Gold catalysis: First applications of cationic binuclear gold(I) complexes and the first intermolecular reaction of an alkyne with a furan

Article abstract of DOI:10.1002/adsc.200606012

Reaction of the cationic binuclear gold(I) complex [(Ph₃PAu) ₂Cl]BF₄ with ω-alkynylfurans furnished phenols as the major product and anellated furans as the side-products. The analogous trimesitylphosphane complex [(Mes

Full text of DOI:10.1002/adsc.200606012

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DOI: 10.1002/adsc.200606012
Gold Catalysis: First Applications of Cationic Binuclear Gold(I)
Complexes and the First Intermolecular Reaction of an Alkyne
with a Furan
a,
a
a
a
´
A. Stephen K. Hashmi, * M. Carmen Blanco, Elzen Kurpejovic, Wolfgang Frey,
Jan W. Batsb
a
Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax: (þ49)-711-685-4321, e-mail: hashmi@hashmi.de
b
Institut für Organische Chemie und Chemische Biologie, Johann Wolfgang Goethe-Universität, Marie-Curie-Str. 11,
60439 Frankfurt am Main, Germany
Received: January 11, 2006; Accepted: February 8, 2006
Abstract: Reaction of the cationic binuclear gold(I)
complex [(Ph3PAu)2Cl]BF4 with w-alkynylfurans fur-
nished phenols as the major product and anellated
furans as the side-products. The analogous trimesityl-
phosphane complex [(Mes3PAu)2Cl]BF4 selectively
afforded only the phenols. The latter catalyst then
enabled the first intermolecular gold-catalyzed phe-
nol synthesis.
Keywords: alkynes; arenes; furans; gold; homogene-
ous catalysis
Scheme 1. The analogy of the gold-catalyzed phenol synthesis
The first step of the gold-catalyzed phenol synthesis,[1] and the gold-catalyzed enyne cyclization.
an isomerization reaction transformingan w-alkynylfur-
an 1 to an anellated phenol 2 by breakingfour bonds and
formingfour new bonds, is the intramolecular reaction
that the barriers of activation do not differ much for
of the alkyne-gold complex with the olefin part of the both oxidation states.[7]
furan ringto form intermediate A, as later demonstrated
Specifically, AuCl3,[1,2] AuIII-pyridine complexes (the
latter beingthe most efficient catalysts for this transfor-
by most elegant work of Echavarren.[2,3]
The 1,n-enyne substructure of 1 (shown in bold in mation known so far)[1e] and a number of other d8-com-
Scheme 1) underlines the analogy to the beautiful ex- plexes of other transition metals[1b,1e,2] were reported to
tension of the 1,n-enyne cycloisomerization chemistry catalyze this transformation, but with gold(I) in the
subsequently developed by Echavarren[4] and others[5] form of Et3PAuCl/AgBF4 we had initially only observed
on the basis of gold catalysts; the first step to the cyclo- a slow, sluggish and unselective reaction, which might
propylcarbene complex B is in complete analogy to have been based on traces of AuIII.[1a] Thus we decided
the formation of A, the reaction pathways divert only to investigate the possibility of a gold(I)-catalyzed phe-
in the subsequent steps.
nol synthesis more closely.
While the phenol synthesis so far was only reported to
While several new tests with Et3PAuCl/AgBF4,
be catalyzed by gold(III)[1,2] and related d8-complex- Ph3PAuCl/AgBF4, THTAuCl/AgBF4 and dinuclear
es,[1b,1e,2] the enyne cyclization was reported to proceed complexes like 4[8] or 5[9,10] (Scheme 2) still gave no
with gold(I) complexes.[4] Due to the similarity of the acceptable results, first promisingexperiments with
first step this is difficult to understand, especially since the Uson–Laguna[11] salt [(Ph3PAu)2Cl]BF4 (6) then
many other gold-catalyzed reactions can be induced by led to the use of the Schmidbaur–Bayler[12] salt
complexes in both oxidation states[6] and theoretical cal- [(Mes3PAu)2Cl]BF4 (7), which proved to be superior. It
culations for other gold-catalyzed reactions indicated is amazingthat Ph PAuCl/AgBF4 did not react well
3
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A. Stephen K. Hashmi et al.
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while 6 was reactive – both should be a source for the cat- clear proof for the constitution of that product; in the di-
alyst [Ph3PAu] þ. The difference might be that with hydroisobenzoindoles known so far (like 9), the NTs
Ph3PAuCl/AgBF4 the formation of [Ph3PAu] þ is irrever- group is symmetrically connected with the phenol ring
sible; once this cation is set free, it can only react with the by one methylene group on each side, in 12 this local
substrate or enter catalyst deactivatingpathways. Salt 6, symmetry is lost. Compound 12 has one methylene teth-
on the other hand, sets free [Ph3PAu] þ in a reversible er and one ethylene tether, a safe spectroscopic distinc-
manner, thus offeringthe re-formation of 6 as a poten- tion of the connectivity shown for 12 (CH2 ortho to OH,
tial restingstate in competition to a possible catalyst de-
activation.
CH2CH2 meta to OH; the isomer in accord with
the mechanism) from the other conceivable isomer
While with catalyst 6 and the substrate 8 the conver- (CH2CH2 ortho to OH, CH2 meta to OH, not expected)
sion was fast but unselective (leadingto 9 and 10), would have needed significantly more effort.
with catalyst 7 only the desired product 9 and no 10
was obtained (Scheme 3). A similar observation was
made with substrate 11, with 6 mainly the tetrahydroiso-
quinoline 12 was formed, with 7 only 12. Chloroform was
superior to acetone or acetonitrile as solvent, in the lat-
ter two the reaction was much slower and small amounts
of uncharacterized side-products were observed.
We succeeded to obtain single crystals for a crystal
structure analysis of 12 (Figure 1),[13] which provided
Figure 1. Crystal structure of the tetrahydroisoquinoline 12 as
a clear proof of the connectivity.
It was also interestingto notice that the seven-mem-
bered side-product 13 is readily and highly selectively
isomerized to 14 in CDCl3 at room temperature without
a catalyst, while the six-membered analogue 10 did not
Scheme 2. Unreactive gold(I) complexes 4 and 5.
show this isomerization under the same conditions.
Thus we have for the first time experimentally proved
that, when the proper ligand is chosen, AuI can selec-
tively and efficiently catalyze the phenol synthesis, too.
So both oxidation states AuIII and AuI show a compara-
ble activity, this is experimental evidence for either sim-
ilar barriers of activation or the in-situ conversion of one
oxidation state as a pre-catalyst to the other oxidation
state as the catalytically active species.
Finally, we investigated the intermolecular reaction of
phenylacetylene 15 and 2,5-dimethylfuran 16. With the
AuIII catalysts all previous efforts had failed, while
with AuI in catalyst 7 for the first time a small amount
(38%) of the ortho, ortho-disubstituted biaryl com-
pound 17 was observed. The hydroarylation[14] product
18 was also obtained (30%, Scheme 4).
A crystal structure analysis[13] of the 1,2,3,4-tetrasub-
stituted arene in 17 unambiguously proved the sequence
of the four substituents to be in accordance with the
mechanistic proposal (Figure 2).
This observation is unique, as these reactions proceed
via the initial formation of a gold carbene,[2,3] it is the first
example of a formation of such a gold carbene C (the an-
alogue of A) by an intermolecular reaction. So far only
an intermolecular trappingof such gold carbenes, them-
selves generated in-situ by intramolecular reactions as
Scheme 3. Isomerization of g- and d-alkynylfurans with the
Uson–Laguna and the Schmidbaur–Bayler salts 6 and 7.
shown for B in Scheme 1[15] or, as shown for G, from
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First Intermolecular Reaction of an Alkyne with a Furan
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Scheme 4. Formation of 17 as evidence for the first generation of gold carbenes by an intermolecular alkyne/alkene reaction.
reaction was monitored by 1H NMR, after two hours the sub-
strate was consumed. The solvent was removed under vacuum
and the residue purified by flash chromatography on silica gel
(petrol ether/diethyl ether, 8:1) to give known 9[1a] (yield:
74.5 mg, 50%) and 10 as a yellow oil (yield: 15.3 mg, 10%).
Product 10: Rf (petrol ether/diethyl ether, 5:3)¼0.38; IR
(film): n¼2923, 2359, 1656, 1347, 1162, 1090, 949, 836, 708,
1
660 cmꢀ1; H NMR (acetone-d6, 500 MHz): d¼2.20 (s, 3H),
2.39 (s, 3H), 3.96 (s, 2H), 4.30 (s, 2H), 4.92 (s, 1H), 5.00 (s,
3
3
1H), 6.09 (s, 1H), 7.35 (d, JH,H ¼8.1 Hz, 2H), 7.67 (d, JH,H
¼
8.1 Hz, 2H); 13C NMR (acetone-d6, 500 MHz): d¼13.3 (q),
21.4 (q), 44.3 (t), 50.1 (t), 102.8 (d), 106.8 (t), 119.9 (s), 128.5
(d, 2C), 130.4 (d, 2C), 134.4 (s), 135.4 (s), 144.6 (s), 146.7 (s),
153.2 (s); MS [DCI (þ)]: m/z (%)¼332 (5), 318 (5), 304.1
(100) [Mþ], 147 (55), 91 (5); HR-MS (70 eV): m/z¼304.0999;
calcd. for C16H18NO3S (Mþ þH): 304.1008.
Figure 2. Crystal structure of the 1,2,3,4-tetrasubstitued arene
17.
Gold-Catalyzed Conversion of 11
In an NMR tube a solution of 11 (112 mg, 353 mmol) and 6
(9.2 mg, 8.8 mmol, 2.5 mol %) in CDCl3 (700 mL) was prepared.
The reaction was monitored by 1H NMR, after two hours the
startingmaterial had been consumed and the solvent was re-
moved under vacuum. The residue was purified by flash chro-
matography on silica gel (petrol ether/diethyl ether, 8:1) to
give 12 (yield: 66.5 mg, 59%) as colourless crystals and the 13
compounds of type 19[16] was successful, their generation
by an intermolecular alkyne/alkene reaction is unprece-
dented.
The phenol synthesis can be catalyzed by gold(I) pre-
catalysts of type 6 and 7, the bulkier phosphane showing
a higher selectivity for the phenolic products. Catalyst 7
then enabled the first intermolecular reaction of such a (yield: 17.0 mg, 15%) as colourless oil.
By evaporation from petrol ether/ethyl acetate/dichlorome-
type, which still needs optimization but opens the door
to a synthetically promisingfield.
thane single crystals of 12 suitable for a crystal structure anal-
ysis were obtained. Mp 155–1568C; Rf (petrol ether/ethyl ace-
tate/dichloromethane, 10:0.5:3)¼0.10; IR (film): n¼3493,
3496, 2914, 1582, 1495, 1458, 1443, 1431, 1375, 1353, 1332,
1320, 1297, 1255, 1127, 1205, 1192, 1155, 1108, 1016, 989, 951,
Experimental Section
874, 815, 796, 757, 729, 654, 606 cmꢀ1 1H NMR (CDCl3,
;
500 MHz): d¼2.20 (s, 3H), 2.42 (s, 3H), 2.88 (t, J¼5.7 Hz,
2H), 3.31 (t, J¼5.7 Hz, 2H), 4.23 (s, 2H), 5.01 (bs, 1H), 6.59
(d, J¼7.7 Hz, 1H), 6.91 (d, J¼7.7 Hz, 1H), 7.32 (d, J¼
8.2 Hz, 2H), 7.75 (d, J¼8.2 Hz, 2H); 13C NMR (CDCl3,
Gold-Catalyzed Conversion of 8
To a solution of 8 (150 mg, 495 mmol) in CDCl3 (700 mL)) in an
NMR tube, 6 (12.9 mg, 12.4 mmol, 2.5 mol %) was added. The
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A. Stephen K. Hashmi et al.
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125 MHz): d¼16.69 (q), 22.95 (q), 30.26 (t), 44.86 (t), 44.99 (t),
120.44 (s), 121.18 (d), 121.94 (s), 129.20 (d, 2C), 130.00 (d),
131.12 (d, 2C), 133.91 (s), 134.85 (s), 145.00 (s), 151.72 (s);
MS (70 eV): m/z (%)¼347 (24) [Mþ], 161 (100), 134 (54), 91
(22); anal. calcd. for C17H19NO3S (317.41): C 64.33, H 6.03, N
4.41; found: C 64.23, H 6.02, N 4.42.
Product 18: Rf (petrol ether/ethyl acetate/dichloromethane,
10:1:0.5)¼0.70; IR (film): n¼2358, 2338, 1577, 1492, 1445,
1
1217, 890, 778, 698, 641, 627, 579 cmꢀ1; H NMR (CDCl3,
2
250 MHz): d¼2.08 (s, 3H), 2.25 (s, 3H), 5.21 (d, JH,H
¼
1.5 Hz, 1H), 5.41 (d, 2JH,H ¼1.5 Hz, 1H), 5.86 (s, 1H), 7.33 (m,
5H); 13C NMR (acetone-d6, 300 MHz): d¼13.0 (q), 13.3 (q),
108.6 (d), 114.1 (t), 122.4 (s), 128.3 (d, 2C), 128.7 (d), 129.2
(d, 2C), 142.2 (s), 143.5 (s), 148.2 (s), 150.3 (s); MS (EI): m/z
(%)¼214 (25), 199 (20) [Mþ þH], 171 (30), 128 (15), 105
(30), 77 (20), 43 (100), 28 (20); HR-MS (70 eV): m/z¼
198.1044; calcd. for C14H14O: 198.1045.
Product 13: Rf (petrol ether/diethyl ether, 5:3)¼0.39; IR
(film): n¼2923, 2360, 1608, 1305, 1098, 1069, 958, 814, 707,
1
582 cmꢀ1; H NMR (acetone-d6, 500 MHz): d¼2.17 (s, 3H),
2.40 (s, 3H), 2.92 (t, 3JH,H ¼5.9 Hz, 2H), 3.58 (t, 3JH,H ¼5.9 Hz,
2H), 4.18 (s, 2H), 5.02 (s, 1H), 5.15 (s, 1H), 6.08 (s, 1H), 7.37
3
3
(d, JH,H ¼8.1 Hz, 2H), 7.72 (d, JH,H ¼8.2 Hz, 2H); 13C NMR
(acetone-d6, 500 MHz): d¼13.1 (q), 21.4 (q), 29.8 (t), 48.2 (t),
56.4 (t), 106.6 (d), 111.9 (t), 122.1 (s), 127.9 (d, 2C), 130.5 (d,
2C), 138.5 (s), 138.7 (s), 144.0 (s), 149.5 (s), 151.0 (s); MS
(EI): m/z (%)¼317 (65) [Mþ], 253 (5), 162 (100), 133 (45),
120 (30), 105 (25), 91 (50), 77 (20); HRMS (70 eV): m/z¼
317.1085; calcd. for C17H19NO3S: 317.1086.
Acknowledgements
This work was supported by the Fonds der Chemischen Indus-
trie and by the AURICAT EU-RTN (HPRN-CT-2002-00174).
References and Notes
Isomerization of 13 to 14
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2923, 2360, 1184, 1096, 1044, 910, 814, 743, 617, 572 cmꢀ1; 1H
NMR (acetone-d6, 300 MHz): d¼1.95 (d, 4JH,H ¼1.2 Hz, 3H),
3
2.20 (s, 3H), 2.42 (s, 3H), 2.95 (t, JH,H ¼5.2 Hz, 2H), 3.61 (t,
4
3JH,H ¼5.2 Hz, 2H), 6.04 (d, JH,H ¼0.9 Hz, 1H), 5.15 (s, 1H),
4
3
6.45 (d, JH,H ¼1.1 Hz, 1H), 7.42 (d, JH,H ¼7.9 Hz, 2H), 7.77
(d, JH,H ¼8.3 Hz, 2H); 13C NMR (CDCl3, 250 MHz): d¼13.3
3
(q), 20.1 (q), 21.5 (q), 30.5 (t), 43.7 (t), 105.6 (d), 115.6 (s),
119.3 (s), 122.0 (d), 126.8 (d, 2C), 129.8 (d, 2C), 143.6 (s),
148.8 (s), 149.8 (s); MS (EI): m/z (%)¼317 (30) [Mþ], 162
(100), 118 (10), 91 (20); HR-MS (70 eV): m/z¼317.1080; calcd.
for C17H19NO3S: 317.1086.
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16
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´
´
Product 17: Rf (petrol ether/ethyl acetate/dichloromethane,
10:1:0.5)¼0.57; IR (film): n¼3558, 2922, 2852, 1732, 1600,
1489, 1259, 1092, 1025, 801, 702, 646 cmꢀ1; 1H NMR (CDCl3,
500 MHz): d¼2.03 (s, 3H), 2.25 (s, 3H), 4.75 (s, 1H), 6.76 (d,
´
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´
3
3JH,H ¼7.6 Hz, 1H), 7.03 (d, JH,H ¼7.6 Hz, 1H), 7.29 (m, 2H),
´
3
4
7.42 (tt, JH,H ¼7.6 Hz, JH,H ¼1.4 Hz, 1H), 7.51 (m, 2H); 13C
NMR (CDCl3, 500 MHz): d¼16.0 (q), 20.2 (q), 121.3 (d),
121.5 (s), 127.6 (q), 128.1 (d), 129.5 (d, 2C), 129.8 (d), 130.3
(d, 2C), 134.4 (q), 135.6 (q), 150.8 (q); MS (EI): m/z (%)¼
198 (100) [Mþ], 183 (45), 165 (25), 83 (20); HR-MS (70 eV):
m/z¼198.1044; calcd. for C14H14O: 198.1045.
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asc.wiley-vch.de
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Adv. Synth. Catal. 2006, 348, 709 – 713

First Intermolecular Reaction of an Alkyne with a Furan
COMMUNICATIONS
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[13] CCDC 293003 (12) and 293003 (24) and contain the sup-
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data can be obtained online free of charge (or from the
Cambridge Crystallographic Data Centre, 12, Union
Adv. Synth. Catal. 2006, 348, 709 – 713
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