Ynamides: Stable ligand equivalents of unstable oxazol-4-ylidenes (novel mesoionic carbenes)

Article abstract of DOI:10.1039/c2cc33319h

Deprotonation of an oxazolium salt induces a ring opening process leading to the corresponding ynamide. Although the expected mesoionic carbene is not obtained, the acyclic ynamide readily reacts with various transition metals yielding robust mesoionic carbene complexes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc33319h

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Ynamides: Stable Ligand Equivalents of Unstable Oxazol-4-ylidenes
(Novel Mesoionic Carbenes)
Gaël Ung, Daniel Mendoza-Espinosa and Guy Bertrand*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Deprotonation of an oxazolium salt induces a ring opening
process leading to the corresponding ynamide. Although the
expected mesoionic carbene is not obtained, the acyclic
ynamide readily reacts with various transition metals yielding
10 robust mesoionic carbene complexes.
The development of mesoionic carbenes (MICs)1 has raised
considerable interest during the past decade,2,3 mainly because of
their strong electron donating properties, which are even superior
to those of the widely used N-heterocyclic carbenes (NHCs).4 We
15 have recently shown that several types of MICs A-C (Figure 1)
are stable and can be isolated as crystalline species.5 However,
we have also observed that MICs D6 and E7 are not stable enough
to be spectroscopically observed. In order to better understand the
factors governing the stability of MICs, we chose to study the
20 stability of oxazol-4-ylidenes F.
Scheme 1 Synthesis of ynamide 3.
This ring-opening process is reminiscent of that observed in the
40 deprotonation of isoxazolium G,9 isothiazolium H,10 and 1,3-
dithiolium salts I7a (Scheme 2). Therefore, it seems that an
oxygen or sulfur atom in the position  to the carbene center does
not allow for the isolation of MICs. This experimental
observation is not readily explained by the strength of the bond
45 that is broken, since C–O are stronger than C–N bonds, nor by the
aromaticity of heterocycles, since sulfur derivatives are generally
more aromatic than their nitrogen containing analogues.
Fig. 1 Stable crystalline MICs A-C, unstable MICs D-E, and targeted
MIC F.
By analogy with the synthetic route used for the preparation of
25 MICs A-E, the oxazolium salt 2 was chosen as a potential
precursor for the desired carbene F1. The cationic heterocycle 2
was obtained by alkylation of the nitrogen atom of the readily
available oxazole 18 (74 % yield; Scheme 1). Clean deprotonation
of 2 with potassium bis(trimethylsilyl)amide was achieved, as
1
30 shown by the disappearance of the oxazolium proton in the H
NMR spectrum. However, the 13C NMR spectrum displayed
signals at 75.7 and 83.7 ppm, far from the range expected for
mesoionic carbenes (~200 ppm).5 These data are consistent with
the presence of sp hybridized carbons, leading to the conclusion
35 that the deprotonation of 2 yields the ynamide 3 and not the
expected MIC F1.
Scheme 2 Ring opening pathway of isoxazolium G, isothiazolium H and
50
of 1,3-dithiolium I leading to ethynylcarbamodithioate J.
Following our report that a variety of transition metal fragments
induce the ring closure of acyclic ethynylcarbamodithioates J,
leading to the corresponding MIC complexes, we envisaged that
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3 should behave in the same way.
To validate our hypothesis, ynamide 3 was reacted with half an
mesoionic oxazol-4-ylidene F1. The infrared CO vibration
frequencies of 5 (νav = 2035.3 cm−1) indicate that ligand F1 is a
equivalent of [RhCl(cod)]2. The disappearance of both acetylene 25 stronger electron donor than classical NHCs (νav = 2039–2041
carbon resonances in 13C NMR and the appearance of a doublet
(1J(C-Rh) = 48 Hz) at 156 ppm, characteristic of MIC-Rh
complexes,2 were strong evidence that the expected ring-closure
occurred and that complex 4 had been generated. Furthermore,
after treatment of
corresponding rhodium(I) dicarbonyl chloride complex 5 was
10 quantitatively obtained. Single crystals of 5 were grown from
CHCl3/pentane and the X-ray diffraction study unambiguously
confirmed the cyclic structure of the carbene ligand (Figure 2).
cm−1),11 but is the weakest donor of the recently uncovered MIC
family (νav = 2016–2031 cm−1).2e,7a This is in line with the higher
electronegativity of oxygen compared to nitrogen and sulfur.
Note that ynamides are useful building blocks for the introduction
5
4
with excess carbon monoxide, the 30 of a nitrogen fragment in organic molecules,12 but their synthesis
is still mainly limited to rather complex procedures (for example:
uses of alkynyliodonium13 or cross coupling14 chemistry) and the
scope of these reactions is often narrow. The results reported
herein provide the synthetic community with a convenient and
35 simple synthesis of these compounds.
Additionally, these results prove that ynamides and most likely
other acyclic analogues can act as a ligand equivalent of the
corresponding cyclic mesoionic carbenes. We are currently
investigating the possibility of using other acyclic molecules for
40 the preparation of various 5- and 6-membered MIC-complexes,
as well as the catalytic activity of metal complexes bearing these
ligands.
This work was supported by the NSF (CHE-1112133) and
DOE (DE-FG02-09ER16069).
45 Notes and references
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957),
Department of Chemistry, University of California, Riverside CA
92521-0403, USA. E-mail: guy.bertrand@ucr.edu;
Fax: (+1) 202 354 5267
50 † Electronic Supplementary Information (ESI) available: Preparation and
spectroscopic data for all compounds. See DOI: 10.1039/b000000x/
1
For the origin of the acronym MIC, see: S. Araki, Y. Wanibe, F. Uno,
A. Morikawa, K. Yamamoto, K. Chiba and Y. Butsugan, Chem. Ber.,
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55 2 For reviews on MICs and related species, see: (a) O. Schuster, L.
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Fig. 2 Structure of 5 in the solid state, hydrogen atoms have been omitted
15
for clarity. Ellipsoids are drawn at 50% probability.
3
For MICs as ligand for transition metal catalysts, see for examples:
(a) H. Lebel, M. K. Janes, A. B. Charette and S. P. Nolan, J. Am.
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Krueger, A. Neels and M. Albrecht, Chem. Commun., 2010, 46, 315;
(e) X. Xu, B. Xu, Y. Li and S. H. Hong, Organometallics, 2010, 29,
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Chem. Commun., 2011, 47, 328; (g) J. Cai, X. Yang, K. Arumugam,
C. W. Bielawski and J. L. Sessler, Organometallics, 2011, 30, 5033;
(h) T. Nakamura, T. Terashima, K. Ogata and S.-I. Fukuzawa, Org.
Lett., 2011, 13, 620; (i) R. Saravanakumar, V. Ramkumarand and S.
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V. Huynh, Organometallics, 2012, 31, 405.
For recent books and reviews, see for example: (a) N-Heterocyclic
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To test the scope of this ring closing process, ynamide 3 was
reacted with other electrophilic metal precursors. Complexes of
gold 6 and palladium 7 were readily obtained in good yields (77
and 91 % respectively; Scheme 3).
4
20
Scheme 3 Complexation of ynamide 3.
Ynamide 3 can therefore act as a synthetic ligand equivalent of

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5
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