An aromatic-antiaromatic switch in P-heteroles. A small change in delocalisation makes a big reactivity difference

Article abstract of DOI:10.1039/b516836h

The low aromaticity of phosphole can be switched to low antiaromaticity by oxidizing the phosphorus atom. This subtle change in the mode of delocalisation alters substantially the chemical behaviour of these heteroles. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516836h

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An aromatic–antiaromatic switch in P-heteroles. A small change in
delocalisation makes a big reactivity difference†
La´szlo´ Nyula´szi,*a Oldamur Hollo´czki,a Christophe Lescop,b Muriel Hisslerb and Re´gis Re´au*b
Received 28th November 2005, Accepted 30th January 2006
First published as an Advance Article on the web 10th February 2006
DOI: 10.1039/b516836h
The low aromaticity of phosphole can be switched to low an-
tiaromaticity by oxidizing the phosphorus atom. This subtle
change in the mode of delocalisation alters substantially the
chemical behaviour of these heteroles.
of these assemblies is their relatively low thermal stability which is
probably due to the isomerisation of the 1H-phosphole hearts into
very reactive 2H-phospholes.5 Interestingly, r4-thiophospholes 2a,
b (Scheme 1) exhibit higher thermal stability allowing these com-
pounds to be used as multifunctional materials for OLEDs.4c This
is likely due to the fact that the 1H-phosphole → 2H-phosphole
isomerisation process is less favoured for r4-phospholes.5b Hence,
chemical oxidation of the P-centre is a clue towards the opti-
mization of the physical properties of these P-materials towards
optoelectronic applications and we have tried to extend this
methodology to other phosphole-based oligomers.
Aromaticity and antiaromaticity are fundamental and fruitful
concepts to rationalise the stability and reactivity of annulenes.1 In
the case of heterocyclic derivatives, the nature of the heteroatom
usually determines their aromatic or antiaromatic character.1b,c
For example, borole A (Scheme 1) is strongly antiaromatic while
pyrrole B exhibits a large aromatic stabilisation.2 The aromaticity
of phosphole C (Scheme 1) has long been debated. It is accepted
now that it exhibits a small aromaticity arising from the interaction
of the p-system not with the P-lone pair, but with the exocyclic
r(P–R) bond (hyperconjugation).3 In this paper, we show that
this unique situation offers the opportunity to switch from
“slightly” aromatic to “slightly” antiaromatic species by chemical
modification of the P-centre and that this switch has a considerable
impact on the chemical properties of these heterocycles.
In contrast to what is observed in the thienyl and phenyl
series, oxidation of the P-atom of 2-pyridyl- and 4-pyridyl-capped
phospholes 1c, d (Scheme 1) results in a decrease of their thermal
stability. For example the decomposition temperature in the solid
state, as estimated by thermogravimetric analysis, of r3-phosphole
◦
1c is 210 ◦C against 145 C for its r4-derivative 2c. The same
trend is observed in solution. r3-Phospholes 1c, d are stable in
◦
refluxing THF for weeks. In contrast, in THF at 40 C, r4-
thiophospholes 2c, d quantitatively transform within two days into
the corresponding phospholenes 3c, d (Scheme 1). The structure
of these novel derivatives was established by multinuclear NMR
spectroscopic data (Table 1), high resolution mass spectrometry
1
and elemental analysis. For example, the 13C{ H}NMR spectra
of the diastereoisomeric mixture of 3c shows two doublets due
to a PCH moiety (+55.4 ppm, JPC = 51.3 Hz and +59.9 ppm,
JPC = 51.1 Hz) and only three remaining signals assignable to
the CH2 groups of the fused carbocycle. These NMR data are
very similar to those of structurally related free or coordinated r3-
phospholenes.6 It is interesting to note that two diastereoisomers
are also observed for 3d at low conversion (31P NMR: d, +64.8
and +66.2). However, after completion of the reaction only one
is obtained (Table 1) indicating a thermodynamically controlled
process. These results prompted us to reinvestigate the thermal
behaviour of 2-thienyl- and phenyl-capped r4-thiophospholes 2a,
b (Scheme 1) in detail. These compounds are stable in refluxing
xylene (138 ◦C) for days. It is likely that the pyridine groups of 2c, d,
acting as a base, can catalyse the 1,3-shift leading to phospholenes
3c, d via the formation of an allylic anion [Scheme 2, eqn (1)].6,7a
Scheme 1 Conditions: i, 3a: refluxing xylene, Et2NH, 1 day; 3b: refluxing
xylene, Et2NH, 4 h; 3c, d: 40 ◦C, THF, 2 days. Dotted lines show the
connectivity of the R1 substituents.
Phosphole-based derivatives 1a–c (Scheme 1) have recently
emerged as a novel class of p-conjugated systems.4 One drawback
aDepartment of Inorganic Chemistry, Budapest University of Technology
and Economics, H-1521 Budapest, Szt. Gelle´rt te´r 4, Hungary. E-mail: nyu-
laszi@mail.bme.hu; Fax: +36 14633642; Tel: +36 14633281
Table 1 Selected NMR Data (CD2Cl2) for phospholenes 3a–d
P–CH
bUMR 6509 CNRS-Universite´ de Rennes 1, Institut de Chimie, Campus
de Beaulieu, 35042, Rennes Cedex, France. E-mail: regis.reau@univ-
rennes1.fr; Fax: +33 2 23236939; Tel: +33 2 23235784
d31P, ppm
d1H, ppm (JPH, Hz)
d 13C, ppm (JPC, Hz)
† Electronic supplementary information (ESI) available: Detailed descrip-
tion of the experimental procedure and total energies (in hartree) and
optimized structures (in cartesian coordinates) of compounds 2a, c; 3a,
c; 4a–e, 5a–e, 6a–e and 7a–e at the given levels of the theory. See DOI:
10.1039/b516836h
3a
3b
3c
3d
+65.8/+64.6
+66.9
+66.4/+64.8
+64.8
5.41 (13.6)/4.90 (20.0)
4.66 (22.7)
5.02 (16.5)/4.82 (11.8)
4.45 (10.8)
54.6 (56.1)/54.3 (51.2)
59.6 (52.6)
59.9 (51.1)/55.4 (51.3)
54.0 (50.3)
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Table 2 DE: Relative energies of 4a–e and 5a–e; E(TS2) and E(TS3):
relative energy of the transition states (6a–e) in reaction (2) and (7a–e) in
reaction (3) (Scheme 2) with respect to 4a–e. B3LYP/6-311+G** energies
in normal text and CCSD(T)/cc-PVDZ//B3LYP/6-311+G** energies
(all in kcal mol−1) in bold text. Bird aromaticity indices (BI), nucleus
independent chemical shifts (NICS) and ISEc homodesmotic B3LYP/6-
311+G** reaction energies (kcal mol−1) for 4a–e
DE
4.7/5.2
−2.3/−0.9
−3.6/−1.9
−2.4/−0.5
−3.2/−1.6
E(TS2)
E(TS3)
ISEc
BI/NICS
4a
4b
4c
4d
4e
87.7
86.0
86.1
88.6
85.4
55.2
49.1
51.5
46.8
51.5
−2.6
4.4
5.7
4.5
5.3
44/−5.5
33/−2.1
31/−1.0
25/−1.5
32/−1.1
Scheme 2 Possible isomerisation mechanisms.
Hence, an excess of diethylamine was added to the refluxing xylene
solution of 2a, b. Indeed, under these harsh reaction conditions,
the formation of the corresponding phospholenes 3a, b is observed
(Scheme 1, Table 1). The conversion of 2a is not complete (yield
ca. 50%) whereas 2b is quantitatively transformed into 3b.
intermediate allylic anion6,7a [Scheme 2, eqn (1)] as supported by
the fact that this reaction is base-catalysed in the case of 2a, b
(Scheme 1). Hence, the conclusions of this theoretical study fit
nicely with all the experimental observations.
Related r3-phosphole–r3-phospholene isomerisation is known
to proceed in the coordination sphere of transition metals.6,7b How-
ever, this type of r4-phosphole–r4-phospholene transformation is
unprecedented. In order to get more insight into this surprising iso-
merisation, quantum chemical calculations (B3LYP/6-31+G*)8
were carried out for the 2-thienyl- and 2-pyridyl-capped series.
Indeed, they revealed that r4-thiophosphole 2c is less stable than
r4-thiophospholene 3c by 1.3 kcal mol−1, while 2a is somewhat
(by 0.9 kcal mol−1) more stable than 3a. These results are in
agreement with the experimental facts that the isomerisation is
almost quantitative for 2c and only partial for 2a (conversion ca.
50%). Furthermore, these calculations show that one of the two
diastereoisomers of 3c (the one with a relative trans arrangement
of the sulfur at P and of the pyridyl group) is more stable than the
other one by 3.4 kcal mol−1. This rationalizes that, in some cases
(Table 1), only one diastereoisomer is obtained at high conversion.
To understand the origin of these effects and the isomerisation
mechanism, the parent ring has been investigated. The r3-
phosphole 4a (Table 2) is more stable than its 2-phospholene
isomer 5a by ca. 5 kcal mol−1 (Table 2). In contrast, r4-
thiophosphole 4b (Y = S) is slightly less stable than its phospholene
isomer 5b (Table 2). Oxidation of the P-atom results in an
inversion of the phosphole–phospholene thermodynamic stability.
Note that (i) all r4-phospholes 4c–e are also less stable than the
corresponding phospholenes 5c–e and (ii) the relative stability of
4b–e decreases when the electronegativity of Y increases (series
b/c, series d/e). The activation energy for the non-catalysed 4b–
e → 5b–e transformation via a monomolecular [intramolecular
1,3-H shift, Scheme 2, eqn (2)] and a bimolecular [intermolecular
H-transfer + isomerisation, Scheme 2, eqn (3)] processes is 85–89
kcal mol−1 and 47–55 kcal mol−1, respectively, depending on the
nature of Y (Table 2). These high energy barriers discard these two
reaction paths since the isomerisation of 2c, d into 3c, d (Scheme 1)
occurs at 40 ◦C, and there is no reason to believe that the presence
of the R1 substituents would lower these barriers considerably.
It is thus probable that the isomerisation mechanism involves an
The next question was to elucidate what is the driving force
of the above isomerisation? Schleyer has recently shown that
the energy difference between a conjugated ring with a methyl
substituent and its isomer with exocyclic double bonds (termed
as isomeric stabilisation energy, ISE) is an excellent measure
of aromatic stabilisation, if corrected by the diene anti–syn
mismatches between the cyclic conjugated system and its isomer.9
The correction is in fact included in the homodesmotic reaction
[see eqn (4)] energy termed here as ISEc. The ISEc results (Table 2)
show that r3-phosphole 4a exhibits a slight aromatic stabilization,
while all the r4-phospholes 4b–e are slightly antiaromatic!
(4)
Since the ISEc energies are small, we investigate further
aromaticity measures to gather additional evidence about the
change in the aromaticity upon P-oxidation of r3-phospholes. We
selected the Bird indices,10 a geometric criteria, and the NICS(0),11
a magnetic criteria. The Bird index of 4a is considerably larger
than for 4b–e (Table 2). Phosphole 4a itself has a moderately small
negative NICS, while NICS values for r4-phospholes12 4b–e are
more positive by 3–4 ppm. Hence, both criteria show a similar
tendency: r3-phosphole 4a has a small aromaticity, which is shifted
towards a small antiaromaticity in the case of r4-phospholes
4b–e. All these data suggest that the inversion of phosphole–
phospholene stability upon oxidation of the P-atom is due to an
aromatic–antiaromatic switch.
It is noteworthy that the antiaromatic character of the r4-
phospholes 4b–e increases with the electronegativity of the Y
fragment (Table 2). This behaviour is formally understandable,
−
+
=
considering the ylide form of the Y P bond (↔ Y –P ).
The presence of a positively charged phosphorus results in a
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cyclopentadienide cation-like structure, which is antiaromatic,
explaining the destabilization of the r4-phospholes. In other words,
the positive partial charge at phosphorus, which increases with
the increasing electonegativity of the Y substituent, lowers the
energy of all orbitals at phosphorus. This prevents the interaction
of the endocyclic p-system with the occupied orbitals of the P-
fragment, while the interaction with the unoccupied r* orbitals
(electron demand) is getting more important resulting in an
antiaromatic character. This electronic situation is reminiscent of
what is observed for 1,1-difluorocyclopentadiene which is also
an antiaromatic species.13 It is also worth mentioning that the
aromaticity of the ring carbomers14 of phosphole and that of
phosphole oxide has recently been investigated, and for C14H4PH a
strong antiaromaticity, while for the phosphole oxide C14H4P(O)H
a moderate aromaticity has been observed.
It is clear that the aromaticity of heterocycles such as A
and B (Scheme 1) can be influenced by variation of sub-
stituents of the heteroatom. However, the case of the phosphole
(aromatic–antiaromatic switch) is unique, even in P-heterocycle
chemistry. For example, r2-phosphabenzene is highly aromatic,
while the r4-phosphabenzene was shown to be slightly (with
H or alkyl substituent) to significantly (with halogen or OR
substituents) aromatic.3b,15 Thus, variation of substituents reduces
but not suppresses aromaticity as illustrated by the fact that
r4-phosphabenzene derivatives have reduced Wittig-reactivity in
comparison with their saturated counterparts,16 in agreement with
an aromatic energetic stabilization.
In conclusion, oxidation of the P-atom of slightly aromatic
r3-phospholes affords r4-derivatives exhibiting a small antiaro-
matic character. Although this switch in aromatic–antiaromatic
stabilisation is small, it has a significant impact on phosphole
properties. Firstly it destabilises r4-derivatives 2a–d with respect
to their phospholene isomers 3a–d. Secondly, since the aromatic
character of building blocks is a crucial parameter influencing the
HOMO–LUMO gap of p-conjugated systems,17 it is very likely
that this aromatic–antiaromatic balance is at the origin of the
tuning of optical properties of phosphole-based oligomers upon
P-modifications.4
99, 586; L. Nyula´szi, Tetrahedron, 2000, 56, 79; F. G. N. Cloke, P. B.
Hitchcock, P. Hunnable, J. F. Nixon, L. Nyula´szi, E. Niecke and V.
Thelen, Angew. Chem., Int. Ed., 1998, 37, 1083.
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C. Fave, M. Hissler, J. Rault-Berthelot and R. Re´au, Org. Lett., 2003,
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C.-C. Wu and R. Re´au, J. Am. Chem. Soc., 2003, 125, 9254; (d) C.
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Hissler, P. Dyer and R. Re´au, Topics in Current Chemistry New Aspects
in Phosphorus Chemistry V, Springer, Berlin, 2005.
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6871.
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Halet and R. Re´au, Chem. Commun., 2003, 14, 1774; (b) F. Leca, C.
Lescop and R. Re´au, Organometallics, 2004, 23, 6191.
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(b) K. D. Redwine and J. H. Nelson, Organometallics, 2000, 19, 3054.
8 Computations were carried out by the Gaussian 98 suite of programs
(M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M.
Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J.
Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T.
Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L.
Andres,C. Gonzalez, M. Head-Gordon, E. S. Replogle and J. A. Pople,
GAUSSIAN 98, Revision A.5, Gaussian, Inc., Pittsburgh PA, 1998) at
the B3LYP/6-311+G** level of the theory. All structures were fully
optimized, and the nature of the stationary points obtained (minima
or first order saddle point) was characterised by calculation of the
second derivatives matrix (having zero or one negative eigenvector,
respectively). Subsequent CCSD(T)/cc-PVDZ//B3LYP/6-311+G**
calculations were also carried out to obtain improved relative energies
for 4 and 5. 6a–e, 7a–e were computed at B3LYP/6-31+G*//B3LYP/3-
21G*.
9 P. v. R. Schleyer and F. Puhlhofer, Org. Lett., 2002, 4, 2873; M. K.
Cyran´ski, Chem. Rev., 2005, 105, 3773.
10 Bird indices with a 0 to 100 (fully aromatic) scale were calculated
by using the original procedure of Bird (C. W. Bird, Tetrahedron,
1985, 41, 1409). The bond lengths used for the Gordy equation (; W.
Gordy, J. Phys. Chem., 1947, 15, 305) were obtained by calculating
the B3LYP/6-31+G* bond lengths of the simplest single and double
bonded molecules (in case of the CC bond ethane and ethene,
respectively).
11 The NICS (P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao and N.
v. E. Hommes, J. Am. Chem. Soc., 1996, 118, 6317) is the negative of
the computed magnetic shielding at the centre of the ring. The NICS
of cyclopentadiene is −3.1 (see ref. 12); Z. Chen, C. S. Wannere, C.
Corminboeuf, R. Puchta and P. v. R. Schleyer, Chem. Rev., 2005, 105,
3842.
We thank the CNRS, the MNERT and OTKA T 049258.
Support from the Scientific and Technological French-Hungarian
Bilateral Cooperation (BALATON program) is also acknowl-
edged.
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998 | Org. Biomol. Chem., 2006, 4, 996–998
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The Royal Society of Chemistry 2006
©