A stable acyclic ligand equivalent of an unstable 1,3-dithiol-5-ylidene

Article abstract of DOI:10.1002/anie.201100420

Substitutes you can rely on: Mesoionic carbenes (MICs) are not always stable. However, the acyclic ethynylcarbamodithioate 2 formed (instead of the corresponding MIC) by deprotonation of dithiolium salt 1 underwent cyclization to its precursor under acidic conditions and reacted with a variety of transition-metal centers to yield robust MIC complexes 3; (see scheme; Tipp=2,4,6-triisopropylphenyl). Copyright

Full text of DOI:10.1002/anie.201100420

DOI: 10.1002/anie.201100420
Mesoionic Carbenes
A Stable Acyclic Ligand Equivalent of an Unstable 1,3-Dithiol-5-
ylidene**
Gaꢀl Ung, Daniel Mendoza-Espinosa, Jean Bouffard, and Guy Bertrand*
Dedicated to Professor Didier Astruc on the occasion of his 65th birthday
During the last two decades, N-heterocyclic carbenes
(NHCs), such as A (Scheme 1), have played a prominent
role as ligands for transition-metal catalysts.[1] Their popular-
ity is mainly due to their strong s-donor properties and the
without the need for kinetic protection. No derivatives of the
1,3-dithiol-2-ylidene F are known owing to their dimerization
into derivatives of tetrathiafulvalene G.[7] Herein, we report
our attempts to prepare a free MIC isomer by carbenes of
type F, namely, a 1,3-dithiol-5-ylidene E. We show that this
compound is unstable owing to spontaneous ring opening to
form the corresponding ethynylcarbamodithioate. Impor-
tantly, the latter reacts with a variety of metals to give 1,3-
dithiol-5-ylidene–metal complexes and therefore is a ligand
equivalent of E.
In analogy with the classical synthetic route used to
prepare NHCs and MICs, we chose the readily available
dithiolium tetrafluoroborate salt 1a as
a
precursor
(Scheme 2).[8] Deprotonation with potassium bis(trimethylsi-
lyl)amide proceeded cleanly, as shown by the disappearance
1
of the signal for the dithiolium-ring proton in the H NMR
Scheme 1. Structural framework of classical NHCs (A), types of MIC
that have been isolated previously (B–D), the targeted 1,3-dithiol-5-
ylidenes (E), and their unknown 1,3-dithiol-2-ylidene isomers (F),
which dimerize to tetrathiafulvalenes of type G.
robustness of the corresponding complexes. These two
features result from the presence of the electropositive
carbon center and the strength of the carbon–metal bond.
Therefore, other types of carbon-based L ligands are highly
desirable. The simplest method for the preparation of metal
complexes featuring a given L ligand is by ligand substitution
at the metal center; however, the availability of stable
compounds with a lone pair of electrons at a carbon center
is very limited.[2] It has recently been shown that mesoionic
carbenes (MICs)[3–5] B–D can be isolated as free species.[6] In
contrast to “normal carbenes”, no obvious dimerization
pathway can be foreseen for MICs. Consequently, a variety
of these unusual carbenes should in principle be available
Scheme 2. Deprotonation of the dithiolium salt 1a did not enable the
isolation of MIC 3, but led to ethynylcarbamodithioate 2. The addition
of trifluoromethanesulfonic acid to 2 induced ring closure to afford the
dithiolium salt 1b. Tipp=2,4,6-triisopropylphenyl, Tf=trifluorometh-
anesulfonyl.
spectrum. The 13C NMR spectrum displayed a signal at d =
81.5 ppm: significantly further upfield than those observed for
other MICs (B: d = 200 ppm, C: d = 115 ppm, D: d =
200 ppm).[6] However, the 13C NMR chemical shift for car-
benes is unpredictable[2b] (ranging from d = 77 ppm[9] to d =
326 ppm[10]). Therefore, in the hope of confirming quickly the
MIC structure of the product, we added trifluoromethane-
sulfonic acid. We were pleased to observe the quantitative
formation of the dithiolium triflate salt 1b (Figure 1).[11]
However, when single crystals of the deprotonation product
of dithiolium salt 1a were obtained, an X-ray diffraction study
revealed that it was not the expected cyclic 1,3-dithiol-5-
ylidene 3, but the acyclic ethynylcarbamodithioate 2 (see the
Supporting Information). The true identity of 2 rationalizes
[*] G. Ung, Dr. D. Mendoza-Espinosa, Dr. J. Bouffard, Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California, Riverside
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/
guybertrandwebpage/
[**] We are grateful to the NSF (CHE-0808825) and the NIH (R01 GM
68825) for financial support of this research. We thank B.
Donnadieu for his assistance with X-ray diffraction studies.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100420.
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Communications
THF proceeded cleanly, but did not afford the expected
complex 4, in which the heterocycle acts as an X ligand as in
vinyl–gold complexes. Instead, the MIC–gold(I) complex 5
was isolated in 68% yield. The 13C NMR spectrum of 5
showed a signal at d = 146.9 ppm: a chemical shift com-
parable to that observed for the [(MIC B)AuCl] complex
(d = 153.7 ppm)[6a] and at significantly higher field than
those of vinyl–gold complexes (d = 178–199 ppm).[14] Sim-
ilarly, an X-ray diffraction study of 5 (Figure 1) revealed
that the gold–carbon bond distance (1.978(4) ꢀ) is similar
to that found in [(MIC B)AuCl] (1.98 ꢀ)[6a] and
[(NHC)AuCl] (1.94–2.00 ꢀ),[17] and slightly shorter than
that in vinyl–gold complexes (2.04–2.06 ꢀ).[15]
These results show that with a gold(I) complex,
ethynylcarbamodithioate 2 acts as a ligand equivalent of
1,3-dithiol-5-ylidene 3. To test the scope of this finding, we
treated compound 2 with the less electrophilic complexes
[{PdCl(allyl)}2] and [{RuCl2(p-cym)}2] (Scheme 4). MIC
complexes 6 (Figure 1) and 7 were isolated in 69 and
83% yield, respectively. To evaluate the donor properties of
Figure 1. Molecular structures of 1b (top left), 5 (top right), 6 (bottom
left), and 9 (bottom right) in the solid state (hydrogen atoms are omitted
for clarity).
the 13C NMR spectroscopic data; furthermore, the infrared
spectrum shows a band at 2160 cmÀ1 characteristic of a C C
ꢀ
triple bond. The formation of 2 is reminiscent of the ring-
opening reaction observed in the deprotonation of isoxazo-
lium[12] and isothiazolium salts.[13] Monitoring of the addition
of potassium bis(trimethylsilyl)amide to 1a by NMR spec-
troscopy showed, even at À608C, the instantaneous formation
of 2. Note that the deprotonation/ring-opening process might
be concerted and therefore does not necessarily imply the
transient formation of MIC 3.
Scheme 4. The MIC–palladium, ruthenium, and rhodium complexes 6–
9 were readily prepared. Thus, acyclic ethynylcarbamodithioate 2 is a
ligand equivalent of MIC 3. p-cym=para-cymene, cod=1,5-cycloocta-
diene.
The proton-induced cyclization of 2 into 1 prompted us to
study the reactivity of the ethynylcarbamodithioate 2 with
gold(I) complexes, which are well-known alkynophilic
p acids.[14] We were particularly interested in the apparent
suitability of compound 2 as a precursor for the formation of
the 1,3-dithiol-5-ylidene ligand 3, we prepared the corre-
sponding rhodium(I) dicarbonyl chloride complex
9
1
2
3
[15]
=
stable
vinyl–gold
complexes
[(R R C CR )AuL]
(Figure 1) by the addition of half an equivalent of [{RhCl-
(Scheme 3), which are still rare, although they are believed
to be key intermediates in gold-catalyzed alkyne activation.[16]
The reaction of 2 with (tetrahydrothiophene)gold chloride in
(cod)}2] to 2, followed by treatment with excess carbon
monoxide. The CO vibration frequencies for
9 (nav =
2030.8 cmÀ1) indicate that 3 is a stronger electron donor
than classical NHCs (nav = 2039–2041 cmÀ1)[18] and cyclic
(alkyl)(amino)carbenes (CAACs; nav = 2036 cmÀ1),[19] but is
weaker than other MICs (nav = 2016–2025 cmÀ1).[2a]
The 1,3-dithiol-5-ylidene–metal complexes reported
herein are thermally robust (m.p. = 272 (5), 219 (6), 217 (7),
194 (8), 1868C (9)) and not air-sensitive. The precursor,
namely, the acyclic ethynylcarbamodithioate 2, can be
prepared in gram-scale quantities within a day and is stable
for several weeks in the solid state under an inert atmosphere
and in solution for up to 1 h at 1408C.
These results suggest that the variety of isolable free
mesoionic carbenes will be limited by their propensity to
undergo ring-opening reactions. However, the reverse pro-
cess, triggered by transition metals, should be of broad
applicability. Since many different analogues of ethynylcar-
Scheme 3. The gold-induced cyclization of 2 did not afford the
expected vinyl–gold complex 4 but the 1,3-dithiol-5-ylidene complex 5.
THT=tetrahydrothiophene.
ꢀ
bamodithioate 2 (R-C C-X-C(Y)R’, in which X and Y are
heteroatoms with a lone pair of electrons) can readily be
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prepared, numerous MIC complexes will be available. The
catalytic study of metal complexes supported by MIC 3 is a
subject of current investigations in our laboratory.
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Experimental Section
All manipulations were performed under an atmosphere of dry argon
through the use of standard Schlenk or dry-box techniques. Solvents
1
were dried by standard methods and distilled under argon. H and
13C NMR spectra were recorded on a Varian Inova 500 spectrometer
at 258C. Mass spectra were performed at the UC Riverside Mass
Spectrometry Laboratory. Melting points were measured with a
Bꢁchi melting point apparatus system.
Synthesis of 2: THF (35 mL) was added to a solid mixture at
À788C of potassium bis(trimethylsilyl)amide (336 mg, 1.68 mol,
1 equiv) and 1a (800 mg, 1.68 mmol, 1 equiv). The resulting light-
yellow solution was stirred for 5 min at À788C and then allowed to
warm to room temperature. The THF was then evaporated, and the
residue was extracted with pentane (40 mL). Evaporation of the
pentane under vacuum yielded 2 (627 mg, 1.62 mmol, 96%) as a pale-
yellow powder. Single crystals were obtained by slow evaporation of a
saturated solution of 2 in hexanes/diethyl ether (9:1). M.p.: 1018C; 1H
(500 MHz, C6D6): d = 0.76–0.84 (m, 2H), 0.84–1.00 (m, 4H), 1.05 (d,
J = 7.0 Hz, 6H), 1.23 (d, J = 7.0 Hz, 12H), 2.61 (sept, J = 7.0 Hz, 1H),
2.96 (br s, 2H), 3.73 (br s, 2H), 3.93 (sept, J = 7.0 Hz, 2H), 6.95 ppm
(s, 2H); 13C (125 MHz, C6D6): d = 24.1 (CH(CH3)2), 24.2 (CH2), 24.4
(CH(CH3)2), 25.8 (br, CH2), 32.8 (CH(CH3)2), 35.4 (CH(CH3)2), 52.5
(br, NCH2), 53.4 (br, NCH2), 81.5 (Cquat), 103.2 (Cquat), 119.4 (Cquat),
121.2 (CHAr), 150.8 (Cquat), 152.8 (Cquat), 190.3 ppm (Cquat); IR (THF):
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À1
ꢀ
~
n = 2160.4 cm
(C C); HRMS: m/z calculated for C23H34NS2:
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[11] CCDC 798263 (1b), 798264 (5), 798265 (6), and 798266 (9)
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.
388.2127 [M+]; found: 388.2135.
Preparation of metal complexes: THF (5 mL) was added to a
solid mixture of 2 (0.5 mmol, 1 equiv) and a precursor metal complex
(0.5 mmol, 1 equiv) at room temperature. The resulting solution was
stirred for 14 h at room temperature. The solvent was then
evaporated, and the residue was washed with diethyl ether (4 ꢂ
10 mL). The resulting powder was dried under vacuum. Complexes
5, 6, 7, and 8 were obtained in 68, 83, 69, and 66% yield, respectively.
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Received: January 18, 2011
Published online: April 6, 2011
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Keywords: cyclization · gold · mesoionic carbenes ·
transition metals
.
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