Towards the stability limit of cyclic diphosphonium bis-ylides

Article abstract of DOI:10.1016/j.crci.2010.04.005

A typology of molecules containing two phosphonium ylide groups (bis-ylides) is proposed, versus the bridge length (fused, ω- and α-bisylides) and their relative topographical orientations (head-to-head, tail-to-tail or head-to-tail). The formal electrost

Full text of DOI:10.1016/j.crci.2010.04.005

C. R. Chimie 13 (2010) 1091–1098
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Towards the stability limit of cyclic diphosphonium bis-ylides
Mohammed Abdalilah a,b, Yves Canac a, ,b, Christine Lepetit a,b, Remi Chauvin a,b,
*
*
a Laboratoire de chimie de coordination (LCC), CNRS, 205, route de Narbonne, 31077 Toulouse, France
b UPS, INP, LCC, universite´ de Toulouse, 31077 Toulouse, France
A R T I C L E I N F O
A B S T R A C T
Article history:
A typology of molecules containing two phosphonium ylide groups (bis-ylides) is
Received 5 February 2010
Accepted after revision 7 April 2010
Available online 31 May 2010
proposed, versus the bridge length (fused,
v- and a-bisylides) and their relative
topographical orientations (head-to-head, tail-to-tail or head-to-tail). The formal
electrostatic constraint occurring in the head-to-head series is systematically
addressed for cyclic representatives based on the o-bis(diphenylphosphonio)benzene
Keywords:
framework. After a survey of previously reported results in the fused and
b-bis-ylide
Phosphonium ylide
Carbodiphosphorane
Yldiide
series, emphasis is given to the cyclic -bis-ylides. The non-substituted, non-stabilized
a
version escaped isolation through spontaneous fragmentation to bis(diphenylpho-
sphino)benzene and acetylene. Inspired by this result, the reverse process was
employed for the generation of stabilized representatives with ethoxycarbonyl and
benzoyl substituents at the ylidic carbon atoms. The stability and stereochemistry of
Diylide
Carbon ligands
the obtained head-to-head
a-bis-ylides was investigated by NMR techniques
and reproduced and analyzed by DFT calculations. The role of electrostatics in
determining structural and reactivity features of cyclically constrained species is here
illustrated.
ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
ligands of transition metals [1]. They are today attracting a
renewed interest from the standpoints of stereochemistry
In a primary description, ylides can be represented as
-zwitterionic species where a positively charged hetero-
and catalysis [1]. Depending on the charge delocalization
exerted by the substituents, ylides are classified in three
families constituted by the non-stabilized, the semi-
stabilized and the stabilized versions. Due to their
intrinsic stability and their ambidentate character (C- vs
a
atom X with an octet electron count (such as P, S, N) is
covalently bonded to carbanionic center. In this
a
category, phosphonium ylides Fo possessing an easily
pyramidalized carbanion stabilized by an adjacent tetra-
hedral phosphonium center (Scheme 1), received a long-
standing attention not only in the field of organic
synthesis, but also of fundamental coordination chemis-
try. Beyond their ubiquitous role in the Wittig-type
olefination of carbonyl compounds, phosphonium ylides
are indeed intriguing stable electron-rich C-sp3 carbyl
O- or Ncoordinating modes), complexes of
a-stabilized
phosphonium ylide ligands have been naturally more
exemplified [1].
Ideally C2-symmetric chelating ligands provide their
complexes with particular kinetic stability (entropic
factor) and stereochemical control (cis vs trans). Along
the same lines as those governing the design of numerous
diphosphines, diphosphonium bis-ylides deserve a par-
ticular typology. Three types of neutral bis-ylides can thus
be distinguished (Scheme 1): the
v-type V1- V3, the a-
* Corresponding authors.
type A1–A3 and the fused type F1–F3. The ionic forms I0–I3
of the respective types F0–F3, in particular yldiides I0 and
E-mail addresses: yves.canac@lcc-toulouse.fr (Y. Canac),
chauvin@lcc-toulouse.fr (R. Chauvin).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.04.005

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M. Abdalilah et al. / C. R. Chimie 13 (2010) 1091–1098
diylides I2, should also be mentioned. Each type gathers
three forms depending on the regiochemistry of the
coupling between two ylide units F0, consisting each in a
carbanionic head and a phosphonium tail: head-to-head,
tail-to-tail or head-to-tail.
reactivity features are, however, basically similar to those
of yldiides I0. Ionic P-alkylated versions of bis-ylides F2 are
the so-called diylides I2, acting as stable ligands in
transition metal complexes [8] possibly serving as
catalyst precursors [8b]. The head-to-tail form F3, with
a neutral sp3-C or -P center, is only a formal bis-ylide, i.e., a
Among general
v
-bis-ylides, the head-to-head form V1
is exemplified by
b
-bis-ylides involving various bridging
b
-zwitterionic form where the ylidic character is no
longer apparent.
In the -bis-ylides series, the head-to-head (A1) and
moieties (X: CO, PR, PR2+, PYZ, P+) [2]. When X = CO or when
the ylides moieties are independently stabilized, the
ambidentate abilities of such ligands are greatly enhanced.
a
tail-to-tail (A2) forms exhibit strong Cꢀ/Cꢀ and P+/P+
electrostatic repulsion, respectively (Scheme 1). Head-
to-head representatives A1 may behave as genuine
carbene transfer reagents upon coordination: cleavage
of a P C bond leads to an ylido-carbene and a phosphino
group anchoring to the metal center [9]. The tail-to-tail
form A2 was first exemplified by Karsch et al., in 1988
[10]. In these species, both the repulsion of the adjacent
cationic phosphorus atoms and the steric bulk of the
carbanionic substituents (Me3Si) may explain the ob-
Several
v
ꢀ bis-ylide ligands of the tail-to-tail form
V2 (X:
(CH2)n, RPPR,. . .) are also known, including
b-representa-
tives (X = RP) [3].
Fused bis-ylides are represented by their head-
to-head (C-fused) form F1, corresponding to the carbo-
diphosphoranes [4]. These species contain two cumulat-
ed ylide functions, with a formally divalent central
carbon atom with two negative charges stabilized
by two phosphonio groups. The presence of two electron
pairs at the central carbon atom induces both a bent
structure and a very strong nucleophilicity. Carbodipho-
sphoranes can be regarded as neutral stabilized
yldiides of type I0, where a generic C-substituent is
replaced by a phosphonium group [5]. Due to their
strong nucleophilicity, carbodiphosphoranes are poorly
exemplified and undergo easy protonation or alkylation
+
+
˚
served length of the P –P bond (2.278(1) A), even longer
than the P–P bond between five-coordinate P atoms in a
bis-phosphorane (2.264(2) A). To the best of our
knowledge, the sole example of fully characterized
˚
head-to-tail
a-bis-ylide form A3, with a triple charge
alternance (Scheme 1), has been reported by Schmidbaur
et al. in 1980 [11].
to the corresponding
type I1.
b
-diphosphonium monoylides of
In contrast to linear diphosphonium bis-ylides (Scheme
1), their cyclic versions have attracted less attention [12].
Tail-to-tail (P-fused) bis-ylides of the F2 type, first
exemplified by Appel et al. in 1982, correspond to s3l5
bis-(methylene)phosphorane derivatives [6]. According
to Schoeller’s theoretical studies, these polarized
systems, isoelectronic to allyl anions, possess a weakly
pyramidalized phosphorus atom [7]. Their structural and
The case of cyclic head-to-head b-, a- and fused bis-ylide
-
types (B1c, A1c, F1c, respectively) is hereafter systematically
addressed (Scheme 2). The corresponding diphosphonium
precursors based on the rigid, cis-spacing, insulating 1,2-
phenylene bridge have been recently shown to be easily
accessible [13].
p
-
Scheme 1. Typology of acyclic and cyclic phosphonium bis-ylides.

M. Abdalilah et al. / C. R. Chimie 13 (2010) 1091–1098
1093
formation of the expected cyclic C-fused bis-ylide 2 [14].
This carbodiphosphorane was identified by 31P and 1H
NMR spectroscopy at ꢀ30 8C, and by the regeneration of
the starting diphosphonium 1.Br2 upon quenching with
one equivalent of HBr. The thermal unstability of 2 is
naturally explained by geometrical and electronic factors;
the acute PCP angle in the five membered ring (ca 1088)
indeed enforces the sp3 (vs sp) character of the central C
atom, thus increasing the weight of the doubly zwitterionic
resonance form, and finally a destabilizing charge separa-
tion.
Scheme 2. Cyclic head-to-head
b-, a- and fused bis-ylides on the o-
phenylene framework.
2.2. Non-stabilized head-to-head
diphosphoniums
b-bis-ylides of cyclic
2. Results and discussion
Elongation of the aliphatic carbon chain between the
ylidic carbons and electron delocalization over two carbon
atoms instead of one, should result in both geometrical and
electronic relaxation. Following this principle, it was
recently shown that addition of two equivalents of n-BuLi
in THF at low temperature to the less strained dipho-
sphonium 3.(TfO)2 afforded the corresponding cyclic head-
2.1. Non-stabilized head-to-head fused bis-ylides of cyclic
diphosphoniums
The diphosphonium 1.Br2 was first obtained by
Schmidbaur et al. in 61% yield by refluxing o-bis(diphe-
nylphosphino)benzene (o-dppb) and dibromomethane in
toluene over 10 days [14]. We recently reported that
replacing CH2Br2 by CH2I2 allowed for the isolation of the
analogous diphosphonium 1.I2 in 96% yield over 2 days
only (Scheme 3) [13]. A rapid H/D exchange of the
methylene protons was observed in the 1H NMR spectrum
of 1.I2 in acidic deuterated solvents: the acidity of these
to-head
b-bis-ylide 4 (Scheme 3) [15]. The structure of 4
was unambiguously established by multi-nuclear NMR
spectroscopy, and in particular by the presence of a unique
31P signal at +24.4 ppm, which confirmed the onio
character and the equivalence of the two phosphorus
atoms. In the 1H and 13C NMR spectra, the simultaneous
2
protons (d1H(P+CH2P+) = +5.38 ppm, t, JPH = 11.7 Hz) sug-
presence of a triplet of triplets at
dH = +0.59 ppm (JHP = 26.8,
gests the possible existence of the di-conjugated base,
namely the cyclic carbodiphosphorane 2. And indeed,
according to Schmidbaur’s results, the treatment of 1.Br2
with an ylide base (Et3P=CHMe) in THF at ꢀ78 8C led to the
JHH = 3.3 Hz) and of a triplet at C = +21.9 ppm (JCP = 4.4 Hz)
d
was assigned to the bridging methylene group of 4. This
bis-ylide was reported to act as a strongly donating C,C-
chelating ligand of Rh(I) centers [15]. In spite of its strong
Scheme 3. Deprotonation of the cyclic diphosphoniums 1, 3 and 5.

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M. Abdalilah et al. / C. R. Chimie 13 (2010) 1091–1098
Scheme 4. Fragmentation of cyclic head-to-head o-phenylene-diphosphonium
a-bis-ylides, R1, R2 = H, CO2Me, CO2Et (see calculated data in Table 1).
internal charge repulsion and its low stability (t1/
4). The stability of such a-bis-ylides, however, cancels if
2 ꢁ 45 min. at +20 8C), the
to undergo selective transformations. Further bridge
shortening from to was thus the next challenge.
b
-bis-ylide 4 is stable enough
the alkoxycarbonyl groups are replaced by benzoyl groups
(Table 1). These trends are in agreement with the relatively
‘‘stabilized’’ character of the corresponding mono-ylides in
general. Although the positive Gibbs energies for the
b
a
2.3. Non-stabilized head-to-head
diphosphoniums
a
-bis-ylides of cyclic
dissociation of ester-stabilized a-bis-ylides are quite small
in the gas phase, solvent dielectric constant effects are
expected to provide additional stabilization of these highly
polar species. Experimental efforts in this direction are
reported in the next section.
The cyclic o-phenylene-diphosphonium 5 with two
carbon atoms in the aliphatic bridge, is easily prepared by
reaction of o-dppb with 1,2-dichloro-ethane [13]. The
corresponding head-to-head
a
-bis-ylide 6 was targeted by
2.4. Stabilized head-to-head
diphosphoniums
a-bis-ylides of cyclic
deprotonation of 5 with two equivalents of n-butyllithium
in THF at ꢀ78 8C. The 31P NMR spectrum indicated the
disappearance of 5 (d31P = +12.6 ppm) and the presence of
a much more shielded new signal at d31P = ꢀ10.0 ppm,
which was attributed to o-dppb (Scheme 3). Multi-nuclear
NMR monitoring in THF-d8 at ꢀ78 8C showed the
immediate appearance of the o-dppb signal, along with
Preparation of head-to-head
a-stabilized a-bis-ylides
is envisaged through the reverse reaction of their
fragmentation depicted in Scheme 4. Preparation of the
corresponding diphosphonium precursors by adaptation of
the previously used method [13] is indeed a priori tricky,
and calculations suggest that the transformation is a priori
not thermodynamically forbidden (Table 1). Moreover,
arylphosphines are known to react with activated unsatu-
rated substrates such as simple acetylenic Michael
a
1H signal attributable to acetylene, and this even when
only a catalytic amount of n-butyllithium was added.
Considering the chemical existence of the more strained
fused bis-ylide 2 with two geminal negative charges, the
non-detection of 6 even at low temperature, can be
explained by electrostatic factors, in particular by the
enhanced repulsion between the two vicinal negative
charges: instantaneous fragmentation results in a neutral
acceptors [9]. The envisioned process is formally
[2+1+1] cycloaddition which proceed in two steps,
initiated by a Michael-type addition [16].
a
o-dppb molecule and a formal
a
-bis-carbene H–C(:)–C(:)–
2.4.1. Ester group-stabilized head-to-head
cyclic diphosphoniums
a-bis-ylides of
H in resonance with acetylene (Scheme 4). Phosphonium
ylides are indeed recognized as phosphino-carbenoid
species [12].
As anticipated above, addition of one equivalent of
diethyl acetylenedicarboxylate (DEAD) to o-dppb in
toluene gave, after 10 min at room temperature, nearly
quantitative yield of the targeted bis-ylide 7 as an apparent
70/30 mixture of two stereoisomers (Scheme 5). The
occurrence of these two isomers likely results from a
restricted rotation about the exocyclic C–CO2Et bonds.
Strong delocalization of ylidic electron pairs is indeed
This extrusion reaction is supported by the calculation
of a strongly exergonic dissociation energy (6 ! o-dppb +
acetylene:
31G** level (Scheme 4, Table 1).
D
G(298 K) = ꢀ21.2 kcal/mol) at the B3PW91/6-
Upon substitution of one of the ylidic carbon atoms
of
6 by an ester group (CO2Et or CO2Me), the
calculated dissociation energy remains exergonic
classically observed in
a-stabilized phosphonium ylides.
(
D
G(298 K) = ꢀ8.6 kcal/mol), and it becomes slightly end-
According to this interpretation, the three Z/Z, E/E and Z/E
stereoisomers could, however, be expected.
ergonic upon substitution of both ylidic carbons (Scheme
Table 1
Dissociation energies (in kcal/mol) of cyclic head-to-head
calculated at the B3PW91/6-31G** level1.
a-bis-ylides into the corresponding diphosphine (o-dppb) and acetylene derivatives (Scheme 4),
R1 = R2 = H
R1 = H, R2 = CO2Me
R1 = R2 = CO2Et
R1 = R2 = CO2Me
R1 = R2 = COPh
D
D
D
H (0 K)
ꢀ6.93
ꢀ9.47
8.53
6.14
22.02
19.39
2.00
22.89
20.14
2.60
20.25
17.56
ꢀ0.67
HZPE (0 K)a
G (298.15 K)
ꢀ21.19
ꢀ8.60
Dissociation energies were calculated from the most stable conformer of bis-ylides, diphosphines and acetylene derivatives
a
Zero-point energy corrected values.

M. Abdalilah et al. / C. R. Chimie 13 (2010) 1091–1098
1095
Scheme 5. Synthetic approach of
a-stabilized diphosphonium a-bis-ylides 7 and 8 from o-dppb.
The structure of 7 in CD3CN solution was first assigned
energies. As anticipated experimentally, the three most
stable conformers rank in the energetic order: Z/E < E/E
< Z/Z (Table 2). This confirms the assignment empirically
proposed on the basis of elementary electrostatic con-
on the basis of the observed 31P NMR chemical shifts of two
broad singlets at
typical range for
structure of 7 was then confirmed by 1H and 13C NMR
spectroscopy and by mass spectrometry (electron spray
[MH]+: 617.2).
d
P = +0.52 and +2.21 ppm, which are in the
-stabilized phosphonium ylides. The
a
siderations (see above): Z/Z isomer at
dPb = +0.60 ppm
(22%); E/E isomer at Pc = +2.23 ppm (37%).
d
The 31P NMR chemical shifts of the three stereoisomers
of 7 have been calculated at the B3PW91/6-31+G** level
(Table 2). The chemical shift values obtained for the
inequivalent phosphorus atoms of the E/E isomer in its
frozen optimized C1-symmetry have been averaged to
A variable temperature 31P NMR experiment indicated a
reversible coalescence of the two broad signals at +60 8C
corresponding to an activation barrier of ca. 15 kcal/mol,
thus demonstrating the dynamic behavior of the different
isomers of 7.
By decreasing the temperature to 0 8C, decoalescence in
four broad singlets was observed, two of them having the
same integral. These signals can thus be globally assigned
to the expected three stereoisomers of bis-ylide 7 in a 41/
22/37 ratio with respective chemical shifts
dPa = +0.36 and
d
Pa’ = +2.40 ppm/ Pb = +0.60 ppm/ Pc = +2.23 ppm (Fig. 1).
d
d
Due to dissymmetry, the Z/E isomer exhibits two non-
equivalent phosphorus atoms and is therefore assigned to
the set of the Pa and Pa’ signals. In contrast, the symmetric
Z/Z and E/E isomers exhibit only one type of phosphorus
atom and are globally assigned to the Pb and Pc signals.
Considering both attractive P+/Oꢀ and repulsive Oꢀ/Oꢀ
electrostatic interactions, the E/E isomer is expected to be
more stable than the Z/Z isomer: the major signal at
d
Pc = +2.23 ppm (37%) is therefore tentatively assigned to
the E/E configuration, while the minor signal at
Pb = +0.60 ppm (22%) would correspond to the Z/Z
d
configuration.
The structure of the three stereoisomers of bis-ylide 7
has been investigated at the B3PW91/6-31G** level of
calculation.
A crude conformational analysis on the
CO2Me- substituted bis-ylide model allowed to rule out
Cs-symmetric conformers with planar or boat conforma-
tions of the aliphatic ring: these conformers are indeed
more than 10 kcal/mol higher in energy than C2-symmetric
conformers with a twisted aliphatic ring. Conformational
analysis of the bis-ylide 7 itself restricted to C2 symmetry
yielded several equilibrium structures of quasi-degenerate
Fig. 1. Variable temperature 31P NMR spectra of the bis-ylide 7 in CD3CN
at 202.53 MHz. For assignment, see text.

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M. Abdalilah et al. / C. R. Chimie 13 (2010) 1091–1098
Table 2
Calculated structures of the three stereoisomers of the head-to-head a
-bis-ylide 7 (B3PW91/6-31G**).a 31P NMR chemical shifts calculated at the B3PW91/
6-31+G** level. For the E/E isomer, the given value is the mean of the calculated chemical shifts (in square brackets) for the frozen optimized C1-symmetric
structure.
Geometry
Z/E
E/E
Z/Z
Symmetry
C1
C1
C2
Relative energy (kcal/mol)
0.0
0.28
0.57
ꢀ7.54
Calcd. 31P
(ppm)a
d
ꢀ11.9
3.9
ꢀ8.57
[(ꢀ16.8 + 0.33)/2]
Exptl. 31P
0.36
2.40
2.23
0.60
d
(ppm)
regenerate the apparent symmetry due their respective
fluxionality at the NMR time scale. The calculated 31P NMR
chemical shifts are systematically more shielded than the
experimental values. However, although the extreme
chemical shifts calculated for the Z/E isomer (ꢀ11.9 and
3.9 ppm) correspond to the extreme chemical shifts
observed for the same isomer (0.36 and 2.40 ppm), the
rigid approximation is not sufficient for reproducing the
relative experimental values with further accuracy. This is
explained by the narrow range of chemical shifts observed
in all the three isomers (
D
= 2.40–0.36 ꢁ 2 ppm). Calcula-
tions of 31P NMR chemical shifts of strained polar bis-
ylides appear therefore less straightforward than those of
related diphosphoniums [13].
2.4.2. Keto group-stabilized head-to-head
cyclic diphosphoniums
a-bis-ylides of
Fig. 2. Calculated structure of the Z/Z stereoisomer of the
a-bis-ylide 8
(B3PW91/6-31G**).
The procedure employed with the DEAD substrate was
applied with another electron-poor symmetric alkyne, the
1,4-diphenylbut-2-yne-1,4-dione [17]. Reaction of this
diketone with o-dppb in toluene at room temperature
symmetry is found much more stable than the Z/E and E/E
isomers, lying 4.95 and 10.5 kcal/mol higher in energy,
respectively (Fig. 2). The calculated 31P NMR chemical shift
of 8 (+6.6 ppm at the B3PW91/6-31+G** level) is quite
shielded for a P(IV) nucleus, in qualitative agreement with
the experimental value (ꢀ0.1 ppm).
over 10 min thus afforded the corresponding cyclic
a-bis-
ylide 8 in nearly quantitative yield (Scheme 5). The bis-
ylide was obtained as a single stereoisomer as indicated
by the observation of
d
a
unique 31P NMR signal at
P = ꢀ0.08 ppm. A variable temperature 31P NMR experi-
ment (ꢀ40 ! +50 8C) did not indicate any significant
change, except a slight sharpening of the signal at low
temperatures which can be due to a higher rigidity of the
structure of the diketo-bis-ylide 8 as compared to that of
the bis(ethoxycarbonyl) bis-ylide 7 (section 2.5). On the
basis of empirical geometrical and electrostatic considera-
tions, the configuration of the observed stereoisomer of 8 is
assigned to Z/Z, with two cis-Ph2P+–C = C–Oꢀ stereogenic
units (Scheme 5).
3. Conclusion
The synthesis and stability of diphosphoniums head-to-
head bis-ylides constrained in five-, six- and seven-
membered rings have been systematically compared. In
the non-stabilized series, it has been shown that while the
fused bis-ylide 2 and the
b-bisylide 4 are relatively stable,
the -bisylide 6 is unstable due to the repulsion of two
a
vicinal negative charges and undergoes spontaneous
fragmentation to o-dppb and acetylene. Introducing
electron-withdrawing substituents at the two ylidic
carbon atoms allowed for the isolation of stabilized
bis-ylides 7 and 8. The latter were obtained by a formal
The structure of the three stereoisomers of bis-ylide 8
has been calculated at the B3PW91/6-31G** level. In
contrast to the bis-ylide 7, but in agreement with the
above-proposed assignment, the Z/Z stereoisomer of C2
a-

M. Abdalilah et al. / C. R. Chimie 13 (2010) 1091–1098
1097
[2+1+1] reaction of o-dppb with electron-poor alkynes
following a stepwise mechanism. The stabilization effects
and the stereochemical behaviour of the a-bis-ylides have
Har), 7.00 (m, 2H, Har), 7.17–7.29 (m, 4H, Har), 7.40 (m, 3H,
Har), 7.50 (m, 1H, Har), 7.58 (m, 2H, Har), 7.66 (m, 1H, Har),
7.77–7.90 (m, 9H, Har), 8.11 (m, 4H, Har). 13C NMR (CD3CN,
been reproduced and analyzed at the theoretical level. In
particular, it has been shown that, by contrast to
ꢀ40 8C):
d = 124.4 (d, JCP = 73.0 Hz, Car), 126.5 (brs, CHar),
128.1 (brs, CHar), 128.8–129.7 (m, CHar, Car), 130.9 (brs,
CHar), 131.7 (brs, CHar), 131.9 (brs, CHar), 132.7 (brs, CHar),
133.5 (m, CHar), 136.1 (brs, CHar), 141.9 (brs, Car), 185.7
(pseudo t, JCP = 9.4 Hz, CO). MS (ES+): m/z 681.2 [MH]+;
HRMS (ES+) calculated for C46H35O2P2 681.2112; found,
681.2115.
unsubstituted parents,
a-bis-ylides stabilized by ester
groups exhibit endergonic dissociation. These results
illustrate how electrostatic effects (e.g., repulsion between
adjacent negative charges) may govern reactivity in
geometrically strained frameworks [13].
4. Experimental section
4.2. Computational details
4.1. General remarks
Geometries were fully optimized at the B3PW91/6-
31G** level of calculation using Gaussian 03 [18].
Vibrational analysis was performed at the same level of
calculation as the geometry optimization. The magnetic
shielding tensor was calculated at the B3PW91/6-31+G**
level using the gauge-independent atomic orbital (GIAO)
method implemented in Gaussian 03 [18]. The 31P NMR
chemical shifts were estimated with respect to the usual
H3PO4 reference.
Toluene and diethyl ether were dried and distilled over
sodium/benzophenone. All other reagents were used as
commercially available. All reactions were carried out
under argon atmosphere, using schlenk and vacuum line
techniques. The following analytical instruments were
used: Bruker ARX 250, DPX 300, AV 500. NMR chemical
shifts
d are in ppm, with positive values to high frequency
relative to the tetramethylsilane reference for 1H and 13C
and to H3PO4 for 31P.
Acknowledgements
7. To a solution of 1,2-bis(diphenylphosphino)benzene
(o-dppb) (0.45 g, 1.0 mmol) in dry toluene (10.0 mL) at
room temperature was added diethyl acetylenedicarbox-
ylate (DEAD) (0.33 mL, 2.05 mmol). The reaction mixture
was then stirred for 10 min. After evaporation of the
solvent under vacuum, the reddish-yellow solid residue
was washed by dry diethyl ether (2 ꢂ 20 mL) affording 7 as
a yellow solid product (yield: 0.60 g, 98%).
The authors thank the French ministe`re de l’Enseigne-
´
ment superieur et de la recherche, the French Centre national
de la recherche scientifique, the ANR program (ANR-08-JCJC-
0137-01) for financial support. The authors also thank
Calcul intensif en Midi-Pyre´ne´es (CALMIP, Toulouse, France),
´
institut du developpement et des ressources en informatique
scientifique (IDRIS, Orsay, France) and centre informatique
31P NMR (CD3CN, 25 8C):
NMR (CD3CN, 0 8C): = +0.36 (s) and +2.40 (s); +0.60 (s),
+2.23 (s) ppm. 1H NMR (CD3CN, 0 8C):
= 0.36 (t,
d P
= +0.52 (s), +2.21 (s) ppm; 31
´
de l’enseignement superieur (CINES, Montpellier, France) for
d
computing facilities. M.A. thanks UNESCO, the French
d
´
Consulate in East Jerusalem, and the conseil regional de
JHH = 7.1 Hz, CH3), 0.41 (broad t, JHH = 7.0 Hz, CH3), 1.07
(broad t, JHH = 7.0 Hz, CH3), 1.11 (broad t, JHH = 6.9 Hz, CH3),
3.33 (m, CH2), 3.54 (m, CH2), 3.78 (m, CH2), 7.18–7.37 (m,
Har), 7.44–7.69 (m, Har), 8.12–8.28 (m, Har). 13C NMR
´
´
Midi-Pyrenees for financial support.
Thanks are also due to one of the referees of this article
who draw our attention to the point mentioned in
reference [16].
(CD3CN, 0 8C): d = 13.3 (s, CH3), 13.4 (s, CH3), 14.8 (s, CH3),
14.9 (s, CH3), 56.5 (s, CH2), 56.6 (s, CH2), 56.9 (s, CH2), 128.1
(m, CHar, Car), 128.3 (d, JCP = 12.2 Hz, CHar), 128.8 (d,
JCP = 15.3 Hz, CHar), 130.2 (d, JCP = 88.1 Hz, Car), 130.9–131.8
(m, CHar, Car), 132.2 (s, CHar), 133.1 (d, JCP = 10.0 Hz, CHar),
133.3 (d, JCP = 10.0 Hz, CHar), 133.7 (m, CHar), 134.4 (pseudo
ttt, JCP = 7.6 and 24.0 Hz, CHar), 169.5 (d, JCP = 25.7 Hz, CO),
170.1 (d, JCP = 28.4 Hz, CO), 170.5 (d, JCP = 27.4 Hz, CO),
170.6 (d, JCP = 27.0 Hz, CO). MS (ES+) m/z 617.2 [MH]+.
HRMS (ES+) calculated for C38H35O4P2: 617.2011; found
617.2037.
8. To a solution of 1,2-bis(diphenylphosphino)benzene
(o-dppbo) (0.45 g, 1.0 mmol) in dry toluene (10.0 mL) at
room temperature was added 1,4-diphenylbut-2-yne-1,4-
dione (0.48 g, 2.05 mmol). The reaction mixture was then
stirred for 10 min. After evaporation of the solvent under
vacuum, the reddish-yellow solid residue was washed by
dry diethyl ether (2 ꢂ 20 ml) giving 8 as a yellow solid
product (yield: 0.61 g, 90%).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.crci.2010.04.005.
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