Ambiphilic compounds: Synthesis and structure of a phosphane-borane with a flexible diphenyl ether tether

Article abstract of DOI:10.1002/ejoc.200700538

The preparation of desired phosphane-borane 1 involves selective lithiation trapping of 2-bromo-2′-iododiphenyl ether 3 to give bromophosphane 4 followed by a second lithiation/electrophilic trapping to install the boron fragment. The structure of the new

Full text of DOI:10.1002/ejoc.200700538

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200700538
Ambiphilic Compounds: Synthesis and Structure of a Phosphane–Borane with a
Flexible Diphenyl Ether Tether
Magnus W. P. Bebbington,[a] Ghenwa Bouhadir,[a] and Didier Bourissou*[a]
Keywords: Ambiphilic derivatives / Phosphanes / Boranes / Tethered compounds / Lithiation / Solvent effects
The preparation of desired phosphane–borane 1 involves se- lecular cyclization of lithiated intermediate 7 in THF was pre-
lective lithiation trapping of 2-bromo-2Ј-iododiphenyl ether
3 to give bromophosphane 4 followed by a second lithiation/
electrophilic trapping to install the boron fragment. The
structure of the new phosphane–borane was verified by sin-
gle-crystal X-ray diffraction studies. A problematic intramo-
vented by the use of toluene as the solvent. This result will
also be of interest to chemists seeking to prepare related un-
symmetrical diphosphanes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
rigid PSB derivative D of Emslie et al.[14] The synthesis of
1 includes a lithiation protocol of more general interest to
those working in phosphane ligand development, as illus-
trated by the synthesis of an unsymmetrical diphosphane
POPЈ.
Introduction
Diphosphane ligands have played a key role in the devel-
opment of transition-metal-catalyzed reactions, including in
many industrially important processes.[1] Recently, we initi-
ated a research program aimed at exploring the use of phos-
phane–boranes (PB) A as so-called ambiphilic ligands for
transition metals (Figure 1).[2] The resultant metal com-
plexes possess in some cases novel coordination modes in-
volving PǞMǞB and PǞM–XǞB[3] bridging interac-
tions. Besides the synthetic challenges and fundamental
questions raised by such transition metalǞborane interac-
tions,[4,5] new insights might also be expected for the use of
such ambiphilic ligands in organometallic catalysis. Indeed,
ligands featuring pendant Lewis acid moieties open promis-
ing ways to activate M–X bonds[6,7] and to anchor sub-
strates in the coordination sphere.[8] In addition, PB deriva-
tives A–C have recently become of major interest as metal-
free systems capable of dihydrogen activation,[9] as readily
tunable fluorescent systems[10] and as direct precursors for
photoisomerizable heterodienes.[11] It is most likely that fur-
ther developments will emerge in the near future that take
advantage of the high structural modularity of ambiphilic
compounds. In this regard, essentially rigid spacer groups
(such as the ortho- or para-phenyl group in A–C) have so
far been used to assemble PB derivatives.[12] With the aim
to extend the structural variety of PBs to flexible systems,
we thus became interested in using the diphenyl ether tether
that has been extensively employed for diphosphanes.[13] In
this communication, we detail the preparation and X-ray
structure of POB compound 1, which is related to the more
Figure 1. Representative PB ambiphilic compounds A–D with rigid
spacers and target phosphane–borane 1 containing a flexible di-
phenyl ether tether.
Results and Discussion
DPEphos and related diphosphanes are usually prepared
directly and efficiently by metallation of diphenyl ether with
nBuLi·TMEDA followed by trapping with the appropriate
P electrophiles.[13a,13b] All our attempts to extrapolate this
strategy to related PBs and even unsymmetrical diphos-
phanes only led to intractable mixtures. The preparation of
target 1 was therefore envisaged from sequential lithiation
of 2-bromo-2Ј-iododiphenyl ether (3) (Scheme 1) by analogy
with the synthetic route developed to obtain PBs A.[2] Com-
pound 3 was to be accessed in one step from known aniline
2.[15] Thus, the synthesis begins with a SNAr reaction of
commercially available 2-bromophenol and 1-fluoro-2-
[a] Laboratoire Hétérochimie Fondamentale et Appliquée du
CNRS (UMR 5069), Université Paul Sabatier
118, Route de Narbonne, 31062 Toulouse Cedex 04, France
Fax: +33-5-61558204
E-mail: dbouriss@chimie.ups-tlse.fr
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M. W. P. Bebbington, G. Bouhadir, D. Bourissou
SHORT COMMUNICATION
nitrobenzene.[16] Reduction of the nitro group under stan-
dard conditions[15] gave aniline 2 quantitatively. Diazotiza-
tion of the aniline and subsequent treatment with KI[17]
gave a mixture of iodobromo compound 3 and an unidenti-
fied aromatic impurity (ca. 20% by GC–MS) in moderate
yield. Repeated chromatographic attempts to further purify
this compound were unsuccessful.
Scheme 3. Postulated mechanism for the formation of cyclic phos-
phane 6 and borane 5 from lithiated phosphane 7.
NMR of the reaction mixture following the addition of
Mes2BF. Instead, the major product had δ = –17 ppm, only
a slight shift relative to the starting material (∆δ = –1 ppm).
The 11B NMR spectrum showed a broad signal at δ =
+80 ppm, consistent with a triarylborane,[21] and the EI
mass spectrum had the expected molecular ion at m/z =
602. Crystallization of the product from Et2O gave analyti-
cally pure PB 1 (55% isolated yield) and allowed unambigu-
ous confirmation of its structure by X-ray diffraction analy-
sis (Figure 2). The torsion angle between the two phenyl
rings of the spacer is about 60°, and the absence of intra-
molecular PǞB interaction, suggested by the NMR spec-
troscopic data, is clear from the large separation of the P
and B atoms (5.37 Å). This is not unexpected given the ste-
ric bulk surrounding the boron atom.[22] Presumably the
change in product profile results from a difference in the
aggregation state of the intermediate aryllithium in the two
solvents. The extent to which the ortho oxygen serves to
stabilize the aryllithium may also be important.[23]
Scheme 1. Preparation of 2-bromo-2Ј-iododiphenyl ether (3).
Pleasingly however, lithium–halogen exchange with
nBuLi at low temperature in THF, followed by trapping
with chlorodiphenylphosphane gave the desired bromo-
phosphane 4 as a stable white solid amenable to chromato-
graphic purification (37% isolated yield). Spectroscopic
data were consistent with the structure, notably including a
single peak in the 31P NMR spectrum at δ = –16 ppm and
a molecular ion at m/z = 433 in the EI mass spectrum. Our
initial attempts to prepare the target 1 by a second low-
temperature lithium–halogen exchange in THF were unsuc-
cessful. In THF, the 31P NMR spectrum showed a major
product with δ = –54 ppm, and the desired molecular ion
(at m/z = 602) was not detected by mass spectrometry. An
apparent molecular ion at m/z = 276, and crystallization
from the crude reaction mixture of a second product shown
to be phenyl dimesitylborane 5 by an X-ray diffraction
study suggested the concomitant formation of cyclic phos-
phane 6, a structure consistent with the 31P NMR chemical
shift (Scheme 2).[18] Product 6 is thought to arise from an
intramolecular cyclization of the first-formed aryllithium 7
with expulsion of phenyllithium (Scheme 3). The PhLi is
then trapped by Mes2BF to give borane 5. An analogous
reaction was reported for the related biphenyl system, in
which rotation about the aryl–aryl C–C bond and hence
variation of the torsion angle between the aromatic rings is
also unimpeded.[19]
Scheme 4. Preparation of phosphane–borane 1.
Scheme 2. Preparation and lithiation in THF of bromophosphane
4.
Some experimentation was therefore required to prevent
this undesired process. Prior addition of TMEDA to a solu-
tion of nBuLi in hexanes/THF and then inverse addition of
bromophosphane 4 did not significantly alter the product
Figure 2. Thermal ellipsoid diagram (50% probability) of 1.
In view of the widespread use of diphosphanes as li-
profile. However, a change to toluene as the solvent brought gands,[1] we elected to examine the possibility of preparing
about a significant breakthrough (Scheme 4),[20] as only a unsymmetrical DPEphos[13] derivatives by this route, none
trace amount of the cyclized product was observed by 31P of which appear in the literature to the best of our knowl-
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Ambiphilic Compounds: Synthesis of Phosphane–Boranes
edge[24] and which would be of general interest to those
working in the field of diphosphane chemistry. We first con-
ducted control experiments to check that the cyclization re-
action observed above was also problematic in the reaction
of P electrophiles. Accordingly, the addition of iPr2PCl fol-
lowing lithium–halogen exchange with 4/nBuLi in THF
gave a large proportion (ca. 70% by 31P NMR) of cyclic
product 6, together with some starting material (ca. 20%)
and some unidentified products. However, upon employing
was filtered to remove the Li salts. The solvent was evaporated and
crystallization from pentane gave colourless crystals of ligand 1
suitable for X-ray diffraction analysis (143 mg, 55%). M.p. 171–
173 °C. 1H NMR (300.1 MHz, CDCl3): δ = 7.31–7.04 (m, 14 H,
3
CH-Ar), 6.96–6.86 (m, 2 H), 6.65 (s, 4 H, Mes), 6.48 (d, JH,H
=
6.0 Hz, 1 H), 6.45–6.40 (m, 1 H), 2.19 (s, 6 H, 2ϫMe), 1.93 (s, 12
H, 4ϫMe) ppm. 13C NMR (75.5 MHz, CDCl3): δ = 160.5 (C-O),
2
159.1 (d, JC,P = 18 Hz, C-O), 142.6 (Cquat), 140.4 (Cquat), 138.9
(Cquat), 138.3 (Cquat), 136.9 (d, JC,P = 13 Hz, Cquat), 134.7 (CH-
Ar), 133.9 (d, JCP = 20 Hz, CH-Ar), 133.4 (CH-Ar), 131.8 (CH-
toluene as solvent, we observed clean conversion to diphos- Ar), 130.2 (CH-Ar), 129.9 (d, JC,P = 15 Hz, Cquat), 128.8 (CH-Ar),
phane 8 (31P NMR signals at δ = –16.5 and + 9.9 ppm,
128.5 (d, JC,P = 7 Hz, CH-Ar), 128.0 (CH-Ar), 123.7 (CH-Ar),
122.5 (CH-Ar), 119.1 (d, JC,P = 2 Hz, CH-Ar), 116.4 (d, JC,P
=
Scheme 5). An analytical sample was recrystallized from
Et2O/pentane. Despite the presence of an electron-rich di-
alkylphosphane group, compound 8 is somewhat surpris-
ingly quite stable to aerobic oxidation (no oxidation ob-
served by 31P NMR after 3 d standing in air, Ͻ5% oxi-
dation overnight in a solution of undistilled CDCl3).[25] No-
tably, formation of cyclized product 6 in toluene in the ab-
sence of any electrophile was very slow (Ͻ5% after 1 h)
even at room temperature.
1 Hz, CH-Ar), 23.0 (2ϫMe), 21.2 (4ϫMe) ppm. 31P{1H} NMR
(121.5 MHz, C6D6): δ = –17.3 ppm. 11B NMR (96.3 MHz, C6D6):
δ = +80 ppm. MS (EI): m/z (%) = 602 (7) [M]+, 587 (15), 278 (100),
185 (50), 77 (10). HRMS (EI): calcd. for C42H40BOP 602.2910;
found 602.2915.
Synthesis of Unsymmetrical Diphosphane 8: A solution of nBuLi
(1.4  in hexanes, 0.33 mL, 0.46 mmol) was added dropwise to a
solution of bromophosphane 4 (200 mg, 0.46 mmol) in toluene
(2 mL) at
– 60 °C. After stirring for 1 h, iPr2PCl (71 mg,
0.46 mmol) was added dropwise. After 2 h at –60 °C, the mixture
was brought to r.t. over 30 min, and after 1 h at r.t., water (2 mL)
was added. Toluene (10 mL) was added, and the organic layer was
separated and dried (Na2SO4). The solvent was evaporated, and
the crude product was purified by column chromatography (Et2O/
pentane, 3:97) to afford diphosphane 8 as a white powder (120 mg,
1
55%). M.p. 147–148 °C. H NMR (300.1 MHz, CDCl3): δ = 7.40
Scheme 5. Preparation of unsymmetrical diphosphane 8.
(td, 3JH,H = 7.8 Hz, 3JP,H = 1.8 Hz, 1 H, CH-Ar), 7.30–7.08 (m, 12
H, CH-Ar), 7.00–6.90 (m, 2 H, CH-Ar), 6.74–6.66 (m, 2 H, CH-
3
2
Ar), 6.61 (d, JH,H = 7.0 Hz, 1 H, CH-Ar), 1.99 (septd, JP,H
=
3
3
1.5 Hz, JH,H = 7.0 Hz, 2 H, CHiPr), 0.89 (dd, JP,H = 15.0 Hz,
Conclusions
3JH,H = 7.0 Hz, 6 H, 2ϫMe), 0.79 (dd, JP,H = 15 Hz, JH,H
=
3
3
7.0 Hz, 6 H, 2ϫMe) ppm. 13C NMR (75.5 MHz, CDCl3): δ =
We developed a synthesis of a phosphane–borane with a
flexible diphenyl ether tether, thus extending the structural
diversity of this potentially useful class of compounds. In
doing so, we have circumvented a problematic intramolecu-
lar cyclization reaction, thus opening a general route to
flexible unsymmetrical diphosphanes.
2
2
160.5 (d, JC,P = 6 Hz, Cipso-O), 159.6 (d, JC,P = 17 Hz, Cipso-O),
136.9 (CH-Ar), 136.7 (CH-Ar), 136.5 (Cquat), 134.2 (CH-Ar), 133.9
(CH-Ar), 130.3 (CH-Ar), 129.1 (Cquat), 128.8 (CH-Ar), 128.4 (d,
JC,P = 7 Hz, CH-Ar), 127.3 (d, JC,P = 23 Hz, Cquat), 123.6 (CH-
Ar), 122.8 (d, JC,P = 8 Hz, CH-Ar), 118.6 (CH-Ar), 118.2 (CH-
Ar), 23.2 (d, 1JC,P = 11 Hz, CHiPr) 20.7 (d, 2JC,P = 21 Hz, CH3iPr),
2
20.3 (d, JC,P = 11 Hz, CH3iPr) ppm. 31P{1H} NMR (121.5 MHz,
C6D6): δ = +9.9 (PiPr2), –17.0 (PPh2) ppm. MS (EI+): m/z (%) =
471 (Ͻ1) [M]+, 427 (100), 199 (35), 43 (40) ppm. HRMS (ES):
calcd. for C30H33OP2 471.2007; found 471.2018.
Experimental Section
General Procedures: All reactions were performed under an inert
atmosphere of argon by using standard Schlenk techniques. Dry,
oxygen-free solvents were employed. 1H, 11B, 13C and 31P NMR
spectra were recorded with Bruker AC200, Avance 300 or AMX
X-ray Crystallography: Data for 1 were collected at low tempera-
tures by using an oil-coated shock-cooled crystal with a Bruker-
AXS CCD 1000 diffractometer with graphite-monochromated Mo-
Kα (λ = 0.71073 Å) radiation. The structure was solved by direct
methods using SHELXS-97[26] and refined with all data on F2 using
SHELXL-97.[27] All non-hydrogen atoms were treated anisotropi-
cally. The hydrogen atoms were geometrically idealized and refined
by using a riding model.
1
500 instruments. H and 13C chemical shifts are reported in parts
per million (ppm) relative to Me4Si as external standard, and 11B
and 31P chemical shifts are reported in ppm relative to external
BF3·OEt2 and 85% H3PO4, respectively. Mass spectra were re-
corded with a Hewlett–Packard HP5989 spectrometer. Aniline 2
was prepared in 84% yield over two steps according to literature
procedures.[15]
1: monoclinic, space group P21/n; T = 173 K; a = 9.1254(7) Å, b =
18.5458(13) Å,
c = 20.7458(15) Å; β = 101.865(2)°; V =
Synthesis of Phosphane–Borane 1: A solution of bromophosphane
4 (186 mg, 0.43 mmol) in toluene (1 mL) was added dropwise to
a solution of nBuLi (1.6  in hexanes, 0.27 mL, 0.43 mmol) and
TMEDA (50 mg, 0.43 mmol) in toluene (2 mL) at –60 °C. After
stirring for 15 min, a solution of dimesityl boron fluoride (116 mg,
0.43 mmol) in toluene (1 mL) was added dropwise. The mixture
was brought to r.t. over 30 min, and after 1 h at r.t. the mixture
3436.0(4) Å3; Z = 4; R [IϾ2σ(I)] = 0.0475, wR2 (all data) = 0.1062
for 5230 unique reflections, 412 parameters, GooF = 1.006.
CCDC-650271 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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M. W. P. Bebbington, G. Bouhadir, D. Bourissou
SHORT COMMUNICATION
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Acknowledgments
The Centre National de la Recherche Scientifique, Université Paul
Sabatier and Agence Nationale de la Recherche (project BILI) are
warmly acknowledged for financial support of this work.
M. W. P. B. thanks the European Union for a Marie Curie Intraeu-
ropean Postdoctoral Fellowship. We also thank Dr. H. Gornitzka
for his assistance with the X-ray diffraction analysis.
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Received: June 12, 2007
Published Online: August 3, 2007
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