Polycyclic phosphiranes: Synthesis of C-substituted BABAR-Phos compounds

Article abstract of DOI:10.2533/000942903777679451

BABAR-Phos is a very stable polycyclic phosphirane which is easily synthesized in few steps from dibenzosuberenone. BABAR-Phos is remarkably stable and is not oxygenated with O₂ nor does it react with sulfur in boiling toluene. BABAR-Phos can be used as a ligand in homogenous catalysis. Substituents at the carbon of the PC₂ heterocycle can be introduced and asymmetric BABAR-Phos were prepared. The coordination chemistry of rhodium complexes containing these as ligands was investigated.

Full text of DOI:10.2533/000942903777679451

LAUREATES: AWARDS AND HONORS (SCS)
187
CHIMIA 2003, 57, No. 4
Chimia 57 (2003) 187–190
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Polycyclic Phosphiranes: Synthesis of
C-Substituted BABAR-Phos Compounds
Florian B. Läng* and Hansjörg Grützmacher
Abstract: BABAR-Phos is a very stable polycyclic phosphirane which is easily synthesized in few steps from
dibenzosuberenone. BABAR-Phos is remarkably stable and is not oxygenated with O2 nor does it react with
sulfur in boiling toluene. BABAR-Phos can be used as a ligand in homogenous catalysis. Substituents at the
carbon of the PC2 heterocycle can be introduced and asymmetric BABAR-Phos were prepared. The coordi-
nation chemistry of rhodium complexes containing these as ligands was investigated.
Keywords: Coordination chemistry · Phosphanes · Phosphiranes · Rhodium
phosphinidene magnesium complexes of
the type [R(trop)N-P=Mg] [4] which un-
Introduction
Synthesis
Phosphiranes are three-membered rings
The synthesis of BABAR-Phos starts dergo an intramolecular cycloaddition with
containing a strained PC2 heterocycle. Var- from the commercially available dibenzo- the C=C double bond of the seven-mem-
ious methods for their synthesis have been suberenone 1 which is converted via the bered ring of the trop unit (trop = 5H-diben-
developed and their reactivity has been chloride 2 and amine 3 to the amino- zo[a,d]cyclohepten-5-yl). Due to the spatial
explored [1][2]. A typical property is their (dichloro)phosphanes 4 (Scheme 1). De- proximity of the phosphinide and olefinic
tendency to undergo [2+1] cyclorever- halogenation of the PCl2 compounds 4 with unit within the rigid boat conformation of
sions which has been used to generate magnesium turnings in THF leads to for- the dibenzotropylidene unit, this key-step
phosphinidenes, R–P:, as reactive inter- mation of the phosphiranes 5. This new is favored. The reaction proceeds at room
mediates. Frequently, this reaction is even method most likely proceeds via (amino)- temperature with excellent yields (> 90%).
facilitated in phosphirane complexes and
phosphinidene complexes, [M(=PR)Ln], are
obtained. However, this decomposition im-
pedes the use of phosphiranes as ligands.
We therefore embedded the heterocyclic
PC2 unit into a polycyclic framework and
obtained very stable phosphiranes 5, which
we named BABAR-Phos [3]. These com-
pounds can be handled and stored in air
without decomposition and are not sensi-
tive against sulfurization with elemental
sulfur in boiling toluene or strong alky-
lating agents like MeOSO2CF3. They also
resist aqueous acids or bases. However,
degradation is observed when solutions of
BABAR-Phos in halogenated solvents are
exposed to light and air for longer periods
of time.
*Correspondence: Dipl.-Chem. F.B. Läng
Laboratory of Inorganic Chemistry
Department of Chemistry
ETH-Hönggerberg
CH–8093 Zürich
Tel.: +41 1 633 41 59
Fax: +41 1 633 10 52
E-Mail: laeng@inorg.chem.ethz.ch
Scheme 1. Syntheses of various BABAR-Phos compounds with sterically demanding sub-
stituents at the N-atom.

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CHIMIA 2003, 57, No. 4
Due to the strong pyramidalization at
the phosphorus center [Σ°(P) 240–250°],
one might expect that BABAR-Phos be-
haves as a π-acceptor but also as a rather
poor σ-donor because the donor-orbital
(HOMO) at the phosphorous has a high
s-character and is strongly contracted [5].
In a recent thermochemical study combined
with DFT calculations, we could show that
BABAR-Phos is the most weakly binding
Scheme 2. Synthesis of the sterically less demanding tropamine 3f as precursor for a N-methyl
BABAR-Phos 5f.
phosphane known to date in the series
[Pt(cod)Me2] + 2 PR3 → [PtMe2(PR3)2] +
cod; with PR3 = BABAR-Phos: ∆Hr =
The alcohol 9 turned out to be a versa- minus). The phosphirane ring was success-
–11.8 kcal/mol, [6]. We also found that tile reagent in palladium-catalyzed Suzuki fully formed with all aryl substituents.
metal centers can be reversibly inserted in- cross coupling reactions [10]. The reaction However, with the rather bulky 1-naphtyl
to one of the P–C bonds of the PC2 cycle with arylboronic acids, ArB(OH)2 (Ar = substituent side products were observed,
[7]. This equilibrium, metal BABAR-Phos aryl = phenyl, naphtyl, anisyl) provides the presumably polyphosphanes, because the
←
complex
metallaphosphetane, is very C-substituted alcohols 10a–c in very good C=Ctrop double bond is less accessible. The
→
sensitive with respect to the coordinating yields (> 80 %).
observed 31P chemical shifts of the C-sub-
metal center and the electronic properties The C-substituted alcohols 10a–c were stituted phosphiranes are in the range of
of the further co-ligands. With both the subsequently converted to the correspon- –128 to –132 ppm. Note that free rotation of
complexes containing the intact BABAR- ding chlorides and further transformed in- the 1-napthyl and the 2-anisyl substituents
Phos ligand as well as with the metal- to the final iPr-BABARAr-Phos 11a–c in the phosphiranes 11b,c is frozen on the
laphosphetanes, we could perform promis- according to Scheme 1 (in our notation, the NMR time scale and hence two 31P NMR
ing hydroboration and hydrosilylation ex- N-bonded substituent is given as prefix and signals are observed, each indicating one
periments [6–8].
the C-bonded one as superscript at the ter- rotational isomer.
Results and Discussion
To further investigate the properties of
BABAR-Phos as a ligand, we started to
introduce various substituents R into the
polycyclic framework. Evidently, changing
the N-bonded group R is simple and ex-
perimentally realized using different suffi-
ciently bulky primary amines in the reac-
tion with the chloride 2 (see Scheme 1 for
with R = alkyl or arene). However, the re-
action presented in Scheme 1 is not prac-
tical with small amines (i.e. MeNH2), as
one ends up with the expected mixture of
di- and tri(alkylated) amines. In this case,
tropolone 1 is converted to the correspon-
ding imine 6 which can be reduced with
NaBH4 to the desired amine 3f (Scheme 2).
Although the N-bonded substituent has
a significant influence on the activity and
selectivity of the catalyst in hydroborations
[9] a chiral group as in 5c gave disappoint-
ingly no enantioselectivity. We therefore
thought to introduce one substituent at the
C-atom of the PC2 heterocycle which also
would lead to a break-down of the mirror
symmetry of BABAR-Phos.
To functionalize the C-atom of the
heterocycle, we prepared the 10-bromo sub-
stituted dibenzocycloheptenol 9 which is
easily accessible in three steps from diben-
zosuberenone 1 via bromination to give 7,
dehydrobromination to give 8, and reduc-
tion of ketone 8 with NaBH4 in almost
quantitative yield (Scheme 3).
Scheme 3. Synthesis of the iPr-BABARAr-Phos 11a–c

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CHIMIA 2003, 57, No. 4
In these reactions, the phosphiranes
11a–c are obtained as a racemic mixture
of both enantiomers (see Scheme 4 where
we denoted the stereochemistry at each
phosphorus center as (R) or (S), respective-
ly). We introduced an additional stere-
ogenic center at the N-substituent in the
hope of being able to eventually separate
the diastereomers. This approach was real-
ized by transforming the alcohol 10a into
the corresponding chloride and then react-
ing this with enantiomeric pure (R)-(+)-1-
phenylethylamine. Unfortunately, howev-
er, we were not able to separate the finally
obtained diastereomeric phosphiranes by
chromatography.
We started to investigate the coordi-
nation chemistry using the C-substituted
phosphiranes 11a–c as ligands for rhodium
complexes. When the racemic mixture of
(R/S)-iPr-BABARPh 11a was reacted with
[Rh2(µ2-Cl)2(cod)2] in acetonitrile as sol-
vent, independently of the molar ratios of
the reactants, the Rh(I) complex 12a was
obtained exclusively as a brown powder
which rapidly precipitates out of the reac-
tion mixture.
The isolated complex consists of a Rh(I)
center which is coordinated by one chloro
and three iPr-BABARPh ligands 11a. Us-
ing two-dimensional NMR methods, the
structure was determined to be the (R,R,S)-
isomer and the enantiomeric (S,S,R)-form,
respectively, as is shown in Scheme 4.
Scheme 4. Synthesis of rhodium(I) complexes 12a with the racemic mixture of 11a.
When this isolated complex is re-dissolved
in THF, complicated 31P NMR spectra are
obtained which slowly evolve with time. We
assign these to a number of different con-
formational and rotational isomers formed
slowly in equilibria involving complexes of
type 12a. Each of these species shows three
inequivalent 31P resonances and each of
them is split into an eight-line pattern (ddd)
by coupling with two 31P nuclei and one
103Rh nucleus. Finally, on storing a THF so-
lution of 11a layered with n-hexane over
two weeks at room temperature, single
crystals of 12a were obtained. The result
of an X-ray structure analysis is given in
Fig. 1, which shows this isomer to have an
(R,R,R)-conformation (the (S,S,S)-enan-
tiomer, which is also included in the crystal
lattice, is not shown). Note, however, that
also these all-(R)-, respectively, all-(S)-iso-
mers show three different 31P resonances
indicating hindered rotation around the
P–Rh bonds. The otherwise slow rate of the
isomerization of the initially formed kinet-
ic product 12a with (R,R,S/S,S,R)-confor-
mation can be increased to minutes when
AgOTf is added to the THF solution.
As the free phosphiranes, the complex-
es 12a show an unusually high robustness
against oxidation and are not decomposed Fig. 1. Structure of the (R,R,R)-isomer of [RhCl(iPr-BABARPh)3] 12a.

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[6] C. Laporte, G. Frison, H. Grützmacher,
A.C. Hillier, W. Sommer, S.P. Nolan,
Organometallics, in press.
[7] J. Liedtke, H. Rüegger, S. Loss, H. Grütz-
macher, Angew. Chem. 2000, 112, 2596,
Angew. Chem. Int. Ed. Engl. 2000, 39,
2479.
[8] J. Liedtke, S. Loss, H. Grützmacher,
Tetrahedron (Symposium in Print) 2000,
56, 143.
[9] H. Grützmacher, J. Liedtke, G. Frasca,
F. Läng, N. Pè, Phosphorous, Sulfur and
Silicon 2002, 177, 1771.
even under 5 atm. of pure oxygen in the
[1] F. Mathey, ‘Phosphorus-Carbon Hetero-
solid state. In accordance with the results
from the calorimetry experiments cited
above [6], in solution the complexes 12a
are rather labile and the iPr-BABARPh-
Phos ligands are quantitatively displaced
when stronger ligands such as PPh3 or
P(OMe)3 are added. Interestingly, while we
could observe complexes with a heterolep-
tic coordination sphere when [PtMe2(iPr-
BABAR)2] was reacted with phosphanes,
PR3, comparable complexes, i.e. [RhCl(iPr-
BABARPh)3-x(PR3)x], were not observed.
Only the homoleptic complexes 12a beside
[RhCl(PR3)x] (x = 2,3) and free iPr-BABARPh
were detected. This finding together with
the observation that in the syntheses of
rhodium(I) BABAR-phos complexes fre-
quently the per-substituted complexes
[Rh(BABAR-Phos)4]+ or [RhCl(BABAR-
Phos)3] are formed even when a less than
stoichiometric amount of ligand is used,
leads us to suspect that additional van-der-
Waals interactions within the ligand sphere
augment the stability of the products. This
aspect as well as catalytic reactions with the
BABAR-Phos compounds described here
are under current investigation.
cyclic Chemistry: The Rise of a New Do-
main’, Pregamon Press, Amsterdam, 2001.
[2] K.B. Dillon, F. Mathey, J.F. Nixon, ‘Phos-
phorous: The Carbon Copy’, Wiley, New
York, 1998.
[3] J. Liedtke, S. Loss, G. Alcaraz, V. Gram-
lich, H. Grützmacher, Angew. Chem. 1999,
114, 1724, Angew. Chem. Int. Ed. Engl.
1999, 38, 1623. The name BABAR-Phos
is a symbiosis out of the related hydro-
carbon barbaralane and the obvious re-
semblance of BABAR-Phos to the famous
green elephant from the children’s books
(see sketch below).
[10] F. Läng, unpublished results of the planned
PhD thesis.
Fig. 2. Structure of barbarlane, C9H10, and
BABAR-Phos the ‘elephant’.
[4] H. Bock, M.Bankmann, J. Chem. Soc.,
Chem. Commun. 1989, 1130.
[5] B.J. Dunne, R.B. Morris, A.G. Orpen,
J. Chem. Soc. Dalton Trans. 1991, 635.
Received: February 7, 2003
Chimia 57 (2003) 190–192
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Stabilization of Molecular LiF, LiFHF, and
Na2SiF6 Using Metallamacrocyclic Hosts
Marie-Line Lehaire* and Kay Severin
Abstract: The molecular forms of LiF, LiFHF and Na2SiF6 have been stabilized using trinuclear metalla-
macrocyclic complexes of (cymene)RuII, (Cp*)RhIII and (Cp*)IrIII as specific receptors. The host–guest com-
plexes were characterized by NMR spectroscopy and single crystal X-ray diffraction. Based on these results,
a highly selective chemosensor for fluoride anions has been developed.
Keywords: Chemosensor · Fluoride · Lithium · Metallamacrocycle · Sodium · Stabilization
1. Introduction
in all cases. They possess three oxygen
donor atoms positioned in close proximity
*Correspondence: M.-L. Lehaire
Institut de Chimie Moléculaire et Biologique
École Polytechnique Fédérale de Lausanne
CH–1015 Lausanne
Tel.: + 41 21 693 93 13
Fax: + 41 21 693 93 05
Recently, we have investigated the self- to each other and can thus be considered as
assembly of (cymene)RuII, (Cp*)RhIII and organometallic analogues of 12-crown-3
(Cp*)IrIII complexes using 3-hydroxy-2- (Scheme). The metallacrown complexes
pyridone as the bridging ligand [1–3]. Tri- were attested to be powerful ionophores
nuclear metallamacrocycles were obtained with outstanding affinities toward Na+
E-Mail: marie-line.lehaire@epfl.ch