Gold-catalyzed synthesis of 1-naphthylcarbenoids and their synthetic utilization in cyclopropanation reactions

Article abstract of DOI:10.1002/adsc.201400104

1-Naphthylcarbenoids are generated via 1,2-acyl migration and subsequent carbene shift from simple, easily available diyne starting materials. These highly reactive species were allowed to react with differently substituted alkenes in both intermolecular and intramolecular fashions. In the intermolecular cases even a simple 1:1 ratio of the starting materials delivered the corresponding cyclopropylnaphthalenes in high yields by the use of a gold(III) catalyst. The methodology offers a completely new approach to these valuable targets, the new route represents an efficient alternative to common methods that are based on cross-coupling strategies.

Full text of DOI:10.1002/adsc.201400104

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DOI: 10.1002/adsc.201400104
Gold-Catalyzed Synthesis of 1-Naphthylcarbenoids and Their
Synthetic Utilization in Cyclopropanation Reactions
Tobias Lauterbach,a Michael Ganschow,a Matthias W. Hussong,a
Matthias Rudolph,a Frank Rominger,a and A. Stephen K. Hashmia,b,*
a
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢀt 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 (KAU), Jeddah 21589, Saudi Arabia
b
Received: January 27, 2014; Published online: March 3, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400104.
Abstract: 1-Naphthylcarbenoids are generated via
1,2-acyl migration and subsequent carbene shift
from simple, easily available diyne starting materi-
als. These highly reactive species were allowed to
react with differently substituted alkenes in both in-
termolecular and intramolecular fashions. In the in-
termolecular cases even a simple 1:1 ratio of the
starting materials delivered the corresponding cy-
clopropylnaphthalenes in high yields by the use of
the ability to substitute hazardous diazo compounds
that are widely used as carbenoid precursors.[6]
Interestingly, in spite of a report of Toste, who in
2007 could show that a gold carbenoid generated by
the decomposition of a diazo compound could be
transferred over a pendant alkyne,[7] only recently
have reports on the utilization of this methodology
based on gold-catalyzed rearrangements of propargyl-
ic esters as precursors been presented. Earlier this
year, Hirao, Chan, Oh and our group independently
reported on first applications of this interesting reac-
tivity.[8] It could be demonstrated that indeed after
generation of a vinyl carbenoid of type A, if a second
alkyne is intermolecularly offered, a subsequent reac-
tion takes place. In the case of aromatic backbones fi-
nally 1-naphthylcarbenoids B are generated. In this
contribution we report our latest studies on the ex-
ploitation of these useful intermediates. Due to the
high importance, from the perspective of a synthetic
chemist, we regarded an intermolecular cyclopropana-
tion reaction as highly attractive. Nearly all of the
a goldACHTUNGTRENNUNG(III) catalyst. The methodology offers a com-
pletely new approach to these valuable targets, the
new route represents an efficient alternative to
common methods that are based on cross-coupling
strategies.
Keywords: carbene shift; cyclopropanation; gold
carbenoids; gold catalysis; naphthalenes
The first reports on the gold-catalyzed[1] generation of common synthetic strategies for the synthesis of val-
carbenoids by 1,2-migration in propargylic esters are uable arylcyclopropanes are based on cross-coupling
now ten years old (Scheme 1, left).[2] Triggered by the strategies.[9] Thus an alternative strategy towards
pioneering work of Fꢁrstner[3a] and Toste,[4] this sector naphthylcyclopropane derivatives by an as yet rather
of gold catalysis has experienced an impressive evolu- unexplored[10] disconnection would offer an attractive
tion over the years.[5] This process is based on the alternative.
simple availability of the starting materials, as well as
To enable this strategy we considered substrates
bearing two terminal alkynes as subunits for the gen-
Scheme 1. Generation of gold vinyl carbenoids (left); synthesis of gold aryl carbenoids (right).
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Gold-Catalyzed Synthesis of 1-Naphthylcarbenoids
Table 1. Screening of reaction conditions.
Entry
[Au]
AgX
Solvent
Yield[a]
1
2
3
4
5
5
6
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuNTf2
IPrAuCl
–
–
–
–
DCM
DCE
dioxane
nitromethane
benzene
DCE
29%
67%
[b]
–
–
–
[b]
[b]
–
AgNTf2
AgPF6
traces
[b]
IPrAuCl
DCE
–
7
8
IPrAuCl
IPrAuCl
AgSO3CF3
AgBF4
DCE
DCE
traces
–
[b]
9
IPrAuCl
–
AuCl
PPh3AuNTf2
BrettPhosAuNTf2
AuCl3
AgSbF6
AgNTf2
–
–
–
–
DCE
DCE
DCE
DCE
DCE
DCE
traces
–
55%
8%
14%
71%
[b]
10
11
12
13
14
15
–
DCE
80%
[a]
Yields were determined by GCMS using hexamethylbenzene as internal standard.
Decomposition.
[b]
eration of 1-naphthyl gold carbenoids as ideal. Be- thane, benzene) were completely unsuitable (en-
sides the easy preparation of the starting material, we tries 3–5). A completely different picture was ob-
assumed that this might serve as a perfect precursor tained if an in situ activation of the catalyst with
for the reaction with externally offered alkenes. This a silver salt as halide scavenger was applied. Not only
can be rationalized by the consideration that for un- for the triflimide counter ion (which should generate
substituted 1-naphthyl carbenoid I no common com- exactly the same active catalyst as in the previous ex-
peting intramolecular trappings of the carbenoid in- periment) but also with other applied silver salts de-
termediate are reasonable (Table 1). We assumed that composition of the starting material was observed
in this case even equimolar amounts of alkene might (entries 5–9).[12] The negative effect of silver was un-
be sufficient for an efficient interception, which derlined by the corresponding control experiment
would strengthen the synthetic application of these with silver triflimide that led to a complete decompo-
transformations. As a test system for the catalyst opti- sition of the starting material (entry 10). As a conse-
mization we chose diyne 1 in combination with quence we tested simple gold(I) chloride which was
1 equivalent of 2,3-dimethyl-2-butene as alkene. The also an effective catalyst but in comparison with the
screening for the best reaction conditions is summar- carbene ligand the yields were slightly lower
ized in Table 1. To our delight with the common (entry 11). Only poor results were obtained with
IPrAuNTf2 catalyst the desired reaction took place at phosphane-based ligands (entries 12 and 13). Finally,
room temperature in dichloromethane (entry 1). But we applied gold catalysts in the oxidation state III.
despite the full conversion of the starting material, Based on the additional coordination sites in these
yields were only low. A remarkably strong solvent square planar compounds no further activation is nec-
effect for the reaction was observed which might be essary for the catalysts and thus negative silver effects
explained by competing pathways of the carbenoid can be ruled out. To our delight AuCl3 delivered
with the applied solvent.[11] Switching from DCM to a promising result (entry 14). By far the best result
dichloroethane led to an immense increase of the (80% yield) was obtained with goldACHTUNGTRNEUNG
(III) complex 3.[13]
yield without a change in any other parameter At preparative scale (400 mmol) under the optimized
(entry 2). While other solvents (dioxane, nitrome- conditions the reaction delivered 74% of a crystalline
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Tobias Lauterbach et al.
Figure 1. Solid state molecular structures of 2a (left) and cis-2f (right)
solid (Table 1, entry 1). The result of an X-ray crystal lized and analyzed via X-ray diffraction.[14] Electron-
structure analysis gave us the final proof for the for- deficient styrenes like para-fluoro or para-chlorostyr-
mation of the desired 1-cyclopropylnaphthalene deri- ene could be transformed in good yields and accepta-
vate (Figure 1).[14]
ble diastereoselectivities as well (entries 9 and 10).
Next we turned to the evaluation of the substrate The possibility to install a chloride at the aromatic
scope of the reaction. The results are summarized in system is noteworthy as competing oxidative addition
Table 2. The reaction with methylenecyclohexane de- reactions might occur if this substituent would be in-
livered 87% of spiro compound 2b (entry 2). Cyclo- stalled via common cross-coupling strategies.[17]
A
pentene turned out to be suitable as well, but yields chloro-substituent in the benzylic position was tolerat-
were slightly lower for this system and in addition the ed, too. With 4-vinylbenzyl chloride as styrene deriva-
diastereoselectivity turned out to be poor (entry 3). A tive product 2k was obtained in good diastereoselec-
much better result was obtained with indene as tivity and reasonable yield (entry 11). 1,1-Diphenyl-
alkene unit that delivered a high yield of 82% in an ethylene reacted as well, but yields turned out to be
excellent diastereoselectivity (entry 4). Almost exclu- lower compared to the cyclopropanations with mono-
sively the cis-compound could be obtained. Styrene substituted styrene derivatives (entry 12). Finally, nor-
derivatives as starting materials turned out to be suit- bornene was applied as starting material but no com-
able as well (entries 5–12). Styrene itself furnished the plete conversion was possible with the goldACTHNUTRGNEU(GN III) cata-
cyclopropanation product in a yield of 84% in good lyst. In this case a shift to gold(I) chloride led to
diastereoselectivity (cis:trans=8:1).[15] Considering better results. Longer reaction times were necessary
that only one equivalent of styrene is used, the high but finally product 2m could be obtained in moderate
yield is remarkable. No polymerization of styrene yield (entry 13).
seems to take place under the reaction conditions.[16]
Finally, we focussed on a related intramolecular
Even electron-rich styrenes, which should be prone to process. Due to its easy accesibility we applied start-
polymerization, could be converted with high efficien-
cy (entries 6–8). In comparison to styrene, the poor
diastereoselectivity in the case of para-methoxy-sub-
stituted styrene (entry 6) is hard to understand as the
other electron-rich styrene derivatives (para-methyl in
entry 7; 4-tert-butyl in entry 8) delivered an even
better selectivity. Based on the different polarity of
the two diastereoisomers a separation was possible
for this substrate. While the trans-compound was ob-
tained as an oil, the cis-compound could be crystal- Scheme 2. Gold-catalyzed intramolecular cyclopropanation.
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Gold-Catalyzed Synthesis of 1-Naphthylcarbenoids
Table 2. Scope of the reaction.
Entry
1
Alkene
Product
Time [h]
1
Yield (cis:trans)
2a
74%
2
3
2b
2c
1
87%
2.5
68% (1:1)
4
2d
2
82% (25:1)
5
6
7
2e
2f
2g
3
1
1
84% (8:1)
83% (1:1)
71% (20:1)
8
9
2h
2i
2
3
77% (13:1)
73% (11:1)
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Tobias Lauterbach et al.
Table 2. (Continued)
Entry
Alkene
Product
Time [h]
3
Yield (cis:trans)
10
2j
74% (9:1)
11
12
2k
2
67% (12:1)
2l
1.5
90
65%
13[a]
2m
65% (15:1)
[a]
No complete conversion could be achieved using 3; 5 mol% AuCl were employed instead.
ing material 4 as test compound (Scheme 2). Under cates that the formation of the naphthyl carbenoid is
the usual conditions 49% of the interesting pyrroli- a fast process. The solid state molecular structure of
dine derivative was obtained. It is notworthy that no the obtained product is depicted in Figure 2.[14]
competing enyne cyclization was observed which indi-
A different picture was obtained with enyne start-
ing material 6. For this substrate no cyclopropanation
was observed, instead the product of a competing hy-
dride shift was obtained. After elimination of the gold
catalyst, vinyl ether 7 is obtained as a mixture of dia-
stereoisomers (all the NMR data fit to this assign-
ment, but unfortunately no crystals could be obtained
for a final proof). Here a competing elimination of
a proton followed by protodeauration instead of the
formation of the cyclopropane ring seems to be fav-
oured which leads to the formation of a double bond
(Scheme 3).
In summary we have presented a very attractive ap-
proach for the synthesis of cyclopropylnaphthalene
derivatives. The opportunity to use easily available
diynes in combination with alkenes delivers a powerful
Figure 2. Solid state molecular structure of compound 5.
Scheme 3. Formation of vinyl ether 7.
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Gold-Catalyzed Synthesis of 1-Naphthylcarbenoids
[M+NH4]+ (100), 295 [M+H]+ (8); HR-MS [DART (+)]:
m/z=312.1960, calcd. for [C20H22O2 +NH4]+: 312.1958.
alternative to commonly used cross-coupling strat-
egies. The methodology enables the fast synthesis of
highly complex derivatives based on simple alkene
precursors instead of the application of preformed cy-
clopropyl precursors which are applied in cross-cou-
pling strategies together with functionalized naphtha-
lene precursors.
Acknowledgements
Gold salts were generously donated by Umicore AG & Co.
KG.
Experimental Section
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