Bridging the Gap between Bisylides and Methandiides: Isolation, Reactivity, and Electronic Structure of an Yldiide

Article abstract of DOI:10.1002/anie.201501818

Bisylides and methandiides are two unique families of carbon bases that have found a variety of applications in recent years. Metalated ylides (yldiides) are the link between these types of compounds. Yet, only little is known about their properties, reactivities, and particularly their electronic structure. Here, we report the preparation of the metalated ylide [Ph₃P-C-SO₂Tol]^- (1) with different alkali metal counterions. The compounds have been studied by X-ray diffraction analysis and NMR spectroscopy and the first structures of a sodium and potassium yldiide are presented. The electronic structure of 1 was explored by DFT calculations confirming its relation with other divalent carbon species. Reactivity studies demonstrate the strong nucleophilicity of the yldiide and its capability to act both as a σ- and π-donor. Metalation makes a difference. The metalation of a stabilized phosphorus ylide yields the isolable yldiide [Ph₃P-C-SO₂Tol]^-, which was isolated with different alkali metal counterions. Its electronic structure shows a clear relation to other divalent carbon species such as bisylides. Reactivity studies demonstrate the strong nucleophilicity of the yldiide and its capability to act both as a σ- and π-donor.

Full text of DOI:10.1002/anie.201501818

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201501818
German Edition: DOI: 10.1002/ange.201501818
Ylide Ligands
Bridging the Gap between Bisylides and Methandiides: Isolation,
Reactivity, and Electronic Structure of an Yldiide**
Thorsten Scherpf, Regina Wirth, Sebastian Molitor, Kai-Stephan Feichtner, and
Viktoria H. Gessner*
Abstract: Bisylides and methandiides are two unique families
of carbon bases that have found a variety of applications in
recent years. Metalated ylides (yldiides) are the link between
these types of compounds. Yet, only little is known about their
properties, reactivities, and particularly their electronic struc-
ture. Here, we report the preparation of the metalated ylide
[Ph3P-C-SO2Tol]À (1) with different alkali metal counterions.
The compounds have been studied by X-ray diffraction
analysis and NMR spectroscopy and the first structures of
a sodium and potassium yldiide are presented. The electronic
structure of 1 was explored by DFT calculations confirming its
relation with other divalent carbon species. Reactivity studies
demonstrate the strong nucleophilicity of the yldiide and its
capability to act both as a s- and p-donor.
have been isolated and structurally characterized: the silyl-
substituted yldiide A synthesized by 1,2-addition of n-
butyllithium to a carbene by the group of Bertrand11 and
phosphoranes Ba/b reported by Niecke (Figure 1A).12
S
ince the first synthesis of an ylide more than a century ago[1]
and their use in Wittig-type reactions[2] these compounds have
been applied in a variety of important reactions, for example,
for the synthesis of natural products and heterocycles.[3] In
addition to their synthetic utility, their electronic structure has
also been the subject of intensive research.[4] For a long time
ylides have been described by two canonical structures—
Figure 1. A) Isolated yldiides, B) structures of bisylides and methan-
diides, and C) ylide-stabilized divalent Group 14 compounds.
+
À
=
À
ylene (e.g. R3P CR2) and ylide (e.g. R3P C R2). While it has
been shown that the contribution of the ylenic structure is
minimal,[5] the description of ylides as phosphine-stabilized
carbenes (R3P!CR2) with a donor–acceptor interaction
between phosphorus and carbon has found renewed inter-
est.[6,7] Although ylide chemistry is now well-established, only
very few reports have appeared on their metalated congeners,
the so-called yldiides. Early studies by Corey revealed an
enhanced reactivity in Wittig reactions with sterically hin-
dered ketones[8] and Bestmann and co-workers demonstrated
their potential in a series of multistep syntheses of alkynes
and carbacycles.[9] The applicability of yldiides as ligands for
transition metals has only been studied once by Chauvin and
co-workers. Here, the yldiide was synthesized in the coordi-
nation sphere of the metal.[10] At present, only two yldiides
In a structural sense, yldiides can be viewed as the link
between bisylides, such as carbodiphosphoranes (CDPs, for
example, C),[13] and methandiides such as D (Figure 1B).[14,15]
Both have been shown to be very promising as carbon bases
and ligands.[16] For example, CDP C is applicable in the
activation and coordination of small molecules[17] such as
CO2, BH3, and GeCl2, and also as a ligand in transition metal
complexes.[18] Despite the limited number of isolated yldiides,
these compounds are likewise attractive as ligands. Owing to
their structure they should exhibit unique s- and p-donor
abilities. This has been indicated by Driess, Kawashima, and
Fürstner with the synthesis of ylide-stabilized silylene E[19]
and carbenes F and G (Figure 1C), respectively, although
here no isolated yldiide ligand was used.[17c,20] For a general
application of yldiides, however, the access of readily
available and isolable representatives is necessary. Here we
report the synthesis and isolation of [Ph3P-C-SO2Tol]À as such
a readily accessible yldiide. We present the first structure
elucidation of a sodium and potassium yldiide as well as the
study of their electronic structure and reactivity, which
confirms the capability of yldiides to act as s- and p-donors.
In order to access an isolable yldiide, we aimed at the
synthesis of the sulfonyl-substituted compound 1. We
assumed that the efficient coordination ability of the sulfonyl
moiety towards alkali metals should support the stabilization
[*] T. Scherpf, R. Wirth, S. Molitor, K.-S. Feichtner, Dr. V. H. Gessner
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: vgessner@uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
forschungsgruppen/dr_v_gessner/
[**] This work was supported by the Boehringer Ingelheim Foundation
(Exploration Grant), the Deutsche Forschungsgemeinschaft (Emmy
Noether Grant), and the Fonds der Chemischen Industrie.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501818.
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different aggregates. Sharp signals, however, were observed
for the 18-crown-6 adduct, suggesting the formation of one
distinct species. The same holds true for the sodium salt 1-Na
in [D8]THF. The metalated carbon atom gives rise to a broad
signal at dC = 44.1 ppm (1-K) and 41.2 ppm (1-Na), and is thus
downfield-shifted relative to the analogous carbon atom im
ylide 1-H (dC = 34.5 ppm).
Scheme 1. Preparation of yldiide 1 from the phosphonium salt 1-H2
and ylide 1-H; Tol=p-C6H4CH3: a) nBuLi, À788C, THF or NaH, RT,
THF; b) nBuLi, LiHMDS, NaHMDS, or KHMDS, THF or benzene, RT;
A=Li, Na, or K.
Single crystals of [Ph3P-C-SO2Tol]À (1) could be obtained
from THF solutions of the sodium and potassium salt,
respectively. The latter required the use of one equivalent
of 18-crown-6 for complexation of the metal. Both molecular
of the metalated ylide and thus facilitate its isolation. The
required phosphonium salt 1-H2 and the corresponding ylide
1-H were prepared by modified literature procedures
(Scheme 1, see the Supporting Information for experimental
details).[21] Both compounds could be isolated in crystalline
form and fully characterized by multinuclear NMR spectros-
copy, elemental, and single-crystal X-ray diffraction analysis
(Figure 2).[22] The second deprotonation to yldiide 1 was
accomplished by using either a second equivalent of butyl-
ꢀ
structures (triclinic space group P1) confirm the metalation of
the central carbon atom (Figure 3). The sodium compound
Figure 2. Molecular structures of the cation of phosphonium salt 1-H2
and ylide 1-H. For selected bond lengths and angles, see Table 1.
Displacement ellipsoids drawn at the 50% probability level.
lithium or alternatively lithium, sodium, or potassium hexa-
methyldisilazide (LiHMDS, NaHMDS, KHMDS). The reac-
tion was accompanied by a color change from colorless to
yellow. 31P{1H} NMR spectroscopic studies showed quantita-
tive deprotonation after only 1–6 h reaction time depending
on the base used. Isolation of the product was found to be
more convenient with NaHMDS and KHMDS due to the
insolubility of the lithium compound in all common organic
solvent. The sodium salt 1-Na was thus obtained in 76% yield
by crystallization from benzene. 1-K, on the other hand, could
be isolated in 87% yield in crystalline form after addition of
18-crown-6 or as an amorphous solid in 88% yield after
washing with hexane. When isolated, yldiide 1 was found to be
stable in solution (benzene, toluene, THF) even at elevated
temperatures and can be stored under an inert atmosphere for
weeks.
Yldiide 1 is characterized by the absence of the signal of
any methylene proton and by an upfield shift in the
31P{1H} NMR spectrum from dP = 14.1 ppm in 1-H to dP =
À12.1 ppm for the potassium salt 1-K and dP = À11.1 ppm
for the sodium compound 1-Na. 1-K features only broad
signals in all NMR spectra recorded from C6D6 solution, thus
indicating fluxional behavior in solution or the presence of
Figure 3. Molecular structures of [1][K·18-crown-6] (A) and
{[1]3Na3·THF}2 (B). Non-coordinating THF molecules and hydrogen
atoms omitted for clarity. Displacement ellipsoids drawn at the 50%
probability level.
forms a Ci-symmetrical dimer, with three yldiides in the
asymmetric unit. The central structural motif consists of two
face-connected (NaO)4 cubes, in which the Na ions form
contacts with the ylidic carbon atoms (C1) and the oxygen
atoms of the sulfonyl moieties. [1][K·18-crown-6] forms
a monomeric structure. Here, the potassium ion is coordi-
nated by the crown ether as well as by the metalated carbon
atom and one oxygen atom of the sulfonyl moiety. The
coordination of the yldiide results in an unsymmetrical
À
complexation of the crown ether with the K O bond lengths
ranging between 2.790(1) and 3.022(1) Š. The K–C1 bond
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length of 3.154(2) Š is in the range of other organopotassium
compounds.[23]
Figure 4 displays the two highest occupied molecular orbitals
(HOMOs) of 1. Both orbitals represent lone pairs of electrons
at the central carbon atom, one of p- (HOMO) and one of s-
symmetry (HOMOÀ1). This is also reflected by the natural
bond orbital (NBO) analysis of 1, which showed two lone pair
The most interesting features of the molecular structures
of 1 concern the P-C-S linkage. Due to the higher quality of
the data obtained for the potassium salt, those bond lengths
and angles are only discussed here. However, analogous
observations were made for the sodium compound (see the
Supporting Information). The P-C-S linkage undergoes
a dramatic contraction upon metalation (Table 1). A contin-
À
orbitals at the central carbon atom and two P/S C s-bonding
orbitals (Table 2). This electronic structure is not only
Table 2: Results of the NBO analysis and proton affinities (PA) of CDP C,
methandiide D, and yldiide 1 [energies in kcalmolÀ1; M062X/6-
311+g(d)].
Table 1: Experimental and calculated [values in brackets; M062X/6-
311+g(d)] bond lengths [Š] and angles [8], calculated NBO charges, and
NMR spectroscopic data of yldiide [1][K·18-crown-6] (C6D6), 1-H, and
[1-H2][I] (CD2Cl2).
1-H2
1-H
1
qC
qP
WBI C P
À1.40
1.35
1.30
À1.50
À1.33
P-C1
S-C1
P-CPh
S-CPh
1.808(3) [1.833] 1.700(3) [1.699] 1.646(2) [1.650]
1.809(3) [1.827] 1.683(3) [1.703] 1.626(2) [1.660]
1.792(3) [1.800] 1.804(3) [1.823] 1.835(3) [1.850]
1.759(3) [1.775] 1.776(3) [1.799] 1.801(2) [1.822]
1.441(2) [1.453] 1.446(2) [1.465] 1.478(1) [1.484]
115.9(2) [118.6] 124.2(2) [119.9] 124.3(1) [118.1]
1.60
1.52
À
[a]
1.34
1.42
1.650
118.1
P C [Š]
1.668
1.647[a]
127.7[a]
sp1.70 (58.0)
sp2.89 (1.64)
p (1.55)
274.1
À
S-O[a]
P-C-P/S [8]
124.7
sp(CCP)/(%C)
sp1.79 (56.8)
sp2.12 (55.1)
sp1.43 (1.70)
p (1.51)
351.7
P-C1-S
sp(CLP1)/(Occ.) sp1.92 (1.70)
sp(CLP2)/(Occ.) p (1.52)
dH,2JPH [ppm, Hz] 6.19, 12.4
dC,1JPC [ppm, Hz] 51.3, 45.9
2.93, 13.9
34.5, 123.0
–
44.1 (br)[b]
41.2, 38.0[c]
À1.33
1st PA
2nd PA
429.4
325.9
179.2
247.2
qC (calcd)
À0.95
À1.05
À
[a] Experimentally determined P C bond length (1.635(5) Š) and
P-C-P angle (131.7(3)8).
[a] Average values. [b] For 1-K. [c] For the sodium compound 1-Na in
[D8]THF.
uous bond shortening is thus observed in the series from 1-H2
obtained for the “free” anion [Ph3P-C-SO2Tol]À, but also
for its sodium and potassium salts 1-Na and 1-K, which
account for the additional metal–carbon interaction. Due to
À
À
to 1-H and 1. As such, the P C1 and the S C1 distances
decrease from 1.808(3) Š and 1.809(3) Š in the phosphonium
salt to 1.646(2) Š and 1.626(2) Š, respectively, in 1. This
shortening is due to electrostatic interactions within the P-C-S
moiety upon metalation and the increase of the negative
charge at the ylidic carbon atom. This is reflected by the NBO
charges qC at the central carbon atom. Additional negative
hyperconjugation effects in 1-H and 1 result in a lengthening
À
the predominantly ionic nature of the C M bond, the metal
ion has only a small influence on the overall electronic
structure of the yldiide. It is interesting to note that despite
the anionic nature of the methandiide and yldiide, the highest
negative atomic partial charge at the central carbon atom (qC)
is found for the neutral CDP.[24] This can be attributed to the
strong anion-stabilizing ability of the substituents in 1 and D.
The calculated gas-phase proton affinities (PA) rank the
basicity of 1 between those found for CDP C and methandiide
D. Both the first and second PAs are higher than those of the
corresponding CDP and lower than those of the methandiide,
thus suggesting an intermediate reactivity.
À
À
of the S O and P/S CPh bonds (Table 1). The P-C-S angle also
increases upon deprotonation, thus indicating a change in
hybridization from sp3 to sp2.
The utility of bisylides and methandiides is strongly
connected to their basicity and the two pairs of electrons at
the central carbon atom. To gain insight into the electronic
structure of yldiides, density functional theory calculations
[M062X/6-311 + g(d)] on [Ph3P-C-SO2Tol]À (1) as well as on
the related CDP C and methandiide D were performed (for
computational details see the Supporting Information).
Preliminary reactivity studies were performed to exper-
imentally evaluate the nucleophilicity and coordination
ability of yldiide 1 (Scheme 2, for details see the Supporting
Information). As suggested by the high negative charge at the
central carbon atom, the proton affinities, and frontier
orbitals, [Ph3P-C-SO2Tol]À should be able to act as a strong
s- and p-donor. The nucleophilicity of 1 is demonstrated by its
reactivity towards water to afford ylide 1-H. Interestingly, this
protonation reaction is accompanied by the formation of
phosphine oxide 2. This behavior is analogous to that
observed for CDP C.[25] This nucleophilicity of 1 can also be
used for the preparation of functionalized ylides. As such,
treatment of 1 with one equivalent of hexachloroethane
selectively provides the chlorinated ylide 3, which could be
isolated as a colorless, air and moisture-stable solid in 69%
Figure 4. Kohn–Sham orbitals of 1 (HOMO (left) and HOMOÀ1
(right); isosurface values at 0.035 [M062X/6-311+g(d)].
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Figure 5. Molecular structure (left) and Kohn–Sham orbitals (right) of
Scheme 2. Reactivity studies of yldiide 1.
the ylide-stabilized borane 6. Displacement ellipsoids drawn at the
50% probability level. Selected bond lengths [Š] and angles [8]: C1–B
1.528(4), C27–B1 1.513(4), P1–C1 1.731(2), P2–C27 1.743(2), S1–C1
1.711(2), S2–C27 1.709(2); C27-B1-C1 131.9(2), S1-C1-P1 113.9(1),
S2-C27-P2 112.6(1).
yield. Likewise, reaction with diphenylchlorophosphine
afforded the phosphanyl-substituted ylide 4 in 48% yield as
a crystalline solid. The unique reactivity of the yldiide in
comparison to simple ylides becomes evident from the
reaction with benzaldehyde. Instead of the clean formation
of the Wittig product, ketone 5 turned out to be the main
product of the reaction and was isolated in 44% yield. This
acylation formally arises from the elimination of sodium
hydride from the alcoholate intermediate formed in the first
reaction step.[26] The hydridic character and the reducing
nature of this alcoholate can be attributed to the additional
p-donor ability of the ylidiide ligand. This results in prefer-
ential hydride elimination over the formation of the Wittig
product, which was only formed in small amounts (see the
Supporting Information). The same reactivity is observed
with pentafluorobenzaldehyde. However, in this case the C6F5
In summary, we have successfully synthesized and char-
acterized the sulfonyl-stabilized metalated phosphorus ylide
[Ph3P-C-SO2Tol]À (1). Employment of various metalating
agents allowed the preparation of its different alkali metal
salts and the first structure elucidation of a sodium and
potassium yldiide. DFT studies confirmed the presence of two
lone pairs of electrons at the central carbon atom and the
strong basicity of the compound. Reactivity studies proved
the unique donor capacity of the yldiide and its capability to
act as both a s- and p-donor. Hence, it represents a link
between bisylides and methandiides. Given their applicability
in main-group element and transition metal chemistry, further
applications of metalated ylides as unique carbon bases can be
expected.
À
moiety acts as leaving group in a remarkably facile C C bond-
cleavage reaction.[27] This results in the formation of formyl-
phosphorane 5b, which was isolated in 62% yield.
The unique donor capacity of the yldiide ligand is also
evident in the reaction of 1-Na with BH3·THF. Instead of
formation of the corresponding sodium borate, selective
formation of borane 6 and sodium borohydride is observed.
Thus, for complete conversion to 6 a 2:3 yldiide/borane ratio
has to be employed. Borane 6 was isolated in 78% yield and
characterized by various spectroscopic methods. Most inter-
estingly, X-ray crystallography revealed that 6 is a rare
example of a monomeric organoborane.[28] In the molecular
structure (Figure 5), 6 features a planar three-coordinate
boron atom. The two ylide units are twisted with respect to
Keywords: alkali metals · carbanions · lithium ·
structure elucidation · ylides
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8542–8546
Angew. Chem. 2015, 127, 8662–8666
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[24] When M–C interactions are included in the calculations of D
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increase.
[25] W. Petz, F. Öxler, K. Aicher, B. Neumüller, Z. Anorg. Allg.
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[26] The reducing nature of the intermediate alcoholate results in the
reduction of benzaldehyde to benzyl alcohol. Thus 2 equiv of
benzaldehyde must be used. The reaction is accompanied by the
formation of considerable amounts of 1-H, thus limiting the
conversion to 5. Note, that this reactivity is in contrast to that
observed by Corey and co-workers with an in situ generated
yldiide (Ref. [8]).
À
[27] Only few other examples exist in which C6F5 acts as a good
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Received: February 25, 2015
Revised: April 9, 2015
Published online: June 10, 2015
8546
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