Synthesis, Structure, and Reactivity of a Stable Phosphonium–Sulfinyl Yldiide

Article abstract of DOI:10.1002/ejic.201700648

The synthesis of a stable lithium phosphonium–sulfinyl yldiide was explored. The compound was fully characterized by NMR spectroscopy and X-ray crystallography. The electronic structure of 2 was analyzed by DFT calculations, which indicated strong ylidic character. Yldiide 2 was stable enough to catalyze the hydroacylation of benzaldehyde. More interestingly, the Wittig reaction with activated ketones proceeded smoothly at room temperature owing to the presence of the lithium cation.

Full text of DOI:10.1002/ejic.201700648

DOI: 10.1002/ejic.201700648
Communication
Stable Yldiides
Synthesis, Structure, and Reactivity of a Stable Phosphonium–
Sulfinyl Yldiide
Azucena Garduno-Alva,[a] Romaric Lenk,[a] Yannick Escudié,[a] Mariana Lozano González,[a]
Laura Bousquet,[a] Nathalie Saffon-Merceron,[b] Cecilio Alvarez Toledano,[c] Xavier Bagan,[d]
Vicenç Branchadell,[d] Eddy Maerten*[a] and Antoine Baceiredo*[a]
Dedicated to the 50th anniversary of Laboratoire Hétérochimie Fondamentale et Appliquée, LHFA
Abstract: The synthesis of a stable lithium phosphonium–sulf- to catalyze the hydroacylation of benzaldehyde. More interest-
inyl yldiide was explored. The compound was fully character- ingly, the Wittig reaction with activated ketones proceeded
ized by NMR spectroscopy and X-ray crystallography. The elec- smoothly at room temperature owing to the presence of the
tronic structure of 2 was analyzed by DFT calculations, which lithium cation.
indicated strong ylidic character. Yldiide 2 was stable enough
Introduction
Since more than a century, ylide chemistry has attracted consid-
erable interest,[1] and without any doubt, the Wittig reaction is
widely recognized as one of the most important reactions in
organic chemistry.[2] Bisylide derivatives I are species containing
two cumulated ylide functions, and they formally possess a cen-
tral carbon atom with two negative charges, which are stabi-
lized by two positively charged fragments.[3] Recently, consider-
able work on their electronic structure has shed new light on
these compounds.[4] Indeed, rather than the classical zwitter-
ionic representation, their description as divalent carbon(0)
complexes featuring two L ligands seems to fit better with their
reactivity.[5]
Figure 1. Bisylides, yldiides, and isolated yldiides. Tipp = triisopropylphenyl.
Following our work on neutral mixed phosphonium–sulf-
onium bisylides,[11] in which we demonstrated their utility as
asymmetric atomic carbon sources, we became interested in
the synthesis and reactivity of the corresponding anionic phos-
phonium–sulfinyl yldiides.
The replacement of a neutral L ligand by an anionic moiety
(R) gives metalated congeners II,[6] which are called yldiides or
metalated ylides (Figure 1). Despite limited examples (only
three yldiides have been isolated),[7] some interesting applica-
tions were already described for organic synthesis as highly re-
active Wittig-type reagents for sterically hindered ketones,[8] in
transition-metal chemistry as ligands,[9] and in main-group
chemistry for the synthesis of stable borenium cations.[10]
Results and Discussion
Precursor phosphonium sulfinyl ylide 1 was prepared in good
yield (66 %) by reaction of the chlorophosphonium salt with
methyl(triisopropylphenyl) sulfoxide in the presence of a non-
nucleophilic strong base [i.e., lithium diisopropylamide (LDA,
2 equiv.)]. Deprotonation of 1 with butyllithium cleanly afforded
desired lithium yldiide 2 (70 % yield) as a white solid
(Scheme 1). As expected, in the 31P NMR spectrum, the chemi-
cal shift of the signal for 2 (δ = 18.0 ppm) appears at a higher
field than that for 1 (δ = 46.8 ppm). In the 13C NMR spectrum,
the metalated central carbon atom exhibits a characteristic
doublet at δ = 48.3 ppm (JPC = 30.6 Hz), which is in the same
range as that of the related sulfonyl-yldiide reported by Gessner
et al.[7c]
[a] Université de Toulouse, UPS, and CNRS, LHFA UMR 5069
118 route de Narbonne, 31062 Toulouse, France
E-mail: maerten@chimie.ups-tlse.fr
baceired@chimie.ups-tlse.fr
http://www.lhfa.cnrs.fr/
[b] Université de Toulouse, UPS, and CNRS, ICT FR2599
118 route de Narbonne, 31062 Toulouse, France
[c] Instituto de Química-UNAM, Circuito Exterior, Ciudad Universitaria,
Coyoacán C.P. 04510 Ciudad de México, México
[d] Departament de Quimica, Universitat Autonoma de Barcelona
08193 Bellaterra, Spain
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201700648.
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Communication
tures were confirmed by X-ray diffraction analysis (Figures 2 and
3).[12]
X-ray analysis of 2 confirms metalation of the central carbon
atom and reveals a dimeric structure incorporating a Li2O2 core.
The Li1–Li2 interatomic distance is greater (2.716 Å) than the
sum of the covalent radii (2.68 Å).[13] The C–Li2 bond lengths
(C1–Li2 2.158 and C2–Li2 2.139 Å) are slightly longer than those
reported in previously described lithium yldiides (2.060–
2.085 Å) probably because of the coordination of oxygen atoms.
Scheme 1. Synthesis of lithium yldiide 2.
Both compounds were isolated in crystalline form from a Notably, the sulfinyl oxygen atoms coordinate less strongly to
toluene/pentane solution at –20 °C for 1 and from a pentane Li2 (O1–Li2 2.065 and O2–Li2 2.074 Å) than the two THF mol-
solution with a minimum amount of THF for 2, and both struc- ecules to Li1 (O4–Li1 1.965 and O3–Li1 1.972 Å). More classical
interactions are observed with Li1 (O2–Li1 1.894 and O1–Li1
1.896 Å).
The P–C distance (1.649 Å) in [2·THF]2 is shorter than that in
1 (1.677 Å), and the value is consistent with those previously
reported for metalated ylides III–V (1.630–1.646 Å).[7] The S–C
bond length also undergoes a shortening upon going from 1
(1.736 Å) to [2·THF]2 (1.682–1.693 Å), and the P–C–S angle de-
creases significantly upon metalation from 120.82° in 1 to
117.51–115.53° in [2·THF]2. To gain more insight into electronic
structures of 1 and 2, DFT calculations were performed at the
M06-2X/6-31+G(d,p) level of theory (see the Supporting Infor-
mation). The structure of 2 was considered with and without a
countercation. In the presence of the cation, two minima were
found depending on the coordination of the lithium ion, either
on the central carbon atom (the most stable) or on the oxygen
atom (ΔE = 5.1 kcal mol–1 and ΔG = 5.9 kcal mol–1; see the
Supporting Information). We consider 2calcd. without a cation
for the rest of the discussion, as the ionic character of the C–Li
Figure 2. Molecular structure of 1. Thermal ellipsoids represent 30 % probabil-
bond has a minor influence on the rest of the electronic param-
ity. H atoms (except that on C1) and disordered atoms are omitted for clarity.
eters. The optimized geometries, 1calcd. and anionic 2calcd., agree
Selected bond lengths [Å] and angles [°]: P1–C1 1.677(2), P1–C10 1.811(2),
S1–O1 1.500(2), S1–C1 1.736(2), S1–C16 1.8162, P1–C1–S1 120.83(13).
quite well with the experimental crystal structures (Table 1).
Figure 3. Molecular structure of [2·THF]2. Thermal ellipsoids represent 30 % probability. H atoms, solvent molecules, and disordered atoms are omitted for
clarity. Selected bond lengths [Å] and angles [°]:O1–Li1 1.896(6), O1–Li2 2.065(6), O2–Li1 1.894(6), O2–Li2 2.074(6), Li1–O4 1.965(7), Li1–O3 1.972(10), Li1–Li2
2.716(8), C1–Li2 2.158(6), C2–Li2 2.139(7), P1–C1 1.649(3), P1–C11 1.823(3), P2–C2 1.649(3), P2–C40 1.826(4), S1–O1 1.546(2), S1–C1 1.682(3), S2–O2 1.554(2),
S2–C2 1.693(3), P1–C1–S1 117.51(18), P2–C2–S2 115.53(19), S1–C1–Li2 85.6(2), Li2–C1–P1 142.3(3), S2–C2–Li2 86.0(2), Li2–C2–P2 142.8(3).
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The only noticeable difference concerns the S–C bond length, field doublet at δ = 11 ppm (JCP = 12.2 Hz) in the 13C NMR
which is overestimated by 0.04 Å in the calculation. The varia- spectrum. In the same vein, yldiide 2 can be easily re-
tion in the Wiberg bond indices on going from 1calcd. to 2calcd. protonated, and the addition of 1 equivalent of water immedi-
(P–C, +0.36; C–S, +0.11) is in good agreement with an increase ately and quantitatively affords precursor phosphonium–sulfinyl
in the negative charge on the ylidic carbon atom. The value ylide 1.
obtained for the atomic population on the central carbon
atom (–1.358) is reminiscent of that reported by Gessner et al.
(–1.33).[7c] The two highest occupied molecular orbitals
(Figure 4) correspond to the two lone pairs at the central
carbon atom in 2, and the HOMO–1 and HOMO present σ and
π symmetry, respectively.
Interestingly, 1 equivalent of trifluoroacetophenone readily
reacts with 2, as indicated by the immediate appearance of a
new signal at δ = –39 ppm in the 31P NMR spectrum, which
slowly evolves over 24 h at room temperature toward a new
signal at δ = 22 ppm. This high-field chemical shift is in good
agreement with the formation of oxaphosphetane 4 with a
pentacoordinate phosphorus atom, which is the key intermedi-
ate of the Wittig reaction. The new signal in the 31P NMR spec-
trum was assigned to the corresponding phosphine oxide by
comparison with an authentic sample, and after workup, cis-
vinyl sulfoxide 5 was isolated in 68 % yield (Scheme 2). Notably,
decomposition of a related oxaphosphetane, obtained by reac-
tion of a P,S-bisylide with trifluoroacetophenone, requires much
stronger thermal activation (150 °C).[11b] The evolution of 4 at
much lower temperature (room temperature) can be explained
by the interaction of the lithium cation with the carbanion cen-
ter, which triggers the Wittig reaction.[14]
Table 1. Selected geometrical parameters of the experimental crystal struc-
tures of 1 and [2·THF]2 and the related optimized structures.
1
1calcd
[2·THF]2
2calcd
P–C [Å]
S–C [Å]
P–C–S [°]
1.677(2)
1.687
1.649(3)
1.649(3)
1.682(3)
1.693(3)
117.51(18)
115.53(19)
1.642
1.736(2)
1.750
117.7
1.720
113.4
120.83(13)
In marked contrast, the reaction of 2 with 1 equivalent of
benzaldehyde did not afford the Wittig product, but 1H NMR
spectroscopy revealed the formation of benzyl benzoate exhib-
iting a characteristic signal at δ = 5.4 ppm. The formation of
benzyl benzoate can be rationalized by the hydroacylation of
benzaldehyde; this transformation can be performed by using
a catalytic amount of yldiide 2 (see the Supporting Information
for kinetic studies). The reaction probably proceeds by a
hydride transfer that is initiated upon the formation of 6, which
is trapped by a second equivalent of aldehyde to generate lith-
ium alcoholate 7. Then, the reaction between 6 and 7 gives rise
to benzyl benzoate and regenerates catalyst 2 (Scheme 3). A
similar catalytic transformation was already reported by using
N-heterocyclic carbenes and P,S-bisylides as catalysts.[11b,15]
Figure 4. HOMO–1 (left, –2.86 eV) and HOMO (right, –2.10 eV) of 2calcd. calcu-
lated at the M06-2X/6-31+G(d,p) level of theory ( 0.05 isosurfaces).
The nucleophilic character of yldiide 2 was confirmed by a
quantitative reaction with methyl iodide, which cleanly afforded
corresponding C-methylated ylide 3 as a white solid in 95 %
yield (Scheme 2). The 31P NMR spectrum exhibits a singlet at
δ = 48.6 ppm, and the C-methyl protons appear as a character-
istic doublet signal at δ = 2.02 ppm (JPH = 13.2 Hz) in the
1H NMR spectrum. The central carbon atom displays a high-
Scheme 3. Proposed mechanism for the catalytic transformation of benzalde-
hyde into benzyl benzoate.
Conclusions
In conclusion, a stable lithium phosphonium–sulfinyl yldiide
was synthesized and structurally characterized by X-ray crystal-
lography. DFT calculations evidenced strong nucleophilic char-
acter, which was experimentally confirmed. Metalation of the
central carbon induced some chemical behavior differences rel-
ative to its neutral P,S-bisylide analogue, and particularly, Wittig-
type reactions could be performed at room temperature with-
Scheme 2. Nucleophilic character of yldiide 2 and Wittig reaction with tri-
fluoroacetophenone.
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Acknowledgments
This work was supported by the Centre National de la Recher-
che Scientifique (CNRS) and the Université de Toulouse, UPS.
“Ministère de l'Education nationale, de l'Enseignement supéri-
eur et de la Recherche” is acknowledged for a Ph.D. grant to
Y. E. We would also like to thank the CONACyT-México 207613-
FOIINS-France project and Conacyt-México for the Ph.D. grant
extended to M. L.-G. and A. G.-A. Financial support from the
Spanish Ministry of Economy and Competitiveness (grant
CTQ2016-77978-R) is acknowledged.
Keywords: Ylides · Yldiides · Structure elucidation ·
Phosphorus · Wittig reactions
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Received: June 2, 2017
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Stable Yldiides
A stable lithium phosphonium–sulfinyl
yldiide is isolated and fully character-
ized. Between methandiides and bisyl-
ides, the metalated center allows Wit-
tig-type reactions under soft condi-
tions. Theoretical calculations indicate
strong ylidic character, which is experi-
mentally confirmed through addition
reactions with alkyl halides and its use
as a catalyst for the hydroacylation of
benzaldehyde.
A. Garduno-Alva, R. Lenk, Y. Escudié,
M. L. González, L. Bousquet,
N. Saffon-Merceron, C. A. Toledano,
X. Bagan, V. Branchadell,
E. Maerten,* A. Baceiredo* .............. 1–5
Synthesis, Structure, and Reactivity
of a Stable Phosphonium–Sulfinyl
Yldiide
DOI: 10.1002/ejic.201700648
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