Expanding the phosphorus-carbon analogy: Formation of an unprecedented 5-phosphasemibullvalene derivative

Article abstract of DOI:10.1039/c6cc02461k

We could access for the first time a 5-phosphasemibullvalene derivative via quantitative and selective photochemical di-π-methane rearrangement from the corresponding phosphabarrelene. Due to the striking analogy between phosphorus and carbon, this hitherto unknown transformation of vinyl-phosphorus species provides the possibility to prepare novel, chiral and conformationally rigid organophosphorus cage compounds in a straightforward manner.

Full text of DOI:10.1039/c6cc02461k

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Expanding the phosphorus–carbon analogy:
formation of an unprecedented 5-
Cite this:DOI: 10.1039/c6cc02461k
phosphasemibullvalene derivative†
Received 21st March 2016,
Accepted 12th April 2016
Massimo Rigo, Manuela Weber and Christian Muller*
¨
DOI: 10.1039/c6cc02461k
www.rsc.org/chemcomm
We could access for the first time a 5-phosphasemibullvalene deriva-
tive via quantitative and selective photochemical di-p-methane
rearrangement from the corresponding phosphabarrelene. Due to
the striking analogy between phosphorus and carbon, this hitherto
unknown transformation of vinyl-phosphorus species provides the
possibility to prepare novel, chiral and conformationally rigid organo-
phosphorus cage compounds in a straightforward manner.
For several decades, many main-group-chemists endeavor the
often very challenging and tedious preparation of phosphorus
analogues of extraordinary hydrocarbon or nitrogen-containing
molecules.1 Fascinating compounds have been synthesized
and, in particular, the chemistry of compounds containing
PQC double and PRC triple bonds has been investigated to
quite some extent.1,2 This also includes the access to phos-
phorus heterocycles and polycyclic cages.1–3 The inspiration to
this field is mainly based on the diagonal relationship between
phosphorus and carbon and is justified by the isovalency
between a phosphorus atom and a CH-moiety. In fact, some
of these species have received tremendous interest due to their
particular stereoelectronic properties, which can differ significantly
from their related carbon- or nitrogen-containing counterparts.4
During the course of our investigations on low-coordinate
phosphorus heterocycles, we came across the remarkable
semibullvalene(tricyclo[3.3.0.02,8]octa-3,6-diene (2, 20)).5,6 This
compound has been the subject of intensive studies both from
Scheme 1 Photoinduced rearrangement of barrelenes to semibullvalenes.
Fig. 1 Literature-known phosphabarrelene and semibullvalene derivatives.
an experimental and theoretical point of view.7 Semibullvalene however, are usually not selective and the formation of byproducts
was first synthesized by Zimmerman et al. via the photosensitized is observed in many cases.10
di-p-methane rearrangement of barrelene (1) and shows rapid
Phosphorus derivatives of 1 and 2 are also known (Fig. 1).
degenerate Cope rearrangement (Scheme 1).6,8,9 Substituted Phosphabarrelene 6 has first been synthesized by Markl and co-
barrelenes, such as benzobarrelene 3, undergo similar inter- workers through a [4+2] cycloaddition reaction of the aromatic
conversions upon irradiation with UV-light. These transformations, 2,4,6-triphenylphosphinine (7) with in situ generated benzyne.11
Derivatives of 6 were later applied very successfully by Breit et al.
¨
¨
¨
Institut fur Chemie und Biochemie - Freie Universitat Berlin, Fabeckstr. 34/36,
as monodentate phosphorus–ligands in the Rh-catalyzed hydro-
formylation of internal alkenes.12 Related to 6 is phosphatripty-
cene 8, reported by Bickelhaupt and co-workers in 1974.13
The formation of tetraphosphasemibullvalene 9 was observed
by Regitz et al. during thermal cyclotetramerization of the
14195 Berlin, Germany. E-mail: c.mueller@fu-berlin.de
† Electronic supplementary information (ESI) available: Experimental procedure
for the preparation of all compounds as well as X-ray crystallographic informa-
tion. CCDC 1438074. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6cc02461k
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Fig. 2 Time-dependent 31P{1H} NMR spectra for the irradiation of
6 in THF.
corresponding phosphaalkyne and its constitution was deter-
mined by means of NMR spectroscopy.14 The only existing
mono-phosphorus containing semibullvalene derivative known
until now is dibenzo-1-phosphasemibullvalene 10, reported by
Gru¨tzmacher and co-workers.15 10 was synthesized via a multistep
procedure, starting from 5-chloro-5H-dibenzo[a,d]cycloheptene
and tris(trimethylsilyl)phosphine (P(SiMe3)3). This compound
turned out to be thermally unstable and its coordination
chemistry has not been investigated any further.
Fig. 3 Molecular structure of 12 in the crystal. Displacement ellipsoids are
shown at the 50% probability level. Selected bond lengths (Å) and angles
(1): P(1)–C(1): 1.842(2); P(1)–C(4): 1.816(2); P(1)–C(5): 1.801(2); P(1)–Se(1):
2.0814(6); C(1)–C(2): 1.505(3); C(1)–C(7): 1.536(3); C(2)–C(7): 1.559(3);
C(2)–C(3): 1.479(3); C(3)–C(4): 1.341(3); C(5)–C(6): 1.403(3); C(6)–C(7):
1.504(3); C(1)–P(1)–C(5): 94.77(9); C(1)–P(1)–C(4): 94.60(10); C(4)–P(1)–C(5):
96.64(9).
Although phosphabarrelenes of type 6 have been known for
several decades (vide supra), we were very surprised that no
investigations on photosensitized di-p-methane rearrangements
of these compounds to phosphasemibullvalene-derivatives have
been described in the literature so far. As 31P{1H} NMR spectro-
scopy would provide a perfect tool for following this rearrange-
ment reaction, we started to investigate the irradiation of a
solution of 6 in THF with UV-light and the course of the reaction
is depicted in Fig. 2.
Most strikingly, phosphabarrelene 6 is indeed gradually
consumed upon irradiation and a new single species (11) is
formed quantitatively within 15 h of reaction time. Compound
11 shows a resonance peak at d (ppm) = 33.1 in the 31P{1H}
NMR spectrum, which is almost 105 ppm downfield of the
signal of the starting material (d (ppm) = ꢀ71.8). In the 13C{1H}
NMR spectrum of 11, three resonance peaks can be detected in
the aliphatic region at d = 47.82, 60.43 and 63.68 ppm. These
signals can be attributed to three sp3-hybridized carbon atoms
of a cyclopropane ring, integrated in a semibullvalene derivative.
In contrast, starting material 6 contains only one sp3-hybridized
carbon atom, which shows a single resonance peak in the
aliphatic region of the 13C{1H} NMR spectrum. Likewise, a new
resonance peak, which integrates for one proton, can be detected
at d = 4.20 ppm in the 1H NMR spectrum of the product,
suggesting the successful photochemical di-p-methane rearran-
gement of a phosphabarrelene to a phosphasemibullvalene
derivative. Moreover, the identical molecular weights of 6 and
11 were confirmed by means of mass spectrometry.
Indeed, we were able to obtain crystals of 12 suitable for
single crystal X-ray diffraction via slow evaporation of the solvent
and the molecular structure along with selected bond lengths
and angles is depicted in Fig. 3. The X-ray crystal structure
determination confirms that compound 12 is indeed the desired
5-phosphasemibullvalene-selenide. We can therefore assume
that the photochemical conversion of phosphabarrelene 6 has
led to the exclusive formation of the unprecedented and hitherto
unknown 5-phosphasemibullvalene derivative 11.
Alternatively, we found that 12 can also be obtained quanti-
tatively from the corresponding phosphabarrelene-selenide 13,
as this compound was converted to 12 within 1.5 h upon
irradiation with UV light.17 The transformations described
above are summarized in Scheme 2.
We assume that the photochemical conversion of phospha-
barrelene 6 to 5-phosphasemibullvalene 11 proceeds in an
analogous manner as the acetone-sensitized rearrangement of
benzobarrelene 3 to benzosemibullvalene 4, investigated by
Zimmerman et al. (Scheme 1, vide supra).10 In the case of 6,
Compound 11 was obtained as a pale yellow solid but, unfortu-
nately, no single crystals could be grown for structural characteriza-
tion. We therefore attempted to convert 6 with grey selenium into
the corresponding selenide 12, hoping for a structural proof of this
compound. The product could be prepared as an off-white solid in
97% isolated yield and the presence of a phosphorus–selenium
species was confirmed by the presence of a 31P–77Se coupling
constant (1J = 824.9 Hz) in the 31P{1H} NMR spectrum of 12.16
Scheme 2 Synthetic routes to 12.
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Fig. 5 Ni(CO)3 complexes of 5-phosphasemibullvalene (14), phospha-
barrelene (15), 2,4,6-triphenylphosphinine (16) and triphenylphosphine (17).
Table 1 Selected spectroscopic data
Compound
n~(CO) [cmꢀ1] (CH2Cl2)
TEP w (ligand)
Scheme 3 Proposed mechanism for the di-p-methane rearrangement of
phosphabarrelene 6 to 5-phosphasemibullvalene 11.
16
15
14
17
2079
2075
2074
2069
23
19
18
13
corresponding LNi(CO)3 complexes 14, 15 and 16 were obtained
quantitatively (Fig. 5).
The relevant IR-spectroscopic data (n~ A1) of this series of
Ni(0) complexes are summarized in Table 1, with Ph3PNi(CO)3
(17) as the reference compound. As expected, the aromatic
Fig. 4 The two enantiomers of 11.
the radicals formed during interconversion are most likely 2,4,6-triphenylphosphinine 7 is the best p-acceptor with the
stabilized by the phenyl-groups in the a-position of the phos- lowest net-donor capability, while phosphabarrelene 6 is a some-
phorus heterocycle (Scheme 3). This substitution pattern would what better s-donor. Apparently, 5-phosphasemibullvalene 11
consequently account for the high selectivity observed for the can be considered as a slightly stronger net-donor than 6, while
conversion of 6 to 11, in contrast to the conversion of 3 to 4 triphenylphosphine is the strongest net-donor within this series.
(Scheme 3).
In summary, we have demonstrated the access to novel, chiral
As obvious from Scheme 3, the conversion of the achiral phosphorus cage compounds by photochemical di-p-methane
phosphabarrelene 6 to the 5-phosphasemibullvalene 11 leads rearrangement of phosphabarrelenes. 8,10,11-Triphenyl-1-
to the implementation of a stereogenic phosphorus atom as well phosphatricyclo-[6.2.2.02,7]dodeca-2(7),3,5,9,11-pentaene
(6)
as three stereogenic carbon atoms in one single step. However, was quantitatively and selectively converted to the corresponding,
since all four chiral centers are related to one another, com- hitherto unknown, 5-phosphasemibullvalene derivative (11) upon
pound 10 is only obtained as a racemic mixture (Fig. 4).
irradiation with UV-light. The product was unambiguously identi-
The presence of two enantiomers could indeed be verified by fied by means of single crystal X-ray diffraction of the phosphorus-
analyzing a sample of rac-12 with the help of a chiral HPLC selenide (12). First results on the ligand properties and on the
column. It should be noted here that the introduction of coordination chemistry of 11 have been obtained. The here pre-
chirality into phosphabarrelenes of type 6 is obviously very sented results describe a new approach for the preparation of novel
difficult. Research in this direction has focused on systems chiral and configurationally stable organophosphorus compounds
containing chiral auxiliaries or the introduction of axial chir- based on a di-p-methane rearrangement reaction. Although the
ality.18,19 In phosphasemibullvalenes of type 11, however, the phosphorus carbon analogy has been well established for many
intrinsic chirality based on four stereogenic centers might lead years, it generally refers to compounds with formally sp2 hybridized
to very interesting ligand systems based on phosphorus cages atoms. It is thus interesting to note that the here described results
for applications in asymmetric homogeneous catalysis in the apparently demonstrate also a striking analogy between formally
future. It should be mentioned here that 11 is a very rare sp3-hybridized phosphorus and carbon centers.
example of a configurationally rigid P-chiral compound, as
the two enantiomers cannot interconvert without breaking Training Network SusPhos (317404), the Free University of
any C–C bond.20
Berlin and the Deutsche Forschungsgemeinschaft (DFG).
Financial support was provided by the European Initial
´
In order to evaluate the Tolman electronic parameter w of 11 Marlene Bruce Vazquez Del Mercado is gratefully acknowledged
and to compare it with related phosphorus-compounds, we for fruitful discussions.
investigated the coordination chemistry of 11 as well as of
phosphabarrelene 6 and the related 2,4,6-triphenylphosphinine (7)
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