An isolable mixed P,S-Bis(ylide) as an asymmetric carbon Atom source

Article abstract of DOI:10.1002/anie.201002833

Heart of carbon: The first stable asymmetric bis(ylide), N,N'-(iPr ₂NCH₂CH₂NiPr) (Ph) P-C-SPh₂, has been isolated (see picture; C black, N gray). The presence of the two different ligands causes this carbon (0)

Full text of DOI:10.1002/anie.201002833

Communications
DOI: 10.1002/anie.201002833
Atomic Carbon Sources
An Isolable Mixed P,S-Bis(ylide) as an Asymmetric Carbon Atom
Source**
Nicolas Dellus, Tsuyoshi Kato,* Xavier Bagꢀn, Nathalie Saffon-Merceron, Vicenꢁ Branchadell,
and Antoine Baceiredo*
The transformation of molecules by introduction of atomic
elements is one of the most fundamental reactions in
chemistry. Indeed, the reactions of atomic hydrogen (reduc-
tion, hydrogenation) and oxygen (oxidation, epoxidation) are
exceedingly important in organic synthesis, and numerous
reagents and catalysts for this purpose are available. In
marked contrast, the manipulation of atomic elements with a
higher valence number, such as nitrogen and carbon, remains
a more difficult task.
reactivity and allow easier laboratory handling, atomic carbon
can be stabilized by two donating ligands. These complexes
can be represented either as divalent carbon(0) complexes
(III) or bis(ylide)s (III’).[4] To date, several stable carbon(0)
complexes have been synthesized, and investigations into the
electronic and geometric properties associated with their
coordination chemistry have been reported.[5,6] Surprisingly,
their chemical properties as an atomic carbon source have
received very little attention, with the exception of carbodi-
phosphoranes (CDPs) IIIa.[7] CDPs are known to react with
two equivalents of ketone, leading to the formation of
cumulenes as a result of a double-Wittig reaction. We have
recently described the synthesis of the first persistent mixed
P,S-bis(ylide) of type IIIb.[8] In contrast to the symmetrical
CDPs IIIa, bis(ylide) IIIb features two different ylide
functions (phosphonium[9] and sulfonium[10]), each of which
have significantly different chemical behavior.[11] Therefore, it
is expected that this type of unsymmetrical bis(ylide) IIIb
behaves as a source of atomic carbon with a masked
asymmetric dicarbene character. Herein we present the
synthesis of a stable and isolable P,S-bis(ylide) 3 and its
reactivity as an asymmetric atomic carbon source.
Although several C1 sources have been intensely inves-
tigated, only a few atomic carbon synthetic equivalents, such
as unsaturated diazo derivatives I[1] and II,[2] have been
P,S-bis(ylide) derivative 3 was synthesized from the
corresponding carbon-atom-protonated cationic salts 2,
which are readily obtained by the reaction of chlorinated
phosphonium salts 1 with two equivalents of diphenylsulfo-
nium ylide (Scheme 1). Deprotonation of 2 using potassium
described. It is also possible to generate genuine atomic
carbon by the carbon arc method.[3] However, such a species is
so reactive that it requires the use of sophisticated apparatus
and careful control of the reaction conditions. To reduce the
[*] N. Dellus, Dr. T. Kato, Dr. A. Baceiredo
Universitꢀ de Toulouse, UPS, and CNRS, LHFA UMR 5069
118 route de Narbonne, 31062 Toulouse Cedex 9 (France)
Fax: (+33)5-6155-8204
Scheme 1. Synthesis of P,S-bis(ylide) 3. KHMDS=potassium hexame-
thyldisilazane.
E-mail: kato@chimie.ups-tlse.fr
baceired@chimie.ups-tlse.fr
Homepage: http://hfa.ups-tlse.fr
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, Structure Fꢀdꢀrative Toulousaine en
Chimie Molꢀculaire, SFTCM FR2599
118 route de Narbonne, 31062 Toulouse (France)
hexamethyldisilazane (KHMDS) results in the clean forma-
tion of P,S-bis(ylide) 3, which was successfully isolated as pale
yellow crystals in good yield (72%). In the 31P NMR
spectrum, derivative 3 displays a signal shifted to higher
field (d = 39.6 ppm) compared to 2 (d = 49.2 ppm). The
X. Bagꢁn, Prof. V. Branchadell
Departament de Quꢂmica, Universitat Autꢃnoma de Barcelona
08193 Bellaterra (Spain)
central carbon appears as a doublet at d = 16.6 ppm (1JPC
=
[**] We are grateful to the CNRS (LEA 368, and ANR LEGO), the
Ministerio de Ciencia e Innovaciꢄn (CTQ2007-61704/BQU), and the
Generalitat de Catalunya (2009SGR-733 and XRQTC) for financial
support of this work. Time allocated in the Centre de Super-
computaciꢄ de Catalunya (CESCA) is gratefully acknowledged.
22 Hz) in the 13C NMR spectrum.
The structures of both 2 and 3[12] were confirmed by X-ray
crystallography (Figure 1). Interestingly, the angle at the
central carbon decreases significantly from the salt 2 (116.68)
to the bis(ylide) 3 (109.88). This observation is in contrast to
the trend in CDPs IIIa, in which the angle tends to become
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002833.
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oxaphosphetane 4a, as indicated by the 31P NMR signal,
which appears at high field (d = À33.0 ppm), which is typical
for pentacoordinate phosphoranes.[17] In the 13C NMR spec-
trum, the ylidic carbon of 4a appears at 41.0 ppm as a doublet
with a large coupling constant of 176.7 Hz. It is interesting to
note that the sulfonium ylide function remains intact even in
the presence of excess ketone. Like oxaphosphetanes derived
from CDPs IIIa,[18] compound 4a shows high thermal stability.
In fact, its decomposition, associated with the elimination of
the phosphine oxide, requires thermal activation (3 h at
1608C). Owing to the harsh conditions, the reaction is not
selective and a complicated mixture of unidentified products
was formed, although only one peak corresponding to the
phosphine oxide (d = 24.5 ppm) in the 31P NMR spectrum was
observed. In contrast, the oxaphosphetane 4b, obtained from
3 and benzaldehyde, is more reactive. It slowly reacts with
another equivalent of aldehyde to give benzyl benzoate 5, and
regenerate bis(ylide) 3. This catalytic hydroacylation reaction
probably proceeds via a hydride transfer from the electron-
rich oxaphosphetane 4b to benzaldehyde, leading to the
transient ion pair 6. Subsequent nucleophilic attack of the
anion gives rise to the benzyl benzoate 5, and regeneration of
bis(ylide) 3 (Scheme 2). Similar hydroacylation reactions
catalyzed by N-heterocyclic carbenes (NHC) have already
been described.[19] This catalytic hydroacylation reaction is
probably the reason that no Wittig reactions between CDPs
IIIa and aldehydes have been reported.[20]
Figure 1. Molecular structures of a) 2 and b) 3. Hydrogen atoms
(except for that on C of 2) and the triflate anion (of 2) are omitted for
clarity. Selected bond lengths [ꢅ] and angles [8]: 2: C–P 1.690(4), S–C
1.687(3); S-C-P 116.6(2). 3: P–C 1.667(3), S–C 1.684(3); P-C-S
109.78(17).
larger (121–1808) upon deprotonation.[13] In fact, the P-C-S
fragment is so strongly bent that it nearly matches that of the
highly strained five-membered cyclic CDP (1058).[14] The P C
À
bond of 3 (1.667 ꢀ) is slightly shorter compared to the
precursor 2 (1.690 ꢀ), but is longer than those of CDPs
(1.549–1.635 ꢀ).[15] In contrast, the C S bond length remains
almost unchanged (DCÀS = 0.003 ꢀ).
À
To gain more insight into the electronic structure of the
molecule, DFT calculations were performed on 3. The
geometry optimization at the M05-2X/6-31 + G(d,p) level
À
À
closely reproduced the X-ray structure (P C 1.671 ꢀ, C S
1.692 ꢀ; P-C-S 110.78; Figure 1).[16] The Wiberg bond index
À
À
for C1 S (1.214) is lower than for P C1 (1.415). The
electronic structure has been analyzed by means of the
natural bond orbital (NBO) method, which localizes a C1
lone pair nearly in the P-C-S plane (nsC), forming a dihedral
angle of about 1608 with the sulfur lone pair, and an out-of-
plane lone pair npC. The geometry of the molecule allows
stabilization of both carbon lone pairs through interactions
À
À
À
with s*(S C4,5), s*(P C3), and s*(P N) orbitals as deter-
mined by second-order perturbation analysis of donor–
Scheme 2. Synthesis of oxaphosphetanes 4a,b and the reaction of 4b
with benzaldehyde.
acceptor interactions in the NBO basis. In particular, nsC
À1
À
interacts with s*(P N1) (16.7 kcalmol ) whereas npC inter-
À1
À
À
acts with s*(S C5) (31.4 kcalmol ), s*(P N2) (28.5 kcal
À1
À1
À
mol ), and s*(P N1) (20.8 kcalmol ), indicating the much
less stabilized nsC by the ylidic interaction with adjacent
substituents compared with npC. Meanwhile, the nsC has a
significantly enhanced s character (42.5%), indicating its
hybridization close to sp. In contrast, the out-of-plane lone
pair npC has a 100% p character. These results demonstrate
that the nsC is efficiently stabilized by the strongly bent
structure rather than by the ylidic interactions. Indeed, the
energy difference between the bent and the linear structures
was also calculated (DEPÀCÀS(1108À1808): ca. 10 kcalmolÀ1) and
found to be much larger than those for CDPs IIIa (DEP-C-P(1368–
1808): 0.3–3.0 kcalmolÀ1).[5b]
In the solid-state structure of oxaphosphetane 4a
(Figure 2), the pentacoordinate phosphorane has oxygen
and nitrogen atoms at the apical position. The most striking
À
À
features are the short P C1 (1.758 ꢀ) and elongated P O
bonds (1.837 ꢀ) compared with those of classical oxaphos-
À
phetanes 7 derived from phosphonium ylides (P C 1.820 ꢀ,
[21]
À
P O 1.745 ꢀ). The alteration of bond lengths in 4 relative
to 7 is certainly caused by the additional negative charge on
the carbon atom in 4, which is delocalized towards the
phosphorane center (4’ in Scheme 3). As a consequence, the
À
À
P C bond is strengthened, and the P O bond is weakened,
which could explain the increased reluctance of 4 to undergo a
Wittig-type reaction as for 7.[7]
The next logical step was to compare and contrast the
reactivity of bis(ylide) 3 and CDPs especially Wittig-type
reactions. The bis(ylide) 3 reacts with carbonyl compounds
exclusively on the phosphonium ylide side. Indeed, the
reaction of 3 with trifluoroacetophenone cleanly forms
Taking into account the electronic features of 4a, we
considered the use of a Lewis acid catalyst to accelerate the
Wittig reaction. Indeed, the formation of adduct 8 by
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Figure 2. Molecular structure of 4a. Hydrogen atoms are omitted for
clarity. Selected bond lengths [ꢅ] and angles [8]: P–C1 1.758(2), S–C1
1.669(2), C1–C2 1.527(3), P–O 1.8372(14), C2–O 1.415(2), S-C1-P
124.98(11), P-C1-C2 95.74(13), C1-C2-O 94.15(14), C1-P-O 73.65(8),
C2-O-P 96.43(11), S-C1-C2 137.61(15).
Scheme 4. Reactions of P,S-bis(ylide) 3.
Scheme 5. Reactivity of transient vinylsulfonium species 13 and 14.
Scheme 3. Representation of oxaphosphetanes 4, 4’, 7, and 8.
sulfonium ylide 15 which reacts with another equivalent of
thiophenol to give the bis(adduct) 11.
complexation of the remaining lone pair on the central carbon
atom should destabilize the system (Scheme 3). Therefore,
the reaction of bis(ylide) 3 with trifluoroacetophenone was
performed in the presence of a catalytic amount of copper(I)
triflate (5%). The reaction proceeds smoothly at room
temperature with elimination of phosphine oxide and diphe-
nylsulfide, and involves both ylide functions of 4a. The
formation of allene 9 can be rationalized by dimerization of a
transient vinylidene.[22] Probably, after the first Wittig reac-
tion, the resulting unsaturated sulfonium ylide decomposes
into the corresponding vinylidene carbene, catalyzed by
copper(I)[23,24] (Scheme 4).
The Wittig reaction can also be triggered by weak acid
species, such as thiophenol. The reaction goes to completion
almost immediately upon the addition of thiophenol (1 equiv)
to 4a at room temerpature, and quantitatively affords the
vinyl phenyl sulfide 10 as a result of the formal insertion of the
Of particular interest, bis(ylide) 3 reacts with benzalde-
hyde (1 equiv) and N-tosyl-g-amino-aldehyde (1 equiv) dia-
stereoselectively, affording the cis-fused bicyclic epoxide 12
(Scheme 4). This result is in good agreement with the
transient formation of a sulfonium ylide, which undergoes
an intramolecular epoxidation reaction.[25] This is, to the best
of our knowledge, the first reaction of bis(ylide)s as an atomic
carbon source, thus allowing the formation of a stereogenic
quaternary carbon center.
In summary, we have successfully synthesized and char-
acterized a stable mixed P,S-bis(ylide), which can be involved
in Wittig-type reactions with carbonyl derivatives. Interest-
ingly, Lewis acids efficiently promote the decomposition of
the transiently formed oxaphosphetanes, and this cumulenic
bis(ylide) can be used in multicomponent reactions. Clearly,
P,S-bis(ylide) 3 behaves as an unsymmetrical atomic carbon
source, allowing a direct creation of quaternary carbon
centers. Given the wide variety of known ligands, the results
shown here should be a major breakthrough, thereby allowing
the preparation of a large variety of carbon(0) complexes as
asymmetric single atomic dicarbene sources with tunable
properties simply by changing the combination of ligands.
À
vinylidene carbene into the S H bond (Scheme 4). Further-
more, this efficient triggering system can also be applied in the
case of benzaldehyde. However, in contrast to the previous
case, the reaction consumes two equivalents of thiophenol to
give the a,b-disulfide 11. The different reaction patterns
observed between trifluoroacetophenone and benzaldehyde
can be rationalized by the inverted polarization of the
vinylsulfonium intermediate 13 owing to the presence of the
strong electron-withdrawing trifluoromethyl substituent
(Scheme 5). Therefore, in the case of 13, the thiolate
nucleophilic attack occurs at the a carbon, affording 10,
whilst vinylsulfonium 14 undergoes the nucleophilic attack at
the b carbon. This reaction leads to the corresponding
Received: May 10, 2010
Revised: June 10, 2010
Published online: August 16, 2010
Keywords: carbon · phosphorus · reaction intermediates ·
.
sulfur · ylides
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