Synthesis of annelated phospholes through intramolecular C-H activation by monovalent phosphorus

Article abstract of DOI:10.1002/anie.201410603

Electrophilic terminal phosphinidene complexes [Ar-Ar-P-W(CO)₅] (Ar-Ar: biaryl or an analogue thereof) undergo a spontaneous insertion of the phosphorus atom into the vicinal C-H bonds to give annelated phospholes. Twelve examples are described, including biphenyl, thienyl, pyrrolyl, and benzofuryl groups as biaryl moieties. The activation energy of the insertion reaction is quite low (about 2 kcal mol^-1).

Full text of DOI:10.1002/anie.201410603

Angewandte
Chemie
DOI: 10.1002/anie.201410603
À
C H Activation
Synthesis of Annelated Phospholes through Intramolecular
À
C H Activation by Monovalent Phosphorus**
Xinda Wei, Zongming Lu, Xu Zhao, Zheng Duan,* and Francois Mathey*
Abstract: Electrophilic terminal phosphinidene complexes
[Ar-Ar-P-W(CO)5] (Ar-Ar: biaryl or an analogue thereof)
undergo a spontaneous insertion of the phosphorus atom into
performed in neat acetylenedicarboxylate for extended
periods of time. The cycloaddition takes place on the tungsten
side of the phosphole ring, contrary to what happens with
phenyl or methyl substituents.
À
the vicinal C H bonds to give annelated phospholes. Twelve
examples are described, including biphenyl, thienyl, pyrrolyl,
and benzofuryl groups as biaryl moieties. The activation
energy of the insertion reaction is quite low (about 2 kcal
molÀ1).
A
nnelated phospholes are recognized as valuable precursors
for the preparation of a variety of optoelectronic devices,
which explains the upsurge of interest in the synthesis of these
species in the recent literature.[1] A wide range of annelated
phospholes, including some species that were never described
before, were now prepared following a new methodology. The
carbene-like chemistry of electrophilic terminal phosphini-
dene complexes [R-P-M(CO)n] (M = Cr, Mo, W, Fe; n = 4,5)
is well developed,[2] but so far the formation of single P C
À
À
bonds by insertion of phosphinidenes into C H bonds
remains scarce.[3] Champion and Cowley described the intra-
I
À
molecular insertion of P into the C H bond of the vicinal tert-
butyl group of the transient [Fe(CO)4]/supermesitylphosphi-
nidene complex.[3a] This report led us to consider the
possibility that such insertions may be favored by proximity
effects. A logical testing ground for this idea was the family of
1,1’-biaryl-2-phosphinidene
complexes
[1,1’-Ar-Ar-2-P-
W(CO)5]. Thus, we decided to investigate this family of
phosphinidenes using the classical 7-phosphanorbornadiene
route.[4] These precursors were synthesized as shown in
Equation (1). Because of the bulkiness of the biaryl substitu-
ent, the synthesis of these 7-phosphanorbornadienes must be
[*] X. Wei, Z. Lu, X. Zhao, Prof. Dr. Z. Duan, Prof. Dr. F. 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)
and
Division of Chemistry and Biological Chemistry
Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: duanzheng@zzu.edu.cn
The thermolysis of 3a gives the complex of the parent
À
dibenzophosphole 4a, whose P H bond is immediately
visible on the 31P NMR spectrum: d31P(4a) À43.3 (CDCl3),
1JPH = 341 Hz. Similar thermolyses were carried out with 3b–
l to give 4b–l with satisfactory yields, as shown in Equa-
tion (2). This result highlights the potential of this strictly
neutral approach to annelated phospholes.
fmathey@ntu.edu.sg
[**] 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.
À
In biaryl analogues with a thiophene moiety, C H
activation can either take place at the a or b position (see
products 4 f and 4g). However, when both positions are
available, there is a 100% preference for the a position
[Equation (3)], as demonstrated by the synthesis of 4 f.
Compounds related to 4 f have been reported recently,[5]
Supporting information for this article (including the experimental
section, NMR and UV-Vis data, emission spectra, and the X-ray
crystal structure of compound 4 f) is available on the WWW under
http://dx.doi.org/10.1002/anie.201410603.
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Figure 1. X-ray crystal structure of 4 f. Main bond lengths (ꢀ) and
angles (8): P1–W1 2.4845(14), P1–C6 1.817(5), P1–C12 1.811(5), C6–
C7 1.380(7), C12–C13 1.384(6), C7–C13 1.456(7), C12–S1 1.743(5),
S1–C15 1.710(11), C13–C14 1.479(11), C14–C15 1.343(11); C6-P1-C12
89.4(2), C12-S1-C15 93.0(4).
Table 1: NMR data of 4a–k in CDCl3.
d
31P
d 1H (PH)
1JHP [Hz]
1JPW [Hz]
4a
4b
4c
4d
4e
4 f
4g
4h
4i
À43.3
À41.4
À42.7
À48.3
À41.84
À48.9
À52.9
À62.8
À63.4
À67.7
À62.2
6.79
6.78
6.78
6.87
6.84
6.92
6.76
6.79
6.75
6.97
6.74
341
341
341
343.7
339.6
349
225
227.8
224.8
228.2
224.6
229.3
225
228.3
228.5
235.4
225.5
346
343.5
350.4
347.5
343.6
4j
4k
and the structure of 4 f has been confirmed by X-ray crystal
analysis (Figure 1).[9]
The synthesis of previously unknown annelated hetero-
phospholes such as 4j is also quite noteworthy. The most
significant NMR data of 4a–k are collected in Table 1. The
NMR data of the dibenzophospholes 4a–e are quite homo-
geneous. Only the CF3-substituted compound, 4d, displays
significant differences. According to reported data,[6] the
(W) level (Figure 2a).[7] The two aryl rings form an angle of
448. The separation between P and H29 is relatively small at
2.73 ꢀ. This proximity favors the insertion, which proceeds
through a transition state that is shown in Figure 2b. In this
transition state, the phosphole ring is already formed, but the
migrating hydrogen atom is still closer to the carbon than to
the phosphorus atom. The transition state lies only 2.0 kcal
molÀ1 above the starting phosphinidene (corrected for zero-
point energy). The corresponding reaction profile is shown in
1
higher JPW coupling of 4d indicates its stronger acceptor
properties in comparison to the other compounds. On the
same basis, the a-connected phosphole 4 f has stronger
acceptor properties than the b-connected thiophene deriva-
tive 4g. The other compounds 4h–k display significantly
different NMR parameters. The heterophosphole 4j appears
to be the most powerful acceptor of the group.
In order to test the limits of the method, we investigated
whether simpler structures, such as phosphindoles, could also
be obtained by this approach. This is indeed the case, as
shown by Equation (4).
À
Figure 2c. The insertion of the P atom into the C H bond
appears to be very easy. It is clear from these data that the
relatively high temperature that is needed to perform the
synthesis of these annelated phospholes 4 results in the
collapse of the 7-phosphanorbornadiene 3. Once the phos-
phinidene is generated, its cyclization is instantaneous.
To gain information on the insertion reaction, we com-
puted the structure of the biphenylphosphinidene complex
obtained from 3a by DFT at the B3LYP/6-31G(d)-Lanl2dz
2
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In order to explain the specific formation of 4 f [see
Eq. (3)], we also computed the electronic structure of the
3-thienylphenylphosphinidene complex. It appears that the
cyclization occurs under charge control. The electrophilic
phosphorus atom (Mulliken charge + 0.323) preferably
interacts with Ca (À0.383) rather than Cb (À0.135).
The ability of monovalent phosphorus to easily activate
À
vicinal C H bonds (comparable with that of transition
metals)[8] opens quite exciting perspectives. The only question
that remains is whether it is possible to decomplex the
annelated phospholes or not. This reaction is indeed possible,
as shown in Equation (5), although the conditions have not
been optimized.
Obviously, this new approach toward annelated phosp-
holes has huge potential and supplements the existing routes[1]
to these species whose potential as conjugated materials for
optoelectronic applications is now well established. The
emission spectra of several phospholes (see the Supporting
Information) suggest that these species deserve further
attention.
Received: October 30, 2014
Published online: && &&, &&&&
À
Keywords: biaryls · C H activation · insertion · phosphinidenes ·
phospholes
.
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1.408, C25–H29 1.088, H29–P1 2.731; C13-P1-W12 112.77, C13-C15-
C23-C25 44.03. b) P1–W12 2.555, P1–C13 1.785, P1–C25 1.878, P1–
H29 1.638, C25–H29 1.302; C13-P1-C25 90.00, P1-H29-C25 78.50.
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4
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À
C H Activation
X. Wei, Z. Lu, X. Zhao, Z. Duan,*
F. Mathey*
&&&& — &&&&
Proximity matters: Electrophilic terminal
phosphinidene complexes (left, with Ar-
Ar being biaryl or an analogue thereof)
undergo a spontaneous insertion of the
annelated phospholes. The latter com-
Synthesis of Annelated Phospholes
pounds are valuable precursors for the
preparation of a variety of optoelectronic
devices.
À
through Intramolecular C H Activation
by Monovalent Phosphorus
À
P atom into the vicinal C H bond to give
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!