Versatile synthesis of Phospholides from open-chain precursors. application to annelated pyrrole- and silole-phosphole rings

Article abstract of DOI:10.1021/acs.orglett.5b00598

Phospholides are easily obtained by treatment of the open-chain acetylenic phosphines shown by an excess of lithium at room temperature in THF (12 examples).

Full text of DOI:10.1021/acs.orglett.5b00598

Letter
pubs.acs.org/OrgLett
Versatile Synthesis of Phospholides from Open-Chain Precursors.
Application to Annelated Pyrrole− and Silole−Phosphole Rings
Youzhi Xu,† Zhihua Wang,† Zhenjie Gan,† Qiuzhen Xi,† Zheng Duan,*,† and Francois Mathey*,†,‡
̧
†College of Chemistry and Molecular Engineering, International Phosphorus Laboratory, Joint Research Laboratory for Functional
Organophosphorus Materials of Henan Province, Zhengzhou University, Zhengzhou 450001, P. R. China
‡Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: Phospholides are easily obtained by treatment of the open-chain acetylenic
phosphines shown by an excess of lithium at room temperature in THF (12 examples).
already been described in the literature.5 The second family (4),
hospholides are one of the two fundamental aromatic rings
1
Pof phosphorus−carbon heterocyclic chemistry. However, incorporating a Z-enyne functionality, was prepared from the
appropriate bromo enynes (3) whose synthesis has been
recently described6 (Scheme 2). These two families of
phosphines have a backbone similar to those of secondary
phosphine oxides that have been previously used in a synthesis
of phosphindoles.7
the number of synthetic routes to these species is very limited
in spite of their key role in organophosphorus synthesis and
coordination chemistry. In fact, they are practically always
synthesized by reductive cleavage of the P−R exocyclic bond of
phospholes,2 the driving force being the conversion of the
nonaromatic phosphole into the aromatic phospholide.
Alternatively, it is possible to couple the easy [1,5] shift of
the sp2-carbon substituent of phospholes from P to Cα with a
deprotonation of the resulting 2H-phosphole.3 To the best of
our knowledge, the only case where a noncyclic precursor has
been used concerns the formation of a calcium phospholide by
reaction of a calcium bis(trimethylsilyl) phosphide with a 1,3-
diyne.4 We wish to report hereafter on a versatile access to
phospholides starting from easily made noncyclic phosphines
and allowing to synthesis of hitherto unknown annelated
species.
Scheme 2. Synthesis of Diphenylphosphinoenynes
Our work began with the synthesis of two families of open-
chain phosphines incorporating a carbon−carbon triple bond at
the γ position from the phosphorus center. The first family (2)
derives from triphenylphosphine. Its synthesis relies on a
classical Sonogashira coupling (Scheme 1). Phosphine 2a has
This led us to investigate the reaction of lithium with our two
families of phosphines. A statistical cleavage of the P−C bonds
would lead to a 66% yield of the anion 5 whose cyclization
would give a phosphole or a phosphindole (Scheme 3).
The actual results of our experiments with a 5- to 6-fold
excess of lithium were more satisfactory than expected. In fact,
the formation of Ph2PLi, as detected by its 31P resonance at
Scheme 1. Synthesis of o-Alkynyltriphenylphosphines
Scheme 3. Proposed Synthetic Scheme
Received: February 26, 2015
Published: March 23, 2015
© 2015 American Chemical Society
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DOI: 10.1021/acs.orglett.5b00598
Org. Lett. 2015, 17, 1732−1734

Organic Letters
Letter
−22 ppm,8 is negligible, and the cyclization almost exclusively
leads to the phospholide and phosphindolide anions 6 and 8
which are easily detected by their characteristic 31P resonances
at low fields.9 The observed shifts are collected in Figure 1. The
experiments are described in Schemes 4 and 5.
at the UB3LYP/6-311G(d,p) level. Radicals such as 10 are
known to be involved in the reduction of triarylphosphines by
alkali metals.10 The geometrical structure of 10 is shown in
Figure (2). The most striking characteristic of this structure is
Figure 2. Computed structure of radical anion 10. Main distances (Å)
and angles (deg): P12−C13 1.875, P12−C3 1.835, P12−C24 1.835,
C24−C25 1.456, C25−C34 1.411, C34−C35 1.216; C3−P12−C13
101.38, C3−P12−C24 105.06; C13−P12−C24 102.92 C24−C25−
C34 122.39.
Figure 1. 31P chemical shifts of phospholides and phosphindolides (in
THF with Li+ as counterion).
that the P−C13 (P−Ph) bond at 1.875 Å is significantly longer
than the two other P−C bonds at 1.835 Å. The SOMO (Figure
3) is antibonding on this P−C13 bond, whereas it is bonding
Scheme 4. Synthesis of Phosphindoles
Scheme 5. Synthesis of Phospholes
Figure 3. SOMO (Kohn−Sham) of radical anion 10.
and highly localized on the two other P−C bonds. These data
indicate that the radical evolves preferentially by cleavage of this
P−Ph bond.
A recent theoretical paper has stressed the interesting
properties of ladder-type heterotetracenes with dual bridging
atoms for optoelectronic applications.11 A mixed phosphorus−
silicon species 11b was included among the studied molecules.
It is possible to use this new route to phospholides to
synthesize 11b and its derivatives as shown in Scheme 6.
Due to the use of an excess of lithium, the crude solutions
containing the anions 6 or 8 also contain phenyllithium which
is formed by reduction of the initial phenyl radical by the excess
of metal. Treating the solution of 6a by iodine and sulfur led to
7i (R = R1 = Ph, X = S). In order to investigate why lithium
almost selectively cleaves the P−Ph bonds of 2 and 4, we
decided to study the electronic structure of radical 10 by DFT
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Organic Letters
Letter
In conclusion, we have developed the first general synthesis
of phospholides and annelated phospholides starting from
open-chain precursors. This synthesis has served to prepare a
new phosphorus−silicon heterotetracene of interest for
optoelectronic studies and a new phosphole−pyrrole annelated
system. Many more applications of this new scheme can be
envisaged.
Scheme 6. Synthesis of P,Si-Tetracenes
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section, NMR data for 2−14 and X-ray data for
11c This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
The initially formed β-lithiated phosphindole spontaneously
cyclizes by demethylation of the trimethylsilyl substituent to
build the silole ring. Somewhat similar demethylations leading
to benzosiloles have been described in the literature.12,13 The
X-ray crystal structure of 11c is shown in Figure 4. The core of
the molecule is slightly bent by 3.2° around the C7−C8
junction between the phosphole and the silole planes.
■
*E-mail: duanzheng@zzu.edu.cn.
*E-mail: fmathey@ntu.edu.sg.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation (21272218), Specialized Research Fund for the
Doctoral Program of Higher Education (20134101110004),
and Zhengzhou Science and Technology Department
(131PYSGZ204) of China.
DEDICATION
■
This work is dedicated to Prof. Tamotsu Takahashi on the
occasion of his 60th birthday.
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Another original species 14b incorporating fused pyrrole and
phosphole rings has also been obtained as shown in Scheme 7.
Scheme 7. Synthesis of an Annelated Pyrrole−Phosphole
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