR
5069, 118 route de Narbonne, 31062 Toulouse, France.
E-mail: dbouriss@chimie.ups-tlse.fr; Fax: +33 5-6155-8204;
Tel: +33 (0)561557737
b Sasol Technology UK, Purdie Building, North Haugh, St Andrews,
UK KY16 9ST
w Dedicated to the memory of Pascal Le Floch, a brilliant scientist and
a great person.
z This article is part of a themed issue on Main Group chemistry.
y Electronic supplementary information (ESI) available: Experimental
details and characterization data for 2. CCDC reference number
766316. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0nj00117a
z Present address: School of Engineering and Physical Sciences,
William Perkin Building, Heriot-Watt University, Edinburgh EH14
4AS, UK.
Fig. 1 Molecular view of 2 in the solid state (thermal ellipsoids at
50% probability). The phenyl and mesityl groups are simplified.
Hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (1): C(1)–P(1) 1.814(6), C(6)–B(1) 1.565(9), C(6)–B(1)–
C(32) 122.4(5), C(6)–B(1)–C(23) 115.2(5), C(32)–B(1)–C(23) 122.1(5).
8 Present address: CNRS, LCC, 205 route de Narbonne and
Universite de Toulouse, UPS, INPT, 31077 Toulouse, France.
´
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(l : b 1.2–1.3) were found for both ligands (Table 1), suggesting
that the BMes2 group of 3 does not participate as a Lewis acid,
but rather as a sterically demanding substituent.20
In conclusion, the phosphine-borane Ph2P–(1,10-ferrocenyl)–
BMes2 2 has been prepared and characterized both spectro-
scopically and crystallographically. Precatalysts based on the
combination of this new ambiphilic ligand or the related
system Ph2P–(o-phenylene)–BMes2 3 with [Rh(acac)(CO)2]2
promote the hydroformylation of 1-octene. The higher activity
and selectivity in favor of the linear product are found
with the flexible 1,10-ferrocenyl vs. o-phenylene bridge. The
phosphine-borane 3 and Buchwald-type biarylphosphine 4
were shown to behave very similarly, and future investigations
will seek to compare the properties of these two classes of
ligands in other transformations such as Pd-catalyzed cross-
coupling reactions.
Chart 1
Fe - B interaction is also apparent from the long FeB
distance (3.37 A) and from the slight deviation of the CipsoB
bond out of the plane of the Cp ring (by 71).14
Until now, phosphine-boranes and related ambiphilic
ligands have only been rarely evaluated in transition metal-
mediated catalysis.7,8,15,16 Here, we decided to study the
rhodium-catalyzed hydroformylation of 1-octene and evaluated
both 2 and the related phosphine-borane Ph2P–(o-phenylene)–
BMes2 3 described previously (Chart 1).10 The reactions were
performed in toluene at 90 1C and 40 bar of syngas with
[Rh(acac)(CO)2], 20 equiv. of the ligand and 1-octene : Rh
ratio E 4000. With the flexible ligand 2, the rhodium
concentration was fixed at 0.5 mM in order to maintain the
temperature at 90 1C. Under these conditions, complete
Experimental
General considerations
All reactions were performed using standard Schlenk techniques
under an Argon atmosphere. NMR spectra were recorded at
293 K on a Bruker Avance 300 spectrometer. Chemical shifts
are expressed with a positive sign, in parts per million, relative
to residual 1H (7.24 ppm) and 13C (77.16 ppm) solvent signals
and external BF3ꢂOEt2 (0 ppm) and 85% H3PO4 (0 ppm),
respectively. The mass spectrum was recorded on a Hewlett
Packard 5989A apparatus. Pentane was dried over CaH2,
tetrahydrofuran and toluene over sodium and distilled prior
to use. All organic reagents were obtained from commercial
sources and used as received. The 1,10-dibromoferrocene,11a
1-bromo,10-diphenylphosphinoferrocene 1,11b 1-dimesitylboryl-
2-diphenylphosphinobenzene10 and biaryl diphenylphosphine
ligand 417 were prepared according to literature procedures.
conversion was reached in about 45 min (TOF 6400 hꢀ1
)
and the linear product was obtained with 72% selectivity. The
related borane-free ligand, namely diphenylphosphinoferrocene,
afforded the same rate and product distribution within
experimental error (Table 1). This indicates that the BMes2
group of 2 does not play any role in this case, most probably as
the result of the flexibility of the 1,10-ferrocenyl spacer. The
properties of the related rigid phosphine-borane 3 were then
evaluated in order to assess the influence of the linker. The
1-octene was completely consumed within about 150 min
under the same conditions (except for [Rh] = 1 mM and
1-octene : Rh E 2000) to afford a 54 : 46 mixture of linear and
branched aldehydes. Thus, the presence of a BMes2 group
maintained in close proximity to the metal coordination
sphere induces a decrease of both activity and selectivity. In
order to determine whether the influence of the BMes2 group
was essentially steric or electronic in origin, the related
sterically encumbered Buchwald-type biarylphosphine 417
was tested under the same conditions.18 Quasi-identical
rates (TOF E 1700–1800 hꢀ1) and product distributions
Preparation of 1-dimesitylboryl-10-diphenylphosphino-
ferrocene (2)
A solution of n-butyllithium (1.4 M in hexanes, 357 mL,
0.50 mmol) was added dropwise to a solution of 1-bromo-10-
diphenylphosphinoferrocene (224 mg, 0.50 mmol) in THF
(2.5 mL) at ꢀ70 1C. After 10 min, a yellow precipitate
appeared. After a further hour at this temperature, a solution
of Mes2BF (135 mg, 0.50 mmol) in THF (1 mL) was added
dropwise. The mixture was then brought to room temperature
over 30 min and the solvent was then removed at reduced
pressure. The residue was dissolved in toluene (3 mL) and
filtered by cannula to remove Li salts. The product 2 was
recrystallised at ꢀ40 1C from pentane and isolated as a purple
powder (198 mg, 64% yield). Mp 155–156 1C (dec.). 1H NMR
(300.1 MHz, CDCl3): d = 7.45–7.30 (m, 10H, HPh), 6.85 (s,
4H, HMes), 4.65 (s, 2H, HFerr), 4.50 (s, 2H, HFerr), 4.46 (s, 2H,
Table 1 Catalytic results for the hydroformylation of 1-octene.
Reactions were carried out in toluene at 90 1C and 40 bar (1 : 1 feed
syngas)
Ligand
TOF/hꢀ1 c
K/hꢀ1
l : bf
Linearity (%)
2a
6400d
6240d
1700e
1785e
2.74
2.57
1.56
1.78
2.5
2.6
1.2
1.3
72
73
41
46
H
Ferr), 4.08 (s, 2H, HFerr), 2.39 (s, 12H, o-CH3), 2.34 (s, 6 H,
a
p-CH3); 13C NMR: d = 142.7 (Cq), 139.1 (Cq), 138.7 (d,
JCP = 10 Hz, Cq), 137.1 (Cq), 133.6 (CH), 133.4 (CH), 128.6
(CH), 128.2 (CH), 80.1 (CH), 77.3 (d, JCP = 9 Hz, Cq), 75.4
(CH), 73.9 (d, JCP = 14 Hz, CH), 72.5 (CH), 24.6 (o-CH3),
21.1 (p-CH3), the quaternary carbon atom of ferrocene bearing
the boron atom is not observed; 11B NMR (96.3 MHz,
FcPPh2
3b
4b
a
b
c
With [Rh] = 0.5 mM. With [Rh] = 1 mM. Average turnover
d
frequency. Calculated from 0 to 30 min. Calculated from 0 to
f
60 min. Linear to branched product distribution.
e
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CDCl3): d = 84.7; 31P NMR (121.5 MHz, CDCl3) d = ꢀ17.5;
m/z (EI): 618 (M+ꢃ), 432 (M+ꢃ ꢀ PPh2), 368 (M+ꢃ ꢀ BMes2),
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ꢃ
Catalytic experiments
A 50 mL s/s autoclave equipped with gas entraining stirrer and
sampling valve, was charged with [Rh(acac)(CO)2] and placed
under an atmosphere of N2. Dry, de-gassed (d/d) toluene
(10 mL) was then added. In a glovebox, a Schlenk was charged
with ligand (L : Rh = 20), then transferred to Schlenk line.
Toluene (d/d, 10 mL) was added to the ligand and the solution
added to the autoclave under flow of N2. At room temperature,
the vessel was pressurised to 20 bar with 1 : 1 syngas, then
heated to 90 1C. Once the reaction temperature was reached,
1-octene (d/d, 10 mL) was injected and the reactor pressure
increased to 40 bar. During the reaction, the pressure in the
vessel was maintained via a temperature compensated ballast
vessel and the rate of catalysis was assessed by measuring the
pressure drop in the ballast vessel which was logged with a
polling frequency of 1 s. When gas uptake had ceased, the
vessel was cooled to RT, the excess pressure vented, the vessel
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Acknowledgements
Financial support from the Centre National de la Recherche
Scientifique, the Universite de Toulouse and the Agence
´
Nationale de la Recherche (ANR-06-BLAN-0034) are
gratefully acknowledged. D. B. thanks the COST action
CM0802 PhoSciNet for supporting this work, and M. W. P.
B. thanks the European Union for a Marie Curie Intra-
european Postdoctoral Fellowship. We also thank Dr P. W.
Dyer for helpful discussions and Dr H. Gornitzka for his
assistance with the X-ray analysis.
8 1,10-Bis(diphenylphosphino)ferrocenes featuring
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