Gold-catalyzed hydroarylating cyclization of 1,2-Bis(2-iodoethynyl)benzenes

Article abstract of DOI:10.1002/adsc.201400749

1,5-Diynes bearing halogen-substituted alkynes were synthesized and converted in the presence of a gold catalyst. In contrast to the corresponding hydroarylating aromatization reaction with terminal alkynes, a totally different reaction mode was observed.

Full text of DOI:10.1002/adsc.201400749

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DOI: 10.1002/adsc.201400749
Gold-Catalyzed Hydroarylating Cyclization of
1,2-Bis(2-iodoethynyl)benzenes
Pascal Nçsel,a Vanessa Mꢀller,a Steffen Mader,a Setareh Moghimi,a
Matthias Rudolph,a Ingo Braun,a Frank Rominger,+a
and A. Stephen K. Hashmia,b,*
a
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢁt Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg,
Germany
Fax : (+49)-6221-54-4205; e-mail: hashmi@hashmi.de (homepage: http://www.hashmi.de)
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
b
+
Crystallographic investigation.
Received: July 29, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400749.
Abstract: 1,5-Diynes bearing halogen-substituted al- vinylidene species or a gold carbenoid is involved.
kynes were synthesized and converted in the pres- By the incorporation of one solvent molecule, diiodi-
ence of a gold catalyst. In contrast to the correspond- nated aromatic products are obtained in high selec-
ing hydroarylating aromatization reaction with termi- tivity.
nal alkynes, a totally different reaction mode was ob-
served. Instead of the expected dual catalysis path-
way, only one gold center is needed and a 1,2- Keywords: gold catalysis; gold vinylidenes; 1,2-halo-
halogen migration is initiated in which either a gold gen migration; iodoalkynes; naphthalenes
Introduction
dual catalysis conditions. The results of the cyclization
with iodoalkynes as starting materials are summarized
Amidst the continuous stream of new homogeneous in this contribution (Scheme 1).
gold-catalyzed reactions the field of diyne cyclizations
is steadily gaining importance. The reactions can
either be initiated by p-activation alone[1] or by a syn- Results and Discussion
ergistic interplay between two gold fragments in
which s/p-activation (dual catalysis) takes place.[2] As An initial screening was performed with the 1,2-bis-
an expansion of the dual catalysis approach, we could
ACHTUNGTREN(NGNU iodoethynyl)-4,5-dimethylbenzene derivative 7a.
show that iodoalkynes were also suitable substrates Indeed the formation of one major product was de-
for these kinds of transformations. This was demon- tected with most of the applied catalysts. A summary
strated by the synthesis of iodofulvenes 6 which were of the screening (GC yields) is depicted in Table 1.
formed from the corresponding iodoalkynes 5. The Our first choice of catalyst was DAC (dual activation
crucial steps in this transformation were shown to be catalyst) 8[3] as these types of catalyst turned out to be
the formation of a gold acetylide. This only took the best choice for the synthesis of iodofulvenes 6.[4]
place in the presence of an organogold compound as Indeed 48% of product was detected by GC with
additive and a catalyst transfer from the aurated 2.5 mol% of 6, which corresponds to 5 mol% of
product to the iodoalkyne which closes the catalytic monoACTHUNTGRNEUNGnuclear gold(I) complexes. Shifting to the corre-
cycle. Based on these findings, we were curious if the sponding “normal” cationic gold complex IPrAuNTf2
same reaction principle was also feasible for related 9 [now 5 mol% instead of 2.5 mol% of 6 as 9 is
intermolecular processes. As model systems we inves- a monoACHTNUTRGNEnNUG uclear gold(I) complex] delivered a rather
tigated iododiyne 7 as suitable substrate. The corre- unexpected picture. While in the case of the iodoful-
sponding terminal diyne system 3 has already been vene synthesis organogold additives or dual catalysts
shown to add benzene in a b-selective fashion under were crucial for the transformation of the iodoalkyne
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Scheme 1. Overview of gold-catalyzed transformations of aromatic diynes.
Table 1. GC yields for the conversion of 7a to 15a employing tion pathways, we performed a reaction under the
various catalysts.
same conditions except that the starting material was
added via syringe pump to keep its concentration as
low as possible (Table 2, entry 1). This procedure al-
lowed the isolation of 15a as pure compound in 70%
yield. Crystals suitable for a single crystal X-ray struc-
ture analysis could be obtained for a secure structural
assignment.[5] Figure 1 depicts the solid state molecu-
lar structure of compound 15a. Indeed the expected
benzene incorporation into the cyclization product
took place, but unlike the related hydroarylating aro-
matization of the substrate not bearing iodine sub-
stituents,[6] for the diiodo starting material the aromat-
ic solvent was selectively placed in the peri-position
of the naphthalene system. Furthermore, a 1,2-iodine
migration from the former position next to the alkyne
into the second peri-position of the naphthalene
system must have taken place. It is noteworthy that
5% of the product containing three iodine atoms on
the naphthalene system was observed as disorder in
the XRD structure.
Entry
1
Catalyst
Yield [%]
48
8 (2.5 mol%)
2
3
4
5
6
7
8
IPrAuNTf2
AuCl
Ph3PAuNTf2
BrettPhosAuNTf2
t-Bu-XPhosNTf2
AuCl3
9 (5 mol%)
10 (5 mol%)
10 (5 mol%)
11 (5 mol%)
12 (5 mol%)
13 (5 mol%)
14 (5 mol%)
76
50
39
0
0
51
0
AgNTf2
starting materials, for this transformation an even
In order to check the generality of this transforma-
faster reaction was detected for the mono-metallic tion different diiododiynes 7 were converted under
complex 9 and in addition yields were also better the optimized conditions (Scheme 2, Table 2). First
than for the corresponding DAC. As a consequence we examined substrate 7b (entry 2) with no additional
our further screening was conducted with monomeric substituents at the benzene backbone. A clean con-
gold complexes. Simple AuCl 10 was also efficient version delivered the desired product 15b in high
(entry 3), but yields turned out to be lower than for yield. Electron-withdrawing halides at the benzene
the NHC ligand. In a series of phosphane ligands only backbone were also tolerated. With the more electro-
the common Ph3PAuNTf2 10 showed significant prod- negative fluoride substituent yields were only moder-
uct formation (entry 4), while Buchwald ligands ate (entry 3) but shifting to bromide substitution ena-
turned out to be inefficient (entries 5 and 6). Gold in bled good yields again (entry 4). Problems occurred
the oxidation state III was also effective, but yields with diester-substituted starting material 7e. Due to
were only moderate (entry 7). The control experiment low solubility no syringe pump addition was possible
with only silver showed no conversion (entry 8).
and furthermore decomposition of the catalyst
Next we performed the transformation on a prepa- IPrAuNTf2 was observed. Shifting to 10 mol% AuCl
rative scale (190 mmol) under the optimized condi- delivered the target compound 15e but yields were
tions in order to completely characterize the obtained still poor (entry 5). Next we investigated substrate 7f
species. Unfortunately, the isolated yield now was sig- containing two electron-donating methoxy groups.
nificantly lower than the GC yield (54%). This might These were tolerated well and the corresponding
result from a partly decomposition of the starting ma- product was isolated in 69% yield (entry 6). 1,3-Ben-
terial, which turned out to be not stable under the re- zodioxole derivative 7g delivered only a complex mix-
action conditions. To suppress competing decomposi- ture which might be due to competing reactions in-
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Gold-Catalyzed Hydroarylating Cyclization of 1,2-Bis(2-iodoethynyl)benzenes
Table 2. Conversion of various halodiynes under optimized conditions.
Entry
Substrate
Product
Yield [%]
70
1
2
85
3
48[b]
4
78
5
36[a]
6
69
7
unselective
8
53 (isomeric mixture 2:1)
45 (isomeric mixture 3:2)
74
9
10
11
43[b]
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Table 2. (Continued)
Entry
Substrate
Product
Yield [%]
12
32
41
13
14
53[c]
15
no conversion[c]
[a]
10 mol% AuCl; no addition via syringe pump due to poor solubility in benzene.
Contains traces of inseparable by-products.
20 mol% AuCl at 808C.
[b]
[c]
duced by the acetal functionality.[7] Unsymmetrically
substituted starting materials 7h and 7i delivered the
corresponding products in acceptable yields (entries 8
and 9) but no strong effect on the regioselectivity
could be induced via the electronic nature of the sub-
stituents and therefore inseparable regioisomeric mix-
tures were obtained for both test substrates. Next we
turned our focus to non-aromatic backbones as possi-
ble precursors for diiodobenzene derivatives. The
gold-catalyzed transformation of cyclohexene sub-
strate 7j smoothly delivered the desired benzene de-
rivative 15j in 74% (entry 10). Unfortunately, the re-
action with the corresponding cyclopentene substrate
7k only delivered a moderate yield and furthermore
the product was contaminated by traces of insepara-
ble by-products (entry 11).
Figure 1. Solid state molecular structure of compound 15a.[5]
Next we evaluated the possibility to apply other ar-
omatic solvents. When used as solvents, both mesity-
lene (entry 12) and para-xylene (entry 13) delivered
the expected products but the yields turned out to be
significantly lower than in the case of benzene as sol-
vent. Finally, we tested the suitability of other halo-
gens for this transformation. With dibromo derivative
7n (entry 14) a corresponding reactivity was obtained,
but in this case higher temperatures combined with
higher catalyst loadings were necessary (in this case
AuCl turned out to be the catalyst of choice). The di-
Scheme 2. Hydroarylating cyclization of halodiynes 15.
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Gold-Catalyzed Hydroarylating Cyclization of 1,2-Bis(2-iodoethynyl)benzenes
Scheme 3. Formation of the tetraiodinated product 16.
chloro derivative 7o was completely unreactive
(entry 15).
As electrophilic iodine itself is known to be a good
p-activator as well,[8] we also performed test reactions
with diiodoalkyne 7a in the presence of electrophilic
Figure 2. Solid state molecular structure of compound 16.[5]
iodine sources instead of the gold catalyst (Scheme 3). It is highly likely that the incorporation takes place
In the presence of 2.5 equivalents of the Barluenga via protodeauration, thus a gold fragment should be
reagent and an excess of aqueous HBF4 for activation located at this position at a late stage of the reaction.
the tetraiodinated compound 16 was formed in low If one compares the deuterium labelling of the iodo
yield (Scheme 6). The surprising structure of 16 was reaction with the related hydroarylating reaction to-
confirmed by crystal structure analysis (Figure 2).[5]
wards a-substituted naphthalenes (that are formed if
Our next efforts focused on the elucidation of more high catalyst loadings are applied) (Scheme 4, right),
mechanistic features. Scheme 4 displays the reaction it becomes obvious that a completely different reac-
in deuterated benzene under the normal reaction con- tion mechanism must take place as in this case no
ditions. About 80% of the deuterium label was incor- deuterium incorporation at this position takes place,
porated at the position between the two iodine atoms. instead the two adjacent naphthalene positions are
Scheme 4. Deuterium labelling experiments.
Scheme 5. Possible mechanisms.
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Pascal Nçsel et al.
the iodine centers undergoes a 1,2 iodine migration.
This methodology provides a fast access towards syn-
thetically useful diiodonaphthalene derivatives which
can be further functionalized by metal-mediated
cross-coupling strategies.
Scheme 6. Suzuki cross-coupling of substrate 15a with phe-
nylboronic acid.
Experimental Section
deuterated to a degree of about 20% each.[9] Based General Procedure for the Gold-catalyzed
on these findings, two mechanistic scenarios are rea- Conversion
sonable (Scheme 5). Path a involves a gold(I) vinyli-
Five mol% of IPrAuNTf2 were dissolved in 0.5 mL of ben-
dene species I that is generated via a 1,2-iodine shift.
zene and heated to 608C. The diiodo compound 7 was dis-
The formation of a gold(I) vinylidene species by
solved in additional 1.5 to 2 mL of benzene (depending on
a 1,2-iodine shift was already discussed by Fꢀrstner’s
group in the gold-catalyzed synthesis of halophenan-
threnes[10] and in a recent publication on the synthesis
of 3-iodo-2H-chromene derivatives by the Gonzꢃlez
group.[11] Induced by the high electrophilicity of the
generated vinylidene species I, cyclization takes place
in the next step. Upon addition of benzene to the a-
position a proton/deuterium is released which proto-
deaurates the catalyst and closes the catalytic cycle.
Path b is initiated by an arylating cyclization that
would be related to the synthesis of a-substituted
napththalenes from the corresponding terminal start-
ing materials under “a-conditions”.[12] Upon addition
of benzene a proton/deuterium would be released
that could protonate in b-position to the gold frag-
ment which would generate a gold carbene intermedi-
ate V. After selective 1,2-iodine migration and subse-
quent elimination of the catalyst, product III would
also be formed. Like for path a this pathway would
be in accordance with the observed deuterium label-
ling. Therefore we cannot rule out this mechanistic
scenario even if it is highly speculative that instead of
protodeauration, protonation and carbene formation
should be favored.[8,13] The absence of protodeaura-
tion products originating from intermediate IV could
be evidence in favor of path a. In the context of these
reactions, it should be kept in mind that previous in-
vestigations on the iodine-induced cyclizations of dia-
the solubility) and added dropwise (about 4 drops per
minute) using a syringe pump. The reaction mixture was
stirred at 608C until completion was indicated by TLC.
Then the mixture was taken up with DCM and adsorbed
onto Celiteꢄ. The solvents were removed under reduced
pressure and the crude product purified by flash column
chromatography.
2,4-Diiodo-6,7-dimethyl-1-phenylnaphthalene (15a): Ac-
cording to the general procedure, 1.00 equiv. of 1,2-bis(io-
doethynyl)-4,5-dimethylbenzene (7a, 76.0 mg, 187 mmol) dis-
solved in 1.5 mL of benzene were added dropwise to the
catalyst (8.10 mg, 9.36 mmol). The mixture was stirred for
2 h at 608C. After flash column chromatography (SiO2, PE)
15a was obtained as a yellow solid was obtained; yield:
60.0 mg (124 mmol, 70%). Rf (PE/EA 10:1)=0.64; decompo-
sition at 2008C; IR (KBr): n=3055, 2971, 2937, 1623, 1546,
1
1496, 1443, 1325, 1266, 1130, 1027, 869, 699 cmÀ1; H NMR
(300 MHz, CD2Cl2): d=2.28 (s, 3H), 2.45 (s, 3H), 7.09 (s,
1H), 7.20–7.23 (m, 2H), 7.47–7.55 (m, 3H), 7.84 (s, 1H),
8.51 (s, 1H); 13C NMR (75 MHz, CD2Cl2): d=20.1 (q), 20.3
(q), 97.1 (s), 98.8 (s), 127.7 (d), 128.4 (d), 128.8 (d, 2C),
130.3 (d, 2C), 132.1 (d), 132.7 (s), 132.9 (s), 138.3 (s), 138.7
(s), 143.2 (s), 144.7 (d), 145.0 (s); MS (EI+, 70 eV): m/z
(%)=484 (100) [M]+, 230 (26), 215 (23); HR-MS (EI+,
+
70 eV): m/z=483.9194 [M]+, calculated for C18H14I2 :
483.9179 [M]+.
Acknowledgements
lkynylarenes gave different products, and also iodoal- The authors thank Umicore AG & Co. KG for the generous
donation of gold salts.
kynes have been dimerized or been used in cycloiso-
merization reactions.[14]
To demonstrate briefly that the iodinated catalysis
products can be suitable substrates for further metal- References
mediated cross-couplings, substrate 15a was coupled
with phenylboronic acid in an acceptable yield.
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place. Instead products are obtained in which one of
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8 Gold-Catalyzed Hydroarylating Cyclization of 1,2-Bis(2-
iodoethynyl)benzenes
Adv. Synth. Catal. 2014, 356, 1 – 8
Pascal Nçsel, Vanessa Mꢀller, Steffen Mader,
Setareh Moghimi, Matthias Rudolph, Ingo Braun,
Frank Rominger, A. Stephen K. Hashmi*
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