Gold(III)- versus gold(I)-induced cyclization: Synthesis of six-membered mesoionic carbene and acyclic (aryl)(heteroaryl) carbene complexes

Article abstract of DOI:10.1002/anie.201207961

The golden state: Selective 5-exo- and 6-endo-cyclizations of an alkynyl benzothioamide have been achieved. The selectivity is controlled by the oxidation state of the gold precursor (+I or +III), yielding two new types of carbene ligand: an (aryl)(heteroaryl)carbene and a six-membered mesoionic carbene (see scheme). Mes=2,4,6-trimethylphenyl. Copyright

Full text of DOI:10.1002/anie.201207961

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Angewandte
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DOI: 10.1002/anie.201207961
Gold Complexes
Gold(III)- versus Gold(I)-Induced Cyclization: Synthesis of Six-
Membered Mesoionic Carbene and Acyclic (Aryl)(Heteroaryl)
Carbene Complexes**
Gaꢀl Ung, Michele Soleilhavoup, and Guy Bertrand*
In the last decade, gold complexes have been widely used as
catalysts for promoting a variety of organic transformations.[1]
We have recently demonstrated that the gold-induced cycli-
zation of heteroatom-substituted alkynes A and B provides
the corresponding gold complexes d-[M] and E-[M], respec-
tively (Scheme 1).[2,3] These complexes feature a recently
By analogy with the preparation of complexes d-[M] and
E-[M], we envisioned that an alkyne of type C could undergo
a 6-endo-dig cyclization, affording complex F-[M] featuring
a thiopyryl-3-ylidene ligand (Scheme 1). However, examina-
tion of the recent literature on gold catalysis showed that
examples of 6-endo-dig cyclization are rare,[8] and that the
competitive 5-exo-dig cyclization is more likely to occur.[9]
The latter pathway would nonetheless yield a very rare
example of carbene gold complex G-[M].
To force a cis geometry between the alkyne and the thione
of C, we chose a benzene fused system. 2-bromoiodobenzene
was cross-coupled with mesitylacetylene, giving alkyne 1 in
good yield (92%). Treatment of 1 with tert-butyllithium and
subsequent addition of dimethylcarbamothioyl chloride
afforded the desired alkynyl benzothioamide 2 (93% yield)
(Scheme 2).
Scheme 1. Transition-metal-induced cyclization of ethynyl dithiocarba-
mate A and ynamide B to the corresponding complexes 1,3-dithiol-5-
ylidene D and oxazol-4-ylidene E. Alkyne C acts as a potential precursor
of the thiopyryl-3-ylidene ligand F or carbene gold complex G.
discovered type of carbon-based ligand, namely a mesoionic
carbene (MIC),[4,5] in which a positive charge is delocalized
throughout the ring atoms, and the formal negative charge is
associated with a carbon atom of the ring. In contrast to their
N-heterocyclic carbene (NHC) cousins, which are known to
exist as 4- to 8-membered rings,[6] MICs have been mostly
limited so far to the 5-membered series.[7] We report herein
the selective preparation of a cyclic 6-membered mesoionic
carbene gold complex, as well as a very rare example of an
acyclic (aryl)(heteroaryl)carbene gold complex. Both com-
pounds are prepared from the same precursor by an unusual
gold induced 6-endo-dig ring closure process and by a more
classical 5-exo-dig cyclization, respectively. The selectivity
observed is dictated by the oxidation state of the gold
promoter.
Scheme 2. Synthesis of alkynyl benzothioamide 2. Reagents and con-
ꢀ
ditions: a) HC CMes, [PdCl2(PPh3)2], CuI, NEt3, 16 h, RT; b) tBuLi,
Et2O, 15 min, ꢁ788C, then ClC(S)NMe2, Et2O, 2 h, ꢁ788C!RT;
Mes=2,4,6-trimethylphenyl.
Compound 2 was first reacted with [(tht)AuICl] (tht =
tetrahydrothiophene). The reaction proceeded cleanly in
CH2Cl2; however, the 13C NMR spectrum of the product
showed peaks at d = 92.9 and 93.1 ppm, indicating that the
alkyne moiety was still present and, consequently, that the
expected cyclization did not occur. Nevertheless, slight but
notable shifts in the 13C NMR spectrum indicated that the
coordination of 2 to gold did take place. Single crystals were
grown from a dichloromethane solution, and an X-ray
diffraction study revealed that 2 displaced the tetrahydro-
thiophene ligand yielding complex 3, in which the sulfur atom
is coordinated to the metal (Figure 1, left; Scheme 3).[10]
Despite the aurophilicity of sulfur, we reasoned that the
desired MIC–AuI complex would be thermodynamically
favored over a thioamide–AuI complex. After heating 3 at
408C for 16 h, we observed the quantitative conversion into
a new product. No alkyne carbon signals were detected in the
13C NMR spectrum, and the CS signal shifted drastically
upfield, indicating that a cyclization process occurred. A
single-crystal X-ray diffraction study showed that it was
[*] G. Ung, Dr. M. Soleilhavoup, Prof. G. Bertrand
UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555),
Department of Chemistry and Biochemistry, University of California
San Diego
La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
[**] The authors gratefully acknowledge financial support from the NIH
(R01 GM 68825) and DOE (DE-FG02-09ER16069)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207961.
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Figure 2. Structure of 5 in the solid state. Ellipsoids are set at 50%
Figure 1. Structures of 3 (left) and 4 (right) in the solid state.
Ellipsoids are set at 50% probability. Hydrogen atoms and co-crystal-
lized solvent molecules are omitted for clarity. Selected bond lengths
[ꢀ] and angles [8]: 3: C1-C2 1.192(4), Au1-Cl1 2.2795(7), S1-Au1
2.2547(7), C3-S1 1.714(3), C3-N1 1.306(3); S1-Au1-Cl1 174.03(2). 4:
Au1-Cl1 2.307(3), Au1-C1 1.990(11), C1-C2 1.325(15), C2-C3 1.457(15),
C3-C4 1.436(14), C4-C5 1.426(15), C2-S1 1.781(11), C5-S1 1.733(11),
C5-N1 1.299(14); C1-Au1-Cl1 179.0(4).
probability. Hydrogen atoms and co-crystallized solvent molecules are
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: Au1-Cl1
2.3286(14), Au1-Cl2 2.3602(17), Au1-Cl3 2.3048(15), Au1-C1 2.077(7),
C1-C2 1.462(9), C3-C2 1.432(9), C3-C4 1.455(9), C1-C5 1.308(10), S1-
C5 1.754(7), S1-C4 1.734(7), N1-C4 1.325(8), Cl1-Au1-Cl2 90.10(6),
Cl3-Au1-Cl2 91.41(6), C1-Au1-Cl1 93.34(19), C1-Au1-Cl3 85.01(19), C5-
C1-C2 124.0(7).
for a [(vinyl)AuIIICl3] complex (2.2743(9) ꢀ),[9b] and
slightly longer than for [(NHC)AuIIICl3] complexes
(1.98–2.01 ꢀ).[15] The gold center features a weakly
distorted square planar geometry (sum of angles =
359.98), and the six-membered MIC ring is only slightly
twisted, which is due to the proximity of the bulky
mesityl substituent with the chloride on the gold center.
Although NHCs are oxidized by AuIIICl3,[16] preventing
the direct synthesis of NHC–AuIII complexes, the
formation of metallic Au0 was never observed in the
reaction leading to 5. Interestingly, the reaction of 2
with H[AuIIICl4] also gave rise to complex 5 without the
loss of purity or yield. This result demonstrates the
robustness of 5 towards protodeauration.
In summary, we have shown that the regioselectivity
of the gold-promoted cyclization of the alkynyl benzo-
thioamide 2 is controlled by the oxidation state of the
metal. Complex 4 is the first example of a diarylcar-
bene–gold complex, and more generally of a diary-
Scheme 3. Reactivity of 2 towards AuI and AuIII complexes. tht=tetrahydrothio-
phene.
complex 4, resulting from the usually encountered 5-exo-dig
cyclization[1,9] (Figure 1, right).[10] However, in contrast to
previously reported studies on gold-mediated cyclization,[11,12]
this complex is better described as the (aryl)(heteroaryl)car-
bene complex 4 rather than the zwitterionic vinyl gold
lcarbene–metal complex obtained without using a diazo
precursor[14,17] or an oxidative addition process.[18] Further-
more, 5 is the first metal complex featuring a non-nitrogen-
containing[7] six-membered mesoionic carbene. The availabil-
ity of a variety of analogues of 2 that feature different
heteroatoms should allow for the preparation of numerous
six-membered MIC complexes. Both complexes 4 and 5 are
very robust (m.p. 4 and 5: 2068C), which is a key feature for
possible catalytic applications, which are under current
investigation in our laboratories.
ꢁ
complex 4’ (Scheme 3). The Au1 C1 bond (1.990(11) ꢀ) is
shorter than in vinyl gold complexes (2.04–2.06 ꢀ),[9a,12] and in
the range observed for NHC[13] and MIC gold complexes[2a,5c,g]
(1.94–2.01 ꢀ). Complex 4 is a very rare example of non-
heteroatom-substituted carbene–coinage-metal complex.[14]
Recently, Hashmi, Nolan et al. reported an interesting
switch of selectivity between 5-exo-dig and 6-endo-dig
cyclization using two different NHC–AuI complexes.[8d] This
prompted us to test other gold precursors, and we chose to
simply change the oxidation state of the metal. A clean
reaction was observed when AuIIICl3 was added to 2, and
alkyne carbon signals were absent in the 13C NMR spectrum,
indicating that a cyclization process had occurred. Indeed,
when single crystals were obtained from a saturated acetone
solution, an X-ray diffraction study proved that the desired 6-
endo-dig cyclization took place, yielding the mesoionic
Experimental Section
All manipulations were performed under an atmosphere of dry argon
using standard Schlenk or dry box techniques. Solvents were dried by
standard methods and distilled under argon. 1H and 13C NMR spectra
were recorded on a Varian Inova 500 or Bruker 300 spectrometer at
258C. Mass spectra were performed at the UC San Diego Mass
Spectrometry Laboratory. Melting points were measured with
a Bꢁchi melting point apparatus.
Synthesis of complexes 3 and 4: CH2Cl2 (10 mL) was added to
a Schlenk tube loaded with benzothioamide 2 (210 mg, 680 mmol)
and [(tht)AuCl] (220 mg, 680 mmol), and the mixture was stirred for
14 h. The reaction mixture was filtered and the filtrate was
carbene complex 5 (Figure 2; Scheme 3).[10] The Au1 C1
ꢁ
bond (2.077(7) ꢀ) is considerably shorter than that observed
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evaporated under vacuum. The residue was recrystallized from
a saturated solution of dichloromethane. Complex 3 was obtained
as orange crystals (240 mg, 65% yield). m.p. 2008C (dec); 1H
(300 MHz, CDCl3) d = 2.26 (s, 3H, CH3para), 2.38 (s, 6H, CH3ortho),
3.21 (s, 3H, NCH3), 3.58 (s, 3H, NCH3), 6.86 (s, 2H, CHar), 7.30–7.36
(m, 3H, CHar), 7.38–7.40 ppm (m, 1H, CHar); 13C (75 MHz, CDCl3)
Angew. Chem. 2011, 123, 10097; Angew. Chem. Int. Ed. 2011, 50,
9923; c) G. Ung, G. Bertrand, Chem. Eur. J. 2012, 18, 12955.
[3] G. Ung, D. Mendoza-Espinosa, G. Bertrand, Chem. Commun.
2012, 48, 7088.
[4] For reviews on MICs and related species, see: a) O. Schuster, L.
Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev. 2009, 109,
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Ed. 2010, 49, 8810; f) D. Martin, M. Melaimi, M. Soleilhavoup,
G. Bertrand, Organometallics 2011, 30, 5304.
[5] For our contribution to MIC chemistry, see: a) V. Lavallo, C. A.
Dyker, B. Donnadieu, G. Bertrand, Angew. Chem. 2008, 120,
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ꢀ
d = 21.2 (CH3), 21.4 (CH3), 44.6 (NCH3), 46.1 (NCH3), 92.9 (C C),
q
q
ꢀ
93.1 (C C), 118.9 (C ), 119.1 (C ), 125.9 (CHar), 127.8 (CHar), 128.8
(CHar), 130.1 (CHar), 132.5 (CHar), 138.8 (Cq), 140.1 (Cq), 143.2 (Cq),
197.7 ppm (C = S). A CH2Cl2 solution of 3 was heated at 408C for
16 h. The solvent was evaporated under vacuum. The residue was
recrystallized by layering pentane on top of a saturated solution of
dichloromethane/THF (1:1). Complex 4 was obtained as yellow
crystals (238 mg, quant.). m.p. 2068C (dec); 1H (500 MHz,
[D6]DMSO) d = 2.08 (s, 6H, CH3ortho), 2.24 (s, 3H, CH3para), 3.41 (s,
3H, NCH3), 3.88 (s, 3H, NCH3), 6.86 (s, 2H, CHar), 7.67 (t, J = 8.0 Hz,
1H, CHar), 7.91 (t, J = 8.0 Hz, 1H, CHar), 8.34 (d, J = 8.0 Hz, 1H,
CHar), 10.03 ppm (d, J = 8.0 Hz, 1H, CHar); 13C (125 MHz,
[D6]DMSO) d = 20.0 (CH3), 20.6 (CH3), 46.5 (NCH3), 50.1 (NCH3),
127.9 (CHar), 128.6 (CHar), 131.1 (CHar), 135.6 (CHar), 144.7 (Cq),
145.4 (Cq), 166.9 (Cq), 174.2 (Cq), 179.8 ppm (Cq).
Synthesis of complex 5: THF (10 mL) was added at room
temperature to a solid mixture of benzothioamide 2 (175 mg,
0.57 mmol) and the gold(III) precursor (AuCl3: 173 mg; H[AuCl4]:
305 mg, 0.57 mmol) was added under an atmosphere of argon. The
reaction mixture was stirred for 16 h, and was allowed to decant. The
supernatant was removed by cannula filtration, and the resulting
yellow solid was washed with diethyl ether and dried under vacuum.
The residue was recrystallized from a saturated solution of acetone.
Complex 4 was obtained as pale yellow crystals (275 mg, 79% yield).
m.p. 2068C; 1H (500 MHz, [D6]DMSO) d = 2.33 (s, 6H, CH3ortho), 2.34
(s, 3H, CH3para), 3.62 (br s, 3H, NCH3), 3.91 (br s, 3H, NCH3), 7.04 (s,
2H, CHar), 7.85 (t, J = 8.0 Hz, 1H, CHar), 8.15 (t, J = 8.0 Hz, 1H,
CHar), 8.25 (d, J = 8.0 Hz, 1H, CHar), 8.63 ppm (d, J = 8.0 Hz, 1H,
CHar); 13C (125 MHz, [D6]DMSO) d = 20.8 = (CH3), 45.8 (br, NCH3),
50.3 (br, NCH3), 120.9 (Cq), 125.7 (Cq), 128.3 (CHar), 128.7 (CHar),
128.8 (CHar), 129.3 (Cq), 130.3 (Cq), 133.2 (CHar), 134.9 (CHar), 138.5
(Cq), 139.6 (Cq), 140.6 (Cq), 176.4 ppm (Cq).
Received: October 2, 2012
Revised: October 25, 2012
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Published online: November 21, 2012
Keywords: cyclization · gold · intermediates ·
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mesoionic carbenes
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