A direct ylide transfer to carbonyl derivatives and heteroaromatic compounds

Article abstract of DOI:10.1002/anie.201002919

Power transfer: The title reaction proceeds under mild conditions (room temperature, short reaction times) and directly affords sulfonium ylides from active methylene compounds and heteroaromatics in a single step and in high yields. A detailed comparative structural analysis of a variety of ylides is presented and the implications of structure on the reactivity of these compounds are discussed.

Full text of DOI:10.1002/anie.201002919

Angewandte
Chemie
DOI: 10.1002/anie.201002919
Ylide Chemistry
A Direct Ylide Transfer to Carbonyl Derivatives and Heteroaromatic
Compounds**
Xueliang Huang, Richard Goddard, and Nuno Maulide*
Since the pioneering work of Corey et al. and Johnson et al. in
the 1960s,[1] sulfonium and sulfoxonium ylides have occupied
a prominent place as cornerstone, textbook reagents in
organic chemistry.[2] In addition to the construction of
simple small rings such as epoxides,[3] cyclopropanes,[4] and
aziridines,[5] new methods to access more complex architec-
tures through cascade cyclizations using sulfonium ylides with
suitable reagents have also been developed in recent years.[6]
Perhaps less well-known but equally interesting are reports
highlighting antimicrobial properties of sulfur ylides.[7] More
recently, increased interest has focused on the potential of
sulfonium/sulfoxonium ylides to behave as metal carbene
precursors under suitable conditions.[8] Indeed, whilst the
arena of carbenoid donors to metals is almost exclusively
dominated by a-diazocarbonyl derivatives,[9] their toxic and
hazardous nature has long plagued applications to synthesis.
In particular, the rapid evolution of nitrogen gas upon their
decomposition is prone to generate uncontrolled exotherms
that limit their suitability for large-scale processing.[8e]
technology for preparation of sulfonium and sulfoxonium
ylides is still essentially the same as that introduced by Corey
et al. and Yao and co-workers more than 40 years ago,[12] and
entails deprotonation of a sulfonium/sulfoxonium salt (which
must be prepared by a substitution reaction) by a strong base.
Ironically, the most competitive process to the aforemen-
tioned method is perhaps the decomposition of diazo com-
pounds in the presence of sulfides to produce sulfonium
ylides—with its own shortcomings, such as the requirement
for (often) stoichiometric amounts of metal salts and the prior
synthesis and handling of toxic and explosive diazo com-
pounds.[13] Herein, we report on simple, high-yielding ylide
transfer reactions of Martinꢀs sulfurane (1; Scheme 1) that
allow direct access to aliphatic and aromatic ylide derivatives
with interesting structural properties.
Martinꢀs sulfurane (1), named after the scientist whose
group was the first to prepare it in 1971,[14] is widely used as a
reagent for dehydration (mostly of alcohols) in organic
synthesis.[15,16] We speculated that 1 might behave as a ylide
transfer reagent to suitable carbonyl derivatives, thus yielding
diphenylsulfonium ylides (2, R3 = Ph). To the best of our
knowledge, Martinꢀs sulfurane (1) has not been previously
employed for this purpose.[17]
To test this hypothesis, we began our study using readily
available ethyl acetoacetate 3a as a model substrate for
reaction with 1. Gratifyingly, treatment of 3a with 1.5 equiv-
alents of 1 in diethyl ether afforded a single compound in
quantitative yield [Eq. (1)]. After careful analysis, the prod-
uct was found to have a structure consistent with 4a (as
additionally supported by NMR spectroscopy and high-
resolution mass spectrometry). Unambiguous proof of struc-
ture and geometry was achieved by single-crystal X-ray
analysis (Figure 1).[18]
It is particularly intriguing to note that, whereas a-
diazocarbonyl compounds are typically synthesized in a single
step by one out of a number of so-called “diazo transfer
reactions”,[10] the analogous concept of “ylide transfer” has
been scarcely developed (Scheme 1).[11] In fact, standard
Scheme 1. Comparison of the synthesis of diazocarbonyl compounds
and carbonyl-stabilized sulfur ylides. R1–R3 =alkyl, aryl, electron-with-
drawing groups; RF =C6H5(CF3)2C.
[*] Dr. X. Huang, Dr. R. Goddard, Dr. N. Maulide
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr (Germany)
Fax: (+49)208-306-2999
Encouraged by this result, we investigated the scope of
this ylide transfer reaction. As compiled in Scheme 2, a broad
variety of substrates afforded the corresponding sulfonium
ylides in good to excellent yields. Different ketoesters (3a–d)
including tetronic acid 3i reacted smoothly with 1. The
reactions of acyclic (3e–g) and cyclic (3h) diketones bearing
aromatic or aliphatic groups gave the corresponding ylides
(4e–h) in good to quantitative yields. Pleasingly, dimethyl
E-mail: maulide@mpi-muelheim.mpg.de
Homepage: http://www.kofo.mpg.de/maulide
[**] We are grateful to the Max-Planck Society and the Max-Planck-
Institut fꢀr Kohlenforschung for generous funding of our research
programs. Invaluable assistance from our HPLC, NMR, and X-Ray
departments is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002919.
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8979

Communications
bears a leaving group. Similar compounds are reported as
potent fungicides, and previous preparations relied on
bromination/deprotonation of the pre-synthesized, unsubsti-
tuted ylides.[19] In sharp contrast, 4n is available in a single
step through the present methodology.[20]
It is worthhwile noting at this juncture that sulfurane 1
appears to be particularly effective in this ylide transfer
process; alternative methods (e.g., combinations of sulfoxides
and diverse dehydrating reagents) are known[11] to be plagued
by low yields, harsh reaction conditions, and somewhat
narrow substrate scopes. Mechanistically, the reactions in
Scheme 2 may proceed either by initial deprotonation of the
carbonyl derivative by dissociated Martinꢀs sulfurane with
subsequent attack on sulfur, or direct nucleophilic displace-
ment by an enol tautomer.
Figure 1. X-ray crystal structure for 4a. The thermal ellipsoids are
drawn at 50% probability. Hydrogen atoms are omitted for clarity.
Intrigued by the simplicity of this procedure, we then set
our sights on more challenging classes of substrates. In
particular, we hypothesized that the powerful ylide transfer
ability of sulfurane 1 might be amenable to Friedel–Crafts-
like dearomatization of suitable heterocycles. Indole 5a
reacted smoothly with 1 within 2 hours at room temperature,
affording the indole-3-sulfonium ylide 6a in quantitative yield
[Eq. (2)].[21] Once again, the strikingly facile access to this
compound encouraged us to examine related substrates in
more detail.
As can be seen, this reaction appears to possess a broad
scope (Scheme 3). Indoles 5a–h bearing substituents ranging
from electron-donating to electron-neutral and strong elec-
tron-withdrawing groups were tolerated. Moreover, consid-
erable flexibility is allowed regarding the substituent location.
All the ylides 6a–h were obtained in quantitative yields.
In addition, pyrrole (5i) and its derivatives 5j,k also
performed competently in this reaction. As might be
expected, pyrrole 2-sulfonium ylide 6i was obtained as the
exclusive regioisomer.[22] Interestingly, the presence of an
ester substituent as in 5k led to the production of small
amounts of the 4-sulfonium ylide isomer, presumably owing
to the strong meta-directing effect of the ester moiety. The
robustness of this reaction is additionally apparent from the
ease of scale-up: indole derivative 5d was easily converted
into the corresponding sulfonium ylide 6d on a multigram
scale.[23]
Scheme 2. Direct ylide transfer to active methylene compounds using
1. Yields refer to pure, isolated compounds. All reactions were run in
diethyl ether using 1.5 equivalents of 1 unless mentioned otherwise.
[a] Solvent was chloroform. [b] Solvent was dichloromethane.
malonate (3j) and the analogous malononitrile (3l) and
Meldrumꢀs acid (3k) were perfect candidates for this reaction,
furnishing 4j–l in high yields. Furthermore, for acetophe-
nones 3m,n and phenylacetate derivatives 3o,p, the corre-
sponding ylides 4m,n and 4o,p were also obtained in high
yields. Compound 4n, derived from a-bromoacetophenone,
merits notice because of the fact that its ylidic carbon atom
In addition to the crystal structure of 4a, the crystal
structures of 4g, 4j, 4l, 4n, and 4m were determined,[24] and
they reveal an interesting trend, which is noticeable in the
geometry about the ylidic carbon atom. Whereas 4m,
containing electron-withdrawing groups (R1 = CN and R2 =
=
COPh), has a relatively long S C distance of 1.734(1) ꢁ and a
=
large C-C( S)-C angle of 126.4(1)8, 4n, having the weaker
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1
2
=
Table 1: The dependence of S C distances and R -C-R angles upon
substituents R1 and R2.
Compd.
R1
R2
d [ꢁ]
a [8]
b [8]
4n
4l
4j
Br
CN
COOMe
COMe
COPh
CN
COOMe
COMe
1.688(2)
1.718(1)
1.724(2)
1.729(1)
1.723(1)
1.7300(6)
1.734(1)
118.1(2)
121.8(1)
121.0(2)
126.5(1)
127.3(1)
126.25(5)
126.4(1)
101.8(1)
103.2(4)
104.5(1)
105.0(1)
105.2(1)
105.65(3)
102.8(1)
4g
4a
4m
COMe
CN
COOEt
COPh
charged oxygen atom onto the proximal, activated aromatic
ring (Scheme 4).[27,28] This intriguing behavior was not unique
to 4a, and other analogues (Scheme 4) also smoothly
isomerized into their tetrasubstituted olefin counterparts
7d–m. The geometries of the final products were variable,
likely resulting from facile E/Z isomerization of the push–pull
olefins formed.[29]
Scheme 3. Direct ylide transfer to heteroaromatic compounds using 1.
Yields refer to pure, isolated compounds. All reactions were run in
toluene using 1.5 equivalents of 1 unless mentioned otherwise.
[a] 16% of the 4-sulfonium ylide was also obtained.
In summary, we have developed a new ylide transfer
reaction. This process allows a direct, operationally simple
electron-withdrawing group (R1 = Br), has a
=
shorter S C distance of 1.688(2) ꢁ and a
=
smaller C-C( S)-C angle of 118.1(2)8
(Table 1). Consistent with build-up of negative
charge on the ylidic C, the C atom in 4n is
slightly pyramidal (0.15 ꢁ out of the plane
=
through S, Br, and C( O)(Ph)), indicating that
this C atom is very basic; that is, the ylidic
resonance form dominates. In contrast, the
angles around the S atom remain essentially
constant down the series, suggesting that little
electron density is transferred from the ylidic C
back to S.[25] In the case of 4g, the structure of a
selenurane analogue has been determined,[26]
=
wherein the C-C( X)-C angle is significantly
larger (134.108 cf. av. 126.9(2)8). Interestingly,
not only is the angle larger in the selenurane,
but in contrast to 4g, which contains E- and Z-
configured carbonyl groups in each of the two
independent molecules in the unit cell, both
carbonyl oxygen atoms point towards the Se (Z,
Z). Clearly, the geometry of the ylide is
particularly sensitive to the electronic situation
at the carbanion.
Scheme 4. Phenyl migration from S to O for 4a.
The structural complexity of the prepared compounds
gives rise to unusual reactivity profiles. For instance, ther-
molysis of compound 4a triggers a clean migration of a phenyl
group from S to O, thus leading to (E)-7a (Scheme 4). This
migration correlates with the short distance between the
ketocarbonyl oxygen atom and the diphenylsulfur ylide that is
observed in the crystal structure.[24] We thus believe that this
rearrangement proceeds by ipso-attack of a negatively
synthesis of sulfur ylides from active methylene compounds
and heteroaromatics, and provides a powerful sulfur equiv-
alent to the well-known diazo transfer reactions. Indeed, 1
appears uniquely suited to direct, high-yielding syntheses of
sulfur ylides and the reactions described herein should find
broad applications in synthesis. Furthermore, we have uncov-
ered a pronounced impact of the electron density variations
upon the ylide geometry which may influence future synthetic
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Communications
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Received: May 14, 2010
Revised: August 9, 2010
Published online: October 11, 2010
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Keywords: rearrangement · structure elucidation · sulfuranes ·
transfer reactions · ylides
.
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allowed the formation of 4a in excellent yields. See the
Supporting Information for details.
3a with 1.0 equivalents of 1 gave ylide 4a in 85% yield. See the
Supporting Information for details.
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[24] CCDC 776545 (4a), 776546 (4g), 776547 (4j), 776548 (4l),
776550 (4m), 776549 (4n), 785393 (4o), 776551 (6d), 776552
(6k), 776553 (7m) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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[27] Such a process is mechanistically related to the Smiles rear-
rangement: a) J. F. Burnett, R. E. Zalher, Chem. Rev. 1951, 49,
273; b) W. E. Truce, E. M. Kreider, W. W. Brand, Org. React.
1970, 18, 99.
[28] a) M. Takaku, Y. Hayasi, H. Nozaki, Tetrahedron 1970, 26, 1243.
For Grignard reagent-initiated rearrangements, see ref. [17d];
b) For recent reviews on sulfur-mediated rearrangements, see:
Topics in Current Chemistry, Springer, Berlin, 2007, 274 and 275.
[29] The geometry of (E)-7a was determined through NOE experi-
ments. See the Supporting Information for details.
[23] The amounts of sulfurane 1 could be reduced to 1.0 equivalents
with only a marginal decrease in yield. For example, reaction of
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