Photoisomerizable heterodienes derived from a phosphine borane

Article abstract of DOI:10.1002/anie.200605078

(Chemical Equation Presented) Stable relations: Reaction of phenylazide with a phosphine borane derivative gives rise to an intramolecularly stabilized phosphazide 1 (R = iPr, R′ = mesityl) that undergoes an unprecedented photoisomerization process associ

Full text of DOI:10.1002/anie.200605078

Angewandte
Chemie
DOI: 10.1002/anie.200605078
Phosphorus Heterocycles
Photoisomerizable Heterodienes Derived from a Phosphine Borane**
Magnus W. P. Bebbington, SØbastien Bontemps, Ghenwa Bouhadir, and Didier Bourissou*
The coupling of tertiary phosphines and azides, commonly
referred to as the Staudinger reaction (Scheme 1), provides a
straightforward route to phosphazenes.[1] Numerous applica-
reversibility of these phenomena are demonstrated with a
related phosphazine adduct.
The phosphine borane derivative 1[7] reacted cleanly with
phenylazide, although a large excess of PhN3 was necessary to
achieve complete conversion in a reasonable time at room
temperature (Scheme 2). The 31P NMR signal for the result-
Scheme 1. The Staudinger reaction.
tions of this reaction have been made in organic synthesis (for
=
the preparation of C N bonds, in conjunction with the aza-
Wittig reaction) as well as in chemical biology (for the
chemoselective ligation of biologically relevant moieties).[2]
Considerable effort has also been devoted to detecting
phosphazide intermediates A. Accordingly, substituent
effects, ring strain, and/or coordination to transition metals
were eventually found to impart sufficient stabilization on
phosphazides to permit their structural characterization.[3,4]
More recently, computational studies have provided more
insight into the whole process.[5] In particular, combined
experimental and theoretical studies by Grützmacher and co-
workers suggested that phosphazides can be significantly
stabilized when the nitrogen atom in the a position to the
phosphorus is coordinated intramolecularly to a Lewis
acid.[5b] This prompted us to investigate the reaction of
azides with phosphine borane derivatives ortho-(R2P)C6H4-
(BR’2) that we have recently introduced as ambiphilic ligands
for transition metals.[6] Here we report that the resulting
phosphazide has high thermal stability, which allows the study
of its photochemical properties. Accordingly, such hetero-
dienes were found to undergo an unprecedented photo-
isomerization process associated with a change from an Na!
B to an Nb!B interaction. The generality and possible
Scheme 2. Synthesis, thermaldecomposition and photoisomerization
of the phosphazide 2. Mes=mesityl=2,4,6-trimethylphenyl.
ing product 2 (d = + 60.6 ppm) appears at significantly lower
field compared to that of 1 (d = + 5.5 ppm), which unambig-
uously shows the oxidation of the phosphorus atom. The large
shift of the 11B NMR signal (from d = + 75.0 ppm in 1 to d =
+ 2.1 ppm in 2) indicates a change from a tricoordinate to a
tetracoordinate boron atom, and mass spectrometry demon-
strates the retention of the three nitrogen atoms. All the
spectroscopic data for 2[7] support a phosphazide structure,
whose precise nature was deduced from X-ray analysis
(Figure 1a).[8] The nitrogen in the a position to the phospho-
rus atom strongly interacts with the boron center as indicated
ꢀ
by the short Na B bond (1.65 Š) and the noticeable
pyramidalization of the boron environment (340.78).[9] This
Na!B interaction results in an almost planar five-membered
ring. Notably, the heterodiene system of 2 adopts an s-cis
[*] Dr. M. W. P. Bebbington, Dr. S. Bontemps, Dr. G. Bouhadir,
Dr. D. Bourissou
Laboratoire HØtØrochimie Fondamentale et AppliquØe du CNRS
(UMR 5069)
UniversitØ PaulSabatier
118, route de Narbonne, 31062 Toulouse Cedex 09 (France)
Fax: (+33)561-558-204
E-mail: dbouriss@chimie.ups-tlse.fr
=
conformation with a terminal N N bond of E configuration.
Phosphazide 2 was then subjected to thermal decompo-
sition. Only 10% conversion was reached after heating for
18 h in toluene at reflux, and no decomposition of 2 was
observed in the solid state under vacuum until the melting
point was reached at around 2208C. At this stage a rapid but
clean loss of nitrogen occurred to give the corresponding
phosphazene 3 in 72% yield. The 31P and 11B NMR data for 3
are rather similar to those of 2. The X-ray analysis of 3[10]
revealed the retention of the N!B interaction.
[**] The CNRS, UPS, and ANR are warmly acknowledged for financial
support of this work. M.W.P.B. thanks the European Union for a
Marie Curie Intraeuropean Postdoctoral Fellowship. We also thank
Dr. H. Gornitzka for his assistance with X-ray analyses.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Intrigued by the possible reversibility of the isomerization
between 2 and 4, we next attempted to convert 4 back into 2
under various thermal and photochemical conditions. How-
ever, complex mixtures were obtained and only small
amounts of the Na-coordinated phosphazide 2 could be
observed.
To demonstrate the generality and eventually the rever-
sibility of the unusual photoisomerization process observed
for the phosphazide 2, the formation and photochemical
behavior of a related phosphazine were then investigated.
The phosphine borane derivative 1 was treated with ethyl
diazoacetate to afford the corresponding adduct 5 in 83%
yield (Scheme 3).[7] The 31P and 11B NMR data for 5 (d 31P =
Figure 1. Molecular views (thermal ellipsoids set at 50% probability)
of a) 2 and b) 4 in the solid state. Hydrogen atoms are omitted for
clarity.
The rather harsh thermal conditions required
for the loss of dinitrogen prompted us to inves-
tigate the photochemical behavior of 2 as an
alternative. Although UV irradiation is a standard
method for activating azides and diazo com-
pounds, to the best of our knowledge, this
method has not been reported so far with phos-
phazides as substrates. Irradiation of a solution of 2
in THF at room temperature for four hours (l =
Scheme 3. Synthesis and reversible isomerization of the phosphazine 5.
312 nm) led to the establishment of a photostationary
equilibrium which consisted of about 15% of 2 and 85% of
a new product with a 31P NMR signal (d = + 34.7 ppm) that is
markedly different from that of the related phosphazene 3.
The new compound was isolated by precipitation from
pentane. The 11B NMR spectrum (d = + 5.4 ppm) indicates
the retention of the tetracoordinate boron center and the
mass spectrum showed no loss of dinitrogen, which suggests
that an isomerization process had occurred. An X-ray analysis
allowed us to assign the structure of the new product as the
phosphazide isomer 4 (Figure 1b). Derivative 4 is only the
second example of an Nb-coordinated phosphazide.[4b] The
isomerization from Na!B to Nb!B coordination does not
noticeably affect the strength of the N!B interaction, as
+ 64.9 ppm and d 11B = + 2.9 ppm) are very similar to those of
2. The X-ray analysis performed on 5[8,12] established the
presence of the Na!B interaction, the s-trans conformation
of the heterodiene framework, and the E configuration of the
=
C N bond. As anticipated, the phosphazine 5 was found to
have very high thermal stability (no decomposition occurred
upon heating 5 under vacuum, even above the melting
temperature of the solid). By analogy with the related
phosphazide, the phosphazine 5 was found to isomerize
readily upon irradiation (l = 312 nm) of the solution in THF,
and a photostationary equilibrium between the starting
material 5 and a new product 6 of around 20:80, respectively,
was obtained within 24 h at ꢀ608C. The 31P NMR chemical
shift for 6 (d = + 29.9 ppm) is very similar to that of the Nb-
coordinated phosphazide 4, which suggests that a related
isomerization of Na!B to Nb!B coordination had occurred.
This was unambiguously confirmed by low-temperature
isolation and X-ray analysis of 6,[8] which provided the first
example of an Nb-coordinated phosphazine.[13] Notably, the
ꢀ
deduced from the very similar N B bond length and boron
pyramidalization (Table 1). The six-membered ring which
resulted from the Nb!B interaction strongly deviates from
planarity and adopts a flattened-boat conformation. Notably,
the isomerization from Na!B to Nb!B coordination is
accompanied by a change of both the phosphazide confor-
=
isomerization of 5 to 6 affected the configuration of the C N
=
mation (from s-cis to s-trans) and the N N bond configuration
bond, but not the conformation of the heterodiene. When the
stability of 6 was monitored by 31P NMR spectroscopic
analysis of the solution in THF at room temperature and in
the dark, the spontaneous and quantitative reisomerization of
6 into 5 with first-order kinetics (t1/2 ꢁ 16 h) was revealed.[7]
In conclusion, the Staudinger reaction between phenyl-
azide and a phosphine borane derivative was shown to give a
phosphazide with high thermal stability as a result of an
intramolecular Na!B interaction. This heterodiene under-
goes an unprecedented photoisomerization process associ-
ated with a change from an Na!B to an Nb!B interaction.
The generality and possible reversibility of such behavior
have been demonstrated. Indeed, for the related phosphazine
adduct obtained from ethyl diazoacetate, the Nb!B coordi-
nation, which was photochemically generated, could be
(from E to Z).[12] Although the interconversion between E
and Z azo compounds is a well-known process, the conversion
of 2 into 4 provides the first example of phosphazide
isomerization.[10]
Table 1: Selected bond lengths [Š] and angles [8] for compounds 2–6.
Compound
2
3
4
5
6
ꢀaB
340.8(2) 340.1(6) 340.6(2) 339.9(6)
1.687(2) 1.631(2) 1.633(4) 1.663(2)
1.649(2) 1.713(4) 1.684(7) 1.690(3)
340.2(9)
1.609(3)
1.662(4)
=
P Na
ꢀ
N B
P-NN-X
1.4(2)
–
163.2(3) ꢀ169.2(2) ꢀ162.6(2)
(X=N or C)
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Angewandte
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Farkens, A. Fischer, P. G. Jones, R. Schmutzler, Z. Anorg. Allg.
thermally transformed back to the Na!B interaction in
solution. These results extend the synthetic interest of
ambiphilic compounds which have recently led to spectacular
achievements not only as bifunctional organocatalysts,[14] but
also as molecular probes[15,16] and dihydrogen activators.[17]
Accordingly, ambiphilic derivatives may be considered gen-
eral scaffolds for new photoisomerizable systems,[18] whose
physicochemical properties might be finely tuned. With this in
mind, further investigations will focus on the structural and
electronic factors governing the coordination isomerization of
such heterodienes.
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Experimental Section
All reactions and manipulations were carried out under an atmos-
phere of dry argon, using standard Schlenk techniques. All NMR
spectra were recorded at 293 K unless otherwise stated.
2: Phenylazide (240 mg) was added to a solution of 1 (150 mg) in
toluene (1.5 mL) at room temperature. After 48 h, the solvent and
excess azide were removed under vacuum. The resulting solid was
washed with pentane to give spectroscopically pure material (180 mg,
94% yield); m.p. 220–2228C; 31P{1H} NMR (121.5 MHz, CDCl3): d =
+ 61.0 ppm; 11B NMR (96.3 MHz, CDCl3): d = + 2.1 ppm; EIMS
(70 eV): m/z (%): 561 (< 1) [M+], 442 (25) [MꢀMes]+.
3: A powdered sample of 2 (50 mg) was heated to melting point
under vacuum until no further loss of nitrogen was observed
(approximately 5 min). The resulting light yellow solid was washed
with pentane and recrystallized from diethyl ether at ꢀ188C to give 3
as colorless crystals (36 mg, 72% yield); m.p. 217–2198C; 31P{1H}
NMR (121.5 MHz, C6D6): d = + 55.8 ppm; 11B NMR (96.3 MHz,
C6D6): d = + 6.8 ppm; EIMS (70 eV): m/z (%): 533 (< 1) [M+], 490
(< 1) [MꢀiPr]+, 414 (100) [MꢀMes]+.
[7] See the Supporting Information for details.
[8] CCDC-631086–631090 contain the supplementary crystallo-
graphic data for compounds 2–6, respectively, 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.
4: A solution of 2 (60 mg) in THF was irradiated with a 312 nm
UV lamp at room temperature for 4 h. The solution was warmed to
room temperature and the solvent was removed under vacuum. The
resulting solid was washed with pentane and recrystallized from THF
at ꢀ188C to give colorless crystals (50 mg, 83% yield): m.p. 208–
2108C; 31P{1H} NMR (121.5 MHz, CDCl3): d = + 34.7 ppm; 11B NMR
(96.3 MHz, CDCl3): d = + 5.4 ppm; EIMS (70 eV): m/z (%): 561 (< 1)
[M+], 442 (25) [MꢀMes]+.
[9] This Na!B interaction is apparently enforced by the proximity
of the N and B atoms. Aryl dimesityl boranes do not normally
bind fragments larger than fluoride which makes them suitable
groups for optical and electronic systems. For example, see: C. D.
Entwistle, T. B. Marder, Chem. Mater. 2004, 16, 4574 – 4585.
[10] In such systems, the Na!B (or Nb!B) interaction is inherently
=
related to a formal E (or Z) configuration of the P N skeleton,
Received: December 15, 2006
Revised: January 31, 2007
Published online: March 22, 2007
with the phenyl ring at the P atom being antiperiplanar (or
synperiplanar) to the Nb atom. As a result, the geometry of such
phosphazide and phosphazine skeletons can be described by
three independent parameters: the s-cis/s-trans arrangement of
Keywords: boranes · isomerization · phosphazides ·
.
=
=
the PNNN or PNNC fragment, the E or Z terminal N N or N C
bond, and the Na!B or Nb!B interaction.
phosphorus heterocycles · photochemistry
[11] Rapid s-cis or s-trans equilibration has been suggested for a
macrobicyclic tris(phosphazide) on the basis of VTP NMR
experiments; see Ref. [3i].
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=
=
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