A new versatile route for the conversion of phospholes into phosphinines

Article abstract of DOI:10.1002/chem.201001305

(Figure Presented) En route to phosphinines: An efficient route to convert 1-phosphanorbornadienes into phosphinines is described (see scheme). This opens up the possibiltiy to use the broad and versatile phosphole chemistry as a starting point for an equ

Full text of DOI:10.1002/chem.201001305

COMMUNICATION
DOI: 10.1002/chem.201001305
A New Versatile Route for the Conversion of Phospholes into Phosphinines
Huaiqiu Wang,[a] Chuan Li,[a] Dingjin Geng,[a] Hui Chen,[a] Zheng Duan,*[a] and
FranÅois Mathey*[a, b]
As a result of their aromaticity, phosphinines play a cen-
tral role in phosphorus heterocyclic chemistry and, since the
demonstration of their extraordinary efficiency as ligands in
the rhodium-catalyzed hydroformylation of olefins,[1] the
need for new versatile synthetic methods that allow the
tuning of their electronic and steric properties becomes
even more evident. We have, some time ago, demonstrated
that the chemistry of the easily available phospholes was
governed for a large part by the readily attained equilibrium
between 1H- and 2H-phospholes.[2] From a synthetic stand-
point, a major consequence of this equilibrium is that
1-phosphanorbornadienes become easily available through
the reaction of 2H-phospholes with alkynes [Eq. (1)].
availability of 1-phosphanorbornadienes designated them as
natural starting points for the synthesis of phosphinines by
extrusion of the carbene bridge. The strain of the bridge, the
À
relative weakness of the P C bonds, and the aromaticity of
the phosphinine product suggested a rather easy transforma-
tion. Unfortunately, during a first series of attempts some
time ago, we met a limited success in this direction. It ap-
peared that only the 1-phosphanorbornadienes with a diphe-
nylcarbene bridge made from 1,2,5-triphenylphosphole are
susceptible to be transformed into phosphinines and, even
in this case, the reaction conditions are drastic (2308C) and
the versatility is very limited.[4] Even worse, the 1-phospha-
norbornadienes produced from 1-phenyl-3,4-dimethylphos-
AHCTUNGTREGpNNNU holes proved to yield only decomposition products upon
thermolysis. We reasoned that this extrusion is so difficult
because the ground state of carbenes is a triplet,[5] thus im-
plying that the extrusion is not a concerted but a stepwise
process. On the basis of this working hypothesis, we decided
to completely change our synthetic approach.
Our aim was to change the reaction pathway so that the
extruded fragment would have a singlet ground state. Since
there is no practical way to synthesize the 1-phosphanorbor-
nadienes with aminocarbene bridges, we were led to investi-
The versatility, the reaction conditions, and the efficiency
of this cycloaddition chemistry heavily depend on the substi-
tution scheme of the phosphole partner. By far the best
phosphole in this respect is the readily made[3]1-phenyl-3,4-
dimethylphosphole. From another standpoint, the ready
À
gate the oxidation of the strained P C bridge bond as previ-
ously described in the literature.[6] This oxidation has two
adverse effects for the kind of chemistry that we wanted to
perform: it reduces the strain of the bridge and replaces a
À
À
relatively weak P C by a stronger P O bond. Nevertheless,
we were confident that the concerted extrusion of a carbon-
yl derivative would prove easier than the non-concerted ex-
trusion of a carbene. This is indeed the case. Our prelimina-
ry experiments were carried out with compound 1a, easily
made from 1-phenyl-3,4-dimethylphosphole and diphenyl-
[a] H. Wang, C. Li, D. Geng, H. Chen, Prof. Z. Duan, Prof. F. Mathey
Chemistry Department, International Phosphorus Laboratory
Zhengzhou University, Zhengzhou 450052 (P.R. China)
Fax: (86)371-6778-3391
E-mail: duanzheng@zzu.edu.cn
[b] Prof. F. Mathey
ACHUTNGERNaNUG cetHCATUNGTRENNyUGN lene. The oxidation was performed by hydrogen perox-
Division of Chemistry & Biological Chemistry
School of Physical & Mathematical Sciences
Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
Fax: (65)65-6791-1961
ide in xylene at 808C. The oxidized product 2a was then
converted into the corresponding P=S derivative 3a by a
stoichiometric amount of P4S10 at 708C. In line with our ex-
pectations, it proved possible to perform the reduction-ex-
trusion on 3 a at only 1808C using 1,2-bis(diphenylphosphi-
no)ethane (dppe) as the reducing agent (Scheme 1).
E-mail: fmathey@ntu.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001305.
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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of the phosphorus lone pair:
the higher the nucleophilicity,
the lower the coupling.[7] As ex-
pected, both complexes are on
the high side with respect to tri-
phenylphosphine (245 Hz), but
the influence of the substitution
pattern appears to be higher
than expected in phosphinines.
Changing the substituents on
the side of the alkynes is the
most evident way to get a range
Scheme 1. Synthesis of phosphinines from 7-unsubstituted 1-phosphanorbornadienes.
À
Phosphinine 4a was characterized as its P W(CO)5 com-
plex (made by reaction with [W(CO)
5A
THF). This chemistry was duplicated without any problem
with 5-decyne to give the tetraalkylphosphinine 4b. We
were eager to check whether this route involving milder
conditions than our older scheme (180 vs. 2308C) was com-
patible with functionalities. The new scheme was easily
transposed for the synthesis of the 2-ethoxycarbonylphosphi-
nine 4c. In this case, the phosphinine is more resistant
toward oxidation and was characterized as such. The superi-
ority of the new scheme was confirmed by our experiments
with phenyl(trimetylsilyl)acetylene. Phosphanorbornadiene
1d was converted into the 2-trimethylsilylphosphinine 4d
with an overall yield of 51%, whereas the former route ap-
peared to be incompatible with the silyl substituent. For the
tin derivative 1e (R1 =Ph, R2 =SnBu3), the conversion fol-
lowed a different path (Scheme 2). Indeed, the tin substitu-
Figure 1. X-ray crystal structure of the pentacarbonyltungsten complex of
À
phosphinine 4e. Main bond lengths [ꢁ] and angles [8]: P1 W1
À
À
À
À
2.4710(11), P1 C1 1.731(3), P1 C5 1.703(4), C1 C2 1.399(5), C2 C3
À
À
1.415(5), C3 C4 1.407(5), C4 C5 1.379(5); C1-P1-C5 103.65(17).
of different phosphinines by
this method. Using the versatili-
ty of the 1,5-shifts in phosphole
chemistry[2] is another possibili-
Scheme 2. Synthesis of a 2-unsubstituted phosphinine from a tin 2-substituted 1-phosphanorbornadiene.
ty. We have chosen to illustrate
this possibility by replacing the
phenyl by the 9,9-dimethyl-2-
fluorenyl substituent in the
ent was lost during the sulfurization step. Replacing P4S10 by
the milder Lawesson reagent did not prevent this loss. It
must be noted, however, that the 2-unsubstituted phosphi-
nine 4e (R2 =H) is difficult to make from phenylacetylene
due to the poor yield of the synthesis of the starting phos-
phanorbornadiene.
starting phosphole. The transposition works nicely as shown
in Scheme 3.
Finding an efficient route to convert 1-phosphanorborna-
dienes into phosphinines means that we can now use the
broad and versatile phosphole chemistry as a starting point
for an equally broad variety of phosphinines. Several meth-
ods have already been described in the literature for the
conversion of phospholes into phosphinines,[8] but none of
them has the same potential as this one. Among the most
evident applications of this scheme, we can propose the con-
version of 2,2’-bis-phosphanorbornadienes[9] into 2,2’-biphos-
phinines, the coordination chemistry of which has already
proven its worth, and the use of phosphanorbornadienes
À
The P W(CO)5 complex of 4e was characterized by X-
À
ray crystal structure analysis (Figure 1). We note that the P
C bond lengths 1.703(4) versus 1.731(3) ꢁ are significantly
different. In another vein, we noticed that the JPW coupling
1
constant is significantly lower when comparing the complex
of 4e (264.7 Hz) with the complex of 4a (275.5 Hz). This
coupling is known to be correlated with the nucleophilicity
10660
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Phosphorus Chemistry
COMMUNICATION
Keywords: phosphinines
phosphorus phospholes
synthetic methods
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·
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Experimental Section
Full experimental details can be found in the Supporting Information.
CCDC-777086 (4e) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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Acknowledgements
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[10] S. R. Gilbertson, D. G. Genov, A. L. Rheingold, Org. Lett. 2000, 2,
2885.
The authors thank the National Natural Science Foundation of China
(No 20702050), Zhengzhou University (P. R. China) and the Nanyang
Technological University in Singapore for the financial support of this
work.
Received: May 14, 2010
Published online: July 30, 2010
Chem. Eur. J. 2010, 16, 10659 – 10661
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10661