Mesoionic thiazol-5-ylidenes as ligands for transition metal complexes

Article abstract of DOI:10.1039/c1cc14165a

The first examples of thiazol-5-ylidene complexes featuring group 9, 10 and 11 metal centers, have been prepared by deprotonation of a series of 2,3,4-triaryl-susbtituted thiazolium salts in the presence of the corresponding transition metal precursor.

Full text of DOI:10.1039/c1cc14165a

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Mesoionic thiazol-5-ylidenes as ligands for transition metal complexesw
¨
Daniel Mendoza-Espinosa, Gael Ung, Bruno Donnadieu and Guy Bertrand*
Received 11th July 2011, Accepted 27th July 2011
DOI: 10.1039/c1cc14165a
The first examples of thiazol-5-ylidene complexes featuring
group 9, 10 and 11 metal centers, have been prepared by
deprotonation of a series of 2,3,4-triaryl-susbtituted thiazolium
salts in the presence of the corresponding transition metal
precursor.
2,3,4-Triaryl-substituted thiazolium salts 3a–c are conveniently
prepared in two steps from thioamides 1a–c (Scheme 1).
Addition of triethylamine in the presence of bromoacetophenone
affords ketones 2a–c in 55 to 76% yield. Then, cyclization under
acidic conditions produces the desired thiazolium salts 3a–c in
42 to 60% yields. In order to increase their solubility, a salt
metathesis reaction with silver trifluoromethanesulfonate was
carried out, producing 4a–c in excellent yields. The formation
of the thiazolium salts was confirmed by the characteristic
downfield signal between 8.64 and 9.20 ppm (thiazolium C–H)
in the 1H NMR spectra and, in the case of salt 3b, by a single
crystal X-ray diffraction study (Fig. 2, left).z
In 2001, Crabtree et al. reported the preparation of complex
B,1 featuring an imidazol-5-ylidene C as ligand. It was quickly
demonstrated that these types of ligands2 are even stronger
electron-donors than their imidazol-2-ylidene isomers, the
so-called NHCs A3,4 (Fig. 1). Recently, it has been shown that
derivatives of type C,5 as well as pyrazolin-4-ylidenes D6 and
1,2,3-triazol-5-ylidenes E,7 can be isolated. Since no reasonable
canonical resonance forms showing a carbene can be drawn
without additional charges, and since they are mesoionic
compounds, they can be named mesoionic carbenes (MICs).8
Importantly, in contrast to regular carbenes, no obvious
dimerization pathway can be foreseen, and therefore it was
reasonable to believe that unhindered MICs with various
heteroatom-containing skeletons could be stable. To test this
hypothesis, we attempted the preparation of 1,3-dithiol-5-
ylidenes F, but observed a spontaneous ring opening yielding
the corresponding ethynyl dithiocarbamate G.9 The formation
of G is reminiscent of the ring-opening reaction observed in
the deprotonation of isothiazolium salts,10 and therefore is
probably due to the weakness of the sulfur-carbon bond.
These results prompted us to investigate the preparation of
thiazol-5-ylidenes H, since a similar ring-opening process would
imply the cleavage of a carbon–nitrogen bond. Moreover,
although 1,3-dithiol-2-ylidenes I have never been observed, it
is noteworthy that 1,3-thiazol-2-ylidenes J have been used for
decades as organocatalysts,11 beginning with the seminal work by
Breslow,12 before their isolation by Arduengo.13 Here we report the
synthesis of a series of 2,3,4-aryl-substituted thiazolium salts, and
their use for the preparation of thiazol-5-ylidene transition
metal complexes.
All attempts to deprotonate salts 3 and 4 with a variety of
n
bases [MHMDS (M = Li, Na, K), BuLi, KOtBu and LDA]
resulted in complex mixtures. Even monitoring the deprotonation
reaction by NMR spectroscopy, at ꢀ50 1C, did not allow for the
observation of the desired thiazol-5-ylidenes of type H.
However, when salts 4a–c were treated at ꢀ78 1C with an
equimolar amount of LiHMDS, in the presence of
(THT)AuCl or [Pd(Allyl)Cl]2, the corresponding thiazol-5-
ylidene transition metal complexes were obtained and isolated
in moderate to good yields (51 to 90%) (Scheme 2). The 13C
NMR chemical shifts of the carbene carbon of AuI (149.5 to
151.4 ppm) and PdII (156.0 to 159.5 ppm) complexes are in the
range observed for previously reported metal complexes
featuring MICs C–F,5,9,14 and as expected, at much higher
field than those of classical NHCs.15
To unambiguously confirm the existence of the new thiazol-
5-ylidene ligands, single crystals of complex 5b, obtained by
slow diffusion of hexane into a concentrated chloroform
UCR-CNRS Joint Reserach 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
Fig. 1 NHCs (A), Crabtree’s imidazol-5-ylidene complex (B), types
of MIC that have previously been isolated (C–E), transient 1,3-dithiol-
5-ylidene (F) and its ring opened isomer (G), the targeted thiazol-5-
ylidenes (H), the never-observed 1,3-dithiol-2-ylidenes I, and the
isolated 1,3-thiazol-2-ylidenes J.
w Electronic supplementary information (ESI) available: Preparation
and spectroscopic data for all compounds, selected X-ray data for
compounds 3b and 5b. CCDC 833533 and 833534. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc14165a
c
10614 Chem. Commun., 2011, 47, 10614–10616
This journal is The Royal Society of Chemistry 2011

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Scheme 3 Synthesis of the rhodium(I) complexes 7a–c [Ar=Phenyl
(a); 2,4,6-trimethylphenyl (b); 2,6-diisopropylphenyl = Dipp (c)].
of salts 4a–c, at ꢀ78 1C, with LiHMDS, in the presence of half
an equivalent of [Rh(COD)Cl]2, followed by treatment with an
excess of carbon monoxide (Scheme 3).
The average CO vibration frequency for 7a–c (nan = 2034
cmꢀ1) indicates that the electron-donor capabilities of thiazol-
5-ylidenes are slightly superior to those of conventional NHCs
(nan = 2039–2041cmꢀ1),16 similar to those of 1,3-dithiol-5-
Scheme 1 Synthesis of thiazolium salts 3a–c and 4a–c [Ar = Phenyl
(a); 2,4,6-trimethylphenyl (b); 2,6-diisopropylphenyl (c)].
ylidene F (nan = 2030 cmꢀ1 9
) and cyclic (alkyl)(amino)-
carbenes (nan = 2036 cmꢀ1),17 but inferior to those of MICs
C–E (van = 2016–2025 cmꢀ1).2d
Although thiazol-5-ylidenes cannot be observed, the readily
available 2,3,4-triaryl-substituted thiazolium salts are modular
precursors for the synthesis of a variety of thiazol-5-ylidenes
transition metal complexes. This novel type of MIC ligand
features stronger donor properties than classical NHCs, and
gives rise to complexes, which are thermally- and air-stable.
Research in our laboratory is currently underway to extend
the scope of mesoionic carbenes, and explore the catalytic
activity of the corresponding complexes.
Fig. 2 Crystal structures of thiazolium salt 3b (left) and gold complex
5b (right) with ellipsoids shown at 50% probability. Hydrogen atoms,
except for the CH of 3b, are omitted for clarity. Selected bond
distances [A] and angles [1]: 3b: C(1)–S(1) = 1.716(6), C(1)–C(3) =
1.363(6), C(3)–N(1) = 1.397(3), N(1)–C(2) = 1.361(3), C(2)–S(1) =
1.684(2), C(3)–C(1)–S(1) = 112.0(4); 5b: C(1)–S(1) = 1.697(14),
This work was supported by the NIH (R01 GM 68825) and
DOE (DE-FG02-09ER16069).
Notes and references
C(1)–C(3)
1.351(15), C(2)–S(1)
Au(1)–Cl(1) = 2.293(4), C(3)–C(1)–S(1) = 110.0(10).
=
1.364(16), C(3)–N(1)
=
1.414(15), N(1)–C(2)
=
z Crystal data: 3b: C27H30BrNO2S, M = 512.49, monoclinic, a =
9.4960(14), b = 28.054(4), c = 9.6399(14), a = 90, b = 104.693(2),
g = 90, V = 2484.1(6) A3, T = 100(2) K, space group P21/c, Z = 4,
20 859 reflections measured, 6277 unique (Rint = 0.0276), which were
used in all calculations. The final R [I 4 2sigma(I)] was 0.0319. 5b:
C24H21AuClNS, M = 587.89, monoclinic, a = 18.170(4), b =
9.7695(15), c = 24.734(4), a = 90, b = 99.134(3), g = 90, V =
4334.9(12) A3, T = 200(2) K, space group C2/c, Z = 8, 11 994
reflections measured, 4607 unique (Rint = 0.0501), which were used
in all calculations. The final R [I 4 2sigma(I)] was 0.0648.
=
1.695(14), Au(1)–C(1)
=
2.005(14),
1 S. Grundemann, A. Kovacevic, M. Albrecht, J. W. Faller and
¨
R. H. Crabtree, Chem. Commun., 2001, 2274.
2 For reviews on imidazol-5-ylidenes and related species, see:
(a) O. Schuster, L. Yang, H. G. Raubenheimer and M. Albrecht,
Chem. Rev., 2009, 109, 3445; (b) M. Albrecht, Chem. Commun.,
2008, 3601; (c) P. L. Arnold and S. Pearson, Coord. Chem. Rev.,
2007, 251, 596; (d) M. Albrecht, Chimia, 2009, 63, 105;
(e) M. Melaimi, M. Soleilhavoup and G. Bertrand, Angew. Chem.,
Int. Ed., 2010, 49, 8810.
Scheme 2 Synthesis of metal complexes 5a–c and 6a–c [Ar = Phenyl
(a); 2,4,6-trimethylphenyl (b); 2,6-diisopropylphenyl (c)].
solution, were subjected to an X-ray diffraction analysis
(Fig. 2, right). Complex 5b crystallizes in the C2/c space group
displaying a monomeric unit with a Au(1)–C(1) bond distance
of 2.005(14)A, which is slightly longer that those observed for
the (1,3-dithiol-5-ylidene)AuCl [1.978(4)A]9 and (imidazol-5-
ylidene)AuCl [1.98 A].5 The carbene bond angle angle in 5b
[110.0(10)1] is slightly more acute than the corresponding angle
of thiazolium salt 3b [112.0(4)1]. This feature is consistent with
the increased s-character of the s lone-pair orbital on the
carbene atom in 5b as compared with the C–H+ bond orbital
in 3b. Finally, as expected, the Au(I) center is in a linear
environment with an C(1)–Au(1)–Cl(1) angle of 179.2(4)1.
To evaluate the donor properties of thiazol-5-ylidene
ligands, the corresponding rhodium(I) dicarbonyl chloride
complexes 7a–c were prepared by the in situ deprotonation
3 For recent reviews on NHCs, see for examples: (a) T. Droge and
¨
F. Glorius, Angew. Chem., Int. Ed., 2010, 49, 6940;
(b) G. C. Vougioukalakis and R. H. Grubbs, Chem. Rev., 2010,
110, 1746; (c) J. C. Y. Lin, R. T. W. Huang, C. S. Lee,
A. Bhattacharyya, W. S. Hwang and I. J. B. Lin, Chem. Rev.,
2009, 109, 3561; (d) P. L. Arnold and I. J. Casely, Chem. Rev.,
2009, 109, 3599; (e) S. Dıez-Gonzalez, N. Marion and S. P. Nolan,
´ ´
Chem. Rev., 2009, 109, 3612; (f) M. Poyatos, J. A. Mata and
E. Peris, Chem. Rev., 2009, 109, 3677; (g) C. Samoj"owicz,
M. Bieniek and K. Grela, Chem. Rev., 2009, 109, 3708;
(h) W. A. L. van Otterlo and C. B. de Koning, Chem. Rev.,
2009, 109, 3743; (i) S. Monfette and D. E. Fogg, Chem. Rev.,
2009, 109, 3783; (j) B. Alcaide, P. Almendros and A. Luna, Chem.
Rev., 2009, 109, 3817; (k) F. E. Hahn and M. C. Jahnke, Angew.
Chem., Int. Ed., 2008, 47, 3122; (l) D. Bourissou, O. Guerret,
F. P. Gabbaı and G. Bertrand, Chem. Rev., 2000, 100, 39.
¨
c
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4 For recent books on NHCs, see: (a) N-Heterocyclic Carbenes,
From laboratory Curiosities to Efficient Synthetic Tools,
ed. S. Dıez-Gonzalez, Royal Society of Chemistry Publishing,
´ ´
9 (a) G. Ung, D. Mendoza-Espinosa, J. Bouffard and G. Bertrand,
Angew. Chem., Int. Ed., 2011, 50, 4215; (b) G. Ung, G. D. Frey,
W. W. Schoeller and G. Bertrand, Angew. Chem., Int. Ed., 2011,
DOI: 10.1002/anie.201104303.
10 A. Dehope, V. Lavallo, B. Donnadieu, W. W. Schoeller and
G. Bertrand, Angew. Chem., Int. Ed., 2007, 46, 6922.
11 See for recent examples: (a) K. Hirano, A. T. Biju, I. Piel and
F. Glorius, J. Am. Chem. Soc., 2009, 131, 14190; (b) R. Lebeuf,
K. Hirano and F. Glorius, Org. Lett., 2008, 10, 4243; (c) A. T. Biju
and F. Glorius, Angew. Chem., Int. Ed., 2010, 49, 9761; (d) I. Piel,
M. Steinmetz, K. Hirano, R. Frohlich, S. Grimme and F. Glorius,
Angew. Chem., Int. Ed., 2011, 50, 4983.
12 R. Breslow, J. Am. Chem. Soc., 1958, 80, 3719.
13 A. J. Arduengo, III, J. R. Goerlish and W. J. Marshal, Liebigs
Ann./Recl., 1997, 365.
2011; (b) N-Heterocyclic Carbenes in Transition Metal Catalysis,
ed. F. A. Glorius, Springer Verlag, 2007; (c) N-Heterocyclic
Carbenes in Synthesis, ed. S. P. Nolan, Wiley-VCH, 2006.
5 (a) E. Aldeco-Perez, A. J. Rosenthal, B. Donnadieu,
P. Parameswaran, G. Frenking and G. Bertrand, Science, 2009,
326, 556; (b) G. Ung and G. Bertrand, Chem.–Eur. J., 2011,
17, 8269; (c) Y. Wang, Y. Xie, M. Y. Abraham, P. Wei,
H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am.
Chem. Soc., 2010, 132, 14370.
6 (a) V. Lavallo, C. A. Dyker, B. Donnadieu and G. Bertrand,
´
Angew. Chem., Int. Ed., 2008, 47, 5411; (b) I. Fernandez,
C. A. Dyker, A. DeHope, B. Donnadieu, G. Frenking and
G. Bertrand, J. Am. Chem. Soc., 2009, 131, 11875.
14 (a) P. Mathew, A. Neels and M. Albrecht, J. Am. Chem. Soc.,
2008, 130, 13534; (b) A. John, M. M. Shaik and P. Ghosh, Dalton
Trans., 2009, 10581; (c) X. Xu, B. Xu, Y. Li and S. H. Hong,
Organometallics, 2010, 29, 6343.
7 (a) G. Guisado-Barrios, J. Bouffard, B. Donnadieu and
G. Bertrand, Angew. Chem., Int. Ed., 2010, 49, 4759;
(b) J. Bouffard, B. K. Keitz, R. Tonner, G. Guisado-Barrios,
G. Frenking, R. H. Grubbs and G. Bertrand, Organometallics,
2011, 30, 2617; (c) B. K. Keitz, J. Bouffard, G. Bertrand and
R. H. Grubbs, J. Am. Chem. Soc., 2011, 133, 8498.
8 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., 1993, 126, 1149.
15 D. Tapu, D. A. Dixon and C. Roe, Chem. Rev., 2009, 109, 3385.
16 A. Furstner, M. Alcarazo, H. Krause and C. W. Lehmann, J. Am.
¨
Chem. Soc., 2007, 129, 12676.
17 V. Lavallo, Y. Canac, A. Dehope, B. Donnadieu and G. Bertrand,
Angew. Chem., Int. Ed., 2005, 44, 7236.
c
10616 Chem. Commun., 2011, 47, 10614–10616
This journal is The Royal Society of Chemistry 2011